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Porphyry copper deposits consist of low-grade stockwork and disseminated sulfide zones that contain characteristic vein generations formed during the evolution of the magmatic-hydrothermal systems. The present contribution proposes an interpretive framework for the formation of porphyry veins that is based on quartz solubility calculations in the H2O-NaCl system at temperatures of 100° to 1,000°C and pressures of 1 to 2,000 bar. The model predicts that high-temperature (≥500°C) quartz in A veins of deep (≥4 km) porphyry deposits forms as a result ol the cooling of ascending intermediate-density fluids at lithostatic conditions. In deposits of intermediate depths (-1.5-4 km), A vein quartz is mostly formed through cooling of ascending hydrothermal fluids under closed-system conditions or quasi-isobarie cooling under open system conditions within the two-phase field of the 1 LO-NaCl system. In shallow (≥1.5 km) porphyry deposits, rapid decompression can also result in quartz precipitation, forming so-called banded veins. The high-temperature quartz in A veins is associated with potassic alteration. During continued cooling of the magmatic-hydrothermal system, quartz is formed at intermediate temperatures (≥375°-500°C). This quartz overprints earlier A veins and forms B veins. The fluid inclusion inventory of this quartz generation suggests formation at fluctuating pressure conditions, marking the lithostatic to hydrostatic transition, and the change of wall-rock behavior from ductile to brittle conditions. The quartz is precipitated because of cooling and decompression of the magmatic-hydrothermal fluids under K-feldspar-stable conditions. Textural evidence from many porphyry veins suggests that hypogene sulfide minerals present in A and B veins postdate the quartz, as contacts between quartz and sulfide minerals commonly show dissolution textures. Hypogene sulfide minerals in C veins form at conditions of retrograde quartz solubility, explaining why these veins contain little to no quartz. The quartz solubility calculations suggest that C vein formation occurs at temperatures of ~375° to 450°C from low-salinity, single-phase fluids escaping from the lithostatic to the hydrostatic environment. At the upper end of this temperature range, C veins are biotite stable. However, these veins are associated with chlorite, chlorite-K-feldspar, or chlorite-sericite alteration in most deposits. Late quartz is formed during continued cooling of the hydrothermal fluids at £.375°C within the single-phase field of the I LO-NaCl system as quartz solubility under these conditions decreases with temperature. This process is responsible for the formation of quartz in D veins and later base metal-bearing E veins, which are associated with phyllie, advanced argillic, or argillic alteration. [ABSTRACT FROM AUTHOR]

The discovery of the world-class Cu-Au deposit at Tumpangpitu, East Java (1.9 Gt @ 0.45% Cu, 0.45 g/t Au), has reinforced the eastern Sunda arc as a significant metallogenic belt that is highly promising for the discovery of major porphyry deposits. The arc hosts three premier porphyry Cu-Au deposits at Batu Hijau, Elang, and Tumpangpitu (>3001 Au and >5 Mt Cu). These giant porphyry deposits are confined to the eastern segment of the eastern Sunda arc (eastern Java to Sumbawa), constructed on island-arc crust where the Roo Rise is being subducted. By contrast, major epithermal (low-sulfidation dominant) deposits associated with poorly endowed porphyry prospects occur along the western segment of the arc (western Java), developed on thick continental crust on the southern margin of Sundaland associated with subduction of typically thin Indian oceanic crust. Porphyry Cu-Au deposits of the eastern Sunda arc are spatially associated with small, nested, dioritic to tonalitic intrusive complexes with low-K calc-alkaline to weakly alkaline signatures. Hypogene alteration and mineralization developed during three main temporally and spatially overlapping events, termed early, transitional, and late. The hypogene Cu-Au mineralized zone of the giant deposits forms a bell-shaped body ol 0.3% Cu shell greater than 1 km in both diameter and vertical extent that is centered on multiphase, elongate, pencil-like tonalite intrusions, 200 to 1,200 m in diameter and >2 km vertical extent. Large (>20 km²) lithocaps associated with high-sulfidation epithermal systems and postporphyry mineralization diatreme breccias characterize productive porphyry districts at Batu Hijau, Elang, and Tumpangpitu. The known porphyry deposits with metal content exceeding 3001 Au and >5 Mt Cu all formed later than 5 Ma, suggesting an important change in the metallogeny of the arc at this time. These giant porphyry deposits formed relatively rapidly, typically over a period of 30 to 860 k.y. during the Pliocene-Pleistocene, between 5 and 2.5 Ma. Porphyry deposits along the eastern Sunda arc display characteristic geologic features similar to porphyry deposits elsewhere. Significant differences include low-K systems, dominance of hydrothermal biotite over K-feldspar, early intense actinolite alteration associated with ore, widespread shreddy chlorite after secondary biotite, narrow "quartz-sericite-pyrite alteration zones, lack of development of chalcopyrite veins, large overlying lithocaps, and abundant chalcocite-bornite-covellite veinlets associated with late overprinting epithermal systems. Hydrothermal oligoclase is also more abundant than K-feldspar in the central biotite-magnetite zones. [ABSTRACT FROM AUTHOR]

Geologic field relationships combined with spatially resolved geochemistry and high-precision U-Pb geochronology of zircons allow quantification of magmatic processes leading to porphyry emplacement and Au-Cu ore formation at Ok Tedi, the Earths youngest giant porphyry-skarn deposit. Trace element contents in zircons were obtained by laser ablation-inductively coupled plasma-mass spectrometry before selected grains were removed from the sample mount, chemically abraded, spiked, and dissolved for high-precision U-Pb geochronology by chemical abrasion-isotope dilution-thermal ionization mass spectrometry. U-Pb geochronology by laser ablation-sector field-inductively coupled plasma-mass spectrometry on abundant inherited zircon cores that occur in all intrusions together with Hf isotope analyses by solution multicollector-inductively coupled plasma-mass spectrometry permitted further assessment of the degree of crustal assimilation of the upper crustal magma reservoir. The combined chemical data and precise crystallization ages of zircons indicate closed system fractional crystallization and concurrent cooling in a common magma reservoir, from which the intrusions at Ok Tedi were successively extracted. Inherited zircon populations and Hf isotope analyses of Pleistocene zircons record lower crustal assimilation with Proterozoic basement. Based on cathodoluminescence (CL) imaging a chemically distinct zircon population with low Th/U was identified in the youngest synore intrusive rocks. These zircons indicate injection of a distinct magma into the common magma reservoir, which triggered the emplacement of the youngest porphyry, intrusion of a polymictic breccia with partly igneous matrix, and intimately associated hydrothermal Au-Cu mineralization. Crystallization ages of chemically characterized zircon crystals extend over a total time span of 212 ± 44 k.y., recording the minimum duration of fractional crystallization in the large magma reservoir from the time it reached zircon saturation. The youngest zircons in each intrusion resolve the emplacement of early thorite intrusions, low-grade monzodiorite porphyries, including the Sydney monzodiorite and Fubilan porphyry, into three time-resolved pulses separated by a total of ~160 k.y. The Sydney monzodiorite contains barren quartz veins and produced endoskarn and exoskarn; however, the exact timing of skarn-hosted Au-Cu mineralization is not yet clear. The emplacement of the ore-bearing Fubilan porphyry and the intrusive breccia overlap at 1.187 ± 0.022 Ma, constraining the time and duration of porphyry emplacement with associated Au-Cu mineralization. The injection of a distinct, probably more mafic magma batch into the magma reservoir preceded the emplacement of the Fubilan porphyry and the intrusive breccia by less than the resolution of 30 k.y., indicating that this event was the immediate trigger for Au-Cu mineralization at Ok Tedi. [ABSTRACT FROM AUTHOR]

The Wafi-Golpu district of Papua New Guinea contains contrasting high- and intermediate-sulfidation styles of epithermal veins and alteration that have overprinted and partially obscured the giant, high-grade Golpu porphyry Cu-Au deposit. The initial stages of magmatic-hydrothermal activity produced high-grade porphyry-style assemblages diat are zoned about the Golpu diorite intrusions, with most mineralization formed between 8.76 ± 0.02 and 8.73 ± 0.01 Ma. Porphyry mineralization was followed by the eruption of the Wafi diatreme, after which upward-widening zones of epithermal alteration and mineralization have overprinted both the mineralized Golpu diorites and the Wafi diatreme. Above the top of the Golpu porphyry, vein and alteration assemblages include quartz (locally vuggy), alunite, pyrophyllite, kaolinite, dickite, and diaspore, with high-sulfidation mineralization consisting of pyrite-covellite-ehalcocite-tennantite ± enargite ± bornite that transitions outward to a distal assemblage of disseminated pyrite-sphalerite ± tennantite. A zone of intermediate-sulfidation epithermal carbonate-sulfide ± quartz ± adularia veins and montmorillonite-ehlorite ± muscovite ± illite alteration occurs along the margins of the high-sulfidation domain. Most of the epithermal Au occurs near the transition from the high- to intermediate-sulfidation domains. New geochronological results indicate that the main stage of Golpu porphyry mineralization to the last stage of Wafi epithermal veins lasted between 120 and 220 k.y. [ABSTRACT FROM AUTHOR]

The Tumpangpitu porphyry and high-intermediate-sulfidation epithermal deposit is the largest deposit in the Tujuh Bukit district, southeast Java, Indonesia. The porphyry resource contains 1.9 billion tonnes @ 0.45% Cu and 0.45 g/t Au, for 28.1 Moz Au and 19 billion lbs of Cu. There are an additional 2.1 Moz Au and 72.9 Moz of Ag in oxidized high-sulfidation epithermal mineralization. Tumpangpitu is located along a NW-striking structural corridor covering an area of 12 x 5 km that hosts several Cu-Au-Mo mineralized tonalitic porphyries, each with varying degrees of metal enrichment. At least eight discrete intrusions spanning the alteration-mineralization sequence have been identified at Tumpangpitu. What is unusual, however, is the presence of both a premineralization, relatively dry volcanic breccia pipe (Tanjung Jahe) and a late-mineralization diatreme complex associated with a significant, large magmatic-hydrothermal system (Tumpangpitu) in the same district. Magmatism, mineralization, and alteration at Tumpangpitu occurred in response to north-directed subduction of the Indo-Australian plate beneath the Asian continental plate margin. The Tujuh Bukit district is floored by early to late Miocene sedimentary and andesitic volcanic rocks. Volcanic-hydrothermal activity at Tujuh Bukit began with the formation of the weakly altered Tanjung Jahe diatreme complex (U-Pbzircon ages of 8.78 ± 0.22-8.52 ± 0.21 Ma). Mineralization at Tumpangpitu was preceded by the intrusion of a large, equigranular, dioritic batholith (5.81 ± 0.20-5.18 ± 0.27 Ma). Hydrothermal activity associated with mineralization has been constrained by U-Pb age determinations from syn- to late-mineralization porphyries that were emplaced in the early Pliocene from 5.40 ± 0.46 to 3.94 ± 0.69 Ma. High- and intermediate-sulfidation Au-Ag ± Cu mineralization and associated advanced argillic alteration (part of a district-scale lithocap) has overprinted and significantly upgraded the top of the porphyry orebody. 40Ar/3SAr dating of alunite (4.385 ± 0.049 Ma) and Re-Os dating of molybdenite (4.303 ± 0.018 Ma) have defined a short time period between the high-sulfidation epithermal and porphyry mineralization events. This suggests extreme rates of uplift, exhumation, and erosion in the vicinity of the Sunda-Banda magmatic arc. Volcanic-hydrothermal activity associated with the Tumpangpitu diatreme occurred during epithermal mineralization (breccia matrix zircon age of 2.7 ± 1.0 Ma with systematic errors). Clasts of high-sulfidation state mineralized rocks are a minor but significant component of the diatreme, and late-stage epithermal veins cutting the diatreme demonstrate an intermineralization timing with respect to epithermal activity in the district, implying that epithermal mineralization continued intermittently for 1 to 1.5 m.y. after porphyry mineralization ceased at Tumpangpitu. [ABSTRACT FROM AUTHOR]

The late Miocene, calc-alkalic, Wainaulo Cu-Au porphyry deposit of the Namosi district, Fiji, hosts distinct styles of alteration and mineralization that overlapped to produce a substantial porphyry Cu-Au resource. The early stages produced medium-grade Cu, low-grade Au and concentric calc-potassic to propylitic alteration that is zoned around the early-stage diorite intrusions. Discrete zones of high-grade Cu and Au and calc-sodic alteration were then superimposed during the intrusion of the main-stage quartz diorites and the formation of quartz-sulfide and epidote-sulfide veins. As the magmatic-hydrothermal system waned, lower Cu and Au grades, with a weaker intensity of calc-sodic alteration and lower density of veins, were produced coincident with emplacement of subsequent quartz diorite intrusions. Late-stage anhydrite-pyrite veins and chlorite-illite alteration overprinted the quartz diorite intrusive complex. The final hydrothermal event consisted of argillic alteration that was concentrated in and around steeply dipping, ENE-trending shears. These structures appear to have controlled the emplacement of the quartz diorite complex and distribution of high-grade Cu-Au mineralization, suggesting they were active during the pre- and synmineralization stages. Stable and radiogenic isotopic data provide evidence for direct seawater contributions to the magmatic-hydrothermal system. Measured δ34Ssulfide (-5.0 to 3.8‰) and (δ34Ssulfate (9.0-16.8‰) values are consistent with a predominantly magmatic source, whereas an elevated bulk sulfur composition (6.7‰) suggests mixing with an isotopically heavy fluid (e.g., seawater). Estimates of δDfluid derived from epidote (-9.1 to 11.3‰) and δ18Ofluid from epidote and anhydrite (-0.2 to 4.7‰) approach that of Vienna standard mean ocean water, and the initial Sr isotope ratios of epidote (0.70364-0.70378) suggest a component of seawater Sr ranging from 3.2 to 5.8%. These results are consistent with the inferred submarine paleogeographic setting and may explain the abundance of albite- and epidote-rich alteration assemblages at Wainaulo. [ABSTRACT FROM AUTHOR]

The Boyongan and Bayugo porphyry Cu-Au mineral deposits, discovered under postmineralization cover during the previous decade, are part of an emerging belt of intrusion-centered Au-rich Cu mineral deposits and prospects in the Surigao district of northeast Mindanao, Philippines. Since their formation in the early Pleistocene, exhumation and weathering of these deposits have led to the development of a 600-m-thick oxidation profile at Boyongan and a modest (30-70 m) oxidation profile at Bayugo. Debris flows, volcanic material, and fluviolacustrine sediments accumulating in the actively extending Mainit graben subsequently covered the weathered deposits, preserving their supergene profiles. The mineral deposits formed in association with a composite diorite complex containing at least 12 discrete intrusive stages. Three premineralization diorite poiphyry stocks and a silt-sand matrix breccia complex represent early stages of magmatism and brecciation. Significant Cu and Au introduction followed these events and occurred in association with small early-mineralization diorite poiphyry stocks at Boyongan and Bayugo. Within the diorite complex, the two mineral deposits are spatially distinct, separated by approximately 1 km of premineralization diorites. Inter- and late-mineralization intrusions were emplaced as the magmatic-hydrothermal system waned. A characteristic progression of vein and K silicate alteration styles affected each of the synmineralization intrusions. Rare comb quartz unidirectional solidification textures (stage 0) mark the transition from magmatic to hydrothermal conditions. Quartz-poor wispy magnetite-biotite-K-feldspar veinlets characterize stage 1. Stage 2 veins consist of quartz with selvage and/or disseminated magnetite or biotite and K-feldspar halos. Stage 3 quartz veins have K-feldspar halos but generally lack magnetite and biotite. Stage 4 veins consist of massive bornite-chalcopyrite and chalcopyrite-pyrite with K-feldspar halos. Stage 3 quartz veins and stage 4 sulfide veins host Cu-Au mineralization of the greatest volumetric significance, reflecting the general paucity of sulfide in the earlier vein stages. Despite the simplicity of this sequence, detailed paragenetic reconstructions reveal that this characteristic progression of veining and K silicate alteration was repeated with the emplacement of each synmineralization intrusive event, revealing multiple magmatic-hydrothermal cycles of alteration and mineralization. At Boyongan and Bayugo, intense and pervasive illite alteration, in association with pyrite, chalcopyrite, and tetrahedrite-tennantite, developed in narrow structures crosscutting quartz-K-feldspar veins. Debris flows in the burial sequence above and adjacent to the Boyongan/Bayugo complex also contain abundant clasts with intense, pervasive illite and alunite-pyrophyllite-dickite-kaolinite alteration assemblages that have overprinted K silicate-style quartz veins. In both mineral deposits, Cu and Au are associated with intense quartz-vein stockworks composed primarily of K silicate stage 3 veins. Despite this association, not all of these quartz-vein stockworks contain Cu and Au to the same tenor. Quartz-vein stockworks in die eastern high grade of Boyongan have been intersected over a vertical interval of 800 m, having affected much of the early-mineralization stock. However, hypogene Cu grades exceed 0.5% by weight only in the upper 300 m of the stockwork (in the cupola of the early-mineralization stock). Superior grade development in high-grade zones at Boyongan, locally exceeding 2% Cu and 3 g/t Au, developed where fertile vein stages from two or more magmatic-hydrothermal cycles affected the same wall rock. The documented paragenetic relationships demonstrate multiple discrete cycles of K silicate-stage veining and alteration associated with each synmineralization intrusive event. All such events predate formation of feldspar-destructive illite-smectite-chlorite, illite-pyrite, and quartz,-alunite-clay assemblages. Existing geochronological constraints on the timing of magmatism and hydrothermal activity demonstrate that these repeated cycles supporting superior grade development transpired extremely rapidly, in a period of less than 200,000 years (2.3-2.1 Ma). Geologic and geochronologieal constraints on the life cycle of Boyongan and Bayugo describe an extremely dynamic history of emplacement, exhumation, weathering, and burial over a period of 2.3 m.y. The study illustrates the spectrum of metallogenic processes operative over a geologically brief period and highlights some of the key elements responsible for formation of superior grades and for deposit preservation in an extensional setting. [ABSTRACT FROM AUTHOR]

The Black Mountain porphyry Cu-Au deposit in the Baguio district, Northern Luzon (Philippines) is associated with late Miocene to Pliocene intrusive rocks that retain primary crystallization signatures, providing an excellent natural laboratory to study the details of magma chamber processes. Each igneous phase contains disequilibrium textures, including resorptive textures in plagioclase and distinct core rim textures in amphibole. High crystallization temperatures and pressures in felsic (~880°C and ~2.8 kbar) and mafic (~1,030°C and ~7.7 kbar) rocks recorded by amphibole phenocrysts reflect crystallization in the magma chamber. Amphibole in felsic rocks aged 6.39 ± 0.49 to 2.83 ± 0.23 Ma have similar crystallization temperatures and pressures, suggesting a long-lived and hot felsic magma chamber under the Black Mountain deposit. Four independent oxybarometers, including amphibole, coexisting magnetite-ilmenite, magmatic anhydrite, and zircon, consistently show a high oxidation state of >NNO+1 for the felsic magma and ~NNO+O.7 for the mafic magma. Element profiles across phenocrystic plagioclase display repeated oscillatory variations of Ca, Fe, and Sr contents with large anorthite variations and a positive correlation between CaO and FeO. The amphibole is classified as group 1 Mg hornblende to tschermakite-hornblende in felsic rocks and group 2 Mg hastingsite in mafic rocks. The group 1 amphiboles show a variety of major and trace element variations from core to rim in disequilibrated grains and transitional compositions to group 2 Mg hastingsite. Both mineral textures and chemistry recorded by phenocrysts of plagioclase and amphibole indicate repeated mafic magma recharge in the felsic magma chamber. The common occurrence of rounded or elongated sulfide inclusions in the megacrystic amphibole from the mafic rocks (2.81 ± 0.15 Ma) suggests a greater enrichment of Cu in mafic rocks relative to the ore-forming porphyries (2.91-2.83 Ma). This is also consistent with higher Cu contents (3.4 ± 0.9 ppm) in the megacrystic amphibole from the mafic rocks relative to amphibole in felsic rocks (1.0 ± 0.2 ppm) and may indicate up to 3.4 times higher Cu in the mafic magma relative to felsic magma. Therefore, large-scale mafic magma recharge (particularly at ~2.8 Ma), likely introduced ore-forming metals to the long-lived felsic magma chamber, further contributing to the fertility of the felsic rocks and ultimately to Cu-Au mineralization at the Black Mountain deposit. [ABSTRACT FROM AUTHOR]

In the Abitibi subprovince of the Superior craton, Canada, recognition of intrusion-related gold deposits is improving, but their formation remains poorly understood. The recently reopened Lac Bachelor deposit in the northeastern part of the Abitibi subprovince is located on the edges of a syenitic complex, and this deposit is an example of a highly fractionated, Fe-Mg–poor, quartz-syenite to alkali-granite centered deposit. Gold is associated with pyrite in localized alkaline, fluorine, and hematite-rich metasomatized zones in magnetite-rich host rock. Minor mineralization is present in a quartz vein stockwork on the edges of the syenitic complex. A genetic model is proposed, wherein alkaline-oxidized magma, which favors sulfur and gold solubility and transport, migrates through the crust via major and subsidiary faults. The presence of miaroles, transolvus, and low-temperature crystallization conditions confirms the shallow emplacement of the magma. The high fluorine content of the magma, which lowers both viscosity and solidus temperature, favored the shallow emplacement of the magma and permitted the development of a long-lived magmatic-hydrothermal system. Oxygen and hydrogen isotopes, miaroles filling, and mineralized zone paragenesis confirm that oxidized hydrothermal fluids were exsolved from the magma. The planar geometry of the metasomatized ore zones indicates that hydrothermal fluid circulation and metasomatism reactions occurred preferentially in high permeability pathways, such as preexisting faults in the host rock. The redox contrast between the oxidized magmatic-hydrothermal fluid and the intermediate volcanic host rock caused sulfide precipitation and gold deposition. This genetic model is applied to the Abitibi subprovince and other Archean cratons. [ABSTRACT FROM AUTHOR]

The Zijinshan district, Fujian Province, southeastern China, is a major mineral field that contains high- and intermediate-sulfidation epithermal Cu-Au-Ag and porphyry Cu-Mo deposits. This contribution discusses the structural geology and the tectonic regime prevalent during mineralization at Zijinshan. Statistically, the district is dominated by NW-striking fault systems that dip at moderate angles (40°-50°) to the northeast and southwest. They played a first-order role in controlling the emplacement of hydrothermal veins and breccias. Most NW-striking faults were active as oblique normal/strike-slip faults during mineralization, as evidenced by the geometry of syntectonic hydrothermal mineral fibers. NE-striking, steeply dipping faults also controlled the emplacement of hydrothermal veins and breccias, and these faults contain syntectonic mineral fibers indicating predominantly dextral strike-slip movements. The kinematic and dynamic analysis of fault-slip data shows that the predominant tectonic regime during mineralization was extensional, with subvertical σ1 and NNE to NNW-trending 03. Secondary clusters of subhorizontal σ1 suggest that short periods of fault reactivation under a strike-slip regime occurred locally. Although the formation of porphyry Cu-Mo systems is commonly thought to be favored by compressive to transpressive tectonic regimes, which lead to a diminution in the rate of volcanic output and to the entrapment of magmas and volatiles in the upper crust, these results show that at least medium-sized porphyry Cu-Mo systems can form under extensional conditions in previously thickened continental crust. This has important exploration implications globally. Based on current geologic knowledge, the district appears to have a high exploration potential at depth, mainly for intermediate-sulfidation epithermal and porphyry-style mineralization. [ABSTRACT FROM AUTHOR]

The investigation of this paper focused on whether the composition of hydrothermal ore-related magnetite in till could be used to locate porphyry deposits in terrane where glacial overburden overlies rocks that host porphyry Cu-Au mineralization. This hypothesis was tested using 20 till samples collected in a -900-km² area surrounding the Mount Polley poiphyry Cu-Au deposit in south-central British Columbia, Canada. At least 100 magnetite grains were randomly selected from the magnetic fraction of each til! sample. Nineteen trace elements in ~50 magnetite grains in each sample were measured by laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS). The large beam or raster size used for laser ablation (to 100 μm) homogenizes any heterogeneous trace element distributions in magnetite that result from oxy-exsolution and/or and dissolution/reprecipitation, avoiding this issue with the few micron size of an electron beam. Linear discriminant analysis (LDA) performed on a compilation of magnetite compositions measured by LA-ICP-MS from worldwide porphyry deposits and intrusive igneous rocks define the chemical signature (Mg, Al, Ti, V, Mu, Co, Ni) of hydrothermal magnetite exclusive to porphyry systems. Application of our twc LDA models to the 985 magnetite compositions we measured in the till samples surrounding Mount Polley showed anomalous amounts of hydrotnermal magnetite grains in till up to 2.5 km west-southwest and 4 km northwest of the deposit--a pattern that is consistent with the ice-dow history of the region. Our LDA models for magnetite trace element compositions have strong potential to be an effective tool in exploration for buried porphyry systems. [ABSTRACT FROM AUTHOR]

The Ak-5ug porphyry Cu-Au-Mo deposit is situated within the Altai Sayan region that forms part of the Tuva-Mongol arc of the Central Asian orogenic belt. The Ak-Sug intrusive complex is made up of a series of nested intrusions, including diorite, feldspar porphyry, quartz-feldspar porphyry, tonalite I, and tonaliie II. Cu-Au-Mo mineralization occurs mainly within quartz-carbonate ± K-feldspar sulfide veins and later serieite-sulfide veins and alteration. In the northe u zone, sulfide zoning consists of a central bornite core flanked by chalcopyrite and pyrite-chalcopyrite zones, w th a pyrite zone on the hanging-wall side. Re-Os molybdenite dating of an early-stage quartz-carbonate ± K-feldspar sulfide vein and a late-stage molybdenite-only vein gives ages of 517.3 ± 3 and 517.4 ± 3 Ma, respectively. This indicates an Early Cambrian age for mineralization at Ak-5ug, making it one of the eldest porphyry copper deposits in the Central Asian orogenic belt. Published whole-rock geochemical data indicate that intrusive rocks at Ak-Sug and other Cambrian to Early Ordovician intrusive complexes in the Tuva-Mongol arc evolved by hornblende ± titar.ite fractionation due to high magmatic water contents, leading to adarite-like compositions with high Sr/Y and flat to listricshapecl rare earth element patterns that characterize many fertile arc magmas. [ABSTRACT FROM AUTHOR]

The Dabaoshan polymetallic deposits in the Nanling Range, South China, consist of porphyry and skarn-type Mo mineralization genetically related to Jurassic porphyritic intrusions and adjacent strata-bound Cu-Pb-Zn mineralization hosted in mid-Devonian limestone. Porphyry Mo mineralization is characterized by the superposition of multiple generations of crosscutting quartz-bearing veins including: barren quartz veins (VI), quartz-inclybdenite veins with K-feldspar alteration halos that host the bulk of the Mo mineralization (V2), quartz-pyrite veins with muscovite alteration (V3), and late base metal mineralization with argillic alteration (V4), as well as limestone-hosted strata-bound Cu-Pb-Zn mineralization (VS). Fluid inclusion petrography and microthermometry combined with cathodoluminescent textures and trace elements in quartz reveal changes in pressure and temperature of the hydrothermal system that formed the deposit. VI and V2 veins are dominated by low-salinity (1-6 wt % NaCl equiv), CO2-bearing (4-10 mol %) two-phase inclusions with about 35 vol % bubble trapped in the one-phase field above the solvus of the fluid. VI veins are dominated by CL-bright granular quartz mosaics with higher CL intensity than any other vein type. This quartz also contains more Ti tnan any other vein generation (24-89 ppm), while Al, Ge, and Li concentrations overlap with other vein generations. Molybdenum ore-hosting V2 veins are also dominated by CL-bright granular quartz mosaics, but V2 veins display slightly less CL intensity and correspondingly lower Ti concentrations (10-65 ppm). V3 veins have a broader spatial distribution than VI and V2 veins, extending from inside the porphyries out into tire adjacent Limestone. These veins are also dominated by low-salinity (2-6 wt % NaCl equiv), CO2-bearng (4-7 mol %) two-phase inclusions, these containing about 45 vol % bubble trapped above their solvus. V3 veins are dominated by CL-dark quartz with euhedral growth zones of oscillating CL intensity and systematically lower Ti concentrations than previous vein generations (1.5-12 ppm). Minor V4 veins cut all the above vein generations and also occur as late infill in V3 veins. Low-salinity (4-7 wt % NaCl equiv), CO2-bearing (4-5 mol %) two-phase inclusions with about 20 vol % bubble prevail in V4 veins. V4 veins have the lowest CL-intensity quartz of all veil, types, with euhedral growth zones of oscillating CL intensity and the lowest Ti concentration of all vein types (0.54--5,3 ppm). Intersections of fluid inclusion isochores with Ti-in-quartz isopleths indicate that the hydrothermal system evolved from near-magmatic pressures and temperatures of 2.7 ± 0.2 kbars and 6503 ± 4CPC for VI veins to 1.9 ± 0.2 kbars and 530° ± 40°C for V2 veins to 0.65 ± 0.2 kbars and 400° ± 40°C for V3 veins. V4 veins, which lack rutile, formed as the system exiled to 250° to 300°C at maximum pressures of 0.40 to 0.65 kbar. Unlike nearly all other reported porphyry-1ype ore deposits, quartz-bearing veins from the Dabaoshan porphyry Mo deposit contain few halite-bearing or vapor-dominated fluid inclusions in any vein type. The dearth of such fluid inclusions, coupled with the abundance of two- and three-phase CO2-bearing low-salinity fluid inclusions in all vein types is evidence that the formation conditions of VI to V4 veins remained above the V-L surface in the H2O-NaCl-CO2 system, such that fluid unmixing rarely occurred in the Dabaoshan hydrothermal system. This is further supported by only rare evidence for quartz dissolution textures in all vein types, implying that pressures were dominantly higher than zone of retrograde quartz solubility. Taken together, the fluid inclusions, CL textures, and quartz trace element data indicate that the Dabaoshan porphyry Mo deposit is one of the deepest formed porphyry-type ore deposits, having formed at depths of 6 to 7 km below surface. The extreme depth and lack rf fluid unmixing inhibited Cu precipitation in the porphyry system, and instead allowed Cu to remain in solution to precipitate with Pb and Zn upon interaction with the surrounding Devonian limestone. [ABSTRACT FROM AUTHOR]

The Cerro la Mina Au (Cu-Mo) porphyry-high sulfidation prospect is located in Chiapas State, southeastern Mexico, outside of the major metallogenic provinces of Mexico. The prospect is hosted by Pleistocene alkaline volcanic rocks of the Chiapanecan volcanic arc that formed in a complex triple-junction tectonic setting. Cerro la Mina's stratigraphy comprises pyroclastic flows that were intruded by monzodiorites and diorites at 1.04 ± 0.04 Ma tU-Pb, zircon), and that were overlain by debris flows and synvolcanic trachyandesite domes. The volcanic stratigraphy of Cerro la Mina is dominated by pyroclastic flows and rare basalts that are cut by the Cerro la Mina breccia pipe, a matrix-rich granular, vertically oriented, downward-tapering, polymict lithic rock unit that.s host to all of the significant alteration and mineralization. A NW-trending sinistral wrench fault, which was active throughout the history of Cerro la Mina, is responsible for dismembering the prospect after mineralization. The magmatic hydrothermal system was composed of early porphyry-style potassic veins (quartz + K-feldspar ± biotite) and stage 1 pyrite that are preserved in clasts within the breccia pipe, suggesting that brecciation disrupted an embryonic porphyry system. Late potassic alteration occurred after the formation of the breccia pipe, as its matrix is strongly K-feldspar altered. Hydrothermal fluids then produced phyllie alteration composed of cuartz, muscovite, illite, illite-smectite, and chlorite that is associated with stage 2 pyrite ± chalcopyrite ± molybdenite ± quartz veins. An unusual zoned pattern of advanced argillic-argillic alteration overprinted potassic and phyllie alteration. This zoning included a low-temperature (<110°C) halloysite + kaolinite that extends from 800 to 250 m below present-day surface and is deeper than higher temperature (>120°C) quartz + dickite ± kaolinite ± pyrophyllite ± alunite that occurs from 250 m to the present-day surface. The advanced argillic-argillic altered rocks host the most significant Au-Cu mineralization, which is associated with stage 3 marcasite. sphalerite, galena, and barite, and stage 4 arsenian pyrite ± enargite ± covellite. The magmatic hydrothermal system at Cerro la Mina began sometime between monzodiorite emplacement (1.04 ± 0.14 Ma; zircon U-Pb) and the precipitation of porphyry stage 2 molybdenite at 780 ± 10 ka (Re-Os). 40Ar/39Ar dating of biotite (689 ± 13 ka) records the age at which the hydrothermal system cooled below the biotite elesure temperature of ~300°C and provides a maximum estimate for the onset of advanced argillicargillic alteration. Sulfur isotope results of sulfides (-2.5 to +4.9%c; mean +0.7%e; n = 20) and a sulfate (barite; + 10.5%o; n = 1) suggest a magmatic source of sulfur for all four stages of mineralization. The lack of residual quartz, rare alunite, and anomalous halloysite-kaolinite alteration may be explained by the high acid-buffering capacity of alkaline volcanic host rocks, high COa contents of the alkaline magma, and/or potentially by a highly reduced magmatic hydrothermal fluid. At the regional metallogenic scale, the Cerro la Mina prospect along with the nearby Santa Fe mine and Campamento deposit represent parts of a porphyry copper system--specifically, a porphyry/high-sulfidation, proximal skarn and intermediate sulfidation deposit, respectively. The characteristics of Cerro la Mina (i.e., anomalous halloysite-kaolinite alteration) broaden the window for additional discoveries to be made in the porphyry-epithermal environment. [ABSTRACT FROM AUTHOR]

Field relationships and U-Pb data (zircon) are used to constrain the timing of tectonomaginatic events that occurred shortly after the formation of Late Devonian (~372 Ma) porphyry Cu-Au deposits at Oyu Tolgoi (Mongolia). In tire two northernmost of the several discrete porphyry centers (Hugo Dummett deposits), Upper Devonian rocks positioned stratigraphically above copper-gold mineralization preserve a record of uplift, exhumation, and subsequent burial shortly after mineralization. Strongly altered and mineralized quartz mon-zodiorite and volcanic rocks at Hugo Dummett are overlain by an unmineralized dacite sequence (369 ± 2 Ma). The basal breccia and conglomerate of the dacite sequence contain mineralized porphyry clasts derived from erosion of the porphyry deposits. Two zircon grains recovered from one of these mineralized porphyry clasts yielded chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) 206Pb/238U dates of 374.0 ± 1.9 and 371.4 ± 1.6 Ma, which are consistent with previously published ages for mineralized intrusions. This implies that Oyu Tolgoi or other nearby porphyry deposits were exhumed shortly after the main mineralization event. Weakly mineralized granodiorite dikes intruded the postmineralization dacite sequence, as well as Devonian sedimentary and voleaniclastic rocks that structurally overlie the dacite. A weighted mean 206Pb/238U age of 370.6 ± 0.5 Ma for this rock is based on the three youngest grains analyzed by the CA-TIMS technique (MSWD = 1.14). Burial of the Cu-Au deposits at Oyu Tolgoi soon after their exhumation was critical to their preservation. Exhumation and subsequent burial occurred at ~371 Ma, during a short time frame within the overlapping U-Pb zircon ages for the quartz monzodiorite, mineralized porphyry clast, unmineralized dacite sequence, and granodiorite unit. [ABSTRACT FROM AUTHOR]

Porphyry-style Cu (Au, Mo) mineralization is widely associated with igneous suites with Sr/Y > 40 and low heavy rare earth element (HREE) content. This geochemical signature is commonly attributed to garnet or amphibole fractionation in the lower crust where mantle derived melts evolve into intermediate composition arc suites. However, the presence of a residuum related to mineralized porphyry systems where amphibole and garnet are stable is generally inferred from geochemistry and has nowhere been documented directly. We here report on two occurrences of late Miocene garnet-bearing granodiorite porphyry at the Tesorito Au-Cu prospect in the Quinchia district and El Poma Au-Cu prospect in the Middle Cauca Au-Cu porphyry belt in Colombia. These rocks provide proof that garnet was stable in the lower crust at the time of porphyry mineralization. At both studied sites, gamet-bearing I-type granodiorite porphyries were intruded by intramineralization garnet-free plagioclase phyric granodiorite porphyries that were cut by quartz and K-feldspar veins and affected by biotite alteration. Garnet-bearing porphyries predate intramineralization porphyries by 0.4 to 2 m.y. Garnet occurs as unzoned grains, commonly included in large plagioclase phenocrysts, that in some cases also contain inclusions of igneous epidote; the latter indicating pressures of >1 GPa for early crystallized phases. Large plagioclase phenocrysts exhibit two growth stages separated by a resorption boundary. High-Al amphibole phenocrysts as well as titanite and magnetite formed after the first stage but prior to the second stage of plagioclase growth at pressures of ca. 0.9 GPa. Whole-rock geochemical compositions of garnet-bearing porphyries are consistent with only limited garnet fractionation whereas some of the intramineralization porphyries have Y <11 ppm and Sr/Y >40, which is, together with other trace element indicators, taken as evidence for significant garnet fraction at depth during porphyry mineralization. HREE content of zircon in garnet-bearing porphyries are higher than that of intra-mineralization porphyries, which supports the interpretation that garnet fractionation in the lower crust was significant during the formation of the intramineralization porphyries. The phase relationships and geochemical data reflect a geodynamic change from an environment that permitted rapid ascent of oxidized garnet-bearing porphyries prior to Au-Cu mineralization to a regime that favored melt evolution in the lower crust where garnet was a residual phase and at the same time allowed the formation of large mid- to upper-crustal magma chambers. The latter are widely inferred to be essential for porphyry mineralization to occur. [ABSTRACT FROM AUTHOR]

Porphyry Cu ± Mo ± Au and iron oxide copper-gold (IOCG) deposits share many similarities (e.g., Fe, Cu, and Au contents), but also have important differences (e.g., the predominance of sulfide minerals in porphyry deposits and iron oxides in IOCG deposits). Genetic comparisons are complicated by the broad definition of IOCG deposits; here we restrict our study to IOCG deposits that are related to igneous intrusive systems. In the Mesozoic Coastal Cordillera of northern Chile, both porphyry and IOCG deposits occur in close spatial and temporal proximity, offering the chance to examine what controls their different modes of formation. From detailed examination of the timing, geochemistry, and tectonic setting of associated igneous rocks, based on new and published data, we find that rocks associated with mid-Cretaceous IOCG deposits (-125-110 Ma) re largely indistinguishable from those associated with slightly earlier (>125 Ma) and later (<110 Ma) porphyry Cu ± Mo ± Au deposits. Magmas related to IOCG deposits were formed during a brief period of back-arc transtension in the mid-Cretaceous and are, on average, somewhat more mafic (dioritic), locally alkaline, and isotopically primitive compared to granodioritic magmas associated with porphyry deposits formed during normal contractional arc tectonics in the later Cretaceous. However, these compositional ranges overlap, and the differences are not clear enough to be diagnostic. We measured the SO3 content of igneous apatite from selected samples of these rocks to test the hypothesis that the difference in sulfur content of the ore deposits was due to differences in sulfur content of the associated magmas. Early igneous apatite crystals occurring as inclusions in silicate phenocrysts from the Carmen de Andacollo orphyry Cu-Au deposit (Re-Os molybdenite ages of 103.9 ± 0.5 Ma, 103.6 ± 0.5 Ma) are significantly richer in S (0.25 ± 0.17 wt % SO3 , n = 69) than similar apatite crystals from two IOCG deposits (Candelaria, Casualidad) and a sample of regional mid-Cretaceous igneous rock from near Productora (0.04 ± 0.02 wt % SO3 , n = 76). Using published partition coefficients for S between apatite and oxidized silicate melt, we semiquantitatively estimate corresponding magmatic sulfur contents of -0.02 wt % S in the Carmen de Andaeollo magmas versus -0.001 to 0.005 wt % S in the IOCG-associated magmas. This is an order of magnitude difference, and the opposite of what would be expected if the difference were due to bulk magma composition (sulfur solubility is generally higher in mafic magmas, whereas here the S content is higher in the more felsic porphyries). We conclude that the porphyry-forming magmas indeed had higher S contents than the IOCGrelated magmas and suggest that these differences reflect different petrogenetic processes. During normal subduction, magmas derived from the metasomatized mantle wedge are hydrous, moderately oxidized, and S rich, and have the potential to generate S-rich porphyry-type deposits. In contrast, in back-arc extensional settings, upwelling asthenospheric melts carry a weaker subduction signature, includinmore likely to give rise to S-poor IOCG deposits. [ABSTRACT FROM AUTHOR]

Nuevo Chaquiro is a newly discovered blind porphyry Cu-Au-(Mo) deposit located in the Middle Cauca belt, Colombia, approximately 60 km south-southeast of Medellin. The most recent resource estimate was stated as 566 M tonnes (t) averaging 0.64% Cu, 0.31 g/t Au, 4.34 g/t Ag, and 127 ppm Mo. The geology of Nuevo Chaquiro consists of a Miocene volcanic sequence of andesitic tuffs, agglomerates, and flows (Cornbia Formation) intruded by small stocks and dikes of diorite and quartz diorite also of Miocene age. Sparsely occurring outcrops at Nuevo Chaquiro consist of a 1.2 × 0.8 km zone of pervasive sericitic alteration with locally well developed quartz-Fe oxide stockworks. The sericitic alteration grades downward into sericitechlorite alteration and then predominantly potassic (biotite-magnetite) alteration at depths of 250 to 400 m. Calcic-potassic alteration, containing actinolite and epidote, occurs at greater depths. A high-grade sheeted vein complex occurs immediately adjacent to the upper contact of an Early quartz diorite intrusion. Quartz veinlet density in this area reaches up to 80% quartz by volume, accompanied by up to 5 vol % chaleopyrite. Later Intermineral porphyries created a second orebody slightly to the west but contiguous with the Early orebody. Here Cu-Au-Mo mineralization in an inverted bowl shape is associated with potassic alteration that manifests itself as secondary flooding of the tuff wall rock matrix by hydrothermal biotite, accompanied by disseminations and hair veinlets of chaleopyrite, magnetite, pyrite, and molybdenite. The porphyry system is also accompanied by intermediate-sulfidation carbonate-quartz-base metal veins, typically with sericitic alteration selvages, which occur on the upper western flank of the deposit. At Nuevo Chaquiro, multiple intrusions of Early and Intermineral porphyry contain calcic-potassic and potassic alteration, with a later overprint by sericite-chlorite alteration centered on intrusion apexes. The amount of sericite-chlorite overprinting varies with each intrusion. Collectively, these intrusions are all overprinted by a single, later sercitic (phyllic) alteration event that is superimposed as a broad cap upon the entire system. There is a consistent paragenesis for the alteration/mineralization associated with each intrusion, suggesting a relatively stable underlying shallow magma chamber that fluxed geochemically similar pulses of magmatic/ hydrothermal fluid with each porphyry. In many aspects, Nuevo Chaquiro is a "typical" Au-rich porphyry copper system. However, the contrast between the mineralization styles between the Early and Intermineral orebodies as well the timing differences in copper deposition suggest that differences in porphyry mineralization styles also occur at Nuevo Chaquiro. [ABSTRACT FROM AUTHOR]

Porphyry Cu deposits are commonly thought to have formed by magmas that were unusually rich in metal and/or sulfur. In this study, we test this assumption by reconstructing the metal and sulfur content of an ore-related latite magma at Bingham Canyon and comparing it with that of intermediate magmas in several other arc magma systems. The ore-related latite magma at Bingham Canyon records strong evidence for magma mixing and has a major to trace element composition that can successfully he modeled by a mixture of ∼40 wt % mafic magma, which was similar to the most mafic rock found at Bingham Canyon (a melanephelinite containing 45 wt % Si02), and ∼60 wt % felsic magma of rhyolitic composition. Based on the modal abundance of 0.19 ± 0.01 vol % sulfides and laser ablation-inductively coupled plasma-mass spectrometry analyses of unaltered sulfide inclusions preserved within hornblende and plagioclase phenocrysts, the latite magma contained 50 to 90 ppm Cu, 0.8 to 2.0 ppb Au, 2 to 3 ppm Mo, and ∼0.12 to 0.14 wt % S. Whole-rock and melt and sulfide inclusion data suggest that the bulk of copper and Au in the latite magma was derived from the mafic end member, whereas significant amounts of sulfur were also provided by the felsic end member. A rough, independent estimate of the amount of Cu present in die mixed magma can be obtained by taking the Cu content of mafic, sulfide-undersaturated silicate melt inclusions and multiplying it with the mass fraction of mafic magma involved in the magma mixing. Applying this latter approach to two other poqrhyry Cu-mineralized magma systems (Santa Rita, USA; Bajo de la Alumbrera, Argentina) and several modern arc magma systems suggests that ore-forming intermediate magmas in mineralized systems were not unusually Cu rich. Whether or not they were unusually sulfur rich could not be answered with the available data. If the sulfur contents of mineralizing magmas prove to be normal, then the most distinctive feature of fertile magma systems may be the formation of large, long-lived magma chambers at 5- to 15-km depth and the development of vent structures that enable focused fluid flow. [ABSTRACT FROM AUTHOR]

With many new discoveries of giant Mo deposits in the past decade, northeastern China has become the largest molybdenum mineralization region in China. The Xilamulun district is in the southern part of northeastern China and contains 25 Mo deposits and prospects, dominantly of porphyry type. These deposits are mostly along the E-strildng Xilamulun fault. Re-Os dating of hydrothermal molybdenite from four deposits in the northern Xilamulun district revealed mineralization ages of .137 to 129 Ma (129.4 ± 3.4 Ma for Aolunlma, 135.3 ± 2.6 Ma for Shabutai, 136.4 ± 0.8 Ma for Haisugou, and 136.1 ± 6.6 Ma for Banlashan), in general agreement with the crystallization ages of their host granitoids. The compilation of existing data on Mo-bearing deposits in northeastern China, including the new data of this study, shows that Mesozoic Mo deposits in this region have a wide range of ages, from ~250 to 90 Ma. We propose that they are linked to three tectonic-magmatic events: (1) Triassic Mo deposits (250-220 Ma) are mainly distributed along the E-trending Xilamulun fault and are related to posteollisional crustal extension following the final closure of the Paleo-Asian ocean; (2) Jurassic Mo mineralization (200-140 Ma) displays a clear younging trend from southeast to northwest, coincides with the regional magmatism trend, and is interpreted to be related to the northwestward flat-slab subduction of the Paleo-Pacific plate beneath the Eurasian continent that started in the Early Jurassic (ca. 200 Ma); (3) Cretaceous Mo mineralization (140-90 Ma) shows a reversed migration trend from northwest to southeast, and can be explained by the coastward migration of slab rollback related to lower crust delamination, asthenospheric upwelling, and lithospheric thinning in eastern China. The spatial-temporal distribution of Mesozoic Mo mineralization is important for regional metallogeny and exploration. Recently numerous epithermal Au (Cu, Mo) deposits have been recognized in the southeast margin of northeastern China. According to this study, we predict that hidden poqrhyry/skam systems could be located beneath these epithermal deposits. [ABSTRACT FROM AUTHOR]

This study combines results from reconnaissance-scale mapping of hydrothermal alteration, rock types, and structures to provide a district-scale cross section and associated palinspastic reconstruction of an area with two previously undescribed Laramide (~70 Ma) porphyry systems at Sheep Mountain and Copper Basin (Crown King). Extension at the district scale is placed in an arc-scale context using an original compilation of strike and dip measurements on Tertiary rocks to reconstruct the Laramide porphyry belt prior to extension. The study area contains five sequential, partially superimposed sets of normal faults that are nearly planar where exposed. Dips of all normal faults initiated at 60° to 70° and rotated during slip to angles as gentle as 20°. A palinspastic reconstruction reveals that two, spatially distinct hydrothermal systems overlie different cupolas of a Late Cretaceous pluton. Hydrothermal alteration is zoned from greisen to potassic to transitional greisen-potassic assemblages from deep to shallow structural levels. The reconstruction is used to identify two covered exploration targets. The prospects may be porphyry molybdenum systems of the quartz monzonitic-granitic porphyry Mo-Cu subclass, joining others in an arc that is best known for porphyry copper deposits. The Laramide porphyry belt prior to extension displays a variably well-defined axis, ~100 km wide, with gaps and clusters of deposits along its 700-km strike length. The majority of deposits lie along the axis, but others lie in fore- or rear-arc positions. The interpreted preextensional geometry of the Laramide porphyry belt resembles other porphyry belts and the distribution of active volcanoes at convergent margins. [ABSTRACT FROM AUTHOR]

Zircon LA-ICP-MS and CA-TIMS and molybdenite Re-Os geochronology for the El Abra porphyry copper deposit (Chile) document a -8.6 Ma protracted magmatic-hydrothermal history. Initial magmatic events occurred at -45 Ma, and continued to ~36 Ma. An ~1.8 m.y. magmatic lull is recorded in the early stages of magmatism, which is characterized by equigranular rocks from the Pajonal suite. Magmatism from -41 to -36 Ma, corresponding to the El Abra Granodiorite Complex, is more recurrent, ending in a -1.4 m.y. period of porphyritic intrusions, which are coeval with Cu-Mo mineralization. Late porphyritic units reveal subtle zircon inheritance likely associated with early magmatic pulses. The young stages of magmatism indicate system rejuvenation based on plagioclase phenocryst compositions, suggesting that the porphyry system is ultimately linked to a less evolved magma. Although there are multiple porphyry and veining events, molybdenite Re-Os ages define a focused mineralization episode from 36.34 to 36.18 Ma. Molybdenite Re-Os ages from nearby related prospects belonging to the El Abra cluster expand the mineralization period in the district up to -1.0 m.y., suggesting a multistage, long-lived hydrothermal system. Integration of the molybdenite Re-Os dates and those from previous works at the El Abra and Toki clusters, accompanied by available tectonic reconstruction along the Domeyko fault system suggest the development, by late Eocene times, of a single, large (-30 km across) porphyry copper cluster, which was subsequently separated by an offset of -35 km along the West fault. [ABSTRACT FROM AUTHOR]

In this contribution we discuss a series of porphyry and porphyry-skarn mineral systems, which constitute important metallogenic belts along major lithospheric breaks and/or collision zones that form the margins of the North China craton and the Yangtze craton. We describe a selection of porphyry and porphyry-skarn deposits in the Great Xing an Range in northeast China, the Yanshan-Liaoning, Xilamulum belts along the northern margin of the North China craton, the Qinling orogen, and the Middle-Lower Yangtze River Valley along the bonudaries between the North China and the Yangtze cratons. The occurrence of platform carbonate successions may have played some role in the evolution of ore fluids that resulted in the formation of porphyry-skarn deposits. The porphyry deposits of eastern China, although sharing some common features with classic porphyry systems that are related to convergent margins, do exhibit unusual features, particularly in terms of ore fluid compositions, the common presence of fluorite, hydrothermal alteration assemblages, which tend to be less hydrous than the porphyry at convergent margins. On the other hand, the porphyry mineral systems of eastern China have some similarities with the Climax-type porphyry. A major tectonothermal event (Yanshanian), which took place in the Jurassic-Cretaceous, affected eastern China and most of the eastern margin of Asia. This Yanshanian event was responsible for the magmatism and associated mineral systems in the region. The granitoid intrusions and accompanying porphyry and porphyry-skarn deposits were developed along orogenic belts and major structural discontinuities mostly along the tectonic boundaries between the North China and Yangtze cratons and between the northern margin of the North China craton and the Mongolia-Siberia plate. The porphyry and porphyry-skarn deposits herein described include: Chalukou in the Great Xing'an Range orogen, Jiguanshan in the Xilamulun metallogenic belt, Xiaojiayingzi, Lanjiagou, Chehugou, Yuanbaoshan in the Yanshan-Liaoning belt, Nanninhu, Yuchiling, Jinduicheng, Lemeingou in the East Qinling orogen, Tongchakoou, Shaxi from the Middle-Lower-Yangtze River Valley. The role of mantle-lithosphere dynamics in eastern China, which led to the formation of these porphyry and porphyryskarn mineral systems, is discussed and reviewed. [ABSTRACT FROM AUTHOR]

Porphyry dikes and hydrothermal veins from the porphyry Cu-Mo deposit at Butte, Montana, contain multiple generations of quartz that are distinct in scanning electron microscope-cathodoluminescence (SEM-CL) images and in Ti concentrations. A comparison of microprobe trace element profiles and maps to SEM-CL images shows that the concentration of Ti in quartz correlates positively with CL brightness but Al, K, and Fe do not. After calibrating CL brightness in relation to Ti concentration, we use the brightness gradient between different quartz generations as a proxy for Ti gradients that we model to determine time scales of quartz formation and cooling. Model results indicate that time scales of porphyry magma residence are -1,000s of years and time scales from poqrhyry quartz phenocryst rim formation to porphyry dike injection and cooling are ~10s of years. Time scales for the formation and cooling of various generations of hydrothermal vein quartz range from 10s to 10,000s of years. These time scales are considerably shorter than the ~0.6 m.y. overall time frame for each porphyry-style mineralization pulse determined from isotopic studies at Butte, Montana. Simple heat conduction models provide a temporal reference point to compare chemical diffusion time scales, and we find that they support short dike and vein formation time scales. We interpret these relatively short time scales to indicate that the Butte porphyry deposit formed by short-lived episodes of hydrofracturing, dike injection, and vein formation, each with discrete thermal pulses, which repeated over the ~3 m.y. generation of the deposit. [ABSTRACT FROM AUTHOR]

Precise and accurate determination of the timing and duration of ore-forming processes in porphyry systems is a fundamental step in understanding their genesis and placing them in a regional context. Here, we take advantage of the considerable improvements in the field of geochronology over the last decade to provide a robust geochronologic framework for hydrothermal and magmatic events in the Eocene Coroccohuayco porphyry-skarn Cu deposit, and the first robust dating of an ore system in the emerging Andahuaylas-Yauri batholith and metallogenic belt, southern Peru. This batholith and associated porphyry systems were emplaced during the Incaic orogeny, in a context of slab flattening, compression, exhumation, uplift, and the initiation of the bending of the Bolivian orocline. High-precision ages from early skarn (U-Pb, hydrothermal titanite) and later-stage mineralization (Re-Os, molybdenite) in the Coroccohuayco deposit are indistinguishable from each other and from available high-precision U-Pb zircon ages of the porphyries. All together, they indicate that the deposit was formed in less than 100 k.y. between 35.7 and 35.6 Ma. We also highlight a previously unrecognized pre-ore high-temperature hydrothermal event (U-Pb, hydrothermal titanite) that corresponds to the emplacement of a precursor gabbrodiorite complex at ca. 40.2 Ma. A new 40Ar/39Ar age at 26.6 Ma of a post-ore alkali basalt is interpreted as recording the initiation of slab roll-back following the flat slab episode and is therefore not related to the magmatic-hydrothermal system at Coroccohuayco. These data, together with structural measurements at the Coroccohuayco deposit and available regional data, suggest that the Coroccohuayco deposit was formed toward the end of Eocene arc magmatism, in a context of transpressional stress, intense erosion, and exhumation associated with Incaic orogeny. At the scale of the Tintaya ore district (which hosts the Coroccohuayco, Tintaya, and Antapaccay deposits), available data and a new molybdenite Re-Os age obtained for the Tintaya deposit suggest that mineralizing events were spatially focused and episodic over several millions of years, while a single economic deposit may have been formed within less than 100 k.y. [ABSTRACT FROM AUTHOR]

The Diyanqinamu porphyry Mo deposit in the southern Greater Khingan Range of the Central Asian orogenic belt contains 800 million metric tons (Mt) of ore with an average grade of 0.097% molybdenum. The deposit is hosted in Late Jurassic volcanic rocks of tuff, andesite, and volcanic breccia. Multiple-stage hydrothermal activities have resulted in propylitic, phyllic, and argillic alteration in this deposit. Five stages (I-V) of hydrothermal activity are identified. Stage I is represented by a mineral assemblage of epidote, chlorite, and magnetite, with some discontinuous barren veinlets of quartz + K-feldspar ± fluorite ± magnetite ± epidote ± chlorite. Stage II is marked by occurrence of quartz + fluorite + molybdenite + magnetite ± pyrite ± sericite ± siderite veinlets/veins with phyllic halos. Stage III consists of fluorite + siderite + quartz + molybdenite + pyrite ± ankerite ± calcite ± chalcopyrite veins that are commonly related to phyllic alteration and dissemination of fluorite in the altered rocks. Stage IV has an assemblage of fluorite + quartz + pyrite ± ankerite ± calcite ± molybdenite ± chalcopyrite ± sphalerite ± galena in coarse veins (10-20 mm wide). Stage V consists of narrow (≤5-mm wide) veinlets of calcite + fluorite + pyrite ± quartz. Molybdenite mainly occurs in Stages II and III. Re-Os dating results for molybdenite samples from these two stages yielded an isochron age of 156.2 ± 4.2 Ma (2σ, MSWD = 0.96, n = 10). Most molybdenite samples have high δ34S values (≥8.4‰) relative to other sulfide minerals (i.e., galena, sphalerite, pyrite, and chalcopyrite) of Stages II to V (δ34S = 2.5-8.3‰, n = 22). Molybdenite also has low 207Pb/204Pb and 208Pb/204Pb ratios relative to other sulfide minerals although there are minor overlaps. In a diagram of 206Pb/204Pb versus 207Pb/204Pb, these Pb isotope data display a positive trend transecting the growth curves of crustal lead, which could be invoked by mixing of crustal and mantle sources with distinct Pb isotopes. In combination with the S isotope data and mineral paragenesis, we suggest that magmas were the main source of molybdenum, whereas other metals (i.e., Pb, Zn, and Cu) were possibly sourced from the country rocks. [ABSTRACT FROM AUTHOR]

The El Teniente Cu-Mo porphyry deposit, Chile, is one of the world's largest and most complex porphyry ore systems, containing an estimated premining resource of approximately 95 Mt Cu and 2.5 Mt Mo. Although Cu mineralization at the deposit is quite well studied, little work has focused specifically on the distribution and timing of Mo mineralization. Combined grade, vein, and breccia distribution analysis reveals that deposit-wide Mo grades of 0.01 to 0.06 wt % are strongly controlled by the abundance of main mineralization (type 6a) quartz ± molybdenite veins. These show a clear spatial relationship with several felsic-intermediate intrusions and appear to develop outward and upward into Cu-rich (type 6b-7b) quartz-chalcopyrite veins and (type 8) chalcopyrite-anhydrite ± bornite veins with sericitic alteration halos. High-precision Re-Os molybdenite dating reveals that these linked vein types did not develop in a single, deposit-wide evolution, but are diachronous, related to distinct episodes of hydrothermal activity associated with the emplacement of diorite finger porphyries and the composite Teniente Dacite Porphyry. These units acted as effective, short-lived (<100,000 years) conduits for pulses of Mo- and Cu-bearing hydrothermal fluids between 6.3 and 4.6 Ma. The rapid thermal contraction of each system during mineralization led to extensive overprinting of Mo-rich veins by their lower-temperature, Cu-rich equivalents. Separate pulses in magmatic-hydrothermal activity are separated by distinct gaps of up to 300,000 years, during which Mo-mineralizing activity appears to have gone into quiescence. Mo grades exceeding 0.06 wt % correspond to the presence of molybdenite-bearing, late mineralization-stage, tourmaline-cemented (type 9), and anhydrite-carbonate ± gypsum (type 10) veins and breccias. These are abundant at shallow mine levels and show a close spatial relationship with a series of concentric faults associated with the Braden Breccia Pipe. Mineralization in this paragenetic stage is relatively short-lived and occurs in all parts of the deposit between 4.80 and 4.58 Ma. The generally Cu poor nature of the late mineralization stage is attributed to the prior preferential extraction of Cu from the underlying magma chamber in earlier mineralizing events. This led to the late exsolution of oxidized, Mo-rich fluids that may have undergone further enrichment by remobilizing Mo from main mineralization-type veins associated with the Teniente Dacite Porphyry. The formation of the Braden Breccia Pipe is likely to have occurred in a single cataclysmic event at approximately 4.58 Ma, which cut the Mo-rich tourmaline breccias and created a distinct Mo-rich grade halo at shallow mine levels. With the exception of minor mineralization associated with small dacitic dikes at approximately 4.42 Ma, the Braden event marked the termination of Mo deposition. [ABSTRACT FROM AUTHOR]

Porphyry Cu (±Mo ±Au) and epithermal Au-Ag deposits are major sources of mined metals and are commonly formed by magmatic-hydrothermal fluids derived from hydrous magmas in Phanerozoic convergent margin settings. The igneous rock assemblages associated with porphyry mineral deposits are common in modem convergent margin settings, but while many have produced acidic magmatic fluids, very few, past or present, have produced sufficient metal, chlorine, and sulfur enrichments necessary to engender an ore deposit. The reasons for this remain uncertain. We report SHRIMP-RG ion microprobe analyses of hafnium, titanium and rare earth element (REE) abundances in zircon, a nearly ubiquitous and robust trace mineral in crustal magmas. Comparison of the compositions of zircons in ore-forming and barren granitic plutons indicate that ore-forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies (mostly EUN/EUN* ≥0.4). We interpret these small zircon europium anomalies to indicate oxidizing magmatic conditions and hypothesize that in many cases this reflects oxidation due to SO2 degassing from magmas with a relatively low Fe/S ratio. Oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate (S6+) to SO2 (S4+) upon degassing. This interpretation reinforces the important role of oxidized sulfur-rich fluids in porphyry and epithermal mineral deposit formation. Zircon compositions thus may be used to identify ancient magmas that released significant amounts of SO2-rich gases, and regional surveys of zircon composition are potentially a valuable tool for mineral exploration. [ABSTRACT FROM AUTHOR]

Porphyry-related base metal vein and replacement mineralization (i.e., Cordilleran polymetallic mineralization) in the Morococha district, central Peru, is part of a large magmatic-hydrothermal system associated with the emplacement of several late Miocene porphyry intrusions and formation of important Cu-Mo mineralization. Zn-Pb-Ag-Cu veins overprint the giant Toromocho porphyry Cu-Mo deposit in the center of the district and display a typical concentric base metal zonation (Cu → Zn, Pb → Ag) covering an area of approximately 50km². A detailed fluid inclusion study supports the hypothesis that base metal mineralization precipitated from cooled and evolved metal-rich, intermediate-density porphyry-type fluids. In early stages of Cordilleran base metal vein formation, fluid inclusions have low salinities of ~2 to 5 wt % NaCl equiv, CO2 contents of 3 to 10mol %, and homogenization temperatures (Th) of 380° to 340°C. They are similar to intermediate-density fluid inclusions trapped in a milky quartz vein predating Cordilleran polymetallic mineralization, with similar low salinities (3.0-3.8 wt % NaCl equiv) and low CO2 contents (6.5-8 mol %), but higher Th of -420° to 410°C. During cooling of the intermediate-density fluids from 400° to 300°C, the Iithostatic pressure regime changed to a hydrostatic one. The fluids underwent pressure drop as well as phase separation (i.e., unmixing) and lost most of their CO2. They acquired moderate salinities, in some cases intermediate (-up to 16 wt % NaCl) to brine compositions. However, the bulk of the magmatic fluid retained low salinity while it continued to cool under open-system conditions and precipitated tennantite-tetrahedrite, chalcopyrite, enargite, sphalerite, and galena. Upon cooling below 270°C, the fluids deposited abundant rhodochrosite and quartz, while following the boiling curve toward lower P-T conditions. These data record an evolution of mineral precipitation from deep (minimum depth of 2-1.5 km) to shallow environments (300-800 m). Oxygen, hydrogen, and carbon stable isotope data indicate that the hydrothermal fluids have a dominantly magmatic signature and were diluted by meteoric waters during the carbonate stage. Copper (5,000-18,000 µ/g), sulfur (up to 12,000 µ/g), and iron (2,100-6,000 µ/g) concentrations in the intermediate-density fluid inclusions in the milky quartz veins are approximately 5 to 10 times higher than in intermediate-density inclusions of the early Cordilleran base metal veins. The base metals Zn, Pb, and Mn have comparable concentrations between 100 and 1,000 µ/g for both types of fluid. These findings suggest that the fluids identified in Cordilleran polymetallic veins are compositionally similar to the porphyry-type fluids and could have derived from the latter after precipitation of Cu- and Fe-bearing sulfides in a deeper porphyry environment. The new data explain the commonly observed base and precious metal zonation patterns encountered in porphyry-centered districts (e.g., Bingham, Butte) and show that both porphyry and polymetallic mineralization can precipitate from similar magmatic-hydrothermal fluid pulses. [ABSTRACT FROM AUTHOR]

In the Gangdese belt of southern Tibet, Paleocene-Eocene magmas record the final stage of Neo-Tethyan subduction and are associated with few economic porphyry deposits. In contrast, magmas formed during later stages of the India-Asia collision in the Oligo-Miocene are associated with several large porphyry Cu-Mo ± Au deposits, especially in the eastern part of the belt (~89°E-93°E). In a previous study, we showed that these Oligocene-Miocene magmas were more hydrous than the earlier Paleocene-Eocene magmas. In this study, we show that the later magmas were also more oxidized. Paleocene-Eocene rocks from the eastern Gangdese belt are characterized by low zircon Ce4+/Ce3+ ratios (mostly <50; 6.0-66.8; average = 25.7 ± 18.4, n = 26) and low to moderate ΔFMQ values (-1.2 to +0.8, average = -0.2 ± 0.8, n = 5; estimated from ilmenite-magnetite mineral pairs). In comparison, Miocene igneous rocks from the eastern Gangdese belt show higher zircon Ce4+/Ce3+ ratios (mostly >50; 32.3-141.9, average = 74.3 ± 30.1, n = 33) and higher ... values (ΔFMQ 0.8-2.9, average = 1.8 ± 0.8, n = 6; estimated from magnetite-ilmenite pairs). Estimates of magmatic oxidation state from amphibole compositions also show an increase from ΔFMQ 1.2 to 2.1 (average = 1.6 ± 0.2, n = 40) in the Paleocene-Eocene to 2.0 to 2.8 (average = 2.5 ± 0.2, n = 58) in the Miocene. Sparse whole-rock Fe3+/Fe2+ ratios show the same general trend. The amphibole results are systematically shifted to higher ΔFMQ compared to data from magnetite-ilmenite pairs, but their trend is internally consistent. The higher oxidation states (and water contents) of Miocene igneous rocks from the eastern Gangdese belt may explain their unique association with large porphyry deposits in Tibet, because it has been shown from other studies that the potential for hydrous calc-alkaline magmas to transport Au is maximized near ΔFMQ 1.0, and Cu at higher ... (ΔFMQ > 1.0). In comparison, the somewhat less oxidized and less hydrous Paleocene- Eocene magmas would have been less fertile for the formation of such deposits. We suggest that these differences reflect the tectonomagmatic evolution of the Cenozoic Gangdese collisional orogen, from early collision-related magmatism in the Paleocene-Eocene to late collisional magmatism in the Miocene. Asthenospheric upwelling following slab breakoff in the Miocene caused interaction between mantle-derived magmas and previously subduction modified Tibetan lithosphere. The resultant evolved magmas had high oxidation states and water contents, which are favorable properties for the subsequent formation of magmatic-hydrothermal porphyry Cu-Mo ± Au deposits. [ABSTRACT FROM AUTHOR]

SHRIMP-RG U-Pb zircon geochronologic data together with published U-Pb ages from the succession of quartz diorite porphyry intrusions in the western porphyry Cu-Au centers at Reko Diq, western Balochistan, Pakistan, indicate the emplacement of four spatially distinct porphyry Cu centers over ~1 m.y. in the Miocene between ~12.9 and 11.9 Ma. Intrusive activity in the western porphyry cluster began with emplacement of hornblende-rich quartz diorite porphyry associated with the northern and uneconomic H79 porphyry Cu center with U-Pb ages of 12.87 ± 0.27 and 12.72 ± 0.3 Ma. To the south of the H79 porphyry Cu center, successively younger porphyry intrusive centers formed along a south-southwest trend. The economic H15 center has U-Pb zircon ages for four quartz diorite porphyry intrusions between 12.58 ± 0.29 and 12.18 ± 0.19 Ma with three of the porphyry intrusions that span the intrusive sequence in the H15 center having essentially identical ages between 12.58 ± 0.29 and 12.49 ± 0.20 Ma. The adjacent economic H14 center to the south has U-Pb ages on zircons for five quartz diorite porphyry intrusions between 12.22 ± 0.15 and 12.00 ± 0.16 Ma with four of the porphyry intrusions spanning the intrusive sequence having ages between 12.14 ± 0.14 and 12.00 ± 0.16 Ma. The southern end of the trend is the subeconomic H13 complex where mineralized quartz diorite porphyry has a U-Pb age on zircon of 11.93 ± 0.14 Ma. Two new Re-Os ages on molybdenite reported herein agree with the magmatic ages for the H15 (12.43 ± 0.05 Ma) and H14 complexes (11.95 ± 0.05 Ma), whereas previously published Re-Os ages on molybdenite are younger (H15: 11.73 ± 0.04 Ma; H14: 11.82 ± 0.04 Ma) than the youngest dated intrusion in those centers. Published K-Ar ages on hydrothermal biotite from the H79 center (13.0 ± 0.2 Ma), H14 (12.0 ± 0.2 Ma), and H13 (12.0 ± 0.2 Ma) indicate the individual system cooled rapidly. Overall the combined U-Pb geochronology dataset for the four porphyry intrusive centers suggests short-lived magmatic systems characterized each center. However, the available Re-Os ages on molybdenite suggest the potential for complexity in associated hydrothermal systems at the H15 and H14 porphyry Cu-Au centers. [ABSTRACT FROM AUTHOR]

The Biga Peninsula in northwestern Turkey hosts a large number of high and low sulfidation epithermal gold-silver-(copper) and associated copper-gold porphyry deposits and prospects, associated with voluminous, Eocene-Miocene calc-alkaline volcanism and plutonism. Approximately 15 million gold-equiv oz have been discovered within the last 10 years in this area, with a number of new discoveries now in the drill-testing stage and two deposits in the development stage. At the TV Tower property in the central Biga Peninsula, Eocene(?) to Oligocene intermediate to felsic volcanic and volcanielastic strata and associated intrusive rocks overlie metamorphosed basement rocks of the Gamhca Group. The volcanic sequence is broadly divisible into a lower sequence of andesitic dikes and flows intruded by monzonite porphyry stocks and dikes, a middle sequence of andesitic breccias and flows, and an upper sequence consisting largely of dacitic tuffs, flows, and epiclastic strata. The middle and upper sequences are strongly altered over an area covering several tens of km², including argillic, advanced argillic, and massive to vuggy quartz alteration, the latter largely manifested as ledges hosted in dacitic volcanielastic rocks. Moderate- and high-angle normal and oblique faults are common in the area and may have influenced the distribution of mineralization. The Küçükdağ zone was drill-tested in 2010 on the basis of strong surface soil and rock sample data. The second hole returned a near-surface intercept of 136.2 m averaging 4.30 g/t Au, 15.0 g/t Ag, and 0.68% Cu. As of the date of this communication, 169 holes have been drilled to test the zone, which measures approximately 600 x 400 m and consists of a lower gold-rich zone and an upper silver-rich zone. Total mineral endowment is estimated at 966,000 indicated and 351,000 inferred gold-equiv oz. Mineralization is strongly controlled by stratigraphy, with gold hosted primarily within a massive lithic lapilli- bearing tuff unit and silver largely constrained to an overlying fluvial-lacustrine sequence capped by andesite breccias. Copper is distributed throughout the zone in association with both gold and silver. The zone is located primarily in the hanging wall of a moderate-angle fault that separates the dacitic volcanielastic sequence from andesite breccias in the footwall. Gold is associated with tectonic-hydrothermal breccias, zones of vuggy quartz and veins, and alteration minerals consisting primarily of alunite and dickite with sulfides including pyrite, enargite, and coveflite. Spatially, silver is largely separate from gold, and is associated with silicification, very fine grained pyrite, and/or marcasite and enargite. Paragenetic evidence suggests that the silver event may predate the gold event, but that the two may have shared common fluid pathways. A number of other targets are currently being explored on the property, including silicified ledge or rib-style, vuggy quartz-hosted oxide gold, Cu-Au porphyry, and low sulfidation epithermal gold-silver veins. [ABSTRACT FROM AUTHOR]

The Altar porphyry Cu-(Mo-Au) deposit occurs in the Miocene to early Pliocene copper belt of the high Andes of western Argentina, close to the Chilean border. A cluster of late Miocene porphyries intruded a more extensive complex of early Miocene rhyolitic to andesitic volcanic and subvolcanic rocks. Potassic alteration in the Central Porphyry was partially overprinted by feldspar-destructive alteration and finally by argillic to advanced argillic alteration. Early stockwork veins have been cut successively by quartz-pyrite veins, tourmaline veins, and enargite veins. Based on correlation analysis of assay results and mapped abundances of these four vein types, we estimate that approximately 11 to 26% of the copper in the Altar orebody is associated with enargite veins, whereas the remainder is associated with the early stockwork veining and potassic alteration. Gold concentration is low compared with typical Cu-Au porphyries of the Andean back-arc region, but higher than in the giant Miocene deposits of Chile, at an average Au/Cu ratio of 0.14 x 10-4 by weight across the Central Porphyry orebody at Altar. Gold is dominantly associated with chalcopyrite in the domain of stockwork veining and potassic alteration. Small-scale assays in combination with petrographic observations and laser ablation-inductively coupled plasma-mass spectrometry microanalyses show that a significant fraction of the gold in the stockwork veins is enclosed in pyrite as submicroscopic particles attached to inclusions of a Bi-Te-Pb-Ag-enriched Cu-Fe sulfide phase. Microscopic grains of native gold also occur along grain boundaries between pyrite and chalcopyrite, and rarely as larger particles in enargite veins. [ABSTRACT FROM AUTHOR]

The Santa Cruz porphyry Cu-(Mo) system near Casa Grande, Arizona, includes the Sacaton mine deposits and at least five other concealed, mineralized fault blocks with an estimated minimum resource of 1.5 Gt @ 0.6% Cu. The Late Cretaceous-Paleocene system has been dismembered and rotated by Tertiary extension, partially eroded, and covered by Tertiary-Quaternary basin-fill deposits. The mine and mineralized fault blocks, which form an 11 km (~7 miles) by 1.6 km (~1 mile) NE-SW-trending alignment, represent either pieces of one large deposit, several deposits, or pieces of several deposits. The southwestern part of the known system is penetrated by three or more diatremes that consist of heterolithic breccia pipes with basalt and clastic matrices, and subannular tuff ring and maar-fill sedimentary deposits associated with vents. The tephra and maar-fill deposits, which are covered by ~485 to 910 m (~1,600-3,000 ft) of basin fill, lie on a mid-Tertiary erosion surface of Middle Proterozoic granite and Late Cretaceous porphyry, which compose most xenoliths in pipes and are the host rocks of the system. Some igneous xenoliths in the pipes contain bornite-chalcopyrite-covellite assemblages with hypogene grades >1 wt % Cu, 0.01 ounces per ton (oz/t) Au, 0.5 oz/t Ag, and small amounts of Mo (<0.01 wt %), These xenoliths were derived from mineralized rocks that have not been encountered in drill holes, and attest to additional, possibly higher-grade deposits within or subjacent to the known system. The geometry, stratigraphy, and temporal relationships of pipes and tephras, interpreted from drill hole spacing and intercepts, multigenerational breccias and matrices, reequilibrated and partially decomposed sulfide-oxide mineral assemblages, melted xenoliths, and breccia matrix compositions show that the diatremes formed in repeated stages. Initial pulses of basalt magma fractured granite, porphyry, and other crustal rocks during intrusion, transported multi-sized fragments of these rocks upward, and partially melted small fragments. Rapid decompression of magma induced catastrophic devolatilization that ruptured overlying rocks to the surface, and generated fragment-volatile suspensions that abraded conduits into near-vertical cylindrical structures. Fragments entrained in suspensions were milled and sorted, and ejected as basal surge, pyroclastic deposits, and airfall tephra that built tuff rings around vents and filled vent depressions. Comminuted m- to mm-sized fragments of wall rocks in magma and suspensions that remained in conduits solidified as heterolithic breccias. Subsequent pulses of basalt magma ascended through the same conduits, brecciated older heterolithic breccias, devolatilized, and quenched, leaving two or more generations of nested and mingled heterolithic breccias and internal zones of fluidized fragments. Tephra and maar-fill deposits from later eruptions are composed of more hydrous and oxidized minerals than earlier tephras, reflecting a higher proportion of water in transport fluid which, based on fluid inclusion populations in mineralized xenoliths, was saline water and CO2. The large vertical extent (~600 m; ~2,000 ft) of basalt matrix in pipes, near-paleosurface matrix vesiculation, and plastically deformed basalt lapilli indicates that diatreme eruptions were predominantly phreatic. Diatreme xenoliths represent enistal stratigraphy and, as in the Santa Cruz system, provide evidence of concealed mineral resources that can guide exploration drilling through cover. Vectors to the source of bornite-dominant xenoliths containing > 1% Cu and significant Au and Ag could be determined by refinement of breccia pipe geometries, by reassembly of mineralized fault blocks using modal, chemical, and temporal characteristics of hydrothermal mineral assemblages and fluid inclusions, and by paleodrainage analysis. [ABSTRACT FROM AUTHOR]

The eastern Gangdese magmatic belt (between ~88°E and 94°E) in southern Tibet preserves a series of Cenozoic collision-related igneous rocks that have formed since the start of the India-Asia collision at ~55 to 50 Ma. The Paleocene-Eocene intrusive rocks have intermediate [La/Yb]N ratios (average = 12.0 ± 8.6, n = 150, range = 1.15-64.0), intermediate to low Sr/Y ratios (mostly < 40), and negative Eu anomalies (average Eun/Eu* = 0.76 ± 0.19, n = 150, range = 0.23-1.33). They are mainly composed of equigranular pyroxene and plagioclase in mafic intrusions or porphyritic plagioclase and quartz in intermediate-felsic intrusions, with minor interstitial amphibole and biotite. In contrast, Oligo-Miocene intrusive igneous rocks have high [La/Yb]N ratios (Oligocene: average [La/Yb]N = 42.8 ± 17.0, n = 35, range = 23.2-93.2; Miocene: average [La/Yb]N = 28.5 ± 8.8, n = 93, range = 6.7-49.1), high Sr/Y ratios (>40), and weak or absent Eu anomalies (Oligocene: average Eun/Eu* = 0.99 ± 0.27, n = 35, range = 0.67-1.58; Miocene: average Eun/Eu* = 0.95 ± 0.13, n = 93, range = 0.44-1.46), and their mineralogy consists mainly of plagioclase, quartz, and amphibole as phenocrysts. These geochemical and mineralogical characteristics suggest that the Paleocene-Eocene magmas were relatively dry and evolved primarily by fractionation of pyroxene and plagioclase, whereas the Oligo-Miocene magmas were more hydrous and fractionated significant amounts of hornblende and lesser plagioclase prior to upper crustal emplacement. Oligocene and Miocene magmatic rocks crop out as small-volume intrusions and are associated with several large- to giant-sized porphyry Cu-Mo ± Au deposits, as well as numerous smaller porphyry and skarn deposits. In contrast, earlier, more voluminous Paleocene-Eocene magmatism is only known to be associated with three small porphyry deposits. Erosional loss of subvolcanic porphyry systems from the older Paleocene-Eocene sequence is not thought to be a controlling factor in this temporal difference of porphyry deposit distribution, because exhumation rates after 55 Ma were relatively low and coeval Linzizong volcanic rocks are extensively preserved in the Gangdese belt. Instead, we suggest that differences in magmatic history and petrogenesis led to this restricted temporal distribution of porphyry deposits. The Paleocene-Eocene magmas were generated during the onset of collision between India and Asia and were triggered by rollback of the Neo-Tethyan oceanic slab. This magmatism records the final stage of subduction of oceanic lithosphere beneath southern Tibet, and the relatively low water contents of these magmas may reflect final dehydration of the remnant Neo-Tethyan slab. After oceanic slab breakoff at ~40 to 38 Ma, the Greater India slab began to subduct beneath the Gangdese belt until hard collision with the Indian continent at ~35 Ma and breakoff of the Greater India slab at ~25 to 10 Ma. This last event is suggested to have caused asthenospheric mantle upwelling and partial melting of subduction-modified, hydrated Tibetan lithosphere. The resultant partial melts may have remobilized metals residual in the deep lithosphere from previous arc magmatism, giving rise to a suite of postsubduction porphyry-type ore deposits in the Oligo-Miocene. [ABSTRACT FROM AUTHOR]

Re-Os dating of two molybdenite samples from the Squaw Peak porphyry copper-molybdenum prospect in central Arizona returned essentially identical ages of 1,729 ± 7 and 1,738 ± 7 Ma. Therefore the prospect is not a component of the Laramide (Late Cretaceous-early Tertiary) porphyry copper province of southwestern North America as previously presumed. These Paleoproterozoic ages are similar to that of 1,740 ± 15 Ma for the I-type, magnetite-series Cherry batholith, within which the Squaw Peak porphyry stock and associated mineralization are located. Squaw Peak cannot be more than a few million years younger than volcanogenic massive sulfide (VMS) copper deposits in the nearby Jerome district, which are part of the Yavapai Supergroup, host to the Cherry batholith. These volcanic and intrusive rocks and their associated copper mineralization were formed in a juvenile island-arc setting and are now part of the Yavapai province, which was assembled and accreted to the Archean nucleus of North America by ~1.68 Ga. The Paleoproterozoic age for Squaw Peak in conjunction with the existence of the slightly older VMS deposits shows that the Laramide province of southwestern North America first developed its copper metallogenic signature >l,700 m.y. ago. The presence of this Paleoproterozoic copper mineralization maybe taken as further support for recently proposed metasomatism of the mantle lithosphere during Paleoproterozoic subduction as a precursor to formation of at least part of the Laramide porphyry copper province. Similar spatial associations between inferred metal sources in Proterozoic mantle lithosphere and lowermost crust, relatively minor copper and/or molybdenum mineralization in Proterozoic magmatic arcs, and important post-Paleozoic porphyry copper and/or molybdenum provinces have recently been documented elsewhere, particularly in eastern China, and could prove to be of more general exploration significance. [ABSTRACT FROM AUTHOR]

Lorraine is a copper-gold porphyry deposit (6.4 Mt indicated resource at 0.61% copper and 0.23 g/t gold, and 28.8 Mt inferred resource at 0.45% copper and 0.19 g/t gold) in north-central British Columbia--one of the youngest known alkalic porphyry deposits within the Triassie-Jurassic Quesnel arc. The deposit is hosted by the Early Jurassic Duckling Creek Syenite Complex, an alkalic multiphase intrusive dike complex of pyroxenite, syenite, and monzonite composition. The southern part of the complex hosts numerous copper-gold porphyry prospects in addition to Lorraine; together, they constitute the Duckling Creek district. Main-stage Cu-Au porphyry mineralization at Lorraine is hosted by phase 1 of the Duckling Creek Syenite Complex, an SW-dipping, early intrusive suite of silica-undersaturated pyroxenites and syenites. Mineralized zones are up to 2.5 km long by 500 m wide, in the form of northwesterly elongate, SW-dipping panels that generally parallel the phase 1 intrusive contacts. Mineralization is attributed to "blind" porphyries within the phase 1 sequence, which was tilted after main-stage mineralization. Copper sulfides define deposit-scale zonation patterns of bornite-chalcocite cores rimmed by chaleopyrite-rich and outer, weakly developed pyrite-dominated domains. Alteration patterns and intensity are consistent across mineralized zones. Strong, pervasive K-feldspar-biotite (potassic) core zones are rimmed by albite-diopside-magnetite (calc-potassic) alteration that is pervasive near the center of the zones, but varies laterally outward to vein-style alteration. Pervasive albite alteration (sodic) is locally present and is interpreted to be an early alteration event. Mineralization at Lorraine lias been cut by later phases of the Duckling Creek Syenite Complex. A second cycle of pyroxenite-syenite-monzonite intrusion (phase 2) is mineralogically similar to phase 1, but is unmineralized and has vertical intrusive contacts that strike northwest. A change to late, silica-saturated intrusions is marked by subvertical phase 3 silica-saturated leucosyenite and pegmatite dikes. U-Pb zircon and Ar-Ar geochronology constrain the timing of main-stage mineralization at Lorraine to an approximately 1.5 m.y. interval between 180 Ma and 178.5 Ma. Phase 1 intrusions are as old as 181.68 ± 0.95 Ma based on an Ar-Ar age on biotite from pyroxenite, but may range to as young as 180.2 ± 0.28 Ma based on a U-Pb zircon age from a mineralized phase 1 syenite. The upper age limit for mineralization is 178.75 ± 0.21 Ma, based on U-Pb data for a postmineral phase 2 syenite. An alternate interpretation considers the presence of zircon antecrysts in an analyzed phase 1 rock, which leads to a more narrow time window of 0.31 m.y. that includes phase 1 intrusion, main-stage porphyry mineralization, and structural tilting, which was followed by phase 2 intrusion. Magmatic and hydrothermal activity in the Lorraine area persisted until at least 176 Ma, based on the U-Pb zircon age of 176.77 ± 0.36 Ma from a phase 3 leucosyenite dike, and a late biotite-albite-apatite vein with an Ar-Ar biotite age of 175.21 ± 0.91 Ma. Several intrusive-hosted porphyry prospects within the Duckling Creek district share mineralogical and alteration characteristics, as well as relative igneous paragenesis, with Lorraine. The model for magmatic and hydrothermal development at Lorraine may broadly be applied to the district as a whole, providing a geologic framework upon which to base future exploration in the district. [ABSTRACT FROM AUTHOR]

Porphyry Cu-Au-Mo deposits are characterized by numerous fracturing events. Brittle fracture of mineralized intrusions and the surrounding host rocks is a consequence of hydraulic breakage caused by the interplay of the changing physiochemical properties of magmatic fluids and/or the local tectonic regime. We have mapped more than 1,400 quartz-sulfide vein sets at the Ordovician Endeavour 26 North (E26) deposit, central New South Wales, in order to constrain the processes influencing fracture development around small (~200 m) porphyritic intrusions. Veins from intermediate levels in the E26 deposit define complicated patterns within 10 m of the porphyritic intrusions, whereas more systematic patterns occur up to 50 m from the intrusive contact. The outer parts of the ore deposit are characterized by quartz veins with orientations of -340° and subordinate orthogonal sets of bisecting veins at -240° and -290°. The data show a broad range of vein orientations that define a conical distribution around a vertical axis. There is a rare subset of near-horizontal veins that formed early relative to the main stage of ore-bearing vein emplacement. Many quartz veins exhibit fault displacements (at both micro- and macroscopic scales) and fabrics (including serrate fractures, slickenlines, and asymmetric fiber trails). The distribution of veins suggests that the dominant fracture array developed perpendicular to the direction of bulk finite extension, but significant vein dilation also occurred in orientations with a component of shear strain. We propose a physical model that combines intrusion-driven hydraulic fracturing into a preexisting fracture mesh to help explain the observed vein patterns and to better link fracture development and vein emplacement. Hydraulically driven deformation was important locally at E26, contributing to the formation of the irregular fracture stockwork immediately adjacent to the small intrusive bodies. The more predictable vein patterns that occur away from the porphyritic intrusions require part of the hydraulically driven extension to have reactivated the existing fracture mesh. Parts of this more systematic geometry, including the occurrence of flat veins, are probably related to the emplacement of larger (10-km diameter) magma bodies adjacent to E26, with doming causing the subordinate fracture arrays, including the gross conical distribution. Fluctuating fluid pressures, combined with hydraulically driven extension, caused dilation of preexisting and synmineralization fractures and superposition of veins with similar orientations. Vein patterns suggest that the regional stress state that existed prior to or synchronously with porphyry ore deposit formation strongly influenced vein orientation to within tens of meters away from the mineralizing intrusions. Fractures that localized ore in the E26 porphyry, deposit developed in response to the infiltration of fluid batches, including magma and magmatic-hydrothermal fluids, into a regional deformation environment. [ABSTRACT FROM AUTHOR]

The Lower Main zone of the Lorraine alkalic porphyry Cu-Au district, BC, Canada, is hosted in an intrusive complex that comprises pre- and late mineral biotite pyroxenites, monzonites, and syenites. Some of the biotite pyroxenites have interstitial sulfides, which appear similar to net-textured sulfides in magmatie ore deposits; however, the pyroxenites have undergone several stages of hydrothermal alteration. Main-stage mineralization produced sulfide zonation patterns consisting of a bornite-chalcocite or bornite-chalcopyrite core grading outward to domains of chalcopyrite, chalcopyrite > pyrite, and a peripheral domain of pyrite with minor chalcopyrite. Syenite in the inner bornite-chalcopyrite zone typically contains abundant turbid K-feldspar (>70%), whereas syenite marginal to the bornite-chalcopyrite core contains less K-feldspar (50-70%), indicating an increase in K metasomatism of syenites toward the core of the deposit. Main-stage mineralization predominantly occurs as fine-grained disseminated sulfides in syenite, biotite pyroxenite, and fine-grained K-feldspar biotite rock. Textural analyses have shown that (1) primary magmatie diopside in contact with sulfides has corroded and actinolite-altered margins, suggesting alteration of primary minerals, (2) scalloped relicts of biotite and diopside occur as inclusions in sulfides, implying that sulfides have replaced primary minerals, and (3) the deposit-scale sulfide zonation patterns overprinted numerous rock types, including the biotite pyroxenites that contain interstitial sulfides. These results provide evidence for replacement-style mineralization at Lorraine, where primary magmatie biotite and diopside were totally or partially replaced by sulfide minerals during main-stage alteration. U-Pb zircon dates from pre- and late mineral syenite dikes show that the timing of main-stage mineralization occurred between 178.8 and 176.0 Ma. Lorraine is hosted within the Quesnel island arc terrane, and mineralization and magmatism at Lorraine postdate accretion of the Quesnel terrane to ancestral North America by approximately 7 to 10 m.y. The Lorraine deposit represents the youngest known alkalic Cu-Au porphyry deposit within the Quesnel terrane, and appears to coincide with the last gasp of alkalic magmatism within the Quesnel terrane during the Mesozoic. [ABSTRACT FROM AUTHOR]

The Late Triassic Galore Creek porphyry district is the largest accumulation of Cu-Au prospects in British Columbia, Canada, in terms of contained metal. The principal economic resource is the Central zone deposit. It comprises three high-grade centers: the Au-rich North and South gold lenses and the Cu-dominated Central replacement zone. Alteration footprints, ore shells, and porphyry intrusions in these centers dip 45° to 60° to the west in the North gold lens and southwest in the South gold lens and Central replacement zone, indicative of significant postmineral tilting. This inclination created an oblique vertical expression of the Central zone at surface and at depth, providing a +700m depth profile through the deposit. Galore Creek is considered to be the end-member of the silica-undersaturated class of alkalic porphyry Cu-Au deposits. It displays a unique hydrothermal alteration footprint, generally devoid of quartz veining. Despite the lack of crosscutting veins, a paragenetic sequence of several discrete potassic and calcic alteration events has been established. During main-stage alteration, two mineralization events occurred that formed the economic resource in the Central zone. Highly oxidizing fluids derived from porpbyritic syenite to monzonite intrusions produced initial potassic alteration and sulfide mineralization in the North gold lens and South gold lens. Cu-Au ore shells are characterized by a gold-rich core dominated by bornite associated with hematite-dusted orthoclase, specular hematite, and anhydrite. The second period of hydrothermal activity caused brecciation and calcic alteration in the Central replacement zone. The calcic fluids formed hydrothermal cement grading from Ti-rich andraditic to grossularitic garnet in the core of the breccia to a diopside and magnetite-dominated assemblage at the margins. Calcic alteration also formed in the surrounding host rock. This alteration event was followed by potassic alteration and mineralization, characterized by biotite, anhydrite, and chal-copyrite in the Cu-rich ore shell of the Central replacement zone. Most of the Au is hosted in bornite that is spatially associated with oxidized (reddened) rock units dominated by an alteration assemblage of orthoclase ± hematite + specular hematite ± garnet ± anhydrite. High-grade Cu in the Central zone is related to abundant chalcopyrite that has commonly replaced an assemblage of biotite ± magnetite ± diopside. This distinct spatial separation strongly suggests that the local redox environment within the wall rocks governed sulfide and gold distributions. The switch from early Au-rich mineralization to late-stage Cu-dominated mineralization appears to have been controlled by varying redox conditions within the magma. The change from more oxidized alteration assemblages (Fe3+ > Fe2+) to more reduced assemblages (Fe3+ < Fe2+) is indicative of a change in oxidation potential that may have propelled sulfide saturation in the magma and, thus, the subsequent depletion of most of the Au. [ABSTRACT FROM AUTHOR]

Mount Polley is a Late Triassic (~205 Ma) alkalic porphyry Cu-Au-Ag deposit (226.3 thousand tonnes (t) Cu, 21.5 t Au, and 65.11 Ag), hosted by silica-undersaturated to silica-saturated monzonitic intrusions of the Mount Polley Complex, located in British Columbia, Canada. The Northeast ore zone at Mount Polley is hosted by magmatic-hydrothermal breccia. Copper and precious metals occur in sulfide minerals primarily as coarse- to fine-grained breccia cement. Local wall rocks include equigranular to porphyritic diorite, monzodiorite, and monzonite. Alteration, breccia cement, and veins of the Northeast ore zone formed in five paragenetic stages: prebreccia (stage 1), brecciation and main-stage mineralization (stage 2), late-stage mineralization (stage 3), unmineralized postbreccia dikes and veins (stage 4), and epithermal-style veins (stage 5). Intense pervasive K and Fe metasomatism ± calcite and calc-silicate alteration occurred prior to brecciation caused by the intrusion of mega-crystic K-feldspar-phyric monzonite. Stage 2 fluids were silica undersaturated, high temperature (>350°C), CO2 enriched, and of near-neutral to alkaline pH. Potassic, sodic, and cale-potassic assemblages precipitated with mineralization during stage 2 with moderate temperatures at the deposit periphery and in stages 3 and 4. Evidence for more acidic and lower-temperature conditions is preserved in stage 5 veins. The δ34Ssuifide isotope compositions of stages 2 and 3 chalcopyrite, pyrite, and bornite range from -7.1 to +1.4%c. Sulfur isotope compositions of anhydrite and gypsum are mostly between 6.2 and 9.8%o. These values, together with the presence of hematite, are consistent with deposition from an oxidized, sulfate-dominant, high-temperature magmatic-hydrothermal fluid. Limited sulfur isotope geothermometry indicates that Cu sulfides precipitated at temperatures from -480° to ~250°C. Hydrothermal calcite occurs in all paragenetic stages at Mount Polley. Calcite δ13C values range from -0.2 to -10.5%, and δ18O values from 4.0 to 20.9%. The enriched C-O isotope values are not consistent with simple precipitation from an entirely magmatic source of hydrothermal fluid. Interaction of the fluid and/or magma with limestone is considered a likely process to explain the C and O isotope signature. Lead isotope data suggest mixing of mantle and crustal sources during mineralization. Main-stage chalcopyrite and pyrite as well as late-stage galena have 206/204pb values of 18.77 to 18.92, 207/204pb of 15.56 to 15.59, and 208/204pb of 38.22 to 38.32. Strontium isotope data (0.70331-0.70371) provide evidence of a strongly depleted mantle source of Sr with minor crustal input. Epsilon Nd values for main-stage apatite range between 5.9 and 6.5, also indicating a depleted mantle source. Stage 5 carbonate 206/204pb values of 18.96 to 19.04, 207/2044pb of 15.57 to 15.59, and 207/204pb of 38.26 to 38.36 suggest superposition of an epithermal system onto the Northeast ore zone, potentially as late as -100 m.y. after breccia formation. The data presented are consistent with the hypothesis that the silica-undersaturated alkalic Mount Polley Complex formed due to carbonate assimilation prior to mineralization. This process can explain the 1δ13C-δ18O isotope data, calcite precipitation concurrent with Cu-Au mineralization, and silica undersaturation of the magma. The CO2 released during assimilation of carbonate also could have promoted magmatic-hydrothermal brecciation. Silica-undersaturated alkalic porphyry systems may preferentially form in arc terranes built on a carbonate-bearing substrate. [ABSTRACT FROM AUTHOR]

Paired belts of alkalic and calc-alkalic porphyry deposits are genetically linked to early Mesozoic volcanic arcs preserved as the Quesnel and Stikine terranes in western Canada. These parallel arc terranes extend for 2,000 km along the axis of the Canadian Cordillera. They are joined at their northern ends, but are otherwise separated by relics of Tethyan ocean basin and oceanic arc rocks collectively referred to as the Cache Creek terrane. Porphyry Cu ± Au-Mo-Ag deposits are concentrated within the Stikine-Quesnel arc terranes, with most of their economic metal endowment a consequence of a 15-m.y. mineralizing epoch. A particularly prolific period within the mineralizing epoch is a 6-m.y. pulse centered on 20.5 Ma when more than 90% of the known copper endowment was acquired. Distinct trends of Cu-Au ± Ag-Mo mineralization within both arc terranes coincide in time and space with events that are attributed to effects of slab subduction. Stratigraphy within both Stikine and Quesnel arc terranes is equivalent, with greater variation along the arcs than between them. Magmatism within both arcs began in the Late Devonian, following initiation of subduction near eastern Panthalassa and formation of the back-arc Slide Mountain ocean basin margin adjacent to Laurentia. Both terranes record further arc development in the lower Carboniferous and cessation in the Permian. Between the Late Permian and Middle Triassic, both arcs foundered, and then recorded a cryptic Early Triassic deformational event, coincident with closing of the Slide Mountain ocean. Permo-Triassic subduction and arc magmatism are recorded in the Sitlika--Kutcho-Venables arc, but this occurred far to the west, within the Tethyan realm. Early in the Late Triassic (early Carnian), both the Stikine and Quesnel arcs reignited. Later, near the end of the Triassic (late Norian), the arcs were uplifted, picrites were erupted locally, and magmatism was dominantly strongly alkaline. Monzonitic plutons hosting alkalic Cu poq)hyry deposits were emplaced along the arcs, followed by a widespread erosional hiatus that extends throughout much of the latest Triassic (Rhaetian). Late Triassic events culminating in generation of Cu porphyry deposits are attributed to a model wherein delivery of the Permo-Triassic Sitlika-Kutcho-Venables arc to the Stikine-Quesnel arc trench led to stalled subduction and arc-parallel tearing of the slab. Ingress of hot subslab mantle into the tear resulted in high-degree partial melts, as recorded by pierite within the arc circa 210 Ma. Widening of the tear and decay of the initial thermal spike raised temperatures across broad regions of the metasomatized mantle wedge, with low-degree partial melts generated from the most hydrated, metal-enriched portions circa 203 Ma. These melts produced the well-defined belt of alkalic Cu-Au-Ag porphyries. Parts of the slab tear may have nucleated near the fore-arc to generate Mo-rich Cu porphyries, such as the Gibraltar deposit, which partly cuts the accreted Cache Creek terrane. Metal-rich hydrous melts may have traveled along preexisting intrusive pathways to invade batholithic bodies along the arc axis, such as at Highland Valley, where 204 Ma mineralization is hosted within a larger ~210 Ma plutonic complex. Migration of the locus of mineralized porphyry intrusions toward the back-arc may track the leading edge of the slab gap, with the formation of Cu-Au and younger Au-rich poqihyries until ~195 Ma. Late alkalic porphyry formation, following emplacement of the Quesnel arc onto the ancestral North American margin at -186 Ma, may have been produced through cannibalism of mineralized intrusions emplaced at deeper crustal levels during the ~203 Ma Cu porphyry event. Two calc-alkalic porphyry deposits that predate Sitlika-Kutcho-Venables arc collision--Fin and Schaft Creek (~220 Ma)--are temporally related to arc uplift and erosion across the northern Stikine arc, but little is known about the extent and cause of this event and how it relates to porphyry Cu genesis. Slab tears may play a dominant role in the formation of porphyry deposits globally. Previous workers have linked episodes of porphyry formation with subduction of ocean plateaus, changes in subduction polarity, or termination of subduction. In each case, slab tears are the likely underlying cause of porphyry formation. We believe them to be fundamental to mineralization in the early Mesozoic arc terranes of Stikine and Quesnel. [ABSTRACT FROM AUTHOR]

An introduction to the journal is presented in which the author discusses various reports within the issue on topics including alkalic gold-copper deposits, magmatism, and metallogeny particularly in British Columbia and the South Pacific region.

Altar (31° 29' S, 70° 28' W) is a large porphyry Cu-(Au-Mo) deposit with associated epithermal Au-(Ag- Cu) veins located in the Cordillera Principal of southwest San Juan province (Argentina). Altar is a complex magmatic-hydrothermal system formed from several magmatic and hydrothermal pulses during the middle- late Miocene. New LA-ICPMS U-Pb ages in zircons from the Altar porphyries indicate four discrete events of intrusions over an extended magmatic life time of ca. 3 m.y. It comprises a premineralization porphyry (11.75 ± 0.24 Ma), three mineralized porphyries (11.62 ± 0.21 and 11.68 ± 0.27 Ma, 11.13 ± 0.26 Ma, 10.35 ± 0.32 Ma) related to hydrothermal breccias, two postmineralization intrusions, and a postmineralization breccia (8.9 ± 0.4 Ma). The three mineralized porphyries (porphyries 2, 3, and 4) were emplaced within -0.7 to 1.3 m.y. Amphibole phenocrysts from the porphyries crystallized from oxidized magmas (fO2 = NNO +1 to +2) at temperatures of 780° to 850°C and pressures between 0.9 and 1.8 kbars corresponding to depths of -4 to 7 km. Anorthite- and Fe-rich rims in the plagioclase phenocrysts suggest that the magmatic chambers were episodically recharged by a less evolved magma. The middle-late Miocene intrusions are interpreted to have been derived from a deeper and relatively large magmatic reservoir that supplied magmas to smaller chambers located in the upper crust. The focused magmatic output to shallow levels during a period of a few million years in the Altar area has been a main requirement in the formation of this large porphyry copper deposit. [ABSTRACT FROM AUTHOR]

The Yao'an porphyry Au system, Machangqing porphyry Cu-Mo system, and Beiya porphyry-skarn Au system, are spatially and temporally associated with potassic felsic intrusions emplaced during the Eocene to Oli-gocene epochs at 37 to 33 Ma in a postcollisional intracontinental setting in western Yunnan, western Yangtze craton, China. The Yao'an monzonite and quartz monzonite porphyry intrusions are alkaline and potassic with high K2O/ Na2O ratios (1.1-1.5). They have Sr-Nd-Pb isotopes similar to coeval lamprophyres and are characterized by uniform zircon εHf (-6.4 to -8.7) and δ18O values (6.6-7.0‰). They are interpreted as products of fractional crystallization of lamprophyre-like potassic mafic magma derived from ancient metasomatized lithospheric mantle, a scenario similar to the mid-Cretaceous postcollisonal Scheelite Dome gold system in Yukon, Canada. The Machangqing granitic intrusions are high K calc-alkaline and show high Sr, SrA' and La/Yb, but low Y and Yb geochemical signatures. They have Sr-Nd-Pb isotope compositions similar to amphibolite xenoliths hosted by potassic felsic intrusions in western Yunnan. The zircon εHf values of the Machangqing granitic intrusions are positive (0.3-4.7), and the zircon-depleted Hf mantle model ages are 1.1 to 0.8 Ga. They also have mantle-like zircon (δ18O values (5.5-6.4‰). The Machangqing granites were most likely derived from partial melting of Neoproterozoic lower crust. The Beiya granitic intrusions are alkaline, with high K2O/Na2O (1.9-2.7), Sr/Y and La/Yb ratios, high Sr contents, and low Y and Yb contents. They contain abundant zircon inheritance and have variable magmatic zircon εHf (-4 to +4) and the highest magmatic zircon δ18O values (6.6-7.8‰). The Beiya felsic intrusions are interpreted to be derived from partial melting of a K-rich mafic source mixed with a metasedimentary component. The Eocene-Oligocene intracontinental potassic intrusions and associated mineralization in western Yunnan are located proximal to the Mesozoic Jinsha suture, suggesting that this Mesozoic lithospheric boundary may have provided a first-order control on localization of Cenozoic mineral systems. These potassic felsic intrusions are coeval with regional potassic mafic magmatism in western Yunnan and were emplaced between 37 to 33 Ma, after the collision between India and Asia at ca. 60 to 55 Ma. It is therefore postulated that continental collision may have preferentially thickened the continental lithospheric mantle (CLM) adjacent to the Jinsha suture, in which overthickened lower continental lithospheric mantle was subsequently removed during 37 to 33 Ma, inducing melting of residual metasomatized lithospheric mantle as well as lower crust. The gold-rich Yao'an and Beiya intrusions are alkaline and potassic, characterized by high zircon δ18O values (>6.5‰), which is consistent with supracrustal contributions. In contrast, the Cu-Mo-rich Machangqing intrusions are high K calc-alkaline with mantle-like zircon dISO values (<6.5‰) and juvenile enr signatures, indicating negligible supracrustal recycling. Empirically, source compositions played an important role in determing the metal endowment among intrusions formed under the same tectonic setting with similar ages in western Yunnan. In western Ymman, gold tends to be associated with alkaline and potassie melts with a supracrustal contribution, whereas Cu-Mo mineralization seems to be more related with juvenile crustal sonrees with little supracrustal influence. [ABSTRACT FROM AUTHOR]