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The timing and extent of sulfide saturation have been suggested as controlling factors in the formation of economically significant porphyry Cu deposits in subduction zone settings. However, details on the sulfide saturation history in post-collisional porphyry systems remain ambiguous. Accordingly, we have characterized the whole-rock geochemistry, including platinum-group elements (PGE), of igneous intrusions in the post-collisional Chongjiang porphyry Cu–Mo–Au deposit (southern Tibet) and utilize this data in conjunction with zircon U–Pb geochronological results and sulfide chemistry to assess the timing of sulfide saturation, the nature and amount of magmatic sulfide produced. The Chongjiang intrusions (monzogranite, biotite monzogranite porphyry, granodiorite, dacite porphyry, and quartz diorite porphyry) and mafic microgranular enclaves (MMEs) have zircon U–Pb ages of 14.2 to 12.8 Ma. Covariations in whole-rock major and trace elements among the Chongjiang intrusions and MMEs, together with similarities in their Sr–Nd and zircon Hf isotope compositions, indicate that they are co-magmatic and crystallized from a juvenile lower crustal melt that mixed with mafic melt derived from the lithospheric mantle; this hybrid melt subsequently evolved via fractional crystallization. Trace-element ratios in zircon and temperature − ∆FMQ estimates of the different intrusions suggest that they all crystallized from oxidized (average ∆FMQ = 1.9–2.6) and water-rich magmas. Palladium contents and Pd/Pt ratios in the Chongjiang igneous intrusions increase with decreasing MgO up to 3.9 wt % MgO, after which they abruptly decrease. The initial increase in Pd/Pt ratios likely results from the fractionation of a Pt-rich mineral (e.g. Pt–Fe alloy). The decrease in Pd contents and Pd/Pt ratios at 3.9 wt % MgO likely results from sulfide saturation during magma evolution, but prior to volatile exsolution, which occurred at approximately 1.4 to 2.4 wt % MgO. The presence of magmatic sulfide inclusions in amphibole and magnetite in samples with 3.9 wt % MgO, and the geochemical compositions of sulfide inclusions suggest that they represented trapped sulfide liquid and intermediate solid solution. Results of Monte Carlo simulations demonstrate that 0.003 to 0.009 wt % magmatic sulfide is required to have fractionated from the magma to explain the decrease in Pd contents at 3.9 wt % MgO. Highly chalcophile elements, such as Pd, will be sequestered by the magmatic sulfide that saturates at depth, decreasing their concentrations in the residual silicate melt, whereas concentrations of the less chalcophile elements, such as Cu, Mo, and even Au, will not be as significantly affected. Consequently, sufficient concentrations of Cu–Mo–Au will remain in the residual melt and, upon reaching volatile saturation, can be transported by the vapor phase to form porphyry Cu–Mo–Au deposits. In the case of the Chongjiang deposit, sulfide saturation was likely triggered by the high pressures and/or depletion of FeO caused by the thick (~70 km) crust beneath the Gangdese belt. This contribution presents evidence of sulfide saturation in post-collisional magmatic systems, and demonstrates that the amount of magmatic sulfide produced is a critical factor in controlling the formation of post-collisional porphyry Cu deposits. [ABSTRACT FROM AUTHOR]

Subduction zone magmatism is a major control of volcanism, the generation of modern continental crust and the formation of economically important porphyry Cu–(Mo–Au) deposits. Reading the magmatic record of individual arc segments and constraining the rates of magmatic changes are critical in order to fully understand and quantify the processes that drive magma evolution in subduction settings during arc growth. This study focuses on the San Francisco Batholith and the Rio Blanco-Los Bronces porphyry deposit cluster in central Chile, which provides an igneous rock record over ~13.5 Myr of arc evolution. We use whole-rock geochemistry, zircon geochronology and Hf isotope geochemistry to track changes in the crustal magmatic system of this arc segment during crustal thickening and porphyry Cu deposit formation. By combining the analytical dataset with Monte Carlo fractional crystallisation and assimilation fractional crystallisation modelling, we test a model for significant crustal involvement during magma evolution. Systematic and continuous increases in Dy/Yb, La/Yb, V/Sc and Sr/Y in the magmas over time indicate a transition in the main fractionation assemblage from plagioclase-dominated to amphibole-dominated that reflects deeper crystallisation and/or a higher meltwater content. Concomitant decreases in εHf and Th/La as well as increasing Ba/Th are best explained by assimilation of progressively deeper crustal lithologies from low (Chilenia) to high Ba/Th (Cuyania) basement terranes. Our study highlights that an increasingly hydrous magma and a deepening locus of crustal magma differentiation and assimilation, driven by crustal thickening contemporaneous with increased tectonic convergence and ingression of the aseismic Juan Fernandez ridge, can account for all investigated aspects of the multi-Myr magmatic evolution leading up to the formation of the Rio Blanco-Los Bronces porphyry Cu deposits. Our findings corroborate the importance of high-pressure differentiation of hydrous magma for the formation of Andean-style porphyry deposits. Once magmas favourable for porphyry Cu mineralisation were generated in the lower crust, multiple episodes of efficient magma migration into the upper crust fed several, discrete, shallow magmatic-hydrothermal systems over ~3.5 Myr to form the world's largest known Cu resource at Rio Blanco-Los Bronces. [ABSTRACT FROM AUTHOR]

The origin of intermediate to felsic potassic magmas is debated, and not much is known about the volatile content of potassic magmas associated with porphyry Cu (Au, Mo) deposits. To better understand the liquid line of decent of mafic potassic magmas and the behavior of volatiles during magma differentiation, we performed 19 experiments at 1.0 GPa and 1150 °C to 850 °C using piston cylinder presses. We developed a new experimental technique that involves a capsule liner made of single-crystal zircon to prevent the loss of Fe and S in the starting material to the noble metal capsule. The starting material is a high-Mg, basaltic trachyandesite (52 wt% SiO2, 12 wt% MgO, 1.9 wt% Na2O and 5.3 wt% K2O) from the Sanjiang region in southwestern China, doped with geologically realistic amounts of volatiles (i.e. 4.0 wt% H2O, 0.34 wt% CO2, 0.27 wt% Cl and 0.25 wt% S). The addition of 0.25 wt% S in the form of anhydrite internally buffered the experiments at an oxygen fugacity of 2.0 ± 0.5 log units above the fayalite–magnetite–quartz buffer, which is similar to the redox state of the Sanjiang variously evolved potassic magmas. The experimentally produced silicate melts match well with the Sanjiang intermediate to felsic magmas in terms of major, minor and trace element compositions, and also with regard to the S and Cl contents. The sequence of crystallizing minerals (olivine + clinopyroxene –> biotite ± orthopyroxene –> apatite –> K-feldspar) also fits with the one observed in the Sanjiang mafic to intermediate magmas. These results suggest that the Sanjiang intermediate to felsic magmas, including the porphyry Cu (Au, Mo)-forming magmas, can form solely by differentiation of potassic mafic magmas without any involvement of old crustal material. During experimental differentiation at 1.0 GPa, the S content of the evolving silicate melt first increased until ~57 wt% melt SiO2, and then decreased in response to precipitation of sulfides, sulfate melt and/or anhydrite, whereas the H2O and Cl contents of the evolving silicate melt increased exponentially until saturation in a CO2-rich fluid was reached at 60 to 65 wt% melt SiO2 and ~ 8 wt% melt H2O. During further magma differentiation the H2O and Cl contents of the evolving silicate melt remained constant until ~70 wt% melt SiO2, after which point the Cl content of the silicate melt decreased due to increased partitioning of Cl into the fluid phase ± increased fluid/melt ratio. Based on these experimental results and petrographic and geochemical evidence from natural samples, the Sanjiang porphyry Cu (Au, Mo)-forming magmas (65–70 wt% SiO2) are interpreted to have formed through differentiation of primitive, mantle-derived, potassic magmas in the lower crust (≥1.0 GPa), and to have ascended ±directly from the lower crust to shallow crustal levels. They likely contained 8 to 13 wt% H2O, 0.37 to 0.90 wt% Cl and 0.07–0.29 wt% S. This case study on the magma evolution in the Sanjiang region may have implications for the origin and nature of intermediate to felsic potassic magmas in various tectonic settings. [ABSTRACT FROM AUTHOR]

We proposed a stepwise procedure to obtain a multiscale 3-D resistivity model beneath the Caosiyao porphyry Mo deposit. Due to the joint inversion of data collected from 302 audio-frequency magnetotelluric and 33 broad-band magnetotelluric sites, this model can simultaneously combine both the shallow resolution and detection depth. The near-surface part of the model reveals the high-resolution structures. Specifically, a high-resistivity body (>500 Ω m) is interpreted as the late Jurassic porphyry intruding into the metamorphic basement. Two NE-trending high-conductivity (<100 Ω m) belts are interpreted as fault zones. The one in the southeast extends deeply (>15 km) is suggested to be a crust-scale weak zone and conduit for transporting ore-forming magmas and fluids. The other one is shallow (<2 km), but coincides with outcropped orebodies and alteration zones, so it may have played an important role in the hydrothermal alteration and mineralization. A predominant high-conductivity block (<30 Ω m) occurs within the lower part (∼15–30 km) of the model and connects with Caosiyao deposit through the inferred crust-scale fault. Its enhanced conductivity may result from the abundant fluids. We determined it had been a mid-lower crustal magma chamber providing space for the mixing of ore-forming magmas and fluids, as well as the element exchange during the mineralization. Combined with a previous magnetotelluric imaging, we suggested the heats and fluids needed for the mineralization might have been transported from a giant upper-mantle basaltic magma reservoir as indicated by a conductor beneath the Datong volcanic area through a lithosphere-scale shear zone. Summarily, the multiscale resistivity model provides us a chance to depict the multiscale metallogenic system for the Caosiyao porphyry Mo deposit from the aspect of electrical resistivity. [ABSTRACT FROM AUTHOR]

Sulfide-rich hydrous arc cumulates in the lower crust are considered a likely source for porphyry Cu-forming magmas in post-subduction settings. However, it remains controversial whether or not post-subduction porphyry Cu magmas can be produced by partial melting of lower crustal arc cumulates. Here we performed detailed petrographic and geochemical studies on exhumed lower to middle crustal cumulates from the Gangdese and Kohistan arc sections, and then conducted partial melting experiments of the Gangdese sulfide-rich lithologies at 1.5 GPa and 800°C–1150°C using a piston cylinder press. Both the Gangdese and the Kohistan arc-root samples formed as cumulates in the lower to middle crust, but the Kohistan samples were subsequently brought into the garnet stability field during the India–Eurasia continental collision. Most of the Gangdese samples contain relatively high abundances of magmatic sulfides preserved in the form of Cu-poor monosulfide solid solution. In contrast, most of the Kohistan samples contain very low abundances of Cu-rich sulfides preserved as inclusions in garnet, probably due to intense metamorphic dehydration and up to 2 wt% partial melting in the garnet stability field. The original magmatic sulfides at Kohistan appear to have been removed mainly by metamorphic fluids rather than by partial melts, as the arc-root samples could have experienced only very low degree of partial melting and the sulfur solubility in experimentally produced partial melts is low (≤0.1 wt%) even at oxygen fugacities two log-units above the fayalite-magnetite-quartz buffer. Furthermore, the experimental partial melts contain much less K2O, MgO, Cl and ±S than typical post-subduction porphyry Cu-forming magmas. Hence, we conclude that the role of sulfide-rich arc cumulates in the formation of post-subduction porphyry Cu magmas has been severely overestimated. [ABSTRACT FROM AUTHOR]

The Rio Blanco deposit, which is one of the largest porphyry Cu–Mo deposits in northern Peru, formed coevally with the subduction of the Inca Oceanic Plateau at 12–10 Ma. However, the genetic relationship between the subduction of oceanic plateaus and the porphyry deposit formation remains unclear. Igneous rocks emplaced at 23–12 Ma in northern Peru, including the Portachuela batholith (which hosts the Rio Blanco porphyry complex), are normal calc-alkaline to weakly adakitic. In comparison, the 12–8 Ma igneous rocks, including the ore-related Rio Blanco porphyry complex, have typical adakitic signatures, such as high Sr/Y ratios (up to 180) and LaN/YbN ratios (up to 32). The Rio Blanco igneous rocks (Portachuela batholith and Rio Blanco porphyry complex) have uniform zircon εHf(t) values (+0.3 ± 1.2) and δ18O values (6.5 ± 0.14‰). These geochemical characteristics indicate that the Rio Blanco igneous rocks evolved from mantle-derived parental melts in a long-lived, stable, homogeneous isotopic reservoir at the crust–mantle boundary. However, whereas both the Portachuela batholith and the Rio Blanco porphyry complex formed from hydrous parental magmas (>5 wt %; based on plagioclase hygrometry), the ones of the Rio Blanco porphyry complex seem to be more oxidized, hydrous, and sulfur-rich compared with the older batholitic rocks. Reverse zoning in plagioclase phenocrysts, with a systematic core–mantle–rim variation in An (anorthite) and Fe (total iron) contents, are common in the intermineralization rocks. The An content of the mantles of the plagioclase phenocrysts correlates positively with the Fe content, but in the rims, the An contents significantly decrease while Fe remains constant. The apatite inclusions in the mantles are richer in S (0.24 ± 0.06 wt %) and Cl (1.42 ± 0.32 wt %) than those in the phenocryst cores (S: 0.09 ± 0.07 wt %; Cl: 1.03 ± 0.56 wt %) and rims (S: 0.14 ± 0.09 wt %; Cl: 0.83 ± 0. 35 wt %). These systemic geochemical variations in the plagioclase phenocrysts suggest recharge by S- and Cl-rich melts followed by fluid exsolution. This magma recharge and subsequent fluid exsolution may have triggered porphyry Cu mineralization at Rio Blanco. The coincidence of timing between the geochemical transition and collision (initial subduction) of the Inca Oceanic Plateau with the South American plate may indicate a change in the tectonic regime to a compressional state of stress and a thickening of the crust during the collision. The tectonic transition would have facilitated the fractionation of mantle-derived magma in a deep crustal hot zone, resulting in oxidized, volatile-rich residual melts. Replenishment of the upper-crustal magma chamber by such volatile-rich magmas and the subsequent discharge of fluids are interpreted to be fundamental for porphyry Cu mineralization at Rio Blanco and plausibly for the formation of Late Miocene porphyry ore deposits in northern Peru in general. [ABSTRACT FROM AUTHOR]

Mount Hagen is a mid-Pleistocene stratovolcano located in the Papua New Guinea Highlands. It was selected for this study because of (1) its close location to several giant Cu–Au porphyry deposits in the region; (2) its high-K calc-alkaline character (absarokite–shoshonite) and (3) its wide compositional range (2 to 12 wt % MgO). Whole-rock geochemistry, petrography and QEMSCAN observations are consistent with fractional crystallisation of the evolving magma being controlled by the phases now represented by the megacrysts in the lavas: olivine and clinopyroxene at high MgO and hornblende and plagioclase at low MgO. However, the role of plagioclase is enigmatic. Although it is an abundant megacryst phase in the low-MgO samples, plagioclase has little or no influence on fractional crystallisation, suggesting that it may be an antecryst. The appearance of plagioclase megacrysts does not slow the rate of increase in Al2O3 or Sr with decreasing sample MgO and the low-MgO samples do not have Eu anomalies. At ~5.5 wt % MgO, there are significant changes in the system that are inconsistent with simple fractional crystallisation. These changes include hornblende and plagioclase replacing olivine and clinopyroxene as the principal megacryst phases, a sharp rise in platinum-group elements (PGE) concentrations, the brief reappearance of high Fo olivine megacrysts and reversed zoning in the plagioclase megacrysts. These changes are interpreted to have resulted from a new pulse of magma entering the system, with higher water, MgO, SiO2 and PGE concentrations than the original parent magma. Scatter in the PGE data is attributed to the presence of micronuggets in all samples, including the most mafic samples. We conclude that the magma system became sulphide saturated during an early stage in its fractionation history, probably before leaving the crust to deep crustal magma chamber, and then became undersaturated as a consequence of decreasing pressure as magma ascended into a mid-crustal magma chamber. The early saturation episode had a pronounced influence on PGE concentration but had little affect Cu and Au, due to their lower partition coefficient into sulphides. The magma became sulphide saturated again in the crustal magma chamber at ~8.5 wt % MgO, this time affecting all chalcophile elements. Given the early episodes of sulphide saturation, depletion in Au in the Mount Hagen magma system and the absence of a known porphyry system, it is unlikely that Mount Hagen produced economic porphyry mineralisation. [ABSTRACT FROM AUTHOR]

Dabie-type porphyry Mo deposits have recently been identified as a new subtype of porphyry Mo deposits, but several questions remain about the role of ore-related magmas in the formation of this type of deposit, as well as distinctions in genetic processes with the well-studied Climax-type porphyry Mo deposits. Here, mineral and melt inclusions from the giant Qian'echong deposit, Dabie orogen, central-eastern China, were studied in order to improve our understanding of the nature and the role of ore-related magmas in the genesis of Dabie-type porphyry Mo deposits. The magmatic and hydrothermal evolution of the system was reconstructed based primarily on the analysis of quartz-hosted silicate melt and mineral inclusions, in concert with field and petrographic relations as well as previously published U–Pb and Re-Os geochronology. Ore-related magmas at Qian'echong include, from early to late, quartz porphyry dikes, rhyolite porphyry dikes, granite porphyries (stock and dikes) and a newly discovered, deep-seated monzogranite porphyry stock. Based on TitaniQ thermobarometry, these lithologies were sourced from a ~16–19-km deep (500–600 MPa) magma chamber and underwent nearly isothermal decompression to ~7 km (200 MPa), at temperatures ranging from 720°C to 690°C. According to the trace element composition of melt inclusions, in combination with published whole-rock Nd isotopic compositions, the early magma evolved from quartz porphyry to rhyolite porphyry through fractionation crystallization and thus became a crystal mush. This viscous crystal mush was subsequently re-melted to produce the granite porphyries and was injected with melts from a different source to generate the monzogranite porphyry. At Qian'echong, all melt inclusions have low concentrations of Mo (2–8 ppm), >5 wt. % H2O, and little to no F (≤0.26 wt. %), arguing against the requirement for ore-related magmas to be enriched in Mo to form large porphyry Mo deposits. Rayleigh fractionation modeling shows that the concentration of Mo in the quartz and granite porphyries increased through fractionation of quartz, feldspars, biotite, magnetite, and ilmenite and that Mo was subsequently depleted in the melt through fluid exsolution, exclusively in the rhyolite and monzogranite porphyries, as a result of magma ascent and decompression. This suggests that, in addition to ore-related granite porphyry, both the rhyolite and the monzogranite porphyries also contributed to the hydrothermal mineralization. This study confirms that the formation of porphyry Mo deposits does not rely on abnormally high concentrations of Mo in ore-related magmas but instead requires efficient extraction of Mo from large volumes of magmas, with normal concentrations of Mo. Unlike the Climax-type deposits where multiple pulses of ore-forming fluids are delivered from convecting shallow magma chambers, Mo mineralization in Dabie-type deposits was achieved by the assembling of ore-forming fluids from successively emplaced, relatively deep intrusions. Although the Dabie- and Climax-type Mo deposits are respectively associated with I-type and A-type granitoids in the Dabie orogen, it is suggested that both types of magmas are derived from the partial melting of subducted Yangtze continental crust and that it is the tectonic transition from compressional to extensional settings that controlled the different styles of porphyry Mo mineralization in this orogen. [ABSTRACT FROM AUTHOR]

There are more than 80 porphyry (or skarn) Mo deposits in northeastern China with Jurassic or Cretaceous ages. These are thought to have formed mainly in a continental arc setting related to the subduction of the Paleo-Pacific oceanic plate in the Jurassic and subsequent slab rollback in the early Cretaceous. The Jurassic Daheishan porphyry Mo deposit is one of the largest Mo deposits in NE China, which contains 1·09 Mt Mo with an average Mo grade of 0·07 %. To better understand the factors that could have controlled Mo mineralization at Daheishan, and potentially in other similar porphyry Mo deposits in NE China, the geochemical and isotopic compositions of the ore-related granite porphyry and biotite granodiorite, and the magmatic accessory minerals apatite, titanite and zircon from the Daheishan intrusions, were investigated so as to evaluate the potential roles that magma oxidation states, water contents, sulfur and metal concentrations could have played in the formation of the deposit. Magmatic apatite and titanite from the causative intrusions show similar εNd(t) values from –1·1 to 1·4, corresponding to TDM2 ages ranging from 1040 to 840 Ma, which could be accounted for by a mixing model through the interaction of mantle-derived basaltic melts with the Precambrian lower crust. The Ce and Eu anomalies of the magmatic accessory minerals have been used as proxies for magma redox state, and the results suggest that the ore-forming magmas are highly oxidized, with an estimated ΔFMQ range of +1·8 to +4·1 (+2·7 on average). This is also consistent with the high whole-rock Fe2O3/FeO ratios (1·3–26·4). The Daheishan intrusions display negligible Eu anomalies (Eu/Eu* = 0·7–1·1) and have relatively high Sr/Y ratios (40–94) with adakitic signatures; they also have relatively high Sr/Y ratios in apatite and titanite. These suggest that the fractionation of amphibole rather than plagioclase is dominant during the crystallization of the ore-related magmas, which further indicates a high magmatic water content (e.g. >5 wt%). The magmatic sulfur concentrations were calculated using available partitioning models for apatite from granitoids, and the results (9–125 ppm) are indistinguishable from those for other mineralized, subeconomic and barren intrusions. Furthermore, Monte Carlo modelling has been conducted to simulate the magmatic processes associated with the formation of the Daheishan Mo deposit, and the result reveals that a magma volume of ∼280 km3 with ∼10 ppm Mo was required to form the Mo ores containing 1·09 Mt Mo in Daheishan. The present study suggests that a relatively large volume of parental magmas with high oxygen fugacities and high water contents is essential for the generation of a giant porphyry Mo deposit such as Daheishan, whereas a specific magma composition (e.g. with unusually high Mo and/or S concentrations) might be less critical. [ABSTRACT FROM AUTHOR]

Modern arc adakites with high Mg# values (molar 100 × Mg/(Mg + Fe) ratio) are generally considered products of interaction between melts derived from subducted oceanic crust and/or eroded forearc crust and peridotite in the mantle wedge. An alternative model, in which high-Mg# adakitic rocks are produced by garnet fractionation of mantle-derived magmas, has been proposed based on whole-rock geochemical variations; however, magmatic garnet has not been found in high-Mg# adakitic rocks, and little is known about the physical conditions required for this magmatic differentiation. Here we report geochronological, mineralogical and geochemical data for Late Triassic garnet-bearing high-Mg# (Mg# = 45–56) adakitic diorite porphyries and garnet-free non-adakitic diorite porphyries with Mg# > 62 from central Tibet. Consistent compositional correlation between Ca-rich garnet crystals, their host rocks and zircon autocrysts suggests that the garnet crystals grew in their host magmas. Amphibole, garnet, zircon and the host rocks display increasing Dy/Yb ratios with increasing magma differentiation. Pristine magmas in equilibrium with amphibole that crystallized prior to garnet are not adakitic. The garnet-bearing high-Mg# adakitic rocks were probably generated by the fractionation of pyroxene, amphibole and garnet at ∼1 GPa from a primitive andesitic parent that was geochemically similar to the garnet-free diorite porphyries. The primitive andesitic magmas with enriched isotope compositions ([87Sr/86Sr]i > 0·709, ɛNd[t] < −3·4) may be derived from shallow melting of subduction-enriched lithospheric mantle in a post-collisional, extensional setting resulting from oceanic slab breakoff. The most likely scenario for garnet crystallization is that mantle-derived hydrous (H2O >5 wt %) magmas stalled, cooled isobarically and differentiated at the base of the crust. This study provides direct mineralogical evidence for the generation of high-Mg# adakitic rocks by crystal fractionation involving garnet, rather than by interaction between crust-derived melt and the mantle, although the latter is potentially a frequent occurrence in the mantle wedge. [ABSTRACT FROM AUTHOR]

Porphyry systems, the most important reserves of Cu and Mo with significant Au, are genetically linked to the emplacement of hydrous and oxidized intermediate to acidic magmas, in response to temporal and geochemical evolution of crust in orogenic terranes. In this study, comprehensive whole-rock and zircon geochemical and isotopic datasets of intermediate to acid igneous rocks were integrated to characterize the crustal evolution and metallogeny of porphyry deposits in the Central Asian Orogenic Belt (CAOB). The ore-forming porphyries of Cu ± Au ± Mo and Mo deposits have higher but largely overlapped ΔFMQ (the proxy for oxygen fugacity) with those of the barren igneous rocks. However, the ore-forming porphyries of Cu ± Au ± Mo deposits are characterized by distinctly higher whole-rock V/Sc and zircon Eu/Eu* (both are proxies for water content) than barren rocks. Furthermore, the V/Sc ratios positively correlate with the Cu tonnages of calc-alkali porphyry Cu deposits, suggesting that magmatic water contents may yield the first-order control on metal endowment. The general decrease of V/Sc from the Paleozoic to Mesozoic, combined with the negative correlation of V/Sc with K2O and SiO2, also indicates the gradual evolution of crust in CAOB clearly controls the end members of porphyry-type systems (i.e. Cu‒Au and Mo deposits). The crustal residence age (TRes, the time difference between Nd depleted-mantle model age and the crystallization age) of c. 500 Ma from the Nd isotopes is proposed as the threshold distinguishing porphyry Cu ± Au ± Mo (TRes < 500 Ma) and Mo deposits (TRes > 500 Ma) in the CAOB. The coupled zircon Hf isotopes and crustal thickness reveal that the fundamental crustal architecture in the eastern and western CAOB had been built by the Late Permian and Late Carboniferous, respectively, highlighted by the converging trends of εHf(t) commencing at c.250 Ma in the eastern segment and c.300 Ma in the western segment of CAOB, indicating reworking and homogenizing of juvenile crust after collision. In the eastern CAOB, porphyry Cu ± Au ± Mo deposits were formed by juvenile materials in thin island arcs, while porphyry Mo deposits were formed by reworked materials in the thickened orogenic crust after c.250 Ma. In the western CAOB, porphyry Cu deposits in the Balkhash region during the Late Carboniferous were formed in thickened continental crust (generally > 40 km), genetically linked to the culmination of world-wide magmatic addition rates (MARs) triggered by accelerated production of the juvenile crust, in contrast to the porphyry Cu ± Au ± Mo deposits formed in thin island arc (generally < 40 km) during the Early Paleozoic. This study tests the zircon ΔFMQ as proxy for f O2, and zircon Eu/Eu* and whole-rock V/Sc ratios as proxy for water content. It highlights that whole-rock V/Sc ratio is a favorable index for the Cu tonnages of porphyry Cu ± Au ± Mo deposits, and that the distinct porphyry-type mineralization in the CAOB is controlled by the crustal evolution reflected by crustal composition and thickness. [ABSTRACT FROM AUTHOR]

Hydrothermal fluid is essential for transporting metals in the crust and mantle. To explore the potential of Cu isotopes as a tracer of hydrothermal-fluid activity, Cu-isotope fractionation factors between Cl-bearing aqueous fluids and silicate magmas (andesite, dacite, rhyolite dacite, rhyolite and haplogranite) were experimentally calibrated. Fluids containing 1.75–14 wt.% Cl were mixed together with rock powders in Au95Cu5 alloy capsules, which were equilibrated in cold-seal pressure vessels for 5–13 days at 800–850°C and 2 kbar. The elemental and Cu-isotopic compositions of the recovered aqueous fluid and solid phases were analyzed by (LA-) ICP–MS and multi-collector inductively coupled plasma mass spectrometry, respectively. Our experimental results show that the fluid phases are consistently enriched in heavy Cu isotope (65Cu) relative to the coexisting silicates. The Cu-isotope fractionation factor (Δ65CuFLUID-MELT) ranges from 0.08 ± 0.01‰ to 0.69 ± 0.02‰. The experimental results show that the Cu-isotopic fractionation factors between aqueous fluids and silicates strongly depend on the Cu speciation in the fluids (e.g. CuCl(H2O), CuCl2– and CuCl32−) and silicate melts (CuO1/2), suggesting that the exsolved fluids may have higher δ65Cu than the residual magmas. Our results suggest the elevated δ65Cu values in Cu-enriched rocks could be produced by addition of aqueous fluids exsolved from magmas. Together with previous studies on Cu isotopes in the brine and vapor phases of porphyry deposits, our results are helpful for better understanding Cu-mineralization processes. [ABSTRACT FROM AUTHOR]

Most genetic models for magmatic-hydrothermal ore deposits are based on the prerequisite that the parental magmas associated with mineralization are enriched in water (> ∼4 wt %). However, it has been recognized that a number of magmatic-hydrothermal ore deposits also formed within tectono-magmatic settings that produce initially water-poor magmas such as Climax-type porphyry deposits. Here, we present a detailed reconstruction of the Tieshan magma plumbing system related to skarn-porphyry Cu–Fe–Au mineralization in the Edong district, in which primitive magmas typically show water-poor features. Applications of multiple thermodynamic calibrations on various magmatic units from the Tieshan and Tonglushan deposits provide a wealth of information regarding the structure and evolution of the transcrustal magmatic system. Petrographic observations and clinopyroxene-liquid thermobarometry calculations indicate that the Tieshan magmatic-hydrothermal system was fed by a deep crustal magma reservoir. An accurate picture of the evolution of H2O within the magma plumbing system is presented using the plagioclase-liquid hygrometer in combination with the amphibole hygrometer. Three critical stages during the evolution of water within the plumbing system have been recognized, associated with H2O contents of 0·8–1·7 wt %, 2·1–2·8 wt % and 3·2–4·6 wt %, respectively. The first enrichment of water in the magmas can be attributed to the separation and transfer of evolved melts from the deep magma reservoir to the shallow crust. Continuous cooling and solidification of the shallow magma body gave rise to the second enrichment of H2O in residual melts, leading to magmas that were fertile for the formation of ore deposits. The detailed chemical evolution of the magma plumbing system was investigated using mineral trace element compositions in combination with the partition coefficients predicted by the lattice strain model. The earliest equilibrium melts are characterized by high Sr contents (the average = 658 ± 64 ppm), suggesting that high Sr/Y signatures were likely derived from their magma sources or fractionation at deeper levels in initially water-poor environments. Variations of some particular geochemical fingerprints in equilibrium melts such as, Dy/Dy* and Eu/Eu*, also provide fundamental information on the evolution of the magma plumbing system. Our study confirms the critical role of a deep crustal magma reservoir on the formation of magmatic-hydrothermal ore deposits. The fertility of magmas with respect to ore deposit formation was enhanced by the extraction and transfer of evolved magmas from the deep reservoir to shallower levels, particularly due to the enrichment of magmatic water contents. In addition, the presence of a deep magma reservoir also sustains the incremental growth of shallow magma chambers, which provide ore-forming fluids. [ABSTRACT FROM AUTHOR]

The effect of magmatic sulfide precipitation on the potential of magmatic systems to produce porphyry-type ore deposits is still a matter of debate. In particular, we need to know whether magmatic sulfide precipitation has an impact on the Cu and Au content of the exsolving magmatic volatile phases and, by this way, on the Cu/Au ratio of porphyry deposits. The Javorie volcano is a perfect place to explore these questions. First, it hosts several Au-only porphyry-type mineralized occurrences which have among the lowest Cu/Au ratios reported in the literature. Secondly, the geology of the Javorie volcano and the timing of porphyry Au mineralization are well established. The evolution of the Javorie magmatic system was reconstructed by detailed petrographic studies and laser ablation inductively coupled plasma mass spectrometry analysis of minerals, melt inclusions and sulfide inclusions. The Javorie volcano was formed during the post-subduction magmatic activity affecting the Western Carpathians. It is a typical stratovolcano, composed dominantly of basaltic andesites and andesites which were intruded by several small stocks of dacitic to dioritic composition. According to our thermobarometric data, the volcano was fed by a transcrustal magmatic system in which two levels of magma chambers could be identified. Part of the magma evolved in the lower crust as suggested by the occurrence of magmatic garnet antecrysts in some of the studied rocks. The occurrence of magmatic sulfide inclusions in garnet indicates that sulfide saturation was reached in this lower crustal magma chamber. Most of the rocks crystallized in an upper crustal magma chamber (∼2 ± 1 kbar) that was fed by a basaltic to basaltic andesite magmas. A large variation in temperatures, ranging between 820°C and 1025°C, recorded by the extrusive and intrusive rocks suggest either that the upper crustal magma chamber was thermally zoned, or that the temperature of the whole magma chamber varied dramatically during its lifetime. Magmatic sulfide inclusions are present in all minerals and rocks of the upper crustal magma chamber, independent of their timing relative to porphyry Au mineralization (pre-, syn-, post-ore). These observations suggest that the magmatic system was sulfide saturated during its entire evolution. With very few exceptions, the precipitating sulfides were composed of monosulfide solid solution containing 0·2–9·2 wt % Cu and 0·05–11 ppm Au. The presence of these magmatic sulfides, together with results of a numerical model, suggest that the primitive magma feeding the upper crustal magma chamber contained less than 2·75 wt % H2O and that only a minor part of the magmatic sulfides was fractionated out of the system. Finally, the Cu/Au ratios measured in the magmatic sulfide inclusions and the ones predicted for the exsolved aqueous fluids are 10 to 100 times higher than the Cu/Au ratios of the porphyry deposits. Therefore, the extremely low Cu/Au ratios of the porphyry deposits must have been acquired during the hydrothermal stage. [ABSTRACT FROM AUTHOR]

We provide a two-stage approach to extract spectral induced polarization (SIP) information from time-domain IP data. In the first stage we invert dc data to recover the background conductivity. In the second, we solve a linear inverse problem and invert all time channels simultaneously to recover the IP parameters. The IP decay curves are represented by a stretched exponential (SE) rather than the traditional Cole–Cole model, and we find that defining the parameters in terms of their logarithmic values is advantageous. To demonstrate the capability of our simultaneous SIP inversion we use synthetic data simulating a porphyry mineral deposit. The challenge is to image a mineral body that is hosted within an alteration halo having the same chargeability but a different time constant. For a 2-D problem, we were able to distinguish the body using our simultaneous inversion but we were not successful in using a sequential (or conventional) SIP inversion approach. For the 3-D problem we recovered 3-D distributions of the SIP parameters and used those to construct a 3-D rock model having four rock units. Three chargeable units were distinguished. The compact mineralization zone, having a large time constant, was distinguished from the circular alteration halo that had a small time constant. Finally, to promote the use of the SIP technique, and to have further development of SIP inversion, all examples presented in this paper are available in our open source resources (https://github.com/simpeg-research/kang-2018-spectral-inducedpolarization). [ABSTRACT FROM AUTHOR]

An integrated study of the petrography, mineral composition, zircon geochronology, whole-rock geochemistry and Sr-Nd-Hf isotopes was carried out for an unusual suite of igneous rocks, including high-Mg andesites, garnet-bearing dacites and porphyries, rhyolites and leucogranites, from West Qinling, central China. These data, particularly observations from garnets, are used to demonstrate the petrogenetic links among the associated magmatic components which eventually formed the observed lithologies, evaluate the influence of recycling crystal populations and reconstruct the whole magmatic plumbing system. The crystallization ages of these igneous rocks are ~239-244 Ma. The high-Mg andesites are phenocryst-rich and characterized by high Mg# (>40; Mg#=100*mol. MgO/(MgO + FeO)), Cr and Ni abundances, low Sr/Y and (La/Yb)N ratios, and relatively high ISr and negative εNd(t) and εHf(t) values. The petrography, mineral chemistry and geochemical data indicate that the high-Mg andesites were generated by mixing between mantlederived magmas and crustal melts, with subsequent entrainment of xenocrysts (e.g. high Mg# pyroxenes, high An plagioclase and some glomerocrysts) from various sources within the crust. The chemical compositions of the garnet-bearing dacites and porphyries and crystal-poor rhyolites define a common differentiation trend. They become more strongly peraluminous and have more evolved Sr-Nd-Hf isotopic compositions with increasing SiO2 content. Petrological and geochemical data indicate that these peraluminous magmas were likely produced by fractional crystallization of andesitic magma, accompanied by assimilation of crustal materials and, or, entrainment of various phenocryst/xenocryst assemblages. Four types of garnets have been identified, including antecrysts, orthocrysts, peritectic phases and xenocrysts, and the variations in mineral composition and inclusion assemblage indicate a complicated history of magma mixing and mineral-melt interaction/re-equilibrium. The leucogranites are strongly depleted in HREE ((La/Yb)N > 300) and show remarkable negative Eu anomalies (Eu/Eu*=0.42-0.52). These geochemical features are indicative of the presence of both residual garnet and plagioclase in the magma source resulting from muscovite dehydration melting of metapelitic rocks. Together, all these observations consistently reflect magma evolution in several dispersed but interconnected magma reservoirs which formed a complicated trans-crustal magmatic plumbing system. Local magma compositions have been influenced by multiple processes, including crystallization and accumulation, recharging, anatexis, magma mixing and mingling, assimilation, remobilization of crystal mushes and random entrainment of various phenocryst assemblages and crustal xenoliths. Therefore, detailed petrographic information and mineral composition data are needed for interpreting the whole-rock geochemistry properly. The rapid ascent and eruption of crystal-rich and garnet-bearing magmas have been closely associated with an extensional regime in a post-collisional tectonic setting and facilitated by active fault systems. [ABSTRACT FROM AUTHOR]

High-Mg diorites and/or ultrapotassic volcanic rocks are generally associated with postcollisional porphyry copper deposits, but their contribution to the formation of the mineralization remains unclear. A suite of Miocene postcollisional ultrapotassic-potassic lamprophyres, high-Mg diorites, and adakite-like intrusions have been recognized in the Zhunuo porphyry Cu deposit, located in a continental collisional zone within the Gangdese belt, southern Tibet. The post-mineralization ultrapotassic-potassic lamprophyres have zircon U-Pb ages of 12.2±2.1 Ma and contain abundant Proterozoic to Miocene inherited zircons. The ultrapotassic lamprophyres have high K2O (>8.5wt%) and MgO (>8.8wt %) contents, are enriched in light rare earth elements (LREE; La=123 ppm) and large ion lithophile elements (LILE; e.g. Ba=3102 ppm, Th=116.6 ppm, and Pb=140 ppm), and display high Th/Yb and Rb/Sr, and low Ba/Rb and Hf/Sm ratios. They have zircon εHf(t) values of -2.8 to 1.3, δ18O values of 6.5 to 7.4%0', and enriched bulk-rock Sr-Nd-Pb isotope compositions ((87Sr/86Sr)i=0.73134, εNd(t)=-13.7, (206Pb/204Pb)i=19.20). Their parental magmas were derived from partial melting of an enriched mantle source that had been metasomatized by fluids and sediment-derived melts associated with Neo-Tethyan oceanic subduction and subsequent Indian continental lithosphere subduction. The potassic lamprophyres have lower contents of K2O, MgO, REE and LILE than the ultrapotassic lamprophyres and (87Sr/86Sr)i of 0.710993 to 0.711139, εNd(t) of -12.3 to -12.4, and (206Pb/204Pb)i of 18.59 to 18.72. Taken together with observations of a negative trend between eNd(t) and MgO content; positive trends between (87Sr/87Sr)i, (206Pb/204 Pb)i and MgO content from ultrapotassic lamprophyres to potassic lamprophyres; the existence of abundant Miocene inherited zircons showing similar ages and εHf(t) values to the adakite-like intrusions; and variable Hf/Sm ratios with some Hf/Sm ratios similar to adakite-like intrusions, we propose that the potassic lamprophyres were formed by mixing of ultrapotassic lamprophyre magmas with adakitelike magmas. The syn-mineralization high-Mg diorites including diorite porphyry and enclaves hosted by the adakite-like intrusions at Zhunuo have zircon U-Pb ages of 13.0±0.2 Ma and 13·1±0·2 Ma. They show negative correlations between Y, Yb, Dy/Yb and SiO2, and positive correlations between Sr, Sr/Y and SiO2, among which some more evolved samples (such as diorite porphyry) show adakite-like geochemical signatures. The high-Mg diorites are enriched in LREE and LILE, depleted in high-field-strength elements (HFSE), and have (87Sr/86Sr)i of 0·709401 to 0·710362, εNd(t) of -11·1 to -9·9, and (206Pb/204Pb)i of 18·62 to 18·71. Taken together with petrographic observations that show magma mixing, we argue that the high-Mg diorites were derived from previously subduction-modified Tibetan lithospheric mantle with little or no input from Indian continental sediment. Mixing with adakite-like magmas and fractional crystallization of hornblende and/or titanite are also responsible for the differentiation of the high-Mg diorites. The ore-hosting, adakitelike granitic rocks at Zhunuo with zircon U-Pb ages of 14·7±0·3 Ma and 14·6±0·2 Ma have lower concentrations of REE, LILE and HFSE, much higher εNd(t) (-6·1 to -6·9) and lower (87Sr/86Sr)i (0·707325-0·707663) values than the ultrapotassic lamprophyres and the high-Mg diorites. They were derived from remelting of previously subduction-modified Tibetan lower crust with some involvement of hydrous high-Mg dioritic magmas during magma mixing. The postcollisional adakite-like intrusions in the Gangdese belt could be generated by remelting of previously subduction-modified lower crust and mixing with hydrous high-Mg dioritic magmas in a lower crustal MASH zone and/or in an upper-crustal adakite-like magma chamber. The metallogenic potential of postcollisional adakite-like intrusions largely depends on rejuvenation of subductionmodified lower crust by previous arc magmas, differentiation of hydrous high-Mg dioritic magmas, and magma mixing of high-Mg dioritic magmas with lower crustal magmas. [ABSTRACT FROM AUTHOR]

The metal content, volatile content and crystallization conditions of ore-related rhyolite samples from the Climax-type porphyry Mo deposits at Urad-Henderson and Silver Creek (Rico) were reconstructed based on the composition of melt inclusions, mineral inclusions, and titanium-inquartz thermobarometry. Additional melt inclusion data were obtained from contemporaneous mafic dikes. Crystallized melt inclusions were either directly analyzed by laser ablation inductively coupled plasma mass spectrometry, or re-homogenized and then analyzed by electron microprobe. Inter-mineralization rhyolite melts at Urad-Henderson were highly evolved (four times more fractionated than average granite of the continental crust), contained 10-15ppm Mo, 6-7 wt % H2O and 0.5-0.7wt % F, and record crystallization conditions of 730-770°C, 1-3 kbar and an oxygen fugacity about two log units above the fayalite-magnetite-quartz buffer (FMQ+2). Melts from two presumably syn-mineralization rhyolite dikes at Silver Creek were slightly less evolved and contained 3-5ppm Mo, 7-8 wt % H2O and ~0.3wt % F, and record crystallization conditions of 780-800°C, 2-5 kbar. Both datasets are characterized by distinctly lower fluorine contents but higher temperatures and higher water contents than corresponding values reported for ore-related rhyolites at Climax. Calculated melt viscosities (log η=3.5-4.9 Pa s) are lower than those of average granitic melts at the same temperature, which may have facilitated crystal-melt segregation and thus accumulation of large volumes of fractionated, crystal-poor melts in shallow magma chambers, a process that seems to be critical for the formation of Climax-type porphyry Mo deposits. [ABSTRACT FROM AUTHOR]

Geochronological and geochemical studies, including platinum group element (PGE) analyses, were undertaken on samples from the El Abra-Pajonal igneous complex, northern Chile, to investigate the magmatic evolution of the suite. Special attention was paid to identifying the onset of sulfide saturation and to documenting how it influenced the geochemistry of the chalcophile elements and the formation of the El Abra porphyry Cu deposit. The PGE have extreme sulfide melt-silicate melt partition coefficients, making them sensitive indicators of the timing of sulfide saturation in an evolving magmatic system. In arc-related intermediate to felsic magmatic systems, which have the potential to produce porphyry deposits, the timing and extent of sulfide saturation relative to orefluid saturation may control the capacity of these systems to produce economic mineralization and, if they do, whether the deposits are Cu-only or Cu-Au. This study incorporates the first comprehensive analysis of PGE in a felsic magmatic suite associated with an economic porphyry system. The suite comprises a series of quartz monzodiorite to granite intrusions with U-Pb zircon ages between 43 and 35 Ma. Their petrography and major element chemistry, including increasing Sr/Y ratios and rare earth element patterns, suggest that crystal fractionation and crustal assimilation were the key magmatic processes governing the evolution of the El Abra-Pajonal suite. Plagioclase fractionation dominated the oldest intrusions, and their associated granites and aplites. Following the injection of a more primitive, wetter, mafic magma at 41-40 Ma, plagioclase fractionation became suppressed and amphibole became the dominant fractionating phase, leading to the formation of the El Abra porphyry intrusion and Cu deposit. Abundances of Pt and Pd in felsic rocks from the El Abra-Pajonal intrusive complex drop rapidly in samples with MgO values below 25wt %, following sulfide saturation of the magmas, which occurred slightly before ore-fluid saturation and formation of the Cu deposit. Modeling suggests that the amount of sulfide formed was very small, enough to strip the PGE and Au from the magma but not Cu, because of the lower partition coefficient of Cu relative to the precious metals, which explains why the mineralization at El Abra is a Cu-only porphyry deposit, rather than a Cu-Au deposit. [ABSTRACT FROM AUTHOR]

The petrological evolution of magmatic rocks associated with porphyry-related Cu deposits is thought to exert a first-order control on ore genesis. It is therefore critical to understand and recognize petrological processes favourable to the genesis of porphyry systems. In this study we present new petrographic, geochemical (whole-rock and mineral), and isotopic (Pb, Sr, Nd) data for rocks from the magmatic suite associated with the Eocene Coroccohuayco porphyry-skarn deposit, southern Peru. Previously determined radiometric ages on these rocks provide the temporal framework for interpretation of the data. Arc-style magmatic activity started at Coroccohuayco with the emplacement of a composite precursor gabbrodiorite complex at c. 40·4 Ma. After a nearly 5 Myr lull, magmatic activity resumed at c. 35·6 Ma with the rapid emplacement of three dacitic porphyries associated with mineralization. However, zircon antecrysts in the porphyries show that intra-crustal magmatic activity started c. 2 Myr before porphyry emplacement and probably built a large intra-crustal magmatic body with an associated large thermal anomaly. Our data suggest that all magmas underwent a period of evolution in the deep crust before transfer and further evolution in the upper crust. The gabbrodiorite complex was sourced from a heterogeneous deep crustal reservoir and was emplaced at a pressure of 100-250 MPa where it underwent a limited amount of fractionation and formed a chemically zoned pluton. Its initial water content and oxygen fugacity were estimated to be around 3 wt% H2O and NNO ± 1 (where NNO is the nickel-nickel oxide buffer), respectively. The deep crustal source of the porphyries appears to have been more homogeneous. The porphyries are interpreted to be the product of advanced differentiation of a parental magma similar to the gabbrodiorite. Most of this evolution occurred at deep crustal levels (around 800 MPa) through fractionation of amphibole + pyroxene + plagioclase ± garnet, leading to the development of a high Sr/Y signature characteristic of porphyry-related magmatism worldwide. Subsequent upper crustal evolution (100-250 MPa) was dominated by crustal assimilation, cannibalism of previously emplaced magma batches (proto-plutons) and magma recharge. Water content and oxygen fugacity were estimated to be around 5 wt % H2O and NNO + 1 to NNO + 2, respectively, at the end of the period of upper crustal evolution. This high oxygen fugacity is inferred to have favoured sulphur and metal enrichment in the melt. The high thermal regime generated through 2 Myr of sustained magmatism in the upper crust favoured crustal assimilation, proto-pluton cannibalism, and efficient metal extraction upon fluid exsolution. The Coroccohuayco magmatic suite appears to have acquired its metallogenic potential (high fO2, high Sr/Y) through several million years of deep crustal evolution. [ABSTRACT FROM AUTHOR]

The words "mantle source" have been changed to "magmatic source" in two places: Page 1108 (right column, end of first paragraph) and Page 1110 (left column, end of second paragraph). In the "Conclusions" section, number (4), the words "Hf isotopic composition is inferred to fingerprint the source and O the isotope composition of the melting process" have been changed to "Hf isotopic composition is inferred to fingerprint the source while O the melting process". [Extracted from the article]

The Hideaway Park tuff is the only preserved extrusive volcanic unit related to the Red Mountain intrusive complex, which produced the world-class Henderson porphyry Mo deposit. Located within the Colorado Mineral Belt, USA, Henderson is the second largest Climax-type Mo deposit in the world, and is therefore an excellent location to investigate magmatic processes leading to Climax-type Mo mineralization. We combine an extensive dataset of major element, volatile, and trace element abundances in quartz-hosted melt inclusions and pumice matrix glass with major element geochemistry from phenocrysts to reconstruct the pre-eruptive conditions and the source and evolution of metals within the magma. Melt inclusions are slightly peraluminous topaz rhyolitic in composition and are volatile-charged (≤6wt % H2O, ≤600ppm CO2, ~0·3-1·0wt % F, ~2300-3500ppm Cl) and metal-rich (~7-24ppm Mo, ~4-14ppm W, ~21-52ppm Pb, ~28-2700ppm Zn, <0·1-29ppm Cu, ~0·3-1·8ppm Bi, ~40-760ppb Ag, ~690-1400ppm Mn). Melt inclusion and pumice matrix glass chemistry reveal that the Hideaway Park magma evolved by large degrees of fractional crystallization (≤60-70%) during quartz crystallization and melt inclusion entrapment at pressures of ≤300MPa (≤8km depth), with little to no crystallization upon shallow ascent and eruption. Filter pressing, crystal settling, magma recharge and mixing of less evolved rhyolite melt, and volatile exsolution were important processes during magma evolution; the low estimated viscosities (~105-1010Pa s) of these H2O- and F-rich melts probably enhanced these processes. A noteworthy discrepancy between the metal contents in the pumice matrix glass and in the melt inclusions suggests that after quartz crystallization ceased upon shallow magma ascent and eruption, the Hideaway Park magma exsolved an aqueous fluid into which Mo, Bi, Ag, Zn, Mn, Cs, and Y strongly partitioned. Given that the Henderson deposit contains anomalous abundances of not only Mo, but also W, Pb, Zn, Cu, Bi, Ag, and Mn, we suggest that these metals were sourced from similar fluids exsolved from unerupted portions of the same magmatic system. Trace element ratios imply that Mo was sourced deep, from either the lower crust or metasomatized mantle. The origin of sulfur remains unresolved; however, given the extremely low S solubility of rhyolite melts in the shallow crust we favor the possibility that another source of S might supplement or account for that present in the ore deposit, probably the comagmatic, mantle-derived lamprophyres that occur in minor quantities with the voluminous topaz rhyolites in the area. To account for the 437Mt of MoS2 (~1·0×106t Mo) present in the Henderson ore deposit, a volume of ~45km³ of Hideaway Park rhyolite magma would have been necessary to supply the Mo (a cylindrical pluton measuring 3·1km×6·0km) along with sparging of ~6·8×105t of S from ~0·05km³ of lamprophyre magma. Based on a weighted mean 40Ar/39Ar age of 27·58±0·24Ma, similar melt geochemistry, and characteristically F-rich biotite phenocrysts, we conclude that the Hideaway Park tuff was cogenetic with the intrusions at Red Mountain that formed the Henderson deposit. [ABSTRACT FROM AUTHOR]

We have investigated a suite of Miocene high-Mg diorite porphyries from Qulong in southern Tibet, the largest porphyry Cu-Mo deposit in China. Laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating shows that the high-Mg diorite porphyry was emplaced at 15·7 ± 0·2 Ma, which is slightly younger than the Qulong adakite-like Rongmucuola pluton (c. 19·5-16·4 Ma). The Qulong high-Mg diorites have phenocrysts showing disequilibrium textures and include high-Mg# clinopyroxene (0·91-0·97). These porphyry rocks exhibit both ultrapotassic and adakite-like features, and are characterized by high contents of MgO (4·2-5·1 wt %), K2O (3·2-3·6 wt %), and compatible trace elements (e.g. Ni: 115-142 ppm; Cr: 214-291 ppm), as well as by high Sr/Y and La/Yb ratios. The rocks have highly radiogenic isotopic compositions with (87Sr/86Sr)i = 0·707004-0·707198, εNd(t) = -5·1 to -5·5, 207Pb/204Pb = 15·697-15·704, and 208Pb/204Pb = 39·082-39·116, as well as variable zircon εHf values of -3·0 to +5·9. Petrographic, elemental, and isotopic evidence suggests that the Qulong high-Mg diorites were formed by mixing between ultrapotassic and adakite-like melts, derived from metasomatized Tibetan lithospheric mantle and juvenile lower crust, respectively. In contrast, the Qulong pre-ore Rongmucuola pluton is characterized by high SiO2 (66·3-68·9 wt %) and Al2O3 (16·4-17·0 wt %) contents, high Sr/Y ratios (121-151), low compatible element contents (e.g. Ni = 16·0-17·4 ppm; Cr = 14·5-20·2 ppm), low Mg# values (0·44-0·52), positive large-ion lithophile element (LILE) anomalies, marked negative high field strength element (HFSE) anomalies, positive εNd(t) values (+0·4 to +2·5), and low (87Sr/86Sr)i values (0·704847-0·705237). These features indicate that the Rongmucuola pluton was formed by partial melting of subduction-modified juvenile lower crust within the garnet stability field. The newly identified Qulong high-Mg diorite allows us to propose a mixing model for the origin of the Gangdese high-K, adakite-like rocks. In this model, the formation of these rocks occurred in two stages: (1) partial melting of highly metasomatized lithospheric mantle that generated ultrapotassic mafic melts; (2) underplating of such melts beneath thickened juvenile lower crust, which resulted in melting of the lower crust and the generation of adakite-like magmas. Mixing of the adakite-like melt with ultrapotassic magmas elevated the K2O, MgO, and other LILE (e.g. Rb and Ba) contents of the adakite-like melt. Exogenous water necessary for formation of the Gangdese porphyry Cu systems was mainly added during mixing of ultrapotassic magma with adakite-like melt at lower- and/or upper-crustal depths, reflecting the large decrease in the H2O solubility of the ultrapotassic mafic melt upon ascent and decompression. Upper-crustal fractionation of the Rongmucuola magma could also possibly increase the water content of mineralization-related, adakite-like porphyry intrusions at Qulong. Fluid exsolution from the ultrapotassic magma is likely to have been a key process in the generation of the Gangdese porphyry Cu deposits, as well as other porphyry Cu deposits in the Tibetan collisional orogens. [ABSTRACT FROM AUTHOR]

Intrusive rocks related to porphyry copper mineralization are part of the wide diversity of subduction-related, mantle-derived, igneous rocks generated in convergent margin settings. What differentiates them from barren igneous rocks results ultimately from the multi-component and multi-stage processes that condition magma composition in these settings. Unfortunately, the petrogenetic history is largely obscured by the pervasive alteration that affects rocks in these deposits. We address this issue through the study of zircon grains from El Teniente, one of the largest known porphyry Cu–Mo deposits in the world. El Teniente belongs to the Miocene–Pliocene Cu–Mo belt of the Central Chilean Andes, which formed in a short timespan during the Cenozoic constructive period of the orogen. Previously U–Pb dated zircon grains were selected for re-examination of their morphological characteristics and in situ analysis of chemical (rare earth element, Hf, Y and Ti contents) and isotopic (Hf, O) composition. They are from six intermediate to felsic syn- to late-mineralization, intrusive units covering a timespan of ∼1·6 Myr. The El Teniente zircons have compositional and morphological characteristics indicating crystallization from a series of cogenetic melts. However, a minor hydrothermal imprint is documented in the presence of crystals with mottled surfaces that correspond to thin high U–Th overgrowth rims (low-luminescent features in cathodoluminescence images). In terms of any other chemical and isotopic characteristic, these are indistinguishable from the main mineral populations. Zircons define morphological and chemical trends reflecting an evolution towards more differentiated magma compositions, lower crystallization temperatures and increased cooling rates with decreasing age of intrusion. Hf and O isotopic compositions are remarkably uniform at grain, sample and deposit scale. This, together with the general absence of older inherited zircon components, the lack of correlations between isotopic signature and whole-rock composition and high initial &varepsilon;Hf values (total average 7·4 ± 1·2; 2σ), rules out involvement of any significant crustal contamination in the genesis of the El Teniente magmas. The Hf isotopic composition indicates a relatively juvenile source, but with some crustal residence time. The δ18OZrc weighted mean of 4·76 ± 0·12‰ (2σ; 61 analyses) is at the lower limit of the normal mantle zircon range of 5·3 ± 0·6‰ (2σ), and might reflect crystallization from low-18O magmas. Hf isotopic compositions have a restricted range in initial &varepsilon;Hf values between +6 and +10, identical to preceding Cenozoic barren magmatic activity in Central Chile. These igneous rocks are the product of nearly 25 Myr of subduction-related magmatic activity, developed under contrasting tectonic regimes and margin configurations. This suggests a primary control of the isotopic signature by a stable long-lived MASH-type (melting, assimilation, storage and homogenization) reservoir in the deep lithosphere. In the context of the Cenozoic evolution of Central Chile we argue that dehydration melting in the enriched MASH reservoir occurred as a consequence of increasing crustal thickness, and was prompted by a high-temperature thermal regime resulting from long-lasting preceding magmatism. This process can also fractionate O to generate low-18O magmas. At the time of El Teniente formation, dehydration melting occurred coevally with arc migration, which probably influenced the fertility of the magmas by increasing the melt component derived from this process relative to the component derived from primary basalt differentiation. At a regional scale, such reactions are expected to occur as a consequence of progressive crustal thickening ... [ABSTRACT FROM PUBLISHER]

El Teniente, the world’s largest Cu deposit, is hosted in Late Miocene and Pliocene plutons that intrude the older Teniente Volcanic Complex (or Farellones Fm; 14·2–6·5 Ma). The Late Miocene and Pliocene plutonic host rocks of the deposit include, sequentially, the relatively large (>50 km3) Teniente Mafic Complex laccolith (8·9 ± 1·4 Ma), the smaller (∼30 km3) Sewell equigranular tonalite complex (7·05 ± 0·14 Ma) and associated andesitic sills (8·2 ± 0·5 to 6·6 ± 0·4 Ma), small dacitic porphyry stocks (<1 km3; 6·09 ± 0·18 Ma), the unusual Cu- and S-rich ‘Porphyry A’ anhydrite-bearing granitoid stock (<1 km3; 5·67 ± 0·19 Ma), the Teniente Dacite Porphyry dike (<1 km3; 5·28 ± 0·10 Ma), minor latite dikes (4·82 ± 0·09 Ma), and finally a small dacite intrusion (4·58 ± 0·10 Ma). These plutonic rocks are all isotopically similar to each other (87Sr/86Sr = 0·7039–0·7042; ɛNd =+2·5 to +3·5) and also to the Teniente Volcanic Complex extrusive rocks, but distinct from both older Late Oligocene to Early Miocene volcanic rocks (87Sr/86Sr = 0·7033–0·7039; ɛNd = +3·8 to + 6·2) and younger Pliocene post-mineralization mafic dikes and lavas (87Sr/86Sr = 0·7041–0·7049; ɛNd = +1·1 to −1·1). Multiple Cu-mineralized magmatic–hydrothermal breccia pipes were emplaced into these plutonic rocks during the same time period as the felsic porphyry intrusions, between at least 6·31 ± 0·03 and 4·42 ± 0·02 Ma. These mineralized breccia pipes, which formed by exsolution of magmatic fluids from cooling plutons, have their roots below the deepest level of mining and exploration drilling and were derived from the same magma chamber as the felsic porphyries, >4 km below the paleosurface. To produce the ∼100 × 106 tonnes of Cu in the deposit requires a batholith-size (>600 km3) amount of magma with ∼100 ppm Cu. We suggest that both the mineralized magmatic–hydrothermal breccias and the progressively smaller volumes of more fractionated, but isotopically equivalent, Late Miocene and Pliocene felsic plutonic rocks that host the deposit were derived from the roof of a large, long-lived, thermally and chemically stratified, open-system magma chamber, or magmatic plumbing system, recharged from below by mantle-derived magmas. Only when this system fully solidified did post-mineralization mafic olivine-hornblende-lamprophyre dikes (3·85 ± 0·18 to 2·9 ± 0·6 Ma) pass through the system from the mantle to the surface. The significant progressive temporal isotopic evolution, to higher 87Sr/86Sr (from 0·7033 to 0·7049) and lower ɛNd (from +6·2 to −1·1), that occurred between the Late Oligocene and Pliocene in the vicinity of El Teniente for mafic mantle-derived magmas, and by implication their sub-arc mantle-source region, was due in part to increased mantle-source region contamination by subducted crust tectonically eroded off the continental margin. The post-mineralization olivine-hornblende-lamprophyres also imply extensive hydration of the mantle below this portion of the Andean arc by the Pliocene, which may have played a role in producing oxidized volatile-rich magmas and mineralization at El Teniente. [ABSTRACT FROM AUTHOR]

The 37 ± 1 Ma breccia-hosted porphyry Mo(–Nb) deposit at Cave Peak is genetically related to a mafic, alkaline intrusion nearby (the Marble Canyon Stock), thus providing an excellent opportunity to study the influence of mafic magmatism on porphyry Mo formation. Laser ablation inductively coupled plasma mass spectrometry analysis of crystallized melt inclusions allowed reconstruction of pristine concentrations of Mo and other fluid-mobile elements in melts ranging from phonotephrite (∼50 wt % SiO2) to rhyolite (∼75 wt % SiO2) in composition. Incompatible trace element abundances in these melt inclusions define smooth, linear trends reaching up to 100 times average upper continental crust, implying that the felsic magmas evolved from a mafic parent by fractional crystallization. The mafic parent has a geochemical signature typical of primitive melts generated in within-plate tectonic settings. Molybdenum increases from 4 ppm in the mafic endmember to 12 ppm in felsic melts immediately before Mo mineralization. After breccia formation, Mo concentrations in the residual melt decreased gradually to 5 ppm, suggesting that Mo was removed from the residual melts by exsolving fluids. This interpretation is supported by fluid–melt partition coefficients (DMo, fluid/melt = 17–20) measured on assemblages of coexisting fluid and melt inclusions. A high sulfur fugacity in the mafic input magma is indicated by early pyrrhotite saturation. To account for the 0·4–1·5 × 106 t Mo present in the deposit a volume of ∼30–100 km3 alkali feldspar granite magma is needed, or up to 10 times more of the mafic melt. Significantly less magma is required to provide all the sulfur present in the deposit. The results of this study provide direct evidence for derivation of metals (and sulfur) in porphyry Mo deposits from associated mafic, alkaline magmas through fractional crystallization. [ABSTRACT FROM PUBLISHER]

The magmatic processes leading to porphyry-Cu mineralization at Santa Rita are reconstructed on the basis of petrographic studies, thermobarometry, and laser-ablation inductively-coupled-plasma mass-spectrometry analyses of silicate melt and sulfide inclusions from dikes ranging from basaltic andesite to rhyodacite. Combined results suggest that magma evolution at Santa Rita is similar to that of sulfur-rich volcanoes situated above subduction zones, being characterized by repeated injection of hot, mafic magma into an anhydrite-bearing magma chamber of rhyodacitic composition. The most mafic end-member identified at Santa Rita is a shoshonitic basaltic andesite that crystallized at 1000–1050°C, 1–3 kbar and log fO2 = NNO + 0·7 to NNO + 1·0, whereas the rhyodacite crystallized at 730–760°C and log fO2 = NNO + 1·3 to NNO + 1·9. Mixing between the two magmas caused precipitation of 0·1–0·2 wt % magmatic sulfides and an associated decrease in the Cu content of the silicate melt from 300–500 ppm to less than 20 ppm. Quantitative modeling suggests that temporal storage of ore-metals in magmatic sulfides does not significantly enhance the amount of copper ultimately available to ore-forming hydrothermal fluids. Magmatic sulfides are therefore not vital to the formation of porphyry-Cu deposits, unless a mechanism is required that holds back ore-forming metals until late in the evolution of the volcanic–plutonic system. [ABSTRACT FROM AUTHOR]

Mafic to intermediate enclaves are evenly distributed throughout the dacitic 1991–1995 lava sequence of Unzen volcano, Japan, representing hundreds of mafic recharge events over the life of the volcano. This study documents the morphological, textural, chemical, and petrological characteristics of the enclaves and coexisting silicic host lavas. The eruptive products described in this study appear to be general products of magma mingling, as the same textural types are seen at many other volcanoes. Two types of magmatic enclaves, referred to as Porphyritic and Equigranular, are easily distinguished texturally. Porphyritic enclaves display a wide range in composition from basalt to andesite, are glass-rich, spherical and porphyritic, and contain large, resorbed, plagioclase phenocrysts in a matrix of acicular crystals and glass. Equigranular enclaves are andesitic, non-porphyritic, and consist of tabular, medium-grained microphenocrysts in a matrix glass that is in equilibrium with the host dacite magma. Porphyritic enclaves are produced when intruding basaltic magma engulfs melt and phenocrysts of resident silicic magma at their mutual interface. Equigranular enclaves are a product of a more prolonged mixing and gradual crystallization at a slower cooling rate within the interior of the mafic intrusion. [ABSTRACT FROM AUTHOR]

The Dexing adakitic porphyries (quartz diorite–granodiorite porphyries), associated with giant porphyry Cu deposits, are located in the interior of a continent (South China). They exhibit relatively high MgO, Cr, Ni and Sr contents, high La/Yb and Sr/Y ratios, but low Yb and Y contents, similar to adakites produced by slab melting associated with subduction. However, they are characterized by bulk Earth-like Nd–Sr isotope compositions (ɛNd(t) = −1·14 to +1·80 and (87Sr/86Sr)i = 0·7044 – 0·7047), and high Th (12·6–27·2 ppm) contents and Th/Ce (0·19–0·94) ratios, which are different from those of Cenozoic slab-derived adakites. Sensitive High-Resolution Ion Microprobe (SHRIMP) geochronology studies of zircons reveal that the Dexing adakitic porphyries have a crystallization age of 171 ± 3 Ma. This age is contemporaneous with Middle Jurassic extension within the Shi-Han rift zone, and within-plate magmatism elsewhere in South China, indicating that the Dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting. Their high Th contents and Th/Ce ratios, and Middle Jurassic age, argue against an origin from a Neoproterozoic (∼1000 Ma) stalled slab in the mantle. Taking into account available data for the regional metamorphic–magmatic rocks, and the present-day crustal thickness (∼31 km) in the area, we suggest that the Dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust, which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the Shi-Hang rift zone. Moreover, the Dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent, which elevated not only their MgO, Cr and Ni contents, but also the oxygen fugacity (fO2) of the mantle. The high fO2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements, which are required to produce associated Cu mineralization. Therefore, the Cu metallogenesis associated with the Dexing adakitic porphyries is probably related to partial melting of delaminated lower crust, similar to the metallogenesis accompanying slab melting. [ABSTRACT FROM AUTHOR]

Growth histories and residence times of crystals in magmatic systems can be revealed by studying crystal sizes, size distributions and shapes. In this contribution, serial sectioning has been employed on a sample of porphyritic rhyolite from a Permo-Carboniferous laccolith from the Halle Volcanic Complex, Germany, to reconstruct the distribution of felsic phenocrysts in three dimensions in order to determine their true shapes, sizes and three-dimensional size distributions. A model of all three phenocryst phases (quartz, plagioclase, K-feldspar) with 217 crystals, and a larger model containing 1599 K-feldspar crystals was reconstructed in three dimensions. The first model revealed a non-touching framework of crystals in three dimensions, suggesting that individual crystals grew freely in the melt prior to quenching of the texture. However, crystal shapes are complex and show large variation on a Zingg diagram (intermediate over long axis plotted against short over intermediate axis). They often do not resemble the crystallographic shapes expected for phenocrysts growing unhindered from a melt, indicating complex growth histories. In contrast, the three-dimensional size distribution is a simple straight line with a negative slope. Stereologically corrected size distributions from individual sections compare well with stereologically corrected size distributions obtained previously from the same sample. However, crystal size distribution (CSD) data from individual sections scatter considerably. It is shown that CSDs can be robustly reproduced with a sampling size of greater than ~200 crystals. The kind of shape assumed in stereological correction of CSDs, however, has a large influence on the calculation and estimation of crystal residence times. [ABSTRACT FROM AUTHOR]

High-temperaturepressure experiments were carried out to determine the chlorinehydroxyl exchange partition coefficient between hornblende and melt in the 1992 Unzen dacite. Cl in hornblende and melt was analyzed by electron microprobe, whereas OH in hornblende and melt was calculated assuming anion stoichiometry of hornblende and utilizing the dissociation reaction constant for H2O + O = 2(OH) in water-saturated melt, respectively. The partition coefficient strongly depends on the Mg/(Mg + Fe) ratio of hornblende, and is expressed as ln K1 = (Cl/OH)hb/(Cl/OH)melt = 237 - 46[Mg/(Mg + Fe)]hb at 23?kbar and 800850C. The twofold variation in Cl content in the oscillatory zoned cores of hornblende phenocrysts in the 19911995 dacite cannot be explained by the dependence of the Cl/OH partition coefficient on the Mg/(Mg + Fe)hb ratio, and requires c. 80% variation of the Cl/OH ratio of the coexisting melt. Available experimental data at 200?MPa on Cl/OH fractionation between fluid and melt suggest that c. 1218?wt % degassing of water from the magma can explain the required 80% variation in the Cl/OH ratio of the melt. The negative correlation between Al content and Mg/(Mg + Fe) ratio in the oscillatory zoned cores of the hornblende phenocrysts is consistent with repeated influx and convective degassing of the fluid phase in the magma chamber. [ABSTRACT FROM AUTHOR]

Trachybasalt scoria from a cinder cone near the Mexican volcanic front contain phenocrysts of olivine with chromite inclusions, apatite, augite and hornblende, with microphenocrysts of plagioclase. The water-saturated phase relations reproduce the phenocryst assemblage between 1040C and 970C with water contents of between 25 and 45% (50150?MPa). The absence of biotite phenocrysts in the scoria places a tight constraint on the pressuretemperature conditions of phenocryst equilibration, as there is only a small zone where biotite does not accompany hornblende in the experiments. Diluting the fluid phase with CO2 changes the composition of the olivine, indicating that CO2 was only a minor component of the fluid of the scoria. Hornblende is stable to 1040C at oxygen fugacities of NNO + 2 (where NNO is the nickelnickel oxide buffer), but at lower oxygen fugacities, the upper limit is 990C. There is a progressive increase in crystallinity in experimental runs as both pressure and temperature decrease. Isobaric plots of crystallinity show that the onset of hornblende crystallization involves a reaction relation, and also results in a marked ~1540 vol. % increase in crystallinity. Ascending hydrous magmas intersecting the cooler crust could be trapped there by the large increase in crystallinity accompanying the isobaric crystallization of hornblende. [ABSTRACT FROM AUTHOR]

K-silicate alteration in the Far Southeast (FSE) porphyry Cu–Au deposit formed at ∼1.4 Ma, concentric to dikes of quartz diorite porphyry. At ∼2 km paleodepth the hydrothermal system consisted of magmatic hypersaline liquid and vapor at 550°C and ≤50 MPa (lithostatic pressure). Advanced argillic alteration formed at the same time over the deposit at ≤1 km paleodepth from acidic condensates of the vapor. At ≥ 1.3 Ma, K-silicate alteration was overprinted by 350°C magmatic liquid (∼5 wt % NaCl equiv.) at hydrostatic pressure. Sericite alteration and much of the magmatic fluid composition was simulated with a magma-chamber crystallization model. Homogeneous crystallization during early stage convection is assumed, whereas at 50 vol. % crystals the chamber becomes stagnant and crystallizes from rim to core over a narrow crystallization interval. The model calculation, based on a magma chamber with 2 km thickness at 6 km depth (150 MPa) and ≥800°C (saturated melt composition at 30 vol. % crystals, 5 wt % H2O, 0.2 wt % Cl and δD of −40‰), can reproduce the chemical and isotopic compositions of the early and late magmatic fluids. The most critical factor controlling the compositional evolution of the model hydrothermal system is the transition from convective to stagnant magma-chamber crystallization. There is also a sharp decrease in the rate of fluid exsolution associated with this transition, which can account for the thermal collapse of the FSE porphyry system, from K-silicate to sericite alteration. [ABSTRACT FROM AUTHOR]