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This paper provides an overview on the genesis of Mesoproterozoic igneous rocks and associated iron oxide ± apatite (IOA) ± rare earth element, iron oxide-copper-gold (IOCG), and iron-rich sedimentary deposits in the St. Francois Mountains terrane of southeast Missouri, USA. The St. Francois Mountains terrane lies along the southeastern margin of Laurentia as part of the eastern granite-rhyolite province. The province formed during two major pulses of igneous activity: (1) an older early Mesoproterozoic (ca. 1.50–1.44 Ga) episode of volcanism and granite plutonism, and (2) a younger middle Mesoproterozoic (ca. 1.33–1.30 Ga) episode of bimodal gabbro and granite plutonism. The volcanic rocks are predominantly high-silica rhyolite pyroclastic flows, volcanogenic breccias, and associated volcanogenic sediments with lesser amounts of basaltic to andesitic volcanic and associated subvolcanic intrusive rocks. The iron oxide deposits are all hosted in the early Mesoproterozoic volcanic and volcaniclastic sequences. Previous studies have characterized the St. Francois Mountains terrane as a classic, A-type within-plate granitic terrane. However, our new whole-rock geochemical data indicate that the felsic volcanic rocks are effusive derivatives from multicomponent source types, having compositional similarities to A-type within-plate granites as well as to S- and I-type granites generated in an arc setting. In addition, the volcanic-hosted IOA and IOCG deposits occur within bimodal volcanic sequences, some of which have volcanic arc geochemical affinities, suggesting an extensional tectonic setting during volcanism prior to emplacement of the ore-forming systems. The Missouri iron orebodies are magmatic-related hydrothermal deposits that, when considered in aggregate, display a vertical zonation from high-temperature, magmatic ± hydrothermal IOA deposits emplaced at moderate depths (~1–2 km), to magnetite-dominant IOA veins and IOCG deposits emplaced at shallow subvolcanic depths. The shallowest parts of these systems include near-surface, iron oxide-only replacement deposits, surficial epithermal sediment-hosted replacement deposits, synsedimentary ironstone deposits, and Mn-rich exhalite deposits. Alteration associated with the IOA and IOCG mineralizing systems of the host volcanic rocks dominantly produced potassic with lesser amounts of calcic- and sodic-rich mineral assemblages. No deposits are known to be hosted in granite, implying that the mineralizing systems were operative during a relatively short, postvolcanic period yet prior to intrusion of the granitoids. Companion studies in this special issue on mineral chemistry, stable isotopes, and iron isotopes suggest that the magnetite within the IOA deposits formed from high-temperature fluids of magmatic or magmatic-hydrothermal origin. However, the data do not discriminate between a magmatic-hydrothermal source fluid exsolved from an Fe-rich immiscible liquid or an Fe-rich silicate magma. Mineral chemical, fluid inclusion, and stable isotope data from these new studies record the effects of metasomatic fluids that interacted with crustal reservoirs such as volcanic rocks or seawater.

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 volcaniclastic strata and associated intrusive rocks overlie metamorphosed basement rocks of the Camlica 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 volcaniclastic rocks. Moderate- and high-angle normal and oblique faults are common in the area and may have influenced the distribution of mineralization. The Kucukdag 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 × 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 volcaniclastic 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 covellite. 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.

The type examples for the class of deposits termed intrusion-related gold systems occur in the Tombstone-Tungsten belt of Alaska and Yukon, on the eastern side of the Tintina gold province. In this part of the northern Cordillera, extensive mid-Cretaceous postcollisional plutonism took place following the accretion of exotic terranes to the continental margin. The most cratonward of the resulting plutonic belts comprises small isolated intrusive centers, with compositionally diverse, dominantly potassic rocks, as exemplified at Scheelite Dome, located in central Yukon. Similar to other spatially and temporally related intrusive centers, the Scheelite Dome intrusions are genetically associated with intrusion-related gold deposits. Intrusions have exceptional variability, ranging from volumetrically dominant clinopyroxene-bearing monzogranites, to calc-alkaline minettes and spessartites, with an intervening range of intermediate to felsic stocks and dikes, including leucominettes, quartz monzonites, quartz monzodiorites, and granodiorites. All rock types are potassic, are strongly enriched in LILEs and LREEs, and feature high LILE/HFSE ratios. Clinopyroxene is common to all rock types and ranges from salite in felsic rocks to high Mg augite and Cr-rich diopside in lamprophyres. Less common, calcic amphibole ranges from actinolitic hornblende to pargasite. The rocks have strongly radiogenic Sr (initial 87 Sr/ 86 Sr from 0.711-0.714) and Pb isotope ratios ( 206 Pb/ 204 Pb from 19.2-19.7), and negative initial ɛ Nd values (−8.06 to −11.26). Whole-rock major and trace element, radiogenic isotope, and mineralogical data suggest that the felsic to intermediate rocks were derived from mafic potassic magmas sourced from the lithospheric mantle via fractional crystallization and minor assimilation of metasedimentary crust. Mainly unmodified minettes and spessartites represent the most primitive and final phases emplaced. Metasomatic enrichments in the underlying lithospheric mantle are attributes of the ancient North American cratonic margin that appear to be essential prerequisites to this style of postcollisional magmatism and associated gold-rich fluid exsolution. This type of magmatic hydrothermal activity occurs in a very specific tectonic setting that typically sets intrusion-related gold deposits apart from orogenic gold deposits, which are synorogenic in timing and have no consistent direct relationship to such diverse and contemporaneous lithospheric mantle-derived magmas, although they too are commonly sited adjacent to lithospheric boundaries.

Although porphyry copper-gold mineralization in the region has been known for approximately 40 years, the Chagai belt has only recently revealed its true potential with the discovery of a world-class deposit at H14-H15 at Reko Diq. The ~300-km-long, east-west–trending Chagai belt comprises several superimposed magmatic arcs and corresponding porphyry copper- (gold, molybdenum) mineralization generated in consecutive events at 43 to 37 Ma (middle-late Eocene), 24 to 22 Ma (early Miocene), 18 to 16 Ma (early Miocene), 13 to 10 Ma (middle Miocene), and 6 to ~4 Ma (late Miocene-early Pliocene) The tectonomagmatic evolution of the region was marked by major, continental-scale events associated with (1) the northward drift of India and its interaction and final collision with the southern margin of Eurasia, and (2) the collision of Arabia with central Iran resulting in final closure of the Neo-Tethyan Ocean. Initial, precollisional stages of arc evolution between the Middle Jurassic and Paleocene formed kilometer-thick submarine volcanic sequences with important flyschlike packages and intercalations of biohermal limestone. The subalkaline basalt and basaltic andesite of this stage possess a tholeiitic lineage of oceanic island-arc affiliation. Small-scale, submarine volcanogenic copper mineralization of manto- and Kuroko-type formed during the late, mature stages of this arc just prior to collision. Early contact of the leading edge of India and the island arcs in the late Paleocene (~55 Ma) resulted in a reorganization of the arc in the Chagai belt, with the consequent extrusion of subaerial andesite followed by emplacement of magnetite-series, calc-alkaline composition batholiths during the late Eocene. This plutonism accompanied the first manifestations of porphyry-type alteration and mineralization at ~43 Ma, in a rapidly emerging arc of Andean type. The belt is defined by 48 porphyry systems with copper-(gold, molybdenum) mineralization associated with calc-alkaline, biotite- and amphibole-bearing porphyry stocks of predominantly quartz diorite to granodiorite composition. Hydrothermal alteration includes potassic, propylitic, sericite-clay-chlorite, sericitic, and advanced argillic assemblages, the last in transitional epithermal environments. Calcic-potassic assemblages are developed locally at the expense of more mafic, commonly dioritic, rocks. Widespread porphyry copper and copper-gold mineralization during the Miocene, at 24 to 22 and 18 to 16 Ma, formed in a subaerial subvolcanic environment during moderate tectonic uplift, most notably during the early Miocene at Saindak and Tanjeel. Faster rates of regional uplift and exhumation, in conjunction with a period of conspicuous volcanic quiescence, characterized large-scale porphyry copper-gold mineralization at H14-H15 at Reko Diq during the middle Miocene between 13 and 10 Ma. Regional, tectonically triggered uplift and consequent exhumation may have been the result of collision of Arabia with central Iran. In general, mature, cumulative supergene enrichment and oxidation zones seem to be absent in the porphyry copper deposits of the Chagai belt, primarily due to the high neutralization potentials and low pyrite contents of the ore-related potassic alteration. Nevertheless, a supergene chalcocite blanket formed at Tanjeel during the Pliocene, at ~4 Ma, in sericitic alteration that accompanied the hypogene mineralization. The tectonomagmatic characteristics of the Chagai porphyry copper belt, including the multimillion-year history of subduction along an Andean-type margin, together with conspicuous short-lived contraction events and concomitant volcanic quiescence during arc construction and evolution are similar to those in contractional belts at convergent margins containing large-scale porphyry copper deposits elsewhere. Superposition of the Chagai magmatic arcs and corresponding porphyry copper events occurred during the last 55 m.y., a situation that implies that the focus of magma generation above the subducting slab remained essentially stationary. Possible mechanisms responsible for this include seaward migration of the trench axis during much of the Tertiary, especially since the Oligocene, and consequent flattening of the subducted slab.

The giant porphyry copper-molybdenum deposits of central Chile formed within a thick sequence of Cretaceous to Pliocene volcanic rocks. The predominantly calc-alkaline basaltic andesites of the Cretaceous Las Chilcas Formation are characterized by La/Smn ratios of 1.8 to 2.5 and Sm/Ybn of 1.8 to 2.8. The upper Oligocene to lower Miocene Abanico Formation (variously defined as the Los Pelambres, Abanico, or Coya Machali Formations) ranges from basalt to rhyolite in composition and exhibits a broad southward transition from calc-alkaline to tholeiitic. All samples from this formation are characterized by LREE enrichment and moderately, or locally strongly, fractionated HREE (La/Smn = 1.3-4.1; Sm/Ybn = 1.5-5.8). The basaltic andesites and andesites of the middle Miocene Salamanca Formation have REE chemistry similar to that of the Upper Cretaceous strata (La/Smn = 1.5-2.7; Sm/Ybn = 1.6-2.7). The overlying middle Miocene Farellones Formation ranges from tholeiitic to calc-alkaline and from basalt to andesitic and has a similar LREE enrichment and fractionated HREE (La/Smn = 1.7-2.5; Sm/Ybn = 1.7-3.3). The Pliocene La Copa Rhyolite Complex, however, is strongly LREE enriched and HREE depleted (La/Smn = 3.8-3.9; Sm/Ybn = 4.2-4.7). Overall, the trace element geochemistry of the Cretaceous to middle Miocene volcanic rocks is characterized by enriched LREE and negative Nb anomalies, consistent with an arc setting, with only minor differences in the abundance of most elements. Crustal thickening during the Miocene in central Chile has been suggested to have been responsible for a transition from an amphibole- to garnet-dominated residual mineralogy resulting in the release of fluids that enabled the formation of giant copper porphyry deposits. However, the gradual increase in La/Yb through the early, middle, and late Miocene reported in earlier studies and interpreted to be a response to crustal thickening is not observed in the regional data. Instead, a rapid change in the geochemical signature between the end of the eruption of the Farellones Formation and the eruption of the high La/Yb La Copa Rhyolite Complex implies a more abrupt change in the tectonic environment. Isotopic data broadly support this, although lower epsilon Nd values in the Farellones Formation imply a greater role for crustal contamination in younger suites. In the absence of gradual crustal thickening, it is suggested that the subduction of the Juan Fernandez Ridge may have been the key geodynamic process responsible for the genesis of the three middle Miocene to lower Pliocene giant porphyry copper deposits in central Chile, possibly by promoting crustal-scale faulting and even acting as a source of metals.

The Ad Duwayhi gold deposit, located in the east-central part of the Arabian shield, is the newest gold discovery in Saudi Arabia. Exploration to date has identified a gold resource of greater than 1 million ounces (oz) with significant potential for expansion. Gold mineralization was closely associated, in time and space, with emplacement of a late- to postorogenic, intracrustal, northwest-oriented granite body (659 ± 7 Ma) and comagmatic square quartz porphyry (646 ± 11 Ma), a hypabyssal and perhaps younger phase of the granite. Mineralization was largely confined to northeast-striking, southeast-dipping fault zones. Hydrothermal alteration consisted of early biotitic alteration overprinted by sericitic alteration. Sericitic alteration was coincident with gold mineralization and produced a quartz-sericite-carbonate-pyrite-rutile mineral assemblage, found both as vein fill and wall-rock alteration products. Mineralization styles, in the following general paragenetic sequence, include (1) quartz-molybdenite veins in and near the granite stock, (2) low-grade gold-bearing quartz vein breccia in and along the margins of the granite stock, (3) gold-bearing stockwork and sheeted quartz veins, and (4) massive to banded gold-rich tabular quartz veins. The gold-bearing stockwork, sheeted, and tabular veins are generally spatially associated with square quartz porphyry dikes and more distal to the granite stock. Mineralized zones at Ad Duwayhi are characterized by low sulfide and base metal content and gold/silver ratios of approximately 6/1. Gold shows no significant correlation with other metals, except lead, and moderate correlation with silver. Re-Os dating of molybdenite from a quartz-molybdenite vein and a tabular quartz vein with cogenetic gold produced robust ages of 655.6 ± 2.7 and 649.9 ± 2.3 Ma, respectively, documenting that gold mineralization and crystallization of granite and square quartz porphyry were, within uncertainty, coeval events. This age correlation combined with granite textural features, the presence of unidirectional solidification textures in granite and square quartz porphyry, and the nature and time-space distribution of mineralization styles, all indicate that mineralization evolved in and near the interface between a crystallizing magma and the surrounding rocks and, thus, is consistent with an intrusion-related genesis. In light of our findings at Ad Duwayhi, a reassessment of similar intrusion-hosted deposits in the Arabian shield is warranted, and areas of late- to postorogenic plutonism, particularly in the Afif composite terrane, should be considered prospective for intrusion-related gold systems.

The Hemlo deposit is one of Canada’s largest gold camps. Gold is both structurally and lithologically controlled, but shows unusual relationships relative to many other Archean lode deposits. Field evidence suggests that gold was emplaced within mechanical and chemical traps along a jog in a major sinistral shear zone, most likely during early formation of second-generation (G2) structures, which represent the most intense phase of regional deformation. Mineralization was followed by amphibolite-facies regional metamorphism. U-Pb geochronology has been carried out on zircons from rocks associated with the deposit, in order to determine the precise chronology of geologic events and their relationship to mineralization. Zircon populations are commonly affected by both inheritance and hydrothermal/metamorphic crystal growth. Meaningful and precise age information can only be determined by precise single zircon dating on rocks from a well constrained, structural, and stratigraphic context. Gold is mostly found near the deformed contact between the Moose Lake quartz porphyry volcanic complex and stratigraphically underlying metasedimentary rocks. These rocks have been folded by a camp-scale, noncylindrical F2 fold. Zircon from the Moose Lake quartz porphyry is dominated by inheritance and gives ages that extend from ~2800 to ~2690 Ma. Metasedimentary rocks that underlie and overlie the Moose Lake porphyry show a much narrower range of provenance ages, and are dominated by 2,690- to 2,693-million-year-old zircon. Two detrital zircons from a metasedimentary rock directly beneath the porphyry define an age of 2685 ± 4 Ma. This should be an older age limit on volcanism, deformation, and gold mineralization. The four youngest near-concordant zircons from the Cedar Lake pluton define a magmatic age of 2680 ± 1 Ma. This pluton was emplaced prior to or early during G2 deformation, while emplacement of gold was most likely controlled by G2 structures. Therefore, 2680 ± 1 Ma probably also represents an older age limit on gold mineralization. A feldspar porphyry dike that cuts gold ore gives a younger age bracket of 2677 ± 1 Ma. The results of zircon dating suggest that a volcanosedimentary complex was constructed in the area over the period of 2693 to 2685 Ma. This developed on earlier crust, ca. 2720 Ma in age, but there is evidence from detritus and xenocrysts for older rocks dating back to over 2800 Ma. Deposition of supracrustal rocks was followed by granodiorite plutonism at 2680 ± 1 Ma, then major deformation (G2) and gold mineralization, overprinted by amphibolite-facies metamorphism (previously dated at 2676 Ma from titanite ages). Late-tectonic "sanukitoid suite" plutons were emplaced at 2677 ± 1 Ma. Thus, the primary gold-forming event occurred near the beginning of a period of plutonism and crustal reworking that lasted, at most, a few million years. This time association supports the view that granitoid magmas were the source of auriferous fluids. Plutons intruded into an actively deforming crust and were probably also the heat source for regional metamorphism. The most likely environment was a tectonically active, Timiskaming-type sedimentary basin. Similarly aged basins are found in the Timmins area to the east and the Shebandowan greenstone belt to the west.

Results from a multidisciplinary geoscience program since 1994 are summarized for the North Pilbara terrain of the Pilbara Craton. Major findings include the recognition of three separate terranes with unique stratigraphy, geochronological, and structural histories; the ca. 3.72 to 2.85 Ga East Pilbara granite-greenstone terrane, the ca. 3.27 to 2.92 Ga West Pilbara granite-greenstone terrane, and the

Ore-bearing quartz-carbonate veins of the Coeur d’Alene mining district yield 87Sr/86Sr ratios of 0.74 to >1.60 for low Rb/Sr, carbonate gangue minerals, similar to current ranges measured in Middle Proterozoic, high Rb/Sr rocks of the Belt Supergroup. Stable isotope and fluid inclusion studies establish a genetic relationship between vein formation and metamorphic-hydrothermal systems of the region. These extraordinary 87Sr/86Sr ratios require accumulation of radiogenic 87Sr in a high Rb/Sr system over an extended period prior to incorporation of Sr into the hydrothermal veins. Evaluation of the age and composition of potential sources of highly radiogenic Sr indicates that the ore-bearing veins of the Coeur d’Alene district formed during the Cretaceous from components scavenged from rocks of the Belt Supergroup, the primary host rocks of the district. Proterozoic Pb isotope ratios observed in galena from many Coeur d’Alene veins were established when Pb separated from uranium during deposition or diagenesis of the Belt Supergroup at 1400 to 1500 Ma, possibly as disseminated syngenetic deposits. K-Ar and Rb-Sr apparent ages and δ 18O values of Belt Supergroup rocks decrease from the Coeur d’Alene district toward the Idaho and Kaniksu batholiths, approximately normal to the trends of metamorphic isograds, fold axes, foliation, and the major reverse faults of the district. Isoclinal folding, thrust faulting, high-temperature metamorphism, granitic plutonism, and regional-scale metamorphic-hydrothermal activity is documented in the region between 140 and 45 Ma, representing the only such combination of events in the Coeur d’Alene region subsequent to about 1300 Ma. The Sr and oxygen results and geologic evidence favor formation of the ore-bearing carbonate veins by fluids related to a complex metamorphic-hydrothermal system during the Cretaceous. Pb with Proterozoic isotopic compositions was probably mobilized and incorporated like other metals into the hydrothermal veins during this event. The ore-bearing veins were sheared and displaced during early Tertiary northwest-trending dextral strike-slip faulting along the Osburn fault and related structures of the Lewis and Clark line.

A program of geologic mapping and lithogeochemical and geochronological sampling has been carried out over a 745-km2 area of the Atacama Desert surrounding the porphyry Cu deposits at Escondida, Zaldivar, and Chimborazo (Cordillera de Domeyko, northern Chile). The purpose of this study was to examine the regional tectonic and magmatic setting of this preeminent porphyry Cu district for evidence of features or processes that might explain the giant scale of mineralization at Escondida and provide predictive tools for exploration in other areas. The geologic history of this area as recorded by exposed rocks begins with voluminous, intermediate to felsic Permo-Carboniferous volcanism (La Tabla Formation), and these rocks appear to constitute the crystalline basement throughout much of the porphyry belt of northern Chile. Geochemically, they are I-type in character, but the parental magmas were relatively dry, and thus did not generate effective magmatic-hydrothermal systems (few significant ore deposits are known to be associated with them). Andean cycle arc magmatism began in the Triassic, centered on the La Negra magmatic arc (now located near the Chilean coast). Farther inland, near Escondida, back-arc processes led to the eruption of intermediate to felsic lavas and tuffs and the deposition of marine sediments in rift basins. Closure of these basins in the Late Cretaceous resulted in deformation of the volcano-sedimentary sequences and was followed by emplacement of small alkali gabbro stocks and dikes. The axis of arc magmatism moved eastward in the Paleocene (Central Valley arc) and produced widespread calc-alkaline intermediate to felsic volcanism through to the Eocene. East- to northeast-directed convergence maintained a dextral transpressive regime during this period, and early movements in the West Fissure zone, a corridor of orogen-parallel faults that runs the length of the Cordillera de Domeyko (over 1,000 km), reflect this couple. At the end of the Eocene, however, stresses in the arc appear to have relaxed, and by the late Oligocene, strike-slip movement along the West Fissure zone had reversed to sinistral. This period of stress relaxation at the end of the Eocene period coincided with the voluminous emplacement of dioritic magmas at shallow crustal levels and also with porphyry development. Six samples of hornblende from these diorites yield 40Ar/39Ar dates between 38.28 ± 0.32 and 36.94 ± 0.46 Ma (2 σ ). Porphyry emplacement at Escondida, Zaldivar, and Chimborazo was coeval with this dioritic magmatism at ~38 Ma. Where plutonism was intense, the dioritic magma is interpreted to have evolved by processes of assimilation and fractional crystallization to more felsic compositions characteristic of the ore-forming porphyry intrusions. Whole-rock trace element data indicate that hornblende fractionation was an important control on chemical evolution of the diorites and attests to high-magmatic water contents (>4 wt % H2O). Volatile saturation would have occurred during further differentiation of these magmas, evidence for which is provided by the porphyry ore deposits. Porphyry emplacement was localized within a broad zone of intersection between the West Fissure zone and a regionally extensive northwest-trending structural corridor (the Archibarca lineament). It is proposed that the geometry of this junction was conducive to the formation of transtensional pull-apart structures during relaxation or reversal of dextral shear on the West Fissure zone. Such dilational structures would have focused the ascent and pooling of magma in the upper crust and maximized the potential for formation of magmatic-hydrothermal ore deposits. The formation of giant porphyry systems such as Escondida is, therefore, considered to be the result of a fortuitous coincidence of processes, including generation of suitable volumes and compositions of magma, appropriate lithospheric stress conditions, and structural focusing of emplacement; in addition, the development of thick supergene enrichment blankets has been critical to the economic value of these deposits. None of these contributory processes are in themselves unusual or rare, but because they are largely independent of one another, their constructive cooperation in ore formation is not necessarily repeatable at different places and at different times, thus explaining the relative rarity of giant porphyry deposits.

The Endako batholith is a composite batholith that consists of early foliated hornblende ± biotite diorites, intermediate-age unfoliated hornblende ± biotite diorites, and late granodiorites to monzogranites. These latter plutons host the Endako molybdenite deposit, a major porphyry deposit located in the central Canadian Cordillera. Extensive K-Ar dating on the batholith had given relatively reproducible ages centered on 140 Ma, indicating a short life span for magmatic activity. However, a combined U-Pb and 40Ar/39Ar study has resulted in a substantial reappraisal of the age and time span of intrusive activity and its association with molybdenite mineralization. The new data show that the batholith had a protracted emplacement history, covering ~75 m.y., with clear evidence for periods of magmatic quiescence. The oldest magmatic suite of the Endako batholith, the Stern Creek suite, is dated at ~220 Ma and comprises foliated gabbros and diorites. Mafic to intermediate plutons of the Stag Lake suite range in age from 180 Ma to 161 Ma. The Francois Lake suite is divided into two subsuites: the Glenannan subsuite dated at 157 Ma to ~155 Ma and the 149 to 145 Ma Endako subsuite that hosts the Endako molybdenite deposit. 40Ar/39Ar dating of hydrothermal biotite indicates molybdenite mineralization is genetically associated with terminal stages of magmatic activity. Biotite within the mine had its 40Ar/39Ar systematics reset at ~145 Ma by hydrothermal fluids with temperatures in excess of 300°C. This resetting is associated with intrusion of the 145.1 ± 0.2 Ma Casey phase monzogranite, the youngest phase within the Endako subsuite. A further partial resetting of biotite is interpreted to be coeval with Eocene intrusion of the high-level, miarolitic Sam Ross Creek phase and numerous mafic dikes that cut through the region. Hydrothermal fluids associated with this latter event were below biotite closure temperature and do not appear to have any relationship to molybdenite mineralization within the Endako deposit. The new geochronologic data, coupled with detailed mapping, show a general age progression from mafic plutonism at the margins to a central core of high-level granodiorite to monzogranite. The data document the existence of a long-lived, relatively stationary magmatic center. Pulsed magmatism, evident even within individual magmatic suites, may play a major role in concentrating and ultimately depositing molybdenite in some porphyry systems. At Endako, molybdenite would have formed within the higher level portions of the batholith, a position that would normally have rendered the deposit prone to erosional removal. However, subsequent Eocene extensional block faulting may explain the preservation of this Jurassic deposit, which is the oldest economic molybdenite deposit in the Cordillera of North America.

The high gold fineness, geographic distribution, and general lack of bedrock lode sources make the Ninemile district of west-central Montana one of the most interesting gold placer localities in Montana. Analysis of the distribution of the gold-bearing gravels and the petrography and geochemistry of the gold grains leads to the conclusions that the gold is detrital, of very high original purity, and derived from lode deposits hosted by Proterozoic-age Belt Supergroup rocks located within the Ninemile Valley. The limited known lode occurrences, type, and metamorphic grade of bedrock in the Ninemile Valley, and gold grain characteristics are compatible with slate hosted-type, gold-bearing quartz veins.The lode deposits, based upon relationships noted in the Ninemile Valley and elsewhere in the region, were probably emplaced during Laramide-age felsic plutonism. The gold, however, may ultimately have been derived from the Prichard Formation, the basal unit of the Belt Supergroup, which underlies the entire area and hosts gold-bearing quartz veins elsewhere in the Belt basin. The stratigraphic interval encompassing the middle Belt carbonate through the lower part of the Missoula Group may also have been the source for some of the vein gold. We speculate that the distribution of placer deposits resulted from dextral offset of lode sources by the northwest-striking Ninemile fault during the Tertiary. This fault may have displaced the northeast half of the formerly circular- to ellipically shaped lode district up to 15 km to the southeast.

The Hope Brook gold mine is a high-sulfidation epithermal gold deposit hosted by Late Proterozoic rocks of the northern Appalachian Avalon zone. It is located within the Whittle Hill Sandstone, which is intruded by a Late Proterozoic quartz-feldspar porphyry sill-dike complex of the Roti Intrusive Suite. The deposit (45 t Au) is enclosed within a zone of hydrothermal alteration >3 km long and up to 400 m wide. This zone of alteration is characterized by extensive advanced argillic alteration with pyrophyllite, kaolinite, andalusite, and alunite, which is developed mostly in the structural hanging wall of the ore zone, and two main stages of massive silicic alteration. The first, a buff-colored stage of massive silicic alteration, extends for > or = 3 km laterally away from the deposit. It constitutes a barren to weakly auriferous unit and most likely results from the pervasive acid leaching of the original host(s). The second stage of gray massive silicic alteration is spatially coincident with gold mineralization and is characterized by vuggy quartz. The mineralization consists of several percent of pyrite and lesser amounts of chalcopyrite and bornite and some tennantite with local traces of enargite. In addition to Au and Cu, there is a very limited amount of other metals in the deposit, although it has anomalously high amounts of Sb, Bi, Pb, and As.The ages of altered (pre- and late ore) quartz-feldspar porphyry and of an unaltered (postore) intermediate dike cutting altered rocks bracket the age of mineralization-alteration between 578 and 574 Ma. This temporally and genetically links mineralization-alteration and Roti Intrusive Suite plutonism. Despite a strong spatial relationship between a major fault zone (Cinq Cerf fault zone) and gold mineralization, the Hope Brook deposit bears no genetic relationship to major ductile deformation. The deposit was formed approximately 150 m.y. prior to the development of this fault zone.The existence of Hope Brook, along with a few Mesozoic (Pueblo Viejo), Paleozoic (Temora), Neoproterozoic (Brewer), and Paleoproterozoic deposits (Enaasen), highlights the potential for discovery of such Au-Cu intrusion-related deposits in older terranes. Despite their emplacement at high levels in the crust, the potential for preservation of such high-sulfidation deposits in older terranes may be high if tilting and/or burial took place early in the geologic evolution of the host volcano-plutonic arc.Although the nature and distribution of the initial buff silicic stage of alteration are critical indications of a large-scale hydrothermal system, the ore at Hope Brook is only localized in rocks displaying the gray-colored second silicic stage of alteration. Consequently, the recognition of the two stages of massive silicic alteration was critical in understanding the genesis of the Hope Brook deposit and constitutes an important exploration tool in discriminating between potentially mineralized and barren zones.

The Campbell mine and the adjacent Red Lake mine are developed in an Archean gold deposit hosted within volcanic rocks of the Red Lake greenstone belt, situated in the western part of the Uchi subprovince of the Superior province. Geochronological evidence suggests that the Campbell-Red Lake deposit formed after volcanism but prior to late felsic plutonism and regional metamorphism, between 2722 and 2710 Ma. The orebodies at Campbell mine are largely hosted in epithermal-style veins and vein stockworks associated with strike-slip faults.Complex, superimposed phases of wall-rock alteration occurred prior to the emplacement of main stage veins and were controlled by both primary and secondary (contact, fault, and fracture induced) permeability in mafic and ultramafic rocks as well as secondary permeability in rhyolite and diorite. Widespread pervasive carbonatization and potassic (biotite) alteration with distal chloritic alteration is well developed in basalt and ultramafic rocks. In basalt, this alteration was in turn partly overprinted by silicification and aluminous alteration consisting of zoned mineral assemblages of quartz-andalusite-sericite-chloritoid-cordierite-margarite occurring within bleached zones, and quartz-chlorite-chloritoid-garnet-cummingtonite assemblages occurring in flanking chloritic zones. Similarly, zoned aluminous mineral assemblages also occur in diorite and rhyolite. In ultramafic rocks, the aluminous alteration is represented by sericite-fuchsite-cordierite assemblages, which grade out toward chlorite-anthophyllite-cordierite assemblages. This zoning appears to reflect the bulk composition of zoned premetamorphic aluminous (argillic) alteration.Main-stage carbonate (dolomite to ankerite)-quartz veins exhibit well-developed open-space-filling textures characterized by colloform and crustiform banding and cockade breccia infill textures typical of veins which formed in a near-surface environment. Fissure veins occur within fault zones themselves, whereas large snowbank (banded carbonate-quartz) veins formed in dilatant sites associated with a releasing bend in the faults. Fissure veins and snowbank veins are best developed in basalt, whereas veinlet stockwork and sheeted veinlet zones occur in ultramafic rocks adjacent to main-stage vein sites in basalt. The main-stage veins partially sealed remaining permeable zones, and proximal early alteration assemblages were overprinted by distal chloritic alteration as the hydrothermal system collapsed inward. The main-stage veins were then overprinted by silicification + or - tourmaline within breccias and veinlet stockworks.Mineralization is associated with microstockwork sulfide veinlets, strongly disseminated sulfide, and sulfide cemented breccias which cut silicified zones in veins and wall rocks. Native gold occurs on its own or with sulfides in fractures cutting silicification. Arsenopyrite is commonly associated with gold and occurs in ore zones with pyrite, pyrrhotite, and magnetite. Stibnite and sphalerite commonly occur in high-grade zones. Mineralized zones hosted in fissure veins and wall-rock replacement zones tend to be narrow (

Trace and rare earth element (BEE) geochemistry indicates that rocks in the Manitouwadge mining camp were derived from three major lithological units: mafic volcanic rocks, acid volcanic tuffs, and dacites. The Geco, Willroy, Willecho, and Big Nama Creek Cu-Zn-Ag deposits are contained within the 2720 Ma acid volcanic tuffs. Our study suggests that the thickness of the felsic volcanic unit (acid tuffs and volcaniclastics) may be significantly underestimated in the area. Medium- to high-grade metamorphism (upper amphibolite, lower granulite) gave rise to specific alteration assemblages which crystallized during several episodes of deformation and plutonism. Because major element geochemistry of the rocks was modified during ore- forming processes and later by several episodes of metamorphism and tectonism, the present alteration minerals merely reflect the cumulative effect(s) of these episodes and are often misleading in the identification of the protoliths. Thus, due to extensive alteration, the protoliths of various lithological units can be positively identified only from trace and rare earth element geochemistry. Extensive (up to 97%) light REE depletion was observed in the felsic volcanic rocks around the Geco, Willroy, and Willecho deposits. The positive correlation between BEE mobility and mineralization suggests that REE geochemistry can be successfully used in the exploration for volcanogenic massive sulfide deposits around the Manitouwadge mining camp. The absence of a well-defined discordant alteration pipe at Geco and localized enrichment in Fe, Al, + or - K at the Willroy, Willecho, Big Nama Creek, and Geco deposits implies that they may be highly metamorphosed equivalents of Mattabi-type deposits where the original iron-rich carbonates have recrystallized to calc-silicate or amphibole-rich assemblages during regional metamorphism. Localized enrichment in Fe, Al, and K in the felsic tuffs could be used to identify possible alteration pipes or semicomformable alteration zones associated with mineralization.

Structurally hosted lode gold deposits in the Early Proterozoic Reindeer zone, Saskatchewan, have many attributes in common with Archcan counterparts worldwide. The detailed field relationships at the Jolu mine indicate that the development of shear zones and mineralization in the Star Lake pluton clearly postdates eraplacement of the host footwall granodiorite at 1846 + or - Ma and predates the time of regional metamorphism in the Star Lake area estimated at 1783 Ma using a U-Pb titanitc age obtained from the Star Lake pluton. The best estimate for the timing of gold mineralization is 1807 Ma based on U-Pb dating of hydrothermal titanitc found in a mafic dike sample that exhibits a close spatial and temporal relationship to quartz vein mineralization. This age of mineralization is identical to high-grade metamorphism at deeper crustal levels in the Trans-Hudson orogen (e.g., in the Kisseynew domain) and, as has been shown for some Archcan lode gold deposits, provides a temporal link between gold mineralization at high crustal levels and high-grade metamorphism and devolatilization at depth.

Studies of plutonic rocks in Archean terranes such as the southern Abitibi subprovince have been an important component in the development of an accretionary tectonic model for the Superior province. In the southern Abitibi subprovince, pretectonic ( approximately 2.75-2.70 Ga), low K, tholeiitic calc-alkaline, dioritic to tonalitic plutons (e.g., Bourlamaque batholith; Flavrian pluton) are associated with volcanism and are interpreted to be derived from both partial melting of hydrated mantle during oceanic arc formation and fractionation of tholeiitic to calc-alkaline basalt-andesite. Pretectonic, synvolcanic tholeiitic intrusions (e.g., Dundonald sill, Kamiskotia gabbroic complex: 2.72-2.70 Ga) show in situ fractionation trends, and locally, have mixed magma textures with contemporaneous calc-alkaline granitoids. They are believed to be derived from a midocean ridge basalt (MORB)-like depleted mantle, with little evidence for contamination by an older crustal component. Pre- to syntectonic ( approximately 2.72-2.69 Ga) porphyry and tonalite plutons (e.g., Timmins porphyries, Round Lake batholith) have geochemical characteristics typical of Archean tonalite-trondhjemite suites considered to be derived from melting of basaltic rocks at high metamorphic grades. Late to post-tectonic (

The Sigma mine at Val d9Or, Quebec, in the Abitibi belt is typical of vein-type gold deposits in Archcan greenstone belts. Although the relative timing of gold mineralization with respect to magmatism, metamorphism, deformation, and hydrothermal events in the mine is well documented, precise isotopic age constraints have, until recently, been lacking. This paper presents the results of a 40 Ar/ 39 Ar dating study of the Sigma mine and of the Davidson-Tisdale deposit in the Abitibi belt. In the Sigma mine, dates on metamorphic amphibole indicate that the regional metamorphism, which predates the vein formation, occurred at ca. 2.68 Ga. Muscovite from the gold-bearing quartz veins yields a "plateau" date of 2579 + or - 9 Ma (2Sigma ), whereas biotite from the veins shows disturbed, saddle-shaped, age spectra. The mild disturbance of the biotite is attributed to later diabase dike emplacement, and a 40 Ar/ 39 Ar date on a dike indicates that this event is younger than 2.3 Ga. Thermal effects of the Grenville orogeny (1.2-1.0 Ga) may also be present. A muscovite separate from a gold-bearing quartz vein at the Davidson-Tisdale deposit yields a 2607 + or - 10 Ma (2Sigma ) plateau date. Although the amphibole and muscovite dates at the Sigma mine bracket the gold mineralization event at between ca. 2.68 and 2.58 Ga, a number of tectonothermal histories are possible for this time interval. It is concluded, based on all evidence, that the muscovite plateau dates are the time of vein and gold formation, which thus occurred in the Sigma mine ca. 100 m.y., and in the Davidson-Tisdale deposit ca. 70 m.y., after syn- to post-tectonic magmatism and regional metamorphism.

Lead isotope measurements of 13 deposits and prospects from western Tasmania have been undertaken to establish a framework for the use of the lead isotope technique in exploration and to gain an understanding of the processes of ore formation. Lead isotope signatures for the Cambrian volcanogenic deposits (Mount Lyell, Rosebery, Hercules, Que River, Hellyer) differ by more than 1 percent from those characterizing the post-Cambrian mineralization, thought to be related to Devonian-Carboniferous granitoids and/or Tabberabberan metamorphism (e.g., Renison Bell, Queen Hill, Spray mine, Farrell lodes). The differences in lead isotope signatures allow discrimination of the two contrasting styles of mineralization in both surface material and drill core; the isotopic signatures have been utilized in an exploration program at Elliott Bay which is described in companion papers.The following important genetic conclusions can be drawn from the results of this exploration-oriented investigation: (1) A whole-rock Pb-Pb age of 580 + or - 170 m.y. (2 Sigma ) was measured for polymictic volcaniclastic rocks from the Que River area, which, although not tightly constrained because of the large error limits, is consistent with paleontological evidence. (2) The intersection of this whole-rock line with the average Que River ore lead isotope value is consistent with a common source for the lead. (3) The lead isotope data are consistent with an origin of the post-Cambrian mineralization by remobilization from the host volcanic pile in response to Devonian-Carboniferous metamorphism and/or plutonism. (4) All isotopic data for the volcanic rocks, massive sulfides, and post-Cambrian mineralization lie on linear arrays, and on the uranogenic lead plot ( 207 Pb/ 204 Pb vs 206 Pb/ 204 Pb) they intersect the lead evolution (growth) curve for other massive sulfide deposits at approximately 1,000 m.y. This is interpreted as evidence of a Precambrian source for the metals and also of the cogenesis of the volcanics, massive sulfide, and later mineralization. (5) The Proterozoic source age preserved in the lead isotope data for the volcanics (and mineralization) is consistent with their formation in a rifting environment or a continental margin. (6) The 206 Pb/ 204 Pb values of 18.27 to 18.35 are unusually high for Cambrian massive sulfides and the calculated model ages of approximately 270 m.y. are about 50 percent too young and possibly result from an increase in the U/Pb ratio of the source in the last billion years. A calculated 238 U/ 204 Pb (mu ) value for the stage from 1,000 (or 1,500) to 540 m.y. ago is high at approximately 13, similar to that measured in the volcanic rocks today, and may account for the unusual 206 Pb/ 204 Pb ratios. (7) The Th/U value of 4 for volcanics and mineralization is normal for continental crustal rocks but is markedly different from the low Th/U of 1 to 2 in Devonian granitoids. (8) The unusual lead isotope signatures for the deposits cast doubt on the proposed conjunction of Tasmania with the Australian mainland in Cambrian times; the relationships with Antarctica await future investigations.

The Fiji platform has a thick, youthful crust of ensimatic origin. Stratigraphy within the Fiji islands indicates a history restricted to the Cenozoic, with volcanic rocks, principally submarine fragmental types with related sediments, dominating the succession. There are also carbonate sediments and plutonic rocks of gabbroic to tonalitic composition which were intruded during the Tholo orogeny, an important middle to late Miocene tectonic event.Mineral deposits in Fiji include various types of massive sulfide; manganese ores; disseminated porphyry-type Cu-Au deposits; epithermal-telethermal Au-Te-Ag deposits; veins, skarns, and limestone replacements related to plutonism; and residual deposits such as iron sands and bauxite.Preorogenic rocks are generally arc tholeiites, but following the Tholo orogeny there was great chemical diversity--possibly owing to anatexis and contamination--and eruption of arc tholeiite, calc-alkaline andesite, shoshonitic rocks, and oceanic-type basalts. Contamination of magmas by watery metamorphic fluids is thought to be the cause of relative enrichment of chalcophile elements in postorogenic rocks of late Miocene to early Pliocene age. Nearly all the major mineral deposits in Fiji are associated with these rocks.Volcanic and structural factors exert a local control on mineralization. On a regional scale plate collision in Outer Melanesia led to the Tholo orogeny and deep fracturing of the Fiji platform--factors of fundamental importance to Fiji mineralization; however, a simple correlation of island-arc mineral deposits and subduction is not apparent.

Machine-plotted maps of metal distributions, combined with field work, indicate the presence of a previously unrecognized regional zoning pattern in the Taseko Lakes and Pemberton map-areas. Mineral deposits occur in well-defined camps, the most productive of which has been the Bridge River gold camp. A distinctive asymmetric metal and mineral zoning pattern occurs in the Bridge River district. Two elongate, northwesterly trending centers of gold-bearing vein deposits lie within a larger area characterized by antimony minerals. The antimony zone is succeeded to the northeast by a mercury zone. This pattern is explained by mineral deposition under a regional thermal gradient decreasing outward from the eastern margin of the Coast Plutonic Complex, implying that mineralization occurred during or shortly after final cooling of the eastern margin of the Coast Plutonic Complex about 50 m.y. ago.Porphyry deposits occur in geographically separate areas from most vein deposits. Porphyry mineralization occurred during the Late Cretaceous, early Tertiary, and Miocene. Copper-molybdenum-gold porphyry deposits characterize the first two events, whereas porphyry molybdenum deposits formed during the Miocene. Principal movements along the Yalakom fault predated mineral deposition in the Bridge River camp. Late Tertiary porphyry molybdenum deposits appear genetically related to Cascade volcanism rather than to plutonism of the Coast Plutonic Complex.

The Cananea "granite" crops out in the Cananea mining district, which includes some of the most important known copper deposits in Mexico. The area is in northern Sonora, approximately 100 km southeast of the twin cities of Nogales, Arizona-Sonera, on the international border.By means of isotopic analyses of U-Pb systems in cogenetic suites of zircon from two different phases of the pluton, its interpreted age has been firmly established at 1,440 + or - 15 m.y. Cananea granite, actually a quartz monzonite, is lithologically characteristic of a major Precambrian suite of anorogenic, consanguineous plutons of porphyritic granodiorite to granite. Constituents of this previously recognized series, which crop out throughout the southwestern U. S. and into northern Mexico, consistently yield zircon radiometric ages within the interval 1,425-1,475 m.y.Major younger intrusions in and nearby the Cananea district include Cuitaca granodiorite and Chivato monzodiorite. These bodies yield apparent zircon ages of 64 + or - 3 and 69 + or - 1 m.y., respectively. Although more precise geochronologic relationships between young plutons, the adjacent layered sequence, and cross-cutting, brecciated, quartz porphyry plugs are needed, correlation exists between major plutonism and mineralization.The Precambrian age of the Cananea granite precludes its chronological association with the younger mineralized plugs and should be taken into account in future planning of exploration.

Sulfate minerals, hematite, and isotopically fractionated sulfides are relatively rare in rocks of Archean age. Several Archean Au deposits, however, contain one or more of these features indicative of oxidized hydrothermal fluids. They include Hemlo, McIntyre-Hollinger, Macassa, Lake Shore, and Ross which are all in Ontario, Canada, and Kalgoorlie in Australia. Examples of lesser deposits are in Hislop Township and in the Matachewan area, Ontario, Canada. With the exception of Hemlo, these are quartz-carbonate vein deposits spatially and temporally associated with felsic plutonism. Because metamorphic fluids and Archean surface waters were mostly reducing, the most feasible general source for the fluids was felsic magmas.Parent magmas were relatively oxidized, hydrous, and CO 2 rich. The oxidized character may have favored the incorporation of Au into the partial melt and its partitioning into magmatic aqueous fluids. High contents of CO 2 are conducive to the separation of an aqueous fluid from magma at greater depth than otherwise. Fluid inclusion data support the formation of major Archean Au vein deposits at deeper crustal levels than more recent magmatic-hydrothermal mineralization, such as porphyry Cu deposits. CO 2 -rich fluids can unmix into two phases at relatively high temperatures and pressures: a saline aqueous liquid and a low-salinity, H 2 O-CO 2 liquid. This can cause partitioning of Cl-complexed base metals from other metals including Au, Sb, and As. The extensive carbonatization around most deposits has consistent delta 13 C values in the range of juvenile-magmatic CO 2 .

Interpretation of duration of magmatism, relationships between occurrence of volcanism and plutonism

Hydrothermal products, Nevadian plutonism, tangential tensional stress, crustal uplift and rupture

Discussion of genesis, ore types, production and potential, Mesozoic and Tertiary metallogenesis