ABSTRACT Structural analysis of Neoproterozoic to lower Paleozoic rocks near Old Crow, in North Yukon, show that they were affected by widespread, but distributed sinistral shear zone deformation. This tectonic event occurred under brittle-ductile conditions, in the early Paleozoic, prior to intrusion of Late Devonian granitoids of the Old Crow plutonic suite (368–375 Ma). Although outcrops are scattered, the shear zone deformation can be inferred to extend over a broad ~W–E corridor, ~10–20 km-wide and ~145 km long, from eastern Alaska into northern Yukon. The sinistral Porcupine shear zone is interpreted to represent a major, early Paleozoic crustal structure along which elements of NE Laurentian and Caledonian affinities in the Arctic Alaska terrane were transferred across the Arctic region during the Paleozoic. Our observations do not support major Paleogene dextral strike-slip deformation along the Porcupine River near Old Crow.

Abstract The northern Pacific Rim, defined as stretching from southern China to southern Mexico, displays profound differences in both metallogenic character and regulatory environment. The largest Mesozoic and Cenozoic ore deposits of the northwestern Pacific quadrant contain principally tin, tungsten, molybdenum, or gold, whereas those in the northeastern quadrant are dominated by copper, silver, molybdenum, or gold. The regulatory environments of the northwestern Pacific Rim countries are generally considered less attractive for equity-financed exploration than those of the northeastern Pacific, as witnessed by the far fewer active companies and projects in the former compared to the latter. Furthermore, there is little western company exploration anywhere for tin, tungsten, and molybdenum, arguably some of the prime targets in the northwestern Pacific quadrant. Therefore, notwithstanding greater state funding of exploration in the northwestern Pacific quadrant, particularly in eastern China and the Russian Far East, the northeastern Pacific region has been the subject of far more effective exploration during the past two decades. More than two-thirds of the 67 principal metal deposits around the northern Pacific Rim are confined to and help define 20 spatially restricted metallogenic provinces, many the products of several discrete metallogenic epochs. The positions of these provinces may reflect the character of the underlying lithosphere. Nearly 40% of these major deposits had been discovered by the early decades of the 20th century, either accidentally or, chiefly in the western Americas, as a result of traditional prospecting. Since the 1940s, geologic, geochemical, and, to a lesser degree, geophysical techniques became progressively more widely employed. Geologic observation, mapping, and conventional geochemical exploration techniques led to most major discoveries before the mid-1990s. Application of broadly similar methods also resulted in the 15 more recent discoveries, 11 of which are located in the northeastern Pacific quadrant; all except four in the previously known metallogenic provinces. Most of the major discoveries from the mid-1990s onward were near known mineralization, seven of those in the northeastern quadrant involving junior exploration companies and four in the northwestern quadrant by semiautonomous provincial agencies. Eleven of the deposits discovered since the early 1990s were wholly or partly exposed, with the remainder completely concealed beneath pre- and/or postmineralization cover. It is predicted that the northeastern Pacific quadrant will continue to attract the majority of effective exploration attention and, as a consequence, will continue to dominate the northern Pacific Rim discovery record into the foreseeable future. Most major new deposits are likely to be located in the defined metallogenic provinces and to result from conventional exploration programs increasingly underpinned by conceptual geology and robust drilling budgets.

Abstract Large igneous provinces (LIPs) represent significant reservoirs of energy and metals that can either drive or contribute to a variety of metallogenic systems. The relationships between LIPs and these various systems can be divided into four distinct although partially overlapping classifications: (1) LIPs form the primary source of commodities within mineral deposits (e.g., orthomagmatic Ni-Cu-PGE sulfides, or Nb-Ta-REE and diamonds for often LIP-related carbonatites and kimberlites, respectively); (2) LIPs either provide the energy to drive hydrothermal systems or can act as source rocks for hydrothermal ore deposits (e.g., volcanogenic massive sulfide (VMS) deposits)—in some cases LIP rocks can also act as barriers to fluid flow and/or reaction zones causing mineralization (e.g., orogenic Au); (3) weathering can concentrate elements such as Al and Ni-Co within laterites that develop from exposed LIP mafic-ultramafic rocks in tropical climates, and for Nb, Ta, and REE in laterites from associated carbonatites; and (4) indirect links exist between LIPs and ore deposits; here we consider two of these types of links, the first of which involves LIP events that are linked to attempted or successful continental breakup where the LIP barcode record can be used as a correlation tool for reconstructing Precambrian supercontinents and therefore enable the tracing of metallogenic belts between presently separated, but formerly contiguous crustal blocks. A second, more speculative, indirect link is provided by the fact that major continental breakup (linked to LIPs) is associated with distal compression and transpression in the plate tectonic circuit (and the formation of orogenic deposits, such as Au). We discuss the role of LIPs (be it major or contributory) in each of these classifications for the generation of this wide variety of differing mineral deposit types and potential implications of this link between LIPs and metallogenesis for exploration strategies. This review shows how our understanding of LIPs, and the processes that affect LIP magmas and rocks, have direct consequences for mineral exploration and economic geology.

Abstract The Cordilleran orogen of western Canada and Alaska records tectonic processes than span over 1.8 billion years, from assembly of the Laurentian cratonic core of Ancestral North America in the Precambrian to sea-floor spreading, subduction, and geometrically linked transform faulting along the modern continental margin. The evolution of tectonic regimes, from Proterozoic intracratonic basin subsidence and Paleozoic rifting to construction of Mesozoic and younger intraoceanic and continent-margin arcs, has led to diverse metallogenetic styles. The northern Cordillera consists of four large-scale paleogeographic realms. The Ancestral North American (Laurentian) realm comprises 2.3 to 1.8 Ga cratonic basement, Paleoproterozoic through Triassic cover successions, and younger synorogenic clastic deposits. Terranes of the peri-Laurentian realm, although allochthonous, have a northwestern Laurentian heritage. They include continental fragments, arcs, accompanying accretionary complexes, and back-arc marginal ocean basins that developed off western (present coordinates) Ancestral North America, in a setting similar to the modern western Pacific basin. Terranes of the Arctic-northeastern Pacific realm include the following: pre-Devonian pericratonic and arc fragments that originated near the Baltican and Siberian margins of the Arctic basin and Late Devonian to early Jurassic arc, back-arc, and accretionary terranes that developed during transport into and within the northeastern paleo-Pacific basin. Some Arctic realm terranes may have impinged on the outer peri-Laurentian margin in the Devonian. However, main-stage accretion of the two realms to each other and to the Laurentian margin began in mid-Jurassic time and continued through the Cretaceous. Terranes of the Coastal realm occupy the western edge of the present continent; they include later Mesozoic to Cenozoic accretionary prisms and seamounts that were scraped off of Pacific oceanic plates during subduction beneath the margin of North America. Each realm carries its own metallogenetic signature. Proterozoic basins of Ancestral North America host polymetallic SEDEX, Cu-Au-U-Co-enriched breccias, MVT, and sedimentary copper deposits. Paleozoic syngenetic sulfides occur in continental rift and arc settings in Ancestral North America, the peri-Laurentian terranes, and in two of the older pericratonic Arctic terranes, Arctic Alaska, and Alexander. The early Mesozoic peri-Laurentian arcs of Stikinia and Quesnellia host prolific porphyry Cu-Au and Cu-Mo and related precious metal-enriched deposits. Superimposed postaccretionary magmatic arcs and compressional and extensional tectonic regimes have also given rise to important mineral deposit suites, particularly gold, but also porphyries. Very young (5 Ma) porphyry Cu deposits in northwestern Vancouver Island and sea-floor hotspring deposits along the modern Juan de Fuca Ridge off the southwest coast of British Columbia show that Cordilleran metallogeny continues.

Abstract A magmatic and metallogenic framework for the northern Yukon-Tanana terrane of west-central Yukon and eastern Alaska is proposed, which contextualizes syngenetic, intrusion-related, and orogenic styles of mineralization in the region. The framework applies to bedrock gold and base metal enrichments in the Dawson Range, White Gold, Klondike, Sixtymile, and Fortymile districts, which are historically known for their placer gold endowment, but which host few significant bedrock mineral resources. New field and geochronological (U-Pb, 40 Ar/ 39 Ar, 187 Re/ 187 Os) data, along with contributions from exploration companies, provide the key constraints on this framework. Sedimentary exhalative Pb-Zn mineralization and porphyry-style Cu-Au mineralization are associated with Late Devonian to Early Mississippian (365-342 Ma) rocks of the Finlayson assemblage and Simpson Range plutonic suite, respectively—both of which formed in a continental arc built on pre-Late Devonian continental margin sediments (Snowcap assemblage) along the ancient Pacific margin of North America. By the Late Permian, these assemblages had rifted away from North America, and W-dipping subduction of the intervening Slide Mountain Ocean was initiated. Volcanogenic massive sulfide-style Pb-Zn-Cu-(Ag-Au) mineralization formed in subvolcanic to volcanic rocks of the Late Permian (269-253 Ma) Klondike arc assemblage that was built on the Devono-Mississippian arc. Together these assemblages make up the Yukon-Tanana terrane. Gold mineralization formed sparsely with syn- to postmetamorphic Late Permian (253-250 Ma) anatectic melts. Five metallogenic events are recognized that coincide with magmatic episodes superimposed on the Yukon-Tanana terrane: (1) Cu-Au mineralization formed during an Early Jurassic (200-179 Ma) pulse of magmatism and was accompanied by rapid crustal exhumation (e.g., Minto); (2) Au-mineralized breccia complexes, skarns, intermediate-sulfidation epithermal systems, and polymetallic veins are associated with mid-Cretaceous (115-98 Ma) magnetite-series arc magmas in the Dawson Range, whereas age-equivalent Au deposits in the back-arc region to the north are associated with ilmenite-series magmas (e.g., Pogo); (3) variably Cu and Au rich porphyry systems formed within the mid-Cretaceous arc in the early Late Cretaceous (79-72 Ma) (e.g., Casino, Nucleus-Revenue); (4) porphyry Mo and Cu systems and Ag-rich polymetallic veins, carbonate-replacement, and skarn bodies are temporally and spatially associated with NE-trending, sinistral oblique-extensional fault systems in the latest Cretaceous (72-67 Ma); and (5) examples of disseminated U, Cu-Pb-Ag skarn, and Au-Ag epithermal systems are associated with dominantly felsic but locally bimodal Paleocene-Eocene (60-55 Ma) magmatism, emplaced into zones of extension during early activity on the Tintina fault zone. At least two distinct orogenic Au-mineralizing events are recognized. Within a Middle to Late Jurassic hiatus in magmatism, gold mineralization formed at 163 to 155 Ma in brittle-ductile to brittle structures within sinistral fault zones (e.g., White Gold), high-angle reverse faults, and kink folds. A subsequent episode of mid-Cretaceous (96-92 Ma) orogenic gold mineralization formed in structures cutting Paleozoic metamorphic rocks and mid-Cretaceous granitoids (e.g., Moosehorn, Boulevard). Weathering and surficial preservation in this unglaciated region since the Pliocene resulted in economic placer gold endowments in the Klondike, Sixtymile, Fortymile, White Gold, and Dawson Range districts. The framework we describe for the magmatism and metallogeny of west-central Yukon and eastern Alaska provides a testable platform for regional exploration targeting and property-scale exploration in a region with demonstrated mineral potential.

Abstract Cretaceous gold metallogenesis in southwestern Alaska comprises three distinct episodes related to the accretionary evolution of northwestern North America. The oldest mineralizing event is characterized by 112 Ma Cu-Au-Bi-Te porphyry-type(?) veining in the zoned Bonanza and adjacent plutons that intruded rocks of the Nyac terrane. Tectonic reconstructions and limited geological and geochemical data suggest that Cu-Au mineralization in the Nyac district may be related to terminal subduction during accretion of the Togiak-Koyukuk arc. The subsequent 100 to 89 Ma metallogenic event is a product of subduction-related magmatism immediately following accretion of the Peninsular-Alexander-Wrangellia superterrane and includes formation of the giant Pebble porphyry Cu-Au-Mo deposit. Pebble underwent a complex history of highly oxidized magmatism that is isotopically linked to enriched lithosphere or metasomatized mantle sources. Pebble and other porphyryrelated plutons were emplaced as a consequence of changes in plate motion and onset of dextral transpression along the continental margin to the southeast of their present-day locations. The final 75 to 65 Ma metallogenic event is regionally the most extensive. It followed a ~15-m.y.-long magmatic lull and is related to enigmatic subduction-related magmatism in the western part of the Alaska Range and the Kuskokwim basin. This event resulted in the formation of porphyry, reduced pluton-related, orogenic, and possible epithermal Au deposits. In the better-studied relatively low-relief areas of the Kuskokwim basin, many of the mineralized systems are spatially associated with ilmenite-series monzonite to quartz monzonite composite plutons that have isotopic signatures consistent with derivation from crustally contaminated mantle sources. These pluton-hosted Au deposits comprise low-sulfide stockworks, sheeted veins, and/or breccias in the cupolas of moderately differentiated intrusions that contain high Au; anomalous As, Bi, Sb, and/or Te; and have variable, but not uneconomic, Cu concentrations. In addition, Au, Sb, and/or Hg deposits hosted by competent NE-trending granite porphyry dike-sill complexes or along faults in flysch are widespread in the Kuskokwim basin. These deposits, including the giant Donlin Creek Au deposit, formed at shallow crustal levels and are classified here as epizonal orogenic Au deposits and related Hg and Sb lodes. Mesozonal orogenic Au deposits were formed along the margins of the uplifting Willow Creek pluton that is now exposed along the southern side of the Talkeetna Mountains batholith. Recent discoveries in the rugged Alaska Range comprise less well-studied porphyry Au-Cu (Whistler, Island Mountain), epithermal(?) Au (Terra), and reduced pluton-hosted Au deposits (Estelle). The cause of the broad latest Cretaceous magmatism associated with the final metallogenic event is enigmatic, but indicates widespread high heat flow possibly related to flat-slab subduction, lithospheric mantle delamination, or escape tectonics. Plutonism postdated regional folding and coincided with periods of movement along regional faults and formation of the NE-trending structural fabric. Magmatism resulted from initiation of transpressional faulting in more landward positions during oblique subduction of the Kula plate. Crustal contamination of mantle melts, either near their source or during ascent through the thick flysch of the Kuskokwim basin, produced low oxidation state magmas and controlled much of the metallogeny, particularly for those deposits that display similarities to both reduced porphyry Au-Cu deposits and, to a lesser degree, reduced intrusion-related Au systems. High heat flow also induced crustal melting and metamorphic devolatilization to form orogenic Au deposits. A transition to extension at ~60 Ma recorded elsewhere in Alaska temporally corresponds to termination of gold deposit formation in southwestern Alaska. The multiple periods of gold metallogenesis in southwestern Alaska offer a wide variety of targets for exploration within discrete parts of the region.

Abstract Mexico is widely known to be a richly endowed country in both metallic and industrial mineral deposits, the exploitation of which has constituted an economic activity of paramount importance for centuries. This paper presents an analysis of the time and space distribution of over 200 mineral deposits, which is based on the available absolute and relative ages of mineralization and constitutes a modified and updated version of the analysis of Camprubí (2009). Pre-Jurassic ore deposits are relatively scarce and of subordinate economic significance. These include Ti-bearing anorthosites and rare element pegmatites in intracratonic environments, barite sedimentary-exhalative (sedex) deposits, and ultramafic-mafic Cr-Cu-Ni(-platinum group element [PGE]) deposits in oceanic environments. Since the Jurassic, the metallogenic evolution of Mexico can be understood as a product of the evolution of two major regions: the Pacific margin and the Gulf of Mexico. The Mesozoic evolution of the Pacific margin is characterized by rifting and separation of the Guerrero composite terrane from the North American continent and the initiation of arc magmatism in an extensional continental margin setting. The ore deposits emplaced in this period are mostly polymetallic volcanogenic massive sulfide (VMS) and Cr-Cu-Ni(-PGE) deposits associated with ultramafic-mafic complexes. These occur dominantly near the boundaries of the Guerrero composite terrane. Porphyry-type deposits emplaced in the mid- Cretaceous are subordinate and, apparently, small. These likely formed in island arcs that were later accreted to the mainland. A shift from extensional to compressional tectonics resulted in the accretion of the Pacific terranes, most importantly the Guerrero composite terrane, to the Mexican mainland by the Late Cretaceous. This tectonic shift gave rise to the initial stages of the Paleocene boom in porphyry-type and sulfide skarn deposits. The continental arcs in these epochs represent the earliest stages for the Sierra Madre Occidental silicic large igneous province. The earliest known examples of epithermal deposits in Mexico are Paleocene and include, besides intermediate to low sulfidation deposits, the La Caridad Antigua high sulfidation deposit, in association with the giant La Caridad porphyry copper deposit. The Late Cretaceous iron oxide copper-gold (IOCG) deposits formed in northern Baja California and along the Pacific margin in southwestern and southern Mexico, and continued forming in the latter regions into the Paleocene. Contrastingly, some Late Cretaceous IOCG deposits formed several hundreds of km inland in northwestern Mexico, and are suspected cases for emplacement in back-arc environments. The formation of orogenic Au deposits began in the Late Cretaceous, and they kept forming into the Eocene as compressional tectonics progressed. The formation of porphyry-type, sulfide skarn, and epithermal deposits continued during the Eocene, and followed the eastward progression of the magmatism of the Sierra Madre Occidental. The number of known examples of epithermal deposits relative to porphyry-type and sulfide skarn deposits increases with time. The formation of IOCG deposits along the Pacific margin seemingly dwindled during the Eocene, although they began to form close to the Chihuahua-Coahuila border, possibly in association with the earliest stages of mineralization in the Eastern Mexican alkaline province. Also, a group of U-Au vein deposits in Chihuahua, in association with felsic volcanic rocks, is apparently restricted to the Eocene. The maximum geographic extension and climactic events of the Sierra Madre Occidental (for both magmatic and ore-forming events) were attained during the Oligocene, as the arc kept migrating eastward and southward. As magmatism reached the Mesa Central, epithermal and subepithermal, sulfide skarn, Sn veins associated with F-rich rhyolites, IOCG, and Sn-W greisen deposits formed around the main reactivated fault zones, generating the highest concentration of ore deposits known in Mexico. The focus of magmatism and mineralizing processes shifted progressively southward in the Eastern Mexican alkaline province between the Oligocene and the Miocene, and intensified significantly in northern Coahuila and Chihuahua in the Oligocene. This province also includes alkaline porphyry Cu-Mo deposits, REE-bearing carbonatites, and polymetallic skarns. During the Miocene, the magmatism of the Sierra Madre Occidental retracted dramatically southward and began concentrating in an E-W arrangement that corresponds to the Trans-Mexican volcanic belt, while continental extension evolved into the opening of the Gulf of California. During this time, metallogenic processes associated with the Sierra Madre Occidental virtually ceased. From the late Miocene, the formation of epithermal deposits, sulfide skarns, and porphyry-type deposits resumed in the Trans-Mexican volcanic belt and the Eastern Mexican alkaline province, whereas IOCG deposits seem restricted to the latter. The opening of the Gulf of California represents the beginning of a new cycle in metallogenesis, with the formation of shallow analogues of sedex deposits and sedimentary phosphorites along the Baja California peninsula, epithermal deposits near the cul-de-sac of the Gulf, and recent VMS deposits in passive continental margins and mid-ocean ridges. The sedimentary-diagenetic history of the Gulf of Mexico includes the formation of Mississippi Valley-type (MVT) and associated industrial mineral, red bed-hosted U and Cu-Co-Ni, sedimentary phosphorite, and sedex deposits. The emplacement of MVT and red bed-hosted deposits was associated with the emplacement of basinal brines through reactivated faults that controlled basin inversion. These faults also played a significant role as channelways for magmas and associated magmatic-hydrothermal ore deposits of the Eastern Mexican alkaline province.

Abstract Carbonatites are relatively rare igneous rocks that are of considerable economic interest due to their common enrichment in certain elements, such as the rare earth elements (REEs), Nb, Ta, Th, and P. There are more than 37 known carbonatite occurrences in western North America, six of which have published resource estimates: these are the Aley and Upper Fir in British Columbia (for Nb, Ta, P), the Bear Lodge Mountains (for REEs), the Wet Mountains area in Colorado (for Th, Nb, U, and REEs), Iron Hill in Colorado (for REEs, Nb, and Th), and Mountain Pass in California (for REEs). Based on their distribution in geologic space and time, the carbonatites in western North America can be divided into four distinct groups: group 1 (~1450-1375 Ma) is Mountain Pass; group 2 (~813-449 Ma) comprises several carbonatites in Canada and the United States; group 3 (~359-328 Ma) comprises most of the carbonatites in western Canada; and group 4 (~52-37 Ma) comprises carbonatites in the northwestern United States and northern Mexico. It is not possible to distinguish between economically more or less favorable carbonatites based on their intrusion ages and geologic settings alone because all of the above groups contain both potentially mineralized and barren carbonatites. Therefore, other factors, such as the source region of the parental magma, the depth and degree of melting of that region, and the evolution of the parental magma during ascent and emplacement, control whether or not a carbonatite reaches ore grade. In terms of regional exploration, the vast majority of the carbonatites in western North America are situated parallel and in close proximity to the Cordilleran deformation front. These carbonatites are of Neoproterozoic to Paleozoic age, are related to rifting and/or extensional tectonic events, and are commonly hosted by miogeoclinal strata. Carbonatites that are distal to the Cordilleran deformation front are (1) the Eocene carbonatites in Montana and Wyoming, which follow a confined N40°W trend, presumably governed by the edge of the subducted Kula plate, and (2) the Mesoproterozoic Mountain Pass carbonatite, which is unique in terms of age and geologic setting. The repetitive and localized nature of carbonatite and alkaline magmatism makes areas in proximity to known carbonatite occurrences prime exploration targets; furthermore, Th-REE-rich quartz dikes are possibly indicative of buried carbonatitic and alkaline intrusions, and the lack of REE carbonates in amphibolite facies metacarbonatites suggests that these phases are not stable at elevated metamorphic pressure and temperature conditions.

Abstract The formation of giant and supergiant porphyry deposits is interpreted to be genetically linked to the subduction of major transform structures on the sea floor. The ultramafic crust and lithosphere associated with oceanic transforms and fracture zones undergo high degrees of metasomatic alteration (serpentinization) aided by ongoing rupture and enhanced fluid flow relative to that experienced by adjacent, structurally homogeneous oceanic crust. When the highly serpentinized (fluid-enriched) oceanic crust and lithosphere formed at these fracture zones subducts at convergent plate boundaries, the hydrous, lithospheric-scale structural weaknesses may locally develop into vertical slab tears. The formation of vertical slab tears leads to localized mantle flow and elevated temperatures at the edges of the slab. The ensuing thermal perturbations along the slab edges enhance serpentinite breakdown reactions and aids in the liberation of aqueous fluids at approximately 600°C and 80- to 100-km depth. This pressure and temperature range is close to the wet melting curve, thereby increasing the potential for slab melting and the generation of magmas such as adakites with relatively high log f o 2 >FMQ. Large oceanic transforms such as the Mocha-Valdiva fracture zone in the southeastern Pacific are likely to contain the greatest proportion of serpentinite and therefore carry fluid in the form of serpentinite group minerals into the mantle. Ultimately, the volume of fluid liberated during subduction and dehydration of the serpentinized mantle at these fractures far exceeds the volume of fluid produced at adjacent, weakly altered, and “structureless” oceanic crust. We suggest that the combination of extremely large volumes of slab-derived fluids plus the potential for slab melts to develop in these regions represents a primary control on the formation of giant porphyry deposits. These conditions are met along the eastern Pacific Rim, where more than 10 large transforms are currently being subducte. The endowment of the subducting oceanic crust with transforms and equally impressive mineral endowment contrasts markedly with the western Pacific Rim where only three to four equivalent-sized transforms are found. We propose that the variation in mineral endowment between the eastern and western Pacific and the formation of giant and supergiant porphyry deposits is linked to the formation and subduction of large oceanic fractures.

Abstract Porphyry Cu ± Mo ± Au deposits (porphyry) and iron oxide copper-gold deposits formed by magmatic-hydrothermal systems (MH-IOCG) share many features, most notably enrichments in Fe, Cu, and Au, as well as an association with calc-alkaline to mildly alkaline magmas generated in orogenic (subduction to collision) and postorogenic tectonic settings. Differences include the predominance of Fe-sulfide minerals and widespread late-stage acidic alteration in porphyry deposits, compared with Fe-oxide minerals and more extensive near-neutral pH alteration in MH-IOCG deposits. They are also separated by temporal distribution, with MH-IOCG deposits predominantly forming in the Precambrian era and porphyry deposits predominantly forming in the Phanerozoic eon, with only rare examples in older rocks. We propose that these differences between otherwise quite similar deposit types can be explained primarily by a difference in magmatic sulfur content, specifically oxidized sulfur (SO 4 2- ). Hydrous, sulfate-rich magmas exsolve SO 2 -rich aqueous fluids upon upper crustal emplacement and cooling. Further cooling of these fluids causes disproportionation of SO2 to H2S and H2SO4, resulting in the precipitation of Fe ± Cu ± Au-rich sulfide minerals (and sulfates) and the formation of progressively more acidic alteration styles. In contrast, fluids exsolved from hydrous but sulfate-poor magmas precipitate the bulk of transported Fe as oxide phases, with minor (but potentially economically important) Cu ± Au-rich sulfide phases and more restricted development of late-stage acidic alteration styles. We further propose that a broad temporal division between sulfate-poor and sulfate-rich magmas at the end of the Precambrian era reflects the global oxygenation of the deep oceans, which lagged the buildup of oxygen in the atmosphere by almost 2 billion years. Deep seawater sulfate concentrations increased rapidly following the Neoproterozoic Oxygenation Event and would have resulted in the voluminous fixation of sulfate in sea-floor-altered oceanic crust for the first time in Earth’s history. Release of sulfate-bearing fluids during prograde metamorphism of subducted oceanic crust is thought to contribute the bulk of sulfur to the metasomatized mantle source of Phanerozoic arc magmas, but this source of sulfate would not have been widely available during subduction of Precambrian oceanic lithosphere. Consequently, S-rich porphyry deposits would rarely have formed from Precambrian arc magmas, in contrast to S-poor MH-IOCG-type deposits, and vice versa in the Phanerozoic eon. Exceptions may arise: locally oxidizing conditions in the Proterozoic eon (e.g., in shallow subduction zones involving oxidized, upper ocean waters) may explain the rare occurrence of porphyry deposits in older rocks; and locally S-poor source rocks, such as the hydrous lithospheric residues of earlier arc magmatism, or sulfate-poor conditions generated during oceanic anoxia events, may explain the occurrence of locally important MH-IOCG deposits in the Phanerozoic eon. Other differences, such as the greater extent of high-temperature alteration zones and the occurrence of lithophile-element enrichments (e.g., rare earth elements [REEs] and U) in some MH-IOCG deposits, may reflect higher geothermal gradients such as were common in the Precambrian era or formation, in high-heat flow extensional environments and/or in proximity to batholiths. Some deposits show a spatial (if not genetic) association with high-heat-producing granites of predominantly crustal origin. Regardless of the cause, extensive high-temperature alteration can result in a greater degree of crustal metal and/or fluid fluxing. The greatest overlap between porphyry and MH-IOCG deposits occurs for postsubduction Au-rich porphyry systems, whose mildly alkaline magmas are generated by partial melting of hydrous amphibole-rich residues of earlier arc magmatism. Such magmas are S poor relative to arc magmas, because the flux of new sulfur from the subduction zone is no longer present. Small amounts of Au-enriched residual sulfide in the metasomatized lithospheric source readily dissolve in these S-undersaturated second-stage melts, rendering the magmas fertile for subsequent porphyry Cu-Au deposit formation. Such mildly alkaline porphyries are typically magnetite rich, with alteration styles similar to those encountered in MH-IOCG deposits. Thus, the key link between porphyry and MH-IOCG deposits is in the introduction of fluid-mobile components from subduction zones to the upper plate lithosphere. Deposits may form in direct association with arc magmatism or may be generated at some later time by remelting of deep lithospheric residues of prior arc magmatism. Under S-rich conditions in either setting, derivative magmas may go on to form porphyry Cu ± Mo ± Au deposits, whereas under S-poor conditions MH-IOCG deposit formation is more likely.

Abstract The Black Mountain Southeast Cu-Au-(Mo) porphyry system of the Baguio district, Northern Luzon, consists of two orebodies with a total resource of 65 Mt @ 0.40% Cu and 0.38 g/t Au. Detailed mapping, petrography, and geochemistry have identified six intrusive phases within the Black Mountain area. From oldest to youngest these are as follows: the Liw-Liw Creek hornblende megacrystic mafic dikes (Liw-Liw Creek; 3.20 ± 0.02 and 4.73 ± 0.17 Ma), the early mineralization quartz diorite, the plagioclase- and variably hornblende-phyric diorite (2.87 ± 0.08, 2.98 ± 0.02 and 2.83 ± 0.23 Ma), the hornblende megacrystic gabbro (2.81 ± 0.15 Ma), the hornblende-phyric basalt, and the aphanitic to plagioclase microphenocrystic fine-grained mafic dikes. The rocks of the Black Mountain area are low to medium K calc-alkaline intrusions; however, the intrusive history of the Black Mountain Southeast intrusive suite demonstrates an abrupt shift from megacrystic mafic dikes to voluminous stocks and plugs of relatively felsic equigranular and porphyritic intrusions, followed by a gradual transition to mafic fine-grained dikes. Hornblendes from the intrusive rocks fall into two groups: one formed at depth in a mafic magma and the other at shallower levels in a felsic magma. The presence of both groups within a single sample suggests mixing of a mafic and felsic magma. Porphyry mineralization in the Black Mountain area is interpreted to have formed as a result of underplating of a felsic magma chamber by a mafic magma that formed as a result of mantle recharge related to the subduction of the aseismic Scarborough Ridge.

Abstract Some of the world’s largest and highest grade alkalic porphyry Au-Cu-(Mo) deposits and related epithermal Au deposits occur in the southwest Pacific. Alkalic deposits of this region share many geologic similarities in their environments of formation. Source magmas are highly oxidized and alkali rich, being derived from enriched mantle sources that were previously modified by subduction processes. The more Cu rich systems formed by high K calc-alkalic and alkalic magmatism are typically located along the main magmatic arc. These subduction-related fluids and mantle-sourced mafic magmas evolve in an environment associated with a thickened crust. In contrast, more Au rich systems appear to be associated with rifting of oceanic crust in back-arc settings. Here, primitive mantle-derived magmas evolve in upper crustal magma bodies to form Au- and PGE- rich alkalic porphyry and epithermal deposits. The gold-rich alkalic porphyry and epithermal deposits formed in and along the margin of sedimentary basins that were intruded by alkalic dikes and stocks. In the largest example (Cadia East), deep mineralization is hosted by sheeted quartz-sulfide veins associated with potassic alteration, while near-surface mineralization is disseminated in both permeable clastic units and quartz-sulfide veins. Potassic alteration grades laterally into proximal, hematite-bearing propylitic alteration, and transitions upward from deep K-feldspar to shallow biotite-tourmaline. The shallow biotite alteration domain is overprinted by a complex, late-stage assemblage of pervasive K-feldsparalbite-sericite-pyrite, and structurally focused sericite-pyrite. In the alkalic epithermal environment, near-surface K-feldspar-quartz-carbonate-anhydrite (± sericite) alteration associated with epithermal Au-Ag mineralization occurs in and around dikes, fault intersections, and along extensive low-angle faults. Catrastrophic failure of the overlying volcanic edifice has the potential to cause superposition of alkalic epithermal mineralization onto porphyry deposits. Given their potential to form in a back-arc setting, alkalic porphyry deposits are considered more likely to be preserved in the ancient rock record than their calc-alkalic counterparts, due to burial in the sedimentary basins in which they form. Thus, areas of fragmented intraoceanic arc terranes within orogenic belts should be considered prospective for Au-rich alkalic porphyry deposits like those found in the southwest Pacific, particularly when they occur in regions overlain by postmineralization sedimentary and/or volcanic cover. Alkalic epithermal deposits offer more challenging exploration targets, as they are likely to be exhumed and eroded soon after their formation, unless a tectonic switch causes burial before any significant erosion occurs.

Abstract The world-class Zn-Pb-Ag deposits of the Red Dog district, Alaska, occur in severely shortened, late Paleozoic sedimentary rocks of the western Brooks Range fold and thrust belt. Red Dog-style strata-bound mineralization occurs within a localized Mississippian black shale facies of the Lisburne Group informally termed the Ikalukrok unit. Prior work has developed paleogeographic models in which the Ikalukrok unit formed in a starved, second-order basin flanked by carbonate platforms. Collision with and northward (modern coordinates) obduction of the oceanic Angayucham terrane telescoped Devonian to Early Jurassic passive margin sediments along thrust faults with displacements ranging from meters to tens or hundreds of kilometers. The most significant thrusts bound allochthons that juxtapose coeval stratigraphy from previously widely separated parts of the former continental margin. Thrusts within the allochthons created structurally and stratigraphically defined thrust plates and subplates, which are in turn deformed internally by smaller faults and associated folds. Steeply dipping extensional faults cut all compressional structures in the district. The Red Dog mine area and Anarraaq deposits are highly enriched in Zn and Pb owing to the superposition of as many as four phases of sulfide mineralization through carbonate replacement, brecciation, silicification, and veining. Barite and secondary silica are pervasive and intimately associated with base-metal mineralization in the mine area deposits. A giant barite body occurs in the structural hanging wall of the Anarraaq deposit but is spatially separated from it. The Ikalukrok unit hosts other strata-bound Zn-Pb-Ag deposits and occurrences in the Red Dog district that are referred to herein as laminated deposits. Sulfides within laminated deposits such as Su and Lik are typically laminated and brecciated. These deposits lack evidence of associated barite or widespread, multiphase massive base-metal sulfides, which form the high-grade cores of the mine area and Anarraaq deposits. We suggest that differences in character of Zn-Pb-Ag deposits in the Red Dog district can be attributed to variability in the original composition of the Ikalukrok unit host and the location of the deposits within the original subbasin.

Abstract Carlin-type ores have been reported in various locations around the world, but to date, the major economic deposits have been restricted to the Great Basin of the southwestern United States. Recent discoveries in east-central Yukon have many characteristics of Carlin-type deposits, and hold promise of great potential for new discoveries of economic importance. Both regions share commonalities of geologic history, including initial deposition of Proterozoic-Paleozoic calcareous host rocks on the passive margin of the fragmented Rodinian supercontinent. This was followed by compressional tectonism and continental accretion that included thrust faulting and plutonism through the late Paleozoic and Mesozoic. Many deposits in the Great Basin are associated with post-accretionary magmatism as the tectonic environment shifted to an extensional regime. However, at this early stage of investigation, the timing of mineralization in Yukon is not clear; the deposits may be geologically related to Late Cretaceous post-accretionary plutons. Associated gold skarn-style mineralization is present in both regions. Following mineralization, both regions experienced significant right-lateral transcurrent tectonism on their western margins; no known Carlin-type mineralization is associated with this latest tectonism. Mineralization in both areas comprises finely disseminated gold associated with arsenian pyrite hosted in calcareous siltstones-sandstones to silty carbonates, although other rock types locally host significant mineralization. Other hydrothermal minerals present in these systems include realgar/orpiment, stibnite, fluorite, barite, and quartz. Temperature of deposition appears to be near 225°C. Hydrothermal alteration consists of decarbonatization, silicification, and argillization. Gold/silver is typically high at 1:1 or higher. Trace elements that show good correlation with gold include thallium, arsenic, antimony, mercury, and to a lesser extent antimony and silver. In general, the deposits from the two areas are quite similar in terms of their tectonic history, and the processes and geochemistry appear to be very similar. The presence of extension and ore-related magmatism in Nevada appears to be a component that is much less clear in the Yukon Territory.

Abstract Goldrush is a Carlin-type sedimentary rock-hosted disseminated gold deposit located within the Cortez mining district on the Battle Mountain-Eureka trend, Nevada, USA. Goldrush is the third giant gold deposit (>310 metric tons Au or 10 Moz Au) discovered in the district after Pipeline (1991) and Cortez Hills (2002), and contains a measured and indicated resource of 59.8 Mt @ 4.35 g/t and an inferred resource of 39.2 Mt @ 4.52 g/t as of the end of 2012. Goldrush is concealed beneath unmineralized Paleozoic rocks as well as Tertiary and Quaternary postmineral tuffs, volcaniclastic sediments, and gravel ranging from more than 100 m to more than 300 m thick. The mineral system is tabular and continuous over a thickness of up to 70 m, a width of up to 250 m, and extends along strike for at least 4,000 m. Gold mineralization occurs within extensive zones of decarbonatization and silicification spatially associated with a stratigraphic horizon containing fossiliferous debris flows in thrust-faulted and folded Devonian carbonate rocks. The system is marked by a large stratiform silicified and sulfidized breccia horizon from 15 to 70 m thick that extends more than 7 km on a north-northwesterly strike; the strike length and continuity of this breccia zone make Goldrush unique compared with other Great Basin Carlin-type gold deposits. Gold occurs as submicroscopic inclusions within fine-grained pyrite, similar to other Carlin-type gold deposits in Nevada. The Goldrush discovery is attributed to a multiyear program utilizing open-pit and field mapping, detailed field, drill hole, and geochemical observations, and relogging of historic drill holes to construct new district- and deposit-scale geologic models. Barrick Exploration management provided strong support via a systematic, model-driven assessment process and funded deep drilling that ultimately resulted in the discovery. Persistence also played an important role as the discovery emerged over several years.

Abstract The northern Pacific Rim—for the purposes of this contribution—comprises the Mesozoic and Cenozoic magmatic-arc and associated terranes of eastern China, Korea, Japan, the Russian Far East, Alaska, Yukon, British Columbia, the western United States, and Mexico. This ~1,800-km-long segment of the Pacific Rim is marked by a broad spectrum of metallogenic environments and mining jurisdictions, which combine to dictate where and how exploration is conducted and the overriding character of any resulting discoveries. This summary report commences with a brief metallogenic overview of the northern Pacific Rim, with particular attention paid to the world-class Mesozoic and Cenozoic ore deposits that define the region’s premier metallogenic provinces. This is followed by a summary of the relative attractiveness of the region’s various mining jurisdictions, as recorded by recent exploration activity. The major discoveries made along the northern Pacific Rim, particularly during the past half century, are then placed in this metallogenic and regulatory context as a basis for determining the successful exploration methodologies employed. This discovery track record is then used to predict what the future of exploration in this vast and varied region may hold. Much of the northern Pacific Rim, from eastern China and the Russian Far East in the northwest through Alaska to western parts of Canada, the United States, and Mexico in the southeast (Fig. 1), is characterized by a complex array of oceanic, accretionary prism, magmatic arc, and back-arc basin terranes and associated microcontinental blocks accreted to the North China, Siberian, Hyperborean, and North American cratons, mainly during Mesozoic times (Coney et al., 1980; Campa and Coney, 1983; Kojima, 1989; Nokleberg et al., 2005; Yakubchuk, 2009). The metallogeny of these tectonic collages is dictated by various combinations of pre-, syn-, and postaccretion ore-forming events, the last of which are generally preeminent, except in British Columbia (Nokleberg et al., 2005; Nelson and Colpron, 2007). Although the Meso-Cenozoic metallogeny of the northwestern and northeastern Pacific quadrants displays some similarities, it is the contrasts that are most marked. The main contrasts stem from the preeminence of tin, tungsten, and antimony in eastern China, Korea, Japan, and the Russian Far East and of copper and silver in Western Canada, the conterminous United States, and Mexico. Nonetheless, both the northwestern and northeastern Pacific quadrants are exceptionally well endowed with gold and molybdenum deposits. The northeasternmost Russian Far East, Alaska, and Yukon Territory display elements of both northwestern and northeastern Pacific metallogeny (Fig. 1). These metallogenic contrasts between the northwestern and northeastern quadrants result in China being the world’s leading producer of tungsten, tin, bismuth, and antimony, mostly from its eastern Mesozoic metallogenic province.