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The Willow Creek mining district was the third-largest lode gold district in Alaska in the 20th century, having produced 19 metric tons (t) of gold from vein deposits and another 2 t of gold from associated placer deposits. The district is located in the southern Talkeetna Mountains, north of the Castle Mountain strike-slip fault system. Most gold occurs in widespread mineralized quartz veins hosted by granitic rocks of the Willow Creek pluton. A geochronologic study of granitic rocks, alteration minerals, and adjacent metamorphic rocks was conducted in order to better understand the timing of magmatism, thermochronologic evolution, and age of gold mineralization in the district. New U-Pb zircon dates from the Talkeetna batholith, including the Willow Creek pluton, range from 72.1 to 71.3 Ma. The U-Pb dates are similar to reported U-Late Cretaceous Pb and K-Ar dates from elsewhere in the study area, and tightly constrain the age of plutonic rocks that host the Willow Creek lode gold deposits. Granitic rocks that intrude the Willow Creek pluton along its northwestern margin yield U-Pb dates of 75.7 and 70.8 Ma. The 40Ar/39Ar thermochronologic data for the Willow Creek pluton and other granitoids reveal rapid postemplacement cooling through hornblende, muscovite, and biotite closure temperatures. Thermochronologic data from K-feldspars document slower cooling rates through lower temperatures, which may be related to slow uplift and exhumation during the Paleocene-Eocene. The data are compatible with a simple tectonothermal history following emplacement of the plutonic rocks, with essentially no evidence of significant thermal disturbance of the rocks after about 64 Ma. Uranium-thorium-lead dating of hydrothermal phosphate minerals (monazite and xenotime) and 40Ar/39Ar dates from hydrothermal sericite from wall rocks and veins indicate that the main stage of gold mineralization occurred at about 67 to 65 Ma. Combining U-Th-Pb data from monazites from the Independence, Talkeetna, Gold Bullion, and Fern deposits gave an average of 65.4 ± 2.3 Ma; hydrothermal xenotime from the Independence vein also yields a statistically indistinguishable age of 64.5 ± 2.3 Ma. These ages overlap 40Ar/39Ar dates of 67 to 66 Ma obtained from hydrothermal sericite associated with formation of the gold-bearing veins, and the age of a muscovite from a weakly mineralized pegamatite that cuts the Willow Creek pluton. In contrast to previous studies that suggested that at least some mineralization was Eocene in age, we found no compelling evidence for significant gold mineralization within the district at this time. The 40Ar/39Ar and K-Ar dates from graphitic white micas from schist south of the Hatcher Pass fault zone yield ages of 61 to 50 Ma that are probably related to uplift and cooling of these Late Cretaceous metasedimentary rocks. The absence of evidence for postemplacement thermal events affecting the plutonic rocks of the Willow Creek district to the north indicates that the schists and granitoids do not share a similar tectonothermal history, and that they have been juxtaposed by subsequent faulting along the Hatcher Pass fault zone. [ABSTRACT FROM AUTHOR]

West Africa, with presently an approximate 10,000-metric ton (t) gold endowment, is one of the world's great gold provinces and the largest Paleoproterozoic gold-producing region. The gold resources are concentrated within the 2250 to 2000 Ma greenstone belts of the Man-Leo shield, forming the southern part of the West Africa craton. Most of the major orebodies are best classified as orogenic gold deposit types, although there are paleoplacer and porphyry-skarn deposits within some of the greenstone belts, and perhaps local intrusionrelated gold systems. The gold-hosting, mainly greenschist metamorphic facies greenstone belts are dominated by tholeiitic volcanic rocks, with clastic and chemical sediments filling adjacent subbasins. The Paleoproterozoic sequences formed in what was likely a rift or series of rifts in a Precambrian cratonic block; it is not clear whether significant Late Archean lithospheric roots occur below these Paleoproterozoic arcs that formed in the resulting ocean subbasins. Although diachronous across West Africa, the Eburnean orogeny is typically indicated to have been initiated at ca. 2130 Ma, with closure of the subbasins, amalgamation of the Paleoproterozoic arcs, and their accretion back to the continental margin of Archean rocks. Compressional tectonics took place for about 25 to 30 m,y, with widespread crustal thickening along orogen-parallel, commonly NE-trending, firstorder thrust fault systems. This was followed by more than 100 m.y. of transcurrent tectonism and associated exhumation; gold ores mainly formed late during the Eburnean deformation. The most productive orogenic gold deposits are located in greenstone belts in Ghana, Mali, Senegal, Burkina Faso, Cote d'Ivoire (or Ivory Coast), and Guinea. Yielding about 2001 Au per year, West African production has exceeded that of the Yilgarn craton of Western Australia since 2007 and, if grouped together, current annual production from these relatively small countries would only be surpassed by China, Australia, and Russia, demonstrating the global significance of the gold mineralization within this region. With an endowment of >2,500 t Au, the Obuasi deposit represents the largest single Precambrian gold deposit discovered in the world to date, exclusive of the Witwatersrand paleoplacers. More than 25 of the deposits in the Man-Leo shield contain resources in excess of 1001 Au. The gold-bearing brittle-ductile quartz veins, stockworks, breccias, and disseminated orebodies are located adjacent to major faults, typically in areas of second-order shears, large dilational jogs, regional fold systems, and rheological contrast. Mineralogy, alteration, structural geology, stable isotope geochemistry, and P-T conditions of gold deposition are typical of those observed in most orogenic gold provinces. Well-constrained absolute ages for much of the orogenic gold formation remain lacking, but there seems to be a temporal association with the 2100 Ma onset of transpression/strike-slip and exhumation. However, ore deposition was likely spread over at least 130 m.y. throughout the shield, with some ores as old as ca. 2160 Ma (e.g., Wassa) and some no older than ca. 2030 Ma (e.g., Damang). It is unclear as to exactly why such a large gold endowment is present in West Africa and, in particular, the Birimian sequences of the Man-Leo shield, although it is likely the consequence of a combination of key regional factors. Relative to other Paleoproterozoic orogens, the abundance of carbonaceous oceanic sediments that overly the Birimian basalts may represent an exceptionally fertile source of both fluid and metal. The fact that many of the orogenic gold deposits formed from ore fluids with at least 70 to 80 mol % CO2, quite different from deposits elsewhere in the world where H2O >CO2, hints at an atypically large volume of carbon being released during metamorphic devolatilization. A series of closing and subsequently inverting basins, which were important sinks for the carbonaceous material, also resulted in a favorable structural architecture with development of many orogen-parallel, deep-crustal shear zones. Two hundred million years of orogenesis represented a lengthy period of deformation, including strike-slip reactivation events along the older thrusts that may have allowed for diachronous gold-forming events throughout the Eburnean. [ABSTRACT FROM AUTHOR]

The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, the largest historic gold producer ol the North American Cordillera, comprises both steeply dipping E-W veins located along lithologic contacts in accreted ca. 300 and 200 Ma oceanic rocks and shallowly dipping N-S veins hosted by the Grass Valley granodiorite; the latter have yielded about 70% of the 13 million ounces of historic lode gold production in the district. The oceanic host rocks were accreted to the western margin of North America between 200 and 170 Ma, metamorphosed to greenschist and amphibolite facies, and uplifted between 175 and 160 Ma. Large-scale magmatism in the Sierra Nevada occurred between 170 to 140 Ma and 120 to 80 Ma, with the Grass Valley granodiorite being emplaced during the older episode of magmatism. Uranium-lead isotope dating of hydrothermal xenotime yielded the first absolute age of 162 ± 5 Ma for the economically more significant N-S veins. The vein-hosted xenotime, as well as associated monazite, are unequivocally of hydrothermal origin as indicated by textural and chemical characteristics, including grain shape, lack of truncated growth banding, lack of an Eu anomaly, and low U and Th concentrations. Furthermore, the crack-seal texture of the veins, with abundant wall-rock slivers, suggests their formation as a result of episodic fluid flow possibly related to reoccurring seismic events, rather than a period of fluid exsolution from an evolving magma. The N-S veins are temporally distinct from a younger 153 to 151 Ma gold event that was previously reported for the E-W veins. Overlapping U-Pb zircon (159.9 ± 2.2 Ma) and 40Ar/39Ar biotite and hornblende (159.7 ± 0.6-161.9 ±1.4 Ma) ages and geothermobarometric calculations indicate that the Grass Valley granodiorite was emplaced at ca. 160 Ma at elevated temperatures (~800°C) within approximately 3 km of the paleosurface and rapidly cooled to the ambient temperature of the surrounding country rocks (<300°C). The age of the granodiorite is indistinguishable from that of the N-S veins, as recorded by the U-Pb age of xenotime in those veins. Consequently, the N-S veins must have formed between 162 and 157 Ma, the maximum permissive age of magma emplacement and the youngest permissive xenotime U-Pb age, respectively, during an E- to ENE-directed compressional regime. The geochemistry of die Grass Valley granodiorite is consistent with it being the product of arc magmatism. It served as a receptive host for mineralization, but it is has no direct genetic relationship to gold mineralization. Initial uplift of the intrusive mass correlates with the initial voluminous fluid flow event and vein formation at depths of no greater than 3 km. The E-W gold-bearing veins hosted within greenschist-facies country rocks adjacent to the intrusion formed during a second hydrothermal event 5 to 10 million years later than the magmatism and were contemporaneous with a shift to transtensional deformation denoted by sinistral strike-slip faulting. [ABSTRACT FROM AUTHOR]

The Pebble Cu-Au-Mo deposit in southwestern Alaska, containing the largest gold resource of any known porphyry in the world, developed in a tectonic setting significantly different from that of the present-day. It is one of a series of metalliferous middle Cretaceous porphyritic granodiorite, quartz monzonite, and diorite bodies, evolved from lower crust and metasomatized lithospheric mantle melts, which formed along much of the length of the North American craton suture with the Peninsular-Alexander-Wrangellia arc. The porphyry deposits were emplaced within the northernmost two of a series of ca. 130 to 80 Ma flysch basins that define the suture, as well as into arc rocks immediately seaward of the two basins. Deposits include the ca. 100 to 90 Ma Pebble, Neacola, and other porphyry prospects along the Kahiltna basin-Peninsula terrane boundary, and the ca. 115 to 105 Ma Baultoff, Carl Creek, Horsfeld, Orange Hill, Bond Creek, and Chisna porphyries along the Nutzotin basin-Wrangellia terrane boundary. The porphyry deposits probably formed along the craton margin more than 1,000 km to the south of their present latitude. Palinspastic reconstructions of plate kinematics from this period are particularly difficult because magmatism overlaps the 119 to 83 Ma Cretaceous Normal Superchron, a period when sea-floor magnetic data are lacking. Our flavored scenario is that ore formation broadly overlaps the cessation of sedimentation and contraction and the transition to a transpressional continental margin regime, such that the remnant ocean basins were converted to strike-slip basins. The basins and outboard Peninsular-Alexander-Wrangellia composite superterrane, which are all located seaward of the deep crustal Denali-Farewell fault system, were subjected to northerly dextral transpression for as long as perhaps 50 m.y., beginning at ca. 95 ± 10 Ma. The onset of this transpression was marked by development of the mineralized bodies along fault segments on the seaward side of the basins. Geochemical and radiogenic isotopic data for igneous rocks associated with the Pebble porphyry deposit suggest continuous melt derivation from enriched lithosphere of a recently metasomatized mantle. These geochemical characteristics, coupled with the arc-continent-related collisional setting, suggest that lithospheric thickening and postcollisional lithospheric melting are the most likely cause of the ore-related magmatism. Subsequent to translation of the Alaskan margin terranes mad early Tertiary oroclinal bending of Alaska, the northernmost Kahiltna basin and the Pebble deposit, as well as the other porphyry systems, reached their present-day locations along southern Alaska. [ABSTRACT FROM AUTHOR]

The type examples for the class of deposits termed intrusion-related gold svstems 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-hearing monzo-granites, 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 87Sr/86Sr from 0.711-0.714) and Pb isotope ratios (206Pb/204Pb from 19.2-19.7), and negative initial εNd 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. [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue, including one on the evolution of Earth's sulfur cycle and its importance in defining the metallogenic history of several base metal-rich deposit types, one on the long-term relationships between manganese ores and evolving seawater chemistry, and another on a description of four metallogenci periods.

Early Cretaceous orogenic gold deposits in eastern Asia are globally unique in that large Phanerozoic lode gold deposits occur in Archean-Paleoproterozoic cratons. In the northern Pacific region, ca. 125 Ma orogenic gold deposits in the North China, Yangzte, and Siberian craton margins, as well as in young terranes in California, may ultimately relate to the giant Cretaceous mantle plume in the southern Pacific basin and the relatively rapid tectonic consequences along both continental margins from resulting Pacific plate reconfigurations. In eastern Asia, such consequences include reactivation of and fluid flow along major fault systems, with fluid focusing into simultaneously forming, isolated core complexes of uncertain genesis. Deposition of gold ores in previously devolatilized high-grade Precambrian metamorphic rocks requires an exotic source of ore fluid, most likely subducted Mesozoic oceanic crust and/or overlying sediment. An implication is that Phanerozoic metamorphic core complexes in other destabilized craton margins could host large gold resources. [ABSTRACT FROM AUTHOR]

The Scheelite dome intrusion-related gold deposit, western Selwyn basin, Yukon, is hosted in hornfelsed metasedimentary strata that lie adjacent to the exposed apices of a monzogranite to quartz monzonite plutonic complex of the mid-Cretaceous Tombstone-Tungsten magmatic belt, Tintina gold province, Maska and Yukon. A variety of mineralization styles occur throughout a 10- x 3-km east-trending corridor and include reduced Au- and W-rich skarns, Au-, W- and Ag-Pb-Zn-Sb-rich quartz tension-vein arrays, and multiphase fault veins and isolated zones of Au-rich sericite-carbonate altered rock. Integrated U-Pb SHRIMP data for magmatic zircon and Ar-Ar data for magmatic and hydrothermal biotite indicate that gold mineralization occurred within 1 to 2 m.y. of magma emplacement. Fluid inclusion, oxygen isotope, and arsenopyrite geothermometry data indicate that hydrothermal minerals formed at depths of 6 to 9 km over a temperature range from <300° to >550°C. High-temperature Au-rich skarns formed at >400°C, whereas vein-hosted mineralization formed at 280° to 380°C. In skarns, Au is strongly associated with enrichments of Bi, Te, W, and As, whereas a variety of Au-rich veins occur, with Asrich (type 1), and Te- and W-rich (type 2) end members. Silver-Pb-Zn-Sb veins are typically Au poor and represent the latest and lowest temperature phase in the hydrothermal paragenesis. The fluid inclusion data indicate that all mineralization styles were formed from low-salinity (≤4 wt % NaC1 equiv) aqueous-carbonic fluids, consistent with the composition of fluid inclusions within infilled miarolitic cavities in the intrusive rocks. However, the nonaqueous fluid was predominantly CH4 in skarn, CO2 in Au-Te and Au-W veins, and a fluid with roughly equal amounts of CO2, CH4, and N2 in Au-As and Ag-Pb-Zn-Sb veins. Oxygen isotope data are consistent with a mineralizing fluid of predominantly magmatic origin that was variably modified to more positive δ18O values during interaction with 18O-enriched metasedimentary strata. Sulfur isotope data suggest two possible sources of sulfur, a magmatic source characterized by 634S values of approximately -5 to 0 per mil and sulfur from the metasedimentary country rocks characterized by more negative 634S values of approximately -15 to -10 per mil. Collectively the data indicate that gold at Scheelite Dome was deposited from a magmatic-hydrothermal system. Interaction of magmatic fluids with graphitic hornfels rocks resulted in reduction of the ore fluids, higher CH4/CO2 ratios, and modification of the oxygen and sulfur isotope values of the ore fluids toward those of the metasedimentary hornfels. Progressive reduction and cooling of hydrothermal fluids, in addition to phase separation in vein-hosted mineralization, were the mechanisms for gold deposition. Compared to other intrusion-related gold deposits associated with the Tombstone-Tungsten magmatic belt magmatism, exposed mineralization at Scheelite Dome is predominantly hosted by hornfelsed metasedimentary rocks. This results in more diverse mineralization styles and a greater spread of isotope and fluid inclusion data. [ABSTRACT FROM AUTHOR]

Mineral deposit types commonly have a distinctive temporal distribution with peaks at specific periods of Earth history. Deposits of less redox-sensitive metals, such as gold, show long-term temporal patterns that relate to first-order changes in an evolving Earth, as a result of progressively declining heat production and attendant changes in global tectonic processes. Despite abundant evidence for plate tectonics in the early Precambrian, it is evident that plume events were more abundant in a hotter Earth. Episodic growth of juvenile continental crust appears to have been related to short-lived (< 100 m.y.) catastrophic mantle-plume events and formation of supercontinents, whereas shielding mantle-plume events correlated with their breakup. Different mineral deposit types are associated with this cycle of supercontinent formation and breakup. Broadly synchronous with juvenile continental crust formation was the development of subcontinental lithospheric mantle, which evolved due to progressively declining heat flow and decreasing plume activity. Archean subcontinental lithospheric mantle has a distinct mineralogical composition and is buoyant, whereas later lithosphere was progressively more dense. Changes in the buoyancy of both oceanic lithosphere and subcontinental lithospheric mantle led to evolution of tectonic scenarios in which buoyant, roughly equidimensional, early Precambrian cratons were rimmed by Proterozoic or Phanerozoic linear elongate belts of neutral to negative buoyancy. Orogenic gold deposits, which formed over at least 3.4 b.y., had the highest preservation potential of any gold deposit type. The pattern of formation and preservation, from episodic to more cyclic, broadly mirrors that of crustal growth. Early Precambrian (mostly ca. 2.7 and 2.0-1.8 Ga) deposits, protected from uplift and erosion in the centers of buoyant cratons, are rare between ca. 1.7 Ga and 600 Ma due to the change to more modern-style plate tectonic processes, with... [ABSTRACT FROM AUTHOR]

The Donlin Creek gold deposit, southwestern Alaska, has an indicated and inferred resource of approximately 25 million ounces (Moz) Au at a cutoff grade of 1.5 g/t. The ca. 70 Ma deposit is hosted in the Late Cretaceous Kuskokwim flysch basin, which developed in the back part of the arc region of an active continental margin, on previously accreted oceanic terranes and continental fragments. A hypabyssal, mainly rhyolitic to rhyodacitic, and commonly porphyritic, 8- x 3-km dike complex, part of a regional ca. 77 to 58 Ma magmatic arc, formed a structurally competent host for the mineralization. This deposit is subdivided into about one dozen distinct prospects, most of which consist of dense quartz ± carbonate veinlet networks that fill north- northeast-striking extensional fractures in the northeast-trending igneous rocks. The sulfide mineral assemblage is dominated by arsenopyrite, pyrite, and, typically younger, stibnite; gold is refractory within the arsenopyrite. Sericitization, carbonatization, and sulfidation were the main alteration processes. Fluid inclusion studies of the quartz that hosts the resource indicate dominantly aqueous ore fluids with also about 3 to 7 tool percent CO2 ± CH4 and a few tenths to a few mole percent NaCl + KCl. The gold-bearing fluids were mainly homogeneously trapped at approximately 275° to 300°C and at depths of 1 to 2 km. Some of the younger stibnite may have been deposited by late-stage aqueous fluids at lower temperature. Measured δ18O values for the gold-bearing quartz range between 11 and 25 per mil; the estimated δ18O fluid values range from 7 to 12 per mil, suggesting a mainly crustally derived fluid. A broad range of measured 6D values for hydrothermal micas, between -150 and -80 per mil, is suggestive of a contribution from devolatilization of organic matter and/or minor amounts of mixing with meteoric fluids. Gold-associated hydrotherrnal... [ABSTRACT FROM AUTHOR]

Defines the areas of uncertainty in the understanding of gold deposits in metamorphic rocks within orogenic belts. Potential diversity of gold deposits types in metamorphic belts; Most common type of orogenic gold deposits; Information on intrusion-related gold deposits.

The article presents comments on the approach of researchers Stephen E. Kesler and Bruce H. Wilkinson to understanding the role of exhumation in the temporal distribution of ore deposits. It discusses problems with the procedure as applied to orogenic gold deposits, including the low exhumation rate for the deposits. Also stressed is that the function of the methodology should be viewed in the context of changes in the thermal state of the Earth and resultant tectonic and lithospheric evolution.

The article presents a broad scale overview of the Paleoproterozoic gold endowment of Man Leo shield, southern part of the West African craton; and mentions the geology of the region, the historic evolution of gold production and the geologic correlation with the Guiana shield goldfields.

The article presents the text of the speech by Richard J. Goldfarb in accepting the Society of Economic Geologists (SEG) Ralph W. Marsden Award for 2011 delivered on September 24, 2012 at Westin Lima Hotel and Convention Center in Lima, Peru. Goldfarb offered his outlook on receiving the Marsden medal. Goldfarb reflected on volunteer opportunities offered by SEG. He acknowledged several persons who contributed to his success in the field of economic geology.