Abstract The Marigold Au deposits are located in the Battle Mountain mining district at the northern end of Nevada’s Battle Mountain-Eureka trend. The Marigold deposits currently make up the second largest Au accumulation in the district with over 320 tonnes (10.35 Moz) of Au in oxidized rock in a N-trending series of mineralized zones approximately 7.5 km long. Ore is hosted primarily in oxidized Paleozoic siliciclastic rocks between the Roberts Mountain and Golconda thrusts. Most of the ore occurs in quartzite of the Ordovician Valmy Formation. Higher grades but lower tonnages of ore are present in the overlying Pennsylvanian-Permian Antler sequence, including the Battle Formation conglomerate, the Antler Peak Limestone, and debris flows and siltstone of the Edna Mountain Formation. Sedimentary rocks at Marigold are crosscut by a series of WNW- to N-striking quartz monzonite dikes (zircon U-Pb chemical abrasion-thermal ionization mass spectrometry ages 97.63 ± 0.05–92.22 ± 0.05 Ma) and a lamprophyre (biotite 40 Ar/ 39 Ar age 160.7 ± 0.1 Ma). Marigold displays many classic Carlin-type characteristics although the deposits are predominantly hosted in relatively unreactive, carbonate-poor siliciclastic rocks. Sulfidation, minor silicification, and possibly pyritization occurred in association with Au mineralization in quartzite and argillite. Chemically reactive but volumetrically minor carbonate rocks also display these alteration styles as well as significant decarbonatization. Argillic alteration occurred proximal to faults in mudstone and siltstone and at the margins of intrusions. Gold, As, Sb, and Tl are enriched along high-angle structures and structural intersections in the sedimentary host rocks and in faulted dike margins. Gold is present in Au-, As-, and Sb-rich pyrite overgrowths on pre-gold stage trace element-poor pyrite grains. Oxidation extends to depths of 150 to 500 m below surface, and above the redox boundary Au is present natively with iron oxides in voids and fractures. In the cores and margins of the Cretaceous dikes and fault zones, a distinct geochemical association of base metal and Ag minerals is identifiable, characterized by Ag-bearing tetrahedrite-tennantite, chalcopyrite, gersdorffite, pyrite, sphalerite, stannite, and galena. Sericite 40 Ar/ 39 Ar ages of 88.0 ± 0.46 and 79.59 ± 0.16 Ma indicate that hydrothermal alteration occurred along the dike margins at least 4 m.y. after emplacement. On the basis of similarities to other deposits in the district, the base metal and Ag mineralization may have occurred at this time. The Au mineralization occurred sometime after the base metal and Ag event, possibly in conjunction with the Eocene magmatism that occurred elsewhere in the district, although this study found no definitive evidence for a magmatic-hydrothermal origin of the Au.

Abstract This contribution provides brief introductions to research on Carlin-type gold deposits completed since publication of the 2005 review paper on the deposits in the Economic Geology 100th Anniversary Volume ( Cline et al., 2005 ). Major advances in our understanding of the deposits have resulted from these studies that cover a broad range of topics, from the geology of deposits to recent discoveries and current geologic models. Studies of host rocks include expanded application of sequence stratigraphy that is refining our understanding of favorable host rocks, now known to have formed on shallow carbonate platforms during lowstands as well as in deep-water slope to basin environments. Sparse igneous dikes at the surface that were emplaced coincident with formation of deposits of the Carlin trend indicate that a batholith of about 1,000 km 2 underlies the trend. Reactivated and inverted normal Neoproterozoic faults formed anticlines and fed ore fluids into structurally prepared reactive rock types. Collaborative district studies determined that structural preparation of host rocks along the Carlin trend occurred during three discrete contractional events followed by Eocene extension and coincident mineralization. Ore and alteration studies identified systematic trace element and sulfur isotope zoning in ore-stage pyrite rims that formed from temporally discrete ore fluids fed by separate structures. Deposit-scale studies determined that ore minerals in shallowly formed deposits are similar to late ore-stage minerals of typical, more deeply formed Carlin-type gold deposits. Breccias containing high-grade ore formed both by replacement and by calcite dissolution and collapse processes. Halos useful in vectoring toward mineralization include rock quality designation values, trace elements above mineralization in premineral rock and in postmineral clay, oxygen isotope ratios, and soil, soil gas, vegetation, and groundwater chemistry. Isotopic studies have indicated relative timing of ore fluid movement through discrete structures. Deposit ages coincide with spatially related intrusion ages, from about 42 to 35 Ma, and both young from northeast to southwest. Magmatism and deposit formation are interpreted as related to Eocene delamination of subcontinental lithospheric mantle. Apatite fission track data indicate that the Betze-Post deposit, which contained >1, 240 tonnes (40 Moz) of gold, formed in <15,000 to 45,000 years. New geologic maps illustrate structural and stratigraphic relationships that will contribute to exploration efforts and potential new discoveries. Recent Nevada discoveries include South Arturo on the northern Carlin trend, the Long Canyon deposit in Cambrian-Ordovician rocks in the newly recognized Pequop district in northeastern Nevada, the giant Goldrush deposit on the Battle Mountain-Eureka trend, and the North Bullion deposit at the southern end of the Carlin trend. Two potential new districts of deposits are being actively explored in the Yukon Territory, Canada, and the Golden Triangle, southern China. Deposits in the Golden Triangle and prospects in the Yukon are currently much smaller than deposits in Nevada, and the presence of proximal coeval magmatism, now recognized in Nevada, is unclear. Studies of some of the Chinese deposits indicate that they formed at conditions intermediate to Carlin-type and orogenic deposits. Recently published geologic models propose that either shallow, basin-related processes or deep magmatic processes provided gold for the Nevada deposits. Studies evaluating the Harrison Pass pluton and the Emigrant Pass volcanic rocks, both the same age as the Carlin deposits, addressed the magmatic model and provide information about potential magmatic ore fluids and systems that may have formed the deposits.

Abstract The genesis of gold deposits along the Carlin trend is not fully understood. Many of the significant mineral deposits in the Carlin trend were formed during the Tertiary as a result of interrelated high-angle basin-and-range faulting, intrusive igneous activity, and hydrothermal processes (Radtke, 1985). According to Shawe (1991), the linearity of the gold deposits along the Carlin trend and the sub-parallel Battle Mountain-Eureka trend suggests that deep-penetrating regional structures controlled the emplacement of magmas generated in the lower part of the crust or upper mantle, and either provided hydrothermal fluids or caused heating of ground waters that were responsible for transport and deposition of the gold ores. To investigate crustal processes that may have contributed to the genesis of gold deposits along the trend, a regional southwest to northeast profile of magnetotelluric (MT) soundings was acquired in 1996 (line MT-MT′, Fig. 1). Two-dimensional resistivity modeling of the MT profile is being used to place constraints on possible heat or magma sources and possible tectonic controls on the linear distribution of mineral deposits.

Abstract Two subparallel NNW-to NW-trending mineral belts in Nevada, the Battle Mountain-Eureka trend on the SW and the Carlin trend on the NE, are thought to reflect deep-seated, pre-Cenozoic crustal structures. These structures may be pre-Cenozoic faults, Mesozoic and/or Paleozoic fold axes, or uncertain features of the Precambian basement. Both geophysical and geochemical/isotopic studies can be used to complement field based geologic studies of these features. Geophysical studies measure time-integrated physical parameters and attempt to distinguish younger from older features. Isotopie studies have the advantage of investigating time-related features by comparing the isotopie compositions of rocks formed at different times during the geologic history of a region for systematic or significant changes or lack there of. The isotopie signatures of igneous rocks largely reflect the average characteristics of their source regions plus any later interaction with the crustal column through which they moved or into which they were emplaced, and in general reflect lower and middle crustal features. Kistler and Peterman (1973, 1978) and Kistler (1983, 1991) demonstrated that the distribution of Sr isotopie compositions of granitoid rocks in the northern Great Basin delineate crustal structure, particularly the location of the continental-oceanic crustal boundary as marked by the I sr = 0.706 line. Ellison et al. (1990) showed that the I sr =0.706 Ime is correlated with Paleozoic stratigraphy. Farmer and DePaolo (1983, 1984) used combined Nd and Sr isotopie compositions of Great Basin granitoids to study the pedogenesis of these rocks and regional crustal structure; however, these pioneering studies are limited by the

Abstract The gold-rich Superior, Canada, and Yilgarn, Australia, cratons have similar geologic histories dating back to the Mesoarchean and showing strong parallels in the Neoarchean. Orogenesis in each craton is marked by a shift from dominant volcanism to dominant clastic sedimentation above unconformities, followed by granitic plutonism, progressive deformation, and dynamothermal metamorphism. The terminal stages of orogenesis correspond to the intervals of 2660 to 2650 Ma in the Superior craton and 2660 to 2630 Ma in the Yilgarn cra ton. The Yilgarn and Superior cratons contain an estimated 9,200 and 8,500 t Au, respectively. Most of the sig nificant gold deposits (>100 t Au) are concentrated in a few narrow, highly endowed gold belts along which the deposits cluster into camps, commonly spaced every 30 to 50 km. Gold deposits of both cratons show similar tonnages and grades, and their size distributions define a Pareto, rather than log-normal distribution. Large deposits are rare but account for most of the gold endowment of each craton. Three recurring host-rock associations account for a majority of large deposits: iron-rich mafic igneous rocks, iron-rich sedimentary rocks, and felsic to intermediate porphyry stocks and dikes. Most deposits, particularly large ones, occur in greenschist-grade rocks and are associated with shear zones, faults, or folds. Gold miner alization styles include quartz-carbonate veins, sulfidic replacements in banded iron formation (BIF), crusti form carbonate-quartz veins and associated sulfidic replacement lodes, disseminated-stockwork zones, sulfide rich veins and veinlet zones, and massive sulfide lenses. Wall-rock alteration assemblages vary with mineralization style and metamorphic grade. Most deposits consist of a single style of mineralization, but many of the large ones combine two or more of these, and some large deposits are unique in their metal associations. The diversity of styles of mineralization, wall-rock alteration assemblages, and overprinting relationships re quire more than one episode of gold mineralization and more than one ore-forming process. Geologic parage neses, coupled with isotopic age constraints, show that, although the Archean histories of both cratons span >300]m.y., the majority of gold deposits formed during the final 30 to 50 m.y. of that time span, corresponding to the orogenic phase. The majority of gold deposits can thus be regarded as orogenic in timing, but with the available constraints clearly pointing to the existence of more than one mineralizing event and involving dif ferent mineralization types and processes. The best-endowed gold camps (Timmins and Red Lake in the Superior craton; Kalgoorlie, Granny-Wallaby, and Sunrise Dam in the Yilgarn craton) commonly possess an anticlinorial structure, komatiitic and basaltic rocks in the core giving way to stratigraphically higher volcanic and clastic sedimentary rock units. Such camps are further marked by coarse clastic rocks deposited above the metavolcanic rock sequences, by concentrations of shallow-level porphyritic intrusions, by extensive carbonate alteration, by multiple styles and ages of gold mineralization and, in most, by through-going regional faults. However, these characteristics are also shared by a number of less-endowed gold camps. The best-endowed gold belts are distinguished by substantial volumes of komatiite, by a high degree of preservation of supracrustal rocks, by structural highs that juxtapose the lower and uppermost parts of the stratigraphic column, by multiple styles and ages of gold mineralization, and by world-class deposits of other metals. The gold belts commonly are aligned along crustal-scale faults that rep resent long-lived structures, which acted as crustal-scale magma and fluid conduits and also influenced coarse clastic sedimentation. Abundant komatiites may reflect the first tangible connection to the deep crust and mantle, but the nature of the subvolcanic crust, ensimatic in the Timmins-Val d’Or, Superior craton, and ensialic in the Wiluna-Norseman belts, Yilgarn cration, seems unimportant in determining gold prospectivity. Significant uncertainty remains concerning the timing of formation of the deposits, the models that best explain their characteristics, and the fundamental causes of the high concentration of gold in a few areas. Various models have been proposed, invoking volcanic, magmatic, and orogenic (metamorphism and/or deformation) processes. The synorogenic model best accounts for the Au-only quartz-carbonate veins and temporally related mineralization styles. However, synvolcanic and magmatic hydrothermal models are also required to explain the presence of Au base metal deposits and those deposits overprinted by significant deformation and meta morphism. The specific histories of the gold belts and the known constraints on timing of deposits suggest that all of these processes have contributed to the gold endowment, although it is difficult to separate orogenic from magmatic processes because they closely overlap in time and space. Despite the presence of synorogenic quartz-carbonate veins throughout the greenstone belts of both cratons, large deposits of this style are mainly restricted to the gold belts, where they also coexist with large deposits of other styles of gold mineralization. The presence of multiple ages and styles of gold mineralization in the best-endowed gold belts indicate a unique locus of successive formation of gold deposits from various processes operating at different stages of the orogenic phase of the evolution of these belts. This would explain the common overprinting of the early deposit types, potentially of synvolcanic or synplutonic origin, by syn orogenic ones. The concentration of multiple types and ages of significant gold deposits in well-defined gold belts is not a unique feature of the Superior and Yilgarn cratons but is shared by Tertiary gold belts of Nevada, such as the Walker Lane, the Battle Mountain-Eureka trend, and the Carlin trend. This must be a reflection of fundamental crustal structure, and perhaps composition of subcrustal mantle, as much as local ore-forming hydrothermal processes.

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.