This paper reports the structural and stratigraphic history of Buck Mountain, Nevada, and its regional significance in the development of southwestern Laurentia during the late Paleozoic. The two distinct generations of folding have similar style and/or timing to other fold sets in late Paleozoic strata of northern Nevada. Unconformities in the upper Paleozoic strata at Buck Mountain are consistent with unconformities documented in northern and east-central Nevada. Northwest-vergent folds (F 1 ) in the Morrowan–Atokan Ely Limestone are erosionally truncated and unconformably overlain by the middle Desmoinesian Hogan Formation and middle Wolfcampian (Sakmarian) Upper Strathearn Formation. This upper Paleozoic stratigraphic package was subsequently refolded by the Buck Mountain Syncline and associated mesoscale folds (F 2 ). F 2 folds lack tight age control but are interpreted to be associated with the Cretaceous central Nevada thrust belt. Critically, none of these structures are localized above or below low-angle faults. The unconformity between the Ely and Hogan formations is consistent with the C5 regional unconformity. Importantly, it constrains the age of northwest-vergent deformation on Buck Mountain. West-vergent folds and west-directed thrusts are documented at several locations in northern and east-central Nevada, but because of the dominance of the C6 unconformity and/or lack of robust age control, the age of these structures has not been tightly constrained. The evidence at Buck Mountain indicates that west-vergent structures predate the C5 unconformity. Buck Mountain is important because it: (1) precisely brackets the age of west-vergent deformation in Nevada to pre–mid-Desmoinesian (sub C5-unconformity) and (2) defines a southeastern edge to the late Paleozoic west-vergent deformation in northern and east-central Nevada.

Abstract The Cortez district is in one of the four major Carlin-type gold deposit trends in the Great Basin province of Nevada and contains three giant (>10 Moz) gold orebodies: Pipeline, Cortez Hills, and Goldrush, including the recently discovered Fourmile extension of the Goldrush deposit. The district has produced >21 Moz (653 t) of gold and contains an additional 26 Moz (809 t) in reserves and resources. The Carlin-type deposits occur in two large structural windows (Gold Acres and Cortez) of Ordovician through Devonian shelf- and slope-facies carbonate rocks exposed through deformed, time-equivalent lower Paleozoic siliciclastic rocks of the overlying Roberts Mountains thrust plate. Juxtaposition of these contrasting Paleozoic strata occurred during the late Paleozoic Antler orogeny along the Roberts Mountains thrust. Both upper and lower plate sequences were further deformed by Mesozoic compressional events. Regional extension, commencing in the Eocene, opened high- and low-angle structural conduits for mineralizing solutions and resulted in gold deposition in reactive carbonate units in structural traps, including antiforms and fault-propagated folds. The Pipeline and Cortez Hills deposits are located adjacent to the Cretaceous Gold Acres and Jurassic Mill Canyon granodioritic stocks, respectively; although these stocks are genetically unrelated to the later Carlin-type mineralization event, their thermal metamorphic aureoles may have influenced ground preparation for later gold deposition. Widespread decarbonatization, argillization, and silicification of the carbonate host rocks accompanied gold mineralization, with gold precipitated within As-rich rims on fine-grained pyrite. Pipeline and Cortez Hills also display deep supergene oxidation of the hypogene sulfide mineralization. Carlin-type mineralization in the district is believed to have been initiated in the late Eocene (>35 Ma) based on the age of late- to postmineral rhyolite dikes at Cortez Hills. The Carlin-type gold deposits in the district share common structural, stratigraphic, alteration, and ore mineralogic characteristics that reflect common modes of orebody formation. Ore-forming fluids were channeled along both low-angle structures (Pipeline, Goldrush/Fourmile) and high-angle features (Cortez Hills), and gold mineralization was deposited in Late Ordovician through Devonian limestone, limy mudstone, and calcareous siltstone. The Carlin-type gold fluids are interpreted to be low-salinity (2–3 wt % NaCl equiv), low-temperature (220°–270°C), and weakly acidic, analogous to those in other Carlin-type gold deposits in the Great Basin. The observed characteristics of the Cortez Carlin-type gold deposits are consistent with the recently proposed deep magmatic genetic model. Although the deposits occur over a wide geographic area in the district, it is possible that they initially formed in greater proximity to each other and were then spatially separated during Miocene and post-Miocene regional extension.

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.