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.

The upper Paleozoic Havallah sequence of central Nevada is a folded and thrust-faulted association of greenstone, siliceous marine sedimentary rocks, and deep-water clastic rocks. Microfossil assemblages (radiolarians, sponge spicules, and conodonts) are used as tools to unravel the stratigraphy and to interpret the paleoenvironments of the siliceous sedimentary rocks. Nine radiolarian assemblages (Osagean to Guadalupian) are described and used for delineation and correlation of fault-bounded lithotectonic units. The biostratigraphic zonation reveals that the oldest rocks in each lithotectonic unit are progressively younger from the structurally highest to the lowest units, suggesting progressive west-to-east upsection stepping of the Golconda sole thrust with accretion of each unit. Analyses of the radiolarian and sponge spicule faunas permit lateral and temporal comparisons of depositional environments. The lower structural units are coarsening-upward sequences of hemipelagic slope deposits overlain by sponge spicule-rich turbidites derived from a shallow source. The uppermost structural unit is a coarsening-upward basinal sequence. Permian sponge spicules in turbidites of the slope sequences and redeposited fusulinids in the basin sequence are similar to those in adjacent autochthonous (North American) regions. Permian radiolarians and sponge spicules in hemipelagic siliceous argillite of the slope sequences are similar to those in the Northern Sierra terrane to the west; the Havallah basin and the Northern Sierra arc terrane were overlain, therefore, by a similar water mass and may have been in proximity during the Permian. Clastic dikes and sills containing volcanic, metamorphic, and sedimentary rock clasts are Leonardian or younger.