Gold has been discovered recently at Cortez, Nevada, about 45 miles southwest of Carlin in carbonate rocks in a window of the Roberts Mountains thrust. The host rock consists of laminated to thin-bedded dark- to light-gray, silty dolomitic limestone and calcareous dolomitic siltstone in the upper part of the Silurian Roberts Mountains Limestone. These rocks contain sparse pyrite cubes and aggregates and some organic carbon. The rocks have been faulted and folded repeatedly during their complex geologic history. The gold is disseminated in a large zone where the rocks have been fractured and bleached and the pyrite oxidized. During oxidation the iron was redistributed, giving the rock a color ranging from light gray to dark red. The alteration zone envelops a 34-m.y.-old intrusive body of biotite-quartz-sanidine porphyry, which is also altered. No genetic relationship between the mineralization and the intrusive body is known. Silicification, iron-oxide staining, decalcification and, in extreme cases, dedolo-mitization generally accompanied the gold metallization, although any one of these phases of hydrothermal alteration may have been well developed without introduction of significant amounts of gold. Some clay alteration occurred in the igneous rock but none in the ore body. The gold is in micron-sized particles of native gold. Gold is mostly with silica between original silt grains and to a lesser extent in quartz-filled microfractures and hematite-goethite pseudomorphs after pyrite.

The gold was discovered during the examination of an arsenic-antimony-tungsten-mercury geochemical anomaly known in the area. Other gold deposits in north-central Nevada are associated with such anomalies.

The Getchell veins are lenticular replacement bodies lying along arcuate branches of a complex range-front fault system. The fault zone cuts all rocks of the district and is tentatively dated as late Tertiary.

The more intensely mineralized portions of the deposit form a shallow blanket with roots that project downward into areas of sparse mineralization.    The gold shoots are restricted to areas of intense mineralization.

Native gold and native silver are the only economic minerals. The great bulk of the gold occurs in minute but microscopically visible particles. Some gold may also occur in submicroscopic particles and some may be in solid solution in pyrite and carbon.

The ore minerals, dissolved in alkali sulfide solutions, are believed to have been deposited when the sulfide ion concentration in the hydrothermal liquid decreased, making unstable the double sulfides of gold-, iron, and arsenic.

The Getchell deposit is similar in many ways to the Nevada quicksilver deposits and present-day hot-spring deposits. The Getchell ore occurrence may represent a gradation from the common epithermal gold deposit to the cinnabar deposit. It is therefore placed close to the feeblest end of the epithermal group.

Most of the ore deposits of the Manhattan district are gold-bearing veins of a simple type and present no unusual features, but other ores inclosed in a single thin bed of limestone offer interesting problems.

Manhattan is situated in the southern part of the northeasterly trending Toquima Range, about 35 miles north of Tonopah. The Toquima Range is one of the less prominent of the many narrow, isolated mountain ranges which are such notable features of the Great Basin topography. The range is bordered by desert valleys—Ralston Valley and Monitor Valley on the east and Big Smoky Valley on the west. On both sides, but particularly on the west, the boundary between rock in place and valley fill is irregular, in marked contrast to the sharp lines of demarcation on the eastern front of the Toyabe Range, which borders Big Smoky Valley on the west.

Geochemical orientation surveys were completed over covered Carlin-type gold deposits in the Cortez mine area with the expressed aim of identifying and evaluating exploration methods to discover Au ore under transported alluvial cover in Nevada. Orientation tests were designed to assess the utility of geochemical applications at various scales of exploration, both drill targets at the deposit scale and “footprints” associated with deposits at the district scale.

Detailed surveys were completed over the covered Gap deposit, located adjacent to the giant Pipeline deposit. Both Carlin-type gold mineralization and earlier, spatially associated, base metal skarn mineralization at Gap were located by soils, soil gas, and vegetation. Loam soils at 6- to 12-cm depth provided a consistent and uniformly available sample medium. Gold ore under 10 m of cover in the northern portion of the Gap deposit was readily detected by analysis of Au by fire assay and ultra trace aqua regia methods in the -80 mesh fraction of these loam soils. Arsenic anomalies occur over the northern end and over the main ore zone at Gap, where there is 25 to 50 m of alluvial cover. Zinc concentrations in soil show the most coherent spatial relationship with underlying Au ore. Tests of MMI-B and Enzyme Leach selective leaches did not result in significant enhancement of the anomalies relative to aqua regia. CO2 and O2 in soil gas indicate faults and underlying mineralized carbonates at Gap, where weathering reactions likely generated CO2 from acid reaction with carbonate. Elevated Au and As in mixed sagebrush and shadscale occur over most ore zones, with the highest Au concentrations over the main ore zone rather than the shallowly buried northern zone. Like soils, elevated Zn in vegetation shows the most coherent spatial relationship with underlying ore.

This study was undertaken to characterize the mineral paragenesis and metal zoning at the property scale, evaluate the potential sources of ore-related metals, quantify the relationship between intensity of alteration and gold grade, and propose a comprehensive genetic model for the Carlin-type Au deposits at the southern part of the Goldstrike property, Nevada.

Mineralogy, textural relationships, whole-rock composition, and spatial distribution of the studied samples revealed two types of gold ore: Ore I and II. The former, which is hosted by the Roberts Mountains and Rodeo Creek Formations, and the Wispy, Planar, and Upper Mud units of the Popovich Formation, is the most abundant and widespread in the property. Ore I is characterized by intense hydrothermal alteration (e.g., carbonate dissolution, silicification, and precipitation of pyrite) and high amounts of trace elements (e.g., Ag, As, Au, Ba, Cd, Cu, Hg, Mo, Ni, S, Sb, Se, Te, Tl, and Zn). On the other hand, Ore II, which is hosted in the Wispy, Planar, and Soft Sediment Deformation units of the Popovich Formation, is mainly confined to the central-north-northwest portion of the Screamer deposit and is weakly altered with low concentration of trace elements. Both Ores I and II contain similar average concentrations of Au in whole rock (14 and 19 g/t Au, respectively) and in pyrite (290 and 540 ppm, respectively); however, auriferous pyrite from Ore I has higher trace element (As, Ag, Cu, Hg, Ni, Sb, Se, and Tl)/Au ratios than Ore II.

Sediment-hosted gold deposits in Guizhou, China, are hosted in late Paleozoic and early Mesozoic sedimentary rocks along the southwest margin of the Precambrian Yangtze craton. They have characteristics similar to Carlin-type gold deposits in Nevada and are notably enriched in As, Sb, Hg, and Tl. The Shuiyindong and Yata deposits consist of disseminated, strata-bound sulfides in Permian bioclastic limestone and fault-controlled mineralization in Middle Triassic calcareous clastic rocks, respectively. Mineralization in both deposits consists of barren milky quartz veins, disseminated gold-bearing arsenian pyrite and arsenopyrite, stibnite, realgar, and orpiment. The barren milky quartz veins occur in the ore-controlling structures with an envelope of gold mineralization in the host rock consisting of disseminated gold-bearing arsenian pyrite and arsenopyrite and replacement-style quartz veinlets. Later drusy quartz, stibnite, realgar, and orpiment fill fractures and vugs on the periphery of gold mineralization. Petrography, microthermometry, laser Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometric (LA-ICP-MS) analyses of fluid inclusions are used to characterize the chemical evolution of ore fluids at Shuiyindong and Yata.

The McCoy Au-Ag skarn and Cove Au-Ag deposits are located in the northern Fish Creek Mountains, Lander County, Nevada. Through the end of mining in 2001, large-scale open-pit and associated underground production at the two deposits yielded 3.3 million ounces (Moz) of Au and 108 Moz of Ag. Most production was from Cove, making it the fourth-largest Ag producer in the history of Nevada.

Cove is hosted by the middle to early Late Triassic Augusta Mountain Formation, which consists of limestone with lesser dolostone and clastic units. Ore also is present locally in Eocene porphyritic granodiorite dikes and sills. The deposit comprises two distinct ore types: a central core of polymetallic vein-type ore and an outer aureole of relatively Ag rich Carlin-style ore. Polymetallic veins consist of pyrite-sphalerite-galena–dominated Au- and Ag-bearing veins, veinlets, stockworks, crustifications, and disseminations in clastic and carbonate strata and locally in the intrusions. Carlin-style ore comprises disseminated Fe ± As sulfides with arsenian, argentiferous, and auriferous components ± native Au-electrum in silty to sandy carbonate strata. Polymetallic vein-type ore has Ag/Au ratios of >50/1, and Carlin-style ore has Ag/Au ratios that decrease from ~50/1 near the feeder faults to ~1/1 in one of the more distal ore zones. Both types of ore are associated with decarbona-tized, silicified, and illitized rocks. New structural and age data for fresh and altered intrusive rocks indicate that mineralization at Cove occurred during active extension and magmatism at ~39 Ma (40Ar-39Ar). Fluid inclusion and δD and δ18O data for polymetallic vein-type ore indicate that the mineralizing fluids had temperatures of 250° to 370°C and were magmatic in origin.

The Bald Mountain mining district has produced about 2 million ounces (Moz) of Au. Geologic mapping, field relationships, geochemical data, petrographic observations, fluid inclusion characteristics, and Pb, S, O, and H isotope data indicate that Au mineralization was associated with a reduced Jurassic intrusion. Gold deposits are localized within and surrounding a Jurassic (159 Ma) quartz monzonite porphyry pluton and dike complex that intrudes Cambrian to Mississippian carbonate and clastic rocks. The pluton, associated dikes, and Au mineralization were controlled by a crustal-scale northwest-trending structure named the Bida trend. Gold deposits are localized by fracture networks in the pluton and the contact metamorphic aureole, dike margins, high-angle faults, and certain strata or shale-limestone contacts in sedimentary rocks. Gold mineralization was accompanied by silicification and phyllic alteration, ±argillic alteration at shallow levels. Although Au is typically present throughout, the system exhibits a classic concentric geochemical zonation pattern with Mo, W, Bi, and Cu near the center, Ag, Pb, and Zn at intermediate distances, and As and Sb peripheral to the intrusion. Near the center of the system, micron-sized native Au occurs with base metal sulfides and sulfosalts. In peripheral deposits and in later stages of mineralization, Au is typically submicron in size and resides in pyrite or arsenopyrite. Electron microprobe and laser ablation ICP-MS analyses show that arsenopyrite, pyrite, and Bi sulfide minerals contain 10s to 1,000s of ppm Au. Ore-forming fluids were aqueous and carbonic at deep levels and episodically hypersaline at shallow levels due to boiling. The isotopic compositions of H and O in quartz and sericite and S and Pb in sulfides are indicative of magmatic ore fluids with sedimentary sulfur. Together, the evidence suggests that Au was introduced by reduced S-bearing magmatic fluids derived from a reduced intrusion. The reduced character of the intrusion was caused by assimilation of carbonaceous sedimentary rocks.

Tertiary faults dismember the area and drop down the upper part of the mineralizing system to the west. The abundant and widespread kaolinite in oxide ores is relatively disordered (1A polytype) and has δD and δ18O values suggestive of a supergene origin. The deep weathering and oxidation of the ores associated with exhumation made them amenable to open-pit mining and processing using cyanide heap leach methods.

The regional distribution of arsenic and 20 other elements in stream-sediment samples in northern Nevada and southeastern Oregon was studied in order to gain new insights about the geologic framework and patterns of hydrothermal mineralization in the area. Data were used from 10,261 samples that were originally collected during the National Uranium Resource Evaluation (NURE) Hydrogeochemical and Stream Sediment Reconnaissance (HSSR) program in the 1970s. The data are available as U.S. Geological Survey Open-File Report 02-0227.

The Jerónimo sedimentary rock-hosted disseminated Au deposit is located within the Potrerillos district of the Atacama region of northern Chile, east of the Potrerillos porphyry Cu-Mo and El Hueso high-sulfidation Au deposits. Prior to development, the Jerónimo deposit contained a resource of approximately 16.5 million metric tons (Mt) at 6.0 g/t Au. Production began in the oxidized, nonrefractory portion of the deposit in 1997 and terminated in 2002. During that time, approximately 1.5 Mt at 6.8 g/t Au was mined by underground room-and-pillar methods, from which a total of approximately 220,000 oz of Au was recovered by heap-leach cyanidation.