The Meikle mine exploits one of the world’s highest grade Carlin-type gold deposits with reserves of ca. 220 t gold at an average grade of 24.7 g/t. Locally, gold grades exceed 400 g/t. Several geologic events converged at Meikle to create these spectacular gold grades. Prior to mineralization, a Devonian hydrothermal system altered the Bootstrap limestone to Fe-rich dolomite. Subsequently the rocks were brecciated by faulting and Late Jurassic intrusive activity. The resulting permeability focused flow of late Eocene Carlin-type ore fluids and allowed them to react with the Fe-rich dolomite. Fluid inclusion data and mineral assemblages indicate that these fluids were hot (ca. 220°C),of moderate salinity ( 2 S rich. Gold-rich pyrite formed by dissolution of dolomite and sulfidation of its contained Fe. Where dissolution and replacement were complete, ore-stage pyrite and other insoluble minerals were all that remained. Locally, these minerals accumulated as internal sediments in dissolution cavities to form ore with gold grades >400 g/t. Petrographic observations, geochemical data, and stable isotope results from the Meikle mine and other deposits at the Goldstrike mine place important constraints on genetic models for Meikle and other Carlin-type gold deposits on the northern Carlin trend. The ore fluids were meteoric water ( δ D = –135‰, δ 18 O = –5‰) that interacted with sedimentary rocks at a water/rock ratio of ca. 1 and temperatures of ca. 220°C. The absence of significant silicification suggests that there was little cooling of the ore fluids during mineralization. These two observations strongly suggest that ore fluids were not derived from deep sources but instead flowed parallel to isotherms. The gold was transported by H 2 S ( δ 34 S = 9‰), which was derived from Paleozoic sedimentary rocks. The presence of auriferous sedimentary exhalative mineralization in the local stratigraphic sequence raises the possibility that preexisting concentrations of gold contributed to the Carlin-type deposits. Taken together our observations suggest that meteoric water evolved to become an ore fluid by shallow circulation through previously gold- and sulfur-enriched rocks. Carlin-type gold deposits formed where these fluids encountered permeable, reactive Fe-rich rocks.
Abstract The formation of bonanza Au-Ag-telluride ores in adularia-sericite epithermal deposits is hypothesized to be attributed to the input of magmatic fluid into flow systems dominated by barren meteoric water. However, understanding of the role and importance of magmatic fluids in the formation of bonanza ores remains limited. To address these concerns, we conducted Cu isotope analyses of chalcopyrite, which coexists with Au-Ag-tellurides in the Te-rich Sandaowanzi deposit located in northeastern China, as well as ore-bearing quartz veins, coeval igneous rocks, and older igneous rocks that underlie the deposit. To aid interpretation, we use geochemical modeling techniques along with O isotope data from calcite and quartz, as well as thermal outputs from modern geothermal systems. At Sandaowanzi, the δ65Cu values of chalcopyrite vary widely, ranging from 0.48 to 0.86‰. These values are higher than underlying Early Jurassic monzogranite (-0.06 to 0.27‰), as well as coeval Early Cretaceous andesite and basaltic andesite (0.01 to 0.11‰) and Early Cretaceous dacite and granodiorite (0.33 to 0.52‰). The O values of calcite vary from -3.2 to 6.7‰. The present isotope data, together with previous δ18O analyses of quartz, support the idea that the fluids responsible for ore formation at Sandaowanzi were derived from a magmatic source. Progressive input of magmatic fluids into covecting meteoric water explains the telluride precipitation. Subsequent boiling can explain Au, Ag, and Cu precipitation in the upwelling limb of convection cells. Injection of high-temperature magmatic fluids (~300 °C) into shallow meteoric groundwater (~250 °C) and formation of Au-Ag-telluride ores can take place over a relatively short timeframe, typically around 1,000 years. In contrast, the process of electrum precipitation occurs at a later stage compared to the formation of Au-Ag-telluride ores in the boiling zones (<300 °C). These findings indicate that Au-Ag-telluride precipitation occurs at the mixing interface under high temperatures (>300 °C), suggesting that it is located at greater depths compared to typical Au-Ag mineralization in adularia-sericite epithermal systems.
Abstract Quartz chemistry is important for revealing fluid sources and evolution in hydrothermal deposits, but such information is lacking for many epithermal systems and deposit types. To investigate quartz chemistry in this system further, we collected representative samples of quartz from adularia-sericite epithermal Ag deposits in China and determined their chemical compositions. In adularia-sericite epithermal Ag-bearing systems, magmatic quartz from porphyry intrusions and host subvolcanic rocks is SEM-CL spectral peaks at 360 and 415 nm and exhibits homogenous CL or weak zonal textures (alternating growth zones within individual quartz crystal). Trace elements in magmatic quartz have the lowest Sb concentrations (median = 0.1 ppm; n = 80). Hydrothermal quartz can be classified into type I and type II by CL false color and CL spectral peaks. Hydrothermal type I quartz has spectral peaks at 360 and 415 nm, exhibits zonal or sector textures, and is associated with base metal sulfides and minor Ag mineralization. Such hydrothermal type I quartz has low Sb concentrations (median = 4.5 ppm; n = 839), contains liquid-rich fluid inclusions and formed by cooling. The cooling trend is indicated by a positive correlation between the concentrations of Sb and Al, as well as between Li and Al. Hydrothermal type I quartz has an Fe center by electron spin resonance, whereas other centers are missing or weak at room temperature. In general, hydrothermal type II quartz mantles type I quartz. Hydrothermal type II quartz has a very high intensity peak (by several orders of magnitude) at 580 nm, zonal textures, and is associated with abundant Ag mineralization. Hydrothermal type II quartz has the highest Sb concentrations (median = 71 ppm; n = 185), which remain constant as Al decreases on an Sb vs. Al plot. This quartz has colloform, bladed or zonal textures and contains coexisting liquid- and vapor-rich fluid inclusions, indicative of boiling. Additionally, this quartz has a significantly higher E'1 center intensity, suggesting a high concentration of oxygen vacancies associated with rapid crystallization. The mineral paragenesis, analytical results, and geochemical models show that in these Ag-bearing epithermal systems hydrothermal type I quartz associated with base metal sulfides precipitated during cooling whereas subsequent growth zoned hydrothermal type II quartz with high Sb concentrations and Ag-minerals precipitated during boiling. These results suggest that the CL texture and spectra, trace elements, and electron spin resonance data of quartz could identify veins with potential for Ag mineralization in epithermal systems.
Northern Nevada is one of the Earth’s premier gold- and silver-producing regions, and gold and silver are mined from a wide range of deposit types (Table 1). Current production largely comes from Carlin-type sedimentary rock-hosted disseminated gold deposits (Christensen, 1995; Teal and Jackson, 1997; Bettles, 2002), many of which lie along linear trends (Fig. 1; Roberts, 1960, 1966; Berger and Bagby, 1991). The small area of the Carlin trend (approximately 8 x 65 km) is North America’s most prolific gold mining district, producing nearly 125 t (4 Moz) of gold annually. Since large-scale mining began in 1965 with the opening of the Carlin mine, the Carlin trend has produced more than 1555 t (50 Moz) of gold. During 2001, gold production from the entire state of Nevada was nearly 253 t (8.13 Moz), which accounted for approximately 76 percent of the United States and 10 percent of the world production (Price and Meeuwig, 2002). In 2001, only the countries of South Africa and Australia produced more gold than the state of Nevada. In addition, Nevada produced 543 t (17.4 Moz) of silver in 2001—about 31 percent of the United States production and 3 percent of the world production.
In 1996, the Mineral Resources Program of the U.S. Geological Survey began a multidisciplinary project to investigate the origin of gold deposits in northern Nevada. This project was undertaken in collaboration with the Nevada Bureau of Mines and Geology, the Ralph J. Roberts Center for Research in Economic Geology at the University of Nevada, Reno, several other universities, and numerous mining companies active in the region. In addition, several universities and mining companies were conducting research on the origin of gold deposits in northern Nevada. This research built on more than a century of studies by the …
The Tuscarora mining district contains the oldest and the only productive Eocene epithermal deposits in Nevada. The district is a particularly clear example of association of low-sulfidation deposits with igneous activity and structure, and it is unusual in that it consists of two adjoining but physically and chemically distinct types of low-sulfidation deposits. Moreover, Tuscarora deposits are of interest because they formed contemporaneously with nearby, giant Carlin-type gold deposits. The Tuscarora deposits formed within the 39.9 to 39.3 Ma Tuscarora volcanic field, along and just outside the southeastern margin of the caldera-like Mount Blitzen volcanic center. Both deposit types formed at 39.3 Ma, contemporaneous with the only major intrusive activity in the volcanic field. No deposits are known to have formed during any of the intense volcanic phases of the field. Intrusions were the apparent heat source, and structures related to the Mount Blitzen center were conduits for hydrothermal circulation. The ore-forming fluids interacted dominantly with Eocene igneous rocks.
The two deposit types occur in a northern silver-rich zone that is characterized by relatively high Ag/Au ratios (110–150), narrow alteration zones, and quartz and carbonate veins developed mostly in intrusive dacite, and in a southern gold-rich zone that is typified by relatively low Ag/Au ratios (4–14), more widespread alteration, and quartz-fissure and stockwork veins commonly developed in tuffaceous sedimentary rocks. The deposit types have similar fluid inclusion and Pb and S isotope characteristics but different geochemical signatures. Quartz veins from both zones have similar thermal and paragenetic histories and contain fluid inclusions that indicate that fluids cooled from between 260° and 230°C to less than 200°C. Fluid boiling may have contributed to precious-metal deposition. Veins in both zones have relatively high As and Sb and low Bi, Te, and W. The silver zone has high Ca, Pb, Mn, Zn, Cd, Tl, and Se. The gold zone has high Hg and Mo. A few samples from an area of overlap between the two zones share chemical characteristics of both deposit types. The deposit types could represent a single zoned or evolving system in which hydrothermal fluids rose along structures within the silver zone, preferentially deposited Ag and base metals, and then spread into the gold zone. Alternatively, the deposit types could represent two distinct but temporally indistinguishable hydrothermal cells that only narrowly overlapped spatially.
As noted in previous studies, the hydrothermal fluids that generated the Tuscarora and other epithermal deposits could have evolved from Carlin-type fluids by boiling and mixing with meteoric water. If so, the Tuscarora deposit may represent epithermal conditions above Carlin-type deposits, and Carlin-type deposits may lie beneath the district.
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