Выпуск 95
Автор(ы):Cindy L.Williams, Jon. L.Powell, Tommy B.Thompson, W.W. Arren Dunbar
Издание:Economic geology, 2000 г., 14 стр.
Gold-bearing breccias of the Rain Mine, Carlin Trend, Nevada

The Rain mine includes two mined-out open pits that contained 36.4 t (1.17 Moz) gold, averaging 1.8 g/t gold, and underground reserves, including underground production, estimated at 4.9 t (157,000 oz) gold averaging 7.7 g/t. Rain orebodies are localized in a breccia complex within the hanging wall of the Rain fault and hosted within the Mississippian Webb Formation immediately overlying the contact with the Devonian Devils Gate Limestone.

The ore host includes four texturally and genetically distinct breccia types: (1) crackle breccia; (2) hy-drothermal breccia; (3) tuffisite with accretionary lapilli; and (4) collapse breccia. Crackle breccia forms a capping over multistage hydrothermal breccias that are cut by tabular- to pipe-shaped tuffisite dikes, with some containing accretionary lapilli. Pre- and synore hydrothermal breccias formed during at least three episodes of convective fluidization, followed by quartz-sulfide-barite cementation. High-grade gold was deposited as a late phase along the upper portion of the hydrothermal breccia mass and extended into the crackle breccia zone. Collapse breccias occur along the floor of the composite breccia mass and have irregular upper and lower contacts. The lower contact occurs on a dissolution boundary with the Devils Gate Limestone.

ТематикаПолезные ископаемые
МеткиCarlin type gold, Gold, Gold-bearing breccias, Rain Mine, Золото
Выпуск 13
Автор(ы):Poul Emsbo
Издание:Economic geology, 2000 г., 12 стр.
Gold in Sedex deposits

Newly recognized gold-rich sedimentary-exhalative (sedex) mineralization in Nevada, with an average gold grade of 14 g/tonne (t), and the occurrence of significant amounts of gold in classic sedex deposits like Rammelsberg, Germany (30 Mt at 1 g/t), Anvil, Canada (120 Mt at 0.7 g/t), and Triumph, Idaho (? at 2.2 g/t) demonstrate that basin brines can form gold ore. The sedex Au mineralization in Nevada represents a previously unrecognized end member in a spectrum of sedex deposits that also includes large Zn-Pb, intermediate Zn-Pb-Ba ± Au, and barite deposits. Study of ore deposits, modern brines, and chemical modeling indicates that variation in metal ratios and their abundance in sedex deposits are dominantly controlled by the concentration and redox state of sulfur in brines. For example, Au and Ba solubilities are highest in H2S-rich, SO4-poor fluids, whereas base metal solubilities are highest when H2S is not present. Chemical modeling indicates a typical reduced brine (15 wt % NaCl equiv, pH = 5.5, H2S = 0.01 m) at 200°C is capable of transporting as much as 1 ppm Au in solution.

The H2S content in brines is controlled by the rate of its production through thermochemical reduction of sulfate by organic matter and the rate of its removal from the fluid through the sulfidation of reactive Fe in the sediments. Thus, sedimentary basins with high organic carbon and sulfate in rocks low in reactive Fe, such as carbonates and shales, are most likely to produce H2S-rich brines that may form gold-rich sedex deposits. Because of the tremendous scale of sedex hydrothermal systems, evidence that basin fluids can transport gold identifies a new mechanism for concentrating gold in sedimentary basins and opens extensive areas to further gold exploration.

ТематикаПолезные ископаемые
МеткиGold, Sedex deposits, Золото
Выпуск 92
Автор(ы):Mark D.Barton, Robert P.Ilchik
Издание:Economic geology, 1996 г., 20 стр.
An amagmatic origin of Carlin-type gold deposits

Carlin-type deposits are major sources of gold, yet their origins are enigmatic. Suggested genetic models make connections to magmatism, regional metamorphism, or regional extension. Depositional mechanisms are uncertain as well. We propose on the basis of geologic, physical, and chemical reasoning, a genetic model in which meteoric fluids were circulated by heat released during crustal extension. These fluids interacted at depth with the sedimentary rock pile and scavenged gold. Upon upwelling, these fluids interacted with various lithologies and/or other fluids and produced the characteristic alteration and metal suites of these deposits. To test the viability of this amagmatic model, we have investigated certain physical and chemical constraints implicit to the model.

ТематикаПолезные ископаемые
Выпуск 90
Автор(ы):Arthur W.Rose, Carl A.Kuehn
Издание:Economic geology, 1995 г., 20 стр.
Carlin gold deposits, Nevada: origin in a deep zone of mixing between normally pressured and overpressured fluids

Gold mineralization at Carlin is clearly younger than hydrocarbon maturation (pre-Cretaceous) and felsic dike intrusion (Cretaceous), and older than deep oxidation (late Tertiary). Within the episode of gold mineralization, the main gold ore (MGO) stage and late gold ore (LGO) stage are distinguished paragenetically, wi-th a variety of vein and mineralization types in each. MGO stage fluids contained 5 to 10 mole percent COa, appreciable H2S, and 3 ± 1 wt percent NaCl equiv. At least portions of MGO stage mineralization were characterized by two-phase boiling (COa exsolution) at 215° ± 30°C and 800 ± 400 bars. In contrast, LGO stage fluids were gas poor with salinities <1.5 wt percent NaCl equiv and record only nonboiling conditions. MGO stage fluids had 518Oh2o values of 5 to 9 per mil, whereas LGO stage fluids resembled unevolved meteoric water with 518Oh2o values < —3 per mil.

From the MGO stage to the LGO stage, calcite 5180 values shifted from near whole-rock values of 12 ± 3 per mil to around 0 ± 1 per mil as LGO stage fluids flooded the system. Jasperoids also record a large range (9-22%o) in 518Oh2o values. These data indicate the involvement of two very different fluids in ore deposition. Because MGO and LGO stage features are closely associated spatially with each other and with Au, As, Sb, Hg, and other ore elements, both fluids are believed to have both been present during most stages of ore deposition.

At pressures of 80 to 85 percent lithostatic, depths of 3.8 ± 1.9 km are required to accommodate the 800 ± 400 bars of pressure recorded in MGO stage fluid inclusions. Carlin, therefore, is not an epi-thermal or hot spring deposit. Carbon dioxide in gas-rich MGO stage fluids may have originated either directly from buried intrusions or their contact aureoles, or from low-grade regional metamorphism at depth. The water may have been originally meteoric, and Au may be magmatic or derived from leaching of deep metamorphic or sedimentary rocks. Ore deposition appears to have occurred in zones of throttling at a pressure seal between normally pressured and overpressured regimes, where fluids experienced a change from near-lithostatic to hydrostatic conditions. Such pressure seals are common in deep sedimentary basins and may be a key to highly localized gold deposition. Mixing of two fluids and interaction with host rocks along thin permeable bioclastic horizons are believed to have been the major factors in depositing ore.


ТематикаПолезные ископаемые
Выпуск 87
Автор(ы):Kuehn C.A., Rose A.W.
Издание:Economic geology, 1991 г., 25 стр.
Geology and geochemistry of wall-rock alteration at the Carlin gold deposit, Nevada

The Carlin disseminated gold deposit occurs in an autochthonous sequence of Paleozoic sedimentary rocks exposed in a structural window in the Roberts Mountains thrust in north-central Nevada. The upper 175 m of the Silurian Roberts Mountains Formation hosts the majority of ore at Carlin and is characterized by laminated, fine-grained, calcareous and/or dolomitic argillaceous siltstone with local coarser grained siltstones and <0.25- to >50-cm-thick lenticular interbeds of sand- and granule-sized calcareous bioclastic debris or fossil hash. Detailed studies of drill core and exposures in the East pit of the Carlin mine show that alteration and mineralization are zoned away from crosscutting fault conduits and these more permeable bioclastic beds, indicating that these two features were major inflow zones for hydrothermal fluid.

In unoxidized rocks, unaltered calcareous siltstone (1) containing quartz, dolomite, calcite, illite, K feldspar, and pyrite is progressively converted to assemblages of (2) quartz + dolomite + calcite + illite + pyrite, (3) quartz + dolomite + illite-K mica + pyrite, (4) quartz + illite-K mica + pyrite, and (5) quartz + kaolinite-dickite + pyrite adjacent to inflow zones where jasperoids are developed. Gold most consistently enriches the zone of calcite and dolomite removal (3 and 4 above), though it occurs in all zones, locally in high concentrations. This zoned alteration was accomplished by a C02-rich acidic fluid. This acidic alteration enhanced the passage of fluids by extensive carbonate removal to form zones of higher permeability.

Oxidation is wholly a supergene effect related to deep weathering, because the oxidation is superimposed on both mineralized and altered rocks with only minor effect on the major element chemistry; it has produced low-temperature goethitic Fe oxides rather than higher temperature hematite and is not spatially related to Au distribution at the mine or on a district scale.

Because of extensive carbonate removal leading to local volume reduction through collapse and/or compaction, geochemical effects are examined using ratios to relatively immobile elements such as Al and Ti. Extensive depletion of Ca, Mg, and C02 and introduction of Si, Au, and S have occurred. Potassium is depleted in the conversion of illite to dickite-kaolin-ite in proximal silicified inflow zones, and Fe enriches some pyritized rock. Carbonate removal and silicification are two separate processes, both of which are spatially associated with mineralization. Mineralized decarbonated rocks and barren footwall rocks commonly are not silicified, and intensely silicified proximal alteration zones are generally low grade.

ТематикаПолезные ископаемые
Выпуск 85
Издание:Economic geology, 1990 г., 26 стр.
Geology and geochemistry of the Vantage gold deposits, Alligator Ridge-Bald Mountain mining district, Nevada

Geologic relationships, major element data, and isotopic geochemistry of a group of Carlin-type Au deposits in the Alligator Ridge-Bald Mountain district of east-central Nevada were investigated to help constrain the origin and relative timing of Au mineralization and associated alteration. The Vantage gold deposits were the largest of 18 known sediment-hosted, disseminated gold deposits and prospects that are distributed over a strike length of 40 km. The district consists predominantly of Paleozoic carbonate and siliciclastic sedimentary rocks. Minor amounts of Tertiary volcanic and volcanic-related sedimentary rocks and a small granitic stock in the northern end of the district are also present. The intrusion and its surrounding aureole also host gold mineralization, but most gold deposits in the district are not spatially associated with intrusive rocks.

ТематикаПолезные ископаемые
МеткиAlligator Ridge-Bald Mountain mining district, Gold, Vantage gold deposits, Золото
Выпуск 75
Автор(ы):Arthur S.Radtke, Dickson F.W., Robert O.Rye
Издание:Economic geology, 1980 г., 32 стр.
Geology and stable isotope studies of the Carlin gold deposit, Nevada

The Carlin gold deposit, largest of the epithermal disseminated replacement-type gold deposits discovered to date in the United States, formed as a result of hydrothermal processes associated with a shallow-seated late Tertiary igneous event. The orebodies formed by the replacement of carbonate minerals, principally calcite, in thin-bedded argillaceous arenaceous dolomitic beds favorable for mineralization within the upper 245+ m of the Roberts Mountains Formation. Early hydrothermal fluids dissolved calcite and deposited quartz. Fluids during the main hydrothermal stage introduced Si, Al, K, Ba, Fe, S, and organic materials, plus Au, As, Sb, Hg, and Tl; quartz and pyrite were deposited, potassium clays formed, and more calcite was dissolved. Sulfides and sulfosalts containing As, Sb, Hg, and Tl, and base metal sulfides of Pb, Zn, and Cu probably formed later in the paragenesis.

The main stage of ore deposition was terminated with the deposition of barite veins and the onset of boiling. The fluids lost H20, C02, H2S, and other components, leading to the production of H2S04 in the upper levels of the deposit and to subsequent intense acid leaching and oxidation of rocks and ore near the surface. Within this zone, calcite and large amounts of dolomite were removed, sulfides and organic compounds oxidized, kaolinite and anhydrite formed, and silica was added. After the hydrothermal event, the upper part of the deposit underwent weak oxidation by cooler ground water.

ТематикаПолезные ископаемые
Выпуск 64
Автор(ы):James E.Elliott, John D.Wells, Lee R.Stoiser
Издание:Economic geology, 1969 г., 12 стр.
Geology and geochemistry of the Cortez gold deposit, Nevada

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.


ТематикаПолезные ископаемые
МеткиCortez gold deposit, Gold, Золото
Выпуск 46
Автор(ы):Peter Joralemon
Издание:Economic geology, 1951 г., 44 стр.
The occurrence of gold at the Getchell mine, Nevada

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.

ТематикаПолезные ископаемые
Выпуск 16
Автор(ы):Ferguson H.G.
Издание:Economic geology, 1921 г., 36 стр.
The limestone ores of Manhattan, Nevada


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.

ТематикаПолезные ископаемые
МеткиGold, Limestone ores, Mamhattan, Золото
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