Выпуск 104
Автор(ы):Bin Xia, Heinrich C.A., Ruizhong Hu, Thomas Pettke, Wenchao Su, Xingchun Zhang
Издание:Economic geology, 2009 г., 22 стр.
Sediment-hosted gold deposits in Guizhou, China: products of wall-rock sulfidation by deep crustal fluids

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.

ТематикаПолезные ископаемые
МеткиCarlin type gold, Gold, Guizhou, Wall-rock sulfidation, Золото
Выпуск 103
Автор(ы):David L.Emmons, Keith Jones, Marcus K.Johnston, Tommy B.Thompson
Издание:Economic geology, 2008 г., 24 стр.
Geology of the cove mine, Lander County, Nevada, and a genetic model for the McCoy-Cove hydrothermal system

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.

ТематикаПолезные ископаемые
МеткиGold, Lander County, McCoy-Cove hydrothermal system, Золото
Выпуск 102
Автор(ы):Hofstra A.H., Nutt† C.J.
Издание:Economic geology, 2007 г., 27 стр.
Bald mountain gold mining district, Nevada: a Jurassic reduced intrusion-related gold system

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.

ТематикаПолезные ископаемые
Выпуск 101
Автор(ы):Boris Kotlyar, Helen Folger, Mary Jane Coombs, Steve Ludington, Thomas G.Hildenbrand, Victor G. Mossotti
Издание:Economic geology, 2006 г., 26 стр.
Regional surficial geochemistry of the Northern Great Basin

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.

ТематикаПолезные ископаемые
Автор(ы):John F.H.Thompson, Richard M.T., Vanessa G.Gale, William A.Wright
Издание:Economic geology, 2004 г., 22 стр.
Characteristics and formation of the Jerónimo carbonate-replacement gold deposit, Potrerillos District, Chile

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.

ТематикаПолезные ископаемые
МеткиCarlin type gold, Gold, Jerónimo carbonate-replacement gold deposit, Potrerillos District, Золото
Выпуск 99
Автор(ы):Ballantyne G.H., Charles E.Barker, Charles G.Cunningham, Charles W.Naeser, Gerry W.Austin, Robert G.Stamm, Robert O.Rye
Издание:Economic geology, 2004 г., 18 стр.
Formation of a paleothermal anomaly and disseminated gold deposits associated with the Bingham Canyon Porphyry Cu-Au-Mo system, Utah

The thermal history of the Oquirrh Mountains, Utah, indicates that hydrothermal fluids associated with emplacement of the 37 Ma Bingham Canyon porphyry Cu-Au-Mo deposit extended at least 10 km north of the Bingham pit. An associated paleothermal anomaly enclosed the Barneys Canyon and Melco disseminated gold deposits and several smaller gold deposits between them. Previous studies have shown the Barneys Canyon deposit is near the outer limit of an irregular distal Au-As geochemical halo, about 3 km beyond an intermediate Pb-Zn halo, and 7 km beyond a proximal pyrite halo centered on the Bingham porphyry copper deposit. The Melco deposit also lies near the outer limit of the Au-As halo. Analysis of several geothermometers from samples collected up to 22 km north of the Bingham Canyon porphyry Cu-Au-Mo deposit indicate that most sedimentary rocks of the Oquirrh Mountains, including those at the gold deposits, have not been regionally heated beyond the “oil window” (less than about 150ºC).

ТематикаПолезные ископаемые
МеткиBingham Canyon Porphyry Cu-Au-Mo system, Carlin type gold, Gold, Золото
Выпуск 55
Автор(ы):Wide A.
Издание:Economic geology, 2003 г., 5 стр.
The golden triangle of Southeast China: another Carlin Trend?

Numerous gold deposits and occurrences in southeast China are located in an area known as the “Golden Triangle,” mainly in Guizhou province (Fig. 1). Comparisons have been made with sedimentary rock-hosted or “Carlin-type” deposits of northeast Nevada (e.g., Li and Peters, 1998), and at least one major multinational mining company has explored for Carlin-type deposits in this area. Such deposits represent attractive mineral exploration targets owing to the size of the contained resources, high grades, and amenability to open-pit mining operations. Furthermore, the deposits tend to occur as clusters (or “trends”). Discovery of a new mineralized cluster would represent a major coup. In this paper I provide a short summary of aspects of the geology of the southwest Chinese deposits and comment on their similarities and differences compared to the deposits of Nevada.

ТематикаПолезные ископаемые
Выпуск 94
Автор(ы):Asadi H.H., Hale M., Voncken J.H.L.
Издание:Economic geology, 1999 г., 8 стр.
Invisible gold at Zarashuran, Iran

Although micron-size particles of metallic gold are observed at the Zarshuran gold deposit, northwest Iran, the quantities do not account for the gold concentrations determined by chemical analyses. The presence of invisible gold has been established by means of trace element electron microprobe analyses of pyrite, arsenian pyrite, orpiment, realgar, stibnite, getchellite, sphalerite, and lead sulfosalts. Quantitative point analyses indicate that invisible gold is present in anhedral pyrite, arsenian pyrite overgrowth rims on gold- and arsenic-free euhedral pyrite, in massive, network, and colloform arsenian pyrite, and in massive and colloform sphalerite intimately intergrown with colloform arsenian pyrite. Gold in these forms adequately explains the measured gold concentrations at Zarshuran. The invisible gold owes its origin to solid-solution deposition and/or encapsulation of submicron-size particles of metallic gold.

ТематикаПолезные ископаемые
МеткиCarlin type gold, Gold, Zarashuran, Золото
Выпуск 6
Издание:Economic geology, 1988 г., 6 стр.
Sediment hosted replacement gold deposit

Sediment hosted replacement gold deposits, also termed Carlin-type gold deposits from where they were first described, have been major gold producers in the western U.S. (98.8 million ounces discovered; Singer, 1993). Most deposits lie along deep crustal fracture systems which define the Carlin and Battle Mountain Trends (Madrid and Roberts, 1990). Significant new discoveries within the Carlin Trend include the Betze-Post and Meikle ore systems (Bettles and Lauha, 1991), with production of 7.1 million ounces andreserves of 28 million ounces of gold at the end of 1994 (Volk et al, 1995). Reviews of this style of gold mineralization by Bagby and Berger (1985), Sawkins (1984), Sillitoe and Bonham (1990), Berger and Bagby (1991), and Kuehn and Rose (1995) present geological models for this deposit type. Critical in the development of these models has been the recognition of similar deposit types in other settings (e.g., Bau, Sarawak; Wolfenden, 1965; Sillitoe and Bonham, 1990: China; Cunningham et al., 1988: Melco and Barney's Canyon deposits, Bingham District, U.S.; Babcock et al., 1992: Mesel, North Sulawesi; Indonesia; Turner et al., 1994; Garwin et al., 1995: and elsewhere in the eastern and western Pacific Rim, G. Corbett and T. Leach, unpub. data; Gemuts et al, 1996: Fig. S.l).

ТематикаПолезные ископаемые
Выпуск 92
Автор(ы):David A.Mako
Издание:Economic geology, 1997 г., 2 стр.
Characterization and dating of argillic alteration in the Mercur gold district, Utah - a discussion

Wilson and Parry (1995) present data pertaining to clay alteration and K-Ar age dates for samples from the Mercur gold district. Their data record a wide spread of K-Ar ages for illite ranging from 98.4 to 226 Ma. They estimate the age of gold mineralization to be between 140 and 160 Ma and explain the wide range of ages as functions of partial thermal resetting of the clay minerals and the distance from the hydrothermal conduits. Morris and Tooker (1996) in their discussion of this paper, point out that a Mesozoic age for gold mineralization at Mercur is incompatible with several lines of long-standing regional geologic evidence that suggest a Tertiary age. In their reply, Wilson and Parry (1996) defend their position for a Mesozoic age of mineralization in part by relying on new 40Ar/39Ar age data and the fact that none of the 22 age dates is Tertiary. Although the research by Wilson and Parry may represent a good study of samples in the laboratory, there are several tenuous assumptions and contradictions of the geologic observations at Mercur that must be addressed.

ТематикаПолезные ископаемые
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