Jacob Margolis, 1997. "Gold Paragenesis in Intrusion-Marginal Sediment-Hosted Gold Mineralization at Eureka, Nevada", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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The Archimedes gold deposit, discovered in 1992 by Homestake Mining Company, is about 1 mile northwest of Eureka, Nevada in the northern part of the Eureka minin district (Nolan, 1962). The deposit, described in further detail by Dilles et al. (1996), is a largely stratiform body of silicification within non-carbonaceous, cherty Ordovician limestones of the Pogonip Group. This zone, West Archimedes, is approximately 2600 feet long at N60°W, and averages 260 feet inwidth and 75 feet in height; the zone plunges gently east. Gold mineralization has low concentrations of Ag-Pb-Zn-Cu and correlates strongly with silicification, which is generally surrounded by decalcified but unsilici:fied limestone containing lower grade. Published reserves for the western zone are 7.62 million short tons grading 0.099 ounces per ton (opt) Au; the mineralization is oxidized. Values above 0.25 opt Au are common in the central part of the zone. This stratiform silicification is concealed by as much as 500 feet of alluvium and locally intersects the bedrock surface. No intrusive rocks or calc-silicate alteration are present within the West Archimedes deposit, although an altered sill lies more than 1000 feet below. This gold-bearing silicification is herein referred to as Type A. North-northeast and west-northwest trending normal faults are thought to have provided ingress for hydrothermal fluids in the deposit area (Dilles et al., 1996); however, the lack of bedrock exposure at the deposit has precluded thorough analysis of the structures.
More strongly brecciated gold-bearing silicification (Type A) on the east side of West Archimedes is underlain
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Including past production, reserves and resources, the Carlin Trend forms the largest and most prolific accumulation of gold deposits in North America. More than 40 separate deposits have been delineated since disseminated gold mineralization in carbonate rocks was discovered in 1961. From this discovery, a classification for this style of gold mineralization has come to be referred to as “Carlin-type” deposits. To date, more than 25 million ounces of gold have been mined on the Carlin Trend from 26 separate operating, or past producing mines (Table 1 ). Open pit mining on the Carlin Trend began in 1965 at the Carlin Mine, and underground mining began in 1993 on the same deposit.
The scope of this paper is to first present a regional overview of the Carlin Trend, followed by summary descriptions of some of the more recent discoveries of deep, predominantly refractory gold deposits. As part of a concluding discussion, a spectrum of Carlin Trend deposits are categorized on a quaternary diagram to illustrate the I relative influence of structural and stratigraphic controls on each deposit.
The Carlin Trend is a 60 kilometer long north-northwest trending alignment of gold deposits located in northeastern Nevada, within the larger Great Basin physiographic province of the western United States (Figs. 1, 2). Gold deposits are hosted in a variable stratigraphic package of Ordovician through lower, Mississippian rocks. Within specific deposits, gold mineralization hosted in Cretaceous and Tertiary dike swarms and the Jurassic-Cretaceous Goldstrike granodiorite stock constitutes up to 15% of the mineralized material.
Regional Tectonic Development Regional stratigraphic and isotopic data indicate that northeastern Nevada was situated along a stable paleo-continental margin during much of the Cambrian through Early Mississippian (Stewart,1980). During this period, a westward-thickening, prism-shaped package of sediments were deposited from the outer margins of the paleo-continental shelf into an oceanic basin. Within this depositional environment, sedimentary facies graded from western eugeoclina1, to eastern miogeoclinal sequences.
During Late Devonian through Middle Mississippian time, eastward-directed compressional tectonism associated with the Antler orogeny resulted in regional scale folding and imbricate thrusting of the western eugeoclinal assemblage of predominantly siliciclastic rocks, over the eastern autochthonous assemblage of silty carbonate rocks (Roberts et al., 1958). The resultant accretionary mass formed the emergent Antler highlands which shed an eastward directed overlap assemblage of clastic rocks during Middle Mississippian to Early Pennsylvanian (Smith and Kettner,1975).
Late Paleozoic tectonism during Early to Middle Pennsylvanian time (Humboldt orogeny) was followed by deposition of shelf carbonate sequences during the Late Pennsylvanian and Permian (Smith and Kettner,1975; Kettner, 1977).