Albert H. Hofstra, 1997. "Isotopic Composition of Sulfur in Carlin-type Gold Deposits: Implications for Genetic Models", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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The goal of this report is to consider new and existing sulfur isotope data from Carlin-type gold deposits within the context of a modem understanding of the geology of the region and processes of ore formation. Increased understanding of the source(s) of sulfur and mechanisms of H2S generation may help to discriminate between the various models proposed for the deposits. The source of reduced sulfur is very important because available evidence indicates the deposits formed from H2S-rich (SO4-poor) fluids that transported gold as a bisulfide complex. High H2S-concentrations in ore fluids are indicated by (1) the common occurrence of high sulfidation state minerals such as orpiment and realgar, (2) mass transfer studies which show that sulfur was one of the most abundant elements introduced by the hydrothermal fluids, (3) gas analyses of fluid inclusions which show the fluids contained 10-1 to 10-2m H2S, (4) petrographic and geochemical evidence which indicates that sulfidation of host rock iron was an important (but not the only) gold depositional mechanism in most of the deposits, and (5) chemical reaction path modeling which shows that sulfidation can produce the gold grades observed in the deposits (Hofstra et al., 1991; Hofstra, 1994). The predominance of H2S over sulfate is important because the isotopic composition of H2S is then representative of the bulk composition of sulfur in the ore fluids. Given that gold was transported as a bisulfide complex, knowledge of the sites of H2S production and migration pathways will also likely provide information on the source
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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).