Ore Fluids at the Getchell, Carlin-Type Gold Deposit, North-Central Nevada
Jean S. Cline, Albert Hofstra, Gary Landis, Robert Rye, 1997. "Ore Fluids at the Getchell, Carlin-Type Gold Deposit, North-Central Nevada", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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The Getchell gold deposit is one of several Carlin-type gold systems located along the Getchell Trend in north-central Nevada. Deposits located along this trend exhibit all of the characteristics typical of Carlin-type systems including the presence of submicron-sized gold particles in arsenic-rich pyrite. Although Carlin-type deposits have been mined for thirty years and are currently responsible for Nevada being one of the leading gold pr~ducers in the world, there is little consensus concerning the geological processes that concentrated gold in these deposits.
The lack of understanding of formation conditions stems from the submicron size of the gold particles, a paucity of ore stage fluid inclusions in most systems, and a lack of readily datable minerals that are unequivocally related to gold mineralization. The lack of visible gold, even under the microscope, has made it difficult to identify minerals and textures associated with gold precipitation, complicating determination of gold precipitation mechanisms. Fluid inclusions are sparse or absent in the fine-grained replacement ore present in most systems, making it difficult to determine system pressure and temperature, and ore fluid chemistry. The lack of datable minerals has prevented identification of key geological events responsible for gold concentration.
The Getchell deposit is somewhat unusual in that a significant amount of open-space-filling mineralization is hosted within the Getchell fault zone. This mineralization exhibits cross-cutting and textural relationships that provide constraints for the ore paragenesis. Additionally, these slightly more coarse-grained minerals trapped several populations of fluid inclusions, including primary inclusions contained within growth zones. Examination of
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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).