Role of Sulfidation in Deposition of Gold, Twin Creeks Sediment-Hosted Micron Gold Deposit, Nevada
David P. Stenger, Stephen E. Kesler, John Fortuna, Dean R. Peltonen, 1997. "Role of Sulfidation in Deposition of Gold, Twin Creeks Sediment-Hosted Micron Gold Deposit, Nevada", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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Despite important advances in our understanding of sediment-hosted micron gold deposits, debate continues over the processes that deposited their gold. Possible processes include cooling or boiling of the ore fluid, dilution, oxidation or other changes caused by mixing two fluids, or wallrock reactions such as carbonate dissolution and sulfidation. Several recent studies have called attention to the role of sulfidation in deposition of SHMG ores. At Jerritt Canyon, support for fluid mixing and sulfidation was provided by the relation between δ18O values and gold grades in ore-stage jasperoids (Northrop et al, 1987) and chemical reaction-path modeling of ore and alteration mineralogy (Hofstra et al, 1991). At Carlin, studies of fluid inclusions by Kuehn and Rose (1995) suggested that mineralization accompanied by sulfidation took place when an acidic, CO2-rich, mineralizing fluid mixed with cooler, gas-poor meteoric(?) water as it underwent episodic leakage from a deep overpressured reservoir.
The best way to assess the role of sulfidation in SHMG deposits is through comparison of the abundance and distribution of elements at the scale of a deposit. We report here the results of comparisons of this type based on a database comprised of 20-foot composite samples from hundreds of drillholes at Twin Creeks, all of which were analyzed for Au, Fe, S (total and reduced) and other elements of interest and labeled according to rock type and degree of oxidation. From the standpoint of chemical compositions, one of the most important differences among the depositional mechanisms listed above is the degree to which
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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).