Relation Between Pyrite and Gold at the Twin Creeks SHMG Deposit, Nevada
Grigore Simon, Stephen E. Kesler, Dean R. Peltonen, Stephen L. Chryssoulis, Hui Huang, James E. Penner-Hahn, 1997. "Relation Between Pyrite and Gold at the Twin Creeks SHMG Deposit, Nevada", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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Most gold in sediment-hosted micron gold (SHMG) deposits cannot be imaged by conventional reflected light, secondary electron (SEM), or transmitted electron (TEM) microscopy. Gold particles with a diameter of about 200 Å were observed by Bakken et al. (1989) as inclusions in pyrite, cinnabar, quartz and illite from the Carlin mine, although considerable gold could not be accounted for. Arehart et al. (1993) showed that much of this “invisible” gold at Post/Betze and Gold Quarry is in As-rich growth zones in pyrite, possibly in solid solution. At Twin Creeks, we have observed mat the gold content of ores appears to vary with the texture of pyrite. We report here results of a preliminary study on the relation between gold values and pyrite textures at the Twin Creeks SHMG deposit, based in part on ion probe measurements of gold concentrations in pyrite and X-ray Absorption Near-Edge Spectroscopy (XANES) measurements to determine the oxidation state of gold in the pyrite.
SHMG mineralization in the Megapit (see Fig. 1 of Hallet al., this volume) area of the Twin Creeks deposit, where our work was concentrated, is hosted by calcareous black shales and interlayered volcanic rocks of the Ordovician Comus Formation, which form a large fold. Gold mineralization is concentrated in the nose of this fold and in favorable stratigraphic horizons in the limbs of the fold. Gold is associated with arsenic-bearing pyrite (arsenian pyrite) that was deposited along with various combinations of quartz, adularia, sericite, realgar, orpiment, and stibnite. The Megapit deposit is
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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).