Age of Mineralization at the Twin Creeks SHMG Deposit, Nevada
Chris M. Hall, Grigore Simon, Stephen E. Kesler, 1997. "Age of Mineralization 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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This paper will explore the significance of thermochemical sulfate reduction (TSR) to the origin of sedimenthosted disseminated gold (SHDG) deposits. TSR provides a means of generating abnormally high concentrations of H2S at moderate temperatures (e.g., 100 to 200°C). A high H2S content is a prerequisite for mobilizing gold and other metals associated with SHDG deposits (Ag, Hg, Sb, As), and is also necessary to form ore bodies with a high Au/Ag ratio. Field and sulfur-isotopic evidence supporting the TSR hypothesis will be reviewed. The discussion will focus on SHDG deposits in the Carlin Trend and similar deposits in China, as well as the Hemlo gold deposit, Ontario, a possible Archean analog of this deposit type. If valid, the TSR hypothesis has significance to the minerals exploration industry as a means to discriminate between favorable vs. unfavorable source rocks.
Mineralization at Twin Creeks mine is hosted by rocks of the Ordovician Comus Formation, which consists of dolomitic to calcareous black shales with tuffaceous interbeds and interlayered basalt flows and sills. These rocks have been deformed into a northwest-trending fold system with a known length of about 4.5 km (3 miles). Gold mineralization post -dates this folding, and is concentrated in the nose of this fold and in favorable stratigraphic horizons in the limbs of the fold. In the most intensely mineralized part of the deposit, the Megapit, the fold is cut into three sections by right-lateral the TC (northern) and DZ (southern) faults trending NE-SW (Fig. 1) and dipping steeply to
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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).