Eocene Magmatism of the Tuscarora Volcanic Field, Elko County, Nevada, and Implications for Carlin-type Mineralization
Christopher D. Henry, David R. Boden, 1997. "Eocene Magmatism of the Tuscarora Volcanic Field, Elko County, Nevada, and Implications for Carlin-type Mineralization", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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The Tuscarora volcanic field (TVF), the largest Eocene volcanic field in Nevada, lies just north of major gold deposits of the Carlin Trend and west of deposits in the Independence Mountains (Fig. 1). Ongoing detailed mapping documents at least five voluminous volcanic-intrusive centers in the southeastern part of the TVF (Figs. 1, 2). 40Ar39 Ar dates show that the five centers developed between 39.9 and 39.2 Ma during a brief, intense period of magmatism. Precious-metal mineralization at Tuscarora formed at about 39.3 Ma, contemporaneous with a major intrusive episode, and is the oldest Tertiary volcanic-hosted epithermal deposit in Nevada. Compilation of 40Ar39 Ar dates indicates that magmatism was particularly intense between 39 and 40 Ma throughout northeastern Nevada. Given that several Carlin-type deposits are now interpreted to have formed in the Eocene (Hofstra, 1995~ Emsbo, 1996~ Leonardson and Rahn, 1996~ Phinisey et al., 1996~ Rota, 1996), contemporaneous with this activity, we suggest that Eocene magmatism was directly or indirectly the heat source to drive hydrothermal circulation that generated Carlin-type deposits (see also Hofstra, 1995).
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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).