Although relatively small, the sedimentary rock-hosted Sterling Mine has been the largest gold producer in the Bare Mountain district in southern Nye County, with total production of about 130,000 troy ounces between 1983 and 1996. Other gold production in the district has come from sedimentary and volcanic rock-hosted ore at the Mother Lode Mine (Mapa, 1990) and the Fluorspar Canyon deposits (Greybeck and Wallace, 1991) about 10 km north and northwest of the Sterling Mine. The district has also produced fluorspar and minor amounts of mercury.
At the Sterling Mine, mineralization is controlled by the intersection of north-northeast striking, high-angle normal faults with a thrust fault that separates siliciclastic rocks of the Late Proterozoic to Early Cambrian Wood Canyon Formation in the upper plate from carbonate rocks of the Middle Cambrian Bonanza King Formation in the lower plate. This mineralization is associated with an alteration assemblage that includes kaolinite, halloysite, illitesmectite, sericite, and alunite (Odt, 1983). Jarosite and limonite are present in most mineralized rock, but primary sulfide ore occurs in places and contains pyrite (Fig. 1 ), marcasite, arsenopyrite, and traces of sphalerite and galena (Joe Marr, personal communication, 1996). Samples ofunoxidized ore consist of quartz± dolomite± kaolinite and contain pyrite± marcasite± stibnite ±galena. On the basis of SEM examination, an unoxidized sample withl38 ppm gold contains pyrite with 1- to 2-micron-thick rims and veins of arsenian pyrite (see the figure below). Very rare micron-size gold is present, but the grade is too high to be accounted for by
Figures & Tables
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).