Mineralogy and Paragenesis of Ore Stage Mineralization in the Betze Gold Deposit, Goldstrike Mine, Eureka County, Nevada
G.C. Ferdock, S.B. Castor, R.W. Leonardson, T. Collins, 1997. "Mineralogy and Paragenesis of Ore Stage Mineralization in the Betze Gold Deposit, Goldstrike Mine, Eureka County, Nevada", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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The Betze gold deposit in the Goldstrike Mine, north-central Carlin Trend, Eureka County, Nevada, is the largest known, sedimentary rock-hosted, disseminated gold deposit containing 32 million ounces of gold (Leonardson and Rahn, 1996)(Fig.1). The paragenesis of hydrothermal mineralization characterizes the nature of gold mineralization in the deposit. This paper describes stages of gold ore mineralization determined from hand specimen and thin section petrology, XRD analysis and SEM studies; mineral formulae reported are those from Frondel (1962) and may not be exact. The stages result from a poly-phase, mineralizing event that has determined the conditions under which gold was precipitated.
Previous studies of paragenesis and associated hydrothermal minerals of Carlin-type deposits were conducted by Arehart et al. (1993a), Bakken and Einaudi (1986), Kuehn and Rose (1992), Peters (1996) and Peters et al. (1997) who determined relative paragenesis of mineralization in a general one or two stage hydrothermal event. Our investigations suggest that three stages of hydrothermal mineral development are associated with gold ore mineralization in the Betzedeposit: Stage I: Early (pre-primary gold deposition); Stage II: Main (primary gold deposition), and Stage III: Late (post-primary gold deposition). Pre-gold stage mineralization events include diagenesis of the sediments and metasomatic processes associated with the intrusion of the dioritic Goldstrike stock. Hydrothermal minerals associated with an individual stage generally are limited to each stage; however, overlap of the major gangue minerals, such as quartz, illite, and sericite (Fig. 2) is common. Some mineralogical overlap between stages is due to cycling of circulating fluid that
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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).