An Amagmatic Origin for Carlin-Type Gold Deposits
Robert P. Ilchik, Mark D. Barton, 1997. "An Amagmatic Origin for Carlin-Type Gold Deposits", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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Suggested genetic models for Carlin-type deposits make connections to magmatism, regional metamorphism or regional extension (Sillitoe and Bonham, 1990; Seedorff, 1991; Ilchik and Barton, in press). Depositional mechanisms are uncertain with mixing, wall-rock reaction, and cooling each consistent with at least some of the available evidence (e.g. Hofstra et al., 1991; Kuehn and Rose, 1995). Based on geological, physical, and chemical reasoning, we propose a genetic model in which meteoric fluids were circulated by heat released during crustal extension during the mid-Tertiary. These deeply circulated fluids interacted with the sedimentaryrock pile and scavenged gold under chemically favorable conditions. Upon upwelling, these fluids interacted with various lithologies and/or other fluids and produce the characteristic alteration and metal suites of these deposits. To test the viability of this amagmatic model, we have investigated certain physical and chemical constraints implicit to the model.
Heat balance and thermal convection calculations indicate that ample surface waters could be heated to appropriate temperatures and circulated during rapid crustal extension (Figure 1). Additional heat from magmatism or elevated subcrustal heat flow could have contributed to the overall energy balance but is not necessary. The critical feature needed to generate major hydrothermal systems is a rapid increase in permeability. Such an increase likely accompanied the time-transgressive crustal extension across the northern Great Basin in the late Eocene and early Oligocene. Mass balance calculations indicate that solution transport efficiencies of <10% can account for several times the amount oflmown alteration and mineralization. In contrast, magma-driven or metamorphic fluids appear
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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).