Contrasting Siliceous Replacement Mineralization, East-Central Nevada
Mark D. Barton, Eric Seedorff, Robert P. Ilchik, Gregory Ghidotti, 1997. "Contrasting Siliceous Replacement Mineralization, East-Central Nevada", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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Fine-grained siliceous replacement of carbonate-bearing rocks (‘jasperoid’) occurs in most mineral districts in east -central Nevada. In many of these occurrences, jasperoid contains Au and(or) Ag and little or no base metals, although concentrations and ratios range markedly. Here we compare and contrast these systems based on the geological and geochemical characteristics of the siliceous rocks and, where present, associated igneous and other types of hydrothennally altered rocks. At issue are both the broad patterns of regional metallogeny and specific links between Carlin-type mineralization and other geologic phenomena. These results are based on our own work and published data.
Broadly, two end-members are distinguished: (1) silicification as an intermediate- to late-stage part of complex alteration associated with igneous centers, and (2) jasperoids lacking other associated alteration and having few or no associated igneous rocks. Within this region, siliceous replacements are found with nearly all metallic(± magmatic) suites. The principal types are summarized in Table 1 and localities with selected characteristics are shown in Figure 1.
Igneous-related systems have well-developed time-space patterns of high-temperature proximal alteration overprinted and surrounded by lower temperature alteration assemblages. Jasperoids and other siliceous bodies are prominent in the latter suite, showing considerable differences in their mineralogical, geochemical, and textural characteristics. Pronounced zoning occurs over distances of a few hundred meters to no more than a few kilometers. These systems also exhibit strong correlations between style and composition of alteration, the composition of the igneous rocks, and associated metals (Fig. 1 ). Igneous-related systems show a pronounced
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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).