Pb and Sr Isotopic Mapping of Crustal Structure in the Northern Great Basin
J.L. Wooden, R.M. Tosdal, R.W. Kistler, 1997. "Pb and Sr Isotopic Mapping of Crustal Structure in the Northern Great Basin", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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Two subparallel NNW-to NW-trending mineral belts in Nevada, the Battle Mountain-Eureka trend on the SW and the Carlin trend on the NE, are thought to reflect deep-seated, pre-Cenozoic crustal structures. These structures may be pre-Cenozoic faults, Mesozoic and/or Paleozoic fold axes, or uncertain features of the Precambian basement. Both geophysical and geochemical/isotopic studies can be used to complement field based geologic studies of these features. Geophysical studies measure time-integrated physical parameters and attempt to distinguish younger from older features. Isotopie studies have the advantage of investigating time-related features by comparing the isotopie compositions of rocks formed at different times during the geologic history of a region for systematic or significant changes or lack there of. The isotopie signatures of igneous rocks largely reflect the average characteristics of their source regions plus any later interaction with the crustal column through which they moved or into which they were emplaced, and in general reflect lower and middle crustal features. Kistler and Peterman (1973, 1978) and Kistler (1983, 1991) demonstrated that the distribution of Sr isotopie compositions of granitoid rocks in the northern Great Basin delineate crustal structure, particularly the location of the continental-oceanic crustal boundary as marked by the Isr = 0.706 line. Ellison et al. (1990) showed that the Isr=0.706 Ime is correlated with Paleozoic stratigraphy. Farmer and DePaolo (1983, 1984) used combined Nd and Sr isotopie compositions of Great Basin granitoids to study the pedogenesis of these rocks and regional crustal structure; however, these pioneering studies are limited by the
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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).