Sequence of Deformational Events and the Recognition of Eocene(?) Deformation in the Alligator Ridge Area, East-central Nevada
Constance J. Nutt, 1997. "Sequence of Deformational Events and the Recognition of Eocene(?) Deformation in the Alligator Ridge Area, East-central Nevada", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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The Alligator Ridge district hosts disseminated gold deposits (Alligator Ridge, Yankee deposits) in Mississippian and Devonian clastic and carbonate rocks. The deposits are about 70 miles (113 km) northwest of Ely, Nevada, along the southern projection of the Carlin trend (Fig. 1), and east of the Roberts Mountain thrust. The area is underlain by lower-plate Paleozoic miogeoclinal carbonate and clastic rocks, Eocene(?) sedimentary rocks; and Oligocene and younger volcanic rocks. Jurassic plutonic rocks are exposed to the north at Bald Mountain (Hitchbom et a/., 1996) (Fig. 2). Hitchbom eta/. (1996) propose that at Bald Mountain tungsten, molybdenum, zinc, and precious metal deposits, including disseminated gold, are associated with the Jurassic pluton and are Jurassic in age, and that Alligator Ridge and Yankee are related distal deposits.
The well-exposed Alligator Ridge area is ideal for unraveling the geologic history of ea.stem-facies rocks, and, in particular, documenting Mesozoic and Tertiary deformation and their controls on ore. An improved understanding of the tectonic history in the Alligator Ridge district may also help to identify Mesozoic and Tertiary structures in the Carlin trend which is underlain by eastern- and western-facies rocks that were deformed during the late Devonian-early Mississippian Antler and late Permian Sonoma orogenies, as well as during the Mesozoic and Tertiary tectonic events. Previously published geologic maps have depicted the Alligator Ridge-Bald Mountain area as a relatively undeformed uplift between the isoclinally folded and thrusted Diamond Mountains on the west side of Newark Valley and the broad syncline comprising the Butte Mountains
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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).