Mississippian Sedimentary Rock-hosted Gold Deposits of the Eastern Great Basin: Relative Importance of Stratigraphic and Structural Ore Controls
Brian J. Maher, 1997. "Mississippian Sedimentary Rock-hosted Gold Deposits of the Eastern Great Basin: Relative Importance of Stratigraphic and Structural Ore Controls", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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Disseminated gold deposits hosted by sedimentary rocks of Mississippian age are common throughout the central and eastern portion of the Great Basin (Fig. 1). Although these deposits have a wide geographic distribution, the similarities in stratigraphic setting, size, tenor and the spatial distribution of gold and other hydrothermal minerals allow them to be grouped as a subtype of the “Carlin-type” of golddeposit. In particular, the presence of quartz or silica in a basal jasperoid is a characteristic feature of Mississippian sedimentary rock-hosted gold deposits. These deposits also have grade-tonnage relationships and total gold content which differs from the “average” Carlin-type gold deposit. Only three of the Mississippian sedimentary rock-hosted gold deposits studied have the size and grade which allow them to be economically significant in the context of worldwide gold production. While the larger and higher grade deposits share many geologic characteristics with the smaller and lower grade Mississippian sedimentary rockhosted gold deposits, it is apparent that the relative importance of structure and stratigraphy in ore-forming processes varies from deposit to deposit. Such differences are important, especially in the context of designing and executing exploration programs for the deposit subtype.
This paper, an outgrowth of earlier research on the Chert Cliff gold deposit (Vikre and Maher, 1996), describes the Mississippian sedimentary rock-hosted gold deposit subtype and elucidates important differences between the larger and higher grade economically significant Mississippian sedimentary rock-hosted gold deposits and their smaller brethren. The paper will also examine the stratigraphic positioning of individual deposits in space
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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).