Geologic Characteristics of Sediment-Hosted, Disseminated Precious-Metal Deposits in the Western United States
Published:January 01, 1985
William C. Bagby, Byron R. Berger, 1985. "Geologic Characteristics of Sediment-Hosted, Disseminated Precious-Metal Deposits in the Western United States", Geology and Geochemistry of Epithermal Systems, B. R. Berger, P. M. Bethke
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Sediment-hosted precious-metal deposits are typically formed in carbonaceous, silty dolomites and limestones or calcareous siltstones and claystones. Gold mineralization is disseminated in the host sedimentary rocks and is exceedingly fine grained, usually less than one micron in size in unoxidized ore. Primary alteration types include silicification, decalcification, argillization, and carbonization. Supergene alteration is dominated by oxidation resulting in the formation of numerous oxides and sulfates and the release of gold from its association with sulfides. Commonly associated trace elements are arsenic, barium, mercury, antimony, and thallium. Deposits of this type are commonly referred to as either Carlin-type deposits, after the large bulk- minable, disseminated-gold deposit in northern Nevada, or as fine-grained or “invisible-gold” deposits. We refer to deposits of this type as sediment-hosted, disseminated precious-metal deposits.
This chapter presents a classification scheme and reviews the geologic characteristics of sediment- hosted, precious-metal deposits. The influences of geology on both mining and the development of genetic and exploration models are discussed. Although deposits of this type occur throughout the western United States, the largest concentration of deposits and also the best understood are in Nevada. We have chosen, therefore, to use selected individual deposits from Nevada as type examples to support the classification scheme and to provide the student with an understanding of the similarities and differences that occur in these deposits. This chapter is thus designed to develop and nurture the knowledge of the comparative geology of sediment-hosted, disseminated precious-metal deposits. This is accomplished by reviewing and comparing regional-, district
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Geology and Geochemistry of Epithermal Systems
In the context of exploration for epithermal deposits, why study geothermal systems at all? After all, not one exploited system to date has been shown by drilling to harbor any economically significant metal resource--but then until recently not one had been drilled for other than geothermal energy exploration.* The latter involves drilling to depths of 500-3000 meters in search of high temperatures and zones of high permeability which may sustain fluid flow to production wells for steam separation and electricity generation. In many cases such exploration wells have discovered disseminated base-metal sulfides with some silver and argillic-propylitic alteration equivalent to that commonly associated with ore-bearing epithermal systems (Browne, 1978; Henley and Ellis, 1983; Hayba et al., 1985, this volume). In general, however, geothermal drilling ignores the upper few hundred meters of the active systems and drill sites are situated well away from natural features such as hot springs or geysers, the very features whose characteristics (silica sinter, hydrothermal breccias) are recognizable in a number of epithermal precious-metal deposits (see, for example, White, 1955; Henley and Ellis, 1983; White, 1981; Berger and Eimon, 1983; Hedenquist and Henley, 1985; and earlier workers such as Lindgren, 1933). Knowledge of the upper few hundred meters of active geothermal systems is scant and largely based on interpretation of hot-spring chemistry. Tantalizingly, in a number of hot springs, transitory red-orange precipitates occur which are found to be ore grade in gold and silver and which carry a suite of elements (As, Sb, Hg, Tl) now recognized as characteristic of epithermal gold deposits (Weissberg, 1969).