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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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