Scientists say more work is required before they can gain an understanding of the genesis of the sediment-hosted, disseminated deposits that comprise a major portion of gold production and reserves in the U.S.
These deposits are characterized by extremely fine-grained, disseminated gold hosted primarily by arsenian pyrite. Other metals are virtually absent, although anomalous concentrations of certain elements are used as exploration tools. The host rocks are silty carbonates, but ore concentrations are also present in siliceous and silicified rocks, as well as intrusive rocks.
Until the last few years, these deposits were considered near-surface, epithermal-type deposits. Because of their fine-grained nature and lack of macroscopic features, such as veins, extracting geochemical data clearly related to their genesis has been difficult.
New views are being developed based on fluid inclusion data (which indicate pressures corresponding to depths of 2-5 km under lithostatic conditions), as well as from stable isotope data from alteration minerals and fluid inclusions.
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