Abstract

The DeLamar silver mine, located in southwest Idaho, produced over 17 million ounces of silver and 230 thousand ounces of gold by bulk mining methods between 1977 and late 1987. The mine occurs within the Silver City rhyolite--an extensive and complex sequence of middle Miocene silicic volcanic flows and domes that unconformably overlie a thick pile of Miocene alkali olivine basalt. The basalt overlies Late Cretaceous granite which is correlative with the Idaho batholith. All three major rock types contain epithermal silver-gold veins, although rhyolites such as those at DeLamar, are the only hosts for low-grade, large tonnage mineralization.The DeLamar mine occurs within a sequence of six pervasively altered silicic volcanic units. From oldest to youngest these units have the following informal designations: (1) porphyritic latite, (2) quartz latite, (3) tuff breccia, (4) porphyritic rhyolite, (5) banded rhyolite, and (6) millsite rhyolite. The porphyritic rhyolite unit is spatially, temporally, and genetically related to silver-gold mineralization and is the principal host rock at DeLamar. This unit was emplaced as coalescing and overlapping lava domes and was preceded by cogenetic eruption of tuff breccia in the form of tuff rings. The emplacement of tuff rings and lava domes was controlled by structures related to a shallow level magma body and regional basin-and-range faulting. Onlap of banded rhyolite preceded hydrothermal activity and served as an important control on localizing ore deposition. The mineralogic character of ore and alteration at DeLamar is similar to adularia-sericite-type volcanic-hosted epithermal deposits, but the geologic setting is more like that of acid-sulfate deposits.

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