TERTIARY EPITHERMAL-MESOTHERMAL AU-AG MINERALIZATION IN HOMESTAKE MINE, LEAD, SOUTH DAKOTA

Tertiary epithermal-mesothermal Au-Ag deposits in the northern Black Hills are spatially associated with intrusive domes in a WNW-trending alkalic igneous province, and include sediment-hosted, igneous- and breccia-hosted, and schist-hosted varieties. A 2,424 m (8000 ft) deep exposure of intrusion-related, schist-hosted mineralization is in the Homestake mine.

Fluid inclusion trapping temperatures decrease gradually upwards from 480^°C to 350^°C in schist-hosted mineralization to the Precambrian-Cambrian nonconformity, where there is a sharp decrease in temperature to 170° to 300 °C in sediment-hosted deposits. Salinities are mostly less than 10 wt%, with minor occurrences of inclusions containing daughter halite and sylvite. Fluid composition changes from saline at depth, to a CO₂-bearing fluid at intermediate levels, to a dilute aqueous fluid above the nonconformity.

Igneous rock δ¹⁸O values are 8-13 per mil, and Tertiary vein quartz δ¹⁸O values are 11.4 to 16.9 per mil. Ore fluid δ¹⁸O values are 8.5 to 12.4 per mil below the 3000 level but decrease sharply to 1.7 to 4.4 per mil in the Deadwood Formation above the nonconformity. Together with fluid inclusion δD values of -47 to -91 per mil, the isotope data are consistent with the deep fluid being magmatic. The decreases in δ¹⁸O, temperature, and salinity at the nonconformity suggest that the magmatic fluid mixed with a meteoric water, a likely cause of ore deposition in sediment-hosted deposits.

Associated metals Mo, W, and Bi are suggestive of a magmatic source. Previous lead isotopic data indicate a source from hydrothermal leaching of the Precambrian crust or crustal contamination of the Tertiary magmas. Although the Tertiary mineralization is clearly related to the alkalic igneous activity, the identity of the causative magma is unclear but is suspected to be deeper than presently exposed mine levels.∂