Ore Microscopy of the Silver Minerals in the Epigenetic Ag-W-Sn Deposits in the Silver Mine District, Southeastern Missouri, U.S.A.

Abstract

The Precambrian epigenetic Ag-W-Sn deposits of the Silver Mine district in southeastern Missouri occur as erratically distributed pods, lenses, and irregularly shaped bodies in quartz veins, contiguous greisen, and greisenized tuff. Ore microscopic examination of 200 polished sections prepared from specimens from nine mines and prospects in the district shows that the primary ores consist of 24 minerals deposited during several stages. Greisen and early vein stages involved the crystallization under xenothermal conditions of quartz, topaz, apatite, sericite, cassiterite, rutile, ilmenite, wolframite, scheelite, zinnwaldite, arsenopyrite, specular hematite, and magnetite. Subsequent stages of base and precious metal deposition consisted of pyrite, marcasite, chlorite, fluorite, sphalerite, pyrrhotite, argentiferous tennantite, antimonpearceite, argentiferous galena, chalcopyrite, berryite, and additional quartz. Fluid inclusion homogenization temperatures for fluorite and sphalerite indicate that the late sulfides and silver-bearing minerals formed under epithermal conditions.

Argentiferous tennantite (Cu_11.9Ag_0.9As_3.71Sb_0.95Bi_0.01S₁₃) occurs as 100–400 µm grains disseminated in quartz. Antimonpearceite (AS_18.8Cu_4.28As_1.34Sb_0.76Bi_0.01S₁₁) partially surrounds and veins some argentiferous tennantite grains. Argentiferous galena and argentiferous chalcopyrite (0.17% Ag) surround and partly replace argentiferous tennantite and antimonpearceite. The youngest silver mineral is berryite (Pb_3.03Cu_3.04Ag_3.09Bi_9.43Sb_0.05S₁₆), which is not found in association with argentiferous tennantite and antimonpearceite. Berryite has formed by the replacement of galena and chalcopyrite and locally it contains tiny irregularly shaped remnants of those two minerals.