MINERALOGICAL AND ISOTOPIC COMPARISON OF A SILICA-SULFIDE BRECCIA AND PEBBLE DIKE FROM THE NORTHERN SHOSHONE RANGE, LANDER COUNTY, NEVADA

The northern Shoshone Range is underlain by lower Paleozoic siliceous and siliciclastic rocks of the Roberts Mountains allochthon and intruded by Eocene intermediate-felsic igneous rocks. Historic mining activity exploited precious- and base-metal–bearing ore bodies which commonly occur as or with sulfidic breccias and pebble dikes. In this study, we mineralogically and isotopically compare a silica-sulfide breccia and pebble dike recovered from the same mineralized area within the northern Shoshone Range.

The silica-sulfide breccia consists of <10 cm-wide, bleached, recrystallized, angular fragments of chert>>quartzite>siltstone in a matrix of black, fine-grained silica + patchy sulfides + euhedral, clear quartz. Textural relationships support the following breccia paragenesis: 1) bleaching and recrystallization of country rocks, 2) fragmentation of altered host rocks, 3) deposition of black, fine-grained silica + sulfides between clasts, 4) deposition of clear, euhedral quartz within residual open spaces between clasts and within matrix. Whole rock assay of the silica-sulfide breccia reveals (in ppm): Au, 4.6; Ag, 9.2; As, 8630; Cu, 173; Pb, 214, and Sb, 192. Electron microprobe analysis of sulfides in the matrix reveals an intergrowth of galena, sphalerite, stibnite, pyrite, chalcopyrite, lautite, ferroselite, and tetrahedrite. Primary fluid inclusions within the late-stage, clear, euhedral quartz homogenize to liquid at 345.5°C (avg., n=4) with a salinity of 3.2 equiv wt % NaCl. Calculated isotopic composition of the source fluid from which the euhedral quartz precipitated the Th and measured δ¹⁸O is forthcoming.

The pebble dike is locally matrix supported and consists of <20 cm-wide, bleached, recrystallized, angular fragments of chert (± quartz veins)>>quartzite>siltstone>porphyry in a matrix of black, fine-grained silica with no visible sulfides. Assay data of the altered rock clasts reveals (in ppm): Au, 0.08; Ag, 0.7; As, 30.7; Cu, 26.4; Pb, 18.5, and Sb, 1.79; concentrations of these elements are consistently higher in the clasts than in the matrix material. The black color of the matrix is due to organic C (0.53%, δ¹³C = -27.6‰), not sulfides as in the breccia.

Both samples were recovered from mine dumps and were not in-situ, therefore we cannot determine the relative ages between them.