57 Economic and Engineering Geology (Posters)

Wednesday, 23 March 2016
8:00 AM - 12:00 PM
Convention Hall

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

Paper number: 57-4
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 48, No. 1

doi: 10.1130/abs/2016NE-272341

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