“BLEACHING” AT THE COMSTOCK DISTRICT, NEVADA: DISTINGUISHING MULTIPLE CAUSES

Outcrops in the Comstock Lode Mining District, Nevada are mostly grey, propylitized andesites with low whole-rock d¹⁸O values (+3 to –3 ‰) and variable magnetic susceptibility (k) that ranges from approximately 0.1 to 32 x 10^-3 SI. Also common are pale, yellowish rocks, that have very low k, variable d¹⁸O, and are separated from the surrounding grey andesites by sharp boundaries. This “bleaching” is produced by at least three distinct processes at Comstock.

1) Hypogene bleaching occurred during the formation of Miocene quartz-alunite alteration zones, likely related to acid-sulfate fumaroles and springs that accompanied the near neutral, chloride-type regional hydrothermal system. These rocks are characterized by high d¹⁸O values (+8 to +12 ‰), except where they were later resilicified by low ¹⁸O fluids.

2) Supergene bleaching developed as cool, rain water reacted with pyrite, producing sulfuric acid that leached the andesites, leaving a quartz-kaolinite rich residue. These bleached rocks have mostly inherited the d¹⁸O values of their protoliths, but are typically elevated by 1 to 3 ‰ due to low T silica and clay precipitation and the destruction of mafics. This type of bleaching is readily observed at an abandoned rail road tunnel in southern Virginia City (Sec. 32 D) that collapsed as the rocks were weakened by this process. Red leachate collected there had a pH of 2.

3) Historical development of supergene bleaching is recorded in mine dumps, most of which date back to the late 1800’s. Andesite samples collected in the deep mines while they were in operation are usually dark grey, and must typify the rocks that were originally dumped on the surface. Over the past ~125 years, pyrite oxidation fostered bleaching and mechanical breakdown, producing a light, yellowish-brown color. At the Yellow Jacket shaft, subsurface samples have consistently low d¹⁸O values (-1.3 ± 0.8 ‰), while "soil" cores from its dump have d¹⁸O values of +0.7 ± 0.5 ‰. Due to the short duration of the mine dump bleaching, these rocks still contain feldspar and chlorite, but are shifted to moderately higher d¹⁸O values. The k of samples from the Ward and Yellow Jacket dumps (0.4 ± 0.1 x 10^-3 SI) are slightly higher than those of bleached outcrops (0.04 ± 0.02 x 10^-3 SI), probably indicating incomplete oxidation.