Hydrothermal processes at the Comstock Lode mining district, Virginia City, Nevada, United States of America

The Comstock Lode mining district, just south of Reno, Nevada, hosted one of the most incredible concentrations of gold and silver ever discovered. Although the bonanzas that once drew Mark Twain, John Mackay, and thousands of miners to the Comstock district in the nineteenth century have long since been mined, this ground has provided geologists with intriguing questions for over a century.; Most unaltered igneous rocks have similar 18O to 16O ratios, represented by whole-rock δ18O values of +7 ± 1.5 ‰, but can be shifted to lower values by exchange with heated groundwaters circulating near magmatic or volcanic activity. This study uses this effect to investigate the Miocene hydrothermal system that formed the famous Comstock Lode ore bodies and produced low δ 18O values in the wall rocks.; More than 900 oxygen isotope ratio analyses on a network of surface and underground samples provide unparalleled resolution of δ18O variations in the Comstock district. This database is used to produce the first three dimensional visualizations of δ18O variation in a hydrothermal system. For example, a detailed 3-D visualization model of oxygen isotope data within a cubic kilometer block defines δ 18O isosurfaces that are conformable to the Big Bonanza, the most valuable ore body at Comstock. On a district-wide scale, patterns of 18O depletion in the hanging wall of the Comstock fault define a highly symmetrical pattern in three dimensions. This symmetry is consistent with strong fluid convection above the Comstock fault, manifested as longitudinal rolls of fluid flow that are superimposed on a typical unicellular hydrothermal system.; Normal faults perturb and offset the regional pattern of δ 18O contours produced by pervasive hydrothermal alteration. These faults are delineated by low 18O belts and high isotopic gradients perpendicular to strike, a result of increased permeabilities that promote the advection of fluids and heat. A series of cross-sections through the district shows that the Comstock, Occidental, and Coryell faults are each bound by an approximately 200 m wide, fault-parallel zone of low 18O rocks in the hanging wall. A mathematical model is developed to deconvolve the observed oxygen isotope data from sample profiles across normal faults, thereby quantifying the effects of enhanced fluid-rock interaction and post-hydrothermal displacement.