O¹⁸/O¹⁶ evidence for meteoric-hydrothermal alteration and ore deposition in the Tonopah, Comstock Lode, and Goldfield mining districts, Nevada

Abstract

Oxygen isotope analyses were obtained for 50 whole-rock samples, 26 vein quartz samples, 4 calcites, and 1 adularia from the volcanic rocks, intrusives, and hydrothermal veins at Tonopah, Goldfield, and the Comstock Lode. At Tonopah, the delta O 18 values are as follows: Mizpah trachyte (-5.9 to +3.3); West End rhyolite (--4.7 to +1.5); Sandgrass andesite (--6.0); main-stage vein quartz (--5.1 to --1.8); late, euhedral quartz crystals (--3.2 to +0.7). The delta D values of 5 of these rocks vary from --135 to --152. These materials are thus depleted in O 18 by about 5 to 15 per mil and in D/H by about 60 to 70 per mil, relative to values in "normal" igneous rocks. The postmineralization Oddie rhyolite and Fraction volcanic breccia, however, have much higher delta O 18 values (+2.5 to +9.5). The hydrothermal alteration and ore deposition therefore must have been produced by heated meteoric ground waters (delta O 18 [asymp] --13, delta D [asymp] --100) that had undergone very little or no O 18 enrichment as a result of exchange or addition of magmatic water. Various models suggest that this involved very high water/rock ratios (>2) and occurred at about 250 degrees -300 degrees C. Although the delta O 18 of quartz is higher (+1.5 to +3.0), the range of whole-rock delta O 18 values at the Comstock Lode is similar to that at Tonopah: Alta andesite (--4.1 to +4.7); Davidson diorite stock (--2.6 to --0.8). At Goldfield, somewhat higher whole-rock delta O 18 values are observed: altered volcanic rocks (+1.7 to +5.8); ore zones (--3.8 to +3.8). These data also indicate a dominant meteoric ground-water component in the hydrothermal fluids, but they suggest smaller water/rock ratios at Comstock and lower temperatures of alteration at Goldfield. It is concluded that most epithermal gold and silver ore bodies in highly faulted piles of altered volcanic rocks probably were deposited by meteoric-hydrothermal solutions heated and set into convective circulation by interaction with epizonal igneous intrusions.

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