268 Economic Geology

Wednesday, 28 September 2016
8:00 AM - 12:00 PM
Room 301

MAGMATIC-HYDROTHERMAL HISTORIES OF MIOCENE EPITHERMAL GOLD-SILVER DEPOSITS IN THE TONOPAH AND DIVIDE DISTRICTS, WESTERN NEVADA

Paper number: 268-6
Program: Technical Sessions
Time: 9:15 AM – 9:30 AM

Authors

Abstract

New field, geochemical, and geochronological data redefine the geologic setting and magmatic-hydrothermal histories of the Miocene Tonopah and Divide gold-silver districts. The main Tonopah district is within the Fraction caldera, formed by eruption of the ~20.2 Ma Fraction Tuff. Tonopah Formation rhyolites and Mizpah Formation andesite domes form the caldera floor and host epithermal quartz-adularia veins in the main Tonopah district (~1.86 Moz Au, 176 Moz Ag produced). The Divide and SW Tonopah (Three Hills area) districts are in the Klondyke caldera, formed by eruption of the ~17.3 Ma Heller Tuff. Divide Andesite and volcanics of Donovan Peak form resurgent lava domes and hypabyssal plugs in the central Klondyke caldera. Brougher and Oddie rhyolite domes partly outline the caldera margin. Both calderas are partly filled with megabreccia and Siebert Formation tuffaceous sedimentary rocks. In the Klondyke caldera, Heller Tuff, Siebert Formation, and Oddie Rhyolite host shallow Au-Ag deposits in the Divide district (~30 koz Au, 300 koz Ag produced) and Three Hills area. New 40Ar/39Ar dates (Ma) are: Mizpah Formation: 21.35±0.06, 20.54±0.20; Fraction Tuff: 20.19±0.10 to 20.01±0.18 (5 samples); Heller Tuff: 17.35±0.15 to 17.31±0.10 (4); Divide Andesite: 17.34±0.02; volcanics of Donovan Peak: 16.9±0.2; Oddie Rhyolite: 17.29±0.03 to 16.6±0.3 (3); Brougher Rhyolite: 17.18±0.02 to 16.55±0.11 (4); rhyolite ring dike near Klondyke: 17.04±0.03; Siebert Formation tuffs in Klondyke caldera: 17.5±0.5, 17.2±0.3; Belmont mine adularia: 20.56±0.12 to 19.81±0.14 (3); King Tonopah mine adularia: 20.08±0.05; Divide district adularia: 17.28±0.07 to 16.96±0.01 (3); Three Hills mineralization age bracketed by dated rocks: 17.5±0.5 to 16.59±0.06. SHRIMP U-Pb zircon ages are 20.0±0.4 Ma for a rhyolite dike on the north Fraction caldera wall and 18.6±0.2 Ma for a rhyolite breccia block near the south Klondyke caldera wall. These data show (1) Fraction caldera formation and vein emplacement in main Tonopah district are essentially coeval and (2) Klondyke caldera resurgent magmatism and mineralization in the Divide and SW Tonopah districts are approximately contemporaneous. Both suggest a strong genetic relationship of low-sulfidation epithermal Au-Ag deposits to magmatism.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-281757

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