EXAMINING THE RELATIONSHIP BETWEEN SEDIMENTARY-HOSTED AU DEPOSITS AND MAGMATISM IN THE BATTLE MOUNTAIN MINING DISTRICT USING GEOCHEMISTRY, GEOCHRONOLOGY, AND THERMOCHRONOLOGY

Although Carlin-type gold deposits in Nevada have produced nearly 200 Moz since 1965, better understanding of mineralizing processes would help refine exploration efforts. The critical question is whether these deposits formed from magmatic hydrothermal fluids or circulating meteoric/metamorphic fluids. While difficult to test directly, geochronology and (U-Th)/He thermochronology allow us to determine the timing of magmatism and thermal events such as hydrothermal fluid flow, respectively. If thermal events in mineralized zones and magmatism are consistently contemporaneous, this implies magmatism is a necessary control on Carlin-type deposit formation. The Battle Mountain mining district provides an ideal area in which to test this relationship, as the district contains deposits that display characteristics of distal-disseminated, skarn, and Carlin systems.

Rhyolite dikes dated at Lone Tree yielded zircon U-Pb CA-TIMS ages of 40.95 ± 0.06 Ma and 40.94 ± 0.05 Ma. Zircon and apatite (U-Th)/He dates generally fall between 31 and 39 Ma, suggestive of a cooling magmatic system with accompanying hydrothermal fluids. At Brooks, our zircon (U-Th)/He dates are Cretaceous (96.58 ± 1.39 Ma to 80.84 ± 1.69 Ma), contradicting mine site interpretations that the intrusions were contemporaneous with those at Lone Tree. Two granodiorite intrusions at Marigold yielded Cretaceous zircon U-Pb CA-TIMS ages as well (96.59 ± 0.07 Ma, 92.22 ± 0.08 Ma). Quartz and sulfide textures evidence at Marigold suggests a Cretaceous-base metal event was overprinted by a younger Au event, hypothesized to be Eocene. Apatite and zircon (U-Th)/He dates from the margins of Au-mineralized intrusions at Marigold and Valmy support Eocene hydrothermal fluid flow. One granodiorite intrusion in the transition zone between the Marigold-Valmy deposits and Trenton Canyon, along the Oyarbide fault, has preliminary zircon U-Pb LA-ICP-MS dates that indicate Cretaceous emplacement, yet again the apatite (U-Th)/He dates indicate younger heating (42.92 ± 1.98 to 21.57 ± 2.31 Ma). We report the first zircon U-Pb LA-ICP-MS analyses from Trenton Canyon Au here, which support Eocene emplacement of granodiorite intrusions. Zircon (U-Th)/He dates are contemporaneous, however, the apatite (U-Th)/He dates predate the zircon U-Pb dates, possibly due to He implantation. Apatite and zircon (U-Th)/He dates from the margins of the Cretaceous Trenton Canyon stock indicate the rheologic contrast between the stock and country rock likely served as a crustal conduit for Eocene fluids.