The oxygen and hydrogen isotopic composition of Eocene and Miocene freshwater cherts in the western United States records regional climatic variation in the Cenozoic. Here, we present isotopic measurements of 47 freshwater cherts of Eocene and Miocene age from the Great Basin of the western United States at two different sites and interpret them in light of regional climatic and tectonic history. The large range of ␦ 18 O of terrestrial cherts measured in this study, from 11.2‰ to 31.2‰ (SMOW: Standard Mean Ocean), is shown to be primarily the result of variations in ␦ 18 O of surface water. The following trends and patterns are recognized within this range of ␦ 18 O values. First, in Cenozoic rocks of northern Nevada, chert ␦ 18 O records the same shift observed in authigenic calcite between the Eocene and Miocene that has been attributed to regional surface uplift. The consistent covariation of proxies suggests that chert reliably records and retains a signal of ancient meteoric water isotopic composition, even though our analyses show that chert formed from warmer waters (40°C) than coexisting calcite (20°C). Second, there is a strong positive correlation between ␦ 18 O and ␦D in Eocene age chert from Elko, Nevada and Salina, Utah that suggests large changes in lake water isotopic composition due to evaporation. Evaporative effects on lake water isotopic composition, rather than surface temperature, exert the primary control on the isotopic composition of chert, accounting for 10‰ of the 16‰ range in ␦ 18 O measured in Eocene cherts. From authigenic mineral data, we calculate a range in isotopic composition of Eocene precipitation in the north-central Great Basin of Ϫ10 to Ϫ14‰ for ␦ 18 O and Ϫ70 to Ϫ100‰ for ␦D, which is in agreement with previous estimates for Eocene basins of the western United States. Due to its resistance to alteration and record of variations in both ␦ 18 O and ␦D of water, chert has the potential to corroborate and constrain the cause of variations in isotope stratigraphies.
Numerous studies have proposed that movement along the 3400-km 2 low-angle (<38) Heart Mountain detachment fault of north-western Wyoming and south-western Montana was facilitated by the presence of lubricating fluids. A recent stable isotope study suggested that the fluids along the Heart Mountain fault originated from large hydrothermal systems associated with Eocene intrusive centers. Herein, we present results from a combined stable isotope, fluid inclusion, and 40 Ar/ 39 Ar geochronology study of the relationship between shallow crustal fluids from Eocene intrusive centers in the New World Mining district and fluids associated with the Heart Mountain fault. Our results suggest that: (i) Eocene intrusive bodies within the New World Mining District focused hydrothermal fluids along specific units and structures from the ore deposit centers along the Heart Mountain fault detachment surface; (ii) hydrothermal waters were focused along the Heart Mountain fault from the breakaway region near Silvergate, Montana toward Heart Mountain in Cody, Wyoming; and (iii) hydrothermal activity along the Heart Mountain fault occurred at the same time as emplacement of the Eocene intrusives, between 50.1 and 48.1 Ma. These data, taken together, suggest that the hydrothermal activity generated by intrusion of Eocene rhyodacites and dacites provided the source of fluids found along the Heart Mountain fault plane.
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