TRIPLE OXYGEN AND CARBONATE CLUMPED ISOTOPES OF MIOCENE CALDERA LAKE SEDIMENTS AND IMPLICATIONS FOR CALDERA LAKE LITHIUM ACCUMULATION

Miocene calderas in north-central Nevada and southern Oregon contain caldera lake sediments recording hydrothermal conditions and records of past climate during the warm Middle Miocene, and contain lithium concentrations far exceeding upper continental crust values (~35 ppm). One important observation from these sediments is that the McDermitt Caldera (~16.5 Ma) contains high lithium concentrations in clay-rich caldera lake sediments (>2000 ppm) at the Thacker Pass site whereas the diatomite and ash rich sediments in the younger Whitehorse Caldera (~15.5 Ma) do not. These observations suggest differences in the caldera lake hydrology and hydrothermal systems, processes we interrogate here isotopically.

In this contribution we present three new datasets. 1) Stable isotope measurements of meteoric and hot spring waters collected from the region, 2) triple oxygen isotope measurements of diatomite samples from the Whitehorse Caldera, and 3) traditional stable and clumped isotope measurements of carbonates from drill cores from Thacker Pass in the southern McDermitt Caldera. Carbonate stable isotopes values suggest that the Thacker Pass sediments represent variable evaporative enrichment (>10‰ spread in δ¹⁸O) indicative of a closed lake system, with deeper core samples associated with hydrothermally altered clays and carbonate veins showing a magmatic δ¹³C signature and warm (>40 °C) formation temperatures. In contrast, the δ’¹⁸O-Δ’¹⁷O systematics of the Whitehorse Caldera diatomites suggest a balance-filled lake setting. The implications of these findings with respect to lithium accumulation in clay-rich sediments are that a caldera lake’s hydrology (closed vs. balance-filled) and the resultant style of sedimentary clay or silica deposition place an important control on initial conditions prior to subsequent hydrothermal alteration of the sediments. Thacker Pass therefore represents an end-member in the spectrum of lithium enrichment from caldera lake systems.