CHARACTERIZING GEOTHERMAL SINTER, TUFA AND TRAVERTINE DEPOSITS IN THE GREAT BASIN REGION, WESTERN USA

The Great Basin region in the western USA is home to more than 400 known geothermal systems, and currently generates ~800 MWe of electricity (gross generation) from geothermal resources. Estimates suggest that ~4300 MWe of geothermal resources are still to be discovered in the region, however the challenge is locating them. Over a third of the known geothermal systems in the Great Basin are ‘blind’ with no current thermal manifestations at the surface. Thus, new approaches are required to improve the identification of potential new resources, and reduce their exploration risk. Silica sinter, tufa and travertine deposits are common in the Great Basin, forming through the precipitation of silica or calcium carbonate from cold, warm or hot springs that are discharging at or near the surface (forming sinter or travertine) or sub-aqueously (forming tufa). They are often preserved long after spring flow has ceased, and can be an indication of a geothermal reservoir beneath, thus are a useful exploration tool. In high-temperature geothermal systems (>180 °C), silica sinter is often characterized by banded, crystalline (opaline) silica and is usually associated with voluminous spring discharge. In the Great Basin, a greater range of sinter characteristics are present, including silica-cemented sands and muds, silica-cemented gravels, silicified root casts, and silicified organic material, in addition to pure opaline silica. Many sites are not actively discharging today, and may have been active during different hydrological regimes (e.g., related to the Pleistocene high-stands of Lake Bonneville and Lake Lahontan, and higher water tables in the region). Here we review the characteristics and distribution of known sinter, tufa, and travertine occurrences in the Great Basin, and explore how these are related to geothermal fluid chemistry, temperature, geothermal system hydrology, and likely proximity to deep, geothermal up-flow. Unraveling these inter-relationships will facilitate improved interpretation of the significance of these deposits when encountered in the field and their use as an exploration tool.