| Abstract |
Siliceous sinters (hot spring rocks) are common in geothermal areas where alkali chloride reservoir water discharges at the surface and deposits amorphous silica (opal-A) to form sinters. The amount of silica in hot spring water is not accounted for by the solubility of opal-A, but is controlled by the solubility of quartz at depth. For example, 210°C alkali chloride reservoir water in equilibrium with quartz at depth becomes saturated with respect to opal-A when the water temperature decreases to 100°C. As hot spring water discharges, it cools to less than 100°C facilitating the precipitation of opal-A. The opal-A entombs everything it comes into contact with silicifying all biotic and abiotic components present in hot spring water such as microbes, pollens and sinter surfaces. The silicification results in the preservation of a variety of textural fabrics reflecting specific hot spring settings such as: (1) near-vent, high temperature pools ( more than 60°C); (2) mid-slope, mid-temperature (35-60°C) pools; (3) low-temperature ( less than 35°C) distal-apron settings. Sinters remain preserved at the surface for thousands of years after hot spring flow has ceased. Also, the deeper hot reservoir fluid can exist at depth long after hot spring activity has stopped. Therefore, sinters provide valuable records of paleo-biologic, paleo-hydrologic and paleo-environmental conditions in extinct hot spring settings and infer a reservoir temperature of more than 210°C. The recognition of temperature significant sinter textures enables the locations of high versus low temperature hot spring sites to be identified. There are many examples around the world where geothermal power plants successfully operate with no present-day discharging hot springs, but sinters in the area indicating historic hot spring flow. Sinters undergo silica phase-changes from opal-A to opal-A/CT to opal-CT to opal-C to quartz. The transformation rates of opal-A to quartz differ between locations. Therefore, mineralogical maturation alone cannot be used as an indicator of the age of a sinter and Accelerator Mass Spectroscopy 14C dating is required to determine the timing of fluid flow to the surface. Regardless of the silica phase and age, textural preservation usually persists. To date, the study of sinters has been limited to outcrops and cores. Ground Penetrating Radar (GPR) images a continuous cross-sectional view of the shallow subsurface. Our results show GPR was successful in: (1) imaging through opal-A to quartz sinters to a depth more than 10 meters; (2) identifying the spatial extent and thickness of completely buried and partially exposed sinters; (3) distinguishing between altered versus unaltered sinters; (4) imaging vent directionality; (5) mapping vent to distal apron areas. GPR has demonstrated promising results as a non-invasive, cost-effective method for imaging a range of geothermal features in the shallow subsurface. The new and innovative combination of techniques that includes GPR imaging of sinters, 14C sinter dating and sinter textural mapping greatly assists in the initial stages of geothermal exploration. This provides useful information about reservoir temperatures, timing of hot spring flow, volume and distribution of flow, flow migration pathways, as well as identifying high versus low temperature sinter textures which allows reconstruction of paleo-flow hot spring settings. |