The purpose of this study is to determine the distribution of trace elements (Hg, As, Au, Sb, Cu, Pb, Zn, Mn, Li) in stream sediments, soils, hydrothermally altered rocks and drill cuttings from The Geysers geothermal field.
A careful review of the abundant and somewhat contradictory international literature concerning the physical and chemical properties, as well as the behavioural characteristics of trace elements at high temperatures was very useful for the interpretation of the analytical data obtained as a result of a detailed sampling program.
The results of sampling indicate that the elements providing the most interesting anomalies in the Geysers system are mercury and arsenic, and to a lesser degree gold, antimony, copper and lithium.
Mercury in stream sediments confirmed the southern limit of the geothermal field.
Mercury and arsenic soil surveys on a regional scale revealed distinct anomalies within the Big Sulphur Creek area. Similar geochemical anomalies could not be defined by the same sampling methods in the northern part of the field where currently exploration and production are also taking place. This somewhat negative evidence may be due to the presence of a thicker self-sealed cap, thus suggesting a deepening of the reservoir zone to the north.
At Little Geysers, the presence of a shallow intrusive body may enhance subsurface fracturing, hence favoring the circulation of geothermal fluids and consequently the increased mobility of mercury and arsenic towards the surface.
Hydrothermally altered rocks are in general consistently enriched in mercury and arsenic and locally exhibit slightly anomalous gold, antimony and copper concentrations, due most likely to hydrothermal fluid circulation in the liquid-dominated system which preceded the present vapor-dominated system.
In drill cuttings to a depth of 1000 feet, mercury and lithium are very slightly enriched in one well, whereas arsenic is subject to lithologic control in several wells.
Trace element geochemistry provides good evidence in regard to the preexistence of a hot-water system which boiled down-structure from the Mercuryville fault zone into the Big Sulphur Creek fault zone where the geothermal system currently vents.
