Precious Metals in High Temperature Geothermal Systems

High temperature geothermal systems are the modern analogue of epithermal precious metal ore deposits. In these systems, gold and silver are transported primarily as bisulfide complexes, with the precious metals being deposited in response to boiling and mixing of the deep geothermal fluid. In order for the geothermal system to produce an economic precious metal deposit within the typical lifetime of a geothermal system, the deposition process must be efficient, and the gold and silver concentrations in the geothermal fluid must be sufficient. Thermodynamic data has been measured for gold and silver bisulfide complexes, but the question remains as to whether the deep geothermal fluid is saturated with respect to these complexes. Measurement of gold and silver concentrations in geothermal fluid sampled at the surface shows very low concentrations, as these metals precipitate down the well. Indirect measurements of the deep reservoir gold and silver concentrations have been calculated by estimating precipitate concentrations on scale deposits recovered from high pressure apparatus at the surface. These estimates produce precious metal concentrations that are below saturation. We therefore designed and built a downhole sampling device specifically to measure precious and other trace metal concentrations in the deep reservoir fluids. The device is manufactured from titanium to be chemically inert, and therefore capable of scavenging the trace metals through acid rinsing. Analyses for precious and related metals on deep waters obtained from Kawerau and Ngawha geothermal systems have been measured. Solutions were analysed by ICP-MS. Lab testing using blank solutions shows minimal sources of contamination from materials used in the construction and sampling procedure (i.e. titanium, MilliQ water and aqua regia). At Kawerau, samples were obtained from deep wells at ≥1000 m depth at temperatures of 260 to 295° C, while at Ngawha samples were taken from > 800 m depth at temperatures of 225° to >235° C. Gold, silver and thallium are at ppb levels, while arsenic, antimony and copper range from hundreds to thousands of ppb. Comparing these results with calculated solubilities of gold, silver and acanthite, suggests deep waters are undersaturated in gold but close to saturation in silver.