Geochemical Indicators of a High-Temperature Geothermal System

Abstract

The intensity and distribution of hydrothermal alteration are frequently used during the exploration and assessment of a geothermal prospect to estimate the size, shape and temperature of a thermal system. Geochemical and petrographic observations used to characterize the hydrothermal alteration include the mapping of both trace- and major-element dispersion patterns and the distribution of secondary mineral assemblages. This paper describes the trace-element and mineralogical distributions common to many of the high-temperature systems (> 150° C) that we have studied. However, examples of important geochemical relationships are primarily drawn from our detailed investigations of the Roosevelt Hot Springs thermal system in southern Utah. The hydrothermal fluids at Roosevelt Hot Springs are enriched in sodium chloride and contain approximately 9000 ppm total dissolved solids. The reservoir, with a base temperature near 270° C, is located in fractured gneisses and granites. At Roosevelt Hot Springs, the surface discharges consist of opaline and chalcedonic sinter, and alluvium cemented by silica, calcite, Mn oxide and Fe oxide. The geochemistry of these surface deposits is extremely variable, but locally they contain up to 5.5 ppm Hg, 858 ppm As, 18.8% Mn, 230 ppm Cu, 290 ppm Sb, 294 ppm W, 17 ppm Li, 68 ppm Pb, 26 ppm Zn, 4.9% Ba and 100 ppm Be. High concentrations of Au and Ag, although not present in the sinters at Roosevelt Hot Springs, occur in hot spring deposits from other chemically similar systems such as Steamboat Springs, Nevada. Mercury and As are the most widely distributed trace elements in the surface samples. Their distribution in soils overlying the thermal system expands the area of interest and helps define the high-temperature portion of the system. The highest concentrations of Hg and As, of up to 5.5 and 26 ppm, respectively, occur in soils within 300 m of the thermal discharges. A broader area extending up to 1000 m from the surficial thermal activity also contains anomalous Hg but with lower concentrations ranging from 50 to 800 ppb. Mercury anomalies tend to mark the location of faults within the uppermost portions of the reservoir and areas where the thermal fluids move laterally away from the thermal system toward the adjacent valley. Depletions of Mn, Cu and Zn are found in the acid-altered soils and in alluvium associated with the hot spring deposits and fumaroles. The acid alteration occurs locally in areas of surficial thermal activity and persists to depths of less than 60 m. Alteration minerals within these zones include alunite, jarosite, native sulphur, opal, chalcedony, kaolinite, sericite, montmorillonite, and mixed-layer clays. The formation of acid waters occurs near the surface and results from the oxidation of H 2 S contained within gases evolving from the fumaroles or within waters discharged by the hot springs. The locally intense acid-sulphate alteration and scavenging of metals within the soils occurs as the fluids percolate downward. Alteration mineralogy at depth is determined through examination of down-hole samples which penetrate the geothermal system to depths in excess of 2 km. Reservoir rocks of temperatures below about 210°C contain an alteration assemblage with mixed-layer clays, montmorillonite, sericite, pyrite, hematite, magnetite, calcite, chlorite, quartz, and potassium feldspar. At higher temperatures, mixed-layer clays and montmorillonite disappear and anhydrite appears locally. Altered rocks within the high temperature portions of the thermal field are characterized by anomalous concentrations of As and Li. Selective chemical leaching of the altered rocks and electron microprobe analyses indicate that As is contained primarily in pyrite or iron oxides after pyrite whereas Li occurs in clays and micas. Mercury exhibits an inverse relationship with temperature and is concentrated in the cooler portions of the thermal system to depths marked approximately by the 200° C isotherm. This distribution is similar to the distribution of clay minerals in the reservoir rock. Heating experiments indicate that Hg occurs primarily as Hg° and that it is readily mobilized by the thermal system at temperatures in excess of 200–250° C.