Trace-element distribution in an active hydrothermal system, Roosevelt hot springs thermal area, Utah

Abstract

Chemical interaction of thermal fluids with reservoir rock in the Roosevelt Hot Springs thermal area, Utah, has resulted in the development of characteristic trace-element dispersion patterns. Multielement analyses of surface rock samples, soil samples and drill cuttings from deep exploration wells provide a three-dimensional perspective of chemical redistribution within this structurally-controlled hot-water geothermal system. Five distinctive elemental suites of chemical enrichment are recognized, each characteristic of a particular combination of physical and chemical conditions within the geothermal system. These are: (1) concentrations of As, Sb, Be, and Hg associated with siliceous material at locations of liquid discharge, fluid mixing or boiling; (2) concentrations of Mn, Ba, W, Be, Cu, Co, As, Sb and Hg in manganese and iron oxide deposits; (3) high concentrations of Hg in argillized rock near fumaroles and lower concentrations in a broad diffuse halo surrounding the thermal center; (4) concentrations of As in sulfides and Li in silicate alteration minerals immediately surrounding high-temperature fluid flow-controlling fractures; (5) deposits of CaCO 3 at depth where flashing of brine to steam has occurred due to pressure release. The geochemical enrichments are not, in general, widespread, pervasively developed zones of regular form and dimension as are typical in many ore-forming hydrothermal systems. As the geothermal system develops, changes and eventually declines through time, the chemical deposits are developed, remobilized or superimposed upon each other, thus preserving within the rocks a record of the history of the geothermal system. Recognition of trace-element distribution patterns during the exploration of a geothermal system may aid definition of the present geometry and interpretation of the history of the system.