Trace element and isotope geochemistry of geothermal fluids, East Rift Zone, Kilauea, Hawaii. Geothermal research at the Puna Facility: Final report.

摘要

A research program has been undertaken in an effort to better characterize the composition and the precipitation characteristic of the geothermal fluids produced by the HGP-A geothermal well located on the Kilauea East Rift Zone on the Island of Hawaii. The results of these studies have shown that the chemical composition of the fluids changed over the production life of the well and that the fluids produced were the result of mixing of at least two, and possibly three, source fluids. These source fluids were recognized as: a sea water composition modified by high temperature water-rock reactions; meteoric recharge; and a hydrothermal fluid that had been equilibrated with high temperature reservoir rocks and magmatic volatiles. Although the major alkali and halide elements show clearly increasing trends with time, only a few of the trace transition metals show a similar trend. The rare earth elements, were typically found at low concentrations and appeared to be highly variable with time. Studies of the precipitation characteristics of silica showed that amorphous silica deposition rates were highly sensitive to fluid pH and that increases in fluid pH above about 8.5 could flocculate more than 80% of the suspended colloidal silica in excess of its solubility. Addition of transition metal salts were also found to enhance the recovery fractions of silica from solution. The amorphous silica precipitate was also found to strongly scavenge the alkaline earth and transition metal ions naturally present in the brines; mild acid treatments were shown to be capable of removing substantial fractions of the scavenged metals from the silica flocs yielding a moderately pure gelatinous by-product. Further work on the silica precipitation process is recommended to improve our ability to control silica scaling from high temperature geothermal fluids or to recover a marketable silica by-product from these fluids prior to reinjection.