Measurement of the concentrations of rare earth elements (REE) in geothermal fluids and associated altered rocks holds promise as a potential aid in exploration for and exploitation of geothermal fields of economic importance. We have initiated a research program which will provide the basis for assessing this potential. The project involves measuring the REE content of fluids from a variety of geothermal systems from around the world and relating measured contents to such parameters as fluid chemistry and the type of rock through which the fluid flows. The overall objectives of the research are to: 1) establish baseline information on REE contents in geothermal fluids from around the world; 2) determine whether there are any distinctions in REE contents of fluids from producing or potentially producing geothermal fields, and geothermal fields which are not economically viable; 3) establish any relationships between REE contents and fluid chemistry and temperature which may exist; 4) establish relationships between REE contents and host (aquifer) rocks; 5) determine whether there are seasonal changes in REE geochemistry of geothermal fluids; and 6) ascertain whether REE geochemistry changes systematically over the production history of a producing geothermal field.

Our methodology is to collect both filtered and unfiltered samples preserved with 2% high-purity acid in acid-cleaned bottles. Temperature, conductivity, and pH are measured in the field. Alkalinity, selected anions and cations, and silica are measured in the laboratory. The REE's, Th, and U are determined after a preconcentration step involving addition of about 60 mg ferric iron to the preserved sample followed by addition of high-purity ammonia which precipitates the iron as ferric hydroxide and co-precipitates the REE's, Th, and U. The sample is filtered to recover the precipitate which is then redissolved in high purity acid and analyzed by ICP-Mass Spectroscopy. We have demonstrated sample detection limits better than of 0.01 mg/L or 0.05 nmole/Kg for most of the REE's, Th and U.

We present preliminary results from geothermal areas in and around the Taupo Volcanic Zone, New Zealand. Low-pH, acid-sulfate geothermal fluids have been found to have elevated, light REE-enriched patterns while geothermal fluids having near neutral and higher pH have much lower REE contents.

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