Preliminary Measurements of Concentrations of Lanthanide Elements in Geothermal Fluids from the Taupo Volcanic Zone, New Zealand

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 μg/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 REEenriched patterns while geothermal fluids having near neutral and higher pH have much lower REE contents. COLLECTION OF THE SAMPLES These samples were collected in March of 1998 and analyzed at facilities at the University of Idaho and Washington State University. The methodology is modified after that developed by van Middlesworth and Wood (1998). Before going out into the field, High Density Polyethylene (HDPE) bottles for containing acidified metal samples were cleaned by soaking overnight in 10% nitric acid. Bottles for anion and silica samples were soaked in deionized water (DIW) overnight. All bottles were then rinsed at least three times with DIW. Sediments and suspended solids were excluded from the cation, anion, and silica samples. For the New Zealand samples this was accomplished by decanting the aliquot for analysis after allowing the well mixed sample to settle for a few days. All subsequent samples are now filtered in the field directly from a designated sampling bottle into the appropriate sample containers. At each site the following fluid samples were taken: 1) a 1-liter, filtered sample (filtered using a 0.45 μm membrane), preserved with 2% Seastar Baseline HNO3 for REE analysis 2) a 1-liter, unfiltered sample, preserved with 2% Seastar Baseline HNO3 for REE analysis 3) a 250-mL unfiltered sample, preserved with 1% trace metal grade HNO3 decanted later for alkali, alkaline earth and transition metal analyses 4) a 250-mL unfiltered sample decanted later for anion analysis and alkalinity determinations 5) a 5 mL unfiltered sample diluted with 50 mL of DIW for silica analysis The following measurements were made in the field at each site: temperature, conductivity, pH, Eh and alkalinity. Except for samples from geothermal wells, pH, Eh and conductivity determinations were made at, or as close as possible to, the emergence temperature of the fluid. The pH and Eh buffers used to calibrate the respective electrodes were also maintained at as close to the emergence temperature of the fluids as possible. Alkalinity measurements were made within a week of sample collection using an automatic titrator at IGNS in Taupo. Location and Description of Samples The sites selected for sampling are located on the north island of New Zealand. Two sites, Miranda and Te Aroha were selected because they are thought to result from circulation of fluids along deep regional faults, where heating is predominately derived from the geothermal gradient as opposed to the shallower circulation typical of most of the thermal areas in the Taupo Volcanic Zone (TVZ), where heating is dominated by subvolcanic sources. Two other sites, Morere and Te Puia, outside of the TVZ proper, were sampled because they are associated with dewatering of sediments accompanying the Pacific plate, which is being subducted underneath the Indian plate just off the eastern coast of the North Island of New Zealand. Nine thermal areas within the Taupo Volcanic Zone were sampled; Waimangu, Waiotapu, Waikite, Te Kopia, Orakeikorako, BroadlandsOhaaki, Rotokawa, Wairakei, and Tauhara-Taupo. Waikite, Te Kopia, and Orakeikorako are associated with faults (Simmons 1995). Waimangu, Wairakei, and Taupo-Tauhara are associated with caldera boundaries (Simmons 1995). Waiotapu Thermal Reserve The Waiotapu Reserve contains the largest chloride spring in the region (the Champagne Pool) and a number of acid sulfate springs. We sampled the outlet of the Oyster pool which is fed by the outflow of Champagne pool and a number of smaller springs. Also sampled was a small hot spring feeding directly into the pool below the Oyster pool at the end of the Oyster pool side trail. The Oyster pool springs are strongly acidic with moderately high conductivity. The Champagne Pool was sampled as the definitive chloride feature in this reserve. The Champagne Pool has very high conductivity and has a low Eh as demonstrated by the As, Sb, and ferrous iron sulfide precipitation on the margins of the pool. Field measurements and elevation differences between different pools in this reserve make it clear that the underlying plumbing systems are capable of producing relatively isolated water chemistries from the same system. This area has a high maximum deep temperature of 295°C as well as high heat flow of 600 MW with an estimated thermal reserve of 6100 PJ. Surface water flow is greater than 250 L/s (Mongillo and McClelland, 1984).