| Title | Rare Earth Elements and Yttrium Distribution in Surface and Geothermal Waters of Bacman Geothermal Field, Philippines |
|---|---|
| Authors | Val Maverick ABECIA, Michael BAU, Simona REGENSPURG, Uwe ALTENBERGER |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | rare earth elements, geothermal, aqueous geochemistry, geothermal exploration, geochemical proxies |
| Abstract | Advancements in inductively coupled plasma-mass spectrometry (ICP-MS) technology and sample preparation techniques in the past decades made rare earth elements (REE) and trace elements analyses possible in fluids, opening the possibility of studying them as geochemical tools in geothermal exploration and production field monitoring. In order to evaluate this potential, rare earth elements and yttrium (REY) in rivers, hotsprings, and well fluids from the Bacon-Manito Geothermal Field (Bacman), Philippines, were sampled using state-of-the-art solid phase pre-concentration and separation techniques. Filter residues were also acid-pressure-digested at high temperature to evaluate the potential impact of suspended particles on REY distribution and transport in surface and geothermal fluids. REY concentrations in the brine from geothermal wells are 2-3 orders of magnitude lower compared to hotsprings and rivers, due to REY partitioning into vapor during phase separation. Bulk rock- as well as chondrite-normalized REY plots from wells also show large positive Europium (Eu) anomalies, suggesting discrimination of Eu from the crystal lattice of secondary minerals at reservoir conditions due to the predominance of divalent Eu at temperatures exceeding about 250°C. The absence of Eu anomalies in the hotsprings may indicate water-rock interaction at temperature below about 200°C or admixture of large amounts of low-temperature surface waters. Relative light rare earth elements (LREE) depletion and heavy rare earth elements (HREE) enrichment in one (1) well sample apparently reflect the signature of reinjection returns. Filter residue-normalized REY patterns in eluates suggest that the transport of REY in Bacman surface and geothermal waters is dominantly controlled by adherence to nanoparticles and colloids smaller than 0.2μm. The presence of a positive Eu anomaly can be used as a geochemical indicator of a high temperature fluid source (≥250°C). Hence, REY geochemistry shows strong potential for “fingerprinting†different fluid sources but should always be used with utmost caution, and should be complemented with other geochemical proxies. |