| Title | Evidence for Magmatic Fluid Input into the Rotokawa Geothermal System, Based on REE, Trace Element and Mineral Isotopic Signature of Its Hydrothermal Minerals |
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| Authors | I Chambefort, A McCoy-West, LE Ramirez, AJ Rae, G Bignall |
| Year | 2011 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Rotokawa, epidote, anhydrite, REE, trace elements, stable isotopes |
| Abstract | Geothermal systems of the Taupo Volcanic Zone (New Zealand) are dominated by meteoric-sourced waters, although rare magmatic fluid input is inferred in some systems (e.g. Ohaaki). Hydrothermal minerals can provide insights on magmatic and/or meteoric fluid origin and controls on fluid flow, with their compositions dependent on host rock and fluid composition, water-rock ratio etc. In this study, EPMA and LA-ICP-MS data, and oxygen, hydrogen and sulphur isotope data for secondary quartz and epidote (in veins, and replacing primary plagioclase and pyroxene) and in relict anhydrite (in epidote-quartz veins) in Rotokawa Andesite has been used to test if the hydrothermal mineral assemblage records the signature of contrasting fluid types in the Rotokawa Geothermal Field. Hydrothermal epidote at Rotokawa exhibit epidote-clinozoisite substitution, with Fe-rich epidote replacement of phenocrysts and in epidote at vein margins, and Al-rich clinozoisite in vein cores, whilst trace element contents reveal extreme fractionation in REE and Sr abundances. Oxygen isotopes in epidote and quartz range from -2.9 to 3.3‰, and 9.4 to 13.6‰, respectively. 18O in anhydrite varies from 13.9 to 17.9‰, with respective 34S of 10.1 and 13.0‰. Preliminary interpretations suggest that a variation in major element chemistry of the epidotes is linked to fluid-rock interactions with the Fe-rich (andesitic) host rock. Trace element variations are consistent with a fluid-dominated system in the southern part of the Rotokawa geothermal system, and rock-dominated fluid-rock interactions at the time of epidote crystallisation further to the north. Oxygen and hydrogen stable isotopes are heavier in the south-eastern part of the Rotokawa Geothermal Field, and may be related to (episodic?) “pulses” of magmatic fluid into the system, in agreement with the isotopic signature of the hypogenic anhydrite in disequilibrium with the present day composition of fluids in the Rotokawa wells. |