Volatile behavior and trace metal transport within the linked magmatic-geothermal system at Mt. Edgecumbe (Putauaki), New Zealand

Abstract

The study focuses on the volatile behavior in the linked volcanic and geothermal system of Mt. Edgecumbe (Pūtauaki) and the Kawerau geothermal field, in the Bay of Plenty, New Zealand. The volcano is composed of several domes, cones, lava flows, and pyroclastic flow deposits, and has been suggested as the heat source of the Kawerau geothermal system for the last ~9 ka (Milicich et al., 2013). Major element (EMPA) and trace element (LA-ICP-MS) analysis of phenocryst phases (plagioclase, orthopyroxene, clinopyroxene, minor quartz, amphibole, and biotite) and silicate melt inclusions provides a means to understand the complicated magmatic and volatile evolution of the Mt. Edgecumbe complex, and to determine the trace metal budgets between the melt, crystalline, and fluid phases distributed between the magmatic and geothermal system. Coupling thermobarometry to ore metal concentrations is a new method aimed at constraining the conditions for volatile exsolution as trace metals are mobilized through the magmatic system, and may draw inferences to proto-porphyry formation. A lack of mafic components with the presence of silicic melt inclusions in intermediate rocks suggest that the Mt. Edgecumbe lavas are the product of mixing of at least two previously evolved magmas, unlike the mixing of mafic and felsic magmas from other intermediate volcanoes in the TVZ. Amphibole thermobarometry and two-pyroxene thermobarometry suggest multiple crystallization histories from both deep and shallow magma reservoirs in the Mt. Edgecumbe system. Major and trace element geochemistry of phenocrysts and the combination of in situ and modeled melt analyses has identified that a deep rhyodacite melt may have ascended from the base of the crust, stalled near the brittle-ductile boundary, and incorporated crystals from a shallow trachyte mush before assimilating rhyolitic xenocrysts upon extrusion. The sources of mineral phases from the Mt. Edgecumbe magmatic system host relatively low ore metal concentrations, with the exception of compatible Cu in biotites and Zn in amphiboles and pyroxenes. Trace metal concentrations coupled with amphibole and pyroxene thermobarometry do not show distinct variation with depth in either shallow or deep crystallizing conditions in the Mt. Edgecumbe magmatic system, and show that trace metal mobilization by a fluid or vapor phase from the depth constraints studied are unlikely. However, fluid models constructed for low and high salinity phases show that many ore metals have the potential to be mobilized by a chloride phase, given the presence of Cl in the system. Cl concentrations in phenocryst-hosted melt inclusions are similar to that of rhyolite in the TVZ, and indicate that the system may be capable of exsolving a hydrosaline brine or vapor phase. Metal budgets from the fluid models suggest that some metals (Cu, Zn) could be in sufficient quantities at Mt. Edgecumbe to be contributed to the geothermal system, however, their mobility is largely related to salinity.