CHEMISTRY OF MAGMATIC FLUIDS IN ARC VOLCANIC GROUND WATERS: EVIDENCE FROM ACTIVE AND FOSSIL HYDROTHERMAL SYSTEMS

Magmas beneath volcanoes expel H2O, CO2, SO2, H2S, and HCl into overlying groundwater, yielding acidic, sulfidic and CO2-rich waters that shape the evolution of the volcano-ground water system as the fluids ascend, hydro-frac and react with enclosing rocks, boil, mix with shallow waters and disperse. The manifestations of such magmatic fluid injection are directly visible in ubiquitous arc geothermal systems such as those of the Taupo Volcanic Zone, NZ. The deepest inner workings of such systems are visible in eroded fossil hydrothermal systems exposed as giant sulfur and metal anomalies in porphyry copper deposits (PCD). Magmatic signatures in H and O isotopes in hydrothermally altered rocks, combined with SEM-cathodoluminescence and fluid inclusion studies of PCD vein quartz show that moderately saline (5 wt% NaCl equiv, CO2-rich) magmatic fluids are injected into the hydrothermal system in repeated episodes of lithostatic-to-hydrostatic pressure fluctuation at temperatures of 600° to 700°C. The magmatic fluids react with host rocks to form a range of feldspar and mica mineral assemblages with descending temperature.

H and O isotopic data show that magmatic fluids condense into overlying meteoric ground waters and commonly make up the 20 to 30% of the fluid in near-surface geothermal systems at ~300°C. Analyzed waters and numerical models of fluid-rock reaction (program Chiller) demonstrate that magmatic SO2 reacts with H2O and disproportionates to H2S and H2SO4. The acid reacts with the rock, yielding quartz, kaolinite, smectite. Total aqueous sulfur commonly decreases to ~1% of its original concentration by precipitation of sulfates and pyrite. Sulfate is reduced to H2S where rock ferrous iron is oxidized to hematite, magnetite and epidote; H2S then reacts with iron forming pyrite. Among the original magmatic gases, only CO2 substantially escapes the system. Magmatic CO2 condenses into the deep groundwater, then boils out as the water rises. At the periphery of geothermal systems, CO2 re-condenses into cold water (e.g. Waiotapu and Broadlands, NZ, Hedenquist, et al.), yielding distinctive moderately acidic, low-Cl, carbonate-rich waters. Some boiled-out H2S condenses with H2O in the vadose zone, where it oxidizes, forming sulfuric acid, which then descends into the system and altering the rock to clays.