Chemical vectoring in continental geothermal systems: Composition of altered rocks and illite as guides to magmatic degassing

Abstract

The Taupō Volcanic Zone, New Zealand, is one of the most voluminous volcanic regions on Earth. Over the last 1.8 Myr, bimodal volcanism in this rifted-arc setting has been dominated by voluminous rhyolite ignimbrite eruptions (>6000 km3 cumulative) and minor arc-type andesite and dacite, underpinned by basaltic intrusions. The combination of the magmatism and extensionally thinned arc crust with high heat flow has resulted in more than 23 active geothermal fields in the region. The fluids sampled in the geothermal systems are dominated by dilute meteoric fluids that are low in components such as acids, alkalis, and some trace metals commonly ascribed to magmatic sources but can contain large amounts of deep sourced magmatic volatiles such as CO2.This study was designed to assess the long-term (>20,000yr) fluid and metal input into two Taupō geothermal systems, and the amount and proportion of magmatic-derived components in these fluids. We collected major and trace element compositions of hydrothermal altered whole rocks and clay minerals from two active geothermal systems and compared them with hydrothermally altered rocks of the fossil (0.6 Ma) Ngatamariki magmatic-hydrothermal system, which has a demonstrated large magmatic contribution. The Ohaaki and Rotokawa geothermal systems have both been drilled to a depth of 3000 m, with sampled reservoir fluids containing low concentrations of chloride (∼1000 mg/kg) but high levels of gas (CO2, N2) that have previously been interpreted to reflect subduction components derived from arc-type magmas. Whole-rock samples lack enrichment in chloride-transported metals such as Cu, Pb or Zn but have minor anomalies of bisulfide-complexed Au, Sb, and As in the uppermost 1500 m of the geothermal reservoirs. Hydrothermal water-rock interactions in the deep reservoirs of both geothermal systems at temperatures between 220 and 300 °C produced assemblages of illite + albite + adularia + calcite + pyrite that are in equilibrium with the observed neutral to slightly acidic pH (6 ± 1) of the present-day fluids. In the lower temperature end of this range, Mg and Fe increasingly enter the illite crystal lattice via Tschermak-type (phengite) substitution, whereas a few high-temperature (>300 °C) samples contain scarce muscovite compositions. The illites commonly have low contents of most trace metals, although minor amounts of Cs, Li, Cu, Sb and Sn occur in near-surface samples. Based on these data, over the >20,000-year lifetime of the Ohaaki and Rotokawa geothermal systems, fluids were dominated by chloride-poor meteoric water and contained little magmatic contributions other than conducted heat, some gases (CO2 - N2 ± H2S), and a small fraction of the total H2O of geothermal waters. Therefore, the inferred magma bodies of intermediate to silicic composition that lie at shallow depth beneath these geothermal systems currently are not, and likely have not been for >20,000 years, degassing significant water and chloride despite the high water and chloride contents of the magmas. By inference, the intermediate to silicic magmas at depth have not transferred large amounts of volatiles to the geothermal systems over this period, and rather are storing them in the magmatic bodies to be released in volcanic eruptions that are commonly explosive and pyroclastic in nature and highly hazardous.