Alteration of the Big Lake Suite granite: An answer to enrichment?

Abstract

Alteration of the Big Lake Suite granite: An answer to enrichment? A. W. Middleton*a, I. T. Uysala, S. Bryanb, C. Hallc and S. D. Goldingd a Queensland Geothermal Energy Centre of Excellence, The University of Queensland, Queensland 4072, Australia (*correspondence: alexander.middleton@uqconnect.edu.au) bBiogeoscience, Queensland University of Technology, 1 George Street, Brisbane, QLD 4001, Australia cDepartment of Earth and Environmental Sciences, University of Michigan, 1100 N. University Ave. Ann Arbor, MI 48109-1005, USA d School of Earth Science, The University of Queensland, Queensland 4072, Australia The Warburton–Cooper–Eromanga basins host one of the most prospective hot dry-rock geothermal resources (Big Lake Suite granite; BLS) that stems from unusual enrichment in radiogenic elements (up to 144 ppm Th and 30 ppm U). Previous studies estimate such concentrations of heat-producing elements may derive from primary enrichment of minerals such as zircon. However, consequent work found unusually elevated concentrations of Th and U in areas of the granite devoid of zircon. By conducting geochemical and geochronological analyses on authigenic illite from the highly altered BLS, this study elaborates on the region’s thermal and tectonic evolution whilst providing an alternative hypothesis to granite enrichment. Calculated ä18O and äD fluid isotopic values of illite samples are compatible with an influx of meteoric waters in an extensional environment. Integrated geochronology (Sm–Nd, Rb–Sr, and Ar–Ar) consistently indicates the Nappamerri Trough underwent episodic tectonism at approximately 128 Ma, 95 Ma and 86 Ma. Chemically distinct periods of fluid flow are similarly inferred by distinct rare earth elements patterns for each of the age populations. Altered granite samples have substantially higher radiogenic element concentrations than the unaltered granite. Further trace element analyses also show illite samples contain unusually elevated concentrations of Th (~40 ppm) and U (~10 ppm) that contribute to ~50% of the whole-rock radiogenic element budget. A multifaceted analytical study of authigenic illite produced geologically significant ages attributed to an anomalous thermal event in the Nappamerri Trough of the Warburton–Cooper–Eromanga basins. Cretaceous ages are consistent with episodic rifting of Gondwana and opening of the Tasman Sea along the eastern margin of Australia. Such consistency suggests plate-wide transmission of tensional stress to intra-continental crust weakened by abnormally high geothermal gradients, associated with the BLS, and preceding basinal tectonism. Extensional tectonism allowed an influx of evolved meteoric water that caused widespread alteration and substantial enrichment of the granite in heat-producing elements (HPE). Although the source of HPE is currently unknown, it is likely such elements were leached by high temperature hydrothermal fluids from the surrounding country rock and deposited in the granite upon illite precipitation.