Unusually radiogenic granites can act as alternative, “green” sources of geothermal energy. The enrichment of these granites in heat-producing elements (K, Th and U) is not completely understood. While the majority of these elements reside in accessory phases, small concentrations can reside in major mineral phases. Radiogenic heat-production can drive the circulation of hydrothermal fluids that upon interaction with a granitic rock will cause chemical, mineralogical and textural alterations. Secondary alterations are of particular importance as they can mobilise radiogenic elements responsible for the heat-producing nature of a granitic body. Thus, constraining the timing and geochemical processes of alteration can provide significant information for the evolution of high heat-producing granitic systems that are viable as geothermal resources.
This thesis has three principle objectives. The first addresses the alteration of accessory phases and the mobility of constituent trace elements in heat-producing granitic systems. The second objective attempts to constrain the tectonic, thermal and fluid flow history of the Warburton–Cooper–Eromanga basins, which host a globally prospective geothermal resource. The final objective uses trace element studies to determine if hydrothermal alteration has a positive or negative impact on the radiogenic nature of granitic rocks with specific attention to the “immobile” element, Th.
The Soultz-sous-Forêts monzogranite, France, contains elevated concentrations of rare earth elements and the heat-producing element, Th. Despite its status as one of the more studied enhanced geothermal systems in the world, little attention has been paid to the residency of heat-producing elements. Electron microprobe analysis of primary and secondary accessory phases shows that primary titanite has been altered to two polyminerallic assemblages dominated by fluorocarbonates or phosphates rich in REE and Th. While fluorocarbonate-rich assemblages are found in samples with minor selective alteration, phosphate-rich mineral assemblages are only found in samples exhibiting pervasive alteration, as a result of interaction under high fluid/rock ratio conditions. The influx of CO2-rich fluid was integral for the destabilisation of titanite. Microprobe analyses show fluorocarbonates are depleted in middle REE and Th compared to primary titanite. However, assemblages dominated by phosphate show comparatively similar REE and Th concentrations to titanite. Therefore, we hypothesise that REE and Th show an enhanced mobility in HCO3– or FCO3-3–rich solutions but are relatively immobile in the presence of aqueous HPO42–.
The Warburton–Cooper–Eromanga basins in central Australia host one of the most prospective geothermal resources in the world, the radiogenic Big Lake Suite granite. Currently, little is known about its alteration mineralogy and why it is unusually enriched in heat-producing elements. This is partly due to the complex and poorly understood tectonic history of the Warburton–Cooper–Eromanga basins. Determining past tectonism and basin evolution can help constrain the evolution of the Big Lake Suite granite and its interaction with basinal fluids, which can actively affect the heat-producing element budget. The tectonothermal and fluid flow history of these basins was investigated using a combination of primary and secondary minerals from the granite and surrounding country-rock.
Integrated geochronology of sediment-hosted vein carbonates and granite-hosted primary minerals provide evidence for tectonism and magmatic activity in the Silurian. Dating of vein carbonates by Sm–Nd provided an isochron age of 435 ± 17 Ma almost coincident with ages for primary zircon (U–Pb; 421 ± 3.8 Ma) and uraninite (U–Th–total Pb; 407 ± 16 Ma). Stable and radiogenic isotopes (δ18O, Sr and εNd), as well as trace element studies of carbonate veins indicate that >200°C basinal fluids of evolved meteoric origin circulated through fracture networks of the Warburton Basin. Such an elevated thermal regime derives from extensional tectonism and decompression-related granitic magmatism following cessation of regional orogenesis.
Analysis of authigenic illite from the granite and overlying sedimentary rocks geochronologically and geochemically constrain three major periods of tectonism and fluid flow within the Warburton–Cooper–Eromanga basins. 40Ar–39Ar, 87Rb–87Sr and 147Sm–143Nd geochronology provide evidence for Carboniferous (323.3 ± 9.4 Ma) and Jurassic (201.7 ± 9.3 Ma) tectonism that resulted in basin-wide fluid flow. Subsequent, episodic Cretaceous (~128, 94 and 86 Ma) tectonism caused an influx of high-temperature fluids that were restricted to a synclinal trough. Calculated fluid stable isotope values from the trough, in conjunction with calculated palaeotemperatures, indicate an ingress of mid to high-latitudinal evolved meteoric waters. Such hydrothermal fluids were under extremely high geothermal gradients (~100 °Ckm–1) and high fluid/rock ratios indicative of an extensional environment. Cretaceous hydrothermal circulation caused texturally destructive alteration of the Big Lake Suite and substantially enriched altered zones in the heat-producing element, Th. Episodic Cretaceous fluid flow reflects continent-wide transmission of tensional stress from a >2500 km long rifting event on the eastern and southern margins of Australia. Coincident extension in the synclinal trough likely derives from the transferral of tensional stress to thermally and mechanically weakened areas of the Australian continent. Conversely, Carboniferous and Jurassic fluid flow involved evolved basinal fluids under substantially lower geothermal gradients (~42 °Ckm–1) and fluid/rock ratios. These tectonothermal events are interpreted as westward arms of widespread regional extension that affected much of central and eastern Queensland.
Hydrothermal alteration can significantly affect the mineralogy and chemistry of radiogenic granites. By conducting a combination of geochemical and geochronological analyses, this thesis provides evidence for a metasomatic contribution to enrichment of the Big Lake Suite in heat-producing elements. Such enrichment derived from episodic influx of hydrothermal fluids capable of destabilising primary phases and mobilising trace elements including Th.