The early Mesoproterozoic tectonic and thermal evolution of the eastern Gawler Craton: implications for mineral systems in eastern Proterozoic Australia

Abstract

Eastern Proterozoic Australia contains many large iron oxide-copper-gold (IOCG) deposits formed during the early Mesoproterozoic within the Gawler Craton, Mount Isa and Curnamona provinces. These mineral systems are generally considered independent of one another due to their present-day geographic separation and the variations in the timing and style of mineralisation. However, tectonic reconstructions of Proterozoic Australia suggest these provinces were contiguous during the early Mesoproterozoic when mineralisation occurred. Despite the economic significance of eastern Proterozoic Australia for copper, the relationships between these well-endowed geologic provinces and the processes involved in mineralisation are still not well resolved. Tectonic reconstructions of Proterozoic Australia commonly place the Peake and Denison Domain in the northeastern Gawler Craton between the Mount Isa Province and the eastern Gawler Craton during the early Mesoproterozoic. But despite its ideal location between two regions containing world class copper deposits, the age of hydrothermal alteration systems in the Peake and Denison Domain has not been investigated. U–Pb data from the Peake and Denison Domain indicates a complex history during the early Mesoproterozoic, with multiple hydrothermal alteration events between 1560–1460 Ma, deformation at c. 1500 Ma and magmatism at c. 1470 Ma. These ages present a number of correlations with the Mount Isa Province, supporting tectonic reconstructions indicating these regions were connected during the early Mesoproterozoic, including during the period of extensive copper mineralisation in the Mount Isa Province. These correlations greatly enhance the prospectivity of the Peake and Denison region in the northeastern Gawler Craton for IOCG mineralisation. The eastern Gawler Craton, like much of eastern Proterozoic Australia, has highly anomalous crustal heat flow derived from significant enrichment of heat producing elements in the crust. Numerous studies have investigated the effects of these thermally energetic rock systems in other regions of eastern Proterozoic Australia, but not in the Gawler Craton, or in the context of mineral systems. Andalusite and cordierite-bearing rocks from the Yorke Peninsula region in the southeastern Gawler Craton formed at P–T conditions of 3.6–4.5 kbar and 660–750 °C between 1560–1500 Ma, and cooled to temperatures below c. 450 °C at around c. 1450 Ma. In the context of the thermally anomalous crust in this region, the protracted duration of metamorphism (>60 Mya), high thermal gradients (45–50 °C km-1), and delayed timing of cooling in the absence of significant magmatism are characteristic of a thermal regime driven by crustal heat production. Thermal models of the crust constrained by the regional heat flow data and large geochemical datasets indicate the metamorphic conditions recorded on the Yorke Peninsula could be readily achieved by burial within the thermally energetic crust, without the necessity for external inputs of heat. Long-term maintenance of high crustal temperatures creates an ideal environment for the generation of large volumes of metamorphic fluids. This has important implications for the development of mineralisation due to the potentially prolonged interval over which fluid–rock interaction and extraction of metals from the crust could occur. There is extensive copper mineralisation on the Yorke Peninsula. In the Moonta–Wallaroo region, mineralisation and hydrothermal alteration occurred between 1585–1550 Ma, with later alteration at 1520–1500 Ma. This overall time interval coincides with the period of time high temperatures were maintained by anomalous crustal heat production. Sulphur isotopes from deposits in the Moonta–Wallaroo region indicate that early sulphide mineralisation has δ34S values of -2‰ to 2‰ and later sulphide mineralisation has heavier δ34S values of 4‰ to 6‰. These values and their relative shift toward heavier δ34S values are consistent with an early magmatic fluid source, followed by a later crustally-derived fluid, which is likely related to metamorphism associated with burial of sulphur-bearing sequences within the high heat production crust. These results suggest crustal heat production should be considered more broadly as a thermal driver for fluid–rock interaction for mineral systems models across eastern Proterozoic Australia.