Punctuated geochronology within a sustained high-temperature thermal regime in the southeastern Gawler Craton

Abstract

The Yorke Peninsula region in the southeastern Gawler Craton forms the southern portion of the Olympic Cu-Au Province that hosts the giant Olympic Dam, Carrapateena and Prominent Hill deposits. The Yorke Peninsula hosts substantial Cu-Au mineralisation, with deposits at Hillside and in the Moonta–Wallaroo district that are broadly considered to have formed during the same event as Olympic Dam, the c. 1595–1575 Ma Hiltaba thermal event. Despite the region's economic significance, there is very little information on the nature and history of metamorphism in the Yorke Peninsula region, leaving a considerable gap in the knowledge of the tectonic controls on mineralisation in this area. New U–Pb LA-ICP-MS geochronology on metamorphic titanite, monazite and apatite from Wallaroo Group metasediments indicate the Yorke Peninsula region has a protracted thermal history, with three groupings of metamorphic ages at c. 1585 Ma, 1560–1550 Ma and 1530–1500 Ma, and apatite cooling ages that cluster around c. 1450 Ma. Phase equilibria modelling of andalusite-sillimanite bearing schists and cordierite-sillimanite bearing migmatitic gneisses indicate that metamorphism occurred between 1560 and 1500 Ma occurred at P–T conditions between 3.6 and 4.5 kbar and 660–750 °C. These high thermal gradient conditions (45–50 °C km−1) are recorded at least 20 million years after Hiltaba Suite magmatism; however, they occur within the thermo-chemically anomalous crust of the South Australian Heat Flow Anomaly, which predominantly consists of rock packages with heat production rates 60–100% greater than the global median for rocks of their age. One-dimensional thermal models constrained by the crustal architecture and thermal regime of the SE Gawler Craton suggest the metamorphic conditions recorded between 1560 and 1500 Ma can be achieved by burial of this heat producing element -enriched crust, without the influence of an advective heat source. Within this thermally-anomalous crustal regime in which high thermal gradients can be sustained, small changes in burial depth are considered sufficient to stimulate metal remobilisation in the metal-rich crust of the Yorke Peninsula region. This potential for burial-triggered, thermally-induced fluid liberation and subsequent metal remobilisation may be applicable to the entire eastern Gawler Craton, including the Olympic Cu-Au Province.