The early Mesoproterozoic tectonic systems and IOCG mineralisation in the northern Gawler Craton

Abstract

The Gawler Craton in southern Australia is renowned for the presence of its world-class iron oxide copper gold (IOCG) deposits, which formed at ca. 1595–1575 Ma, in association with voluminous Large Igneous Province (LIP) magmatism. These hydrothermal breccia deposits, including the supergiant Olympic Dam deposit, are dominated by hematite and formed in the upper crust. In the northern Olympic Cu-Au Province, a series of magnetite-dominated deposits/prospects associated with minor Cu-Au mineralisation are hosted in rocks that experienced multiple phases of high temperature metamorphism and deformation. These have long been considered to represent the deeper expression of the hematite-dominated IOCG systems. Cairn Hill Fe (-Cu-Au) deposit, arguably the best example of a deeply eroded IOCG system in the Gawler Craton, is dominated by magnetite hosted in granulite facies rocks. New U-Pb zircon geochronology shows the magnetite-hornblende lodes at Cairn Hill were formed at ca. 1580 Ma at amphibolite facies conditions. The magnetite lodes are cross-cut by ca. 1515 Ma granitic dykes and then deformed by ca. 1490 Ma event at conditions of 4.6–5.3 kbar and 740–770 °C. However, Cu mineralisation at Cairn Hill occurs in brittle fractures, overprinting the 1490 Ma deformation and metamorphism. The spatial but not temporal association between magnetite and Cu has effectively overlain two distinct episodes of mineralisation to create a composite Fe-Cu deposit observed today. The age of the young Cu mineralisation is constrained by in situ apatite Lu-Hf geochronology. The granulite host-rock that predates Cu mineralisation yields apatite Lu-Hf ages of ca. 1490 Ma, while infiltration of Cu-bearing fluids resulted in recrystallisation of apatite, LREE mobilization and formation of secondary monazite at ca. 1460 Ma. The timing of Cu mineralisation coincides with the onset of Nuna fragmentation, representing a previously unrecognized mineralizing system in southern Australia that installed Cu in previously dehydrated crust during a long history of granulite-grade tectonic events. Detailed petrological and geochemical analyses of magnetite, apatite, and fluid inclusions are conducted to elucidate the genesis of iron and copper mineralisation at Cairn Hill. The early magnetite-apatite-hornblende assemblage may have been formed by magmatic fluids, similar to the deep, early magnetite-apatite at the deep and marginal parts of Olympic Dam. Ore-forming fluids responsible for Cu mineralisation are high-temperature, CO2-rich and oxidized, which may have exsolved from intrusions. The tectonic setting within which the Gawler Craton IOCG deposits and newly identified younger deposits/prospects is uncertain. During the time interval of IOCG mineralisation, Mabel Creek Ridge, in the northern Gawler Craton, is a granulite-facies domain recording early Mesoproterozoic metamorphism. New P–T pseudosection results and geochronology, coupled with the regional seismic and airborne magnetic data, reveal that Mabel Creek Ridge represents a record of early Mesoproterozoic extension in the Gawler Craton, during which thermally perturbed lower crustal rocks were exhumed within a gneiss dome. Early Mesoproterozoic extension took place within a complex geodynamic regime resulting from the interplay between final Nuna convergence along the margin of northeast Australia at ca. 1600 Ma, subduction to the southwest at ca. 1630–1610 Ma and plume-driven magmatism that resulted in the generation of voluminous felsic melts and associated IOCG mineralisation within the Gawler Craton-Curnamona Province system over the interval 1595–1575 Ma.