Timing, geochemistry and tectonic setting of Ni-Cu sulfide-associated intrusions of the Halls Creek Orogen, Western Australia

Abstract

The Lamboo Province of the East Kimberley region of Western Australia contains a record of crustal and tectonic evolution from ca. 1912 to 1808 Ma. In this interval, current models indicate an ocean basin was systematically closed from ca. 1880 Ma until collision of the Kimberley (Western Zone) and North Australia Cratons (Eastern Zone) at the ca. 1830 Ma Halls Creek Orogeny. Within this orogen, the Central Zone has two proposed origins: (1) an intra-oceanic island-arc; and (2) an exotic continental fragment. In this study, we investigated the origin of Ni-Cu mineralized mafic-ultramafic intrusions found in the Central Zone. We performed U-Pb SHRIMP geochronology on 21 samples, together with O-isotopes in zircon and whole-rock geochemistry, to understand the formation of the intrusions and their host mineralization within the tectonic evolution of the Lamboo Province. Geochronology demonstrates that the majority of intrusions are synchronous at ca. 1848–1838 Ma (ca. 1844 ± 2 Ma), and were intruded into the <1869 Ma Tickalara metasedimentary package (with 1895–2810 Ma detrital zircons). The Ni-Cu mineralized Copernicus pyroxenite was dated at 1823 Ma. Whole-rock geochemistry indicates a subduction origin for the mafic-ultramafic magmatism, with trace element data trending with modern continental-arc magmas, and lacking distinct assimilation trends with the Tickalara metasedimentary host-rock. Oxygen isotope data (δ18O ca. 5.6–12.8‰), supports this interpretation. The occurrence of more mantle-derived compositions, in early metamafic dykes, suggests a setting for the Central Zone that included both mantle- and arc-derived magmas. This, together with evidence for syn-magmatic high-temperature metamorphism, suggests a continental back-arc setting in the Central Zone at 1845 Ma. This interpretation, as well as the Archean continental zircon detritus identified in the Tickalara metasediments, indicates the Central Zone is a continental fragment rather than an island arc. Subsequently, this study demonstrates that Paleoproterozoic continental arcs are a prospective setting for orthomagmatic Ni-Cu-PGE systems.