Tectonic evolution and gold mineralisation of the Granites-Tanami Orogen, North Australian Craton

The multiply deformed Palaeoproterozoic Granites-Tanami Orogen (GTO) located in the southern part of the North Australian Craton is a significant auriferous province. This PhD study investigated the magmatism, tectonic and thermal evolution, and gold mineralisation event of the GTO.

The early Palaeoproterozoic stratigraphic succession in the orogen is the Tanami Group. The ca 1864 Ma Stubbins, Dead Bullock and Mount Charles formations of the Tanami Group are correlated and assigned to the Dead Bullock Formation. Pillow basalt and associated dolerite sills in the Dead Bullock Formation over a wide distance in the GTO have common geochemical features of enriched back-arc basin basalts. These mafic rocks are interpreted to be the results of a high degree decompressional melting (5-15%) of an asthenosphere source in the spinel stability field. The source of the rocks have primitive mantle signature with input of 3%-4% subduction-related material. Convective flow operating during subduction circulated the enriched material from wedge corner into the asthenospheric mantle beneath the Granites-Tanami back-arc basin and generated the mildly enriched mafic magma that was emplaced as basalt and dolerite sills.

Widespread younger (ca. 1795 Ma) mafic dykes in the region have geochemical characteristics of primitive mafic magma with input of crust-derived material. The interpreted continental-continental collision between the GTO and Arunta Orogen resulted in the thickening of the lower crust and lithospheric mantle root along the Willowra Lineament. The thickened lithospheric mantle root, having a higher density than the upper asthenospheric mantle, lead to gravitational instability and resulted in convective removal. The detached denser lithospheric mantle root sank into the asthenosphere triggered upwelling of the hot asthenospheric mantle material and extension. Melts derived from partial melting of the previously metasomatized lithospheric mantle with crustal assimilation formed the mafic dykes.

Studies of ⁴⁰Ar/³⁹Ar geochronology identified three major Palaeoproterozoic tectonothermal events in the GTO. The ca. 1840 Ma ⁴⁰Ar/³⁹Ar cooling age of metamorphic hornblende from dolerite sills in the Dead Bullock Formation provided precise age constraint for the first tectono-thermal event during the evolution of the Granites- Tanami back-arc basin. This age is consistent with the ca. 1850–1840 Ma subduction and peak metamorphism events associated with the Halls Creek Orogeny and the Tennant Orogeny. The ⁴⁰Ar/³⁹Ar age of 1753 ± 8 Ma for the early biotite from the Dead Bullock Formation is a cooling age record of the peak greenschist facies metamorphism during the ca. 1795 Ma Tanami Orogeny, which was associated with gold mineralisation in the GTO. The biotite recrystallization ⁴⁰Ar/³⁹Ar age of 1718 ± 8 Ma revealed the last tectono-thermal overprint to the GTO, which was the distal thermal influence by the ca. 1723 Ma Strangways Event in the Arunta Orogen.

The Twin Bonanza deposit, which is hosted by the ca. 1795 Ma Buccaneer Porphyry, is studied as it’s a unique quartz monzonite-hosted gold-only deposit in the GTO. The Buccaneer Porphyry at the deposit consists of two separate phases of porphyritic quartz monzonite and abundant mafic microgranular enclaves (MMEs). Geochemical and zircon geochronological data suggest that the two quartz monzonite phases and MMEs are products of progressive crystal fractionation of a co-magmatic system. The source for this gold-rich intrusion complex is the metasomatized lithospheric mantle. Crustal assimilation with magma that forming the Phase 1 quartz monzonite was considerable, but minor with the Phase 2 quartz monzonite and MMEs. The magma for these magmatic units was generated by partial melting of the metasomatized lithospheric mantle in the post-collisional tectonic setting. The post-collisional magmatism is approximately synchronously with the ca. 1795–1780 Ma Ma gold mineralisation and extensive granitic magmatism elsewhere in the GTO.

In situ U-Pb geochronology and Sm-Nd isotope data of hydrothermal monazite associated with gold-enclosing arsenopyrite from disseminated ore at the Twin Bonanza gold deposit have a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1788 ± 8 Ma (interpreted as the age of the gold mineralisation), and _εNd(t) values identical with the ca. 1795 host quartz monzonites. The synchronicity and identical _εNd(t) values indicate that gold was precipitated from the magmatic hydrothermal fluid that sourced from the metasomatized lithospheric mantle. Our study also suggest the episode of ca. 1795–1780 Ma was a single major gold mineralisation event that took place across the GTO and the Pine Creek Orogen.