Summary of Results. Joint GSQ–GA Geochronology Project: Horn Island, Torres Strait, 2016-2017

This work is a part of an investigation of mineralisation associated with the extensive Kennedy Igneous Association (Champion & Bultitude, 2013) in North Queensland. This part of the project involves U–Pb zircon geochronology of magmatic rocks that are associated with gold mineralisation. By doing this we hope to identify key time-periods of magmatic activity that can be used by explorers to better focus their exploration efforts and assist with the development of new tectono-metallogenic models. Earlier results published by Cross et al. (2019) and Kositcin et al. (2016) in the Jardine Subprovince of the Kennedy Igneous Association in Cape York, for the first time, demonstrated a strong association between gold mineralisation and early Permian (285–280 Ma) felsic dykes that intrude either Proterozoic metamorphic rocks or Devonian granites of the Cape York Batholith. The SHRIMP U–Pb zircon results reported here come from three magmatic rocks, Badu Granite (2678819/QFG8689E), Horn Island Granite (2678820/QFG8800A) and unnamed rhyolite (2678818/QFG8798A), that were sampled from exploration drill core, drilled by Alice Queen Limited on behalf of its subsidiary company, Kauraru Gold Pty Ltd between 2016 and 2017 on the western margins of the historic Horn Island gold mine. Prior to this work, magmatic rocks of the Badu Supersuite on Horn Island were attributed to the Jardine Subprovince of the Kennedy Igneous Association (Champion & Bultitude 2013). The Badu Supersuite comprises the Badu Suite (Badu Granite, Horn Island Granite and unmineralised porphyritic dykes; von Gnielinski et al., 1997) and the Torres Strait Volcanic Group. Gold mineralisation on Horn Island is intrusion-related and occurs within narrow quartz veins that contain native gold and sulphide mineralisation (Alice Queen Limited, 2021) that cut both the Badu and Horn Island granites but not the late-stage porphyritic dykes (von Gnielinski, 1996; von Gnielinski et al., 1997). Historical K–Ar ages from 286–302 Ma for Badu Suite intrusives (Richards and Willmott, 1970) were used to imply a late Carboniferous to early Permian age for the Torres Strait Volcanic Group. Recently however, two units from the Torres Strait Volcanic Group, the Endeavour Strait Ignimbrite and the ‘Bluffs Quarry’ rhyolite dyke yielded SHRIMP 206Pb/238U ages of 349.2 ± 3.1 Ma (Cross et al., 2019) and 353.4 ± 2.2 Ma (Kositcin et al., 2016), respectively, placing this group in the early Carboniferous. Two of the samples, the Badu Granite (2678819/QFG8689E) and Horn Island Granite (2678820/QFG8800A) gave indistinguishable 206Pb/238U results within analytical uncertainty (MSWD = 1.6, POF = 0.21) of 342.8 ± 1.9 Ma and 344.4 ± 1.7 Ma, respectively. The unmineralised, cross cutting, unnamed rhyolite (2678818/QFG8798A) has a significantly younger 206Pb/238U age of 309.9 ± 1.5 Ma. These results demonstrate that the Badu Granite and Horn Island Granite are early Carboniferous in age and not early Permian as previously thought. The historical K–Ar ages (302–286 Ma) for Badu Suite intrusives are interpreted to record thermal resetting. Together with the ca 350 Ma crystallisation ages for two units from the Torres Strait Volcanic Group (Cross et al., 2019; Kositcin et al., 2016), these new results reveal that magmatic crystallisation ages for the Badu Supersuite range between ca 350 Ma and 310 Ma. As such, the Badu Supersuite, along with the Black Cap Diorite (350.7 ± 1.3 Ma; Murgulov et al., 2009) near Georgetown, represents the earliest phase of magmatism associated with the early Carboniferous to late Permian, Kennedy Igneous Association. Consequently, the Badu Supersuite including the Badu Suite and the Torres Strait Volcanic Group are now seen to belong to a newly named Torres Strait Subprovince, which is distinctly older than the Jardine Subprovince on Cape York Peninsula. Additionally, these results constrain the timing of gold mineralisation at Horn Island to between a maximum age at ca 344 Ma provided by the host granites and a minimum age at ca 310 Ma constrained by the rhyolite dyke (2678818/QFG8798A). These constraints for the timing of gold mineralisation at Horn Island are further supported by unpublished results presented by Lisitsin & Dhnaram (2019a, b). These workers mention preliminary ca 342–344 Ma Re–Os molybdenite ages from two samples of quartz-molybdenite veins that cut the Badu Granite and an Ar–Ar age from sericite alteration associated with a quartz-sulphide-gold vein at ca 320 Ma that they considered to best represent the timing of gold mineralisation. The new SHRIMP U–Pb zircon ages presented here for magmatic rocks of the Badu Suite, reveal the association between gold mineralisation and early Carboniferous magmatism associated with the newly named Torres Strait Subprovince of the Kennedy Igneous Association.