Recognition of a widespread Paleoproterozoic hydrothermal system in the southern McArthur Basin, northern Australia, by in-situ analysis of fine-grained pyrite and spatially-associated solid bitumen in the Lamont Pass palaeohigh

The Paleoproterozoic hydrothermal system in the southern McArthur Basin, northern Australia, played a key role in the formation of the world-class clastic-dominant (CD)-type McArthur River Zn-Pb deposit, as well as several small-sized deposits nearby in this basin. However, the spatial scale of this hydrothermal system and its role in controlling mineralisation are uncertain. Such understanding is crucial for improving mineral exploration efficiency. In this study, we characterised the geochemistry of the fine-grained (<10 μm) pyrite and the co-existing solid bitumen in the non-mineralised Paleoproterozoic Barney Creek Formation (BCF) intersected by the drill-hole Lamont Pass 3, with a novel combination of Raman spectroscopy and nanoscale secondary ion mass spectrometry (Nano-SIMS), to underpin any potential hydrothermal activities distal (∼50 km) to the McArthur River deposit. In the studied samples, fine-grained pyrites display sub-rounded, elongated and ultrafine-grained morphologies and occur as grain clusters spatially closely-associated with solid bitumen. Elemental maps obtained from Nano-SIMS reveal that some pyrites are enriched in trace elements, such as Pb, Tl, Ni, Co, and Cu, occurring as individual grains or overgrowth over the trace element-depleted pyrites. Fine-grained pyrites in the studied samples show similar trace element enrichment patterns to those often observed in the McArthur River deposit. The trace element-rich and -poor pyrites are interpreted to form as a result of hydrothermal and diagenetic processes, respectively. Formation temperatures of solid bitumen were estimated as ∼ 170 ℃ based on Raman spectral and reflectance data, approximating temperatures of hydrothermal fluids responsible for the McArthur River deposit. It is interpreted that solid bitumen may be either formed syngenetically with diagenetic pyrites and thermally reworked by hydrothermal fluids or formed contemporaneously with hydrothermal pyrite. The in-situ analysis of fine-grained pyrite and spatially-associated solid bitumen indicates that hydrothermal activity in the study area is consistent with the fluids responsible for the McArthur River deposit with respect to the trace element composition and fluid temperature. Hydrothermal fluid flow in the study area was likely a far-field effect of the McArthur River mineralisation extending along the Emu Fault. The widespread hydrothermal activity in the Barney Creek Formation strongly suggests that other factors, such as the fluid migration conduits (e.g. large-scale faults), are important controls for mineralization in this region.