Fortescue Group Gold – Pilbara Region, WA

Gold hosting conglomerate units occur at the base of the Mount Roe Basalt and Hardey Formation volcano-sedimentary rocks. However, the lateral extent of these units has yet to be established due to limited surface exposures and drilling, and a lack of constraints around the regional basin architecture. Drill holes intersect the Fortescue Group, including a gold nugget in the basal conglomerates. In this work we conducted a detailed study investigating and correlating the geochemistry and mineralogy of the stratigraphy intersected in three drill holes gain insights into the local basin stratigraphy and mineral prospectivity.

Correlation of the stratigraphy between the three holes indicates the basin in the study area is characterized by a localized, likely fault bounded, topographic low where thick accumulations of the Hardey Formation were deposited and intersected in drill hole DDH84MF1. Thinner accumulations of the upper Hardey Formation were deposited on the flanks of the basin as accommodation space was exhausted. This flanking region was intersected by the 18ABAD holes which contain no conglomerate bearing lithologies. The basal boulder conglomerates of the Hardey Formation in DDH84MF1 are derived from multiple sources, and nugget gold occurs in the felsic conglomerate unit, implying potential felsic source lithology for placer gold. Hydrothermal alteration of the felsic basement in 18ABAD01 may have implications for regional hydrothermal influence on nugget gold mineralization in the overlying conglomerate lithologies

The Kylena and Tumbiana Formation are uniform in thickness and were likely deposited on a stable palaeosurface when syn-sedimentary rifting has ceased. This allows correlation by geochemistry and mineralogy over at least ~15 km laterally. The correlation of anomalous Cu and phengitic alteration provides evidence for a potentially widespread volcanic-associated stratiform sediment-hosted copper mineral system between the Kylena Formation volcanics and the Tumbiana Formation siltstone and mudstones. This would be one of the oldest known examples of this mineral system style.

Publications/reports:

Stromberg, J., Spinks, S., Pearce, M., 2019. Geochemical Stratigraphy of the Neoarchean Lower and Middle Fortescue Group, Western Australia. Kensington, WA. CSIRO EP192724.

Stromberg, J., Pearce, M., Spinks, S., 2019. Integrated downhole VNIR-SWIR and TIR hyperspectral mineralogy and geochemistry for defining lithostratigraphy and alteration. In: SEG 2019: South American Metallogeny: Sierra to Craton; 7-10 October 2019; Santiago, Chile. Society of Economic Geologists (SEG); 2p. http://hdl.handle.net/102.100.100/260190?index=1

Farkas, J., Klaebe, R., Scarabotti, L., Stromberg, J., Spinks, S., 2020. Neoarchean carbonates as archives of paleo-redox conditions on early Earth: Insights from metal isotope analyses of the Tumbiana Formation, Pilbara, WA. In: Australian Earth Science Convention; February 9-12, 2020; online. GSA; 2021. 1. http://hdl.handle.net/102.100.100/388141?index=1

Stromberg, J., Spinks, S., Pearce, M., 2019. Characterisation of the Neoarchean Fortescue Group Stratigraphy – Integrated downhole geochemical and mineralogical correlation from new diamond drilling. In: AEGC 2019 – 2nd Australian Exploration Geoscience Conference; 2-5 September 2019; Perth, WA, Australia. Australian Society of Exploration Geophysicists (ASEG); 2019. 1-4. https://doi.org/10.1080/22020586.2019.12073203