DETERMINING POTENTIAL GOLD HOST ROCKS IN POORLY EXPOSED PROTEROZOIC METASEDIMENTARY ROCKS

The Tanami region is one of Australia's richest Proterozoic gold provinces in the Northern Territory. Extensive geochemical sampling of metasedimentary rocks within this poorly outcropping region has provided insights into the distribution of potential host rocks to gold mineralisation. Relationships between rare earth elements (REE), high field strength elements and SHRIMP U-Pb detrital zircon populations provide a model to fingerprint and establish the provenance of siliciclastic sedimentary rocks at the Callie gold deposit which is hosted within the fine-grained Proterozoic Tanami Group. This large scale geochemical, geochronological basin model provides an important window into understanding and predicting the best trap rocks for epigenetic gold deposition in fine grained greywackes and black shales of the Tanami region. The highly prospective Blake beds within the Dead Bullock Fm. (DBF) can be identified by geochemical and geochronological characteristics, the most effective of which are: (1) low (<0.2) Zr/Sc values combined with high (>3) Th/Sc values; (2) light REE depletion (relative to PAAS); (3) enrichments of MgO, TiO2, Cr, Sc; and (4) SHRIMP U-Pb detrital zircons at ~3200 & 2500 Ma. In contrast, turbiditic sandstone of the overlying Killi Killi Fm., which is less strongly mineralised, is characterised by: (1) high (>0.2) Zr/Sc values combined with low (<3) Th/Sc values; (2) relatively flat REE patterns; (3) enrichments in Zr, La/Sc, La/Sm, and (4) the presence of a major population of detrital zircons at ~1870 Ma. The Orac and Shim beds, which lie stratigraphically between the Blake beds and Killi Killi Fm, have geochemical signatures that overlap the fields defined by the Blake beds and Killi Killi Fm. The geochemical characteristics together with age of detrital zircons are interpreted to indicate that the DBF was derived from a provenance dominated by a bimodal suite of Archean mafic and felsic rocks, whereas the Killi Killi Fm. was derived from a provenance dominated by felsic Proterozoic rocks. The Th/Sc-Zr/Sc plot can be used as an exploration tool to differentiate the more favourable DBF from the less prospective Killi Killi Fm. These data combined with regional geologic data indicate that a siltstone-dominated stratigraphy is the best host for gold mineralisation in the Tanami region.