Sedimentology, origin and gold potential of the Late Archean Lalla Rookh Basin, East Pilbara Block, Western Australia

Terrigenous clastic sequences comprising the Lalla Rookh Formation rest with angular unconformity on the 3,550-3,000 Myr granitoid-greenstone terrain of the east Pilbara block. Outcrop of the Lalla Rookh Formation is confined to an elongate structural basin with dimensions of 50 km x 12 km that is bounded by high-angle faults and unconformities with older supracrustal rocks and younger ca 2,800 Myr old metasedimentary rocks. The results of a sedimentologic basin analysis suggest that the configuration of the original depository was similar to the present structural basin. The basin fill, attaining a maximum preserved thickness of 3,000 m, consists of five depositional facies; 1) alluvial-fan and talus-slope; 2) braided-stream; 3) flood-plain; 4) fan-delta; and 5) lacustrine. Braided-stream deposits define the depositional axis of the basin and include various proximal-conglomerate to distal-sandstone assemblages. Significant heavy-mineral concentrations are located in: 1) stacked sequences of proximal core-zone conglomerates; 2) specific beds of conglomerate in stacked sequences of proximal to medial core-zones; 3) telescoped sequences of conglomerates in proximal to distal core-zones; 4) specific beds in stacked sequences of distal sandstones; 5) stacked ''levee'' sequences of sandstone lateral to proximal core-zones; and 6) basal diamictites of debris-flow origin in alluvial-fan facies. Potentially economic gold placers are developed on low-angle, intraformational unconformities. The basin analysis indicates that broad exploration targets for gold-pyrite placers can be identified by recognising suitable facies and facies assemblages. Specific targets are delineated using geochemistry which identifies anomalous concentrations of heavy minerals and sulphides. Two lines of evidence suggest that there is low potential to develop gold placers in the Lalla Rookh Formation: the lack of conclusive evidence that it post-dates a major metamorphic, tectonic and mineralisation event, and the anomalously low gold-potential of Pilbara greenstones. 48 refs, 11 figs, 1 tab