The nature of gold-bearing palaeochannel sediments in the Gidji area north of Kalgoorlie, Western Australia

Abstract

Remnants of an extensive palaeodrainage network are preserved in the Kalgoorlie region of Western Australia as buried palaeochannels. At Gidji a major palaeochannel up to 60 m deep, is infilled with sand- and clay-rich facies and overlain by younger alluvial/colluvial sands and gravels, aeolian sands and silts, playa deposits, red soils and lag materials. The palaeochannel sediments consist predominantly of low-crystallinity kaolinite, quartz and minor smectite. They contain very little muscovite, illite or chlorite and can be readily distinguished from underlying in situ saprolite by their chemical characteristics (particularly K/Al and Mg/Al ratios, low rare earth element contents and low K/Rb ratios). These sediments were largely deposited during the Mid-Late Eocene as an homogenised mixture derived from a low relief and highly weathered pre-Eocene landsurface. They have undergone post-depositional weathering resulting in mobilisation and redeposition of iron as oxides/oxyhydroxides. The stratigraphy of the palaeochannel sediments indicates a change from dominantly fluvial to fluvio-lacustrine conditions during deposition. In areas of gold mineralisation, the presence of palaeochannels complicates geochemical exploration. Areas of thick palaeochannel clays mask bedrock anomalies. Placer and supergene concentrations of gold associated with the palaeochannels can be confused with bedrock anomalies unless a multi-element approach is taken. The Gidji palaeochannel environment contains a combination of bedrock and placer/supergene gold anomalies. Primary bedrock anomalies generally have As, Sb and W as associated pathfinder elements. Some pathfinder elements (e.g. As) have been concentrated in ferruginous pisoliths and gravels or mechanically dispersed in near surface alluvial/colluvial materials giving rise to false anomalies.