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

Remnants of an extensive palaeodrainage network are preserved in the Kalgoo rlie region of Western Australia as buried palaeochannels. At Gidji a major palaeochannel up to 60 rn deep, is infilled with sand- and clay-rich facie s and overlain by younger alluvial/colluvial sands and gravels, aeolian san ds and silts, playa deposits, red soils and lag materials. The palaeochanne l sediments consist predominantly of low-crystallinity kaolinite, quartz an d minor smectite. They contain very little muscovite, illite or chlorite an d can be readily distinguished from underlying in situ saprolite by their c hemical characteristics (particularly K/Al and Mg/Al ratios, low rare earth element contents and low K/Rb ratios). These sediments were largely deposi ted during the Mid-Late Eocene as an homogenised mixture derived from a low relief and highly weathered pre-Eocene landsurface. They have undergone po stdepositional weathering resulting in mobilisation and redeposition of iro n 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 palaeoc hannels complicates geochemical exploration. Areas of thick palaeochannel c lays mask bedrock anomalies. Placer and supergene concentrations of gold as sociated with the palaeochannels can be confused with bedrock anomalies unl ess a multi-element approach is taken. The Gidji palaeochannel environment contains a combination of bedrock and placer/supergene gold anomalies. Prim ary bedrock anomalies generally have As, Sb and W as associated pathfinder elements. Some pathfinder elements (e.g. As) have been concentrated in ferr uginous pisoliths and gravels or mechanically dispersed in near surface all uvial/colluvial materials giving rise to false anomalies. (C) 2001 Publishe d by Elsevier Science Ltd.