REGOLITH-LANDFORM PROCESSES AT NORTHPARKES CU-AU MINE, NSW, AUSTRALIA

The Northparkes Mine is a porphyry Cu-Au mine in the Lachlan Fold Belt in central New South Wales, Australia. Detailed mine pit profile and drill hole descriptions and analyses, together with regolith-landform mapping, have revealed a palaeo-landscape with a greater relief (up to 30 m) than the present relief (mostly less than a few metres) developed on weathered bedrock, now largely buried by weathered sediments. Alluvial and colluvial sediments were deposited in palaeo-valleys and on top of palaeo-interfluves in the northern Lachlan Fold Belt. The Northparkes palaeo-valleys are a few kilometres north of the Canobolas Divide, a major drainage divide initiated in the Late Cretaceous due to drainage incision of a regional Jurassic sediment palaeo-plain. The sediments at Northparkes can be inferred to be Cainozoic, as they include reworked Mesozoic quartzose alluvial sediments. The Northparkes palaeo-valleys may have become established in exhumed north-trending, strike-aligned Jurassic palaeo-valleys. Regional studies indicate that much of the sediment is derived from colluvial slope sheet-wash or debris flows on valley sides, and low angle alluvial fans that coalesce to form alluvial plains. Many areas in the north Lachlan Fold Belt have deep leads where headwater tributaries may have eroded mineralised bedrock and supergene enriched saprolite, and then been buried by sediments. Aeolian sediments mixed in with residual or transported soils through pedogenic and transportation processes, may redistribute, mask or dilute mineralisation signatures, or alternately, add an exotic signal to an existing anomaly or otherwise barren ground. Interpretation of geochemical anomalies is difficult and requires detailed understanding of hydromorphic dispersion, physical transport, and chemical coprecipitation processes in relation to variable palaeo-climate and the evolving palaeo-topography. Application of background geochemical values of the weathered lithologies versus geochemical anomalies related to mineralisation at Northparkes has assisted in establishing geochemical thresholds and targeting mineralisation.