The Controls on Element Distribution in the Regolith at the Portia and North Portia Prospects, Curnaniona, South Australia

摘要

The Portia Au and North Portia Cu-Au-Mo prospects are located on a magnetic structure called the Benagerie Ridge Magnetic Complex, comprising varying pyrite-bearing Proterozoic albitite, phyllite and carbonaceous phyllite. The prospects are covered by up to 75 m of Cainozoic sediments, which comprise from top down, surface soils, Quaternary sediments, the Miocene Namba Formation and the (Late-Oligocene?) Portia Unit. The Portia Unit is dominantly clay-rich with local sand-rich base and it fills palaeo-topographic lows and a palaeo-valley that incised into the weathered basement rock (saprolite). This sedimentary unit hosts the majority of the Au grains found at Portia, with additional Au grains also present in the underlying saprolile. The North Portia prospect comprises saprolite hosted Cu-Au-Mo, with elevated concentrations of As-Pb-V present in the northern part of the prospect. These two groups of elements are delineated using factor analysis, and are regarded as the pathfinder elements for the North Portia mineralisation. Secondary Cu minerals including covellite and native Cu are present in the lower saprolite, and are stable under reducing conditions in the saturated zone. In the upper saprolite (unsaturated), Cu, Au, As, Mo and Pb are associated with Fe-oxides, mainly goethite and hematite. Saprolite leached of Fe generally contains low pathfinder elements concentrations. The Au grains found in the Portia Unit and saprolite contain mineral inclusions such as arsenopyrite, galena and altaite (PbTe), and the Ag contents range from one to ten per cent, ie the Au grains are electrum. These mineral assemblages and chemistry suggest that the Au grains are primary. Taking the lithologic hosts into consideration implies that the Au grains present in the saprolite are primary in situ Au, whereas those hosted in the sedimentary Portia Unit are detrital Au (or deep leads). Partial dissolution and mobilisation of Au is suggested to have taken place with subsequent precipitation of Au-rich rims on pre-existing Au grains, and as small diagenetic Au particles (approx 1-2 mu m).