Geochemical dispersion of the DeGrussa deposit within its associated palaeodrainage system

[摘要] In the context of deeply weathered landscapes, sedimentary packages with exotic mineral and geochemical features contribute to increase the total thickness of the regolith profile. This overburden acts as a filter or impermeable barrier to vertical geochemical dispersion. Sedimentary systems associated with ore deposits may act as mechanical dispersion agents of pathfinder elements at local and regional scales.The DeGrussa Cu--‐Au deposit is hosted in Proterozoic turbidites, basalts and dolerites, and is located in the Naracoota Formation in the Bryah Basin, on the margin of a Tertiary palaeochannel system, in the NE of Western Australia. This study seeks to determine if the DeGrussa deposit can be geochemically vectored using the transported cover of the associated palaeochannel system.The in situ regolith to the west of the DeGrussa deposit is subdivided into ferruginous saprolite at depth, which vertically changes into kaolinitic saprolite, sporadically capped by a ferruginous or siliceous duricrust. This in situ regolith is overlain by a sedimentary package of Tertiary palaeochannel clays and magnetic gravels topped by Quaternary alluvium/colluvium that may be partly silicified. The geochemical sedimentary signature of the DeGrussa deposit is highlighted by elevated Cu, Zn, Au, and Ag concentrations, as well as significant enrichments in In, Mo, Se, Te, Bi, As, Cd and Co, and depletion of Nb, Hf, Th, REE, Cr, Ni, V, Sc and Ba. Strikingly, Cr, Ni, V and Sc are widely enriched in the sedimentary cover as well as in the unweathered sedimentary basement rocks (turbidite and mafic volcanic units), although these elements are significantly depleted in the ore. Positive Cu, Au, Ag, Mo, Se, In and Sb anomalies are geochemical features of the ore, and are enriched throughout the palaeochannnel sequence. Sedimentary units above the Tertiary channel deposits have higher anomalous concentrations of Mo (mean ~1.3 ppm ± 0.6), Se (mean ~1.1 ppm ± 0.5), In (mean ~0.15 ppm ± 0.05) and Sb (mean ~1.2 ppm ± 0.6), compared to means of ~1.1 ± 0.35, ~0.4 ± 0.15, ~0.11 ± 0.04 and ~0.8 ± 0.35 ppm, respectively, in the channel clay units. Carbonate--‐dominated bands display characteristic positive anomalies of Se and Cd. The pervasive metal enrichment of the sedimentary cover reflects a fluvio--‐lacustrine drainage system within a restricted catchment with sediment provenance linked to the local Proterozoic basement rocks, which are enriched in Cr, Ni, V, Sc, Cu, Au, Ag, Mo, Se, In and Sb. The geochemical signature of the DeGrussa deposit is present in the palaeochannel infill sequence and alluvium/colluvium unit. However, it remains uncertain if this signature is partially masked by the combination of similar geochemical features present in the basement rocks, the weathering intensity of the sediments and the cannibalistic nature of the sedimentary system associated with the palaeochannel in the study area. The physical and geochemical vectors for element dispersion from the DeGrussa deposit towards the palaeochannel are dominated by: (1) lateral physical geochemical dispersion with a secondary hydromorphic component, and (2) a locally restricted supply of intensely weathered sediment to the palaeochannel infill sequence.In the context of this research, this study concludes that the geochemistry of the transported cover is an effective medium to sample to detect geochemical signatures of mineral deposits found in this specific region.

[发布机构] CSIRO

[效力级别] Exploration Geochemistry [学科分类] 地球科学(综合)