The Viability of Lithogeochemistry and Surficial and Surficial Geochemistry in Detecting CU-AU Porphyry Mineralization Under Cover in the Lachland Fold Belt, Australia

Abstract

The Racecourse prospect is a possibly economic alkalic Cu-Au porphyry system, situated within the Macquarie Arc, NSW, Australia. Similar in age to both the Ordovician Cadia and Northparks districts of NSW, a volcano-sedimentary host sequence was intruded by two monzonitic porphyry complexes through fluid conduits created by the cross arc architecture of the Lachlan Transverse Zone. Prior industry workers have intersected two intrusive suites at Racecourse; the feldspar porphyry remains mineralized, fertile, and postulated to be the main mineralizing suite. Pearce Element Ratio diagrams for the three main lithologies intersected at Racecourse were created to perform a lithogeochemical prospectivity analysis. Two hundred eight whole rock samples representing the three lithologies were selected from data supplied by Anglo American Exploration Australia. There is a lack of hydrolytic alteration, and minimal alteration experienced by the system as a whole as illustrated by the PERs. Instead, the feldspar porphyry carries a relatively unaltered geochemical signature. Economic metals have no apparent relation to type of alteration. Mineralization is hosted by specific rock types, with Cu mineralization related to the feldspar porphyry, and Au occurring at low concentrations throughout all units. Soils and Pinus radiata tree cores were collected above and distal to mineralization in order to outline the covered prospect’s signature. Soil samples were sieved with the <250 m size analyzed, and show distinct anomalous populations of Au, Cu, Mo, Pb, and Zn above prior drilling. Rings of Pinus radiata were counted, with the age interval of 2003 – 2008 chosen to sample. Samples show elevated Cu, Mo, and Zn above the previously drilled soil anomaly, whereas the southwest of the survey area reveals a Cu anomaly corresponding to localized faulting. 206Pb/204Pb values from the Pinus radiata identify distinct anomalous groups spatially associated with discrete metal anomalies. At the Racecourse target, anomalous Pinus radiata samples yield a similar isotopic signature to the faulted southwestern anomaly, potentially linking the source of these two metal anomalies as the feldspar porphyry. When integrated with subsurface geology and the current understanding of the mineralized body, geochemical media suggest that mineralization may continue down plunge at depth.