Chemical mineralogy of the oxidized zones of the E22, E26 and E27 ore bodies at Northparkes, New South Wales

A number of porphyry copper-gold deposits at Northparkes in central New South Wales are associated with finger-like intrusions of 430-440 Ma quartz-monzonite porphyries, which intrude Ordovician age Goonumbla volcanics (Heithersay et al. 1990). Major primary minerals are pyrite, bornite and chalcopyrite, together with economically significant native gold. Both primary and secondary ore from three bodies, E22, E27 and E26 North (E26) have been worked for copper and gold since 1994. The three deposits are covered by a mixture of clays, some of which are in situ weathering products, and transported overburden (Arundell 2004, Heithersay et al. 1990). The region has experienced prolonged and episodic weathering. Two significant weathering events have been identified, one in the Carboniferous and a subsequent event in the Cenozoic (O'Sullivan et al. 2000, Pillans et al. 1999). As a result, the E22, E26 and E27 deposits display well-developed oxidized zones, with intense leaching of the upper sections (Crane et al. 1998, McLean et al. 2004). The base of oxidation extends to 80 m from surface, with strong kaolinization from approximately 5 to 45 m below surface. Upper oxidized zones are dominated by the secondary Cu(II) phosphate minerals libethenite (Cu2P040H) and pseudomalachite (CU5(P04)2(OH)4) and, uniquely to E26, sampleite (NaCaCu5(P04)4C1.5H20). Beneath the phosphates a zone dominated by malachite (CU2C03(OH)2), azurite (CU3(C03)2(OH)2) and chrysocolla (CuSi03.nH20) gives way at depth to a thin native copper-cuprite (Cu20)-chalcocite (CU2S) supergene enriched zone. E26 is exceptional in that the formation of the secondary Cu(II) carbonates was preceded by extensive precipitation of atacamite (Cu2CI(OH)3), this being reflected by the compositions of present ground waters (McLean et al. 2004); those associated with E26 are much more saline (NaCI). We have reconstructed the solution conditions associated with the development of these assemblages to better understand the dispersion of copper. In order to model the environment of phosphate mineral deposition in E26 a stability constant for sampleite at 298K has been measured.