The Wirrda Well and Acropolis prospects, Gawler Craton, South Australia: Insights into evolving fluid conditions through apatite chemistry

Abstract

The Wirrda Well and Acropolis prospects are located ~25km SSE and SW, respectively, from the giant Olympic Dam Cu-U-Au-Ag deposit within the Olympic Cu-Au Province of South Australia. Mineralisation in the two prospects displays a temporal and spatial zonation characteristic of other IOCG-type deposits and prospects within the province, including Olympic Dam, in which an early high-temperature, magnetite-dominant mineralisation is transitional to a hematite-dominant mineralisation style, accompanied by subordinate and often localised carbonate alteration. Both prospects contain conspicuous hydrothermal apatite which is characteristic in terms of host assemblage and geochemical signature. Chondrite-normalised rare earth element (REE) fractionation patterns for apatite depict the evolution of hydrothermal fluids during ore formation. An early generation of apatite is abundant within magnetite-dominant mineralisation in both prospects, and displays a characteristic light-REE enriched fractionation trend. Overprinting of this initial high-temperature assemblage results in the loss of REE and Y (REY), as well as Cl, along fractures within this apatite, as well as the formation of new generations of apatite that are also depleted in these elements. The transition from early reduced, to later oxidised assemblages within both prospects is accompanied by an evolution of the REY signature of apatite in which LREE-enriched patters with negative Eu-anomalies are replaced by convex middle-REE (MREE)-enriched patterns, positive Eu-anomalies and the development of a conspicuous negative Y-anomaly. Comparable trends are recognised elsewhere within the district and are interpreted as the hallmark of productive mineralised IOCG ore systems. The evolution of chondrite-normalised REY patterns can be explained in terms of changes in fluid parameters, speciation of REY in ore-forming fluids, and the capacity of REY to precipitate and partition into apatite in ways that contrast with those expected by simple consideration of crystal structure. Results are concordant with modelling of REY-speciation, which show that, under hydrothermal conditions typical of IOCG mineralisation, a decrease in salinity, pH and temperature is associated with hematite-sericite alteration sufficient to produce MREE-enriched apatite. These data offer encouragement for the use of apatite geochemistry in mineral exploration within the Olympic Cu-Au Province, and potentially in analogous terranes elsewhere, given the clear association between MREE-enriched apatite and often well-mineralised, hematite-dominant domains within these large IOCG systems.