Isotopic characterisation of mineralised skarn in Pernatty Lagoon

Abstract

The Pernatty Lagoon skarn deposit is located in the Stuart Shelf within the Gawler Craton, approximately 50km South of Carapateena. Investigation into the isotopic signatures of mineralised skarns in Pernatty Lagoon will identify the fluid source of mineralisation and aid in mineralogical exploration in the Olympic Dam province. The age of the skarn is proposed to be the product of fluids from the Gawler Range Volcanics or Hiltaba Intrusive Suite interacting with metasediments of the Wallaroo Group at 1577+7 Ma. Mineralisation in the skarn is dominated by chalcopyrite, galena and zinc. A large-scale alteration halo defines the region, and exemplifies a complex geological history involving many stages of fluid-rock interaction. Andradite garnet is highly abundant throughout the skarn alongside quartz and calcite veins, potassium-feldspar, chlorite, talc, and amphibole. In this study δ18O and δD values were obtained from bulk rock samples in Pernatty Lagoon, yielding values ranging between 4.7 to 14.1 and -80.1 to -45.5, with averages of 10.6 and -61.7 respectively, and show mixing of magmatic water and metasediments.

εNd values were calculated from samples obtained from Pernatty Lagoon during this study and were based on the inferred age of skarn formation, and yielded values ranging between -5.72 and -7.55 with a mean of -6.68. These values suggest the skarn and by association mineralisation was formed by evolved crustally-derived fluids. Trace element data obtained from garnets in Pernatty Lagoon suggest the skarn forming fluid was acidic, saline, and likely of granitic origin. Stable and radiogenic isotope data, as well as trace element and REE data, all strongly indicate that skarn mineralisation occurred under reduced, saline and acidic conditions due to the emplacement of the Hiltaba Intrusive Suite into Wallaroo Group metasediments. These findings are of significance as it confirms that the Hiltaba Intrusive Suite was the driving force of mineralisation in Pernatty Lagoon, and that Pernatty Lagoon mineralisation was independent of Olympic Dam.