Compositional characterisation of skarn garnet in the Pernatty Lagoon region, and comparisons with Punt Hill and Hillside

Abstract

The Pernatty Lagoon Region is located on the eastern margin of the Gawler Craton, within the Stuart Shelf of South Australia. The Gawler Craton is host to a range of sub-economic garnet skarn systems: within Pernatty Lagoon is the PLR21/SAR8 drill hole which is characterised by heavy skarn-style alteration with a uniform mineralisation halo (Cu–Pb–Zn). Punt Hill and Hillside both reside within the Wallaroo Group lithological package of the Gawler Craton and are approximately 6km east and 380km south of Pernatty Lagoon, respectively, allowing for a proximal/distal comparison to be drawn. Laser–ablation inductively–coupled plasma mass spectrometry spot analyses and grain scale element distribution mapping determines the concentrations of rare earth elements/incompatibles and their distribution at grain scale. Garnets from SAR8 fall into the grandite range from And99–85Gr0–13Sp1–2 to And74–56Gr23–41Sp1–2. Laser–ablation inductively–coupled plasma mass spectrometry spot analyses and grain scale element distribution mapping show that Gawler Craton garnet skarns are unique for skarn garnet deposits as they are high in ΣREE+Y and U, have similar garnet compositions, a unique REE+Y distribution (cores enriched in light rare earth elements (LREE), flat heavy rare earth elements (HREE) and rims with similar LREE to HREE (only severely depleted in lanthanum and cerium), and an enrichment in W, Sn, As and Cr. YAG–style REE+Y fractionation only partly accounts for the distribution of REE+Y patterns. Xsite ionic radii substitution accounts for the unusual LILE fractionation, with Cl– and F–complexes being a major suppository of La3+ within hydrothermal fluids. The suggested potential for W, Sn, As and Cr enrichment as a vector for mineralisation and fluid flow is rendered inconclusive: instead the enrichment is due to host rock compositions associated with the fluid pathways.