Trace element geochemistry and geochronology of an IOCG mineral system: a case study from the Vulcan Prospect, South Australia

Abstract

The Vulcan prospect is a hematite-rich brecciated iron-oxide copper-gold (IOCG) prospect within the eastern Gawler Craton, South Australia, 30 km from the ca. 1590 Ma Olympic Dam IOCG system. Apatite is abundant within metamorphic clasts and in altered and mineralised rock. In-situ geochronology and trace element analyses of gangue minerals including apatite, florencite and hematite from 41 thin sections from the Vulcan prospect were investigated to determine timing of mineralising events, changes in fluid chemistry, and to evaluate similarities and differences between Vulcan and similar deposits within the eastern Gawler Craton. Apatite petrography revealed three grain populations. Coarse-grained apatite from hematite-sericite dominated lithologies produced 461 ± 9 Ma, 510 ± 8 Ma and 544 ± 46 Ma ages coincident with the Delamarian orogeny, representing cooling during this period. Fine-grained vein-hosted apatite within hematite lithologies produced 985 ± 18 Ma and 1095 ± 10 Ma ages, coeval with late stages of the Musgravian Orogeny indicating a thermal control on the Gawler Craton. Apatite associated with magnetite-chlorite lithologies produced ages of 1522 ± 42 Ma, 1543 ± 25 Ma and 1611 ± 24 Ma within error of ca. 1580 Ma Hiltaba Suite felsic magmatism and ca. 1586 Ma molydenite mineralisation which has an association with Cu-Au. Hematite and zircon in-situ geochronology was attempted but for a variety of reasons, failed. Trace element analysis indicates early apatite grains are light rare earth element (LREE) enriched and this shifts to LREE-depleted, middle REE-enriched in late apatite growths. Evidence suggests LREE remobilised into florencite grains, possibly attributed to a shift from alkaline to acidic fluid conditions as documented at Olympic Dam. Hematite pseudomorphs and elemental maps of apatite support this shift. Comparison of trace element behaviour in apatite and geochronological constraints between Vulcan and Olympic Dam suggest the two mineralising systems share a similar history.