Textures and U-W-Sn-Mo signatures in hematite from the Olympic Dam Cu-U-Au-Ag deposit, South Australia: Defining the archetype for IOCG deposits

摘要

Graphical abstractDisplay OmittedHighlights?First in-depth textural and geochemical study of hematite from the archetype IOCG system at Olympic Dam.?Hematite containing W, U, Sn and Mo is abundant throughout the Olympic Dam deposit.?Chemical and petrographic variations tied to overprinting processes during the main hydrothermal stages.?Hematite overprinting mechanisms are proposed based on textural and compositional characteristics.AbstractThe textural characteristics and trace element geochemistry of hematite with U-W-Sn-Mo signatures from the Cu-U-Au-Ag orebody at Olympic Dam, South Australia, are documented. Olympic Dam is the archetype for iron-oxide copper–gold (IOCG) deposits where hematite is by far the most abundant mineral in the orebody. The deposit is located within hematite-bearing breccias (>5% Fe) hosted by the ～1.6?Ga Roxby Downs Granite (RDG). Although such breccias are mostly derived from RDG, they also include volcanic clasts and sedimentary rocks. Samples cover the ～6?km strike length and ～2?km vertical extent of mineralisation, including hematite from the aforementioned lithologies. Hematite with U-W-Sn-Mo (‘granitophile’ elements) signatures is recognised throughout all lithologies and parts of the deposit. Hematite enriched in granitophile elements is represented by a variety of textures, of which zoned hematite, defined by oscillatory zonation patterns, is the most prominent and can be tied to the age of the RDG, and thus initiation of the IOCG system as confirmed by published U-Pb geochronology. Other categories of hematite with granitophile signatures include hematite resulting from replacement of pre-existing minerals (e.g., carbonates and feldspars), as well as replacement of previous oscillatory-zoned hematites. Matrix and vacuole filling hematite from volcanoclastic-dominated intervals also carry ‘granitophile’ signatures. In addition, some colloform types which likely post-date primary IOCG mineralisation are also rich in ‘granitophile’ elements. Trace element mapping and spot analysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) defines complex trace element signatures of hematite, which, in addition to the ‘granitophile’ elements, also comprise rare earth elements, high field strength elements, chalcogens and transition metals.The distinct geochemical signature, characterised by enrichment in the ‘granitophile’ elements (up to wt% levels of U and W within individual zones, and up to thousands of ppm Mo and Sn) prevails throughout the hematite in the dep