Summary and final report on pyrite, magnetite and hematite mineral geochemistry, South Australia

Sulfide and oxide trace element geochemical analyses have been conducted on more than 290 drill core samples from 39 drillholes across South Australia, as part of a collaborative project between Centre for Ore Deposit and Exploration Science (CODES), University of Tasmania, and the Geological Survey of South Australia. The project aim was to analyse the mineral chemistry of pyrite, hematite and magnetite across a range of deposit styles in SA, and determine whether the technique could be used to provide insights into, and vector within, the various mineral systems. Pyrite analyses showed discernible trends for individual mineralisation types. For example, pyrite which forms in an IOCG-type environment typically contains Co>>Ni, with varying amounts of As, Te, Ag, and Au. Cobalt, Ni, and As are exclusively lattice-bound, whereas Te, Ag, and Au are commonly hosted as inclusions of Au-Ag telluride phases or electrum (AuAg alloy). Hematite and magnetite analyses from different magmatic-hydrothermal systems have revealed that these elements preferentially concentrate a large range of lithophile elements, including the REE’s, with rare to minor incorporation of select chalcophile and siderophile elements (e.g. Co, Ni, As, Zn, Pb, Bi, and Ag). Pyrite and hematite studies on IOCG-style magmatic-hydrothermal mineralisation have been particularly encouraging in the Intercept Hill area, ~90 km south of Olympic Dam. This area, which is very close to Emmie Bluff, contains IOCG-style hematite-magnetite-matrix granite-metasedimentary breccia bodies, together with pyrite-chalcopyrite mineralisation. Pyrite in these zones at Intercept Hill contain high levels (i.e. >>1 ppm) of Au dissolved in the pyrite structure, a feature which is not typical of IOCG-style sulfide mineralisation. Furthermore, the Au content of pyrite varies between drillholes, with the highest amount located near the presumed centre of mineralisation. Pyrite geochemistry thus appears to be a useful vector toward high-grade Au mineralisation at Intercept Hill. Hematite geochemistry also changes with location at Intercept Hill, though the reasons for this variation are not well-constrained. For instance, hematite mineralisation in drillhole IHAD-3 is characterised by fine-grained, colloform hematite crystals in a silicified metasedimentary host rock (possibly a carbonate horizon), with only trace associated sulfides. This style of hematite mineralisation is highly enriched in Mn (average 2 wt %), and is also intergrown with manganite (Mn 3 O 4 ) which is carrying 1 wt % Cu in solid solution. In contrast, hematite from IHAD-2 and IHAD-5 is coarse-grained (i.e. specularite) and is typically associated with locally abundant pyrite and chalcopyrite. This type of hematite is much lower in Mn (~1000 ppm) and is intergrown with and replaces magnetite, not manganite. The results of this study underscore the value of using mineral trace element geochemistry to elucidate mineral paragenesis and provide proximity indices in magmatic-hydrothermal environments across South Australia and elsewhere.