Process Constraints on the Giant IOCG Mineral System of the Eastern Gawler Craton, Australia

Abstract

The aim of this thesis is to map the distribution of alteration minerals and pathfinder elements from deposit to regional scale within the IOCG mi neral system of the eastern Gawler Craton , South Australia, and understand fluid rock interactions that control th at distribution. I present geochemistry petrology and mineral chemistry from a range of metasomatically altered rocks including four protolit h types; siliciclastic metasedimentary rock, calc silicate metasedimentary rock, g ranite and mafic magmatic rock. Techniques employed included; transmitted light microscopy, electron microprobe analyses ( EM), scanning electron microscopy (SEM) and laser ab lation inductively coupled plasma mass spectroscopy (LA ICPM). Thermodynamic modelling using the HCh software was then applied in order to further understand the temperature, pressure, fO2 and fluid-rock ratio conditions responsible for a range of alteration types Multiple, overprinting paragenetic relationships and unusual alteration textures in four contrasting protolith rocks from the central eastern Gawler Craton can be interpreted within the framework of five paragenetic stages (protolith; skarn; stage 1, K feldspar magnetite; stage 2, hematite chlorite muscovite, including the major Cu ore minerals; stage 3, post mineralization) corresponding to different mineral assemblages in the four protolith types. The paragenesis is consistent with successive per iods of Fe K metasomatism, with early higher temperature, more reduced (magnetite stable) alteration being consistently overprinted by lower temperature, more oxidized (hematite stable) alteration and with the bulk of Cu sulphide mineralization occurring a t close to the transition from magnetite to hematite. Minerals that pre-date the main sulphide phase (namely from the protolith, skarn and stage 1 assemblages) typically have major and trace element concentrations within expected ranges for comparable rock types outside the eastern Gawler Craton mineral province. Hematite associated with stage 2 assemblages has higher average concentrations of Ba, Cu, Mo, Nb, Pb, Th, Ta, U and SigmaREE compared to magnetite (between 1 and 2 orders of magnitude higher). In addition hydrothermal hematite contains elevated concentrations of Cu, U, Sb and Bi compared to the average crustal abundance. Hematite is the main host of Sb even when there are co-existing sulphide phases in the rock. Where sulphide minerals are present most chalcophile pathfinder elements (e.g. Ag, As, Bi, Cu, and Se) are dominantly deported in the sulphides, even at low concentrations, far from mineralisation. Pyrite is the most common sulphide, with chalcopyrite increasing in abundance closer to mineralisation. The pyrites are p-type, with S/Featom ratios of > 2 and Co/Ni ratios ranging between 0.4 and 10, but mostly above 1. This is consistent with a moderate-temperature hydrothermal origin for the pyrite. Concentrations of Co, As, Bi, Se, Te and Au in pyrite reach 2 to 3 orders of magnitude above crustal abundance. The chalcopyrite grains show variable enrichment in pathfinder elements and are most enriched in Bi, Se, Te and Ag, with values ranging between 1 and 4 orders of magnitude above crustal abundance. At elevated whole rock concentrations, within altered rocks, the REE are deported in hydrothermal apatite. This is consistent with the extreme capacity of the hydrothermal system to mobilise, and locally accumulate, even the most refractory elements. REE enrichment (up to 2604 ppm) is a good proximity indicator to ore, since it only occurs around the mineral system. Thermodynamic modelling was conducted using the HCh software to calculate equilibrium mineral assemblages predicted for model granite and calc-silicate protoliths mixed with a range of model hydrothermal fluids. Fluid compositions were consistent with fluid inclusion studies from the eastern Gawler Craton. Models were created in the C-H-O-Cl-S-Na-K-Mg-Fe-Si-Al-Cu-Ca-Mn chemical system at temperatures from 150(0) to 500(0)C, fO2 of -34 to -26 and fluid-rock ratios from 10-3 to 104. The modelling provides semi-quantitative constraints for mineral assemblages associated with IOCG mineral systems of the eastern Gawler Craton. The transition from protolith assemblages to magnetite-K-feldspar assemblages to chlorite-magnetite-K-feldspar assemblages to hematite-chlorite-muscovite assemblages with decreasing temperature, increasing fluid-rock ratio and increasing logfO2. This is consistent with the petrologic observations presented in this thesis, namely the consistent overprinting of stage 1, magnetite-K-feldspar alteration by stage 2, hematite-chlorite and muscovite alteration. Although Cu-sulphides are predicted to be stable over a range of temperature and fO2 conditions, they are predicted to be most abundant between temperatures of ~300(0) to 250(0)C, fO2 of -26 to -34 and at fluid-rock ratios >10. This corresponds to alteration assemblages at the magnetite and hematite boundary, with abundant chlorite and muscovite alteration. The distribution of pathfinder elements (measured in whole rock chemistry) within the central eastern Gawler Craton IOCG province can be predicted by combining petrological observations with mineral chemistry and thermodynamic modelling and are consistent with the observations of Fabris (2012, 2013). Elements associated with hematite alteration (notably Sb and W) are expected to have a wide geographic footprint. Elements enriched in pyrite (notably As, but also S and Se) are likely to have a broad geographic distribution in line with the presence of pyrite in both magnetite and hematite stable alteration assemblages. Elements most enriched in copper sulphides (Cu, Ag and Au) are largely restricted to within <5 km of known deposits.