Polymetallic mineralisation in the Chillagoe district of north-east Queensland: insights into base metal rich intrusion-related gold systems

The Chillagoe district, located in northeast Queensland, is host to several polymetallic mineral deposits, which comprise three major metallogenic commodities: 1. Cu-Au, 2. Zn-Pb-Cu and 3. Sn-W-Mo. Metals such as W, Mo, Bi, Te, (± As, Sb) vary in concentrations within and between deposits. The deposits occur in a wide range of styles such as porphyries, breccias, skarns, and veins. Previous workers suggested that granites related to four igneous supersuites of the Permo-Carboniferous Kennedy Igneous Province were the source of the diverse metal commodities, whereas the calc-alkaline, metaluminous to weakly peraluminous, I-type, weakly oxidised to reduced, compositionally evolved nature of the oldest supersuite suggests that it alone could be the source for the polymetallic deposits. The mineralisation could be recognised as having affinities to intrusion-related gold deposits (IRG), adding to the increasing number of such systems recognised within the Tasmanides of eastern Australia. Although an IRG system could explain the polymetallic nature of the deposits in general, the economic grade of base metals hosted by deposits of the Chillagoe district is quite unusual for an IRG system. This thesis investigated three polymetallic deposits (Recap, Mungana and Red Dome) of the Chillagoe district by combining petrographical, electron-microprobe (EMP), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and radiogenic isotope studies (Re-Os, U-Pb and Lu-Hf) in order to improve the understanding of the metal diversity of IRGs, and provide insights into the processes of magma formation and evolution related to such systems. Petrographical studies showed that skarn formation at all three deposits is very similar and show no differences to the processes in other intrusion-related skarn systems. Mineralisation on the other hand differs regarding their major commodities, with Redcap and Mungana being base metal dominant, but also hosting minor gold mineralisation, whereas Red Dome is dominated by copper mineralisation and also comprises gold mineralisation and a very minor base metal commodity. The difference in the metal commodities and economic grade was caused by the emplacement of two compositionally different magmatic phases. The primary magmatic event (~ 326-320 Ma), characterised by weakly reduced and strongly fractionated rocks, provided the metallogenic diversity typical for an IRG system with some of the metals being incorporated into silicate phases (Sn, Zn) and As-Cu-Sb sulphide minerals (Au). The second magmatic event (more oxidised and less fractionated, ~312-305 Ma) caused remobilisation of metals related to the primary event, as well as adding S and possibly Zn, Cu, Pb to the system at Redcap and Mungana, whereas at Red Dome in addition to the sulphur, Cu and maybe Au was introduced. Studies of the mineral chemistry showed that composition of garnets and sphalerite have the potential to be used as exploration tool to locate causative intrusions. Due to the complexity of the systems investigated here, the concept should be tested again to establish its use in other multiphase IRG systems. Trace element concentrations and radiogenic isotope data (Lu-Hf) of zircons on the other hand showed that the diverse metallogeny of the IRG systems can be linked to the most fractionated phase in the magmatic system, with the magma originally being derived from a crustal metasedimentary source (in Chillagoe: ~2.0-2.5 Ga). The great advantage of determining such information from zircons rather than from whole rock data is that the latter can become easily affected by metamorphism, (hydrothermal) alteration and/or weathering, whereas physicochemically robust zircons preserve their primary information.