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Metallogeny of the Cowal district, New South Wales, Australia

Leslie, CD ORCID: 0000-0002-2867-1020 2021 , 'Metallogeny of the Cowal district, New South Wales, Australia', PhD thesis, University of Tasmania.

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Abstract

The Cowal district, central New South Wales, Australia, is underlain by the 40 by 15 km north-south trending, fault-bound, Cambrian to Late Ordovician Cowal Igneous Complex. The Cowal Igneous Complex forms part of the Junee-Narromine volcanic belt, the westernmost volcanic belt of the now dismembered Early Paleozoic Macquarie Arc. The Cowal Igneous Complex is host to three principal alkalic low-sulfidation epithermal Au deposits (E41, E42, and GRE46) and numerous calc-alkalic porphyry Cu ± Mo ± Au prospects (Marsden, E39, E43, Caloola, Corran and Milly Milly). The epithermal deposits form a 4.5 km north-south trend on the western margin of the Cowal Igneous Complex termed the gold corridor, and contain a combined mineral resource of 9.0 million ounces of gold (265 million tonnes at 1.06 g/t Au). The mineral resources for the porphyry prospects are less well-defined -only the Marsden prospect has a defined mineral resource (123 million tonnes at 0.46 % copper and0.27 g/t gold).This thesis comprises three research themes focused on the Cowal district: metallogeny, fertility and hydrothermal processes. The first part of the study presents new zircon U-Pb and molybdenite Re-Os geochronological data together with zircon Hf isotopic data and whole rock geochemical data to provide insights into the tectonomagmatic and metallogenic evolution of the Cowal Igneous Complex. The Cowal Igneous Complex was built on Cambrian intra-oceanic arc crust as evidenced by widespread ca. 506 to 480 Ma inherited zircons with high +ԐHf values (+10.3 to +13.1) and a 496.6 ± 1.8 Ma monzodiorite intrusion at Marsden with high +ԐHf zircon values (+12.3 to +14.5) and arc-like geochemistry (low-Nb, low-Ti, high-Th/Yb). The volcanic rocks of the Cowal Igneous Complex are co-magmatic with, and also intruded by, six unique, intra-oceanic island arc-related (zircon ԐHf values average +11.0) igneous suites (East, Gold Corridor, Central, West, North and Dewars) that evolved through varying degrees of crystal fractionation. The fractionating mineral assemblages determined from whole rock geochemical trends are dominated by early high-level plagioclase fractionation, transitioning to late hornblende fractionation. These trends, from low-Sr/Y (< 20) to high-Sr/Y (> 20), occur twice during the Ordovician. The Middle Ordovician trend comprises intrusions of the East and West magmatic suites with ages ranging from 496.6 ± 1.8 Ma to 455.5 ± 1.7 Ma and is associated with ca. 467 Ma to ca. 455 Ma porphyry-related mineralisation. The Late Ordovician trend comprises intrusions of the Gold Corridor and Central suites with magmatic ages ranging from 461.5 ± 5.1 Ma to 451.2 ± 2.6 Ma corresponding to a trend from low-Sr/Y to high-Sr/Y and is associated with ca. 455 to ca. 445 Ma alkalic epithermal and calc-alkalic porphyry mineralisation.
The temporal relationships between the two fractionation trends are interpreted to reflect an arc-scale shift from compression to extension due to a change from west- to east-dipping subduction between 460 Ma to 455 Ma. Subduction reversal is interpreted to have caused tearing of the west-dipping slab and the emplacement of enigmatic ca. 460 Ma Dewars suite; a fractionated equivalent of a Nb-enriched basalt derived from a hybridised old and fertile asthenospheric and sub-arc lower-crustal source.
In the second part of the study, zircon trace element geochemical compositions obtained from laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) are used to further investigate the crystal fractionation histories of pre- and syn-mineralisation porphyry Cu-related magmas. LA ICP-MS zircon spot data suggest that syn-mineralisation magmas were more oxidised (average zircon ΔFMQ of +1.1) and crystalized at lower temperatures (average TTiZrc of 720oC) compared to pre-mineralisation intrusions (average zircon ΔFMQ of -0.7 and TTiZrc of 795oC). Syn-mineralisation intrusions yield zircons with higher Eu/Eu*(>0.35), higher (Ce/Nd)/Yb (>0.01), and lower Gd/Yb (<0.07) that suggests syn-mineralisation magmas were also more hydrous (e.g., characterised by hornblende fractionation with suppression of plagioclase crystallization). Based on Rayleigh crystal fractionation modelling of zircon compositions, a diagnostic fractionation trend towards lower Gd/Yb and higher Eu/Eu*, suggests that late titanite and/or apatite co-crystallized with zircons in the late syn-mineralisation magmas.
The zonation of trace elements in zircons from syn-mineralisation intrusions spatially correlates with abrupt changes in zircon textures demarked by dissolution surfaces. LA ICP-MS imaging of key fractionation, hygrometer and oxybarometer indices (e.g., Th/U, Gd/Yb, (Ce/Nd)/Yb, and Eu/Eu*) show that multiple crystal-fractionation events are recorded in single zircon grains. Coincident abrupt changes in Hf/Zr across these resorbed boundaries are interpreted to record periods of discrete magma recharge events that affected the temperature and trace element budget of the magma. In many cases, the highest relative magmatic water content and highest oxidation signature is concentrated near the crystal rims which suggests magmatic fertility increased late during the evolution of these magmas. An evolution to cooler, hydrous (core to rim decrease in Gd/Yb and an increase in Eu/Eu*), and oxidized (core to rim increase in (Ce/Nd)/Yb) conditions with transient periods of magma-recharge and higher-temperature conditions (core to rim increase in Hf/Zr with periods of reversal and zircon dissolution), are considered prerequisites for fertile magma petrogenesis in the Cowal Igneous Complex. In the final part of the study, pyrite trace element compositions obtained from LA ICP-MS spot and imaging methods are used together with NaOCl etching to investigate the physiochemical evolution of epithermal- and porphyry-related hydrothermal fluids and to explore the utility of pyrite geochemical data for discriminating porphyry versus epithermal pyrite in complex metallogenic settings. Because the solubilities of Co, Ni and As vary as a function of temperature, ratios of these elements are shown to discriminate high-temperature porphyry-related pyrite from low-temperature epithermal-related pyrite. Epithermal pyrites from the E42 and GRE46 deposits have low-Co/As and high-Co/Ni values, whereas porphyry pyrites from the E39, E43, Marsden and Central deposits have high-Co/As and low-Co/Ni values. These characteristics agree with compiled pyrite data from other porphyry and epithermal/geothermal systems worldwide.
Epithermal-related pyrite from E42 have Au and trace element enriched and texturally complex cores. Based on a principal components analyses (PCA) followed by K-means clustering of PC loadings on LA ICP-MS image pixel data, the cores contain two end-member pyrite compositions; one high in Au-As but low in Cu-Pb-Sb-Zn-Ag relative to the other. Dissolution surfaces and changes in pyrite zoning textures mark boundaries between the two end-member compositions. These cores are overgrown by homogeneous trace element (Au, As, Cu, Pb, Sb, Zn, and Ag) depleted but Co-enriched rims. These changes in textures and chemistry are interpreted to represent early Au-enriched pyrite that formed during boiling and fluid-mixing events followed by pyrite crystallization during water-rock interactions and co-precipitation of base metal sulfides.
Chapters 2 and 3 address the petrogenesis of a diverse suite of magmas in the Cowal Igneous Complex by providing constraints on their mantle source and mid- to upper crustal crystal fractionation processes. These high-level processes have implications for district metallogeny and magma fertility and also the physiochemical evolution of related magmatic-hydrothermal fluids, addressed in chapter 4. These new data can assist with mineral exploration programs in complex porphyry to epithermalsettings by providing a better understanding of the magmatic and hydrothermal processes linked tomineralisation.

Item Type: Thesis - PhD
Authors/Creators:Leslie, CD
Keywords: Macquarie Arc, metallogeny, epithermal, porphyry, igneous petrogenesis, igneous fertility, hydrothermal fluids
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