Petrological, geochemical, and geochronological characteristics of hydrothermal minerals from the Mount Garnet deposit, northeast Queensland, Australia

Abstract

<p>We present multi-disciplinary datasets reporting petrography, major and trace element geochemistry, and U-Pb geochronology for the Mount Garnet Cu-Pb-Zn skarn deposit, NE Queensland, Australia. The deposit is hosted in limestones of the upper Silurian Chillagoe Formation, within the Hodgkinson Province, Mossman Orogen. Its mineralisation has been interpreted to be related to the intrusion of the Kennedy Igneous Association (~250-345 Ma), however, the exact timing is still not determined.</p><p>Petrographic observations and <em>in suit</em> major and trace element analysis using EPMA and LA-ICP-MS on skarn garnets reveal two generations of garnet formation. Garnets from Gt-I generation are anhedral and massive, dark brown to red in colour. They are mostly Al-rich grossular (Adr<sub>6-22</sub>Grs<sub>61-88</sub>) and show no zoning patterns. Garnets from Gt-II generation are euhedral with a yellow-green colour and porous textures. They are Fe-rich andradite (Adr<sub>10-99</sub>Grs<sub>16-77</sub>) and display oscillatory zoning. Gt-I grossulars have an enrichment in LREEs and depletion in HREEs with negative Eu anomalies, while Gt-II andradites have the opposite trend and prominent positive Eu anomalies. Both W and Sn are present in Fe-rich garnet (>10 ppm). <sup>206</sup>Pb/<sup>238</sup>U ages of two types of garnets are ranging from ~220 Ma to 380 Ma, consistent with the zircon U-Pb age range (~295-335 Ma) from ore-related intrusions.</p><p>Our data allows the exploration of relationships between magmatism, tectonic activities, and the chronological sequence of mineralisation-related processes. A general order of events would include the very early silicification occurring within the host rock and accompanied by potential faulting, followed by the prograde and retrograde metamorphic process, which is represented by garnet, clinopyroxene, considerable vesuvianite, and calcite, along with minor wollastonite at the skarn front. Compositional variations (e.g., Mn concentration) of zoned Gt-II constrain the P-T-X condition of fluids and high Eu/Eu* and Ce/Ce* ratios within both garnets indicate a relatively oxidised skarn system. Negative correlations between Ca and REEs suggest that the incorporation and fractionation of REEs in garnet are collectively controlled by crystal chemistry and the presence of hydrothermal fluids. Further results of garnet geochronology would provide additional constraints on the nature of magmatic sources in the region.</p>