Evaluation of the links between Merlin-style Mo-Re mineralisation and magmatism in the Cloncurry Fold Belt, Queensland : implications for exploration

Merlin is the highest-grade Mo and Re deposit in the world, with 6.7 Mt at 1.33% Mo and 23.1 g/t Re. Merlin is located in the Selwyn Ranges, about 90 km south of the town of Cloncurry, in the Eastern Fold Belt of the Mount Isa Inlier. The main host rock to ore is the Kuridala Group, which is composed of interbedded black shales and phyllites. The Staveley Formation underlies the Kuridala Group and is composed of calc-silicate and phyllitic schists. The Mount Dore Granite (1517 ± 7 Ma) forms the eastern boundary and hanging wall to all mineralisation, being thrust over the metasedimentary rocks along an east-dipping reverse fault. The moderately east-dipping Mount Dore Fault Zone (MDFZ) forms a high strain western boundary and general footwall to mineralisation. The Mount Dore Cu deposit is spatially associated with Merlin. The mineralised corridor extends for over 1500 m, and comprises three main zones: the northern Merlin Mo-Re zone, the central and eastern hypogene Cu zone, and the southern Cu oxide zone. The Mount Dore Cu deposit lies in the hanging wall of the Merlin deposit. Unlike quartz vein-hosted and disseminated molybdenite occurrences in porphyry-style deposits, molybdenite at Merlin forms the cement to thick packages of heterolithic breccias. Merlin molybdenite occurs in two main forms: ‘dirty’, inclusion-rich anhedral grains (molybdenite 1), and coarser, subhedral, inclusion-free grains (molybdenite 2). The black shale at Merlin was a mechanically incompetent unit, and its natural weakness localised shear zones and breccias, where bonanza-grade molybdenite ore zones formed as breccia cement within the brecciated black shale unit. Molybdenite and carbonaceous matter are intimately intergrown in the ore zones at Merlin. Molybdenite 1 contains abundant carbonaceous matter that can be detected by Raman spectroscopy as sub-micron-scale inclusions. Carbonaceous matter (± graphite) and molybdenite 1 cannot be distinguished from each other using high spatial resolution microscopy. Only Raman spectral data provides evidence for the complete intergrowth of carbonaceous matter (± graphite) and molybdenite within the host rock (black shale ± phyllite) units. Rhenium concentrations in molybdenite at Merlin vary from 0.03 to 4800 ppm, and Re is closely associated with elevated V (~2 to 280 ppm) in the molybdenite structure. These trace elements do not correlate specifically with either molybdenite 1 or 2 — they are elevated in both textural types. The host rocks at Merlin are carbonaceous and there is an intimate intergrowth of carbonaceous matter with molybdenite 1 at Merlin. Previous Re-Os dating for Merlin provided a wide range of ages (1552 ± 6 to 1503 ± 5 Ma) that are potentially compromised by mixing of Re (and Os?) sourced from old carbonaceous matter in the host rocks and the younger granitic fluids. The preferred age of mineralisation at Merlin is the younger age (1503 ± 5 Ma), which is only slightly younger than the Mount Dore Granite (1517 ± 7 Ma). Oxygen isotope analyses of quartz related to Mo-Re mineralization yielded ∂18O\(_{qtz}\) values of 9.18–15.9‰ (stage 1), 9.82–15.14‰ (stage 2), and 9.54–17.19‰ (stage 3). These correspond to calculated ∂18OH2O values of 6.12–13.65‰ (stage 1; 450°C and 500°C), 0.87–8.25‰ (stage 2; 250°C and 300°C), and 4.24–13.13‰ (stage 3; 400°C and 350°C), consistent with a predominantly magmatic source of fluids, potentially with minor meteoric water input. The Mount Dore Granite is oxidised, highly fractionated I-type granite that has negative correlations between SiO\(_2\) and TiO\(_2\), P2O\(_5\) and MgO. Negative anomalies for Ba, Sr, Ti, and Nb indicate that feldspars and Fe-Ti oxides were fractionated from the parent magma. Radiogenic isotope analyses shows variability in εNd\(_{1517}\)Ma values (–4 and –11) indicating that the Mount Dore Granite was fractionated at the time of its emplacement. Modern εNd values range from –20 to –25, suggesting that both mafic and felsic components are present in the Mount Dore Granite. Merlin formed when magmatic-hydrothermal fluids, possibly with a minor meteoric water component, interacted with carbonaceous matter in the brecciated Kuridala Formation. There was one major stage of Mo-Re mineralization. The key factors for the formation of Merlin were: • A sedimentary host succession, marked by repeated C-rich black shale horizons; • Magmatic-hydrothermal fluid flow and alteration involving the Mount Dore Granite; and • Tectonic-hydrothermal brecciation that facilitated the movement of oxidised Mo-Re bearing fluids and intimate interaction with a reduced, carbonaceous chemical and structural trap