Geological characteristics and origin of the Watershed W deposit, North Queensland

Tungsten is considered a strategic metal by various countries, including Australia. Between 1998 and 2016 Australia has been steadily increasing its tungsten production, but it is still far smaller than those of the main producers (e.g., China, Russia). Watershed with its current resources of 49.2 Mt averaging 0.14% WO₃ is considered one of the biggest undeveloped tungsten deposits outside of China, and if developed would boost Australia's tungsten production. The main goal of this PhD thesis is to improve our understanding of the Watershed tungsten deposit and how to explore for similar deposits in northeast Queensland. This goal was approached by documenting the geological, geochemical and structural characteristics of the Watershed deposit, as well as the timing, mineral paragenesis and fluid characteristics of the mineralizing system. The Watershed tungsten deposit lies within the Mossman Orogen, which comprises multiple deformed Silurian-Ordovician metasedimentary rocks of the Hodgkinson Formation intruded by Carboniferous-Permian granites of the Kennedy Igneous Association. The Hodgkinson Formation in the Watershed area comprises skarn-altered conglomerate, psammite and slate units, which record at least four deformation events ranging from early ductile folding and shearing events (D₁ to D₃) to the development of later brittle-ductile shear zones (D₄) associated with veining and four separate stages of retrograde metamorphism/alteration (Retrograde Stages 1 to 4). Peak metamorphic assemblages (garnet, actinolite, quartz, clinopyroxene, titanate) in the host rocks to mineralisation formed during D₁-₂. Multiple felsic dykes intrude the metasedimentary rocks at Watershed and include: (a) Carboniferous, monzonite dykes (zircon U/Pb age of 350 ± 7 Ma) emplaced during D₁₋₂; and (b) Permian granite plutons and dykes (zircon U/Pb ages of 291 ± 6 Ma, 277 ± 6 Ma and 274 ± 6 Ma), and diorite (zircon U/Pb age of 281 ± 5 Ma) emplaced during D₄. An early (syn-D₁₋₂) mineralization event involved the syn-tectonic growth of disseminated scheelite in monzonite dykes and adjacent skarn-altered conglomerate, and was associated with the emplacement of the monzonite, which appears to have enriched the Hodgkinson Formation in W-Be- B-Sc-Cu-Mo-Re. The bulk of the economic scheelite mineralization formed in syn-D₄ shear-related, quartz-oligoclase veins and associated vein haloes (with a muscovite Ar-Ar age of 276 ± 6 Ma). The veins developed preferentially in skarn-altered conglomerate and terminate abruptly when they encounter slate. Vein opening occurred synchronous with four retrograde alteration stages in skarn-altered conglomerate. The margins of the D₄ veins contain feldspar, scheelite and quartz, which represents Retrograde Stages 1 and 2. During Retrograde Stage 1 early sanidine (overgrown by plagioclase, An₁₅₋₅₅) formed with minor quartz. Retrograde Stage 2 is characterised by intergrown scheelite and plagioclase (An₃₋₄₃) overgrowing early plagioclase, phlogopite and trace apatite. Further vein opening during Retrograde Stage 3 infilled the central part of the vein with quartz, which is cross cut by muscovite, calcite and minor chlorite, tourmaline and fluorite. Fractures that formed during Retrograde Stage 4 cut textures belonging to the previous stages and contain pyrrhotite, arsenopyrite with lesser pyrite, chalcopyrite, and sphalerite. Scheelite can incorporate small amounts of REE, and the origin of the scheelite grains (i.e. intrusion-related vs metamorphic) has been investigated using the relative abundance of contained LREE, MREE and HREE. Using ternary REE plots, early D₁₋₂ scheelite in monzonite coincides with the compositional field for scheelite that forms during magmatic-hydrothermal processes, whereas late D₄ vein-hosted scheelite is compositionally similar to pure hydrothermal scheelite. The Eu and Mo contents of scheelite, coupled with graphite inclusions in scheelite and the presence of pyrrhotite and arsenopyrite in scheelite-bearing veins, show that D₁₋₂ scheelite precipitated from a relatively oxidized fluid, while vein-hosted D₄ scheelite record a shift to more reduced conditions as a result of fluid interaction with carbonaceous shale. Whole rock geochemistry of the various rock types within the deposit indicates that the Watershed deposit is characterized by an enrichment of W-Be-B-Sc-Cu-Mo-Re. These elements were probably introduced by hydrothermal fluids during D₄ veining. The fluid interacted with the skarn-altered conglomerate to leach REE, Y and Nb plus skarn-related elements (i.e., Ca-F-P-Fe-Sr), and add Rb, Cs and Li in vein haloes. Whole rock geochemistry of psammite units along a 2 km transect north of the deposit shows a regional footprint that is characterised by enrichment in W-Cu-Mo-Ca-Fe-Mn-Li. Fluid inclusions in D₄ vein scheelite and quartz from Retrograde Stage 2 preserve a low salinity H₂O-NaCl-CH₄ fluid (XCH₄ < 0.01). The fluid inclusions show evidence for fluid-fluid mixing between low- (close to 0 wt.% NaCl) and medium-salinity (< 8 wt.% NaCl) fluids. The P-T conditions during mineralisation were determined at ca. 300°C and 1-1.5 kbar (i.e. depths of 3.5-6 km) indicating a high geothermal gradient, linked to the emplacement of Permian granites. Those P-T conditions are similar to those recorded in lode-gold deposits in the Hodgkinson Gold Field and elsewhere. The oxygen fugacity was calculated at 0.6 to 0.8 log₁₀ values below the FMQ buffer, consistent with the reduced mineralogy and geochemical signatures. δ¹⁸OVSMOW values obtained for scheelite (+3.4 to +7.3‰), plagioclase (+7.0 to +11.8‰) and quartz (+12.6 to +15.5‰), which formed during Retrograde Stage 2, and δDVSMOW (−73.4 to −62.7‰) and δ¹⁸OVSMOW (+11.5 to +13.2‰) values for muscovite that formed during Retrograde Stage 3 are indicative of a metamorphic origin for the mineralising fluids, with a possible magmatic component. Sulphur isotope (δ³⁴SCDT) values for sulphides formed during Retrograde Stage 4 in veins are consistent with the presence of seawater sulphate (i.e. basinal brine) in the system. Metamorphic fluids probably originated from prograde devolatilization reactions during metamorphism of the Hodgkinson Formation. Our findings indicate that tungsten was sourced from Carboniferous monzonite, which enriched the metasedimentary rock units of the Hodgkinson Formation during the early stages of deformation/ metamorphism. Continued ductile deformation and associated metamorphism during D₁₋₃ caused devolatilization reactions in the host rocks and remobilisation of tungsten. Permian scheelite mineralisation during D₄ involved a metamorphic-hydrothermal fluid with minor magmatic input that deposited tungsten at 300°C and 1-1.5 kbar (<6 km depth). This tungsten was transported as NaWO₄⁻, HWO₄⁻ and WO₄²⁻ complexes along extensional shear zones. Calcium was supplied by the skarn-altered conglomerate that hosts the scheelite-bearing veins. It is proposed that the precipitation of scheelite was promoted by the interaction between the relatively acidic hydrothermal fluids and the alkaline, carbonate-rich, skarn-altered conglomerate host rock, lowering the solubility of the tungsten complexes and co-precipitating scheelite and Na-rich plagioclase during Retrograde Stage 2. Considering a continuum model for this deposit type (i.e. mineralization could form between 2-20 km depth) it is feasible to consider the potential for mineralization at depth

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