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Genesis of the Wolfram Camp tungsten-molybdenum deposit, Queensland, Australia, and the geochemistry of tungsten ore minerals, The
Liu, Kairan
Liu, Kairan
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2021
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2022-09-10
Abstract
The study presented in this thesis is an economic geology research focusing on tungsten, including a comprehensive geological characterization and genesis study of the Wolfram Camp tungsten-molybdenum (W-Mo) deposit in Queensland, Australia, and mineral trace-element geochemical studies of tungsten ore minerals including scheelite and wolframite.The Wolfram Camp deposit is a pegmatite-greisen type W-Mo deposit. The host rock is the Carboniferous James Creek granite, which intruded the Devonian Hodgkinson Formation sandstone and subordinate Carboniferous Beapeo Rhyolite. The orebodies occur as pipe-like bodies or pockets of quartz ± K-feldspar in the roof zone of the intrusion, with progressively weaker greisen alteration in the granite away from the pipes and pockets. All the alteration and mineralization are confined in the intrusion, which suggests that the magma chamber was not broken during the main stage of mineralization, and the magmatic hydrothermal fluids did not leave the magma chamber. Oxygen and hydrogen isotope data indicate the ore-forming fluids have a magmatic source and no meteoric waters were involved at the time of main mineralization. Sulfur isotope composition is also compatible with a magmatic source. Geochronology data, including zircon U-Pb ages of the granite, Ar-Ar dates of muscovite from greisens, and molybdenite Re-Os ages, suggest that the greisen alteration and mineralization occurred about 4 m.y. after zircon crystallization in the James Creek granite. Fluid-inclusion micro-thermometric measurements and granite solidus modeling suggest the Stage 1 unidirectional solidification texture (UST) quartz formed at minimum 660°C and 3,300 bars (12 km depth). Stage 2-1 syn-mineralization quartz is constrained to have formed at temperatures between 425 and 660°C, and a maximum pressure of 1,510 bar, with a large depressurization (ΔP>1,700 bar) from Stage 1. Stage 2-2 post-mineralization quartz formed between 225 and 320°C. It is concluded that the hydrothermal system of Wolfram Camp has a long lifetime, synchronous with the cooling of the large granite pluton, and the deposit was formed during a period of rapid uplift and exhumation (~1.6 mm/year), which caused the significant depressurization.
Scheelite occurs in both magmatic hydrothermal and metamorphic hydrothermal systems. In this study, I propose a method to use scheelite trace-element compositions to distinguish the two major types of deposits. By analyzing scheelite from six magmatic hydrothermal deposits and eight metamorphic hydrothermal deposits and collecting data from literature, a database including 85 deposits and 2,946 Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS) analyses has been established. With this database, it is found that generally, magmatic hydrothermal scheelite has higher concentrations of Mo (10-10,000 ppm), Nb (1-2,000 ppm) and Ta (0.02-1,000 ppm), less Sr (<1,000 ppm) and has lower calculated Eu anomaly values (EuA=(Eu/Eu*)N=EuN/(SmN*GdN)1/2, 0.1-10). In contrast, metamorphic hydrothermal scheelite has lower concentrations of Mo (<100 ppm), Nb (<30 ppm) and Ta (<5 ppm), more Sr (200-10,000 ppm), and its REE contents exhibit higher Eu anomalies (EuA 1-100). Based on these differences, a discriminant diagram is designed to distinguish the fluid source: Mo*(Nb+Ta) versus Sr*EuA. The diagram has a discriminating effectiveness of ~90%.
Trace elements of wolframite from 13 deposits were analyzed by LA-ICP-MS in this study, with 649 analyses in total. The 13 deposits include six proximal magmatic deposits (Type 1), two distal magmatic hydrothermal deposits (Type 2) and five non-magmatic hydrothermal deposits (Type 3). Wolframite in Type 1 deposits contains more Nb (75-15,304 ppm) and Ta (0.6-2,147 ppm) and has lower Ti/Zr ratios (0.9-12) than Type 2 wolframite (Nb, 9-4,202 ppm; Ta, 2.0-145 ppm; Ti/Zr, 1-115). In addition, wolframite in Type 1 deposits has much stronger negative Eu anomaly (lower EuA values) than in Type 2 deposits. These parameters may indicate proximal vs. distal positions of a deposit relative to the causative intrusions. Compared with the magmatic hydrothermal deposits (Types 1 and 2), non-magmatic hydrothermal deposits have wolframite containing lower Nb (<98 ppm) and Ta (<4 ppm), and higher V (4-1,241 ppm) and Ti/Zr (1-4,015). Plots of EuA vs. V and EuA vs. (Nb+Ta) are designed to distinguish proximal magmatic, distal magmatic, and non-magmatic hydrothermal deposits, whereas Ti/Zr vs. V and Ti/Zr vs. (Nb+Ta) plots can distinguish magmatic and non-magmatic hydrothermal fluids.
Two major mechanisms causing intra-crystal geochemical variations in wolframite of Wolfram Camp have been revealed in this study. The wolframite adjacent to the later-formed scheelite is relatively depleted in Co, Gd, Hf, Lu, Mg, Fe, Mn, Mo, Nb, Sc, Rb, Ta, Y and Zr, enriched in In and has lower Fe/Mn ratio and relatively higher Eu anomaly values compared with wolframite non-adjacent to scheelite. These geochemical patterns are the results of the interaction with later hydrothermal fluids. The scheelite has been altered without mineralogical change. Some trace elements in wolframite also change along with concentric zoning and sector zoning. These elements are Sc, In, V, Rb, Sr, Y, Zr, Nb, Hf, Ta, Th, U and REEs. In patches where the concentric zones and sector zones overlap, these trace elements achieve the highest or the lowest values.
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