Abstract
The oxygen isotope and trace element composition of hydrothermal quartz has been integrated with scanning electron microscope-cathodoluminescence (SEM-CL) images and fluid inclusion properties to track fluid sources and hydrothermal processes in the Mt. Leyshon Au deposit, Australia. Oxygen isotope and trace element data were collected on parallel traverses across the same quartz sections, using secondary isotope mass spectrometry (SIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), respectively, with SEM-CL images obtained before analysis for petrographic context. Over the 280 to 650 °C quartz precipitation range suggested by fluid inclusion microthermometry, δ 18O quartz varies from 0.0 to 14.4‰, corresponding to a wide range of equilibrium δ 18O fluid values, from-6.1 to 10.2‰ (vs. V-SMOW). The δ 18O quartz signature varies systematically among distinct SEM-CL quartz generations (both within and between samples), and can be correlated with variations in temperature and fluid composition, but is independent of intra-generational oscillatory zoning. In contrast, Al and Li concentrations correlate broadly with CL intensity in oscillatory quartz, whereas their concentration is unpredictable in sealed fractures and overgrowths. Concentrations of B, Mg, Na, P, Cl, K, Ti, Mn, Fe, Ge, and Sn are independent of Al, Li, and oscillatory CL features, but Ti correlates with quartz precipitation temperature. Although no systematic correlation between δ 18O quartz and trace element concentrations was found, complementary patterns exist in narrow overgrowths of low δ 18O quartz (∼ 0‰) and high Al (> 10,000 ppma). These quartz zones likely formed during the incursion of 18O-depleted meteoric water into the magmatically-dominated Mt. Leyshon hydrothermal system. We interpret the highest Al concentrations as the result of high quartz precipitation rates, triggered by depressurisation of the hydrothermal cell. The decoupling of oxygen isotope and trace element patterns in quartz leads to the suggestion that (1) under most circumstances, temperature and fluid chemistry dominate δ 18O quartz, and (2) the trace element record, and in particular Al and Li, is influenced by the superimposed effects of quartz precipitation rate.
Published Version
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