The mineral chemistry,near-infrared,and mid-infrared reflectance spectroscopy of phengite from the Olympic Dam IOCG deposit,South Australia |
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| Institution: | 1. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3;2. Deep Exploration Technologies CRC, School of Earth and Environmental Sciences, The University of Adelaide, Adelaide 5005, South Australia;3. Mineralogy and Petrography, University of Innsbruck, Innsbruck, Tirol 6020, Austria;4. Department of Manufacturing, Innovation, Trade, Resources and Energy, Adelaide, South Australia 5000, Australia;1. School of Chemical Engineering and Advanced Materials, The University of Adelaide, Adelaide, SA 5005, Australia;2. BHP Olympic Dam, Adelaide, SA 5001, Australia;3. School of Mathematical Sciences, The University of Adelaide, Adelaide, SA 5005, Australia;4. ARC Centre of Excellence in Ore Deposits, School of Natural Sciences, University of Tasmania, Hobart, TAS 7005, Australia |
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| Abstract: | Phengite is the main potassic dioctahedral mica identified at the Olympic Dam iron oxide–copper–gold (IOCG) deposit, South Australia, where its mineral chemistry is quite variable. These differences can be explained by contrasting degrees of hydrothermal alteration. In the heavily-sericitized, ore-bearing rocks, the phengites display a lower-Si content, a higher-Al content, and a lower Mg-number than the phengites from the weakly-sericitized alteration halo that surrounds the deposit. Variations are also observed in the near- and mid-infrared reflectance spectra collected from phengite-bearing rocks. In the near-infrared, high-Al phengite produces a spectral absorption feature at 2.206 μm, and this feature is displaced to 2.213 μm for low-Al phengite. In the mid-infrared, high-Al phengite produces a strong reflectance peak at 9.59 μm, whereas this peak is observed at 9.57 μm in the spectra from low-Al phengite. Additional peaks were also identified at 10.98, 12.22, and 13.33 μm. These were most intense in the spectra from high-Al phengite. A drill core profile was produced using the results of the spectral analysis that shows the change in phengite mineral chemistry and phengite abundance as a function of depth. In general, near- and mid-infrared reflectance spectroscopy can be used to characterize the aluminum content of potassic dioctahedral micas like phengite, and this information can be used to infer the degree of sericitic alteration that has occurred as a result of hydrothermal fluid flow. |
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