Graduate Thesis Or Dissertation
 

Compositional variations in hydrothermal white mica and chlorite from wall-rock alteration at the Ann-Mason porphyry copper deposit, Nevada

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/r494vn74j

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  • The detection of subtle variations in mineral chemistry in zoned hydrothermal alteration associated with the formation of porphyry copper deposits by short-wave infrared spectroscopy and rock chemistry are potentially valuable vectoring tools for mineral exploration. In order to correctly interpret the data collected by these methods, results must be calibrated by mineral data. Hydrothermal white mica, illite and chlorite grains were sampled from the Ann-Mason porphyry copper deposit in the Yerington district, Nevada, a Middle Jurassic porphyry copper system extended and tilted ~ 90° to the west. Mineral compositions vary spatially and record interactions with chemically distinct hydrothermal fluids over a vertical distance of ~5 km and a lateral distance of ~2 km from the ore center. Data suggest short wave infrared spectroscopy and bulk rock geochemical sampling can detect changes in mineral chemistry related to ore deposit formation but both methods have limitations. To relate short wave infrared spectroscopy to mineral compositions, spectra from rock samples were measured. Characteristic features commonly used to identify white mica, illite and chlorite were compared to chemical compositions of mineral grains from 34 samples were determined by electron microprobe analysis. Results demonstrate short wave infrared spectroscopy can be used to detect changes in the aluminum content of micas using the wavelength of the 2200 nm feature and may be used to map fluid pH gradients in rocks with muscovite or illite-bearing assemblages. The following compositional characteristics of white mica/illite were observed in the short wave infrared spectra: (1) an increase in the wavelength of the Al-OH absorption at ca. 2200 nm that is positively correlated with Fe+Mg+Mn (apfu) content and negatively correlated with total Al (apfu) corresponding to Tschermak substitution in both muscovite and illite, and (2) a decrease the wavelength of the ca. 2200 nm absorption to values below 2193 nm attributed to an increase in Na content (apfu) and the presence of paragonite intergrown with muscovite. For this sample set, illite cannot be distinguished from muscovite using short wave infrared spectroscopy. The proportion of Fe:Mg in the octahedral site in chlorite could not be identified in short wave infrared spectra of rocks using the wavelength of the 2350 nm feature and may have been obscured by coexisting highly reflective clays and micas. To compare trace metal gradients in rocks and minerals, trace metal concentrations of more than 600 altered rock samples, collected in a broad geochemical sampling campaign, were measured using inductively coupled plasma-mass spectrometry and inductively coupled plasma-atomic emission spectroscopy. Trace element contents of rocks were compared to trace metal contents of hydrothermal white mica, illite and chlorite determined by laser ablation-inductively coupled plasma-mass spectrometry from a set of 34 samples. Results show Cu, Mo, Te, Se, Bi, Sb, As, W, Sn, Li and Tl are enriched in rocks from the zone of potassic, sericitic and shallow-level advanced argillic alteration that represents the near-vertical pathway of the ore fluid from the mineralized zone (3.5 km depth) to the near the paleosurface (< 0.5 km). Of these elements, W, Sn and Tl enrichment in rock can be attributed, at least partially, to increased concentrations in muscovite and illite. Lithium enrichment can be attributed to increased concentrations in chlorite and more mafic, chlorite-rich wall-rock lithology at less than 1 km depth. Zinc, Mn, Co and Ni are depleted in altered rock above the ore zone and redistributed upward and laterally by both the magmatic hydrothermal fluid and by circulating sedimentary brines as verified by gradients in chlorite compositions from propylitic alteration. Copper is detected in chlorites but at concentrations (average 280 ppm) too low to contribute significantly to the observed Cu anomaly in rock (>1000 ppm). Chalcophile elements Mo, As, Te, Se, Bi are rarely detected in white mica/illite or chlorite in concentrations greater than 1 ppm and, in more than 50% of analyses, levels are below detection.
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file details Cohen_MSc Thesis_Final_091611.pdf 08/08/2017 Public
file details Appendix D_EMPA.pdf 08/08/2017 Public
file details Appendix E_SWIR table.pdf 08/08/2017 Public
file details Appendix F_LA-ICP-MS.pdf 08/08/2017 Public
file details Appendix G_ICP-MS-AES.pdf 08/08/2017 Public