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From proximal to remote sensing: analysing the pyrophyllite-muscovite association in the Buckskin Range, Yerington district, Nevada.

Portela, Bruno Virgilio (2020) From proximal to remote sensing: analysing the pyrophyllite-muscovite association in the Buckskin Range, Yerington district, Nevada.

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Abstract:Hydrothermal mineral deposits such as skarns, porphyry, epithermal and SEDEX systems, are the primary source of mineral commodities of global importance, such as copper, gold, and silver. Due to their importance, these types of mineral deposits have been subject to numerous studies over the years, using different approaches, including hyperspectral proximal (laboratory-based) and remote (airborne) sensing. Since hydrothermal alteration minerals are active in, amongst others, the visible and infrared range, the analysis of spectral absorption features can be used to identify the mineralogy associated with different alteration events. Some events are responsible for the occurrence of mineral commodities, while other events create mineral alteration without any economic concentrations of precious elements. Therefore, it is important to develop a mineral exploration strategy to rapidly identify and map the indicator minerals linked to a mineralising event. This research combined short-wave infrared proximal (laboratory) and remote (airborne) sensing imagery to study how to discriminate intimate intergrowths of pyrophyllite and muscovite that are linked to different alteration events in epithermal gold systems. The main objective of this study was to characterise the occurrence of overprinting patterns for pyrophyllite and muscovite by combining wavelength maps and spectral indices to highlight these alteration patterns at the laboratory (SisuCHEMA hyperspectral imager) as well as at the airborne (ProSpecTIR-VS sensor) scale. For this study, the test area selected was the Buckskin range at the Yerington district, Nevada (USA), a high-sulfidation epithermal system. Wavelength maps in different ranges were used to map the hydrothermal alteration mineralogy at both scales. The results of the airborne data showed outward zoning alteration patterns from an inner zone of alunite ± pyrophyllite towards the surrounding area dominated by muscovite of varied wavelength position of its Al-OH absorption feature. The laboratory data improved the characterisation of the hydrothermal alteration mineralogy, which included alunite, pyrophyllite, muscovite, dickite, kaolinite, chlorite, topaz and zunyite. The spatial distribution of the pyrophyllite-muscovite association and, consequently, the textural relationship of the two minerals was addressed through the development of a novel spectral index. Pervasive and veinlet-controlled textures were characterised and a subtle shift in the wavelength position of the Al-OH absorption feature of muscovite from 2189 to 2195 nm was detected. Although the shift was not a direct indication of overprinting, an intergrowth of pyrophyllite and muscovite sheets was suggested. The temporal relationship of the two minerals was then addressed: first through the interpretation of the alteration texture; second, through the backscattered electron images (BSE) and microprobe analysis data (EMPA), confirming the overprint of muscovite over pyrophyllite. The association of the spatial distribution with the textural relationship of the pyrophyllite-muscovite association allowed the reconstruction of the fluid chemistry and fluid pathway linked to this scenario. An early-stage low pH high-temperature magmatic-hydrothermally derived fluid characterised by the presence of alunite ± pyrophyllite, responsible for an intense acid leaching and, consequently, the generation of permeable zones in the lithocap. These permeable zones worked as channels (feeders) for the emplacement of a late-stage fluid of near-neutral pH and lower salinity, characterised by the occurrence of muscovite ± zunyite. Thus, the characterisation of the pyrophyllite-muscovite association suggested a high sulfidation epithermal context for the Buckskin range, following the current understanding of this system. While no economic Au deposits are known from this part of the Buckskin range, this research clearly demonstrates the importance of characterising and mapping overprinting patterns at different scales to assist the reconstruction of fluid composition and fluid emplacement. Therefore, contributing to an improved understanding of a high sulfidation epithermal system and a better definition of target areas for follow-up mineral exploration studies.
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
Link to this item:https://purl.utwente.nl/essays/90655
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