Portela, Bruno1; Sepp, Michael D.2; van Ruitenbeek, Frank J.A.1; Hecker, Christoph1; Dilles, John H.2
1Department of Earth Systems Analysis, Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, 7514 AE Enschede, The Netherlands. 2College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Admin. Bldg., Corvallis, Oregon 97331, United States of America.
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. These types of mineral deposits have been subject to numerous studies over the years, using different approaches, including hyperspectral proximal and remote 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 alteration events are responsible for mineralisation, while other events do not generate any economic concentrations of precious elements. Therefore, it is important to develop a strategy to rapidly identify and map the indicator minerals linked to a mineralising event.
This research combined shortwave infrared (SWIR) proximal (SisuCHEMA hyperspectral imager) and remote (ProSpecTIR-VS sensor) 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 replacement textures by combining wavelength maps and a spectral index. The test area selected was the Buckskin range at the Yerington district, Nevada (USA), a high-sulphidation epithermal system.
Airborne data results showed outward zoning 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. Laboratory data improved the characterisation of the hydrothermal alteration mineralogy, which included alunite, pyrophyllite, muscovite, dickite, chlorite, topaz and zunyite. 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, the pyrophyllite-muscovite index (PMI). 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 replacement, 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), confirming the muscovite replacement of 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 of short to long-wavelength range. Therefore, this study clearly demonstrates the importance of characterising and mapping replacement textures at different scales to assist in the reconstruction of fluid emplacement and composition. Therefore, contributing to an improved understanding of a high sulphidation epithermal system and a better definition of target areas for follow-up mineral exploration studies.
Bruno Portela has a BSc degree in Geology from the University of Sao Paulo (Brazil) and an MSc degree in Applied Remote Sensing for Earth Sciences from the University of Twente, The Netherlands. Before his MSc, he worked for four years in different roles for BHP at Escondida Mine (Chile).