Chirico, Rita (2023) Multimethod study of areas of mining interest: hyperspectral remote and proximal sensing for mineral exploration. [Tesi di dottorato]

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Abstract

Delineating ore-related hydrothermal alteration zones and supergene caps is fundamental for mineral exploration in remote areas because it gives significant information to identify new mineral concentrations. When outcropping, such large-scale features can be detected through the application of satellite spectral remote sensing techniques. The present study aims to apply a series of innovative technological solutions for the study of areas of mining interest, both historical and newly discovered, combined with commonly adopted mineralogical and geochemical approaches, in order to develop new workflows for identifying mineralized areas (including Critical Raw Materials – CRMs –bearing ones) and characterizing both the ore mineralogy and the hosting country rocks. The study results are useful for targeting the exploration by mapping the alteration assemblages vectoring to the mineralized bodies at local and regional scales, based on their spectral responses in the Visible Near to the Shortwave Infrared (VNIR-SWIR) regions, and defining new exploration prospects which are potentially unknown or unreported so far. The overall strategy of the work included: (1) spectral mineral mapping of alteration patterns through a multi-scale approach, from field-based spectroscopy to spaceborne hyperspectral imaging, and (2) validation through the integration of data from diverse sources, such as mineralogical (XRPD, Optical Microscopy, SEM-EDS) and geochemical analyses. The main goal was to evaluate the capability of the Italian Space Agency (ASI) Precursore IperSpettrale della Missione Applicativa (PRISMA) satellite hyperspectral imagery for mineral exploration, by inspecting its performances in mineral mapping. The test sites include genetically different ore deposit types: (1) the Mississippi Valley Type Jabali Zn-Pb(-Ag) deposit in Yemen, (2) the Iron Oxide Copper Gold Marimaca ore and the Río Blanco-Los Bronces copper-molybdenum porphyry district in the northern and central Andes of Chile; (3) the Punta Corna Project in Italy. Each test area was investigated through the PRISMA satellite hyperspectral images which have a 30m/pixel spatial resolution, a higher spectral resolution compared to multispectral sensors, and cover the mineral-diagnostic wavelength regions (such as the 2100 nm to 2300 nm range) with a Signal to Noise Ratio (SNR) ≥100. The results for the Jabali Zn-Pb carbonate hosted ore deposit and the Punta Corna Co-Ni vein system were integrated with the use of VNIR-to-SWIR reflectance spectra collected from hand specimens, mineralogical (XRPD), geochemical (ICP-AES/ES/MS) analyses, as well as observation in thin section (Optical Microscopy and SEM-EDS microanalysis) on rock samples collected in the test areas, with the aim of validating the remote sensing-based results. The study of the Zn-Pb Jabali deposit was aimed to apply a multi-scale workflow based on hyperspectral data to delineate hydrothermal dolomitization and supergene alteration associated with the Mississippi Valley-Type Zn-Pb(-Ag) deposit of Jabali (Western Yemen). We adopted a combined use of remote and proximal sources of hyperspectral data for defining features possibly able to target carbonate-hosted Zn-Pb ore deposits. The VNIR-SWIR reflectance spectra derived from the laboratory-based and PRISMA satellite data, that were interpreted with the support of analyses on ground samples (XRPD-QPA and ICP-MS/ES), allowed us to (1) delineate the distribution of the dolomitization in the Jabali area, and (2) identify the gossan outcrops overlying the mineralized areas. Spectral mineral maps were produced through the band ratios method using a combination of specific feature extraction indexes. The dolomites' footprint was mapped using a PRISMA Level 2C image, by enhancing the spectral differences between limestones and dolomites in the SWIR-2 region (major features centered at 2340 nm and 2320 nm, respectively). Gossans were detected due to the Fe3+ absorption band in the VNIR region at 900 nm (Crystal Field Absorption - CFA). The Zn-Pb mineralized area, extended for approximately 25 km2, was thus identified by recognizing gossan occurrences in dolomites. The detailed evaluation of the reflectance spectra from mineralized samples, even if their distribution mapping is not achievable at the satellite spatial resolution, helped the definition of the spectral responses of Zn(-Pb)-bearing oxidation-related minerals. Since they are commonly associated with nonsulfide ores related to supergene alteration of sulfide ore bodies, they represent a useful tool for exploration surveys based on field spectroscopy. The results of this study illustrate the advantages of using feature extraction indexes applied to hyperspectral data for the recognition of outcropping geology, which can be used as a powerful tool for mineral exploration in sedimentary environments at regional scale. The purpose of the study concerning the application of both remote and proximal sensing to the study of the area of the Punta Corna Mining Complex (PCMC) was aimed to highlight the distribution of alteration minerals genetically related to hydrothermally driven metallogenetic processes occurred. The results are meant to be used as support for mineral exploration in the area. The PCMC is a brownfield exploration prospect owned by AltaMin Ltd., located in the Servin Valley in the Western Alps (Piedmont, Italy). It is characterized by hydrothermal polymetallic orebodies defined by zoned Fe2+-rich carbonates and Co-Ni sulfide mineralization, hosted by E–W-trending sub-vertical post-metamorphic veins with a maximum thickness of 6–7 m. Laboratory hyperspectral IR spectroscopy and mineralogical (XRPD) and geochemical (ICP-AES/MS) analyses were carried out to define target alteration minerals and to validate the results obtained from the processing of satellite images. The heavily sericitized analyzed samples were characterized by the application of the 2200 nm feature intensity and wavelength indexes, as well as the Fe-oxy-hydroxides-bearing samples. The first ones are characterized by higher contents of Al-rich white mica, which directly alters albite. White mica can occur in close relationship with chlorite, where the latter seems to partially replace Ca-amphibole crystals. Fe(±As)-hydroxides are also present, forming concretions either surrounding white mica crystals or in the interstices between them. The results obtained for the metabasite country rock, both the spectral characterization and mineralogical-geochemical analysis, show that it is affected by intense sericite–quartz-carbonate alteration only in the immediate proximity of the ore-bearing veins. However, areas showing enrichments in Mg-rich chlorite abundances (semi-quantitative XRPD) are in spatial relationship with the known outcropping veins. This is displayed as a more prominent 2250 nm absorptions feature in the spectral dataset and it is associated also with higher bulk Al2O3 concentrations, suggesting a potential chloritization occurred at regional scale. The delineation of the alteration at the km scale was based on the mapping of mineral occurrences in terms of relative abundances using feature extraction spectral indexes applied to PRISMA hyperspectral satellite. The heavy sericitization and the supergene contribution observed at hand specimens scale were mapped by means of satellite images, highlighting the presence of an alteration halo surrounding the main veins known in the area, as well as in zones where the mineralization occurrences, either reported in the literature or observed in the field, are still poorly known. The results of the study, partly validated through fieldwork, assess that a combined and multi-scale approach based on hyperspectral data represents an effective method for mapping alteration minerals associated with the Co-Ni mineralization, i.e., white mica, chlorite, and supergene goethite, by means of their diagnostic absorption features at around 2200 nm, 2250 nm, and 900 nm, respectively. The method can be used as an additional tool for guiding toward prospective areas where Co-Ni-bearing veins occur. The objectives of the application of PRISMA hyperspectral imagery for the study of the areas centered on the “Marimaca Copper Project”, in the Naguayán district in the Antofagasta Province, and the Río Blanco-Los Bronces copper-molybdenum porphyry district, in the Santiago Region of Chile were to identify and map minerals associated with the surface-exposed hydrothermal (sericitic, propylitic, advanced argillic) and supergene (leached caps) alteration zonation patterns related to Cu deposits in the Chilean Andes and widely affecting the rocks hosting the mineralization. The Naguayán district area is a well-known host for several copper, iron, and minor gold, silver, and zinc ore bodies of Late Jurassic to Early Cretaceous age. The Río Blanco-Los Bronces district is the host for several world-class copper-molybdenum porphyry systems, hosted in Late Miocene – Early Pliocene magmatic arc. To map the relative abundances and compositions of specific supergene and hydrothermal alteration minerals such as Fe-oxides and hydroxides (hematite-goethite), di- and tri-octahedral phyllosilicates (e.g., micas-kaolinite-chlorite), hydroxyl-bearing sulfates (e.g., alunite) and epidote, a range of band ratios in the region around 900 nm, from 1480 nm to 1770 nm, and from 2100 nm to 2300 nm, were applied to PRISMA satellite imagery aimed at diagnostic absorption feature extraction and characterization. The results of this study show a close spatial relationship between the distribution of sulfates (alunite), Al-rich to Al-poor white micas, and chlorite-epidote, as well as the Fe-bearing minerals associated with the supergene alteration, from proximal to distal to known Cu-deposits, supporting a causative association with the magmatic-hydrothermal activity-related alteration. The work supports the use of spaceborne hyperspectral imaging spectroscopy for assisting mineral exploration for other copper deposits worldwide, providing targeting information for follow-up sampling and surveys, which could improve the exploration strategies. The study regarding the characterization of the area affected by the collapse of the tailing “Dam B1” of the Córrego do Feijão Mine (Brumadinho, Brazil) aimed to map the land area affected by the flood and the related environmental effects over time by using multispectral satellite images. The collapse of the tailing “Dam B1”, which occurred in January 2019, is considered a large socio-environmental flood disaster, counting numerous people died and seriously affecting the local flora and fauna, as well as agricultural areas along the Paraopeba River. To pursue the aim, Level-2A multispectral images from the European Space Agency (ESA)’s Sentinel-2 sensor were acquired before and after the tailing dam collapse in the period 2019-2021. The pre- and post-failure event analysis allowed evidencing drastic changes in the vegetation rate, as well as in soils and surficial waters, based on the spectral signatures of the minerals composing the mining products, mainly Fe-oxides, i.e., hematite, characterized by the Crystal Field Absorption – CFA – feature at around 842 nm (in band 8 and 8A of Sentinel-2). By combining the information obtained with the study mentioned above, a work focused on the South-West Sardinia, one of the oldest mining districts in the world, and the Quadrilátero Ferrífero mining district was conducted, with the purpose of segmenting relevant imagery classes, for the automatic detection of mining areas using hyperspectral images of the PRISMA mission. The method is focused on a deep learning model - U-Net convolutional neural network. In order to avoid the typical problem of hyperspectral data redundancy and to improve the computational performances without losing accuracy, the Singular Value Decomposition (SVD) is applied to the hyperspectral data cube, taking only the first three singular values, thus projecting the multi-dimensional data cube to a three channels image. The two areas were analyzed to test the transferability of the model to other mining areas worldwide. These techniques open the possibility for quickly classifying areas affected by floods, as well as obtaining significant information potentially useful for monitoring and planning the reclamation and restoration activities in similar cases worldwide, representing additional tools for evaluating the environmental issues related to mining operations in large areas at high temporal resolution. To conclude, the PRISMA imagery evaluated in this Thesis demonstrates the capability of hyperspectral satellite data for mineral exploration and its advantage over previous hyperspectral and multispectral satellite sensors as regards accurate mineral mapping, which is a key factor for successful mineral exploration. However, additional studies are required to improve advanced machine learning techniques for dealing with spatial resolution problems of hyperspectral satellite imagery, while keeping constant the high spectral resolution and SNR, as well as data noise and sensor artifacts management, all impacting the quality of the mapping results. Moreover, the detailed evaluation of the reflectance spectra from both alteration and supergene mineralized samples commonly associated with the hypogene ore bodies, even if their distribution mapping is not achievable at the satellite spatial resolution, can help their definition in the field and represent a useful tool for exploration surveys based on ground-based hyperspectral analyses.

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