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The Furtei high sulphidation epithermal gold deposit (Sardinia, Italy): mineral assemblage and its evolution
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
Most part of the volcano-sedimentary complex in the area has been affected by hydrothermal alteration and mineralization. At Furtei, alteration can be observed over an area of roughly 5 km2 and is controlled by tectonic structures and lithotypes. In particular the pyroclastic formations and breccias are intensely altered. Four types of alteration have been recognized: propylitization, argillic alteration, advanced argillic alteration, and silicification. Hydro-thermal alteration was overprinted by supergenic, secondary argillic alteration, mainly after oxidation of the sulphide phases. The propylitic alteration affected indiscriminately all rocks at various degrees and its products can be grouped into two main assemblages, i.e. a chlorite assemblage and a calcite assemblage. In the first type the main alteration minerals are chlorite and quartz, with variable amounts of calcite, pyrite, and sometimes epidote and sericite. The second type, comprises calcite, quartz and chlorite. Argillic alteration varies from intermediate to advanced argillic, depending on the local intensity of acid leaching. Intermediate argillic alteration commonly overlaps propylitic alteration. It is characterized by clay minerals, both montmorillonite and kaolinite, sometimes accompanied by quartz and pyrite. The advanced argillic alteration is commonly associated with silicification and pyritization. Kaolinite and dickite are the main minerals, usually accompanied by quartz and sometimes pyrite and pyrophyllite. Two main types of silicification can be recognized: silicification by residual enrichment and silicification by deposition, both as vein filling and as impregnation. Sulfides, including predominantly pyrite and clay minerals, mostly kaolinite and dickite, are normally associated with these alterations. Pyritization is widespread in all types of alterations already considered, ranging from slight, but almost always present, dissemination in the propylitic halo, to major concentrations occurring in the innermost areas of intense alteration, where it practically grades into the ore bodies. The gold-bearing mineralization is mainly associated with either massive or replacement silicification and with residual vuggy silica.
Mapping of favourable mining areas in the Tiouit area by multispectral remote sensing and airborne gamma-ray spectrometry coupled with geochemical data (Eastern Anti-Atlas, Morocco)
Published in Applied Earth Science, 2022
Abdelhalim Miftah, Driss El Azzab, Ahmed Attou, Mohammed Ouchchen, Younes Mamouch, Lahsen Achkouch, Ayoub Soulaimani, Saâd Soulaimani, Ahmed Manar
The identification of hydrothermal alteration zones is an important phase in mineral exploration. The alteration produces distinctive mineral assemblages with specific spectral absorption characteristics in the visible and near-infrared (VNIR), across the wavelength regions of the short-wave infrared (SWIR) (0.4–2.5 μm) and/or thermal infrared (TIR) (8–14 μm) (Sabins 1999; Abrams et al. 2002; Ranjbar et al. 2011; Beiranvand Pour and Hashim 2014; Chattoraj et al. 2020; Aisabokhae and Osazuwa 2021). Porphyry deposits are generally associated with zonation of propylitic, argillic, phyllic, and potassic hydrothermal alteration. These hydrothermal deposits can be detected by remote sensing data using the spectral properties of minerals alteration that have specific wavelengths in the range of 2.0–2.5 μm (Sabins 1999; Abrams et al. 2002; Gabr et al. 2010; Ranjbar et al. 2011; Pour and Hashim 2012; Beiranvand Pour and Hashim 2014; Wambo et al. 2020; El-Desoky et al. 2021; Mamouch et al. 2022). The propylitic alteration is manifested by the presence of chlorite, epidote, and calcite, which have a specific absorption at 2.35 μm. Argillic alterations are associated with alunite and kaolinite, characterized by absorptions at 2.16 and 2.2 μm. Phyllic alteration can be discriminated by sericite, with an absorption of 2.2 μm. Ferruginous alterations are mapped by absorption characteristics between 0.4 and 1.1 µm (Hunt and Ashley 1979).
Assessment of magmatic fertility using pXRF on altered rocks from the Ordovician Macquarie Arc, New South Wales
Published in Australian Journal of Earth Sciences, 2021
T. J. Wells, S. Meffre, David R. Cooke, J. Steadman, J. L. Hoye
Hydrothermal alteration is ubiquitous in porphyry systems, with well-defined conceptual models of alteration zonation around causative intrusions (Seedorff et al., 2005; Sillitoe, 2010; Wilkinson et al., 2015). The alteration footprint of porphyry systems can have a total radius of up to 10km comprising several zones and subzones. Each alteration zone is characterised by particular mineral assemblages. The propylitic alteration assemblage is expansive, and its subzones are frequently used by explorers to vector towards porphyry mineralisation. However, the propylitic alteration assemblage can develop in response to barren porphyry emplacement, or other hydrothermal, geothermal and low-grade regional metamorphic events not associated with porphyry mineralisation (Cooke et al., 2014; Wilkinson et al., 2015). Porphyry-related propylitic alteration is difficult to distinguish from greenschist metamorphic assemblages, which are both characterised by chlorite + epidote + albite ± carbonate ± sericite ± montmorillonite ± apatite ± anhydrite ± hematite ± pyrite ± chalcopyrite. Propylitic alteration occurs in response to addition of H2O and CO2 ± S with limited or no H+ metasomatism and frequently results in the alteration of Ca-plagioclase to albite and epidote, where Ca2+, and Sr2+ are replaced by Na2+ in plagioclase with some Ca2+ and Sr2+ likely to be sequestered in the formation of epidote. Strontium in particular is highly mobile during hydrothermal alteration, which has profound implications for assessments of magmatic fertility in rocks that have undergone greenschist or higher metamorphism.
Mineralogical mapping using Landsat-8 OLI, Terra ASTER and Sentinel-2A multispectral data in Sidi Flah-Bouskour inlier, Moroccan Anti-Atlas
Published in Journal of Spatial Science, 2020
Zakaria Adiri, Abderrazak El Harti, Amine Jellouli, Lhou Maacha, Mohamed Azmi, Mohamed Zouhair, El Mostafa Bachaoui
Concerning the propylitic alteration zone, it mainly comprises chlorite and carbonates (especially calcite and epidote). This zone is distinguished by a characteristic absorption between 2.3 µm and 2.35 µm due to CO3 and Mg-OH groups (Pour and Hashim 2011, 2014, Hunt 1977, Mars and Rowan 2006; Zoheir and Emam 2012, Li 2010, Amer et al. 2012, Mars 2014, Li 2010, Bedini 2011). The spectrum of chlorite also shows absorption in the spectral region of NIR, which can be explained by the presence of iron (Hunt 1977, Adiri et al. 2016) (Figure 2).