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Igneous Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
During the cooling of a magma its different constituents unite to form crystals of silicate minerals, such as pyroxenes, amphiboles and feldspars, and oxides such as magnetite. In a basic magma, minerals such as olivine and magnetite are often the first to crystallize, using up some of the silica, magnesium, and iron; the remaining Mg and Fe, together with CaO and A12O3, is used up later in augite (pyroxene), hornblende (amphibole), and dark mica. Such minerals, on account of their composition, are called ferromagnesian or mafic (a word coined from ma for magnesium and fe for iron). In contrast to these dark and relatively heavy minerals the alkalies and calcium, together with A12O3 and SiO2, form light-coloured or felsic minerals, which include feldspars, feld-spathoids, and quartz. Most of the calcium in a basic magna would go to form a plagioclase, a little contributing to augite. In acid rocks felsic minerals predominate and give the rock a paler colour, in contrast to the darker basic rocks. Between acid and basic types there are rocks of intermediate composition.
Compositional characteristics of mineralised and unmineralised gneisses and schist around the Abansuoso area, southwestern Ghana
Published in Applied Earth Science, 2023
Raymond Webrah Kazapoe, Olugbenga Okunlola, Emmanuel Arhin, Olusegun Olisa, Daniel Kwayisi, Elikplim Abla Dzikunoo, Ebenezer Ebo Yahans Amuah
The concession lies within a ductile sinistral shear zone with a brittle component. The mineralisation appears to sit on a granitoid pluton (now gneisses of felsic to intermediate composition) and arenaceous-argillaceous sedimentary rocks (now schist of varying compositions) which have been affected by various stages of metamorphism and deformation. The sequence of alteration invades the rocks via stringers along weak zones probably caused by the shearing. The alteration is predominantly quartz-sericite, carbonate, sulphides (pyrite, arsenopyrite, and galena) with haematite at the fringes where low grade gold is reported. The main mineralised zones are characterised by sericite alteration coupled with fine-grain pyrite and associated with carbonates. The preliminary sequence of alteration has been deduced as chlorite-carbonate-haematite-sericite-chlorite -silica/sulphides with minor albite.
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 Precambrian consists of an ancient bedrock known as the ‘Saghro Group’, surmounted by the Ouarzazate Group (Thomas et al. 2004). The first Group is the oldest bedrock of the Jbel Saghro and is characterized by two different sets, both Cryogenian: (i) a set formed by a turbiditic series and mafic to intermediate metavolcanic facies (Benziane and Yazidi 1992; Tuduri 2005; Massironi et al. 2008; Gasquet et al. 2008; Errami et al. 2009); (ii) granodiorites and low metamorphic tonalities, dated by TIMS U-Pb zircon method between 677 and 645 Ma (Schiavo et al. 2007; Baidada et al. 2017). According to Jettane (1995), the plutonic massifs were affected by important alteration phenomena, the most important of which are haematization, choritization, silicification, and propylitization, to which mylonitization and recrystallization phenomena are added in places. Subsequently, these terrains are intersected by dykes of acid to intermediate composition (rhyolite, andesite, and trachyandesite) along with two preferential directions: NW–SE and SW–NE (El Maghraoui and Popov 1982, Combe and Morrelli 1983, Benkirane 1987).
Matangkaka manganese deposit, Ambitle Island, Feni Island Group, Papua New Guinea: a Quaternary epithermal stratabound manganese oxide deposit
Published in Australian Journal of Earth Sciences, 2022
Trachyitic lava in hand specimen is distinctly porphyritic, with large euhedral plagioclase phenocryts comprising 10–15 vol% by of the rock. A prominent alignment of dark mica (biotite?) is visible to the naked eye and is considered as evidence for a flow-banding (Lindley, 2015, figure 8a). Small (<0.5 cm) accidental clasts of mafic–intermediate composition are present. Trachytic lava from this unit can be distinguished from trachyte clasts contained in the tephra unit by the presence of visible dark mica content in the former. The rock in thin-section exhibits a strong trachyitic fabric, in which microlites of feldspar and biotite are sub-parallel to one another and wrap around phenocrysts, presumably in the direction of flow (Lindley, 2015, figure 8b, c). Large euhedral phenocrysts of plagioclase feldspar are set in a holocrystalline trachyitic groundmass consisting of plagioclase, biotite (2 vol%), minor quartz (<1 vol%) and trace magnetite. It is not unusual to observe flow-oriented laths of biotite and the groundmass moulded around phenocryts (Lindley, 2015, figure 8b). About 40 vol% of plagioclase phenocrysts (and in places biotite) show signs of fracturing and breakage as a result of physical interaction with other phenocrysts, presumably during flow (Lindley, 2015, figure 8c). The rock is non-magnetic.