China
Africa
Explore chapters and articles related to this topic
Industrial minerals
Published in Francis P. Gudyanga, Minerals in Africa, 2020
Pumice is a volcanic rock that consists of highly vesicular rough textured volcanic glass [898,899]. In composition it is commonly [900] silicic or felsic and composed of highly microvascular glass pyroclastic where the following may occur: andesite, dacitic, pantellerite, phonolite, rhyolitic and trachyte. It is a common product of explosive eruptions [901] with 90% porosity.
Textural and mineral chemical evidence of an Upper Carboniferous rapakivi granite in the Erzgebirge/Krušné Hory
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
The formation of plagioclase mantled K-feldspars indicate disequilibrium and quench processes related to longstanding overheating (Rämö & Haapala, 1995). The fact that no plagioclase mantled K-feldspars occur in the Teplice rhyolite although a mixing of mafic and silicic magma already took place but they occur in the microgranites verifies the assertion that the formation of rapakivi feldspar is related to longstanding overheating. Furthermore, the mixing over a long period is evident since mixing features like small nests of mafic enclaves, disequilibrium mineral assemblages, allanite formation, etc. are dispersed throughout the whole rocks and the contacts between the felsic magma and mafic schlieren are progressive.
Formation of Cu–Au porphyry deposits: hydraulic quartz veins, magmatic processes and constraints from chlorine
Published in Australian Journal of Earth Sciences, 2023
G. N. Phillips, J. R. Vearncombe, J. D. Clemens, A. Day, A. F. M. Kisters, B. P. Von der Heyden
A wide variety of igneous rocks host vein-related copper and gold, especially diorites, quartz diorites, monzodiorites and granodiorites, with a range from silicic to intermediate and lesser mafic rock types. This wide range of host-rock composition is evident, even within single deposits such as El Teniente (Camus, 1975; Cannell et al., 2005; Vry et al., 2010). The porphyritic intrusions in Cu–Au porphyry deposits are exclusively of oxidised I-type and magnetite-series affiliation (Ishihara, 1981), and typically metaluminous and medium-K calcalkaline, but may also fall into the high-K calcalkaline and shoshonitic or alkaline fields. Calcalkaline igneous rocks are the main hosts for porphyry Cu–Au deposits, and high-K types host the largest examples (Cooke et al., 2005).
Taupō: an overview of New Zealand's youngest supervolcano
Published in New Zealand Journal of Geology and Geophysics, 2021
Simon J. Barker, Colin J.N. Wilson, Finnigan Illsley-Kemp, Graham S. Leonard, Eleanor R.H. Mestel, Kate Mauriohooho, Bruce L.A. Charlier
The modern gravity anomaly beneath Lake Taupō is interpreted to mostly reflect the huge magma chamber that was discharged during the Oruanui event (Davy and Caldwell 1998: Figure 1B). The dominant silicic magma type (>99% by volume) represented in eruptive deposits is high-SiO2 rhyolite (HSR: whole-rock SiO2 >74 wt%, hereafter: Figure 5; Wilson et al. 2006; Allan et al. 2017). HSR pumice clasts characteristically contain 3–13% crystals with quartz, orthopyroxene, amphibole and Fe-Ti oxides. A second silicic magma type is represented by sparse (∼0.5%) pumices of low-SiO2 rhyolite (LSR: whole-rock SiO2 <74 wt%) that is crystal-poor (∼6 wt. %) and lacks quartz. The LSR is interpreted to represent less evolved magma that had previously fed the HSR magma body (and evolved to HSR compositions: Allan et al. 2017), but which was also tapped from isolated pockets in the underlying crystal mush during evacuation of the HSR magma body. A third silicic magma type is represented by more crystal-rich, biotite-bearing rhyolite, identified as a ‘foreign’ magma, sourced from the nearby NE dome system, that was fed laterally via dikes throughout phases 1 and 2 (Allan et al. 2012; Myers et al. 2019). The rapid timescale for assembly of the Oruanui magma body, and the evidence for syn-eruptive lateral magma transport imply that external tectonic forces associated with rifting played a key role in the rapid assembly of this super-sized magma body, and in initiating, then modulating the early phases of the eruption (Allan et al. 2012, 2013, 2017; Myers et al. 2018, 2019).
Associations between zircon and Fe–Ti oxides in Hiltaba event magmatic rocks, South Australia: atomic- or pluton-scale processes?
Published in Australian Journal of Earth Sciences, 2020
M. R. M. Ferguson, K. Ehrig, S. Meffre, A. R. Cherry
The HS comprise a series of white to pink, medium- to coarse-grained, equigranular intrusive rocks that range from relatively pristine to strongly altered and deformed (Flint, 1993). They are predominantly granitic, but also include monzodiorite, quartz monzonite, granodiorite (Stewart & Foden, 2003), gabbro and norite (Zang, Fanning, Purvis, Raymond, & Both, 2007). Surface exposure of many plutons is limited; near-surface HS bodies are dominated by granites comprising quartz, alkali-feldspar and plagioclase with subordinate to accessory biotite, magnetite, titanite, amphibole, fluorite, apatite and zircon (Flint, 1993). Less silicic lithologies comprise plagioclase, augite, hornblende and minor K-feldspar and quartz, with apatite and zircon the most common accessory phases.