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Metamorphic rocks
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
Staurolite ((Fe,Mg)2Al9O6(SiO4)4(O,OH)2) is found in mudstones which have been metamorphosed under amphibolite facies (medium grade) conditions. Staurolite shows characteristic yellow pleochroism, and usually occurs as euhedral porphy- roblasts. Knee twinning can occur as shown by the large grain in the centre of the micrograph (Figure 218). In crossed polarised light staurolite shows upper first order to low second order colours (Figure 219). Muscovite mica, biotite (brown platey minerals), quartz and an opaque mineral are present. The small green mineral is tourmaline, a common accessory mineral in metamorphosed mudstones (pelites).
3D structural control and spatial distribution of Zn-Pb-Cu grades in the Palmeirópolis VMS deposit, Brazil
Published in Applied Earth Science, 2022
Saulo B. de Oliveira, Ignacio Torresi, Debora A. L. Rossi
The metamorphosed alteration zone in the Palmeirópolis deposit was described in detail by Araujo et al. (1995) and Araujo et al. (1996). These authors pointed out that the former chlorite alteration zone is chemically enriched in Al and Mg and depleted in alkalies and Fe, being represented by anthophyllite, biotite, quartz, garnet, staurolite, sillimanite, cordierite, gahnite, actinolite, and diopside. From these minerals, only quartz, biotite, garnet, and staurolite were systematically described in the logging and are present in the database. The attempts to model 3D envelopes for quartz and biotite showed a high dispersion throughout the acidic metavolcanic unit; furthermore, as these minerals also occur in the basic metavolcanic unit, they would not be distinctive of the zone alteration. Nevertheless, garnet and staurolite together could be modelled in 3D and generate a consistent halo mainly in the C1 and also in the C2 areas. The alteration halo occurs within the acid metavolcanic domain and also could indicate a primary stratigraphic position of the volcanic pile, discussed below. The alteration zone occurs close to the sulfide ore bodies and has a greater spatial extent and can be used as a prospective guide for new discoveries. For example, the southern region of the C1 area indicates a potential area for further investigation (Figure 9). In Brazil, the Mesoproterozoic volcanic-sedimentary sequence of the Serra do Itaberaba group, with same age as Palmeirópolis, presents VMS sulfide occurrences and a similar alteration (Pérez-Aguilar et al. 2005; Pérez-Aguilar and Juliani 2016).
VHMS mineralisation at Erayinia in the Eastern Goldfields Superterrane: Geology and geochemistry of the metamorphosed King Zn deposit
Published in Australian Journal of Earth Sciences, 2019
S. P. Hollis, D. Podmore, M. James, J. F. Menuge, A. L. Doran, C. J. Yeats, S. Wyche
Hydrothermal alteration at King is dominated by quartz–muscovite ± chlorite ± albite ± carbonate in felsic to intermediate banded schists, and quartz–epidote ± chlorite ± magnetite in garnet-amphibolite. Cordierite and anthophyllite also occur in relatively minor amounts (≦5 vol%) in felsic to intermediate footwall rocks. According to Corriveau and Spry (2014), the ‘best documented alteration types associated with metamorphosed VHMS deposits are the cordierite-anthophyllite schists, commonly the amphibolite facies analogues of chloritic alteration pipes’ (p. 181). Their distinct lithogeochemical signature (+Mg, +Fe, –Ca, –Na, –K) results in mineral assemblages that may include cordierite, orthoamphibole/orthopyroxene, Al2SiO5 polymorphs, garnet or staurolite, quartz, biotite and plagioclase, depending on P–T conditions. The aluminous minerals garnet, chloritoid, staurolite and the Al2SiO5 polymorphs (=andalusite, kyanite, sillimanite) commonly occur close to high-T alteration pipes. This reflects the enrichment of Al by leaching of alkalis under high fluid/rock ratios (Dusel-Bacon, 2012). Metamorphosed phyllic, sericitic and argillic alteration zones (+K, +Mg, ± Fe, –Ca, –Na) will result in the formation of diagnostic peraluminous and/or mica-rich metamorphic rocks (e.g. those unusually rich in Al2SiO5 polymorphs, cordierite, garnet, K-feldspars and/or micas) (Corriveau & Spry, 2014).
What the ca. 1.83 Ga gedrite-cordierite schists in the crystalline basement of Lithuania tell us about the late Palaeoproterozoic accretion of the East European Craton
Published in GFF, 2018
Laurynas Siliauskas, Grazina Skridlaite, Bogusław Baginski, Martin Whitehouse, Sabina Prusinskiene
An assemblage of plagioclase, garnet, chlorite, biotite, staurolite and quartz was stable at the peak of 620°C at 7 kbar. Staurolite inclusions in the Lz13/540 garnet and in the matrix suggest that garnets grew within the staurolite zone. Later uplift caused pressure to drop to 5 kbar, and garnet to decompose to cordierite and chlorite by the reaction garnet = chlorite + cordierite + quartz (Fig. 5A). New skeletal garnet growth commenced at the expense of staurolite and cordierite when temperature increased during a subsequent thermal overprint of 630–640°C as recorded by the garnet composition isopleths (Fig. 6B). The pressure did not drop below 3.5 kbar, where an orthoamphibole stability field begins (Fig. 5A); therefore, orthoamphiboles are absent in this sample. The formation of new staurolite in the matrix at the expense of garnet may have occurred during the later retrogression. Retrograde chlorite may have been formed at the expense of cordierite, especially because cordierite rims are mostly altered into chlorite.