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Metamorphism of sulfide ores from the Aggeneys-Gamsberg base metal deposits, South Africa
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
Although mineralogical changes are generally not as common in the sulfide minerals as those encountered in the host rocks (Marshall et al. 2000), a range of characteristic modifications are recognized at Aggeneys-Gamsberg. These include an increased FeS content of the sphalerite, coupled with exsolution of chalcopyrite and pyrrhotite from the sphalerite lattice, formation of gahnite and zincian staurolite, and changes in the initial ratios between iron sulfides.
Cretaceous tungsten-tin mineralisation in the Tin Range, Stewart Island, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2022
Hamish C. Lilley, James M. Scott, Josh J. Schwartz, Rose E. Turnbull, Andy J. Tulloch
The third greisen species is a gahnite-schist, described as a biotite-topaz schist by Williams (1934b), envelopes veins of quartz-topaz greisen extending outward up to ∼2 m either side of the quartz-topaz greisen veins into the Pegasus Group (Figure 6A). In outcrop, the gahnite-schist greisen type is foliated to hornfelsic, and comprises white mica, quartz, sillimanite, gahnite (zincian-spinel: ZnAl2O4) and wolframite, and rare biotite, topaz, ilmenite, apatite and rutile. The rutile commonly hosts up to ~1% ZnO. Sillimanite can form as spherically radiating aggregates made of up of individual 2 mm wide and up to 3 cm long crystals, which commonly host ∼5 mm sized grains of wolframite and gahnite (Figure 6C). Opaque gahnite can form up to 1 cm in size, typically hosting inclusions of fine-topaz. Semi-quantitative energy dispersal (EDS) x-ray analyses of the gahnite show an average endmember composition of Zn72.5Fe24.7Mg2.8. The gahnite-schist greisen type forms peripheral zone of W-Zn enrichment around the core W-Sn enriched quartz-topaz greisen. An analogous greisen type is found within the altered Pegasus Group quartzite. It has a mineralogy of garnet, quartz and white mica with minor wolframite and accessory sillimanite, xenotime, monazite and zircon. Rare, large euhedral opaque brown garnet (1–2 cm) contain authigenic inclusions of xenotime and monazite. Such garnets have a chemistry that is very similar to those in the Tin Range granodiorite.
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
Primary alteration minerals surrounding VHMS deposits include chlorite, sericite, carbonate, quartz and pyrite, with talc, epidote, albite and kaolinite (or in places other clay minerals) commonly present (Barrett, MacLean, & Arebäck, 2005; Galley et al., 2007; Yeats et al., 2017). In upper greenschist- to amphibolite-facies metamorphic terranes, distinctive coarse-grained mineral suites commonly define VHMS alteration zones (Dusel-Bacon, 2012; Galley et al., 2007). These minerals can include, but are not limited to: chloritoid, garnet, staurolite, kyanite, andalusite, phlogopite, and gahnite (zincian spinel). The presence or absence of each of these minerals reflects not only VHMS-style hydrothermal alteration and P–T conditions during metamorphism, but also the thermal gradient during metamorphism (Dusel-Bacon, 2012). A comprehensive list of metamorphosed VHMS deposits under different conditions (e.g. greenschist, granulite, blueschist), and common minerals associated with each, was given by Corriveau and Spry (2014).
On the occurrence of gallium and germanium in the Bergslagen ore province, Sweden
Published in GFF, 2019
Much later, Papish & Stilson (1930), utilising graphite and silver arc spectrography, determined the presence of Ga in gahnite (zinc spinel, ideally ZnAl2O4) from the Falu mine, northern Bergslagen (as well as in gahnite from Orijärvi, Finland). During a wide-ranging study of different minerals, the presence of Ga in Falu mine gahnite was further corroborated by Goldschmidt & Peters (1931), who also identified it as present in braunite (ideally Mn2+Mn3+7SiO12) from the complex Fe-Mn oxide deposit Långban in westernmost Bergslagen, as well as in sensu stricto spinel (ideally MgAl2O4) from the Åker marble quarry in the south-eastern part of the province. A little later, Landergren (1935), by means of X-ray spectrography, determined Ga contents of about 30 ppm in bulk ore from the Saxberget mine, and less than 1 ppm in ore from the Garpenberg mine. He also, quite succinctly, expressed his belief that Ga at Saxberget was predominantly hosted by silicate minerals, rather than in the sulphidic part of the ore.