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Whole-rock compositions of Precambrian iron formations and Phanerozoic ooidal ironstones: Comparative considerations and mineralogical differentiation of subtypes
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
Like the ironstones, the plots of the iron formations (solid symbols) summarized in Fig. 3 can also be classified according to mineralogical aspects. Field I represents the magnetite-free silicate fades iron-formation of Nigeria being composed of manganese-bearing grunerite-cummingtonite, spessartine-almandine and ± quartz. One plot belongs to the gold-bearing Wanderer iron-formation (carbonate fades) of Zimbabwe containing siderite, skelletal magnetite porphyroblasts, quartz and muscovite. This field is separated from a bigger and elongated field parallel to the SiO2-Fe2O3-line. The latter represents 4 subtypes which all contain quartz in strongly varying proportions: IIa contains magnetite and Fe-silicates (magnetite-silicate facies) comprising the Nigerian iron-formations consisting of magnetite, grunerite and spessartine-almandine, the Monarch iron-formation/ Zimbabwe containing magnetite, manganese-bearing grunerite and chlorite and the iron-formation of Gongchangling, Liaolin/China being composed of ferro-actinolite and magnetite, whereas lib is composed of magnetite (magnetite subfacies) and minor silicates. III contains martitized magnetite and newly-formed hematite and comprises banded iron-formations of the Fortescue Group, Pilbara Craton (Australia); Karna-tak, Chikuayakanhalli greenstone belt and Datari, Shingbum, Orissa (India); and Simandu (Guinea) and IV newly-formed hematite which may contain some martite relics (hematite-subfacies). Analogously to the groups II and III, group IV shows that the amount of SiO2 changes drastically from quartz-dominated iron formations [= banded iron-formations: Muro and Obajana (Nigeria); Simandu, Gueridou/ Chaime de Goiny; and Mt. Nimba/Mount Sempere (Liberia); Datari and Gandhamardan/Orissa and Karnatak (India); Neganayi/Keweenaw Peninsula /Michigan and Rapitan/Mackenzie Mountains (USA); and unmineralized ores of Brunos band from Mt. Tom Price and Mt. Sylvia, Dales Gorge member (DB 4 and DB 12) and Joffre Member (J3), Mt. Tom Price, Hamersley (Australia)] to iron formations being composed of nearly only hematite [= massive ore): Section 6 (West Pit), DB 2, and DB 15, Dales Gorge member, Mt. Tom Price, Hamersley (Australia); Daitari and Gandhamardan/Shingbum and Karnatak (India); Krivoi Rog (Ukraine); Faleme (Senegal); Mt. Nimba (Liberia); Itabira, Mutuca, Aguas Claras (Brazil); and El Pao (Venezuela)].
A review of the occurrence of and potential for jade in the New Guinea Mobile Belt
Published in Australian Journal of Earth Sciences, 2022
Gemmologists recognise two rock types encompassed by the term jade. Nephrite is a fine-grained rock consisting predominantly of tremolite–ferro-actinolite [Ca2(Mg,Fe)5Si8O22(OH)2] (Harlow et al., 2014). Nephrite is distinctive for its translucency along thin edges, high specific gravity and toughness. It breaks with a splintery fracture and has a glistening lustre. Its texture is characterised by randomly oriented bundles, tufts and sheaf-like groups of felted and twisted fibres. Minor to trace inclusions in nephrite include magnetite, chromite, diopside, grossular garnet, graphite, apatite, rutile, pyrite, talc and serpentine. Jade composed of the mineral jadeite is known as jadeite jade, jadeite rock, pyroxene jade or jadeitite (Harlow et al., 2014, Leaming, 1978). Jadeite jade is rare and is generally defined as consisting of ∼90 vol% pyroxene containing at least 90 mol% jadeite [jadeite = NaAlSi2O6] (Harlow et al., 2014; Leaming, 1978). The term pyroxene jade may be used when the composition of the pyroxene is in doubt (Harlow et al., 2014). Jadeite jade is a tough, hard, heavy, equigranular aggregate of prismatic pyroxene crystals. Its texture is characterised by granoblastic to feathery microstructures (Harlow et al., 2012, 2014). Chloromelanite is the name for a dark iron-rich pyroxene variety of jade rich in aegirine (NaFe3+Si2O6) and omphacite [(Na0.5Ca0.5)([Mg,Fe]0.5Al0.5)Si2O6 or Jd50(Di + Hd)50] or augite [(Ca,Mg,Fe)2Si2O6] (Harlow et al., 2014). References to chloromelanite are commonly found in New Guinea literature, and its use is retained in this paper.