"Rock of the Month # 35, posted for May 2004" --- GCW sample 1268, collected on 11 September 1990 from the Mickey Doolan pit of the Paddy's Flat mine, in the Murchison province of the Archean Yilgarn craton of Western Australia. Digital image prepared in March 2004. The 1-cent Canadian coin is 18 mm in diameter.

There are at least three lessons to be learned from this rock: (1) the nature of the distinctive mineral, the green mica; (2) a mineralogical association with gold; and especially (3) the nature of the rock type, or more technically the style of hydrothermal alteration, depicted above. The green mica is a species known as fuchsite, a chromium-bearing muscovite or phengite, which may contain several weight percent Cr₂O₃. In formal terms most of these micas may best be described as chromian phengites, but the varietal terms fuchsite and mariposite are well-entrenched in the literature. The latter is named for the Mariposa ("butterfly" in Spanish) area of the Mother Lode district of California. The rock type is a gold ore, or at least a gold-enriched rock, formed by hydrothermal alteration of mafic-ultramafic host rocks. The fresh rock was most probably an olivine- and chromite-bearing rock such as a peridotite or komatiite, pervasively altered by hydrothermal fluids which enriched the rock in CO₂ (thus carbonates, such as ankerite, magnesite and breunnerite), silica (thus quartz) and trace elements such as gold, and effected breakdown of the original minerals, including even the relatively refractory chromite, liberating Cr to serve as colouring agent in the secondary micas. I prefer to refer to these rocks by their alteration style, and predominant minerals, as QAM (quartz- ankerite- mariposite) rocks, although some opt for the historical name listwanite (also spelled "listvenite" or "listwaenite"), first used by Gustav Rose in 1842, concerning rock from the Listvenaya Gora (Larch Hill) occurrence near Miask in the Urals (Halls et al., 1991). The term mariposite was first applied to the green mica in material collected at the Josephine mine in Mariposa county in 1868 (Peabody, 1991).

This particular sample is coarse and massive, with trace to 3 vol.% disseminated sulphides, cut by veinlets of grey and white carbonates. The ore is dominated by carbonate (probably dolomitic or ankeritic, minimally acid-reactive, roughly 62 vol.% of the rock), quartz (30%) and green mica (8%). Accessory minerals include arsenopyrite, chalcopyrite, pyrite and rutile. The ore is relatively low-grade, probably 1.3-1.5 ppm gold, whereas associated `porphyry' ore may grade 5-6 ppm. A fine early description of this style of alteration was provided by Clarke et al. (1916). Fuchsite was first reported in Australia at Kalgoorlie (ibid., p.302), always found in or near deformed and altered ultramafic rocks.

This style of alteration is common in many gold-mining camps around the world, in rocks of Archean and younger ages. Close equivalents of the green-carbonate and porphyry ores at this Australian mine occur at the much larger historical producers of the Kerr-Addison and Dome mines, in the Virginiatown and Porcupine mining districts of northeast Ontario, Canada. Equally impressive associations of chromian micas with gold can be found in some of the Mesozoic ores of the Mother Lode district in California, U.S.A.; in some Russian localities; in the Transvaal district of South Africa (including, confusingly, the Murchison Range); and elsewhere. Further localities occur in British Columbia and the Yukon, Quebec and Newfoundland; Ireland; Morocco; Egypt; and Saudi Arabia (see, e.g., Buisson and Leblanc, 1986).

References

BUISSON,G and LEBLANC,M (1986) Gold-bearing listwaenites (carbonatized ultramafic rocks) from ophiolite complexes. In `Metallogeny of Basic and Ultrabasic Rocks' (Gallagher,MJ, Ixer,RA, Neary,CR and Prichard,HM editors), Inst.Min.Metall., 121-132.

HALLS,C, ZHAO,R, SHINE,C, COOPER,C and HARRINGTON,K (1991) Listvenites and related rocks associated with gold mineralization in Co.Mayo, Ireland. In `Industrial and Environmental Mineralogy' (Bevins,RE and Williams,PA editors), Programme and Abstracts, Mineralogical Society of Great Britain and Ireland, Winter Conference, Cardiff, 64pp., 25-26.

PEABODY,GW (1991) Mariposite, the rock that made California famous. California Geology 44 no.8, 183-186.