Origin of coexisting wustite, MgFe and REE phosphate minerals in graphite-bearing fluorapatite from the Rumburk granite
Seifert, Wolfgang; Thomas, Rainer; Rhede, Dieter Förster
European Journal of Mineralogy Volume 22 Number 4 (2010), p. 495 - 507
published: Aug 1, 2010
ArtNo. ESP147052204005, Price: 29.00 €
An unique mineral assemblage (graphite + wüstite + wagnerite + sarcopside + monazite-(Ce) + xenotime-(Y) + chlorite), included within fluorapatite, was identified in the S-type Caledonian Rumburk granite, Czech Republic. Graphite appears as a finely dispersed phase within apatite and other minerals and as interstitial flakes in the rock matrix; it is probably of restitic origin related to anatectic melting. Wagnerite, wüstite, monazite-(Ce), and xenotime-(Y) are commonly intergrown, indicating their cogenetic formation. Magmatic fluids, equilibrated with graphite, caused partial metasomatism of early-formed fluorapatite and triggered the formation of wüstite and the various phosphate-mineral inclusions within alteration domains in the apatite host (by dissolution-reprecipitation). The formation of wüstite was facilitated by the coexisting graphite, which maintained the required low oxygen fugacity, and the apatite host, which provided a silica-undersaturated environment. The lower temperature limit of the alteration event is defined by the wüstite stability field (>570 °C). Representative mineral formulae are Fe2+0.67Fe3+0.22〈0.11O (wüstite), (Mg0.87Fe0.97Mn0.15)Σ=1.99(PO4)(F0.64OH0.36) (wagnerite), and (Fe2+2.27Mg0.45Mn0.19)Σ=2.91(PO4)2 (sarcopside). The results of this study improve knowledge about the natural occurrences of wüstite and wagnerite and their paragenetic relations; they permit additional insights into mineral-forming processes in granitic magmas under strongly reducing conditions. This paper provides the first record of wüstite occurring in granite.