Origin of coexisting wustite, MgFe and REE phosphate minerals in graphite-bearing fluorapatite from the Rumburk granite

Abstract

An unique mineral assemblage (graphite + wustite + 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, wustite, 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 wustite and the various phosphate-mineral inclusions within alteration domains in the apatite host (by dissolution–reprecipitation). The formation of wustite 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 wustite stability field (>570 °C). Representative mineral formulae are Fe 2+ 0.67 Fe 3+ 0.22 □ 0.11 O (wustite), (Mg 0.87 Fe 0.97 Mn 0.15 ) ∑=1.99 (PO 4 )(F 0.64 OH 0.36 ) (wagnerite), and (Fe 2+ 2.27 Mg 0.45 Mn 0.19 ) ∑=2.91 (PO 4 ) 2 (sarcopside). The results of this study improve knowledge about the natural occurrences of wustite 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 wustite occurring in granite.