Origin of coexisting wuestite, Mg-Fe and REE phosphate minerals in graphite-bearing fluorapatite from the Rumburk Granite

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 degrees C). Representative mineral formulae are Fe (super 2+) (sub 0.67) Fe (super 3+) (sub 0.22) [] (sub 0.11) O (wustite), (Mg (sub 0.87) Fe (sub 0.97) Mn (sub 0.15) ) (sub Sigma =1.99) (PO (sub 4) )(F (sub 0.64) OH (sub 0.36) ) (wagnerite), and (Fe (super 2+) (sub 2.27) Mg (sub 0.45) Mn (sub 0.19) ) (sub Sigma =2.91) (PO (sub 4) ) (sub 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.