Accessory phases from the Soultz monzogranite, Soultz-sous-Forêts, France: Implications for titanite destabilisation and differential REE, Y and Th mobility in hydrothermal systems

The metaluminous Soultz-sous-Forêts monzogranite, France, is highly evolved and contains elevated concentrations of rare-earth elements (REE), Y and particularly Th. Primary accessory minerals include fluorapatite, allanite-(Ce) and Th-rich titanite. Primary titanite has been altered to anatase + calcite + quartz + synchysite-(Ce) ± bastnaesite-(Ce) or anatase + calcite + quartz + monazite-(Ce) + xenotime-(Y) ± thorite. Fluorocarbonate-bearing assemblages are restricted to those samples exhibiting minor selective alteration, whereas those containing phosphate-rich assemblages formed in pervasively altered samples that have experienced high fluid/rock ratios. Comparative electron-microprobe analysis of primary and hydrothermally-derived accessory phases found middle REE, Y and Th concentrations depleted in synchysite-(Ce) relative to primary titanite. Such depletions are not seen in phosphate-rich samples containing monazite-(Ce) and xenotime-(Y). Variability in elemental concentrations may be attributed to distinct fluid chemistries and hence, lead to differential mobility during alteration. Following previous experimental work and mineralogical observations, the ingress of CO2-rich solutions was integral for titanite breakdown and the resultant metasomatic assemblage. The influx of CO2-rich fluids concomitantly with chloritisation of biotite produced fluids enriched in FCO3−. We, therefore, hypothesise that after the alteration of titanite, remnant HCO3− or FCO3−-rich fluids were able to mobilise significant proportions of MREE, Y and Th not accommodated into the synchysite-(Ce) structure. Conversely, those samples rich in monazite-(Ce) and xenotime-(Y) retained their REE, Y and Th concentrations due to the presence of aqueous HPO42− derived from apatite dissolution.