A first issue to discuss is the spectacularly high solubility of the REEs in the parent fluid of xenotime (Fig. 4). The experimentally determined solubility product of end-member xenotime (YPO 4 ) at 100 °C and 0.1 MPa is extremely low at 10−27 (Migdisov et al., 2016). Because the Wolverine xenotime is a REE solid solution, its solubility product will be even lower than that of the end member. Our analyses of 10−3 mol/kg Y and 10−5–10−6 mol/kg for individual REEs therefore imply vanishingly small concentrations of P in the fluid in the presence of stable aqueous complexes of the metals. Both of the anions detected in the inclusions, Cl– and SO 4 2–, are known to be effective ligands for Y and REEs (Migdisov et al., 2016), and the presence of coeval hematite with xenotime suggests that aqueous sulfur was predominantly in the oxidized state. The first formation constant for the SO 4 2– complex is ∼104.6 (Migdisov et al., 2016), and although the corresponding value for the Cl– complex is about three orders of magnitude lower (Migdisov et al., 2016), Cl– is highly concentrated in the Wolverine fluid. Fluoride is also known to complex REEs exceptionally well at low temperatures (Migdisov et al., 2016), but its concentration in the Wolverine fluid must have been very low, as no fluorite has been found despite aqueous Ca being present in excess. Similarly, OH– and CO 3 2– ligands can be ruled out because the observed sericitization of wall-rock feldspar points to low pH. It is therefore most likely that the high total solubilities of Y and REEs are due to complexing by SO 4 2– and Cl–. Unfortunately, current thermodynamic models do not allow the speciation to be calculated rigorously for this high-salinity fluid in equilibrium with the xenotime solid solution.