Timing and sources of skarn mineralization in the Canadian Tungsten Belt: revisiting the paragenesis, crystal chemistry and geochronology of apatite

Five generations of fluorapatite in mineralized skarn and host rocks from the Mactung W (Cu-Au) deposit, Northwest Territories, Canada, are identified based on petrographic, compositional and geochronological (U–Pb) data. These data, coupled with new (in this study) and previously published data on apatite from the nearby Cantung deposit, provide constraints on the timing of skarn mineralization, as well as metal and fluid sources of the Canadian Tungsten Belt. Type-i apatite of the Mactung deposit formed from ~ 106 ± 4 to 103 ± 2 Ma through recrystallization of sedimentary apatite (type-o apatite) during regional metamorphism, pre-skarnification. Type-i apatite is W-rich (up to 47.6 ppm) and occurs with coeval scheelite and titanite, indicating a potential sedimentary source, perhaps from detrital rutile, for W. Apatite crystals in prograde (type-ii) and retrograde (type-iii and type-iv) skarns yield ages from ~ 96 ± 1 to 92 ± 1 Ma, overlapping with Mactung biotite-granite plutons and late-stage felsic dykes and confirming skarn formation during emplacement of the granites over a period of ~ 5 million years. Type-ii apatite contains high rare earth element that increases with increasingly negative Eu anomalies, suggesting prograde fluids were sourced from a felsic melt undergoing fractional crystallization. Retrograde apatite exhibits weak lanthanide tetrad effects with superchondritic Y/Ho ratios (> 38), suggesting retrograde fluids exsolved from a highly evolved magmatic source. Apatite crystals from the Cantung skarn deposit are compositionally and paragenetically similar to those from the Mactung apatite and yield ages similar to the Cantung biotite-monzogranite plutons and late-stage felsic dykes. We conclude prograde fluids were derived from biotite-granites, whereas retrograde fluids exsolved from evolved melts recorded by later felsic dykes.