APATITE-HOSTED MELT INCLUSIONS FROM GRENVILLE CARBONATITIC VEIN DIKES AT THE SCHICKLER OCCURRENCE AND DWYER MINE, ONTARIO, CANADA

Apatite-hosted inclusions dominated by calcite or fluorite from the Schickler occurrence and Dwyer mine, two fluorite-bearing carbonatitic “vein dikes” located near Wilberforce, Ontario, are interpreted to have been trapped as melts. Scanning electron microscopy and X-ray energy dispersive spectroscopy (SEM-EDS) were used to characterize exposed inclusions within polished apatite crystals. Many inclusions are interpreted to have crystallized from trapped melt based on rounded or globular morphology, internal mineralogy, and textures. Most observed melt inclusions are monomineralic, however, more complex polyphase examples are not uncommon. Polyphase melt inclusions may contain combinations of calcite, fluorite, pyrite, hematite, thorite, uraninite, quartz, REE-carbonates (including bastnäsite-Ce), as well as a Y-bearing phase (possibly yttrialite-Y). All phases were identified using SEM-EDS. Textures suggest both mixed trapping of phases (melt and solid) and formation of daughter crystals from melt inclusions. Apatite from the Schickler occurrence additionally hosts a profusion of micro-mineral inclusions, found primarily within regions of alteration. Apatite from the Dwyer mine does not show this same post-growth alteration and inclusion abundance. Most micro-inclusions are segregated into lamellae within apatite which show a distinctive decrease in BSE intensity compared to adjacent, mostly inclusion free lamellae. The concentration of REE, U, and Th is greater in lamellae with higher BSE intensity. These are some of the major constituents of inclusions found in lamellae with lower intensity. All phases having crystallized as daughters within melt inclusions are also present as micro-inclusions within apatite, but not all apatite micro-inclusion phases are represented in melt inclusion daughter mineralogies. Textural and mineralogical characteristics exhibited by samples indicate carbonate-halide melt immiscibility and heterogeneity at these localities, an igneous origin for Grenville vein dikes, as well as alteration by a late-stage hydrothermal fluid (possibly sourced from the magma body during cooling). While some elemental constituents of inclusions within apatite could be sourced from the alteration fluid, others were likely sourced from the apatite itself.