Fluorite Mineralization at the Dwyer Mine, Wilberforce Area, Ontario, Canada: Microtextural Indications of a Fluor-Calciocarbonatite

The ore at the Dwyer fluorite mine, near Wilberforce, Ontario, consists of calcite–fluorite dikes that show clear signs of flowage. Those dikes and the large-scale development of fenites at the expense of a granite–monzonite pluton can only be explained by the existence of a subjacent body of carbonatite. The dikes consist of ribbons of calcite and fluorite and contain subhedral crystals of fluorapatite aligned with the ribbons. The dikes also carry crystals of aegirine-augite, titanite, and bastnäsite-(Ce). Both the fluorapatite and aegirine-augite contain micrometric globules of boundary-layer melt that crystallized in situ to calcite, fluorite, quartz, bastnäsite-(Ce), hematite, and titanite. Fragments of the REE-enriched fenite show signs of incipient rheomorphism at a temperature estimated to be at least 725 °C. The large-scale alkali metasomatism occurred toward the end of the Grenville orogenic cycle, at a time of crustal relaxation, roughly 200 million years after emplacement of a granite–monzonite pluton. By analogy with occurrences elsewhere, it is likely that the carbonatitic melt separated immiscibly from a nepheline syenitic parental melt. Fluor-calciocarbonatitic magmatism likely is genetically linked to the U and Th mineralization in the area and contributed to the unusual geological complexity of the Bancroft–Haliburton region.