Fluoritites Produced by Crystallization of Carbonate–Fluoride Magma

Abstract

Fluorite-dominated rocks are occasionally found in association with carbonatites, but their geologic and petrologic relations are rarely reported. The Dunkeldyk area of the Pamir mountains in south-eastern Tajikistan contains dikes of distinctive rocks composed of calcite, fluorite, celestine-barite, sulfides, apatite, with minor quartz, biotite, and REE-fluorcarbonates. The dikes have sharp contacts with the host (meta-)sedimentary rocks and layering with ribbons, ranging from fluorite-bearing calcite carbonatites to fluoritites (rocks with >50% fluorite). The fluoritites are characterised by high Ca, F, Ba, Sr, REE and S coupled with anomalously low O. The geologic relations and textures suggest a magmatic origin of the dikes from melts close to calcite-fluorite eutectic that experienced nucleation-controlled differentiation during the crystallization of dikes and the formation of fluoritite cumulates in larger intrusions. The Dunkeldyk dikes demonstrate that sizable geological bodies of fluorite-dominated rocks could form from carbonate-fluoride melts originating by differentiation of alkaline silicate magmas.