PATTERNS OF MN ENRICHMENT IN FINE-GRAINED APATITE FROM THE BLACK MOUNTAIN PEGMATITE, MAINE

In the spodumene- and tourmaline-rich Black Mountain granitic pegmatite (western Maine, USA) apatite occurs as one of three primary phosphate minerals in this complexly zoned body. During the examination of the pegmatite using cathodoluminescence, fine-grained apatite was found to be widely distributed throughout the primary zones and replacement unit of the pegmatite. Millimeter- to micrometer-sized apatite was observed mainly disseminated in albite, and sparingly in microcline, quartz, lithian muscovite and montebrasite. Electron microprobe data provides evidence for the chemical evolution of apatite with respect to Mn enrichment during successive stages of crystallization of the pegmatite.

The Mn content of apatite from the wall zone averages 4.1 wt.% MnO. The average Mn content of apatite during crystallization of the subsequent 1st intermediate, 2nd intermediate, and 3rd intermediate zones shows a steady increase of 1.5 wt.%, 2.5 wt.%, and 3.0 wt.% MnO, respectively. Within the tourmaline-lepidolite-albite and lepidolite-albite units of the 3rd intermediate zones, the MnO concentration of apatite drops sharply to about 0.6 wt. %, but increases again to nearly 2.0 wt.% in late lepidolite-bearing units. The last major unit to crystallize, the replacement body, contains apatite with an average MnO concentration of 1.3 wt.%, similar to apatite found in the 1st intermediate zone of the pegmatite.

The non-linear behavior of Mn enrichment in apatite is attributed to the mineralogy of each zone, where accessory minerals may be present that have a higher affinity for Mn over apatite. Co-precipitation of manganocolumbite and elbaite provides competition for Mn in the melt and may be responsible for the sudden depletion of Mn in apatite from the 3rd intermediate zone. The competition of ions between different mineral phases gives insight in the fractionation patterns on Mn in Black Mountain and provides a possible pathway to determining the trace element chemistry throughout the evolution of the pegmatite.