COLUMBITE GROUP MINERALS FROM THE SOUTH BAY PEGMATITES, MANITOBA, CANADA

Pegmatites at South Bay (SB), Manitoba, Canada, were first described in early 2000’s. Although there has been minor exploration work, there have been no mineralogical investigations.

The SB pegmatites intrude supracrustal and plutonic rocks of the Lynn Lake–Leaf Rapids domains. This domain includes volcanic and associated epiclastic and sedimentary rocks dated at 1.90–1.88 Ga and has been broadly correlated with the La Ronge domain in Saskatchewan. In Manitoba, this domain has been further subdivided into two metavolcanic belts with unique stratigraphic successions, chemistry and structure: the Lynn Lake and Rusty Lake belts. Field relationships suggest that the various volcanosedimentary assemblages were aggregated into a tectonic collage before the emplacement of calcalkaline plutons, which include ca. 1876 Ma gabbro stocks and tonalite. The area is affected by greenschist- to upper-amphibolite–facies metamorphism.

Different types of pegmatites were observed at SB: simple quartz-feldspar-garnet pegmatites, banded pegmatites, and Be-Nb-Ta–bearing pegmatites. The Be-bearing pegmatites are by far the most evolved bodies found in this area. Minerals of the columbite-tantalite group (CGM) were only found in the Be-bearing pegmatites. The studied grains range in size from 20 µm to 5 mm and are subhedral to euhedral. They occur disseminated in the groundmass of both the aplitic and pegmatitic textured portions of the pegmatite. Backscattered electron imagery reveals relatively simple zonation: usually a Nb-enriched core surrounded by a rim with higher Ta content. Overall the CGM exhibit little change in their Fe and Mn content with FeO wt.% ranging from 12.72 to 16.56 and MnO wt.% varying from 3.28 to 4.44.Tantalum and Nb content present a wider variation with values of Ta₂O₅ varying from 10.78 to 56.49 wt.% and Nb₂O₅ ranging from 68.95 to 26.84 wt.%. A constant value of Fe and Mn is observed, contrasting with a rapid increase in Ta content. The rise in the Ta content could be explained by fractional crystallization of the pegmatitic melt, whereas the stable values for Fe and Mn could reflect garnet crystallization contemporaneously with the CGM. Crystallization of garnet could have buffered Fe and Mn activities and thus caused local decoupling of Fe and Mn from Nb and Ta.