THE ROLE OF METAGABBRO RAFTS ON TANTALUM MINERALIZATION IN THE TANCO GRANITIC PEGMATITE, MANITOBA
Résumé
Although it is a common observation that tantalum-bearing pegmatites intrude mafic volcanic rocks or gabbros, there have been few studies of the effects of these rocks on Ta mineralization. The Tanco granitic pegmatite, in southeastern Manitoba, not only is emplaced in metagabbros, but it also encloses rafts of the host metagabbro, and mine geologists have observed abnormally high Ta concentrations close to these rafts. We investigate the chemical role of the metagabbro rafts on Ta mineralization in a high-grade Ta part of the mine. The chosen area consists of cells that mimic the Tanco pegmatite, i.e., it is a smaller and simpler version of the whole body. The composition of columbite-group minerals (CGM) show an increase in Mn/(Mn + Fe) (Mn*) and Ta/(Ta + Nb) (Ta*) from the border to the center of each pegmatite cell, which conforms to normal fractionation in LCT pegmatites. Moreover, the increase in Mn* precedes the increase in Ta*. We compared this evolution in different locations with respect to the raft; the Mn* and Ta* evolution seems similar in all areas, even where no rafts are present. Consequently, we conclude that the rafts do not appear to have any chemical influence on the crystallization of CGM. However, the rafts did act as a physical barrier that separated distinct pegmatite cells that evolved independently from the whole body. The wide range of Mn* and Ta* values close to the metagabbro can be explained by boundary-layer effects. Because of the lower diffusivities of Ta and Nb in the melt compared to the rate of crystal growth, the boundary layer reaches a very high Ta* value that is not representative of that of the bulk melt. At the scale of a CGM crystal, the Ta* and Mn* values do not follow typical fractionation-induced trends. We conclude that the evolution of composition at the centimetric scale is controlled not only by fractionation, but also by changes in diffusivity and solubility of trace elements, which may have had important roles.