Textures and chemical evolutions in tantalum oxides: a discussion of magmatic versus metasomatic origins for Ta mineralization in the Tanco Lower Pegmatite, Manitoba, Canada

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Short course on the application of fluid inclusion techniques to the study of hydrothermal ore deposits

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THE ROLE OF METAGABBRO RAFTS ON TANTALUM MINERALIZATION IN THE TANCO GRANITIC PEGMATITE, MANITOBA

International audienceAlthough 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

Trace element geochemistry by laser ablation ICP-MS of micas associated with Ta mineralization in the Tanco pegmatite, Manitoba, Canada

International audienceIn the Tanco pegmatite, one of the world's major Ta deposits, tantalum mineralization shows a complexity that reflects the complex petrogenesis of its host pegmatite. Micas are common in most of the pegmatite units and are intimately associated with the successive stages of Ta mineralization, from the wall zone to the central zones where micaceous replacement is pervasive. Different generations of micas, both primary and secondary, associated with Ta oxides, were selected for electron microprobe and laser ablation ICP-MS investigation. Their chemical trends are used to constrain the magmatic versus hydrothermal processes that played a role in their crystallization and their associated Ta mineralization. Micas range from dioctahedral muscovite to trioctahedral lepidolite through Al↔Li substitution. Unexpectedly, the most evolved compositions (low K/Rb ratios and high Li contents) occur in the wall zone; they are interpreted to reflect nonequilibrium crystallization from an undercooled melt, with or without boundary layer effects. In the central zones, the fine-grained mica–quartz assemblage hosts some coarser-grained Li-muscovite, which has the highest Ta contents (up to 400 ppm). These Li–F–a-rich micas are interpreted to originate from a magmatic metasomatic event, which was also at the origin of the MQM-style Ta mineralization at Tanco. However, the Li–Ta-poor, muscovite end-member compositions of fine-grained alteration micas suggest crystallization from an aqueous fluid, during a metasomatic (hydrothermal) event involving late pegmatitic fluids. The low Ta concentrations (around 50 ppm) of this fine-grained muscovite suggest that this fluid transported at least small amounts of Ta

Behavior of gold in a magma at sulfide-sulfate transition: Revisited

We have investigated experimentally the partitioning of Au between solid and liquid sulfide phases and basaltic melts at 200 MPa, at redox conditions close to the sulfide-sulfate transition, over temperatures between 1050 and 1200 °C, which span the monosulfide solid solution (MSS) - sulfide liquid (SuL) solidus. The measured MSS/basalt partition coefficient of Au (D Au MSS-sil) is about 100-200, whereas the partition coefficient of sulfide liquid/basalt (DAu SuL-sil) is approximately 10 times larger at 2200. Although we find that temperature, pressure, and oxygen fugacity (fO2) exert relatively weak controls on Au partitioning, they exert major indirect influences on Au behavior by controlling the identity of the condensed sulfide phase and b