Pyrochlore-group minerals from the Loe-Shilman Carbonatite Complex, NW Pakistan: implications for evolution of carbonatite system
This study reports new occurrence of niobium mineralization from the Loe-Shilman carbonatite complex, NW Pakistan. The complex is predominantly comprised of calcite carbonatite, intruded by dolomite carbonatite, and minor late-stage carbonatite veins, intruding the earlier two varieties. Pyrochlore group minerals, which are the major Nb phases in the carbonatites and overlying supergene laterite, manifest numerous textural and compositional discrepancies. Pyrochlores observed in calcite and dolomite carbonatites of the complex are typified by oscillatory zoning, and are compositionally Ca-Na-pyrochlore, and therefore, illustrating crystallization under primary magmatic conditions. However, relatively Ta poor (2.42-5.11 wt%) and Nb rich (60.04-62.14 wt%) pyrochlores preserved in dolomite carbonatite are reflecting a more evolved nature of the dolomite carbonatite compared to Ta rich (6.89-10.11 wt%) pyrochlore of the calcite carbonatite. In addition, pyrochlores of the late-stage carbonatite veins record patchy zonation and are characterized by A-site vacancies, likely due to leaching of Na and Ca, and enrichment of Ba-REE-Sr during hydrothermal alteration by a relatively low pH, Na and/or Si poor fluids. Similarly, intensively patchy zoned, porous and Fe3+ bearing A-site deficient bariopyrochlore in supergene laterite are produced by complete leaching of Ca and Na from A-site, and by partial replacement of strongly-bounded Nb at B-site by Fe3+ during supergene conditions.
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