Mineralogy and genesis of pyrochlore apatitite from The Good Hope Carbonatite, Ontario: A potential niobium deposit

摘要

The Good Hope carbonatite is located adjacent to the Prairie Lake alkaline rock and carbonatite complex in northwestern Ontario. The occurrence is a heterolithic breccia consisting of diverse calcite, dolomite and ferrodolomite carbonatites containing clasts of magnesioarfvedsonite + potassium feldspar, phlogopite + potassium feldspar together with pyrochlore-bearing apatitite clasts. The apatitite occurs as angular, boudinaged and schlieren clasts up to 5 cm in maximum dimensions. In these pyrochlore occurs principally as euhedral single crystals (0.1-1.5 cm) and can comprise up to 25 vol.% of the clasts. Individual clasts contain compositionally- and texturally-distinct suites of pyrochlore. The pyrochlores are hosted by small prismatic crystals of apatite (similar to 400-500 mu m x 10-25 mu m) that are commonly flow-aligned and in some instances occur as folds. Allotriogranular cumulate textures are not evident in the apatitites. The fluorapatite does not exhibit compositional zonation under back-scattered electron spectroscopy, although ultraviolet and cathodoluminescence imagery shows distinct cores with thin (<50 mu m) overgrowths. Apatite lacks fluid or solid inclusions of other minerals. The apatite is rich in Sr (7030-13,000 ppm) and rare earth elements and exhibits depletions in La, Ce, Pr and Nd (La/Nd-CN ratios (0.73-1.14) relative to apatite in cumulate apatitites (La/Nd-CN > 13) in the adjacent Prairie Lake complex. The pyrochlore are primarily Na-Ca pyrochlore of relatively uniform composition and minor Sr contents (<2 wt.% SrO). Irregular resorbed cores of some pyrochlores are A-site deficient (>50%) and enriched in Sr (6-10 wt.% SrO), BaO (0.5-35 wt%), Ta2O5 (1-2 wt.%) and UO2 (0.5-2 wt%). Many of the pyrochlores exhibit oscillatory zoning. Experimental data on the phase relationships of haplocarbonatite melts predicts the formation of apatite and pyrochlore as the initial liquidus phases in such systems. However, the texture of the clasts indicates that pyrochlore and apatite did not crystallise together and it is concluded that pyrochlores formed in one magma have been mechanically mixed with a different apatite-rich magma. Segregation of the apatite-pyrochlore assemblage followed by lithification resulted in the apatitites, which were disrupted and fragmented by subsequent batches of diverse carbonatites. The genesis of the pyrochlore apatitites is considered to be a process of magma mixing and not simple in situ crystallisation.