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Pyrochlore-group minerals are relatively common accessory constituents of calcite carbonatite at the Oka Carbonatite Complex, Quebec. This complex is a member of the Cretaceous Monteregian petrological province, and consists predominantly of calcite carbonatite and feldspathoidal silicate rocks. Mineralogical studies of the complex have identified ceroan pyrochlore, ceriopyrochlore, uranoan pyrochlore, uranpyrochlore, thorian pyrochlore and "thoriopyrochlore" as occurring within the NIOCAN and Bond Zone deposits. At Oka, pyrochlore (general structural formula: A16-xB16O48(O,OH,F)8-y) typically occurs as euhedral to subhedral crystals, rarely as aggregates and clusters. Back-scattered electron images, coupled with results of energy-dispersion analyses, reveal complex compositional zoning in pyrochlore, which reflects changes in magma composition during growth. Uranium-rich pyrochlore from Oka exhibits either strong zonation or is devoid of zoning. Large compositional variations were observed for the major oxides of the pyrochlore: CaO (ranging from 4.1 to 34.8 wt.% oxide), TiO2 (2.3-40.4%), Nb2O5 (20.1- 58.1%), ThO2 (0.3-18.2%), and UO2 (0.1-28.0%). Ceriopyrochlore, ceroan pyrochlore and uranpyrochlore exhibit the greatest A-site vacancies, ranging from 8.1 to 62.5%. Of the rare-earth elements, only Ce is present at high levels of concentration (ranging from 2.1 to 15.8% Ce2O3). Of note is the significant content of ZrO2, which ranges from 0 to 16.3 wt.%. An A-site substitution in the pyrochlore-group minerals has been identified between (REE + U + Th) and (Na + Ca), as well as a B-site substitution between (Nb + Ti) and Zr. The wide variety of pyrochlore-group minerals at Oka, coupled with their textural features, indicate that the Oka calcite carbonatite has an extremely complex evolutionary history. The pyrochlore-group minerals crystallized from two or more carbonatitic magmas, enriched in Nb, Ti, LREE, U, and Th. The calcite carbonatites of the NIOCAN and Bond Zone areas are hybrid rocks. No simple hypothesis can be devised to explain the significant concentrations of pyrochlore in particular samples of host rock. Enrichment in specific zones is dependent upon rheological factors rather than compositional controls.

EXTREME COMPOSITIONAL VARIATION OF PYROCHLORE-GROUP MINERALS AT THE OKA CARBONATITE COMPLEX, QUEBEC: EVIDENCE OF MAGMA MIXING?