Nb-mineralization of the Oka Carbonatite Complex, Quebec

Abstract

A representative collection of minerals &om two niobium deposits found within the Oka carbonatite complex: the Bond Zone deposit; and the NIOC AN deposit, were analysed using an electron microprobe, and compared with existing data from the St. Lawrence Columbium deposit. These minerals are good indicators of the petrological evolution of alkaline rocks, and can be used to re-evaluate the paragenesis of niobium mineralization, and the relationships between Nb-bearing minerals. In addition, this study has revealed the presence of zirconolite as an accessory mineral in calciocarbonatite. At Oka, pyrochlore typically occurs as euhedral- to -subhedral crystals, rarely as aggregates and clusters. Back-scattered images, coupled with microprobe analyses, reveal complex compositional zoning in pyrochlore, which undoubtedly reflects changes in the fluid composition during the growth of the mineral. Large compositional variations were observed for the major oxides of the pyrochlores: CaO (ranging f-om 4.1-34.8 wt. % oxide), TiO2 (2.3-40.4 wt. %), Nb2O5 (20.1-58.1 wt. %), ThO2 (0.3-18.2 wt. %), and UO2 (0.1-28.0 wt. %). Ceriopyrochlore, cerium pyrochlore, and uranpyrochlore exhibit the greatest .4-site vacancies, ranging &om 8.1-62.5 %. Of the REEs, only Ce is present at high concentration levels (ranging &om 2.1-15.8 wt. %). Of note is the significant content of ZrO2, which ranges from 0.9-16.3 wt. %. An .4-site substitution in the pyrochlore-group minerals has been identified between (REE+U+Th) and (Na+Ca), as well as a 5-site substitution between (Nb+Ti) and Zr. Latrappite and Nb-rich members of the perovskite-group are found occurring as euhedral to -subhedral crystals. Both oscillatory and patchy zonation in the Oka perovskites has been identified using back-scattered imaging. Small compositional variations are observed in the mtyor oxides of both the latrappite- and -perovsldte end-members. Oka perovskites are slightly enriched in the LREE's, Ce being the dominant LREE (averaging 3.5 wt. % oxide). Zirconolite is commonly found as lath-shaped discrete crystals or intergrown with perovskite and pyrochlore-group minerals. Oka zirconolites have a large range of Nb2O5, ranging from 11.5-25.8 wt. %. Compositional zoning has been identified using back-scattered imaging, correlating with an increase in LREE toward the rim of the crystals. The zirconolite compositions are similar to other calciocaibonatite-hosted zirconolites, with the exception of their higher Nb contents. The crystallization history of the NIOCAN and Bond Zone deposits cannot be deduced from the observed mineral assemblage. The calciocarbonatite does not represent a liquid composition, as it has a bulk composition which is determined by mixing material derived from several batches of magma. The magmas with gave rise to the NIOC AN and Bond Zone seem more evolved than those forming the pyrochlore-group minerals in the St. Lawrence Columbium deposits, as the pyrochlores from St. lawrence Columbium are "less-evolved" in relation to the NIOCAN and Bond Zone pyrochlores. The major conclusion of this work is that the calciocarbonatites at NIOCAN and Bond Zone are hybrid rocks. Enrichment of specific mineralized zones is probably dependent upon rheological 6ctors rather than compositional controls.