Extreme Compositional Variation of Pyrochlore-Group Minerals at the Oka Carbonatite Complex, Quebec: Evidence of Magma Mixing?

Abstract

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: A 16-x B 16 O 48 (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), TiO 2 (2.3-40.4%), Nb 2 O 5 (20.1-58.1%), ThO 2 (0.3-18.2%), and UO 2 (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% Ce 2 O 3 ). Of note is the significant content of ZrO 2 , 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.Keywords: calcite carbonatite, pyrochlore-group minerals, magma mixing, Oka carbonatite complex, Quebec. SOMMAIRELes minéraux du groupe du pyrochlore sont des phases accessoires assez répandues dans la carbonatite calcitique du complexe d'Oka, Québec. Ce complexe fait partie de la province pétrographique Montérégienne, d'âge crétacé, et contient surtout des venues de carbonatite à calcite et des roches silicatées à feldspathoïdes. Nous avons identifié le pyrochlore enrichi en cérium, le cériopyrochlore, le pyrochlore uranifère, l'uranpyrochlore, le pyrochlore thorifère et le "thoriopyrochlore" dans les gisement de NIOCAN et de la zone Bond. A Oka, le pyrochlore [formule structurale générale: A 16-x B 16 O 48 (O,OH,F) 8-y ] se présente généralement en cristaux idoimorphes ...

Evidence for the Multi-Stage Petrogenetic History of the Oka Carbonatite Complex (Québec, Canada) as Recorded by Perovskite and Apatite

“…Given the extremely large variations in trace element abundances recorded by apatite (Table 5) [14], and those depicted by perovskite investigated here (Figures 4-7; Tables 2 and 3), it is difficult if not impossible to attribute these variations to closed-system melt differentiation involving solely one parental melt, regardless of whether this melt was carbonatitic, or a carbonate-rich, alkaline, silica-undersaturated parental melt [14]. Chen and Simonetti [14] advocated for open-system behavior, possibly involving magma mixing, which is an interpretation also put forward by Zurevinski and Mitchell [68] to explain the chemical variations documented by pyrochlore from Oka. In this study, Figures 8 and 9 (and Tables 4 and 5) clearly indicate that the chemical compositions for apatite from alnöite and jacupirangite are distinct relative to those from other rock types.…”

“…In addition, Samson et al [87] also advocated for the occurrence of late-stage hydrothermal activity at Oka as recorded by fluid inclusions within constituent minerals. The highly variable chemical compositions, Nd and Sr isotope ratios, and ages documented for perovskite, pyrochlore, niocalite, and apatite from the different rock types associated with the Oka carbonatite complex indicate that these formed as a result of episodic, small volume partial melting and subsequent magma mixing [14,15,68]. However, Figure 13B shows that there is a positive correlation between the total REE contents and U-Pb ages for the perovskite grains investigated here (excluding the two reversely zoned grains).…”

Ti-Nb Mineralization of Late Carbonatites and Role of Fluids in Its Formation: Petyayan-Vara Rare-Earth Carbonatites (Vuoriyarvi Massif, Russia)

“…(1) Fluorcalciopyrochlore with the fully filled A-sites is the main variety in magmatic calcite [29,[46][47][48] and dolomite [31,49] U-Th and Ca-F-varieties represent early generations of the Petyayan-Vara pyrochlore and are commonly interpreted as magmatism products. However, the structural position of the pyrochlore localized in mineralized veinlets supports its fluid nature, which is also indicated by some of its chemical features.…”

Liandratite from Karkonosze pegmatites, Sudetes, Southwestern Poland

“…It is based on the observation that the primary mineral is altered first to a phase with the composition of the pyrochlore group minerals Table S2. Please refer also to Hogarth and Horne (1989), Lumpkin and Ewing (1996), Uher et al (1998), Chakhmouradian and Mitchell (2002), Zurevinski and Mitchell (2004), Caprilli et al (2006), Monchoux et al (2006), Mokhov et al (2008), Abd El-Naby (2009), Timofeev and Williams-Jones (2015) or directly to Pb-, Bi-, U-rich pyrochlores, and then possibly to liandratite (i.e. Fig.…”