A new model for the origin of pyrochlore: Evidence from the St Honoré Carbonatite, Canada

Abstract

The Neoproterozoic St Honoré carbonatite complex is one of three currently mined niobium deposits. In these and most other carbonatite-hosted niobium deposits, pyrochlore is the principal ore mineral. Four major pyrochlore-bearing rock types are observed in the St Honoré complex, namely, biotitite, magnetite-biotite rock, apatitite and carbonatite. The textural relationships among the different rock types and within each rock type are extremely complex, as are the textures and crystal chemistry of the pyrochlore. Compositionally, there are two major types of pyrochlore. Type-1 is enriched in Ta, U, Zr, Sr, Th, Fe, REE and Cl and Type-2 is a Ca-Na-F-rich pyrochlore, with proportions of other elements close to or below their detection limits. Type-1 pyrochlore is yellow- to red-brown and, in many cases, displays oscillatory zoning. This variety occurs most commonly in the cores of crystals in apatitite, where it has its highest concentrations of the elements listed above. It also is observed in biotitite, but the proportions of the above elements are much lower. Type-2 pyrochlore is light to dark brown and, generally, does not exhibit oscillatory zoning. It forms overgrowths on Type-1 pyrochlore in apatitite and biotitite, and occurs as large crystals in the magnetite-biotite rock (Type-1 pyrochlore is not present in this rock). We propose that Type-1 pyrochlore crystallised from a highly evolved, fluid-undersaturated carbonatitic magma, whereas Type-2 pyrochlore crystallised from a carbonatitic magma that had undergone fluid exsolution, which depleted the magma in elements that tend to partition preferentially into aqueous fluids, i.e., U, Sr, Fe, REE, Ba and Cl.The processes that governed the evolution of the carbonatitic magma (which was mantle-derived and introduced in numerous small batches) and crystallisation of pyrochlore at St Honoré were biotitisation (by the magma), crystallisation of calcite and aqueous fluid exsolution. Biotitisation of the host syenite consumed MgO, H2O and CO2 from each magma batch (the CO2 was released as a gas), leading to the crystallisation of Type-1 pyrochlore in biotitite. This process also locally increased the CaO content of the magma to a level sufficient to saturate it with calcite, leaving a residual phoscoritic magma that crystallised the most Ta-U-enriched variety of Type-1 pyrochlore. Exsolution of the aqueous fluid occurred as a result of prolonged crystallisation and/or pressure release and led to the crystallisation of Type-2 pyrochlore. The evolution of each of the magma batches was different and depended on the extent of biotitisation, on whether calcite crystallised, and on the timing of fluid exsolution. As the two types of pyrochlore and their temporal relationship have been described for many carbonatite complexes, the hypotheses presented in this paper are likely also applicable to the genesis of pyrochlore in these other complexes.