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

Abstract

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 magnesio-arfvedsonite + 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 (~100–500 μm × 10–25 μm) 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 μm) 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/NdCN ratios (0.73–1.14) relative to apatite in cumulate apatitites (La/NdCN > 1.5) 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 (0.5–3.5 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.

Oscillatory- and sector-zoned pyrochlore from carbonatites of the Kerimasi volcano, Gregory rift, Tanzania

“…Correlation between individual elements is present, although not very strong, suggesting different evolutionary trends for different elements. Our new and previously published data (Hogarth et al , 2000; Subbotin and Subbotina, 2000; Zurevinski and Mitchell, 2004; Walter et al , 2018; Mitchell et al , 2020; Lee et al , 2006) have revealed some common features of pyrochlore oscillatory zoning. The zoning is variable: it can be found throughout the whole crystal or in limited regions; there are also crystals without oscillatory zoning.…”

“…Oscillatory zoning is more common than sector zoning in pyrochlore crystals (Hogarth et al , 2000; Zurevinski and Mitchell, 2004; Lee et al , 2006; Sharygin et al , 2009; Walter et al , 2018; Mitchell et al , 2020). Crystals from both Loluni and Kisete are also characterised by a distinct μm-scale oscillatory zoning (Figs 4, 5, 7, 10).…”

Geochemical and spectroscopic investigation of apatite in the Siilinjärvi carbonatite complex: Keys to understanding apatite forming processes and assessing potential for rare earth elements

“…7a-c). This blue CL color is due to REE activation, mostly Eu 2+ (e.g., Marshall, 1988;Mitchell et al, 1997;Blanc et al, 2000;Kempe and Götze, 2002;Mitchell et al, 2014Mitchell et al, , 2020. A stronger activation by Sm 3+ produces violet shades in the earliest zones (cores) of the crystals (Fig.…”

Compositional variation and genesis of pyrochlore, belkovite and baotite from the Sevattur carbonatite complex, India

“…zH 2 O in late-stage pyrochlore (Hogarth et al, 2000;Zurevinski and Mitchell, 2004;Melgarejo et al, 2012;Chakhmouradian et al, 2015;Mitchell, 2015;Mitchell et al, 2020). This compositional variation implies an increase in Ba, Sr, REE, Pb, Si, H 2 O and vacancies resulting in deviation from ideal stoichiometry associated with the progressive alteration of magmatic pyrochlore (Lumpkin and Ewing, 1995;Lumpkin and Mariano, 1995;Williams et al, 1997;Chakhmouradian and Mitchell, 2002;Chakhmouradian et al, 2015;Mitchell, 2015).…”