Three-D Mineralogical Mapping of the Kovdor Phoscorite-Carbonatite Complex, NW Russia: III. Pyrochlore Supergroup Minerals

Abstract

The pyrochlore supergroup minerals (PSM) are typical secondary phases that replace (with zirconolite-laachite) earlier Sc-Nb-rich baddeleyite under the influence of F-bearing hydrothermal solutions, and form individual well-shaped crystals in surrounding carbonatites. Like primary Sc-Nb-rich baddeleyite, the PSM are concentrated in the axial carbonate-rich zone of the phoscorite-carbonatite complex, so their content, grain size and chemical diversity increase from the pipe margins to axis. There are 12 members of the PSM in the phoscorite-carbonatite complex. Fluorine-and oxygen-dominant phases are spread in host silicate rocks and marginal carbonate-poor phoscorite, while hydroxide-dominant PSM occur mainly in the axial carbonate-rich zone of the ore-pipe. Ti-rich PSM (up to oxycalciobetafite) occur in host silicate rocks and calcite carbonatite veins, and Ta-rich phases (up to microlites) are spread in intermediate and axial magnetite-rich phoscorite. In marginal (apatite)-forsterite phoscorite, there are only Ca-dominant PSM, and the rest of the rocks include Ca-, Na-and vacancy-dominant phases. The crystal structures of oxycalciopyrochlore and hydroxynatropyrochlore were refined in the Fd3m space group with R 1 values of 0.032 and 0.054 respectively. The total difference in scattering parameters of B sites are in agreement with substitution scheme B Ti 4+ + Y OH − = B Nb 5+ + Y O 2−. The perspective process flow diagram for rare-metal "anomalous ore" processing includes sulfur-acidic cleaning of baddeleyite concentrate from PSM and zirconolite-laachite impurities followed by deep metal recovery from baddeleyite concentrate and Nb-Ta-Zr-U-Th-rich sulfatic product from its cleaning.

Lamprophyre-Carbonatite Magma Mingling and Subsolidus Processes as Key Controls on Critical Element Concentration in Carbonatites—The Bonga Complex (Angola)

“…Secondary pyrochlore generations are found in many of these lithological units where they have been altered (Table 1). Similar late occurrences of pyrochlore are found in many carbonatites such as Tchivira in Angola [67,68,75] or Kovdor in Kola, Russia [95].…”

Evolution of pyrochlore in carbonatites of the Amba Dongar complex, India.

“…The compositional evolution and heterogeneity of pyrochlore reflects with changes in temperature, redox potential and chemical activities in its crystallisation environment. Variations in cation and anion composition are typical of pyrochlore from intrusive carbonatites and have been studied, for example, at Oka and Aley in Canada, at Khibiny, Lovozero, Kovdor, Vuoriyarvi and Seblyavr in Russia and in Sokli in Finland (Hogarth, 1989;Hogarth et al, 2000;Subbotin and Subbotina, 2000;Chakhmouradian and Mitchell, 2002;Chakhmouradian and Williams, 2004;Chakhmouradian et al, 2015;Zurevinski and Mitchell, 2004;Zaitsev et al, 2012;Lee et al, 2006;Ivanyuk et al, 2018).…”

Three-D Mineralogical Mapping of the Kovdor Phoscorite-Carbonatite Complex, NW Russia: II. Sulfides

“…The Kovdor phoscorite-carbonatite complex has gradual concentric zonation in terms of the rocks modal and chemical composition, as well as grain size, chemical composition, crystallochemical features and properties of rock-forming and accessory minerals [11,21,22,24,[26][27][28][29]. This zonation mainly results from gradual change in chemical composition of the residual phoscorite melt from silicate-rich to carbonate-rich during crystallization from the pipe margins to its axial zone [22].…”