Oscillatory-zoned crystals of pyrochlore-group minerals from the Guaniamo kimberlites, Venezuela

Oscillatory-zoned crystals of pyrochlore-group minerals are found in the La Ceniza diamondiferous kimberlites, Guaniamo region, Venezuela. This is the first known occurrence of the pyrochlore-group minerals in kimberlites worldwide. The minerals occur in the kimberlite groundmass in association with olivine, zoned spinel, phlogopite, carbonates (dolomite, calcite, magnesite), apatite, tetraferriphlogopite, Fe-Ni-Cu-sulfides and serpentine. They form discrete octahedral crystals or intergrowths (up to 50 μm) with a red-brown core and light brown rim. Individual zones in the cores are remarkably enriched in TiO2 (up to 14.3 wt.%), Ta2O5 (up to 10.1 wt.%), ZrO2 (up to 23.7 wt.%), ThO2 (up to 22.3 wt.%) and UO2 (up to 8.0 wt.%) at moderate Nb2O5 contents, whereas the outer regions contain significant CaO (up to 17.5 wt.%), Na2O (up to 5.7 wt.%), Nb2O5 (up to 58.9 wt.%) and F (up to 4.6 wt.%), approaching the ideal formula (Na,Ca)2Nb2O6F. Following the IMA recommendations and the “50% rule”, the interior is classified as uranoan Zr-rich thoriobetafite/thoriopyrochlore or thorian Zr-rich betafite/pyrochlore, and the outer part as pyrochlore or thorian pyrochlore. Strong compositional zoning is consistent with two substitutions: 2(Th,U)4+ + □ + (Ti,Zr)4+ + O2− → 3Ca2+ + Nb5+ + (F,OH)− and 2(Th,U)4+ + □ + 2(Ti,Zr)4+ + O2− → 2Ca2+ + Na+ + 2Nb5+ + (F,OH)−. The zoning patterns and the absence of perovskite indicate the dominant role of F among fluid components and relatively high alkalinity during crystallization of the silicate-carbonate groundmass of the La Ceniza kimberlite. The composition and zoning of the La Ceniza pyrochlore-group minerals are very specific, but in general strongly resemble those of some pyrochlores from carbonatites.