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

Abstract

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 TiO 2 (up to 14.3 wt.%), Ta 2O 5 (up to 10.1 wt.%), ZrO 2 (up to 23.7 wt.%), ThO 2 (up to 22.3 wt.%) and UO 2 (up to 8.0 wt.%) at moderate Nb 2O 5 contents, whereas the outer regions contain significant CaO (up to 17.5 wt.%), Na 2O (up to 5.7 wt.%), Nb 2O 5 (up to 58.9 wt.%) and F (up to 4.6 wt.%), approaching the ideal formula (Na,Ca) 2Nb 2O 6F. 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+ + O 2− → 3Ca 2+ + Nb 5+ + (F,OH) − and 2(Th,U) 4+ + □ + 2(Ti,Zr) 4+ + O 2− → 2Ca 2+ + Na + + 2Nb 5+ + (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.