The association of niobian rutile, columbite-group minerals and uranium-rich pyrochlore in the Rancul granitic pegmatite, San Luis, Argentina

Abstract

We describe the mineralogy of the association of accessory niobian rutile, columbite-group minerals and uranium-rich pyrochlore that occur in the Rancul property, an NYF intragranitic pegmatite of the Potrerillos group, San Luis, Argentina. Other accessory and secondary minerals include biotite, muscovite, beryl, schorl, apatite, ilmenite, pyrite, bismuthinite, clinobisvanite, hematite, and “limonite”. Niobian rutile occurs as rare cm sized prismatic crystals in the core-margin assemblage usually enclosed in quartz or albite. It has Ti# [Ti/(Ti+Nb+Ta)] variable between ~ 0.81 and 0.92 and contents of TiO₂ between 63 and 80 wt.%, Nb₂O₅ ≤ 21.66%, most of the iron as Fe₂O₃, low UO₂ and higher Sc₂O₃. The contents (wt.%) of Ta₂O₅ are variable, reaching up to 7.86, of WO₃ ≤ 1.46, of SnO₂ ≤ 1.12, of Sc₂O₃ ≤ 0.37 and of ZrO₂ ≤ 0.15. The structure is that of monorutile (space group P4₂/mnm) and the cell parameters are a = 4.622 (2) Å; c = 2.983 (2) Å; V = 63.73 (5) ų. The rutile has inclusions of a columbite-group mineral, likely ixiolite or disordered columbite, and a U-rich pyrochlore-group mineral with up to 53.66 wt.% of UO₂ accompanied by niobian rutile with higher Ti# and lower content of trace elements. They are not exsolution products and were produced along subparallel cracks possibly by Ca,Mn,U-rich aqueous fluids that reworked the primary niobian rutile leaching the trace elements and crystallizing the new mineral phases.