Petrogenesis of Zr–Nb (REE) carbonatites from the Arbarastakh complex (Aldan Shield, Russia): Mineralogy and inclusion data

Abstract

The Arbarastakh Neoproterozoic ultramafic carbonatite complex is located in the southwestern part of the Siberian Craton (Aldan Shield) and contains ore-bearing Zr–Nb (REE) carbonatites and phoscorites. Carbonatites are mainly represented by calcite and silicocarbonatite varieties. The primary minerals composing the carbonatites are calcite and dolomite, as well as phlogopite, clinopyroxene, fluorapatite, amphibole, fluorite, K-feldspar and feldspathoids. Olivine (forsterite), Ti-magnetite, apatite, phlogopite, calcite, dolomite and the minor spinel group minerals form the primary phoscorites. The ore-bearing Zr–Nb mineral assemblages of the phoscorites and carbonatites include accessory zircon, zirconolite, perovskite, pyrochlore and baddeleyite. The Ba–Sr–REE hydrothermal mineralisation consists of ancylite-(Ce), bastnaesite-(Ce) and burbankite, as well as barite-celestite, strontianite, barytocalcite, and rare Cu–Fe sulphides. The silicocarbonatites and carbonatites formed in multiple stages from a single alkaline Ca–Na–K–silicocarbonatite melt, while the phoscorites are products of differentiation of the carbonatitic melt and were crystallised from an Fe-rich phosphate–carbonate melt at temperatures of more than 720 °C. The silicate–phosphate–carbonate melts were responsible for the Zr–Nb mineralisation of the carbonatites at temperatures of more than 540–575 °C; the hydrothermal REE-bearing mineral assemblages crystallised from saline (60–70 wt%) carbonatitic fluids of Na–Ca–Mg–F–carbonate composition at a minimum temperature range of 350–300 °C. The Ca–Sr-carbonate as well as the Na–hydro–carbonate fluids were responsible for the Ba–Sr–REE mineralisation of the phoscorites at ~500–480 and 450–430 °C.