Geological and geochemical characteristics of the Miaoya syenite-carbonatite complex, Central China: Implications for the origin of REE-Nb-enriched carbonatite

Abstract

Carbonatites are well known as economic resources for rare metals, such as rare earth elements (REE) and Nb. However, the origin and evolutionary processes of carbonatites and related silicate rocks as well as the relevant rare metals enrichment mechanism are highly controversial. In this study, we present detailed petrographic observations and geochemical data on the Miaoya syenite-carbonatite complex, which hosts both Nb and REE resources.The Miaoya syenites consist predominantly of K-feldspar, with minor zircon, nioboaeschynite, pyrochlore, and Fe-Ti oxides, and have positive Nb, Ta, Zr and Hf anomalies on primitive mantle-normalized patterns. There are two types of carbonatites at the Miaoya Complex: calciocarbonatites and ferrocarbonatites. Calciocarbonatites consist mainly of calcite, apatite, and biotite, together with allanite, monazite, bastnäsite(-Ce), and parisite-(Ce). Ferrocarbonatites which are the later products of the complex and occur as dykes cutting both calciocarbonatites and syenites, contain ankerite, calcite, apatite, and fluorite, bastnäsite-(Ce), and parisite-(Ce). The carbonatites have lower Nb, Ba, U, Th, and Zr, but higher P, Sr and REE concentrations than the associated syenites. However, both rock types have similar zircon U-Th-Pb ages (430–440 Ma), and nearly identical initial 87Sr/86Sr ratios (0.70325 to 0.70413) and εNd(t) values (2.5 to 3.2), implying that they were derived from a common source of carbonated silicate magmas.A genetic model of combined fractionation and immiscibility is proposed for the Miaoya complex. Fractionation of the carbonated silicate magmas lead to Nb enrichment in syenite and the immiscibility of the parental calcium carbonatite melts when the magmas reached a carbonate saturation level, together with partitioning of P, Sr, and REE into the parental carbonatite melts. During further fractionation of the carbonatite melts, REE components were elevated to economic values in the final ferrocarbonatites. This model can be widely applicable to other alkaline-carbonatite complexes and provides a reasonable explanation for extreme enrichment of rare metals in such rocks.