Enrichment of heavy REE and Th in carbonatite-derived fenite breccia

Abstract

Enrichment of the heavy rare earth elements (HREE) in carbonatites is rare as carbonatite petrogenesis favours the light (L)REE. We describe HREE enrichment in fenitised phonolite breccia, focussing on small satellite occurrences 1–2 km from the Songwe Hill carbonatite from the Chilwa Alkaline Province, Malawi. Within the breccia groundmass, a HREE-bearing mineral assemblage comprises xenotime, zircon, anatase/rutile, and minor huttonite/thorite, as well as fluorite and apatite. A genetic link between HREE mineralisation and carbonatite emplacement is indicated by the presence of Sr-bearing carbonate veins, carbonatite xenoliths and extensive fenitisation. We propose that the HREE are retained in hydrothermal fluids which are residually derived from a carbonatite after precipitation of LREE minerals. Brecciation provides a focussing conduit for such fluids, enabling HREE transport and xenotime precipitation in the fenite. Continued fluid-rock interaction leads to dissolution of HREE-bearing minerals and further precipitation of xenotime and huttonite/thorite. At a maximum Y content of 3,100 μg/g, HREE concentrations in the presented example are not sufficient to constitute ore, but the similar composition and texture of these rocks to other cases of HREE enrichment related to carbonatite suggests that all form via a common mechanism linked to fenitisation. Precipitation of HREE minerals only occurs where a pre-existing structure provides a focussing conduit for fenitising fluids, reducing fluid-country rock interaction. Enrichment of HREE and Th in fenite breccia serves as an indicator of fluid expulsion from a carbonatite, and may indicate the presence of LREE mineralisation within the source carbonatite body at depth.