Abstract
AbstractEnrichment of the heavy rare earth elements (HREE) in carbonatites is rare as carbonatite petrogenesis favours the light (L)REE. We describe HREE enrichment in fenitized phonolite breccia, focusing on small satellite occurrences 1–2 km from the Songwe Hill carbonatite, 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 mineralization and carbonatite emplacement is indicated by the presence of Sr-bearing carbonate veins, carbonatite xenoliths and extensive fenitization. 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 focusing 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 3100 µg g−1, 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 carbonatite-related HREE enrichment suggests that all form via a common mechanism linked to fenitization. Precipitation of HREE minerals only occurs where a pre-existing structure provides a focusing conduit for fenitizing 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 mineralization within the source carbonatite body at depth.
Highlights
The rare earth elements (REE) exhibit magnetic and spectroscopic properties useful in a number of technological and industrial applications, and demand for REE is increasing (Goodenough et al 2018)
We present data on breccias from the Nkalonje carbonatite complex which are similar in terms of their texture and major element composition, but do not exhibit heavy rare earth elements (HREE) enrichment
We suggest that the breccia occurrences around Mauze represent small satellite vents to the main Songwe Hill carbonatite (Fig. 10)
Summary
The rare earth elements (REE) exhibit magnetic and spectroscopic properties useful in a number of technological and industrial applications, and demand for REE is increasing (Goodenough et al 2018). While mineral-scale fractionation of the REE has been demonstrated at many carbonatites (Broom-Fendley et al 2016a, 2017a; Andersen et al 2017), the occurrences at Lofdal and Bear Lodge show that fractionation of the REE occurs on the deposit scale It remains unclear if these examples are an exception, or if HREE mineralization around carbonatites is a common, previously overlooked, feature. Fenitized rocks can host xenotime in small amounts (Dowman et al 2017a) In this contribution, we describe HREE-enriched occurrences of fenitized phonolite breccia from the Chilwa Alkaline Province, Malawi, which share textural and geochemical similarities to the Lofdal deposit and Cole occurrence. We present data on breccias from the Nkalonje carbonatite complex which are similar in terms of their texture and major element composition, but do not exhibit HREE enrichment
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