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

This is the final version. Available on open access from Cambridge University Press via the DOI in this recordEnrichment 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.Natural Environment Research Council (NERC)European Union Horizon 202

Litho- and chemostratigraphy of the Flatreef PGE deposit, northern Bushveld Complex

The Flatreef is a world-class platinum-group element (PGE) deposit recently discovered down-dip from existing mining and exploration operations on the northern limb of the Bushveld Complex. Current indicated resources stand at 42 Moz PGE (346 Mt with 3.8 g/t Pt+Pd+Rh+Au, 0.32% Ni and 0.16% Cu) which, in the case of Pt, is equivalent to ~ 10 years global annual production, making it one of the largest PGE deposits on earth. The grade and thickness of the Flatreef mineralised interval is highly unusual, with some drill core intersections containing up to 4.5 g/t Pt+Pd+Rh+Au over 90 m in drill core. Here, we document the down-dip and along-strike litho- and chemostratigraphy of the Flatreef and its footwall and hanging wall rocks, based on a diamond drill core database totalling > 720 km. At the base of the sequence intersected in the drill cores are up to 700-m-thick sills of ultramafic rocks (dunite, harzburgite, pyroxenite) emplaced into pelitic, dolomitic, and locally quartzitic and evaporitic rocks belonging to the Duitschland Formation of the Transvaal Supergroup. Next is an approximately 100–200-m sequence of low-grade-sulphide-mineralised, layered mafic-ultramafic rocks containing abundant sedimentary xenoliths and, in places, several chromite seams or stringers. This is overlain by a ~ 100-m-thick sequence of well-mineralised mafic-ultramafic rocks (the Flatreef sensu strictu), overlain by a laterally persistent mottled compositional analogies at the base of > 1 km of homogenous Main Zone gabbronorite. Based on stratigraphic, lithological and compositional alanalogies to the layered rocks in the eastern and western Bushveld Complex, we correlate the Flatreef and its chromite bearing footwall rocks with the Upper Critical Zone, notably the interval between the UG2 chromitite and the Bastard Reef as found elsewhere in the Bushveld Complex. This includes recognition of a Merensky Reef correlative. The ultramafic rocks below the main chromitite seam (UG2 correlative) in the Flatreef footwall are correlated with the Lower Critical and Lower zones. However, compared to the western and eastern Bushveld limbs, the studied sequence is strongly enriched in sulphide and PGE, many of the rocks show elevated CaO, K2O, Rb and Zr contents, and lateral continuity of layers between drill cores is less pronounced than elsewhere in the Bushveld, whereas ultramafic units are locally considerably thickened. These compositional and lithological traits are interpreted to result from a range of processes which include contamination with calcsilicate and hornfels, intrusion of granitic magmas, and the influence of multiple structural events such as pre- to syn-emplacement regional-scale open folding and growth faults. Evidence for the existence of potholes also exists. In the shallow, up-dip portions of the project area, the entire magmatic sequence below the Main Zone becomes increasingly contaminated to the extent that individual units are somewhat more difficult to correlate between drill cores. This package represents the Platreef as exposed in outcrop and shallow bore holes across much of the northern limb of the Bushveld Complex. The new data presented here thus indicate that the Platreef is a relatively more contaminated up-dip extension of parts of the Critical and Lower zones