The Geological Controls on the Heavy Rare Earth Element Enriched Alteration Zone of Area 4, Lofdal, Khorixas, Namibia
Thesis or dissertation
University of Exeter
The “heavy” rare earths (HREEs) are the most critical of all REEs, and are attracting much market and exploration attention today since China imposed export quotas in 2011. The Lofdal Intrusive Complex, Khorixas, Namibia is enriched with heavy rare earth (HREE) mineralisation within the late hydrothermal zones associated with carbonatites, proximate to the complex’s syenite intrusions, and are generally controlled by older basement structures. One of these enriched HREE alteration zones is “Area 4”, now a proven CIM NI 43-101 compliant indicated and inferred resource, and sits at the top for REE basket price in the TMR world index of REE projects. Brecciated fenite zones are repeatedly overprinted, commonly hosting greater HREE mineralisation amongst the “busiest” overprints. The fenitisation and brecciation providing the ground preparation for REE deposition in Area 4 have similarities to the Kangankunde and Chilwa carbonatite complexes in Malawi, where HREE enrichment can be found in proximal brecciated fenite zones. 90% of the HREE deportment in Area 4 is in the mineral xenotime, and is commonly associated with rutile (+/-Nb), apatite, zircon, aeschynite, synchysite, and thorite. Variations of this assemblage reside in carbonate phases (calcite, dolomite, ankerite), albite, phlogopite-biotite, and magnetite/hematite. This study enhances the characterisation of the HREE mineral resource in Area 4 by combining details of drill core textural features, whole-rock major to trace element data analysis (ICP-MS) through the alteration zone and HREE mineralisation, with petrological and geochemical analysis of the major HREE bearing phases. 41 samples, representing a range of xenotime mineralised intercepts, of have been analysed petrologically via microscopy, cathodoluminescence and SEM/EDS; and geochemically via EPMA. The variability, fractionation, and partitioning between the REE in the Area 4 xenotimes is compared to synthetic analogues, and to other xenotimes of the Lofdal Complex outside of Area 4, and compared to xenotimes from Witwatersrand, South Africa, and from Browns Range, Australia. REE budget in xenotime can vary in proportion, as well as other element substitution and replacement into either the Y-site (A), or PO4 site (T). Substitutions into the xenotime lattice importantly include Th and U. Tested here is whether variations in ionic radii between the REE in xenotime can provide valuable clues to formational environment, and help to delineate differentiation in paragenses. Gd is in a unique position as it marks the change from unpaired to paired electrons in the filling stages of 4f orbitals, and Gd is shared between the second and the third tetrad. How the GdPO4 polymorph relates across the miscibility gap between LREE phosphate (monazite) and HREE (xenotime) in the Area 4 xenotimes appears temperature dependent. Experimental studies here have shown that in greater overprinted sections modifying fluids effectively “steal” Gd from xenotime to be lost to the fluid, and to LREE phases such as synchysite. Any xenotime that re-crystallises from these lower temperature fluids has been shown to have a higher HREE content, lower Th, but smaller grain size. Corrosion and re-precipitation of xenotime leads to Th being expelled from hydrothermal fluid leading Th to form its own phase, i.e. the silicate thorite, albeit proximal to xenotime, but importantly not substituting into the xenotime lattice, and that this is also shown to be temperature dependent. Textural and compositional data indicate hydrothermal modification of earlier REE bearing phases. Suggested REE transporting ligands in carbo-hydrothermal fluids to later modifying hydrothermal fluids are perhaps a combination, and evolution through, HCO3, PO4 and HS, for example at >300°C, to Cl complexing ligands at to lower temperature <300°C . Decompression as a result of pregnant solution migrating into open space in fractures, fluid/rock interaction with albite, and especially dolomite, have been shown to be effective factors in depositing xenotime from solution. The different stages of hydrothermal activity have been shown to have implications on the chemical and physical structure of the xenotime. This variability has economic implications when it comes to targeting ore, and to mineral processing.
MbyRes in Geology