The Genesis of Heavy REE Mineralisation in the Lofdal Carbonatite Complex, Namibia

English Abstract:

The rare earth element (REE) mineralisation in the 765 Ma Lofdal Carbonatite Complex, Namibia, is highly unusual. Whereas carbonatite-associated REE deposits are typically enriched in the light REE, Lofdal hosts an indicated mineral resource of 2.88 Mt total REE oxides at a cut-off grade of 0.1%, of which 76.3% are heavy REEs (HREE). This makes it one of the most HREE-enriched economic or potentially economic deposits in the World. Although the REE were initially assumed to be hosted by carbonatite dykes, textural relationships observed in the field, drill core and microscopically indicate that the REE mineralisation (xenotime-(Y)) is hosted by albitised rocks that formed along planar structures, and that these rocks were subsequently carbonated by hydrothermal fluids. This interpretation is supported by δ¹⁸O and δ¹³C values that are consistent with a hydrothermal rather than magmatic origin for the carbonate minerals. The paragenesis is albitisation followed by two generations of xenotime-(Y), overprinted by carbonate (calcite or dolomite). The carbonate-forming fluids may locally have remobilised the REE, but if this were the case, the remobilisation was minor. The first and most common mode of occurrence of xenotime-(Y) is in veinlets of biotite ± calcite ± pyrite that cut the albitite. A small proportion of xenotime-(Y) occurs in discontinuous veinlets (lenses) of apatite. A model is proposed in which early fluids released from a carbonatite magma rose along faults in the basement, albitising (fenitising) gneisses and later fluids fractured the albitites and deposited xenotime-(Y) with biotite, and locally apatite. The carbonating fluid is also likely to have originated from the carbonatite but we do not rule out the possibility that it was a formational water or a mixture of magmatic and formational water.