Petrochronology and genesis of granite-hosted rare metal (Ta, Nb, Li, Y + REE, Zr)-F-bearing mineral occurrences in the Cameroon Northern Domain of the Central African Orogenic Belt

Abstract

The Ngoutchoumi Granite Complex in the Cameroonian Northern Domain (ND) of the Neoproterozoic Central African Orogenic Belt has long been known to host tungsten mineral occurrences. Here, we show that this granite complex also contains a suite of rare metals (Ta, Nb, Zr, Li, Y + REE) and F-bearing minerals, which have not been previously reported. The key genetic aspects pertaining to the genesis of this mineralised system are unconstrained. In this study, that couples in-situ LA-ICP-MS zircon and columbite U-Pb and mica ⁴⁰Ar-³⁹Ar geochronology with bulk-rock and mineral geochemistry and Raman spectroscopy, the rare metal minerals and their host granite are characterized. The granite is peraluminous and rich in REE (100–445 ppm), with strong LREE enrichment ((La/Yb)_N = 2.38–41.4) and typically displays a pronounced negative Eu anomaly (Eu/Eu* = 0.06–0.10). Rare metal minerals, including ferrocolumbite, Nb-rich rutile, zircon and various REE minerals are intimately associated with fluorite. Zircon and ferrocolumbite from the rare metal-rich granite yield Pan-African ages of 639 ± 11 Ma and 640.2 ± 12.7 Ma, respectively, indicating part of the rare metal minerals is orthomagmatic. Textural and mineral chemistry features indicate that the primary orthomagmatic rare metal minerals were overprinted, thus leading to at least three episodes of rare metal mineralisation. On the other hand, biotite and muscovite from granite crosscutting the rare metal granite yield Ar-Ar plateau ages of 549.02 ± 0.85 and 552.17 ± 0.22 Ma, respectively, which, when combined with our ferrocolumbite age, as well as previously reported rare metal-related ages within the ND, bracket the age of rare metal mineralisation events within the ND, between ca.640–580 Ma. Our findings, therefore, extend both the spatial and temporal fingerprints of the ND rare metal mineralisation beyond those previously reported and have significant implications for the exploration of rare metals in Pan-African terranes. The episodes of formation of rare metals at Ngoutchoumi span from the orthomagmatic stage to the hydrothermal stage, and this genetic model sheds new light on the genesis of rare metal granite-type deposits, where debates have revolved around metasomatic vs. magmatic rare metal enrichment processes.