Evidence for Nb-Ta Occurrences in the Syn-Tectonic Pan-African Mayo Salah Leucogranite (Northern Cameroon): Constraints from Nb-Ta Oxide Mineralogy, Geochemistry and U-Pb LA-ICP-MS Geochronology on Columbite and Monazite

Periclex Martial Fosso Tchunte,François Turlin,Alliance Nicaise Saha Fouotsa,Anne-Sylvie André-Mayer,Hilaire Somtebda Dakoure,Emmanuel Negue Nomo,Olivier Rouer,Rigobert Tchameni,Marc Poujol

doi:10.3390/min8050188

Abstract

The Mayo Salah pluton, which is located in the North-Cameroon domain of Central African Bold Belt (CAFB), is emplaced as a laccolith in volcano-sedimentary schists of Poli series, and displays features of Rare-metal Granite (RMG). It is made of two main rock groups: (1) the metaluminous barren muscovite granite (MsG) and (2) the Nb-Ta bearing peraluminous leucogranite (MsL) which expresses four subtypes. The evolved Rare-element MsL is subalkaline, slightly peraluminous (ASI = 1.01–1.21), and it displays flat REE chondrite-normalized patterns with a strong negative Eu anomaly (Eu/Eu* = 0.02–0.20). It belongs to the peraluminous low phosphorus Rare-element Granites and L-type igneous rocks, as shown by the relatively low Zr/Hf (4.8–14) and Nb/Ta (1.4–9.0) ratios and the positive slope of the Zr-Hf-Nb-Ta profile in spider diagrams. The rare-element-bearing mineral is represented by columbite-group minerals (CGM) and other Nb-Ta-oxides (Nb-rutile and pyrochlore supergroup minerals). The CGM is classified as Mn-columbite, with Ta# and Mn# ratios increasing from core to rim. Two stages of mineralization are identified; the earliest stage (CGM-I) consists in scattered tabular or prismatic euhedral grains that were related to magmatic fractionation. The latest stage (CGM-II) is expressed as a Ta-rich Mn hydrothermal CGM episode represented as rims and/overgrowths around and/or as veinlet crosscutting CGM-I or in cleavage planes of muscovite. The U-Pb dating of columbite and monazite of the Mayo Salah leucogranite indicates a late-Neoproterozoic magmatic-hydrothermal mineralization event from 603.2 ± 5.3 to 581.6 ± 7.2 Ma, as consistent with both late D2 to D3 events that were recorded in the CAFB in Cameroon, and the associated continental collision environment. The Nb-Ta mineralization of the Mayo Salah pluton provides evidence for the presence of RMG in Northern Cameroon of CAFB, and its temporal association with the youngest period of metallogenic epoch of Nb-Ta-ore formation in Africa associated to Pan-African times.

Highlights

Rare-element granites have been divided in three categories by Linnen and Cuney [4]: (1) Peralkaline rare-element granites (ASI < 1, where ASI expresses the alumina saturation index, i.e., the molar (Al/[Na + K]) ratio) are characterized by very high abundances of F, REE, Y, Zr, Nb, high abundances of Th, Sn, Be, Rb, and U, high Nb/Ta, and low P content, and are typically emplaced in anorogenic geotectonic settings; (2) Metaluminous to peraluminous low P rare-element granites have intermediate REE, Y, Zr, Hf, Th, and Pb contents
Twenty-one representative hand-samples were collected on the Mayo Salah pluton for polished thin sections, whole-rock geochemistry, mineral chemistry, and geochronology
The textures of rare-element minerals and the associated mineral assemblage were observed on polished thin sections, using the backscattered electron mode (BSE) of Hitachi S4800 scanning electron microscope (SEM), equipped with an EDS spectrometer at GeoRessources laboratory (Nancy, France) to characterize the paragenetic assemblages of the samples

Summary

Introduction

Rare-element deposits (Li, Be, Ta, Nb, Sn, W, and REE) are commonly found in magmatic rocks: peralkaline to pegmatitic peraluminous granites, carbonatites, and alkaline igneous rocks [1,2,3,4,5].Enrichment of these lithophile elements may be directly linked to magmatic processes, as evidenced for the Beauvoir granite (French Massif Central [6,7,8]), Yichun granite of China [9,10], Czeck Podlesí granite [11,12]; or, magmatic-hydrothermal processes (e.g., [13,14,15].)Rare-element granites have been divided in three categories by Linnen and Cuney [4]:(1) Peralkaline rare-element granites (ASI < 1, where ASI expresses the alumina saturation index, i.e., the molar (Al/[Na + K]) ratio) are characterized by very high abundances of F, REE, Y, Zr, Nb, high abundances of Th, Sn, Be, Rb, and U, high Nb/Ta, and low P (phosphorous) content, and are typically emplaced in anorogenic geotectonic settings; (2) Metaluminous to peraluminous low P rare-element granites have intermediate REE, Y, Zr, Hf, Th, and Pb contents. Rare-element deposits (Li, Be, Ta, Nb, Sn, W, and REE) are commonly found in magmatic rocks: peralkaline to pegmatitic peraluminous granites, carbonatites, and alkaline igneous rocks [1,2,3,4,5]. Enrichment of these lithophile elements may be directly linked to magmatic processes, as evidenced for the Beauvoir granite (French Massif Central [6,7,8]), Yichun granite of China [9,10], Czeck Podlesí granite [11,12]; or, magmatic-hydrothermal processes (e.g., [13,14,15].). Li-rich muscovite or lepidolite, Rb, and Cs contents are generally very high Because of their strongly peraluminous character, they are classically related to the S-type granites of [19]

Methods

Results

Conclusion