Niobium-tantalum oxide minerals in alluvial placer deposits from the Ngoura area,East-Cameroon

Franc¸ois Ndong Bidzang•Elise´Sababa•Tessontsap Teutsong

Abstract This work presents a preliminary mineralogical characterization of Nb–Ta oxide minerals from alluvial placers located in the Ngoura area,eastern Cameroon.The heavy minerals are characteristic of short hydromechanical transport and highlight the stability of Nb–Ta oxides during the weathering and transport.Nb–Ta mineralization in placer deposits from the Ngoura area may derive from local sedimentation.The assemblage of Nb–Ta oxides in the studied placers consists of tapiolite-(Fe)and tantalite-(Mn).Tapiolite-(Fe)exhibits high concentrations in Ta2O5(78.07–80.46 wt%)and FeO(12.18–13.66 wt%)and plots within a narrow range of Ta*(0.900–0.933)and Mn*(0.037–0.119),which correspond to the ranges typically observed in tapiolite-(Fe)worldwide.Tantalite-(Mn)shows Ta2O5 contents ranging from 62.17 to 69.45 wt%,Nb2O5 from 12.09 to 17.37 wt%,MnO from 7.63 to 12.49 wt%and FeO from 1.48 to 6.62 wt%.It is also characterized by a wider range of Mn*(0.538–0.891)and relatively homogeneous Ta/(Ta+Nb)(0.683–0.779)ratios.Texturally,the studied Nb–Ta minerals exhibit oscillatory zoning characterized by bright Ta-rich zones alternating with dark Nb-rich zones.This oscillatory zoning is progressive in some minerals and the alternating bands may appear regular to wavy with gradual transitions.Oscillatory zoning in Nb–Ta oxides from the Ngoura placers is considered to be a primary magmatic feature and is tentatively explained as a result of magmas mixing,rapid cooling or degassing/decompression of the igneous system.The geomorphology and the tropical humid climate of the eastern region offer suitable conditions for the deposition of alluvial placers.Therefore,niobium-tantalum minerals from the Ngoura placers must have been sourced from the weathering and erosion of alkaline granites and pegmatites widespread in the study area.

Keywords Niobium·Tantalum·Tapiolite-(Fe)·Tantalite-(Mn)·Ngoura placer·East Cameroon

1 Introduction

Niobium(Nb)and tantalum(Ta)are transition metals in group 5 of the periodic table and have nearly similar chemical and physical properties(Nowak and Ziolek 1999).Niobium is a soft and ductile metal with a variety of uses such as niobium carbide,steel alloys and conducting magnets(Mitchell 2015).The world’s mining production of Nb ores comes from carbonatites and their weatheredenriched materials(Mackay and Simandl 2014).Tantalum is a hard but ductile metal,extensively used for electronic applications,superalloys,specialty metal products for medical and chemical industries(Melcher et al.2015).It is recovered from oxide minerals that are present in pegmatites,peraluminous granites,as well as their weathered equivalents and placers(Mackay and Simandl 2014;Simandl et al.2018).The main ore minerals of Ta and Nb include pyrochlore group,columbite-tantalite group,tapiolite group,wodginite group,perovskite,ixiolite,and euxenite groups(Mackay and Simandl 2015).Major Nb and Ta deposits are dated 2.8 Ga–85 Ma(Mackay and Simandl 2014).

Cameroon hosts several mineral occurrences such as iron ore(e.g.,Ndong et al.2016;Teutsong et al.2017;Ganno et al.2018;Soh Tamehe et al.2019),bauxite ore(Nguimatsa et al.2019a),gemstones(Kanouo et al.2016;Mbih et al.2016),alluvial rutile(Tonje et al.2014;Mbanga Nyobe et al.2018)and gold(Vishiti et al.2015;Ndong et al.2015;Nguimatsa et al.2019b).The eastern part of the country is known for extensively on-going artisanal mining of alluvial gold/diamond deposits.Columbo-tantalite(coltan)occurrences have been historically identified in Mayo Darle´(Adamawa Region,North Cameroon)and Garga-Sarali(East Cameroon)(Ntep Gweth et al.2003).Fosso Tchunte et al.(2018)recently investigated Nb–Ta oxides in leucogranites from the Mayo Salah region(Northern Cameroon).Despite extensive exploration campaigns conducted in Cameroon,there is a lack of scientific data on these highly demanded metals.This paper is focused on the mineralogical and chemical investigation of Nb–Ta oxides occurring in alluvial placer deposits from the Ngoura area in eastern Cameroon,with the aim of constraining their possible sources and genesis.

2 Geological setting

Ngoura belongs to the Central African Fold Belt(CAFB)in Cameroon,which is defined as a major collisional belt that underlies the region from the West African Craton to East Africa(Van Schmus et al.2008).The CAFB is Neoproterozoic in age,bounded to the south by the Congo craton and to the west by the West African craton.It has been subdivided into three lithostructural units which are from the south to the north:the Yaounde´domain,the Adamawa-Yade´domain,and the Northwestern Cameroon domain(Toteu et al.2004;Van Schmus et al.2008).

The study area is situated within the Adamawa-Yade´lithostructural unit which represents a Paleoproterozoic basement that was dismembered during the Pan-African orogeny.It is dominated by 640–610 Ma syn-to late-collisional high-K calc-alkaline granitoids,which intrude high-grade gneisses(Toteu et al.2004;Njiosseu et al.2005;Van Schmus et al.2008;Asaah et al.2015).This domain is dissected by large NE-striking transcurrent faults thought to be extensions of the major shear zones of northeastern Brazil.Toteu et al.(2004)identified three main groups in the Adamawa-Yade´domain:(i)large remnants of Paleoproterozoic metasediments and orthogneisses with contributions from an Archaean crust similar to the Ntem Complex;(ii)low-to medium-grade metasedimentary and acidic volcanoclastic rocks;(iii)and syn-to late-tectonic granitoids mainly of transitional composition of crustal origin.

At the local scale,the study area belongs to the Boden gold district(Fig.1).It is underlain by a large complex of late-to post-tectonic calc-alkaline to alkaline granite plutons(Suh et al.2006).The early granite plutons are lowgrade metamorphosed,variably deformed and foliated at their margins.Generally,the granites contain xenoliths and inliers of the older schists,pre-to syn-tectonic granites and gneissic basement.Some granite plutons appear sheared and cross-cut by NE–SW trending pegmatitic and quartz veins(Suh et al.2006).Tourmaline alteration is commonly observed in the wall rock.Recent geological investigations in the Ngoura area have recognized orthogneisses and migmatites intruded by syn-to post-tectonic granites and granodiorites(Takodjou Wambo et al.2016,2018).

3 Methodology

Stream sediment samples used in this study were collected at various points within the drainage basin(Fig.2)defining an area of～735 km2.A total of 42 samples were collected at suitable sites of the Koubou and Oudou rivers tributaries.All sampling sites were located using a GPS and coordinates recorded into a GIS platform.During the sampling process,the thickness of the sterile overburden and the gravel was measured.To eliminate larger clasts in gravels collected from the dug pits,samples were sieved with a filter of mesh size 10 mm.All the smaller size grains which passed through the sieve were collected and washed.Samples were placed in a pan and shaken sideways in a circular motion while being held just under the water.Heavy minerals sink to the bottom and light minerals rise and spill out over the top.The analytical procedures are similar to those described previously by McClenaghan et al.(2011).The heavy mineral fraction was retained and placed in a plastic bag together with the sample tag before the bag was stapled shut.At the base camp,the heavy mineral fraction was dried and the Nb–Ta oxide grains were handpicked under a binocular microscope(Fig.3).Niobium-tantalum oxide grains were selected for microanalyses at the laboratory of microanalyses(LMA)of the University of Laval,Canada.Zoning and mineral chemistry of Nb–Ta minerals were performed using wavelength-dispersive spectrometry(WDS)with a Cameca SX-100 electron microprobe.The microprobe was operated at 15 kV and 20 nA,and an electron beam diameter of 5μm.Counting time for Fe,Mn,Nb,Ta,Ti,and Sc was 10 s,for W 90 s,for Sn and Zr 100 s and for Hf 120 s.The following natural mineral and metal standards were used:columbite(Nb Lα),tapiolite(Ta Lα),magnetite(Fe Kα),rhodonite(Mn Kα),scandium(Sc Kα),rutile(Ti Kα),hafnium(Hf Lα),zirconium(Zr Lα),tin(Sn Lα),tungsten(W Lα),and uranium(U Mα).The detection limit for most elements is 200 ppm.The mineral formulae of Nb–Ta minerals were calculated based on six atoms of oxygen per formula unit(Tables 1 and 2).

4 Results

4.1 Morphological features

The depths of the pits are variable from 2 to 4 m with the bedrock comprising of granitoids(Fig.3a).The sterile which is made up of silt and clay has a thickness between 1 and 1.5 m and the gravel located at the bottom of the pit is the mineralized zone.The latter is composed of pebble,sand,silt,and clay.Sample separation and heavy mineral extraction show that the concentrates are mostly composed of quartz and Nb–Ta oxides when observed with the naked eye.The heavy mineral fraction displays well-formed crystals with crystalline growth faces,straight borders and rounded edges(Fig.3d).

Fig.2 Drainage pattern map showing sampling sites

4.2 Mineral chemistry

Electron microprobe data of Nb–Ta minerals from the Ngoura placers are given in Tables 1 and 2.A total of 42 analyses have been carried out on the core of each crystal,corresponding to columbite group minerals(CGM)and tapiolite group minerals.Atomic compositions and ratios were calculated and plotted on the columbite quadrilateral,where two distinct groups of Nb–Ta oxides were identified:tantalite-(Mn)and tapiolite-(Fe)(Fig.4).

Tantalite-(Mn)is characterized by Ta2O5contents ranging from 62.17 to 69.45 wt%,Nb2O5from 12.09 to 17.37 wt%,MnO from 7.63 to 12.49 wt% and FeO from 1.48 to 6.62 wt%(Table 1).Minor elements vary from 0.12 to 0.64 wt%TiO2and 0.01 to 0.37 wt%SnO2.Na,U,Th,and the analyzed REE(La,Ce,Nd)display very low contents.Tantalite-(Mn)from the Ngoura placers is relatively homogeneous in Ta/(Ta+Nb)(0.683–0.779)but it shows wide variations in Mn/(Mn+Fe)ratios(0.538–0.891)(Table 1).According to their Mn/(Mn+Fe)ratios,tantalite-(Mn)crystals can be divided into two groups(Fig.4).Group 1 shows Mn/(Mn+Fe)＜0.60 and is distributed on the eastern drainage network while group 2 is characterized by higher Mn/(Mn+Fe)ratios(＞0.60)and occurs in the center and western parts of the study area(Fig.5).

Tapiolite-(Fe)from the Ngoura area exhibits higher and rather constant concentrations in Ta2O5(78.07–80.46 wt%)and FeO(12.18–13.66 wt%).Slightly variable contents were obtained for Nb2O5and MnO,ranging 3.49–5.21 wt%and 0.52–1.64 wt%,respectively(Table 2).Minor oxides display comparable ranges,namely 0.19–0.61 wt% TiO2and 0.13–0.65 wt% SnO2.Tapiolite-(Fe)compositions show typically very low and variable Mn/(Mn+Fe)but very high and nearly constant Ta/(Ta+Nb)atomic ratios:0.037–0.119 and 0.900–0.933,respectively(Table 2).Tapiolite-(Fe)samples are distributed throughout the study area(Fig.5).

4.3 Textural features

Backscattered electron images illustrate typical zoning patterns of Nb–Ta oxide minerals from the Ngoura placers(Fig.6).Lahti(1987)described three types of zoning in CGM:progressive,oscillatory and patchy-replacement,using columbite-tantalite crystals from the granitic pegmatites in southern Finland.Oscillatory zoning is the most common feature found in the studied Nb–Ta minerals.It is characterized by bright Ta-rich zones alternating with dark Nb-rich zones(Fig.6a).This oscillatory zoning is usually progressive(Fig.6b),evolving from Nb-rich cores to Tarich rims.The episodic zones form thin to medium and well-defined layers from the core to rim,commonly regular and more rarely curved or wavy(Fig.6a).In other cases,alternating zones appear as coarse bands with gradual transitions,truncated in the border zone(Fig.6c).

Fig.3 Overview of the Ngoura alluvial placer and sample collection.a Field photograph of artisanal mining site.b Typical profile of alluvial Nb–Ta deposit in the study area.c Mineralized gravel collected from alluvial profile.d Nb–Ta oxide minerals collected from mineralized gravel

5 Discussion

5.1 Chemical features

The assemblage of analyzed Nb–Ta minerals from the Ngoura placers consists of tapiolite-(Fe)and tantalite-(Mn).Tapiolite-(Fe)plots within a narrow range of Ta*(0.900–0.933)and Mn*(0.037–0.119)values(see Fig.4),which correspond to the ranges typically observed in Tapiolite-(Fe)(Cˇerny´et al.1992;Wise and Cˇerny´1996).Tantalite-(Mn)displays a wider range of Mn*(0.538–0.891),suggesting variable source rocks.Nb–Ta oxide minerals from the Ngoura deposit display higher concentrations in tantalum compared to those of most of African Ta provinces(Melcher et al.2015).Fluorinity is commonly used to decipher fractionation behavior of CGM in pegmatites(Cˇerny´1992;Alfonso et al.1995).Tapiolite-(Fe)is usually found in the F-poor system whereas tantalite-(Mn)occurs in more evolved F-rich environments(Tindle and Breaks 2000).When comparing with the studied Nb–Ta minerals,Nb–Ta oxides from the Mayo Salah leucogranite exclusively consist of columbite-Mn(Fosso Tchunte et al.2018).

5.2 Zoning patterns

Nb–Ta oxides from the Ngoura placers exhibit oscillatory zoning which appears to be progressive in places.These features were also observed in CGM(columbite-Mn)from the Mayo Salah leucogranite(Fosso Tchunte et al.2018).Oscillatory zoning is certainly the most observed type of zoning in CGM and many examples are reported in the literature(e.g.,Lahti 1987;Alfonso et al.1995;Van Lichtervelde et al.2007).Zoning in CGM is commonly considered to be a primary magmatic feature(Tindle and Breaks 2000).In progressive zoning,crystals show smooth compositional variation between the rim and core.In oscillatory zoned crystals,the composition varies periodically between the core and the rim(Lahti 1987).The origin of oscillatory zoning has been explained in different ways by several authors and remains controversial.In igneous systems,this zoning is commonly related to magma mixing,rapid cooling or degassing/decompression of the system(Holten et al.1997;Badanina et al.2015;Llorens Gonza´lez et al.2017).

BG18 BG14 BG10 BG09 BG08 BG07 BG06 BG05 BG03 10.03 4.18 15.42 64.68 0.07 0.09 0.03 0.16 0.41 0.04 0.06 0.00 95.16 11.00 3.05 16.00 63.86 0.05 0.11 0.02 0.11 0.55 0.05 0.06 0.01 94.86 12.23 2.01 17.37 62.17 0.03 0.10 0.03 0.12 0.35 0.05 0.09 0.00 94.54 8.18 5.96 12.36 68.43 0.11 0.09 0.02 0.17 0.33 0.03 0.09 0.00 95.76 7.64 6.53 12.65 68.38 0.12 0.07 0.03 0.15 0.37 0.05 0.05 0.00 96.01 7.67 6.59 13.22 67.59 0.22 0.07 0.02 0.15 0.36 0.02 0.05 0.01 95.95 7.83 6.40 13.08 68.00 0.06 0.07 0.03 0.17 0.51 0.02 0.05 0.00 96.22 7.71 6.62 13.06 67.78 0.07 0.08 0.03 0.11 0.52 0.02 0.05 0.03 96.06 7.78 6.28 12.33 68.96 0.04 0.09 0.03 0.16 0.42 0.04 0.05 0.00 96.16 0.683 0.287 0.561 1.424 0.002 0.002 0.005 0.004 0.025 0.001 0.002 0.000 2.995 2.538 0.704 0.717 0.751 0.206 0.583 1.401 0.002 0.002 0.003 0.003 0.034 0.001 0.002 0.000 2.988 2.403 0.785 0.706 0.832 0.135 0.631 1.359 0.001 0.002 0.006 0.003 0.021 0.001 0.003 0.000 2.993 2.154 0.860 0.683 0.559 0.418 0.452 1.533 0.004 0.002 0.004 0.005 0.020 0.000 0.003 0.000 3.000 3.392 0.572 0.772 0.520 0.454 0.462 1.523 0.004 0.001 0.004 0.004 0.022 0.001 0.001 0.000 2.998 3.297 0.534 0.767 0.522 0.456 0.483 1.500 0.008 0.001 0.004 0.004 0.022 0.000 0.001 0.000 3.002 3.106 0.534 0.756 0.533 0.439 0.477 1.502 0.002 0.001 0.004 0.005 0.031 0.000 0.001 0.000 2.998 3.149 0.548 0.759 0.525 0.455 0.477 1.500 0.003 0.002 0.004 0.003 0.032 0.000 0.001 0.001 3.002 3.145 0.536 0.759 0.531 0.436 0.450 1.536 0.001 0.002 0.004 0.005 0.026 0.001 0.001 0.000 2.993 3.413 0.549 0.773 Table 1 Electron microprobe analyses(wt%)of tantalite-(Mn)from the Ngoura placer BG01 TI10 TI08 TI07 TI05 TI04 TI02 Sample 7.84 6.20 12.23 68.88 0.37 0.08 0.03 0.14 0.43 0.04 0.05 0.01 96.28 0.535 0.431 0.446 1.535 0.014 0.002 0.004 0.004 0.026 0.001 0.001 0.000 3.000 3.442 0.554 0.775 8.17 5.79 12.38 68.65 0.10 0.08 0.02 0.11 0.33 0.05 0.08 0.00 95.73 0.559 0.406 0.453 1.539 0.004 0.002 0.003 0.003 0.020 0.001 0.002 0.000 2.991 3.397 0.579 0.773 8.13 5.96 12.40 68.72 0.10 0.08 0.03 0.11 0.34 0.02 0.05 0.00 95.93 0.555 0.416 0.453 1.536 0.004 0.002 0.004 0.003 0.021 0.000 0.001 0.000 2.995 3.391 0.572 0.772 7.64 6.53 12.65 68.38 0.12 0.07 0.03 0.15 0.37 0.05 0.05 0.00 96.01 0.520 0.454 0.462 1.523 0.004 0.001 0.004 0.004 0.022 0.001 0.001 0.000 2.998 3.297 0.534 0.767 7.63 6.47 12.53 68.43 0.26 0.09 0.02 0.14 0.29 0.04 0.05 0.00 95.95 0.520 0.452 0.458 1.530 0.010 0.002 0.004 0.004 0.018 0.001 0.001 0.000 2.999 3.341 0.535 0.770 7.79 6.46 13.06 67.51 0.06 0.09 0.02 0.15 0.64 0.05 0.06 0.00 95.87 0.531 0.444 0.477 1.495 0.002 0.002 0.004 0.004 0.039 0.001 0.002 0.000 3.001 3.134 0.545 0.758 10.48 3.48 14.10 67.02 0.05 0.08 0.03 0.12 0.33 0.02 0.06 0.00 95.78 0.719 0.237 0.515 1.480 0.002 0.001 0.004 0.003 0.020 0.000 0.002 0.000 2.985 2.874 0.752 0.742 MnO FeO Nb2O5 Ta2O5 SnO2 U2O3 Na2O Nd2O3 TiO2 ThO2 Ce2O3 La2O3 Total Formulae calculated to 6 atoms of O Mn Fe Nb Ta Sn UNa Nd Ti Th Ce La Total Ta/Nb Mn/Mn+Fe Ta/Ta+Nb

Average MB14 MB13 MB12 MB10 MB09 MB06 MB05 MB04 MB03 9.13 5.01 13.69 67.08 0.11 0.08 0.03 0.11 0.36 0.04 0.07 0.00 95.71 8.27 5.80 12.09 69.45 0.08 0.06 0.03 0.09 0.25 0.05 0.08 0.00 96.25 9.64 4.53 13.79 67.06 0.09 0.05 0.03 0.07 0.20 0.07 0.09 0.00 95.63 9.63 4.46 13.68 67.12 0.24 0.07 0.03 0.06 0.12 0.07 0.09 0.00 95.56 9.67 4.59 14.36 66.27 0.19 0.05 0.03 0.07 0.19 0.05 0.09 0.01 95.57 9.79 4.45 14.20 66.20 0.03 0.07 0.03 0.07 0.47 0.07 0.10 0.00 95.48 9.83 4.40 14.23 66.68 0.04 0.05 0.03 0.09 0.34 0.05 0.10 0.00 95.83 9.71 4.61 14.21 66.46 0.04 0.07 0.03 0.03 0.35 0.05 0.09 0.03 95.68 12.49 1.48 15.25 65.70 0.03 0.06 0.03 0.04 0.16 0.05 0.11 0.00 95.39 9.79 4.27 13.47 67.65 0.01 0.07 0.03 0.08 0.25 0.06 0.09 0.00 95.78 0.623 0.348 0.499 1.488 0.004 0.002 0.005 0.003 0.022 0.001 0.002 0.000 2.996 3.015 0.642 0.749 0.563 0.407 0.441 1.552 0.003 0.001 0.005 0.003 0.016 0.001 0.002 0.000 2.994 3.519 0.580 0.779 0.656 0.319 0.502 1.491 0.003 0.001 0.006 0.002 0.012 0.001 0.003 0.000 2.996 2.970 0.673 0.748 0.655 0.317 0.498 1.498 0.009 0.002 0.005 0.002 0.007 0.001 0.003 0.000 2.997 3.008 0.674 0.751 0.658 0.320 0.523 1.468 0.007 0.001 0.005 0.002 0.012 0.001 0.003 0.000 3.000 2.807 0.673 0.737 0.666 0.309 0.517 1.464 0.001 0.002 0.005 0.002 0.029 0.001 0.003 0.000 2.999 2.832 0.683 0.739 0.669 0.303 0.517 1.471 0.001 0.001 0.006 0.003 0.021 0.001 0.003 0.000 2.996 2.845 0.688 0.740 0.661 0.319 0.517 1.468 0.002 0.002 0.006 0.001 0.021 0.001 0.003 0.001 3.000 2.839 0.674 0.740 0.854 0.102 0.556 1.448 0.001 0.001 0.006 0.001 0.010 0.001 0.003 0.000 2.983 2.604 0.893 0.723 0.666 0.301 0.490 1.505 0.000 0.002 0.006 0.002 0.016 0.001 0.003 0.000 2.991 3.071 0.689 0.754 Table 1 continued MB02 MB01 Sample 9.84 4.19 13.38 67.56 0.34 0.07 0.03 0.06 0.26 0.06 0.09 0.01 95.89 0.670 0.296 0.487 1.504 0.013 0.002 0.006 0.002 0.016 0.001 0.003 0.000 2.998 3.088 0.694 0.755 10.22 4.01 16.23 63.48 0.05 0.11 0.03 0.20 0.52 0.02 0.05 0.00 94.92 0.696 0.271 0.590 1.390 0.002 0.002 0.004 0.006 0.032 0.000 0.001 0.000 2.995 2.356 0.720 0.702 MnO FeO Nb2O5 Ta2O5 SnO2 U2O3 Na2O Nd2O3 TiO2 ThO2 Ce2O3 La2O3 Total Formulae calculated to 6 atoms of O Mn Fe Nb Ta Sn UNa Nd Ti Th Ce La Total Ta/Nb Mn/Mn+Fe Ta/Ta+Nb

Average MB11 MB08 MB07 BG17 BG16 BG15 BG13 BG12 0.98 13.02 4.41 79.25 0.31 0.03 0.02 0.09 0.36 0.03 0.02 0.02 98.55 1.09 12.87 4.39 79.02 0.34 0.05 0.01 0.08 0.40 0.04 0.01 0.00 98.28 1.64 12.18 3.49 80.46 0.16 0.04 0.02 0.12 0.22 0.02 0.05 0.00 98.39 1.02 13.09 4.51 79.23 0.20 0.02 0.02 0.04 0.35 0.02 0.00 0.03 98.55 0.72 13.53 5.19 78.51 0.17 0.01 0.02 0.11 0.52 0.02 0.00 0.03 98.83 0.52 13.60 4.64 78.94 0.37 0.03 0.02 0.07 0.30 0.04 0.00 0.03 98.56 0.55 13.66 5.21 78.15 0.47 0.03 0.01 0.07 0.29 0.02 0.00 0.04 98.50 1.23 13.01 4.88 78.07 0.15 0.05 0.01 0.11 0.61 0.03 0.06 0.03 98.23 1.21 12.66 4.17 79.74 0.13 0.04 0.01 0.06 0.39 0.04 0.03 0.03 98.50 0.070 0.915 0.167 1.812 0.012 0.001 0.002 0.003 0.022 0.001 0.000 0.001 3.007 11.006 0.071 0.915 0.078 0.906 0.167 1.812 0.013 0.001 0.002 0.002 0.025 0.001 0.000 0.000 3.007 10.850 0.079 0.916 0.117 0.864 0.134 1.856 0.006 0.001 0.002 0.004 0.014 0.000 0.001 0.000 3.000 13.851 0.119 0.933 0.073 0.919 0.171 1.809 0.008 0.001 0.003 0.001 0.022 0.000 0.000 0.001 3.008 10.579 0.074 0.914 0.051 0.941 0.195 1.776 0.006 0.000 0.003 0.003 0.032 0.000 0.000 0.001 3.010 9.108 0.051 0.901 0.037 0.955 0.176 1.803 0.014 0.001 0.002 0.002 0.019 0.001 0.000 0.001 3.011 10.244 0.037 0.911 0.039 0.956 0.197 1.780 0.018 0.001 0.002 0.002 0.018 0.000 0.000 0.001 3.015 9.036 0.039 0.900 0.088 0.911 0.184 1.779 0.006 0.001 0.002 0.003 0.038 0.000 0.002 0.001 3.014 9.668 0.088 0.906 0.087 0.890 0.158 1.826 0.005 0.001 0.002 0.002 0.025 0.001 0.001 0.001 2.997 11.557 0.089 0.920 Table 2 Electron microprobe analyses(wt%)of tapiolite-(Fe)from the Ngoura placer BG11 BG04 BG02 TI09 TI06 TI03 TI01 Sample 0.72 13.53 5.19 78.51 0.17 0.01 0.02 0.11 0.52 0.02 0.00 0.03 98.83 0.051 0.941 0.195 1.776 0.006 0.000 0.003 0.003 0.032 0.000 0.000 0.001 3.010 9.108 0.051 0.901 1.13 12.81 4.42 79.50 0.34 0.03 0.02 0.10 0.27 0.02 0.00 0.00 98.63 0.080 0.901 0.167 1.819 0.013 0.001 0.002 0.003 0.017 0.000 0.000 0.000 3.004 10.892 0.082 0.916 1.06 12.85 4.37 79.21 0.35 0.04 0.01 0.03 0.26 0.05 0.03 0.03 98.28 0.076 0.906 0.166 1.819 0.013 0.001 0.001 0.001 0.016 0.001 0.001 0.001 3.003 10.958 0.077 0.916 1.14 12.68 3.62 80.17 0.33 0.05 0.01 0.15 0.17 0.04 0.05 0.00 98.41 0.082 0.900 0.139 1.850 0.012 0.001 0.002 0.005 0.011 0.001 0.001 0.000 3.003 13.309 0.084 0.930 0.84 13.05 4.24 79.66 0.61 0.03 0.02 0.14 0.36 0.04 0.00 0.01 98.98 0.060 0.916 0.160 1.818 0.023 0.001 0.002 0.004 0.022 0.001 0.000 0.000 3.008 11.363 0.061 0.919 0.75 13.18 4.32 79.26 0.28 0.05 0.01 0.13 0.48 0.06 0.01 0.00 98.52 0.053 0.926 0.164 1.812 0.010 0.001 0.002 0.004 0.030 0.001 0.000 0.000 3.004 11.049 0.054 0.917 1.14 12.61 3.57 80.38 0.65 0.04 0.02 0.07 0.19 0.04 0.00 0.01 98.69 0.082 0.893 0.137 1.852 0.024 0.001 0.002 0.002 0.012 0.001 0.000 0.000 3.006 13.518 0.084 0.931 MnO FeO Nb2O5 Ta2O5 SnO2 U2O3 Na2O Nd2O3 TiO2 ThO2 Ce2O3 La2O3 Total Formulae calculated to 6 atoms of O Mn Fe Nb Ta Sn U Na Nd Ti Th Ce La Total Ta/Nb Mn/Mn+Fe Ta/Ta+Nb

Fig.4 Compositions of tapiolite-(Fe)and tantalite-(Mn)plotted in the columbite quadrilateral diagram

5.3 Comparative mineralogy

Tantalum-Niobium minerals in Africa are commonly associated with granites and pegmatites.They are mostly recovered from weathered pegmatites or from eluvial and alluvial placer deposits(Melcher et al.2015).The mineralogy of the studied Ta–Nb oxides revealed oscillatory zoned tapiolite-(Fe)and tantalite-(Mn)grains whilst Ta–Nb minerals in placer from the Kokobin deposit(Ghana)consists of complexly zoned and partly homogeneous columbite-(Mn)to tantalite-(Mn)minerals(Melcher et al.2015).When compared to the Ngoura deposit,placers investigated in the Archean–Paleoproterozoic transition zone at the southern margin of the Congo Craton displays a different mineral assemblage,including complexly zoned columbite-(Fe)to tantalite-(Fe),tapiolite,and subordinate U-bearing microlite(Melcher et al.2015).Similarly,the mineralogy of heavy mineral concentrates from the Jos Plateau placer deposit in Nigeria is completely different to that of the Ngoura deposit,and characterized by columbite-(Fe),tantalite-(Fe)and columbite-(Mn),tapiolite,ixiolite,microlite,fergusonite,euxenite and cassiterite(Mu¨cke and Neumann 2006).

5.4 Genesis of Ta-Nb mineralization

Fig.5 Local geology and drainage pattern map showing tapiolite-(Fe)and tantalite-(Mn)distribution throughout the study area

Fig.6 Backscattered electron images of Nb–Ta minerals from Ngoura.a Oscillatory zoning;b progressive oscillatory zoning;c irregular oscillatory zoning truncated in the border zone

Large Ta–Nb deposits are mined from granites,pegmatites,and carbonatites.Placer deposits have historically provided up to 40% of world Ta production(Mackay and Simandl 2014).In the eastern part of Cameroon,there are abundant alkaline granites and pegmatitic rocks that have never been investigated for Ta–Nb mineralization.The mineral assemblage of the Ngoura sediments is characteristic of hydromechanical transport.The high proportion of Nb–Ta oxides in the alluvia may be linked to their stability during the weathering process and in the alluvial environment.The grain size of quartz and Nb–Ta oxides extremely depends on the textural features of their source rocks(Bassis et al.2016).The angular nature of the heavy minerals in alluvia is a conserved primary relict feature during weathering and transportation.These angular grains may probably be a product of short hydromechanical transportation(Kanouo et al.2012).In addition,the geomorphology and the tropical humid climate of the region are favorable for the deposition of alluvial placers currently mined for gold and diamonds.Therefore,we suggest that niobium-tantalum minerals from the Ngoura placers must have been sourced from the weathering and erosion of granites and pegmatites occurring in the study area(Fig.5).

6 Conclusion

The alluvia from Ngoura are rich in niobium-tantalum oxides comprising tapiolite-(Fe)and tantalite-(Mn).Tapiolite-(Fe)plots within a narrow range of Ta*(0.900–0.933)and Mn*(0.037–0.119)typical of tapiolite-(Fe)worldwide whilst tantalite-(Mn)displays a wider range of Mn*(0.538–0.891).Both species exhibit oscillatory zoning which appears to be progressive in places.This zoning is interpreted as a primary magmatic feature resulting from magmas mixing,rapid cooling or degassing/decompression of the igneous system.The geomorphology of the eastern region combined with a tropical humid climate has played a key role in the deposition of alluvial placers.The heavy mineral fraction shows preserved primary features,suggesting a local provenance.It is therefore suggested that niobium-tantalum minerals from the Ngoura placers must have been sourced from the weathering and erosion of alkaline granites and pegmatites outcropping in the study area.

AcknowledgementsWe thank the University of Laval for electron microprobe analyses of niobium-tantalum minerals.

Compliance with ethical standards

Conflict of interestOn behalf of all authors,the corresponding author states that there is no conflict of interest.