Int J Earth Sci (Geol Rundsch) (2015) 104:1123–1138 DOI 10.1007/s00531-015-1157-3
ORIGINAL PAPER
Mineralogy and geochemistry of Neoproterozoic siliceous manganese formations from Ntui–Betamba (Cameroon Pan‑African Fold Belt): implications for mineral exploration Ngnotue Timoleon · Ganno Sylvestre · Gus Djibril Kouankap Nono · Nzenti Jean Paul
Received: 12 September 2014 / Accepted: 5 February 2015 / Published online: 22 February 2015 © Springer-Verlag Berlin Heidelberg 2015
Abstract The siliceous manganese formations from Ntui–Betamba area have been studied with the aim to constrain their mineralogy, geochemistry, and genesis. Neoproterozoic manganese-bearing rocks at Ntui–Betamba include garnet–galaxite-bearing quartzite and galaxite-bearing quartzite that—besides a characteristic quartz, spessartinegarnet, and galaxite (MnAl2O4) assemblage—comprises also graphite and rutile. Galaxite crystals show chemical zoning from core to rim due to the change in composition from galaxite to cryptomelane. Bulk rock geochemistry reveals that the studied rocks are characterized by high SiO2 content (52.83 wt%). They are enriched in Fe2O3 and Al2O3 and display Mn/Fe ratio of 1.2. MnO concentrations are high in galaxite-bearing quartzite (16 wt%), and moderate (8.03 wt%) in garnet–galaxite-bearing quartzite. The transition trace metal (Co, Cu, and Ni) contents are lower, with Co and Cu markedly depleted relative to Ni, and variable contents of Zn. The REE pattern of garnet–galaxitebearing quartzite shows Ce-positive anomaly similar to the hydrogenous sediment, but differs from the later with strong Eu-negative anomaly and the lack of weak positive N. Timoleon Department of Geology, University of Dschang, P.O. Box. 67, Dschang, Cameroon N. Timoleon (*) · G. Sylvestre (*) · G. D. Kouankap Nono · N. J. Paul Department of Earth Sciences, University of Yaoundé I, P.O. Box. 812, Yaoundé, Cameroon e-mail:
[email protected] G. Sylvestre e-mail:
[email protected];
[email protected] G. D. Kouankap Nono Department of Geology, HTTC, University of Bamenda, P.O. Box 39, Bambili, Bamenda, Cameroon
anomaly in Yb, whereas the galaxite-bearing quartzite is characterized by the lack of Ce anomaly and the weak Eunegative anomaly. Both rocks show HREE content close to that of hydrothermal sediment and exhibit flat HREE trend. The high contents of silica, iron, and aluminum, together with the Co/Zn ratios (~0.3) and the contrasting behavior of REE, suggest that the studied manganese-bearing rocks are of sedimentary origin probably derived from a mixed hydrothermal–hydrogenous source. The high content of Mn-bearing minerals in these rocks represents indicators of concealed or buried mineralization and could be used as exploration guide. Keywords Manganese-bearing rocks · Galaxite · Spessartine-garnet · Hydrothermal–hydrogenous source · Ntui–Betamba · Cameroon
Introduction Iron formations and manganese-bearing rocks are important constituents of chemical sediments in many Precambrian supracrustal successions. Manganese is the tenth most abundant elements in the Earth’s crust. Manganese deposits occur throughout geological history and are extensively distributed both on the continents and on the bottoms of present-day oceans, shallow seas, and lakes (Polgari et al. 2009). Tonnages of the karst-hosted deposits are comparatively minor, about 0.5 % of the total reserves, while volcanic-hosted deposits make up about 6 % and sedimenthosted make up 93 % of the total global reserves (Maynard 2008). Oxide-, carbonate- or silicate-dominated manganese rocks can be differentiated on the basis of predominant manganese-bearing mineral (Roy 1981). Most of Mn deposits are closely associated with organic carbon-rich
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beds. Mn carbonate deposits are typically of such associations, though Mn oxides are not totally excluded. Many of the important Mn deposits of older geological ages have been modified by regional and contact metamorphism which hid the original character of the modes of formation (Roy 1981; Polgari et al. 2009). In the Birimian greenstone belt of northern Ghana, manganese-bearing rocks are reported to be associated with chert, epiclastic and tuffaceous rocks, and metavolcanic rocks (Melcher 1995; Mucke et al. 1999). Such rocks were also described in the Neoproterozoic Urucum District, Mato Grosso Do Sul, Brazil, as containing a complex silicate assemblage with braunite, cryptomelane, some pyrolusite, and authigenic aegirine, and with a source concluded to be from typical ocean water with some deep-sea hydrothermal component (Klein and Ladeira 2004); while in western Katanga (Democratic Republic of Congo), the Mn ore of Kisenge deposit consists of manganese oxide-rich caps which result from regional process of lateritization of primary rhodochrositebearing carbonate rocks among others (Decree et al. 2010). In Cameroon, manganese-bearing rocks have received little attention and remained largely unknown till date. The first work that reported Mn-rich rocks was done by Ngnotue et al. (2000), but no detailed study was carried out by these authors. All the samples described here were
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10° Phanerozoic Cover
collected at Ntui–Betamba, an area which belongs to the southern Cameroonian domain of the Pan-African North Equatorial Fold Belt (PANEFB). The main outcrop is located close to Betamba market at UTM, coordinates 32N 785122E, 490800N. The purpose of this paper is to characterize quantitatively the mineral chemistry and whole-rock geochemistry of siliceous manganese formation at Ntui–Betamba. These results, compared to those obtained on the Neoproterozoic manganese formation in the literature, will enable us to: (1) constrain the Mn source and the mode of formation or genesis; (2) examine their geological context and exploration methods aimed at the discovery of potential Mn ore deposit in Cameroon.
Geological setting The siliceous and oxides manganese formation from Ntui–Betamba is hosted by Neoproterozoic high-grade gneisses, which constitute the northward extension of the Yaoundé series which belongs to the southern domain of the PANEFB, and is situated in the northern edge of the Congo Craton (Fig. 1a) (Ngnotue et al. 2000). The Yaoundé series is composed of intensely deformed metasedimentary
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Tertiary and recent volcanism
Pan-African belt: High-grade metasediments
Granitoid
Undifferentiated gneisses + amphibolites
Garoua
Congo craton: Archaean TTG
Paleoproterozoic series
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8°
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Douala
Atlantic Ocean
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Kribi
Fig. 1B 2°
Fig. 1 a Geological map of Cameroon (adapted from Ngnotue et al. 2012) showing the location of the Yaoundé area and the three main lithotectonic domains of PANEFB: (1) southern domain; (2) central
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domain; (3) northern domain. CCSZ Central Cameroon shear zone, SF Sanaga fault, TBF Tibati Banyo fault, NT Ntem complex, DS Dja series, NS Nyong series. b Geological map of Ntui–Betamba area
Int J Earth Sci (Geol Rundsch) (2015) 104:1123–1138
and metaigneous rocks (Nzenti et al. 1988). The metasedimentary rocks are made up of chlorite schist, garnet- and/ or kyanite-bearing micaschist, garnet- and kyanite-bearing gneisses. The protoliths of these rocks are shale and graywacke deposited in an intracontinental rift context or thinned margin (Nzenti et al. 1988; Ngnotue et al. 2000). These metasediments are locally intruded by diorite and/ or granodiorite and metamorphosed under HP–HT conditions (T = 750–800 °C, P = 0.9–1.3 GPa; Nzenti et al. 1984, 1988). Ages obtained for this metamorphism are 620 ± 10 Ma (U–Pb age on zircon; Penaye et al. 1993), 616 Ma (U–Pb age on zircon and Sm–Nd; Toteu et al. 1994), and between 613 ± 33 and 586 ± 15 Ma (Th–U–Pb age on monazite; Owona et al. 2010). In addition, recent works on the Yaoundé series (Yaoundé area) reveal that the melting responsible for the genesis of leucosomes took place between 592 and 658 Ma, while the leucosome hosting rocks (metasediment and metabasite) have been metamorphosed during Tonien–Stenien (911–1127 Ma) period (Ngnotue et al. 2012). The high-grade gneisses of Ntui–Betamba consist of two distinct rock units which outcrop as paving stone or inselberg in the area around Ntui and Betamba (Fig. 1b). The first unit is of sedimentary origin, and it is made up of garnet- and kyanite-bearing gneisses, whereas the second unit is dominated by pyroxene granulite- and sodic amphibole-bearing gneisses which occur mainly as large bodies in the metasedimentary unit. This series corresponds to shallow water, near-shore sedimentary sequence related to an intracontinental extensional basin on the rifted northern extreme of the Congo craton (Ngnotue et al. 2000). Sedimentation was accompanied by alkaline and tholeiitic magmatism which was subsequently metamorphosed to granulite (Ngnotue et al. 1998, 2012).
Materials and methods Fourteen representative and fresh samples of manganesebearing rocks were investigated in terms of their mineral chemistry and whole-rock geochemistry. Efforts were made to minimize cross-contamination and external contaminants of the sample during transportation and preparation. Polished thin sections were prepared from representative samples at CRPG Nancy (France) using conventional techniques. Mineral compositions were determined for the following minerals: garnet galaxite (Mn-spinel) and rutile. The mineral chemical analyses were performed using a CAMECACAMEBAX electron microprobe. Standard operating conditions included an accelerating voltage of 15 kV, a beam current of 15 nA, and a beam size of 5 lm for micas and
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3 lm for feldspars. Detection limits were between 0.05 and 0.1 wt%. For whole-rock geochemical analysis, all the samples were pulverized to obtain a homogeneous sample, out of which 50–60 g was obtained for the analyses. Chemical analysis was performed using the pulp at ALS Minerals Global Group, Vancouver (Canada). Whole-rock analyses for major elements were performed by inductively coupled plasma atomic emission, and trace elements were carried out by inductively coupled plasma mass spectrometry (ICP-MS) from pulps. 0.2 g of rock powder was fused with 1.5 g LiBO2 and then dissolved in 100 mm3 141 5 % HNO3. The REE contents were determined by ICP-MS from pulps after 0.25 g rock powder was dissolved with four acid digestions. Analytical uncertainties vary from 0.1 to 0.04 % for major elements; from 0.1 to 0.5 % for trace elements; and from 0.01 to 0.5 ppm for rare earth elements. The remaining trace elements were analyzed by ICP-MS after 0.5 g split of sample pulp is digested in aqua regia. Loss on ignition (LOI) was determined by weight difference after ignition at 1,000 °C. Various standards were used, and data quality assurance was verified by running these standards between samples as unknowns. Analytical uncertainties are currently better than 1 % for major elements and 5–6 % for trace elements concentrations. Analysis precision for rare earth elements is estimated at 5 % for concentrations >10 ppm and 10 % when lower.
Results Occurrence of manganese‑bearing rocks Manganese-bearing rocks at Ntui–Betamba are concentrated in the upper part of the Betamba hill made up of kyanite–garnet-bearing gneiss which belongs to the lower part of the Ntui–Betamba series. Manganese rocks are closely associated with quartzites of the upper part of this series. They occur as dismantled lenses and are in the form of polyhedric blocks that vary from 50 to 120 cm in diameter (Fig. 2a) with an average trend of N70°E. These rocks are foliated with fine- to medium-grained black manganese-bearing minerals. It alternates with millimetric to centimetric quartz layers, parallel to that of kyanite–garnet-bearing gneiss, garnet- and pyroxene-bearing gneisses of the Ntui–Betamba series. With this characteristic, we assume that manganese-bearing rocks constitute a continuous layer interstratified in the metasedimentary-hosted rocks. Also, primary sedimentological features such as bedding are partly preserved. Generally, manganese-bearing rocks outcrop more massively than the metasedimentaryhosted rocks (Fig. 2b).
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A
B
Gal Gal
Qtz Qtz
Cry Gal
Cry Cry
Gal
Gal
Gal
Cry
Grt Grt
QQtz tz
Qtz
Qtz
QtzGra
Qtz
C
D
Fig. 2 Field occurrences (a, b) and microstructures (c, d) of studied rocks. a Outcrop of Mn-rich rocks as dismantled blocks; b hand specimen of galaxite-bearing quartzite showing their finely layered structure. c Photomicrograph of garnet and galaxite-rich quartzite show-
ing the mineral assemblage (field of view = 0.5 mm across). d Zoned crystals of galaxite with cryptomelane-rich rim (field of view = 1 mm across). Gal galaxite, Grt garnet, Cry cryptomelane, Qtz quartz
Mineralogy of manganese‑bearing rocks
light layers. Galaxite (Al2O3 = 0.2–0.23; MnO = 0.49– 0.51; Table 1) crystals are subhedral, often concentrated in manganiferous–alumina-rich layers. They are surrounded by cryptomelane (K2O = 0.4–0.5; Al2O3 = 0.2–0.3; MnO = 0.71–0.72) rim (Table 1) and show chemical zoning from core to rim. This zoning is caused by the change in composition from galaxite to cryptomelane (Fig. 2c) due to the weathering. The average MnO content in galaxite zone (crystal core) is 49.54 wt%, while it is 67.29 wt% in cryptomelane zone (crystal rim). The zoning patterns with rims enriched in Mn are common in the rock and may indicate the weathering of galaxite in superficial environment. Garnet crystals are euhedral and occur densely packed with grain boundaries touching each other. Some garnet crystals show optical inhomogeneity with rutile-rich inclusion. Most of the garnet crystals are orientated pointing to pre- and syn-tectonic crystals. Microprobe measurements show that the amounts of Mn and Al are similar (19.89– 20.79 wt% MnO; 20.65–21.83 wt% Al2O3), but significantly different to that of Ca, Fe, and Mg (Table 2). FeO contents range from 10.47 to 11.13 wt%. CaO and MgO contents range from 6.98 to 7.80 wt% and from 2.33 to
Petrographic study reveals that Mn-rich rocks from Ntui– Betamba occur as gray dark- to pinkish-colored layers of various thicknesses, interbedded in metapelites of Neoproterozoic age. These metamorphic Mn-rich rocks are fine- to coarse-grained, show a fine compositional banding, and exhibit granoblastic microstructure. They contain abundant coarse quartz ribbons, medium- to coarse-grained spessartine-garnet, galaxite, cryptomelane with accessory rutile and opaque minerals. Based on manganese-bearing minerals, two main types have been distinguished, namely garnet–galaxite-bearing quartzite and galaxite-bearing quartzite. Garnet–galaxite‑bearing quartzite The garnet and galaxite-bearing quartzite at Ntui–Betamba display a granoblastic heterogranular microstructure (Fig. 2c). Mineral assemblage includes quartz (50–60 %), garnet (10–15 %), galaxite (15–20 %), graphite (
