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Mineralium Deposita

International Journal for Geology,

Mineralogy and Geochemistry of

Mineral Deposits

ISSN 0026-4598

Miner Deposita

DOI 10.1007/s00126-012-0426-3

Mineralogical characterization of the

Nkamouna Co–Mn laterite ore, southeast

Cameroon

G. Lambiv Dzemua, S. A. Gleeson &

P. F. Schofield1 23

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12 months after publication.ARTICLE

Mineralogical characterization of the Nkamouna Co–Mn

laterite ore, southeast Cameroon

G. Lambiv Dzemua & S. A. Gleeson & P. F. Schofield

Received: 8 August 2011 /Accepted: 5 June 2012

# Springer-Verlag 2012

Abstract The Nkamouna property is an oxide laterite de-

posit developed on serpentinized peridotite in southeast

Cameroon. It is enriched in Co and Mn, has sub-economic

Ni grades and will be mined primarily for Co. The ore zone

is ca. 10 m thick and comprises the lower breccia (∼3m

thick) and ferralite (7–8 m thick) units sandwiched between

an 8-m-thick ferricrete overburden and a barren hydrated

Mg–silicate saprolite. The ore mineral assemblage includes

Mn oxyhydroxides, magnetite, maghemite, ferritchromite,

goethite, hematite, kaolinite and gibbsite. Lithiophorite is

the most common Mn mineral and is the main host of Co,

Mn and a significant proportion of Ni. It occurs as coatings

in pores and on other mineral grains and as concretions and

impregnations in the matrix. It is invariably associated with

gibbsite in the lower breccia and with magnetite and ferrit-

chromite in the ferralite. Although ore in the lower breccia is

volumetrically less important than the ferralite, it has the

highest grade and Co/Ni ratio. The lithiophorite in the ore

zone is authigenic, and its formation was enhanced by influx

of Al

from the overlying ferricrete. Magnetite and ferrit-

chromite in the ferralite are relicts and contributed to min-

eralization by enhancing the permeability of the ferralite and

providing substrates for the precipitation of the Mn oxy-

hydroxides. The structure and mode of occurrence of the

lithiophorite makes Nkamouna ore amenable to physical

beneficiation, producing a concentrate with Co grades 2.3–

4.5 times higher than the run-of-mine ore.

Keywords Asbolane . Lithiophorite . Nkamouna .

Cameroon . Cobalt laterite . Nickel laterite . Serpentinite

Introduction

Nkamouna is one of seven oxide laterite deposits developed

over serpentinized ultramafic rocks in the Lomié region of

southeast Cameroon (Fig. 1). These deposits are unusually

enriched in Co and Mn but are Ni poor (Yongue-Fouateu et

al. 2006; Lambiv Dzemua et al. 2009, 2011). Nkamouna is

at an advanced exploration stage and has over 54 Mt of

proven and probable reserves, grading at 0.25 % Co, 1.33 %

Mn and 0.69 % Ni (Volk and Bair 2009). It would be the

first laterite deposit globally to produce Co as the main

commodity as well as the first commercial mine in Came-

roon. Similar to other oxide laterite deposits, the ore zone is

a goethite-dominated ferruginous limonite (Yongue-Fouateu

et al. 2006, 2009; Lambiv Dzemua et al. 2009, 2011) al-

though goethite is not an ore mineral. The mineralogy and

geochemical composition of the Nkamouna ore, which are

key factors in the economics and processing of laterite

deposits (Gleeson et al. 2003; Dalvi et al. 2004;Wedderburn

2010), have not been studied to date.

The mineralogy of lateritic deposits is complex and hetero-

geneous at macroscopic and microscopic scales and, thus, is

notoriously difficult to characterise. As grain sizes become

prohibitively small, it is very difficult for traditional methods

of microscopy such as electron microprobe and image analy-

sis to precisely characterise individual phases (e.g. Schofield

et al. 2002). As a consequence, despite obtaining accurate

Editorial handling: R.P. Xavier

G. Lambiv Dzemua (*) : S. A. Gleeson

Department of Earth & Atmospheric Sciences,

University of Alberta,

Edmonton, AB, Canada T6G 2E3

e-mail: lambiv@ualberta.ca

P. F. Schofield

Department of Mineralogy, Natural History Museum,

London SW7 5BD, UK

Miner Deposita

DOI 10.1007/s00126-012-0426-3

Author's personal copyspatial chemical associations, definitive identification ofmany

of the relevant phases remains a challenge.

X-ray powder diffraction (XRPD) is a well-established,

standard method for phase identification within multiphase

material but also possesses certain limitations (e.g. Lan-

franco et al. 2003). Traditional XRPD necessitates destruc-

tive homogenisation of the sample to produce a

representative powder with an approximately even distribu-

tion of particle sizes. In so doing, all spatial information is

lost. Furthermore, XRPD is a bulk technique with detection

limits, even in favourable circumstances, no better than

about 1 % (e.g. Ma et al. 1994; Cressey et al. 1999), and

phase identification becomes increasingly tenuous as the

relative proportion of the phase decreases. Equally, as the

crystallinity of the phases decreases so does the quality of the

diffraction data, making unambiguous identification of minor

phases in a multiphase mixture very difficult. Moreover,

deviations fromchemical endmembers of solid solution series

may lead to misleading or significantly different X-ray dif-

fraction patterns compared to those obtained from pure syn-

thetic standards that are commonly used for identification

purposes. These well-established techniques do not provide

definitive mineralogy at the level required for a full petroge-

netic or geochemical understanding of lateritic deposits. How-

ever, XRPD experiments performed non-destructively on the

same polished sections that have been examined using the

electron microprobe allows for far more precise phase identi-

fication coupled with elemental distributions. Furthermore, by

maintaining the spatial integrity of the sample, these identi-

fications may be put into textural and chronological context.

Asbolane and lithiophorite are common in Ni laterite

profiles and are generally the main carriers of Co and Mn

(Manceau et al. 1987; Llorca and Monchoux 1991; Yongue-

Fouateu et al. 2006; Roqué-Rosell et al. 2010). They are

Fig. 1 Location and regional

geology map showing

Nkamouna and other mineralized

laterites in Lomié district

(modified from Lambiv Dzemua

et al. 2009)

Miner Deposita

Author's personal copygenerally very fine-grained with poor crystallinity and occur

as dark blue to black nodules, concretions and coatings. They

have similar optical properties and considerable overlap in

their principal XRD peaks (Perseil 1972; Chukhrov et al.

1980a, b, 1982; Manceau et al. 1987; Llorca and Monchoux

1991; Yongue-Fouateu et al. 2006; Roqué-Rosell et al. 2010),

and distinguishing between them is very challenging but

imperative for the purposes of ore processing and genetic

studies. Although their crystal structures are broadly similar,

they have clear chemical differences. Lithiophorite ((Al,Li)

MnO2(OH)2) consists of alternating MnO6 and Al(OH)6 oc-

tahedral sheets whereas asbolane is practically an Al-free Mn

hydroxide (Manceau et al. 1987; Post 1999). A combination

of in situ chemical and diffraction analyses is, thus, necessary

to discriminate these phases.

Goethite is the principal ore mineral in oxide laterite

deposits, and the ores are generally not amenable to physical

beneficiation (Beukes et al. 2000; Moskalyk and Alfantazi

2002; Dalvi et al. 2004). However, the nature of the Nka-

mouna ore makes it physically upgradable, and this is an

important economic incentive of the project (Lambiv Dzemua

et al. 2009; Volk and Bair 2009).

This paper documents the ore mineral assemblage of the

Nkamouna cobaltiferous laterite, identifying the principal ore

mineral, its location and mode of occurrence in the profile and

its chemistry and that of associated phases. We also tested the

suitability of the Nkamouna ore for physical beneficiation by

analysing various size fractions of physically upgraded (PUG)

samples for their mineralogical composition.

Location and geologic setting

Nkamouna is located 30 km east of Lomié town in the Upper

Nyong Division of the East Region of Cameroon (Fig. 1). The

climatic conditions in the study area are described in (Lambiv

Dzemua et al. 2009, 2011). The regional topography is dom-

inated by a series of low plateaus (<800 m above mean sea

level) with gentle slopes dissected by the tributaries of the

Congo River drainage basin. Primary rainforest with a thick

stratified canopy dominates the vegetation.

The regional basement rocks belong to the Neoprotero-

zoic Yaoundé series which is one of the major units that

make up the Yaoundé domain—an allochthonous unit which

was thrust southward over the Archean Congo craton (Toteu

et al. 2006). The Yaoundé domain is bounded northward by

undifferentiated Pan-African units dominated by granitoids

and gneisses (Suh et al. 2008). The Yaoundé series com-

prises garnet–mica–chlorite schists, quartzite, gneiss and

amphibolites into which were emplaced tectonic slices of

ultramafic units (Fig. 1) during the Pan-African Orogeny

(Nedelec et al. 1986; Seme Moungue 1998; Lerouge et al.

2006; Toteu et al. 2006). The ultramafic rocks are

completely serpentinized into massive dark green to black

serpentinite dominated by antigorite and magnetite (Lambiv

Dzemua and Gleeson 2012). Tropical weathering of these

serpentinites has produced thick laterite deposits with resid-

ual and supergene Co, Mn and Ni mineralization.

The Nkamouna laterite profile

The weathering profile in Nkamouna profile averages about

20 m thick and comprises a sequence of six regolith units

which have been identified and named by project geologists.

Their terminology does not strictly follow recommended def-

initions for laterites (Eggleton 2001; Eggleton et al. 2008)but

is adopted in this paper. The units include (from bottomto top)

serpentinite, saprolite, ferralite, lower breccia, ferricrete and

upper limonite (Fig. 2). A thin transitional gravelly unit,

known as upper breccia,may be present between the ferricrete

and the upper limonite. A detailed description of these units is

given in Lambiv Dzemua et al. (2009). The ore zone com-

prises the lower breccia and the ferralite.

Sampling and analytical techniques

The weathering profile was studied in over 2,000 explora-

tion test pits, which are manually dug holes about 1 m in

diameter, extending either to the water table or to the base of

the saprolite (∼23 m). Two trenches, about 15 m deep, have

also been excavated in the western and southern edges of the

deposit to test the vertical and lateral continuity of mineral-

ization. The southern trench is about 30 m long and transects

all the regolith units. Over 200 disturbed and undisturbed

samples were collected from the test pits and trenches by the

authors. Undisturbed loose samples were collected using

improvised sample boxes following the procedure described

by Stoops (2003). The samples were consolidated with

EpoThin epoxy resin, cut and polished into uncovered thin

sections. The thin sections were studied using transmitted

and reflected light microscopy, and micromorphological

features were described and interpreted according to Stoops

(2003). Selected mineral phases were analysed in situ for

their chemical compositions using JEOL 8900 and Cameca

SX100 electron microprobes (EMP), equipped with energy

dispersive spectrometers and wavelength-dispersive spec-

trometers. The microprobes were operated at 20 kVacceler-

ating voltage, 15 nA probe current and 3 μm beam diameter,

and the peak and background count times were varied

between 20–50 and 10–25 s, respectively. Data reduction

was performed using Φ(rZ) correction (Armstrong 1995).

They were calibrated using albite 639 for Na, chromite 639

for Cr, cobalt 639 for Co, diopside 639 for Ca and Si,

fayalite EPS1 for Fe, forsterite93 EPS1 for Mg, ilmenite

Miner Deposita

Author's personal copyfor Ti, synthetic Ni metal for Ni, orthoclase for K, pyrope

for Al and willemite for Mn. The calibrations were deemed

successful when the composition of secondary standards

was reproduced within the error margins defined by count-

ing statistics.

Bulk XRPD analyses were performed using two separate

diffractometers. Firstly, a Rigaku Geigerflex 2173 diffrac-

tometer equipped with a cobalt anode (Kα1), graphite

monochromator, an iron filter and an X-ray tube operating

at 40 kV and 30 mA was used to collect diffraction patterns

from 0° to 90° on a 2 theta scale. Cobalt Kα1 radiation was

selected from the primary beam by a germanium 111 crystal

monochromator with the X-ray tube operating at 30 kV and

35 mA. Horizontal and vertical slits were used to restrict the

beam to a height of 0.24 mm and length of 4.0 mm. Meas-

urements were made in reflection geometry with the sample

surface at an angle of 7.5° to the incident beam.

The various mineral phases were analysed in situ on

polished sections using electron microprobe and non-

destructive X-ray microdiffraction (μXRD). The μXRD

data were collected using a Nonius PDS 120 diffraction

system consisting of an Inel-curved, position-sensitive

detector within a static beam-sample-detector geometry

(Schofield et al. 2002). This system allows the simultaneous

measurement of diffracted X-ray intensities at all angles of

2θ across 120°. A 100-μm-diameter beam was selected from

a 300-μm-diameter primary beam by a pinhole at the end of

a collimator evacuated tube to reduce air scattering. High

brightness Cu Kα radiation was generated by a GeniX

system with a Xenocs FOX2D CU 10_30P mirror operating

at 50 kVand 1 mA. Silicon and silver behenate were used as

external standards; calibration and data collection were per-

formed using Diffgrab™ with an average data collection

time of 180 min. Measurements were made in reflection

geometry; the sample surface was brought into the focal

point of the beam using a Zeiss Axio Cam MRc5 CCD

optical system. Although the beam itself was circular, its

footprint on the sample pictured in Fig. 3 is ellipsoidal (with

Fig. 2 A typical profile

(NKM672 located at the centre

of the of deposit) through the

Nkamouna regolith showing the

different units, their average

thicknesses (in parentheses)

and typical composition

Miner Deposita

Author's personal copydimensions of ca. 100×500 μm) because of the inclination

of the sample slide relative to the incident beam. The sample

was viewed with a CCD image capture system and was

manipulated by a manually controlled x–y–z stage move-

ment. The fixed focus, limited depth of field, of the optical

system was used to bring the sample to the correct position

to be intercepted by the X-ray beam. The exact position of

the X-ray beam was first determined by recording its image

on a fluorescent screen marked with a fiducial spot; the

position where both the fiducial spot and the beam are

coincident and in focus marks the point in x–y–z space

where the beam intercepts the specimen. This point is

recorded on the image on a PC monitor. Thereafter, any

object placed in focus at this point will be intercepted by the

microbeam. Phase identifications was by pattern matching

using JCPDS database of the International Centre for Dif-

fraction Data and standard material from the mineral collec-

tions at the Natural History Museum, London, UK.

Limonitic Ni laterites ores, typically dominated by goe-

thite (Gleeson et al. 2004; Freyssinet et al. 2005; Thorne et

al. 2009; Golightly 2010), are generally not amenable to

physical beneficiation; however, the Nkamouna ore can be

physically upgraded due to the nature of the ore. The

Fig. 3 Photomicrograph showing X-ray microdiffraction beam analy-

sing thin section sample in situ. The ellipsoidal shape of the beam is

due to the inclination of the sample slide relative to the incident beam

Fig. 4 Common field textures

and structures of the ore zone. a

Photograph showing lower

breccia consisting of

uncemented nodules. Note the

deep brown core and yellowish

cortex of the inserted nodule. b

Cemented nodules with bluish-

black Mn-oxide coating, over-

grown by a (transparent)

gibbsite coating. c Ferralite

showing its diagnostic stripy

texture with black lenticular

bands embedded in a yellowish

brown matrix. Note the pres-

ence of deep red hematite

selvedges associated with the

bands. d Ferralite crosscuts by

fractures containing bluish

MnO phases. e Termite channel

containing material from the

upper limonite and black Mn

oxide phases along the walls. f

Ferralite showing termite chan-

nels infilled with quartz con-

taining bluish black Mn stains

Miner Deposita

Author's personal copyprocess involves attrition washing of the ore to remove

the fine ferruginous gangue materials and produce a

lithiophorite-enriched concentrate with significantly higher

Co, Ni and Mn grades. Eleven samples (six from the lower

breccia and five from ferralite), selected on the basis of

assay results provided by Geovic,

were used to test the

effectiveness of this process and to study the particle size

distribution of the ore assemblage. The samples were mod-

erately crushed (manually) in an agate mortar, washed in

beakers using water and the fines were decanted off. A bar

magnet, placed around the beaker, enhanced the retention of

finer ore particles. The samples were then dried and vigor-

ously sieved using 212- and 106-μm mesh sieves. Each

fraction was analysed for its mineralogical composition

using XRPD. The mineralogy of some decanted fines (tail-

ings) was also determined.

Results

The ore zone

The ore zone is sandwiched by the ferricrete and the sapro-

lite and comprises the lower breccia and the ferralite. The

lower breccia is a transitional horizon dominated by hetero-

geneous subangular to spherical nodular material. The nod-

ules are typically <2 cm in diameter and are internally

massive or concentrically layered with a dark-brown core

and a yellowish-brown cortex (Fig. 4a). They are either

loose or cemented and commonly contain detrital fragments

of other minerals including quartz, maghemite and ferrit-

chromite (Fig. 4a, b). In mineralized profiles, the nodules

are typically coated with successive layers of bluish black

Mn oxyhydroxides and gibbsite (Fig. 4b).

The ferralite is essentially a plasmic (homogeneous) ho-

rizon with no primary serpentinite fabric and averages 8 to

10 m thick. It is characterized by abundant bluish black

lenticular bands in a yellowish brown lateritic matrix giving

the ferralite a distinctive stripy appearance (Fig. 4c). The

Geovic (www.geovic.net) is the company that owns the Nkamouna

and other laterite deposits in the Lomié area.

Fig. 5 XRD pattern of selected

ore zone samples indicating the

profile name, depth interval

(metres), lithology, mineralogy

and Co assay (from Geovic).

Gbs gibbsite, Gth goethite, Hem

hematite, Kln kaolinite, Lithio

lithiophorite, Mgh maghemite/

magnetite, Qtz quartz, LB lower

breccia, Fl ferralite

Miner Deposita

Author's personal copylenticular bands are generally few (<2) centimetres thick and

several centimetres long, with their long axes oriented gen-

erally parallel to the topography. The matrix is dominated by

soft yellowish to reddish brown limonite consisting of soft

granular porous aggregates. The ferralite is commonly

crosscuts by fractures containing unidentified sooty bluish

black Mn oxyhydroxides (Fig. 4d). Also common in the

ferralite are slickensides and termite galleries that common-

ly contain quartz and/or bluish black Mn phases (Fig. 4e, f).

The Mn phases are generally common along the walls of the

channels (Fig. 4e). The ferralite is separated from the sap-

rolite by a gradual pedoplasmation front as defined by

Anand and Butt (1988).

Mineralogy

The Nkamouna ore consists essentially of Fe, Mn and Al

oxides and hydroxides, with goethite and hematite being the

major phases. Both goethite and hematite occur together and

are poorly crystallized with considerable overlap in their

Fig. 6 Common ore textures. a Reflected light photomicrograph

showing the internal structure of concentric nodules in the lower

breccia. b Simultaneous transmitted and reflected light photomicro-

graph (crossed polar) showing lithiophorite hypocoatings and coatings

superimposed by gibbsite in the lower breccia. Note the diagnostic

yellowish colour of goethite in the upper left corner. c Photomicro-

graph (simultaneous transmitted and reflected light) showing gibbsite

crosscutting lithiophorite coatings and hypocoatings in the lower brec-

cia. d Backscattered image of a nodular concretion with detrital mag-

netite in the centre enclosed by successive layers of laterite matrix,

lithiophorite and euhedral coarse gibbsite in the lower breccia. Note the

presence of abundant fine magnetite fragments in the laterite matrix. e

Reflected light photomicrograph (crossed polarized light) showing

magnetite with lithiophorite overgrowth in the ferralite. f Backscattered

image of ferritchromite with lithiophorite encrustations. Note irregular

boundaries of ferritchromite grains. g Reflected light photomicrograph

(crossed polarized light) showing pseudomorph of lithiophorite aggre-

gate. h Backscattered electrons image showing non-crystalline lithio-

phorite–asbolane intermediate with pyrolusite inclusions. i Reflected

light photomicrograph (crossed polarized light) showing cryptocrys-

talline cryptomelane and coarse pyrolusite in pores in the ferricrete/

lower breccia

Miner Deposita

Author's personal copyphysical characteristics and XRPD patterns. They were dif-

ferentiated using combined microscopy and XRPD; goethite

had a moderately strong peak at 0.418 nm and hematite at

0.18 nm. Under simultaneous transmitted and reflected

light, goethite appeared yellowish whereas hematite retained

its reddish brown colour. Other important iron oxide phases

include magnetite, maghemite and ferritchromite. Most of

the magnetite is variably oxidized to maghemite and martite/

hematite, which are more common in the lower breccia than

in the ferralite. Maghemite was identified by XRPD and

martitic hematite by its strong reflectance and crystal habit.

In polished sections, ferritchromite is characterized by abun-

dant submicroscopic pores that give it a spotted appearance.

Lithiophorite is the main manganese phase and was

detected in all samples with significant Co mineralization.

It occurs as indurated bluish to black concretions associated

with gibbsite and/or magnetite/ferritcromite. Its identifica-

tion was confirmed by strong sharp XRPD peaks at 0.943,

0.471 and 0.237 nm (Fig. 5). It is highly abundant in the

lower breccia and its abundance decreases sharply in the

ferralite. Other manganiferous phases include pyrolusite,

cryptomelane and lithiophorite–asbolane intermediates.

These were less important and were observed only in fewer

samples. Pyrolusite and cryptomelane were observed

mainly in ferritcrete/lower breccia samples from one

profile. Similarly, lithiophorite–asbolane intermediates

were observed (using EMP) only in one ferralite sample

from deeper level. Finally, an unidentified soft and sooty

bluish black manganeferous ‘wad’ was observed in fractures

in the ferralite.

Other components of the ore zone include gibbsite,

quartz and kaolinite. These phases are more prevalent in

the lower breccia and were readily identifiable in hand

specimens, thin sections and XRPD patterns (Fig. 5). Quartz

is common in the ferralite especially towards the base of the

horizon where it characterizes the silcrete subunit.

Table 1 Typical composition of the lithiophorite in the Nkamouna ore zone

n CaO Al2O3 FeO MgO SiO2 MnO CoO NiO Total Co/Ni Unit Lab. Sample

41 0.04 25.61 0.34 0.09 0.11 35.66 6.81 2.86 71.75 2.4 LB U of A LB004

11 0.06 26.03 0.36 0.06 0.40 28.21 7.50 1.51 64.32 5.0 LB U of A LB005

44 0.03 22.31 0.11 0.07 0.13 39.52 4.05 2.05 68.36 2.0 LB U of A 06LDG015

10 0.10 19.67 0.48 0.08 0.50 33.03 10.75 1.65 66.39 6.5 LB U of A 08GLD037

5 0.01 23.96 0.94 0.03 0.03 41.20 7.11 1.69 75.38 4.2 LB NHM 08GLD032

8 0.01 23.62 1.21 0.05 0.09 41.21 5.47 2.91 74.99 1.9 LB NHM 08GLD037

9 0.05 18.24 14.59 0.22 0.70 27.25 3.06 7.49 72.85 0.4 FL U of A 08GLD025

26 0.09 19.33 14.65 0.09 0.81 23.92 5.37 3.18 67.97 1.7 FL U of A 08GLD026

48 0.09 22.74 7.21 0.08 0.76 27.46 6.62 3.77 68.95 1.75 FL U of A 06LDG009

5 0.06 21.90 5.87 0.25 0.39 32.16 3.32 7.86 73.49 0.42 FL U of A 06LDG009

7 0.09 21.52 5.16 0.13 0.31 31.66 6.58 3.69 70.22 1.78 FL U of A 06LDG009

10 0.07 18.00 20.67 0.21 1.03 24.16 4.49 5.37 75.30 0.84 FL U of A FL006

Samples LB004 is from pit NKM626, LB005 from trench no. 2, FL006 is from pit NKM765, 06LDG009 is from a typical mineralized ferralite in

trench no. 2, 08GLD037 is from pit NKM1028, 08GLD025 and 08GLD026 are from pit NKM1212 and 08GLD032 is from pit NKM672

n number of points analysed in the same phase within the same polished section and their average values are reported, UofA University of Alberta,

NHM Natural History Museum, NA not analysed, <dl below detection limit, FL ferralite

Table 2 Typical composition of lithiophorite–asbolane intermediate in the ferralite

n Al2O3 FeO SiO2 MnO Cr2O3 CoO NiO Total Co/Ni Unit Lab. Sample

5 12.34 0.31 0.09 34.88 0.02 16.95 7.53 73.73 2.3 LB NHM 06LDG016

1 11.64 20.01 5.22 35.7 0.78 2.27 0.73 77.3 3.1 LB NHM 06LDG016

1 14.53 29.98 5.93 16.5 1.06 3.83 1.37 74.51 2.8 LB NHM 06LDG016

1 9.51 32.95 2.68 26.3 0.66 3.07 1.49 77.9 2.1 LB NHM 06LDG016

20 10.53 1.96 0.21 30.25 0.00 13.25 10.44 67.50 1.3 FL U of A 08GLD026

Sample 06LDG016 is from the manganiferous lower breccia exposed along a forestry road near Mwa Ekaha creek flowing in the central western

part of the deposit, and sample 08GLD026 is from NKM1212

n number of points analysed in the same phase within the same polished section and their average values are reported, UofA University of Alberta,

NHM Natural History Museum, NA not analysed, <dl below detection limit, FL ferralite

Miner Deposita

Author's personal copyOre textures

The goethite and hematite in groundmass are generally

cryptocrystalline to fine grained. They are also cryptocrys-

talline in the concentric bands that make up some of the

nodules in the lower breccia (Fig. 6a). Hematite also occurs

as veinlets in the ore zone as well as thin selvedges around

bluish black lenticular bands in the ferralite. In both cases,

hematite is predominantly cryptocrystalline.

Lithiophorite occurs as coatings in pores and on other soil

components (Fig. 6b–f). The coatings consist of cryptocrys-

talline to fine-grained radiating crystals occurring in two to

six colloform bands (up to 200 μm; Fig. 6b, c). It also

occurs as overgrowths on detrital magnetite and ferritchro-

mite grains (Fig. 6d–f) and as hypocoatings,

up to 100 μm

thick (Fig. 6b, c). In the ferralite, lithiophorite is mainly

associated with magnetite and ferritchromite in the bluish

black lenticular bands (Fig. 4c), where it forms irregular,

‘spongy’ crust over magnetite and ferritchromite grains

(Fig. 6e, f); the crust is thicker and well developed around

coarse grains and is largely absent around finer particles. In

some cases, lithiophorite appears to be replacing magnetite/

ferritchromite as suggested by the presence of magnetite/fer-

ritchromite vestiges with irregular boundaries in lithiophorite

(Fig. 6f), and the presence of cubic lithiophorite aggregates

apparently pseudomorphingmagnetite (Fig. 6g). Lithiophorite

is also common along termite channels (Fig. 4e)andas

discrete aggregates not associated with magnetite (Fig. 6g).

Lithiophorite–asbolane intermediate detected in this

study occurs as non-crystalline fillings in pores with com-

mon euhedral pyrolusite inclusions (Fig. 6h). Unlike lithio-

phorite, it is neither associated with gibbsite nor magnetite.

Cryptomelane and pyrolusite occur together as fillings in

pores (Fig. 6i). Cryptomelane is cryptocrystalline and forms

two to three narrow colloform bands along the outer walls of

the pore and pyrolusite consists of coarse subhedral to

euhedral crystals occurring.

Gibbsite associated with lithiophorite occurs in bands (up

to 150 μm thick) post-dating lithiophorite (Fig. 4a, b, d, e)

and consists of coarse subhedral to euhedral crystals. At the

upper interface of the lower breccia, gibbsite is common as

impregnations and fillings in pores associated with kaolinite

and detrital quartz. Kaolinite is the only clay mineral ob-

served in the ore zone; it is rare in the ferralite and occurs

mainly as a thin whitish coating on fractures, joints and

slickensides surfaces. Quartz in the lower breccia is largely

detrital and consists of medium to coarse grains. On the

other hand, quartz in the ferralite consists of fine-to-medium

euhedral crystals common inside termite channels (Fig. 4f).

Quartz also occurs in the silica boxwork that characterises

the silcrete subunit at the base of the ferralite.

Magnetite and ferritchromite occur as randomly distrib-

uted grains and aggregates up to several millimetres in

diameter. Magnetite is also common as large aggregates

consisting of interlocking angular fragments commonly

showing peripheral and/or lamellae oxidation to martite.

Mineral chemistry

The chemical compositions of the various manganese phases

identified in theNkamouna deposit are summarized in Tables 1,

2 and 3. Lithiophorite is cobaltiferous with 3–10 % CoO, 0.7–

8%NiOand18–27 % Al2O3 (Fig. 7), and it is the main ore

mineral. It is depleted inMnO (13–41%) relative to lithiophor-

ite reported in other deposits (Larson 1970; Manceau et al.

1987; Llorca and Monchoux 1991, 1993; Rao et al. 2010;

Roqué-Rosell et al. 2010). Lithiophorite in the lower breccia

is higher in Co, Mn and Al and that in the ferralite is

higher in Ni and Fe (Fig. 7). These differences are reflected in

their Co/Ni ratios which range between 2 and 8 in the lower

Hypocoatings are impregnative/depletive pedofeatures that occur in

the matrix but adjacent to natural surfaces such as voids (Stoops 2003).

Table 3 Composition of pyrolusite and cryptomelane in the ore zone

Mineral n K2ONa2OAl2O3 FeO SiO2 MnO CoO NiO Total Co/Ni Unit Lab Sample

Pyrolusite 9 0.43 0.22 0.54 0.31 0.07 57.08 0.67 0.81 60.33 0.83 FL U of A 08GLD026

Pyrolusite 2 0.02 0.01 0.40 0.12 0.43 78.68 1.23 0.08 81.07 15.3 LB NHM 06LDG016

Cryptomelane 1 1.36 0.18 2.04 0.33 0.12 66 3.99 1.030 75.37 3.9 LB NHM 06LDG016

Cryptomelane 1 0.11 1.06 11.9 0.23 0.1 36.3 16.9 6.880 74.13 2.5 LB NHM 06LDG016

Cryptomelane 2 0.4 0.055 10.6 26.48 3.95 31 2.67 1.11 77.6 2.6 LB NHM 06LDG016

Cryptomelane 1 0.58 0.87 8.9 0.71 0.16 44.2 13.4 6.05 75.55 2.2 LB NHM 06LDG016

Cryptomelane 1 0.99 0.32 6.75 1.11 0.23 52.8 7.73 2.700 73.03 2.9 LB NHM 06LDG016

Sample 06LDG016 is from the manganiferous lower breccia exposed along a forestry road near Mwa Ekaha creek flowing in the central western

part of the deposit, and sample 08GLD026 is from NKM1212

n number of points analysed in the same phase within the same polished section and their average values are reported, UofA University of Alberta,

NHM Natural History Museum, NA not analysed, <dl below detection limit, FL ferralite

Miner Deposita

Author's personal copybreccia and 0.4–2 in the ferralite. No significant compositional

differences were observed between lithiophorite in the differ-

ent colloform bands. The lithiophorite–asbolane intermediates

observed in the profile had lower Al content relative to lith-

iophorite. Pyrolusite is relatively pure, containing only minor

amounts Co and Ni whereas associated cryptomelane contains

significant concentrations of K, Co and Ni. Both minerals are

relatively Al free.

The composition of magnetite and ferritchromite is sum-

marized in Tables 4 and 5. Magnetite is fairly homogenous

with ∼4 % MgO and up to 3 % Cr2O3. Significant trace

elements include Ni (∼0.7 % NiO), Mn (0.6 % MnO) and

Co (0.18 % CoO). There are no significant chemical differ-

ences between magnetite in the ferralite and that in the lower

breccia. The composition of ferritchromite is characterized by

a wide variation in Cr content (3–27 % Cr2O3), 0.1–1.0 %

Al2O3 and fairly constant Co and Ni concentrations with

averages of 0.15 % CoO and 0.4 % NiO. Gibbsite was

relatively pure with no significant impurities. Few microprobe

analyses of goethite in the ferralite showed up to 1 % NiO and

7%Al2O3. However, goethitewas not routinely analysed, and

the analyses are unrepresentative and are not presented here.

The spatial relationships between Co, Ni, Mn, Al, Fe and

Cr in the ore are highlighted using element maps in Fig. 8.

Cobalt and Mn are positively correlated throughout the ore

zone and together with Al show a strong spatial association.

They are hosted mainly by lithiophorite whereas Fe and Cr

are hosted by magnetite and ferritchromite.

Table 4 Composition of magnetite (including maghemite and martite) in the ore zone

n Al2O3 FeOt TiO2 MgO MnO Cr2O3 CoO NiO CuO ZnO Total Co/Ni Unit Lab. Sample

18 0.20 84.08 0.02 3.56 0.65 0.96 0.21 0.57 0.04 0.02 90.59 0.4 FL U of A 06LDG009

6 0.20 81.80 0.03 3.33 0.58 1.14 0.18 0.45 0.06 0.03 88.00 0.4 FL U of A 06LDG009

17 0.12 85.88 0.03 3.73 0.54 0.74 0.19 0.45 0.07 0.02 92.02 0.4 FL U of A 08GLD026

9 0.19 84.92 0.03 3.58 0.45 0.89 0.17 0.45 0.04 0.02 90.97 0.4 FL U of A 06LDG010

0.13 76.27 0.04 5.29 0.61 8.19 0.15 0.66 0.03 0.06 92.32 0.2 FL U of A 08GLD025

0.93 78.98 0.04 3.56 0.52 6.41 0.15 0.64 0.02 0.06 91.67 0.2 FL U of A 08GLD032

0.91 71.72 0.04 2.99 0.68 6.58 0.15 0.81 0.04 0.06 84.59 0.2 FL NHM 08GLD037

38a

0.21 80.24 0.03 4.46 0.59 4.32 0.17 0.65 0.05 0.02 91.40 0.3 FL U of A 06LDG009

Sample 06LDG009 is a typical mineralized ferralite sample from trench no. 2 in the southern margin of the deposit, 06LDG010 and 08GLD037 are

from pit NKM1028, 08GLD025 and 08GLD026 are from pit NKM1212 and 08GLD032 is from pit NKM672

n number of points analysed in the same phase within the same polished section and their average values are reported, UofA University of Alberta,

NHM Natural History Museum, <dl below detection limit, FL ferralite

Magnetite associated with ferritchromite; total iron reported as FeOt

Fig. 7 Co–Ni–Al ternary

diagram (weight percent)

showing the Nkamouna

lithiophorite and lithiophorite–

asbolane intermediate. Note the

differences between the lower

breccia (Co-rich) and ferralite

(Ni-rich) lithiophorite. The

asbolane field is also shown

Miner Deposita

Author's personal copyOre beneficiation and particle size analysis

The mineralogical composition of the PUG concentrates and

their particle analysis are summarized in Table 6. The ferra-

lite was more easily upgraded than the lower breccia be-

cause the ore is loose and the clay-size ferruginous particles

were readily washed away to produce a concentrate domi-

nated by lithiophorite, magnetite and maghemite. In addi-

tion to these phases, samples from the lower breccia

typically contained goethite, hematite, gibbsite, quartz and/

or kaolinite. Figure 9 illustrates the components and pro-

cesses involve in the physical beneficiation process.

There is a clear correlation between the presence of lithio-

phorite in the concentrate and Co content; lithiophorite, with

significant Co concentrations, is present in all concentrates, and

it is absent in Co poor samples (Table 6). Minerals in the

concentrates consist of fine to coarse particles with rare lithio-

phorite, quartz and kaolinite in the −106-μm fraction (Table 6).

The tailings decanted from the concentrate consisted mainly of

goethite, hematite and kaolinite.

Discussion

Ore genesis and paragenesis

The ferralite is an in situ weathering of serpentinite whereas

the lower breccia comprises a mixture of materials derived

from both the ferralite and the overlying ferricrete. The

magnetite and ferritchromite in the ore zone are resistate

minerals inherited from the serpentinite, and they are inti-

mately associated with lithiophorite occurring as dark blue

to black lenticular bands common in the ferralite. The inti-

mate association of lithiophorite and magnetite/ferritchro-

mite is a unique feature of the Nkamouna ore and has not yet

been described in any other laterite deposit. During profile

collapse, magnetite and ferritchromite grains resisted com-

paction and developed micro-porosity that facilitated the

ingress of soil solution and the precipitation of Mn oxy-

hydroxides over the skeletal grains.

Under normal environmental conditions, Mn occurs in

aqueous solution as Mn2+

. The Mn4+

species may occur in

solutions under acidic conditions (Cui et al. 2009). Manga-

nese solubility is controlled by several factors, of which Eh

and pH are the most important (Kabata-Pendias and Pendias

2001; Sethumadhav et al. 2010). Under strongly reducing

conditions, commonly associated with several months of

hydromorphism, both Mn and Fe are rendered soluble. This

usually results in mottling characterized by bleaching of

peds and formation of coatings containing both Fe and Mn

in larger pores (Stoops and Eswaran 1985). The absence of

mottling and Fe coatings in the Nkamouna profile suggests

weak hydromorphism commonly associated with short- Table 5 Composition of ferritchromite in the ore zone

n Al2O3 FeOt TiO2 MgO SiO2 MnO Cr2O3 CoO NiO CuO V2O3 ZnO Total Co/Ni Unit Lab. Sample

4 0.06 51.15 0.10 0.87 0.09 0.49 10.63 0.06 0.34 NA 0.04 0.15 64.00 0.2 FL NHM FL001

4 0.73 59.98 0.19 2.02 0.25 1.00 27.71 0.13 0.41 NA 0.06 0.36 92.85 0.3 FL NHM FL003

8 0.40 54.23 0.12 5.42 0.24 1.88 27.56 0.15 0.43 <dl 0.04 0.54 91.13 0.3 FL U of A 08GLD026

7 1.92 49.36 0.13 4.06 0.31 2.06 28.40 0.13 0.38 0.03 0.05 0.62 87.73 0.3 FL U of A 06LDG010

21 2.05 50.97 0.13 4.69 0.23 2.02 28.56 0.13 0.33 0.03 0.04 0.62 89.92 0.4 FL U of A 06LDG009

22 0.75 53.27 0.12 4.87 0.21 1.73 27.63 0.14 0.40 0.06 0.04 0.56 89.91 0.3 FL U of A 08GLD032

17 0.83 53.23 0.12 4.94 0.21 2.18 26.99 0.20 0.47 0.05 0.04 0.55 89.95 0.4 FL U of A 08GLD037

Sample 06LDG009 is a typical mineralized ferralite sample from trench no. 2 in the southern margin of the deposit, 06LDG010 and 08GLD037 are from pit NKM1028, 08GLD026 are from pit

NKM1212, 08GLD032 is from pit NKM672, FL001 is from pit NKM1360 and FL003 is from pit NKM1368

n number of points analysed in the same phase within the same polished section and their average values are reported, FeOt total iron reported, Lab analytical laboratory, UofA University of Alberta,

NHM Natural History Museum, <dl below detection limit, FL ferralite

Miner Deposita

Author's personal copyduration seasonal saturation of the profile in tropical cli-

mates. Under such conditions, Mn is rendered soluble but Fe

is not. Although Nkamouna has a tropical climate with

several months of wet season, the profile is seldom saturated

because of good internal drainage and the high local evapo-

transpiration rate (Lambiv Dzemua 2005). The solubility of

Mn in soil solutions is significantly enhanced by the pres-

ence of organic ligands, e.g. carboxyl- and amino-bearing

compounds (Putilina and Varentsov 1980).

The precipitation of Mn from (soil) solutions is caused by

several factors including oxidation, evaporative dehydra-

tion, microorganisms and the catalytic action of other min-

eral grains. Oxidation is the most important process because

Mn solubility in oxygenated solutions is very low. Under

neutral to strongly alkaline conditions, aqueous Mn2+

oxidized by air and precipitatedinMnminerals(Burns

and Burns 1979). However, oxidative precipitation of Mn

by air alone is very sluggish because of the high activation

energy of the reaction (McKenzie 1989) though the reaction

can be catalysed by the presence of Mn and Fe

phases

(McKenzie 1989). Precipitation begins with nucleation and

formation of a gel of hydrated Mn oxide on solid surfaces

(Larson 1970).

The type of Mn oxide precipitated directly from solution

is controlled by the physicochemical conditions of the solu-

tion and the environment (McKenzie 1989). The presence of

cations other than Mn2+

in solution inhibits the formation of

‘pure’ end member species (e.g. pyrolusite). Consequently

in soils, pyrolusite is commonly associated with other Mn

oxyhydroxides, which are believed to have ‘purified’ the

solution by removing other cations prior to the precipitation

of pyrolusite (McKenzie 1989). This observation is in agree-

ment with the pyrolusite–cryptomelane association in

Nkamouna in which pyrolusite occurs at the centre of

cryptomelane-lined cavities. Birnessite, vernadite, lithiophor-

ite and hollandite (including cryptomelane) are common in

Fig. 8 Element maps showing the distribution of Fe, Al, Co, Mn and

Ni among some components of the ore zone. Warmer colours represent

higher concentrations. A backscattered electron image at the beginning

of each series shows the textural relationship between various compo-

nents. Samples a and b are from the ferralite, and c and d are from the

lower breccia. a and b are from the ferralite and show the distribution

of indicated element between magnetite/ferritchromite and associated

lithiophorite encrustations. Note: high concentrations of Co and Ni

occur in locally different phases. c and d are from the lower breccia.

c shows a sequence of gibbsite and lithiophorite coatings over lateritic

matrix containing detrital magnetite. d shows lithiophorite enveloped

by gibbsite at the ferricrete/lower breccia boundary

Miner Deposita

Author's personal copyTable 6 Physical upgrade and particle size analyses

Assay (wt.%) Physical upgrade Particle size (μm)

Sample Unit CoO NiO MnO Feed (g) Product Mass (g) +212 +106 −106

NKM264_13-13.5 FL 0.36 0.77 1.53 49.64 Concentrate 18.6 L, M, G, Q L, M, G, Q L, M, G, Q

Tailing 31.04 NA

NKM672_15-16 FL 0.20 0.97 1.11 51.73 Concentrate 21.35 L, M, G, Q M, G, Q M, G, Q

Tailing 30.38 NA

NKM765_14-15 FL 0.01 0.50 0.21 18.71 Concentrate 9.02 M, H, G, K, Q M, H, G, Q M, H, G

Tailing 9.69 M, H, G, Ka

NKM765_18-19 FL 0.30 1.00 1.64 15.53 Concentrate 9.81 L, M, H, G M, H, G, Q M, H, G, Q

Tailing 5.72 L, G, Ka

NKM1278_11-12 FL 0.23 0.64 1.21 22.31 Concentrate 8.32 L, M, H, G, Q L, M, H, G, Q M, H, G, Q

Tailing 13.99 NA

NKM1360_14-15 FL 0.01 0.16 0.11 49.54 Concentrate 2.92 M, H, G, Gb, Q M, H, G, Q M, H, G, Q

Tailing 46.62 NA

NKM264_8-9 LB 0.7 0.6 3.3 NA Concentrate NA L, M, K, Gba

Tailing NA NA

NKM765_8-9 LB 0.01 0.32 0.39 NA Concentrate NA M, K, Gba

Tailing NA NA

NKM1212_9-10 LB 0.01 0.22 0.17 NA Concentrate NA M, K, Gba

Tailing NA NA

NKM1278_9-10 LB 0.19 0.49 1.06 NA Concentrate NA L, M, K, Gba

Tailing NA NA

NKM1380_8-9 LB 0.47 0.59 3.72 NA Concentrate NA L, M, K, Gba

Tailing NA NA

FL ferralite, LB lower breccia, NA not analysed, L lithiophorite, G goethite, Q quartz, K kaolinite, Gb gibbsite, M magnetite+maghemite+

ferritchromite

Sample not differentiated into different particle sizes

Fig. 9 Illustration of the

physical beneficiation of the

Nkamouna ore showing various

components and processes

Miner Deposita

Author's personal copysoils, and their structures can accommodate substantial

amounts of other cations and are believe to precipitate directly

from soil solutions (Taylor et al. 1964; McKenzie 1989;

Sethumadhav et al. 2010). Slow oxidation of K+

-bearing

solutions favours the formation of birnessite (McKenzie

1971). Lithiophorite is the most common phase in highly

weathered soils and has led many researchers to conclude that

it is the weathering product of other Mn-oxide minerals as

summarized in Fig. 10 (Taylor et al. 1964;McKenzie 1989;

Parc et al. 1989; Golden et al. 1993; Dowding and Fey 2007).

Its resistance to weathering has been attributed to the presence

of Al and the high proportion of Mn3+

in its structure (Kim et

al. 2002;Neamanetal. 2004; Dowding and Fey 2007).

Aluminium increases the resistance of oxidized Mn to reduc-

tion by inhibiting electron transfer (McBride 1994; Kim et al.

2002;Neamanetal. 2004).

As shown above, the formation of lithiophorite requires a

high input of Al (Golden et al. 1993; Cui et al. 2009). In

New Caledonia, lithiophorite has been identified only

in laterites associated with aluminous protoliths (Llorca

1993). In the Nkamouna deposit, the serpentinite protolith

has a very low Al content (Lambiv Dzemua and Gleeson

2012) and cannot be the source of Al for the formation of

the lithiophorite. The high abundance of lithiophorite in the

lower breccia suggests a genetic link with the ferricrete,

which has a sialic signature and is acidic (Lambiv Dzemua

2005; Yongue-Fouateu et al. 2006; Lambiv Dzemua et al.

2009, 2011). The acidic nature of the ferricrete enhances the

leaching of Al. Based on this study, it is not known if the

original manganese mineral was lithiophorite or not. How-

ever, the superimposition of gibbsite on lithiophorite indi-

cates Al influx postdated the precipitation of the Mn-oxide

phase. It could be argued that lithiophorite was formed by

transformation of an older Mn-oxide mineral through the

late addition of copious amount of Al and that the super-

imposed gibbsite was precipitated from excess Al left after

the formation of lithiophorite. Although gibbsite could have

also been formed from lithiophorite through reductive dis-

solution of Mn by stronger reducing agents such as organic

matter and Fe

(Parc et al. 1989;Goldenetal. 1993;

Dowding and Fey 2007), this is probably not the case as

suggested by the sharp boundaries between lithiophorite and

gibbsite and their crosscutting relationships (Fig. 6b, c).

Cobalt and nickel enrichment

The Nkamouna ore is significantly enriched in Co. The

enrichment is particularly higher in the lower breccia

(Fig. 7), which also contains high concentrations of Al and

Ni, though the Ni enrichment is relatively higher in the

ferralite (Fig. 7). Lithiophorite is the main host of Co and

Mn in the profile, and its abundance is positively correlated

with Co and Mn grades. Supergene manganese oxyhydrox-

ides are generally enriched in metals especially Co (Burns

1976; Larson 1970; Mckenzie 1989; Kabata-Pendias and

Pendias 2001; Dixon and White 2002; Becquer et al.

2006; Rao et al. 2010). Cobalt-rich lithiophorite has been

observed in the Fort Payne Formation, Tennessee Larson

(1970) and in Nishikhal Mn deposit in Orissa, India (Rao et

al. 2010). In the Fort Payne Formation, lithiophorite is also

mildly enriched in Ni; both Co and Ni are enriched prefer-

entially in the finer-grained and cryptocrystalline phases.

The special strong affinity of Co for manganese oxides

and hydroxides has been observed by many researchers (e.g.

McKenzie 1967, 1970; Burns 1976; Manceau et al. 1987;

Llorca and Monchoux 1991; Kabata-Pendias and Pendias

2001; Yongue-Fouateu et al. 2006; Lambiv Dzemua et al.

2009, 2011). McKenzie (1967) observed a significant en-

richment of Co in pedogenic Mn nodules relative to their

host soils and showed experimentally that the Co enrich-

ment increases with aging after absorption and that adsorbed

Co becomes permanently bound. The enrichment of metals

in Mn-oxide minerals is partly due to their fine particle sizes

which offer larger surface area for metal adsorption. Sec-

ondly, Mn-oxide minerals generally have pH-dependent

charge with a very low point-of-zero charge, which leaves

the minerals with a very strong negative surface charge at

pH values common in soils and consequently a higher

affinity for cations, their surface charge capacity being en-

hanced by their (Mn phases) layered structure (McKenzie

1989). Lastly, Mn oxide minerals have specific adsorption

potential, which is enhanced by their structures which easily

accommodate foreign ions (McKenzie 1989; Dowding and

Fey 2007). Cobalt has special affinity for Mn oxyhydroxides

and unlike Ni and other metals; adsorbed Co becomes ‘per-

manently’ fixed with aging after adsorption (McKenzie 1967).

This phenomenon has been explained by the oxidation of

adsorbed Co2+

to Co3+

by Mn4+

and the subsequent replace-

ment of Mn3+

in the crystal lattice by Co3+

(McKenzie 1989;

Murray et al. 1985; Manceau et al. 1987).

Textural evidence presented in this study, and the geo-

chemical composition of the serpentinite at the base of the

profile (Lambiv Dzemua and Gleeson 2012;Lambiv

Dzemua et al., in preparation) suggests the Co and Mn

present in the ore zone are not sourced from serpentinite.

Although the exact source of Co is not yet known, the

following are some of the possible reasons that account for

Fig. 10 Transformation of other manganese oxyhydroxides to lithio-

phorite (modified after Dowding and Fey 2007)

Miner Deposita

Author's personal copythe high enrichment of Co in the Nkamouna ore. Unlike

other laterite deposits, the Nkamouna ore zone is endowed

with pedogenetic lithiophorite, which like other supergene

Mn minerals has a special high affinity for Co. The lithio-

phorite is dominantly fine-grained to cryptocrystalline and

thus provided a large surface area for the adsorption of Co.

The coincidence of good drainage, high precipitation and

the maturity of the profile are also favourable factors. Al-

though the exact age of the profile in Nkamouna is un-

known, similar profiles in West Africa have minimum ages

of 59–45 Ma (Colin et al. 2005; Beauvais et al. 2008). The

enrichment could have also resulted from reductive dissolu-

tion of Mn. Such a process would lower Mn, Ni and other

loosely adsorbed trace element concentrations in the residue

and increased that of Co residually. The Nkamouna lithio-

phorite has a lower Mn concentration because the perma-

nent fixation of Co by Mn oxides is accompanied by

the release of considerable amount of Mn into solution

(McKenzie 1967). Manganese released in the process is

reprecipitated as soft sooty manganese ‘wads’ with very

low Co observed in fractures in the ferralite. Any released

Ni is remobilized to either deeper levels in the ferralite and

saprolite or leached out of the profile completely. However,

the higher Ni content of the ferralite is also due to residual

accumulation of Ni-enriched magnetite and ferritchromite

(Lambiv Dzemua and Gleeson 2012). The higher Ni content

of the ferralite could also be attributed to the presence of

goethite derived from serpentine. In typical oxide laterite

deposits, goethite developed after serpentine generally con-

tains significant Ni grades and is usually the main ore

mineral (Gleeson et al. 2003; Freyssinet et al. 2005; Thorne

et al. 2009; Golightly 2010).

Ore beneficiation

The physical beneficiation of the Nkamouna ore is one of the

main economic drivers of the project. In the process, goethite

and hematite, the dominant gangue components, are readily

separated into tailings by attrition washing with water, pro-

ducing a low tonnage concentrate with significantly higher

Co, Ni and Mn grades. This property is unique to the Nka-

mouna ore because lithiophorite, the principal ore mineral, is

hard and withstands attrition during washing. Secondly, goe-

thite and hematite occur in clay size particles that float inwater

and are readily separated. Finally, the paramagnetic nature of

lithiophorite and its association with magnetite enhances the

separation of the concentrate from the tailings.

The upgrade factor varies between 2.3 and 4.5 depending

on the type of ore (Volk and Bair 2009); it is generally lower

for the lower breccia ore because of the strong cementation

by gibbsite. The hardness of the lower breccia particles,

however, enhances attrition during washing, and the process

has been optimized by blending of the lower breccia and the

ferralite ores in a 1:9 ratio (Volk and Bair 2009). Excessive

scrubbing must be avoided as it would produce ultrafine

lithiophorite particles which are difficult to separate from the

tailings. However, the recovery of ultrafine lithiophorite could

be improved through the use of wet magnetic separation and/

or hydrocyclone, and the final concentrate would be reduc-

tively leached to release Co, Ni andMn from the lithiophorite.

Conclusions

The weathering profile in Nkamouna is mineralogically and

geochemically complex. The ore zone is about 8 to 10 m

thick and comprises the lower breccia and the ferralite.

Goethite is the principal component of the profile and the

ore zone, and unlike typical oxide-type laterite deposits, it is

not an ore mineral.

Lithiophorite is the principal ore mineral. On hand speci-

men scale, it occurs as hard sooty bluish black concretions,

coatings and infillings in pores. It is pedogenic and is signif-

icantly enriched in Co with a very high Co/Ni ratio (up to

. It

is also the main reservoir of Mn and to some extent Ni. In the

lower breccia, it occurs as coatings intimately associated with

gibbsite, and in the ferralite, it is dominantly associated with

coarse relict magnetite and ferritchromite grains. Textural

evidence suggests that the formation of lithiophorite in Nka-

mouna involved the transformation of a pre-existing Mn-

oxide phase through addition of excess amount of Al leached

from the ferricrete. The contrasting physical properties of

lithiophorite and goethite/hematite make the Nkamouna ore

amenable to a unique physical beneficiation process with

important economic implications.

Acknowledgments This work was funded by Geovic Cameroon Plc

and an NSERC discovery grant to Dr Sarah Gleeson. Field work was

facilitated by Geovic Cameroon geologists and field crew. The authors

also wish to express gratitude to Dr Richard Herrington and Dr Hazel

Hunter both of the Department of Mineralogy, Natural History Museum.

GLD’s trip to London, UK for the microanalyses was partly funded by

research grant from the Society of Economic Geologists Student Foun-

dation. An in-depth review by Patrick Williams and two anonymous

reviewers greatly improved the quality of the manuscript.

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