Geological study of gold indices at Ako’ozam, Akom II ... et al.STD2014.Vol15.Ful… · A Ako'ozam-Akom II, dans l’unité du Nyong, ... Heavy minerals encountered in eluvium include

Sciences, Technologies & Développement, ISSN 1029 - 2225

98

Résumé

Une étude géologique a été effectuée en vue de caractériser la minéralisation d'or à Ako'ozam-Akom II (Sud-Cameroun). L'étude inclut une

description macroscopique et microscopique des roches qui affleurent dans le secteur ainsi qu’une description des matériaux éluviaux et

alluviaux ; les minéraux lourds présents dans ces matériaux ont été séparés puis déterminés et une étude morphoscopique des particules d'or a

été effectuée. A Ako'ozam-Akom II, dans l’unité du Nyong, affleurent des gneiss à amphibole recoupés par des veines de quartz, des gneiss à

grenat et des amphibolites à grenat. Ces roches se composent essentiellement de minéraux tels que l’amphibole, le grenat, le quartz et les

feldspaths. Le matériau éluvial montre, dans sa partie supérieure, un horizon brun noir humifère, au dessus d’un horizon brun rouge nodulaire ; les

alluvions présentent de haut en bas une couche limoneuse gris clair à brun jaune, une couche sableuse grise à brun jaune, une couche graveleuse

noir grisâtre et une couche argileuse clair à noir. Les minéraux lourds identifiés dans le matériau éluvial sont le zircon, la muscovite, l’apatite et le

grenat ; alors que hornblende, muscovite, tourmaline, grenat, monazite, zircon, apatite et sphène sont présents dans les alluvions. L’or se

retrouve uniquement dans les alluvions avec une teneur à l’excavé de l’ordre de 3,7g/t ; les grains d'or ont des formes irrégulières, indiquant qu'ils

ont subi un transport court.

ISSN 1029–2225©2014 Sciences, Technologies et Développementos

Mots clés: Alluvion, éluvion, minéraux lourds, or, Akom II.

Abstract A geological study was carried out to characterize gold mineralization at Ako’ozam -Akom II (South-Cameroon). This included a macroscopic and

microscopic description of rocks outcropping in the area, a description of eluvial and alluvial material, with the heavy minerals present in the

material and a morphoscopic study of alluvial gold found in the area. At Ako’ozam-Akom II, part of the Nyong unit, garnet gneiss, garnet amphibolite

and amphibole gneisses occur as decimetric to decametric boulders with quartz veins on the latter. The main minerals in these rocks are

amphibole, quartz, garnet and feldspar. Superficial eluvial material consisted of a dark brown humiferous horizon and a reddish brown nodular

horizon whereas alluvial material was comprised of a light grey to brownish yellow silty layer, grey to brownish yellow sandy layer, white to greyish

black gravelly layer and a light to dark clayey layer. Heavy minerals encountered in eluvium include zircon, muscovite, apatite and garnet whereas

hornblende, garnet, muscovite, tourmaline, monazite, zircon, apatite and sphene were present in alluvium. The gold grains were only found in

alluvium, with an average excavated gold grade of 3.7g/t; they are generally angular indicating they have undergone a short transport.

ISSN 1029–2225©2014 Sciences, Technologies et Développementos

Key words: Alluvium, eluvium, heavy mineral, gold, Akom II, South Cameroon.

1. Introduction 12In Cameroon, gold is known to occur in eastern and

northern regions. It has been mined artisanally since the last

century, with mining focused on alluvial, eluvial deposits and

weathered quartz veins. Recent studies on these areas

provide informations on residual or primary gold

mineralization suggesting that the country is prospective for

gold in East- Cameroon (Suh et al., 2006; Asaah, 2010) and

North - Cameroon (Embui et al., 2013) to south-western

Chad (Tchameni et al., 2013). In the South region, artisanal

mining have been ongoing in a number of localities notably

Abiete, Mefoup, Bilobe, Bipindi, Njabilobe, Mvie, Ako’ozam-

Akom II, in SW Cameroon; but no major study has been

carried out and literature on gold mineralization in that part of

the country is scarce except from ongoing exploration work

by gold exploration companies such as African Aura Mining

Inc. (2009). The present study is a preliminary descriptive

* Corresponding author : [email protected]

study aimed at providing petrographic information on gold

mineralization at Ako’ozam near Akom II.

2. Geologic setting

SW Cameroon consists of the Ntem, Nyong and

Oubanguides complexes (Owona et al., 2011). The Ntem

complex constitutes the NW part of the Congo craton

exposed in SW Cameroon (Maurizot et al., 1986). Akom II

belongs to the Nyong unit (fig.1), which is a Paleoproterozoic

mobile belt resulting from the Eburnean/Transmazonian

tectonothermal event (2400-1800 Ma) assosociated to the

collision between the Congo and Sao Francisco craton

(Toteu et al., 1994; Shang at al., 2004) under amphibolitic

conditions (Owona et al., 2011-2013). This unit is typified by

(i) a greenstone belt (pyroxenites, peridotites, amphibolites,

talcschists and banded iron formations);(ii) foliated series

(tonalite-trondhjemite- granodiorites, orthogneiss); and (iii)

magmatic rocks (augen metadiorites, granodiorites, and

syenites (Champetier de Ribes and Aubague, 1956). It is

affected by D1-D3 polyphase deformation, overprinted by the

Sciences, Technologies et Développement, Volume 15, pp98-106, Février 2014

http://www.univ-douala.com/sdt/

E-mail : [email protected]

ISSN 1029 - 2225

Geological study of gold indices at Ako’ozam, Akom II region (South Cameroon)

Fuanya Christopher1, Yongue Fouateu Rose1*, Kankeu Boniface2

1Department of Earth Sciences, Faculty of Science, University of Yaoundé-1, B.P 812, Yaoundé, Cameroon 2Institute of Geological and Mining Research, B.P 4110,Yaoundé, Cameroon

Received : June 2013 Revised: Febuary 2014 Accepted: Febuary 2014 Available online: Febuary 2014

Fuanya et al., Sciences, Technologies et Développement (Février 2014), Volume 15, 98-106


99

D2 shear deformation during the Eburnean orogeny (Toteu et

al., 1994; Penaye et al,. 2004; Lerouge et al., 2006) with

microstructural evidence of dynamic recrystallization features

that were active under granulite conditions (Owona et al.,

2013).

Figure 1. Geologic map of South-West Cameroon, after Maurizot et al., 1986.

3. Methodology

3.1. Field study

Field study was done with the aid of standard field

equipments. Hammer prospection aided in identifying the

various rock types in the area. At each location the various

rock types observed were systematically described using

observable field parameters (color, mineralogy and structure)

and sampled.

Two eluvial pits were also dug at shallow depth (about 1m)

due to the thick vegetal cover of the area and to the presence

of a hard nodular horizon which rendered digging difficult.

The horizons were described and 30 liters of eluvium

obtained and panned in nearby streams to obtain the

concentrate.

Alluvial prospection in the various streams in the locality was

also carried out. This involved digging 16 pits at favorable

alluvial sites, a detail description of the various layers present

in each pit, the measurement of gravelly layer thickness,

thickness of the barren material, excavated thickness, length

and width of each pit dug. Stream sediments were collected

at points of optimal mineral concentration such as confluence

points, along meanders and rocky portions of the stream. In

each pit, 30 liters of gravel was measured and panned to

obtain the concentrate. Sampling points for eluvium and

alluvium are shown on figure 2.

3.2. Petrographic study

Petrographic study involves the macroscopic and

microscopic description on the field and in the laboratory

respectively. Macroscopic observation starts on the field

whereby, the different rock samples collected from various

outcrops were observed with the naked eye. Microscopic

observation of rock thin sections was carried out under a light

microscope.

The petrography of heavy minerals obtained from eluvial and

alluvial prospection was equally done in the laboratory. The



100

aim of this was to identify the various heavy minerals

associated to the gold mineralisation and also to see whether

they reflect the mineral composition of rocks in the study

zone.

Figure 2: Sketch map of alluvial and eluvial pits.

In order to mount heavy mineral thin sections, a sieve

analyses was carried out on the mineral concentrates as

follows. They were separated according to their grain size in

a column of AFNOR sifters (diameters ranging from 0.2 mm

to 0.35 mm). After the grain size separation of the

concentrates, those with diameter between 0.2 mm to 0.25

mm were used to prepare thin sections before being

identified under a polarizing microscope. Heavy minerals of

the 0.2 mm to 0.25 mm phase were separated from lighter

ones by pouring the concentrate in bromoform (density >2.7

gcm-3) using a separating funnel. Magnetic minerals were

later hand-picked with a magnet. The heavy minerals

collected were washed with 10% hydrochloric acid for about

20 minutes, in order to eliminate the iron oxide film.

3.3. Morphoscopic study

This study was done with the aid of a binocular magnifying

glass of JENA type. The purpose of this study was to

describe the shapes and surfaces of the gold grains

recovered from each heavy mineral concentrate sample,

using the terminology employed by Gary Nichols (2009) in

describing sediments. A total of 371 gold grains were

collected after washing 30 liters of alluvium collected in each

well opened along the Otong-Meyeng, Otong-Bevaa,

Eveeve, Monezingui and Mone-Abontane streams (fig. 2).

3.4. Evaluation of content

It aims at calculating the gold grade in the gravel/clay horizon

and in the excavated material recovered from each

prospection pit. The data used in this evaluation includes the



101

following; thickness of gravel (mineralised horizon), thickness

of barren material, excavated thickness, the surface of each

prospection pit, number of pans of gravel washed and the

weight of gold grains from each prospection pit. The gold

grade in gravel (tgr), is the grade obtained in 1m3 of gravel

and is given by the relation tgr = W×N/n, the gold grade in a

square meter of the gravel (tc) is the product of the gold

grade in the gravel by the thickness of gravel tc = tgr × g and

the gold grade in the excavated material in cubic meter (te) is

that obtained in 1m2 of the whole column and is given by the

relation te= tc / (g + s), where n is number of pans washed, N

number of pans in meter cube, W weight of the grains

collected in the alluvium, g thickness of the gravel, s

thickness of the barren material , H total thickness of the

alluvium (table 1).

Figure 3: Macroscopic and corresponding microscopic view of rocks (a,b: amphibole gneiss; c,d: garnet gneiss; e.f: garnet

amphibolites; Grt =garnet; Pl= plagioclase; Hbl= hornblende; Qtz= quartz)

4. Results

4.1. Petrography

Amphibole gneiss is the most represented rock in the area.

They occurred as decametric boulders and outcrops, with the

latter having quartz veins (Fig.3a). It has a granolepidoblastic

texture containing the following minerals: hornblende (45%),

garnet (15%), plagioclase (15%), biotite (10%), orthoclase

(5%), quartz (5%), microcline (5%), sphene (1%) and opaque

minerals (Fig. 3b).



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Garnet gneiss occurring as metric boulders, with a

heterogranular granoblastic texture comprised of garnet

(25%), plagioclase (10%), quartz (45%), amphibole (15%)

and opaque minerals (Fig. 3c and d).

Garnet amphibolite occurring as decimetric blocks with a

granolepido porphyroblastic texture which consists of

amphibole (60%), garnet (15%), quartz (5%), plagioclases

(15%) and opaque minerals (Fig. 3e and f).

Figure 4. Sketch of eluvial pits

Figure 5. Sketch of alluvial pits.


Dark brown soil with organic matter

Legend

Brown soil with weathered rock fragments and nodules

Reddish brown soil

Pit 1 Pit 2


103

Figure 6. Heavy minerals in alluvium and eluvium (a:sphene, b:andalusite, c:garnet, d- apatite, e- hornblende, f-zircon)

Figure 7. Morphoscopy of gold grains (a: sub angular grain with rough surface from FK2, b: irregular grain with spongy surface from FK4, c: angular grain with rough surface from FK6, d: rounded grain from FK2, e: flattened grain from FK8, f: angular grain from FK14)



104

4.2. Eluvium

Superficial eluvial studies at Ako’ozam enabled the

identification of three soil horizons; a dark brown humiferous

horizon of about 10cm, a pebbly brown soil horizon and a

reddish brown nodular soil horizon of about 50cm (Fig. 4).

4.3. Alluvium

Alluvial prospection enabled the identification of silty, sandy,

gravelly/pebbly and clayey layers of varying thicknesses. The

most representative pits are shown on figure 5.

4.4. Heavy mineral assemblage

Heavy mineral assemblage in eluvium include muscovite,

zircon, apatite, garnet while garnet, zircon, tourmaline,

epidote, hornblende, monazite, andalusite, sphene,

muscovite, corundum, apatite are in alluvium. Photographs of

heavy minerals are shown on figure 6.

4.5. Gold morphoscopy

The morphoscopy of gold grains from each prospection pit

shows that they are generally irregular in shape.

Sample FK2 (Otong-Meyeng stream)

A total of 81 gold grains were obtained in the gravel horizon

of pit FK2, having a weight of 0.41 g. The gold grains have a

brownish yellow color and show a wide range of shapes

ranging from angular to sub-round. The surfaces of the

grains are rough and blunt. Some grains are elongated and

contain dissolution voids, while others appear corroded (Fig.7

a).

Sample FK4 (Eveeve stream)

73 gold grains weighing 0.42 g were obtained in pit FK4 and

they have a bright yellow color. Their shapes vary from

angular to sub-round. Their surfaces are rough and blunt and

some grains are flattened (Fig.7 b)

Sample FK5 (Otong-Bevaa stream)

A total of 12 gold grains having a weight of 0.012 g were

obtained from pit FK5 and they have a bright yellow color.

They have sub-angular to sub-rounded shapes and their

surfaces are rough and blunt.

Sample FK6 (Otong-Bevaa stream)


they have a bright yellow color. The grains vary in size and

shapes. Their shapes range from very angular to angular to

sub-angular to sub-round. The surfaces are rough and blunt

with some grains having dissolution voids. Very few grains

appear flattened (Fig.7 c).

Sample FK8 (Mone-Abontane stream)

A total of 65 gold grains weighing 0.05 g were obtained in pit

FK8 and they have a bright yellow color. The grains are

angular to sub-rounded. Their surfaces are rough and some

grains are flat (Fig.7 e).

Sample FK14 (Mone-Zingui stream)


they have a bright yellow color. The grains are sub-angular,

sub-rounded to round. Their surfaces are rough, though the

grains which appear rounded have smooth surfaces (Fig.7 f).

4.6. Gold grade evaluation

Data used in the evaluation of gold grade and results

obtained after the calculations are shown on tables 1 and 2.

Calculations gave an average gold grade in gravel of 9.633

g/m3 and 3.735 g/m3 in the excavated material.

5. Discussion

The geology of the study zone consists of gneiss and

amphibolite of the Nyong unit, which is a metasedimentary

and metaplutonic rock unit, which underwent the eburnean a

high-grade tectono-metamorphic event at ~ 2050 Ma

associated with charnockite formations (Lerouge et al.,

2006).

Gold deposits in the eastern region of Cameroon occur in

quartz vein and as altered wall rock (Suh et al., 2006), in the

north as hydrothermal quartz veins and in sulphides (Embui

et al., 2013). Freyssinet et al. (1989) studied the dispersion of

gold and base metals in the lateritic milieu in the East and

found that it is difficult to predict the behavior of these metals

in such a milieu and that weathering plays an important role

in the dispersion of these metals leading to the formation of

dispersion halos around the mineralization.

Results of initial geochemical prospection for gold and

platinium-group elements (Au-PGE) in pyroxenites,

amphibolites, and their weathered products in the Nyong unit

suggested that the potential of the mafic-ultramafic bodies of

the Nyong unit as Au-PGE deposits is low (Ebah Abeng et

al., 2012). In the southern part of Gabon, in the Archean

gneisses and Proterozoic greenschist facies, gold is

associated with quartz, pyrite, tourmaline and muscovite; a

50-m-thick lateritic profile formed in situ from these rocks is

mineralized (Edou Minko et al., 1992), the residual gold

particles become free and chemically rounded. According to

Asaah (2010), gold mineralization in South Cameroon is

associated with Archean to Palaeoproterozoic greenstone

belts, BIF and ultramafic rocks which have been deformed by

shear zones and faults. The geochemical prospection of gold

and PGE on pyroxenites, amphibolites and their weathered

products in this context indicate low contents (Ebah Abeng et

al., 2012). This context is also similar to that of the gold

bearing formations within a metamorphosed greenstone belt,

with host rocks consisting of migmatites, gneisses,

amphibolites, magnetite-rich quartzites (Andrianjakavah et

al., 2007). The presence of quartz veins in some sections of

the area may be an indication of the source of gold in this

locality. The flatness of the area prospected and the scarcity



105

of outcrops did not allow a good structural study, thus for a

better understanding of the structures in the area, the study

sector needs to be widened.

Alluvial and lateritic gold deposition in the weathering

environment involves a complex but interrelated sequence of

chemical, physical and biological weathering processes

governed essentially by relief (Strahler and Strahler, 1992).

The various rock types are progressively exposed to

changing environmental conditions and as time passes and

relief diminishes, zones of mineralization undergo complex

sequences of change (Strahler and Strahler, 1992).

The irregular shape of gold particles from the study zone

indicates that they have undergone little or a short distance

of transport from their source. This is also supported by the

fact that the streams from which these gold grains were

obtained (Otong-Meyeng, Otong-Bevaa, Eveeve, Mone-

Abontane and Mone-Zingui) are all first and second order

streams. The morphoscopic results also show that the

mobility of the gold particles is accompanied by a

modification of their surface and shape. The morphology of a

grain of gold is inherited from its primary state and to a large

extend, irregularities of gold grains in source rocks

predetermine grain morphology in an alluvial setting.

Significant morphological changes in gold particles are

attributed to distance of sorting, or time spent in fluvial

systems (Youngson and Craw, 1999). Chemical dispersion of

gold and associated base metals could be produced during

the downward progress of the weathering front where by

mineralized veins are dissolved in the upper horizons and

elements are dispersed and concentrated in the surrounding

horizons (Freyssinet et al., 1989; Colin et al., 1993;

Ouangrawa et al., 1996). During these surficial processes,

resistant minerals like gold, zircon and tourmaline remain as

tracers of the genesis and evolution of laterites (Ouangrawa

and Yongue, 2001).

The fine-grained size of the gold grains in alluvium could be

explained by the process of fractionation and partial

dissolution of the primary gold grains which favors the

increase of fine fraction of the gold as meteoric weathering

progresses (Edou Minko et al., 1992).

The similarity between the heavy minerals in eluvium (garnet,

zircon, apatite, and muscovite) and in alluvium (garnet,

zircon, tourmaline, hornblende, epidote, andalusite,

muscovite, apatite, corundum, sphene) indicates that they

may have originated almost from the same source rocks.

Nevertheless as only the upper part of the eluvium was

sampled, the minerals present only in alluvium may indicate

rocks variety and heterogeneity.

The average crustal abundance of gold is 0.005 g/t, while the

average gold content in the excavated material in the study

zone is 3.735 g/t signifying that the gold indices in the region

may be of economic importance.

6. Conclusion

Ako’ozam-Akom II is mineralized with alluvial/placer gold.

The geologic formations of the area are high grade

metamorphic rocks of the amphibolites facies comprised of

amphibole gneiss, garnet gneiss and garnet amphibolite.

With the action of weathering favored by the high

precipitation in the region, gold was liberated from the

country rocks with their structural potential traps. Its presence

in the alluvial system as secondary mineralization is

accounted for by the erosion and transport of the weathered

materials. The gold grains are less morphologically evolved;

the similarity between the heavy mineral assemblage of

eluvium and those of alluvium indicates probably that the

alluvial gold originates from the weathering of neighboring

country rocks.

Average gold grade in gravel and in the excavated material

indicate that the gold indices at Ako’ozam may be of interest.

The morphoscopic character of the gold (more or less

spongy surface, reduced size, angular form) proves its

weathering and mobilization in a lateritic environment.

Acknwoledgements

The first author wishes to thank his family for their financial

and material support which aided enormously for the

accomplishment of this work. Profound gratitude to Dr. Ngo

Bidjeck Louise-Marie for her help in gold grade evaluation.

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Documents

Geological study of gold indices at Ako’ozam, Akom II ... et al.STD2014.Vol15.Ful… · A Ako'ozam-Akom II, dans l’unité du Nyong, ... Heavy minerals encountered in eluvium include