Study on Hydrothermal Deposit Modeling and Metallogeny
in the Islamic Republic of Mauritania
- Akjoujt area -
1 h- email@example.com
Geological Survey of Japan, AIST
Institute for Geo-Resources and Environment
Report of the second field survey in Mauritania – Akjoujt area
Outline of the akjoujt area and Akjoujt mine ···············································1
Location map of the Akjoujt mine and the Gleitat el khader of Cu prospect
The Akjoujt mine ··································································································2
Rock types of the Akjoujt mine area ······························································3
Limestone (carbonate rock)
Siderite bearing Magnetite ore ········································································7
Sulfide minerals ···································································································8
Occurrences of Copper oxide ··········································································9
Boundary between chlorite-schist and magnetite ore ······························10
Some occurrences of supergene vein ··························································11
Route map ·············································································································12
Route map of the Akjoujt mine
The core storehouse ··························································································13
The Gleitat el khader Cu prospect ··································································15
The Tabrinkout Cu-W prospect in the eastern Akjoujt ······························17
Cu prospects in the southern part of the Akjoujt area ······························18
Outline of the Akjoujt area
Gleitat el khader
Fig.1 Location map of the Akjoujt mine and the Gleitat el khader Cu prospect
Fig.2 View of the Akjoujt mine from the east. Fig.3 Carbon-in-pulp installations at the Akjoujt
mine (cyanide leaching waste is rich in copper)
Fig.4 Open pit of Akjoujt mine
from the southeast.
Arrows show the viewpoints of
1 the photos below (see figures 5
The Akjoujt mine
Fig.5 Wall rock of the pit (view from 1: see figure 4)
Fig.6 Wall rock of the pit (view from 2: see figure 4)
The Akjoujt (Guelb Moghrein) mine : Copper production from the Akjoujt mine began in 1952 by the
Societe des Mines Cuivre de Mauritanie (MICUMA), and chalcopyrite bearing ore was taken from the level of
90 m below the surface. Unfortunately, it had ceased production in 1958 due to poor metallurgical techniques.
From 1967, mining resumed with the property owned by the Societe Miniere de Mauritanie (SOMIMA; on
Anglo-American company), and ore was taken from the openpit between the surface and several tens of meters
depth. During the following period, up until 1978, oxide copper was mined. It is assumed that the total
production of copper ore during the 1950s and the 1970s was 17Mt, with an average ore grade of 2.5% Cu,
3g/t Au and 305ppm Co. During 1994-1995, exploration including drill holes (total 8000 m) had conducted by
General Gold International SA (GGISA) of Australia. As a result of that exploration, copper ore r eserves
23.6Mt at 1.88% Cu, 1.41g/t Au and 143ppm Co, were reported. Production of gold from tailings by
conventional carbon-in-pulp leaching process could restart when the gold price increases beyond 375$/ounce.
The tailing amounts to 3Mt, with an average grade of 3g/t Au. Mine remains closed today.
Fig.7 Malachite is observed in the outcrops within the Fig.8 Malachite and Cu-oxides are developed on the
square (see figure 8). Arrow shows sheared zone in surface and along fractures/fissures.
Rock types of the Akjoujt mine area
Metamorphic rocks in the Akjoujt mine area are composed of chlorite-schist and meta-gabbroic schist with
small amount of amphibolite and limestone. They generally strike NW and dip 30-60° to the W, although
chlorite-schist in the northern part of the mine strikes NE. Some occurrences of these rocks are showing below.
Fig.9 Chlorite-schist composed mainly of chlorite and albite are distributed in the mostly Akjoujt mine area.
Chlorite-schist is locally deformed into phyllite with asbestos around sheared zones throughout the mine area.
Segregation veins occurred as layer or lens forms within the chlorite-schist
Fig. 10 Segregation veins consists of coarse-grained Fig.11 Porphyloblastic texture is locally observed.
Fig.12 Strike of chlorite–schist gradually changes from NNE to
NW in ascending order from northern to southern parts of the
Meta-gabbroic rock, composed mainly of pyroxene and plagiocla se, is massive and hard. Small amounts of
pyrrhotite and pyrite also occur locally.
Fig.13 This rock occurs as blocks or lenses between chlorite-schist. Meta-gabbroic rock is locally deformed into
serpentine around sheared zones throughout the mine area.
Fig.14 Segregation veins are observed in the meta-gabbroic rock. They generally show more ductile texture
relative to that in chlorite schist. Coarse-grained quartz is dominant in these veins.
Fig.15 A boundary between chlorite-schist and
meta-gabbroic rock is concordant with schistosity.
Meta-gabbro does not intrude chlorite-schist.
Amphibolite is sporadically distributed in the mine area. This rock occurs as blocks or lenses in
meta-gabbroic rocks and is composed of plagioclase, amphibole, and clinopyroxene with variable
amounts of quartz.
Fig.16 Amphibolites tend to occur along quartz between vein-quartz and meta-gabbroic rock during
segregation veins. They are coarse-grained and the metamorphism.
equigranular, probably formed by bimetasomatism
Limestone (carbonate rock)
Fig.17 The limestone was yellowish brown to dark brown in color due to the presence of siderite and occurred
as lenses within chlorite-schist. This rock reacted with hydrochloric acid and foamed. In the Akjoujt mine area,
these carbonate layers are considered to be replaced by magnetite ore with siderite. Original carbonate layer
varied in thickness from a few centimeters to several meters.
Siderite bearing Magnetite ore
Fig.18 Typical occurrence of ore, bearing magnetite Fig.19 Appearance of magnetite in broken oxidized
with small amounts of chalcopyrite. Blackish dots in ore. The magnetite luster has suffered from
rock are magnetite. Matrix of the ore is totally weathering.
oxidized due to weathering.
Fig.20 Comparatively fresh ore. Gangue mineral Fig.21 Magnetite ore shows ball-shaped pod. The
consists mainly of siderite or scapolite (?) which has centers of pod are composed of asbestos and talc,
lozenge and long pillar forms and two cleavages which are surrounded by magnetite mantle.
which intersect by about 80 degrees. Hardness of this Magnetite may simultaneously precipitates with these
mineral is estimated around 5-6. asbestos and talcs.
Scapolite is an important gangue mineral in a wide variety of the skarn deposits, including Fe, Au, W, Cu,
Zn-Pb, Sn, Mo, and REE skarns. Scapolite-group minerals have a general formula M 4 T12O24 A, where M=Na and
Ca, T=Si and Al, and A=Cl, CO3 , and SO4 . Marialite (Ma) and Meionite (Me), the only two presently accorded
species status, have respective idealized anhydrous end-member formulas of Na4 Al3 Si9 O24 Cl and
Ca4 Al6 Si6 O24 CO3 (Bayliss, 1987). The parameter of atomic molar percent Me=100Ca/(Ca+Na+K) has been
widely used to quantify the scapolite solid-solution series.
Scapolite occurs most commonly in endoskarns or as part of sodic metasomatism within intrusion, but is
sometimes present in exoskarns. The common association of scapolite with economic mineralization in skarns
makes it potentially useful in mineral exploration.
Fig.22 Euhedral chalcopyrite are recognized in
the magnetite ore (see pen point in photo), and
also recognized as dots in magnetite grains.
Pyrrhotite and pyrite are also major sulfide
minerals in the Akjoujt mine.
Fig.23 Chalcopyrite , pyrrhotite and pyrite are
observed in a coarse-grained quartz vein
(mesothermal vein type). This vein develops
beneath magnetite ore zone (it means foot-wall
of ore zone). Squares in the photograph show
areas that are zoomed below (figure 24 shows
area 1, figure 25 shows area 2).
Fig.24 Sulfides develo ped along with the vein that is Fig.25 Sulfides developed as aggregates in the vein.
concordant with strike and dip of meta-gabbroic Chalcopyrite, pyrrhotite and pyrite tend to be locally
schist. associated with vein.
Occurrences of Copper oxide
Fig.26 Malachite: clastiform banding is formed along
clacks in chlorite schist.
Fig.27 Kaolinite and covelin vein formed in
chlorite schist. Kaolinite and covelin are
interpreted to have formed as a result of
drain-back of an acid fluid that result from
weathering of sulfide minerals such as
chalcopyrite. Such supergene processes also
result in the formation of chalcedony in
chlorite-schist by decomposing albite. Al and Si
in albite were supplied for forming kaolinite,
and the rest of the Si was consumed in the
formation of amorphous silica. Na in albite may
be distributed to zeolite group minerals (see the
occurrences of supergene veins).
Fig.29 Veins of center part of this
picture show a ladder like structure. It
Fig.28 Quartz vein in chlorite-schist consists of well-crystallized suggests that structural activities
quartz, which seems to be a mesothermal vein system. Strike and occurred after formation of veins.
dip of veins are concordant with schistosity. In this mine site, there
are two kinds of veins. One is segregation, the another one is of
hydrothermal origin including chalcopyrite related to Copper
mineralization. Veins in these pictures are segregation type.
Boundary between chlorite-schist and magnetite ore
Fig.30 Some boundaries are recognized in the mine area. These boundaries exhibit concordant contact with
chlorite-schist. It i also suggested that the primary limestone layer between chlorite-schist may have been
replaced by magnetite ore.
Fig.31 Biotite bearing chlorite-schist tends to occur Fig.32 Euhedral magnetite and asbestos occur in the
around contact with the magnetite ore. chlorite –schist near the contact with magnetite ore.
Some supergene vein occurrences
Mauritania has a dry desert climate. In such environment, it is indispensability important for the geological
survey to consider supergene mechanism. The supergene mechanism is considered to be as follows. However,
the past climates of the region were wetter, probable humid tropical during the age of Mesozoic, Cenozoic, etc.
As a result of weathering, primary plagioclase (albite), commonly contained in chlorite-schist and
meta-gabbroic rocks, is partly altered to kaolinite , while primary mafic silicates of chlorite, clinopyroxene and
hornblende are partly altered into asbestos, talc, or serpentine. Residual silica of that weathering process could
have p recipitated as amorphous silica, cementing these supergene vein or host rocks. It is thought that Na
bearing alunite may also be included in the supergene products.
Fig.33 These supergene veins exhibit various
occurrences. Some of these are concordant
with bedding planes of chlorite-schist, others
cut bedding planes almost vertically. Veins
exhibiting vertical bedding suggest that the
solution of supergene origin drains back
along cracks in the host rock due to gravity.
Fig.34 Kaoline-amorphous silica vein in
In the schist, quartz and mica remain, but plagioclase
is not preserved. Leaving many pores with “ghost”
plagioclase shapes. It is interpreted that the kaolinite
may have been caused largely by the decomposition
of albite, which supplied Na, Al, and Si. This suggests
that the decrease of elements in weathered schist is
ascribed to the predominance of Al-, Si-, and
Fig.35 GPS on the supergene kaolinite-amorphous Na-bearing supergene minerals, such as kaolinite,
silica vein. amorphous-silica, and alunite. Therefore, veins of
supergene origin included kaolin (=Al),
amorphous-silica (=Si), alunite (=Na).
In addition, it is possible that an area with these supergene veins may be detected with kaolinite and/or Na
bearing alunite anomalies in the analysis of satellite images. It may be mistaken that kaolinite and/or alunite
reflect the shallower part of the epithermal deposits (e.g., hypogene acid sulfate alteration zone in the high
sulfidation system, steam heated environment of the low-sulfidation style).
On the contrary, it is quite effective for understanding of interpretation to refer PIMA analyses from Kadiar
Fig.36 Arrow shows the same direction of that in the route map below.
Attention: Scale of “1:5000" in the map is not correct (refer the scale bar)
Fig.37 The route map of Akjoujt open pit
The core storehouse
Fig.38 The storehouse of cores for diamond and
RCdrill holes which had been conducted in the
Akjoujt mine during 1994-1995.
Fig.39 DDGM3 17-22m : boundary between Fig.40 DDGM4 17-20m : magnetite ore zone
chlorite-schist and siderite bearing magnetite ore
Fig.41 DDGM4 17-20m : disseminated chalcopyrite Fig.42 RCGM57 86-91m : disseminated chalcopyrite
zone in the siderite bearing magnetite ore zone in the siderite bearing magnetite ore
Fig.43 RCGM73 69-74m : disseminated chalcopyrite Fig.44 RCGM74 104-110m : magnetite megacryst in
zone in the siderite bearing magnetite ore the siderite zone including chalcopyrite dots
The Gleitat el khader Cu prospect
Fig.45 The Gleitat el khader of Cu prospect viewed Fig.46 The Gleitat el khader of Cu prospect: Arrow
from the north side. Arrow shows location of the shows silicified schist (see below).
prospect. In this area, BRGM had conducted
exploration including drilling from 1992 up to 1997.
Fig.47 Chlorite-schist showing NS strike may have
suffered from silicification due to hydrothermal
alteration relate to gold mineralization.
Fig.48 Silicified breccia zone is developed in the
chlorite-mica schist. There are some drill holes where
direction is vertical to the breccia zone.
Fig.49 Quartzite and chlorite-schist are included in the breccia. Matrix of the breccia is mainly composed of iron
oxide. Silicified part looks like chalcedonic quartz. It means that silicification occurs at the lower temperature. It
is reported that gold mineralization averaging less than 1-2 g/t of Au content has occurred in the breccia zone.
Fig.50 Chalcopyrite and copper-oxide are
observed in the matrix.
Fig.51 Zoom-in of the breccia. Quartzite is
Fig.52 Chlorite-schist is also included in the
breccia at the square part of picture.
The Tabrinkout Cu-W prospects in the eastern Akjoujt area
The Tabrinkout area has a known Cu-W occuarence.
Fig.53 View to northeast from gossan in the Tabrinkout area. Gravel of iron oxide is widely distributed in
and around Cu-W prospect.
Fig.54 Some trenchs were carried out by Fig.55 This area had also been tested by drilling
BRGM in the 1990’s by BRGM in the 1990’s.
Fig.56 Crystaline limestone which consits mostly of
calcite occurs as relict block in the siderite bearing
Fig.57 Malachite occurs in the gossan with
Cu prospects in the southern part of the Akjoujt area
1. Magnetite and siderite are dominant in the gossan.
3 2. A lot of iron-concretions, called “Pisoliths”, on the
3. Magnetite in the gossan showsdifferential weathering.
4. Dolerite dyke in the epidotized rock
5. An epidotized rock containg grossular
6. Muscovite-talc schist showing folding
7. Pebbles of q uartzite are dominant in the region which
developd around epidotized rocks. There is a clear
difference of color and composition between photo 2 and 7.
Field-based observation suggests the following features.
1) Volcaniclastic and epiclastic rocks composing chlorite-carbonate sequence are extensively distribute around
Akjoujt mining area. (see from Figures 7 to 19)
2) The ore body varies in size and reaches more than 50 meter in width. The boundary between ore body and
schist is consistent with local schistosity. (see from Figures 30 to 32, and 37)
3) The thicker part of ore body seems to be located at the plunge of the fold hinge within the primary carbonate
layer. (see Figure 37)
4) The Fe-oxide copper-gold ore consists mainly of magnetite and siderite in the Akjoujt mine and occurs as
massive and strata-bound structures. (see from Figures 18 to 21)
5) Magnetic susceptibility was tested at some outcrops in the mine. Siderite-bearing magnetite ore shows
distinctive high values, more than 300 SI unit; while other rocks such as chlorite-schist, meta-gabbro, and
amphibolite (with the exception of pyrrhotite bearing meta-gabbro showing values of 1 to 2 SI unit) show
very low values, less than 1 SI unit. Therefore, Exploration for Fe oxide Cu-Au deposits in Mauritania,
regional aeromagnetics survey is quite effective to detect high magnetic response originated in massive
6) Based on observation of cores from diamond drilling, Chalcopyrite is disseminated in siderite-bearing
magnetite ore. Chalcopyrite surrounding magnetite grains is also observed. (see from Figures 39 to 44)
7) A Chlacopyrite-bearing quartz vein is observed in the central part of open-pit of the Akjoujt mine (see from
Figures 23 to 25). This vein is located at footwall of magnetite zone. It may suggest that vein was formed
from upcoming deeper fluids, which were responsible for copper mineralization.
8) Gossan at some Cu prospects around Akjoujt mine have formed hilltop “leached cap” gossans. It clearly
indicates that magnetite-siderite layers were strongly oxidized to Fe-oxide with various amounts of malachite,
due to the dry climate. Kaolinite bearing amorphous silica veins are also developed in the mine area due to
supergene processes. It is quite necessary to consider that weathering is stronger in the 10,000 years ago,
when this area was a sub-tropical, and also tropical humid zone in Tertiary and Mesozoic time. (see pages 11)
Lecorche et al. (1989) summarized the thermal history in the southern part of the Mauritanides belt. These
Rb-Sr and 40 Ar/39Ar ages on the felsic igneous rocks indicate ca. 675 to 685Ma, while depositional peaks of the
known mineralization of BIFs are at around 1850, 2450, 2700, and 3000Ma. Accordingly, there is no prominent
correlation with BIFs based on the chronology.
Besides, These characters of Akjoujt mine quite similar to that of the Fe oxide Cu-Au deposits in Brazil
(e.g., Serra Pelada, Cristalino, Igarape, Bahia -alemao, Salobo, and Sossego in the Carajas mineral province).
In the Sierra Pelada Au -PGE deposit, Au -PGE ore display a strong structural control, being hosted in
subgreenschist faces carbonaceous and calcareous metasiltstone, within the hinge zone of a reclined synform.
The entire ore body has undergone deep tropical weathering. Gold -PGE mineralization is associated with the
formation of magnetite-, hematite -rich hydrothermal breccias, massive zone of hematite metasomatism, siderite
veining and a jasperoid envelope of amorphous silica alteration. Ore -related mineral assemblages have
undergone intense weathering to hydrated Fe oxides and secondary clay minerals (Grainger et al. 2002).
This kind of deposits not only has Cu-Au but also PGE (Pd-Pt). For exploration, it is strongly
recommended that PGE contents must be checked even in low level geochemical anomalies. Gold and PGE’s
can be transported in saline, oxidizing, and acidic fluids, as chloride complexes in equilibrium with hematite.
Precipitation of gold and Pd-Pt can be induced by a decrease in temperature, with a rapid decline in solubility
below 300•Ž by an increase in pH and /or reduction of the fluid. The most obvious mechanism for deposition of
Au-Pd-Pt ores is reduction caused by the carbonate host rocks, with an increase of pH due to dissolution of
To conclude: the Akjoujt mine is categorized as a Fe oxide hosted Cu-Au deposit.
Lecorche, J. P., Dallmeyer, R. D., Villeneuve, M. (1989) Definition of Tectonostratigraphic terranes in the
Mauritanide, Bassaride, and Rokelide orogens, Weat africa, Geological Society of america specoal paper 230.
Grainger, J. C., Groves, I. D., and Costa, C, H, C. (2002) The epigenetic sediment-hosted Serra Pelada Au-PGE
deposit and its potential genetic association with Feoxide Cu-Au mineralization within the Carajas mineral province,
Amazon craton, Brazil. Economic geology, special publication 9, 47-64.