It is a comprehensive study about Archean, Paleo-Proterozoic, Birimian
base metals, Zn, Cu, Pb sulphides in West Africa. The paper is
published in Geologisches Jahrbuch 2008, Reihe B, Heft 99.
It is well-documented by 28 figures, 3 tables, 2 appendices and one
map of scale 1:2.000.000, covering West Africa.
The Archean and Paleoproterozoic Man Shield in West Africa is an area
of 1.1 million km2 includung localities of Senegal, Guinea, Mali,
Burkina Faso, Niger, Ghana, Ivory Coast, Liberia and Sierra Leone. It
consists of the Archean Kenema-Man Domain at the west (about one
quarter of the Man Shield) and the palaeo- Proterozoic Birimian
Supergroup (BS). There is an Archean- Birimian transition zone in
western Ivory Coast and eastern Guinea where relicts of Archean rocks
are within the BS granite gneiss. The Archean rocks of Kenema Man
Domain consist of the Leonian Cycle (2.9–3.4 Ga) and the Liberian
Cycle (2.6–2.9 Ga). The Archean is subdivided into a basement complex
and supracrustal belts. The basement complex consists of granitoids,
gneiss and migmatite. The supracrustal belts are composed of
greenstones and meta-sediments. The BS is subdivided into an eastern
and a western subprovince. It consists of pyroxenite, amphibolite or
diorite, ultrabasic to intermediate and acid metavolcanic rock,
meta-sedimentary rock with minor volcanoclastic, meta-volcanoclastic
rock, granite, granodiorite, tonalite, quartz diorite, migmatite or
gneiss. About two thirds of the BS consists of granitoids and gneiss,
the rest are meta-sedimentary and metavolcanic rocks. Late granitoid
magmatism was accompanied by regional metamorphism up to upper
greenschist facies.
Birimian MORB or oceanic interplate basalts are of tholeitic type. The
Perkoa Basalt of Boromo belt and basalt, dolerite and gabbro of Sirba
belt are exceptions in the eastern subprovince, where geochemical data
indicate rather a subduction related setting. The Man Shield hosts
seven major Zn and Cu occurrences wich are in the eastern subprovince
of the Birimian and in the Archean-Birimian transition zone of the
western subprovince in Burkina Faso. The Perkoa massive sulphide Zn
deposit has the best economic potential, the four porphyry copper type
chalcopyrite occurrences in Burkina Faso and chalcopyrite
disseminations in gneiss in Ivory Coast are less important.
The seven major and 109 minor occurrences are hosted by
Palaeoproterozoic rocks of the BS. Eleven minor occurrences are known
from the Archean, hosted by low grade metamorphic rocks in Sierra
Leone and granulite facies rocks in Liberia and Ivory Coast. These
sulphide occurrenses are mainly: sulphide-poor deposits of gold, iron,
manganese or titanium, but sulphides in quartz veinlets, sulphide
disseminations and other types of sulphide occurrences are present
too.
The important ore deposits and occurences in the Man Shield are:
1. Perkoa zinc deposit (Burkina Faso): the dominant ore type is
massive sulphide. The major minerals are sphalerite (30%), pyrite 25%,
barite10%, pyrrhotite(5%), magnetite (5%), white mica (5%) and quartz
(5%). Hydrothermal quartz are relevant where the main orebody is close
to the quartz diorite (0,5 m away). Ba-K-feldspar, Ba-rich biotite,
chlorite, turmaline, andradite-rich garnet, ilmenite, rutile, titanite
and galena are subordinate. Unlike other volcanic hosted massive
sulphide deposits, it contains only traces of chalcopyrite,
arsenopyrite, Ag-rich tetrahedrite and molybdenite. The deposit is
located in a steeply NW dipping meta-volcanosedimentary suite with a
few granitoid dikes. The main host of the orebodies is a siliceous
tuffite or tuffaceous sediment which is graphite free and underlies
the carbonaceous schist. In the tuffitic suite 0.5–5m thick bodies of
microdiorite and dikes of dacite, quartz microdiorite and
microtonalite occurs. Adjacent to massive sulphide occur max. 2m thick
body of microdiorite is found and in the massive sulphide there are
small inclusions (<4cm across) of microdiorite.
Close to the massive ore in hydrothermally strongly altered granitoid
dikes disseminated sulphides as well as sulphide veinlets were
developed. A large intrusive quartz-diorite body (2175±1Ma U-Pb zircon
age) grading into tonalite is located below the main ore body. The
contact between tuffitic host rock and quartz diorite (at an average
distance of 15m from the main orebody) is characterised by sulphide
stringers (0.1–10cm). Thick stringers (5–10cm) enclose strongly
corroded fragments of quartz diorite.
As concerning sructural geology the position of orebodies and the
contact between the quartz diorite and tuffite are subparallel. The
main schistosity of the tuffaceous rocks above the deposit is oriented
subparallel or at an acute angle to attitudes of the orebodies and the
quartz diorite contact.
It may be assumed: 1. the deposition of the ore was controlled by
openings produced in a shear zone, 2. the ore formation predates
deformation and so the shearing occurred after ore deposition. The
most likely alternatives are the syndepositional shearing with a
normal sense of movement.
The unusual composition of the Perkoa deposit (Zn>Ba> Pb>Cu can be
explained by high mSoxidized/mSreduced ratios of the ore forming
fluids and/or a surce with high (Zn+Ba/Cu+Pb) ratios such as the
quartz diorite.
2. Gongody copper deposit (Burkina Faso): has features analogous to
porphyry copper deposits. It is composed of pyrite, chalcopyrite and
pyrrhotite, rare molybdenite and cubanite. It is linked to collapse
breccia pipes and fractures in Birimian diorite at the
diabase-microdiorite contact. Alteration minerals are: chlorite,
sericite, epidote and carbonates.
3. Diénéméra copper occurrence (Burkina Faso): disseminated pyrite,
chalcopyrite, pyrrhotite, molybdenite and cubanite occurs in
hydrothermal intrusive breccias in meta-diabase, spilite, keratophyre
at the contact to Birimian diorite. Alteration minerals are similar to
those at Gongondy.
4. Goren copper molybdenum occurrence (Burkina Faso): disseminated
pyrite, chalcopyrite, molybdenite, pyrrhotite and cubanite occurs in a
quartz-diorite body. There is an intense quartzsericite alteration.
5. Wayen copper-molybdenum occurrence (Burkina Faso): disseminated
pyrite and chalcopyrite occur in a Birimian quartzmicrodiorite, which
shows irregular propylitisation and silicification.
6. Tiebelé zinc occurrence (Burkina Faso): is a volcanic hosted
massive sulphide deposit. Disseminated and fracture filling magnetite,
sphalerite and chalcopyrite occur in tuffite of rhyolitic to dacitic
composition. Meta-basalt and graphite schist are part of the
volcanosedimentary sequence intruded by granitoids.
7. Gan lead occurrence (Burkina Faso): the only significant lead
occurrence in the Birimian. The vein composed of massive galena and
very little quartz is emplaced in a Birimian volcanosedimentary
sequence with quartzite, micaschist and amphibolite.
8. Monogaga copper occurrence (Ivory Cost): a metamorphosed porphyry
copper deposit. Disseminated pyrite, chalcopyrite, molybdenite and
magnetite mainly occur in biotite rich portions of the gneiss, but a
minor quantity is hosted by amphibolite and granodiorite.
9. Zeitouo copper occurrence (Ivory Cost): metamorphosed porphyry
copper deposit. Lenses and disseminated pyrite, pyrrhotite,
chalcopyrite, bornite, sphalerite, galena and moybdenite occur in
fine-grained Birimian biotite gneiss, close to a gneissic granodiorite
of Archean age.
10. Copper occurrences in the Falémé iron district (Senegal): these
are the most important ore deposits in the Man Shield. It consists of
9 major and 19 minor orebodies of exo-and endo-skarn types. Exo-skarns
and endoskarn are hosted in dolomitic and calcitic marble and
albitised microdiorite respectively.
The fact that the low grade greenstone belts represent more favorable
setting for massive sulphide deposits than high grade terrains gives
the explanation for the scarcity of sulphide ore in West Africa.
As concerning sulphide ore-potential of the Man Shield the
volcanogenic exhalative setting is considered to be significant, the
porphyry-copper type mineralisation is of secondary importance. About
60 Archean VHMS deposit are known worldwide and most of them are
located in the Superior and Slave provinces of Canada. Some of them
occur in the Yilgarn and Pilbara Blocks of Australia and one is known
in China. Many deposits are located in low metamorphic grade terrains
whereas in the Archean of West Africa the high-grade metamorphism is
characteristic. Comparing the ratio between Archean greenstone belts
and granite-gneiss terrains of West Africa and Canada — where most of
the Archean volcanic hosted massive sulphide deposits are located — it
is much lower in West Africa than in Canada (Superieur Province and
Slave Province). Acid volcanic rocks are associated with many VHMS
deposits in Canada and Australia, but not in West Africa. This fact
may explain the scarcity of sulphide ore in West Africa,
notwithstanding that the age range of the Archean intrusive and
metamorphic rocks (2.7–2.9Ga) is similar to the age range of the
Archean VHMS deposits in Canada (2.7–3.0Ga). Neither Nisulphide
occurences nor komatiite-associated Ni-Cu-sulphide and komatiites with
Zn-rich ferrochromite deposits have been identified in West
Africa. The scarceness of major lead mineralisation is characteristic
for Palaeo-proterozoic and Archean terrains worldwide, so it is
understandable that the large amount of lead — dominantly of
radiogenic origine — located during the formation of younger
continental crust. In the palaeo-Proterozoic BS large ore forming
hydrothermal low-sulphur systems have been preserved. The scarceness
of high sulphur hydrothermal systems is related to the juvenile
character of large parts of the Birimian crust. Juvenile low-S
“pre-subduction” magmatism represents a less suitable environment than
high-S subduction magmatism with sulphur enrichment by the subducted
oceanic slab, or magmatism favouring the assimilation of crustal
sulphur.
Hydrothermal activity at most part of the Birimian was dominated by
low-sulphur fluids. According to the author, the future exploration
must be concentrated on massive sulphide deposition linked to quartz
diorite and tonalite in West Africa.
Éva VETŐ-ÁKOS
Foldtani Kozlony issue 139/3 p. 320-321 (Bulletin of the Hungarian
Geological Society
