Base-metal Sulphide Ore in the Man Shield (Archean and Paleoproterozoic in West Africa)
2008. 63 pages, 28 figures, 3 tables, geological map 1:2000000, 17x24cm, 390 g
(Geologisches Jahrbuch Reihe B, Band B 99)
ISBN 978-3-510-95975-4, paperback, price: 29.00 €
in stock and ready to ship
- ↓ Description of content
- ↓ Review: Foldtani Kozlony issue 139/3 (Bulletin of the Hung. Geol. Soc.)
- ↓ Review: Zentralblatt Geo. Pal. Teil II 2009 Heft 3/4
- ↓ Contents
Review: Foldtani Kozlony issue 139/3 (Bulletin of the Hung. Geol. Soc.) top ↑
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.
Foldtani Kozlony issue 139/3 p. 320-321 (Bulletin of the Hungarian Geological Society
Review: Zentralblatt Geo. Pal. Teil II 2009 Heft 3/4 top ↑
Contents top ↑
2 Geological setting 8
2.1 The Archean 8
2.2 The Birimian (Paleoproterozoic) 12
3 The geology of base-metal sulphide occurrences 21
3.1 The Perkoa zinc deposit 21
3.1.1 Host rock 21
3.1.2 Orebodies 24
3.1.3 Structural geology 38
3.1.4 P-T-X conditions 41
3.1.5 Ore genesis and comparison to other deposits 45
3.2 The Gongondy copper deposit 47
3.3 The Diénéméra copper occurrence 48
3.4 The Goren copper-molybdenum occurrence 48
3.5 The Wayen copper occurrence 48
3.6 The Monogaga copper occurrence 48
3.7 The Zéitouo copper occurrence 49
3.8 Copper occurrences in the Falémé iron district 49
3.9 The Tiébélé zinc occurrence 50
3.10 The Gan lead occurrence 52
4 Sulphide ore potential 52
4.1 Archean 52
4.2 Birimian 53
5 References 56
Base-metal sulphide ore in the Lower and
Middle Precambrian of West Africa 67
Petrological characteristics of the country rock
of the Perkoa deposit 73
Geological sheet map Base Metal Sulphides
in the Archaean and Proterozoic of West Africa
1:2000000 (in pocket)