Lithostructural controls on gold in the Oumé-Fettékro greenstone belt, Côte d’Ivoire
Ouattara Z. 1, Coulibaly Y. 1, Hein K.A.A2, Garofalo P.S. 3
1Laboratoire de Géologie du Socle et de Métallogénie, UFR STRM, Université de Cocody, Abidjan, Côte d'Ivoire, 2School of the Geosciences, University of the Witwatersrand, South Africa, 3Università di Bologna, Dipartimento di Scienze della Terra e Geologico Ambientali, Piazza di porta San Donato, Bologna
ziegbana@hotmail.fr
Gold mineralizations are known to occur in the paleoproterozoic formations [1]. They are termed Birimian in West Africa. In Côte d’Ivoire, where these formations cover almost all the country, exploration has revealed numeric gold deposits. The southern portion of the Oumé-Fettékro greenstone belt, centre of Côte d’Ivoire, presents a geology dominated by volcanic and sedimentary rocks intruded by a porphyritic granite. These rocks have undergone a single phase of progressive deformation characterized by sinistral strike-slip shearing, tight folding about the north-northeast trending axes which are the Birimian main strike and finally metamorphosed to the greenschist facies.
Gold mineralization through this portion is a case of lithostructural controls expressed by the porphyritic granite and the main shearzone.
In the porphyritic granite, gold occurrences are indicated by these three styles of alterations; sericitisation, chloritisation and silicification. Sericitic and chloritic alteration replace the feldspar matrix when another but more selective replacement of biotite with sericite is observed.
The volcanic and sedimentary rocks are mostly barren at the studied area. Their mineralizations are closed to the shearzone where brittle and ductile deformations are associated with both disseminated and controlled sulphides.
References:
1.Milési, J.P., Feybesse, J.L., Ledru, P., Domanget, A., Ouédraogo, M.F., Marcoux, E., Prost, A.E., Vinchon, C., Sylvain, J.P., Johan, V., Tegyey, M., Calvez J.Y., Lagny, P., (1989) Les minéralisations aurifères de l’Afrique de l’Ouest. Leurs relations avec l’évolution lithostructurale au protérozïque inférieur, (Editors) Chronique de la Recherche Minière 497, p3-98.
On the formation assignment of the Lugokan deposit (Eastern Transbaikalia)
Redin Yu.O.
V.S. Sobolev Institute of Geology and Mineralogy sb ras, Novosibirsk, Russia
Redin.u@mail.ru
The problem of the assignment of gold ore deposits to formations, within the limits of both the new and old mining regions, is significant among applied problems. One of the important and promising types of gold deposits, though non-conventional for many ore provinces in the world, is the gold-mercury type [2].
T
he
Lugokan deposit is situated in Eastern Transbaikalia, in the
Gazimurovsky-Zavodskoy region, at a distance of 200 km to the
north-east from the settlement of Gazimurovskiy Zavod. The deposit
was discovered in 1759 and had been mined for some time after its
discovery; it gave about 357 t of the ore with copper content 3,75 to
16,25%, present mainly in hydrocarbonate hypergene compounds.
However, the assignment of the Lugokan deposit to a formation still
remains controversial. At first, the deposit was attributed to the
skarn formation [3,6]. In the opinion of other researchers [4,5], the
Lugokan deposit is a typical representative of the copper-porphyry
formation. The deposit is situated on the Lower Palaeozoic fold base
of terrigenous-carbonate rocks extruded by the Lugokan ore-producing
stock of granodiorite-prophyries of the late Jurassic age,
Shakhtaminsky complex. The stock contains numerous xenoliths of
surrounding rocks. Endo- and exoskarns are ore-bearing [6]. The ores
of the Lugokan ore deposit are characterized by the diverse mineral
composition. More than 40 ore minerals including 30 hypogene ones
were established in the ores. The major ore-forming minerals are
pyrite, chalcopyrite, while the minor ones are arsenopyrite,
magnetite, hematite, marcasite, tetrahedrite, tenantite, less
widespread are molybdenite, galena, blende, bismuthine, pyrrhotine,
bornite, boulangerite, jamesonite, rarely occurring are native gold,
silver and bismuth, tetradymite, tellurobismuthite, antimonite,
graphite etc. The composition of gold and its mercury content are one
of the specific indices for the deposits of gold-mercury type. 37
signs of native gold were found through the dissolution of the
samples (taken from the surface) in hydrofluoric acid. The morphology
of gold is mainly interstitial, vein-plate like. The size of gold
grains does not exceed 0.1 mm; the majority of gold signs get into
the class of microscopic, powder-like gold. The chemical composition
of gold is rather diverse. In general, several types of gold can be
distinguished (Fig.): 1. High-grade gold (900-920); 2. Medium-grade
gold (740-760) 3. Relatively low-grade gold (560-660); 4. Low-grade
or high-silver (electrum) (380-500). Among other admixtures, mercury
is present in significant amounts (up to 2,2%).
A trend may be followed: mercury content increases with a decrease in
gold grade. Such a peculiarity of gold composition and the presence
of mercury, a decrease in gold grade and increase in its mercury
content from early Au-Cu-skarn
to late Au-Cu-Hg; this trend is a specific feature of the Au-Cu
geochemical profile of deposits; the spatial and genetic link with
the Au-Cu-skarn mineralization, the mineral composition of ores allow
us to attribute the Lugokan deposit to the deposit of
gold-copper-mercury formation.
References:
1. Bessonov N.N. Assignment of the molybdenum-copper-porphyry type of mineralization over the territory of south-eastern Transbaikalia // Bulletin of the Chita State University, 2009, No. 1 (52), p. 12 – 19 .
2. Borisenko A.S., Naumov E.A., Obolenskiy A.A., Types of gold-mercury deposits and the conditions of their formation // Geology and Geophysics, 2006, v. 47, No. 3, p. 342—354.
3. Kormilitsyn V.S., Ivanova A.A. Polymetallic deposits of the Shirokinskoye ore field and some aspects of metallogeny of eastern Transbaikalia // Moscow, Nedra, 1968, 176 p.
4. Sazonov V.D. New data on metallogeny of Transbaikalia / V.D. Sazonov. – Chita, 1983.
5. Sizykh Vit.I., Sizykh Val.I. Ore-bearing capacity of post-collision structures of Transbaikalia // Abstracts of the International Scientific Conference.(VII Chteniya A.N. Zavaritskogo): Postcollision evolution of mobile belts - Ekaterinburg: UrB RAS, 2001
6. Skurskiy M.D. Bowels of Transbaikalia. – Chita, 1996, 692 p.