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

[e-mail]

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

[e-mail]

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].

The 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.

Typomorfic features of gold on the Enganepe Ridge (Polar Urals)

Ustyugova K.S.

Syktyvkar State University, Syktyvkar, Republic of Komi, Russia

[e-mail]

In Enganepe-Manitanyrd region of the Polar Urals same-name Cu-Ag-Au mineragenic zone is defined. It consists of isolated Enganepe and Manitanyrd gold-bearing units. Gold-quartz-sulfide (pyrite-arsenopyrite) deposit and several gold occurrences are known on the Manitanyrd Ridge, but on the Enganepe ridge only halos and small placers were found, that’s why prospecting for primary gold here is a topical issue.

The Enganepe Ridge is a brahyanticlinal structure that has Late Riphean-Vendian volcanic and igneous-sedimentary rocks in center and Early Paleozoic sedimentary rocks on wings. District Izyavojski in southern part of the ridge considered being the most perspective for bearing gold, and ZAO “Gold Minerals” pursued here prospecting on load gold at 2007-2009 years. On the first stage of research they executed litho geochemical survey and outlined complex Au-As-Pb-Zn-Cu-Ag anomalies. During confirmation of anomalies by exploring there were found out ones of disseminated pyrite mineralization that contained primary gold.

Sulfide mineralization is connected to submeridional shear zone in volcanic and igneous-sedimentary rocks of enganepe suite (V₂-Є₁ en) that are partly cataclastic, milonitized and have metasomatic changes. Petrology of milonitized rocks was difficult to determine, but according to x-ray diffractometric analysis they contain quartz, illite, chlorite (including ferrous), glaucodot, scorodite, pyrite, chalcosine. Disseminated pyrite mineralization is connected with sulfide and rare-earth metal micro mineralization [1]. 22 gold grains were separated from milonitized rocks by geologists of ZAO “Gold Minerals” and are devolved us for research. We used scanning electronic microscope JSM-6400 with energy-dispersive spectrometer «Link» in the Institute of Geology of Komi SC UB RAS (analyst V.N. Filippov) to investigate morphology of gold, its surface relief and mineral inclusions. Chemical composition was determined on surface and in cut of polished section.

Explored gold is of a fine gradation class; its size is within 0.05-0.3 mm. The most common is highly fine (0.1-0.25 mm) – 63%, extremely fine (0.05-0.1 mm) – 32%, fine gold (0.25-1.0 mm) – 5%. Granulometric distribution of placer gold in the same district slightly differs to the enlargement [2].

Morphology of gold is described using only 18 grains, but they include all types – euhedral, subhedral and anhedral: bar-shaped, lump, platy. Platy gold is dominant; its percentage is 39%. There are thick and thin species, often they are characterized by bandy shape and rugged edges. Lump grains (28%) are also of a different shape, they include isometric, extended gold, grains of irregular shape with spikes on edges. Subhedral gold figures up to 17%, it has at least 2 crystal faces, commonly characterized by isometric and slightly extended form. Than bar-shaped grains make up 11%. Euhedral gold is presented by only one grain of rod-like form. In placers of this district all grains are of anhedral class [2], it could be explained by mechanical transformation during transportation.

Gold in most cases has plane compact surface, only in several parts it becomes porous, cellular and spongy. Such element of relief as other minerals’ prints: of cube and hexagon shape, probably, sulfides, parallel extended form – prints of scale or foliated minerals, drills and rollers along print edges, parallel growth step. In summary gold has an ore outlook, and small spaces of partly dissolved surface point out initial stage of gold transformation in supergene process [3]. It’s reasonable, because gold-bearing rocks are located in near-surface environment.

Main impurity elements in gold are Ag и Hg. Content of Ag varies in wide range – from 9.59 to 27.00 mas.% in central part of grains and from 11.28 to 27.34 mas.% at the edge. Spread of Ag content on gold surface is even more – 0.92 to 28.11 mas.%. Ag content tends to grow slightly to the edge of gold that is a characteristic feature of primary gold. Hg content ranges in 0.71-1.67 mas.%, and it doesn’t vary much in different parts of grains. Addition of Cu in quantity of 0.59 to 3.84 mas.% was determined only in several analyses on the surface, it is often defined in parts of high fineness gold.

Fineness of gold varies from 735 to 889‰. Gold of moderately high fineness (800-899‰) absolutely dominates - 94%, gold of relatively low fineness (700-799‰) is present in small amount – 6%, distinct parts has high fineness (900-1000‰). Fineness of gold in grains and on surface virtually doesn’t differ; exception is for small areas of high fineness gold adjacent to gathering of iron hydroxide that also points out initial stage of supergene gold transformation. Generally argentous gold prevails (70%), considerably rarer argentous-mercurous gold occurs (17%), and copper gold and argentous-copper gold (8 и 4% corresponding).

In placer gold of Izyavojski district small amount of low fineness gold is presented, however mostly gold is of high fineness [2]. Such correlation in fineness of primary and placer gold is well-known and widespread in gold-bearing regions. As well as in newly found primary gold in placer gold of this district argentous gold with Ag content 0-29.1 mas.% is predominant, rarely admixture of Hg (0-1.8 мас.%) is determined in gold. In placer gold in contradiction from primary gold Cu content hasn’t been recognized [2].

During the electronic-microscoping of gold on its surface, inside it and in accretion many mineral inclusions have been found out. They include chalcopyrite and arsenopyrite inside gold and allanite in accretion. Chalcopyrite forms isometric inclusions of less than 1 mkm. Chemical analysis of its composition corresponds formula Cu_0.82Fe_0.8-0.81S₂, lack of cations is due to nanoparticle analysis fault. Arsenopyrite is present by accumulation af nanoinclusions of 50-100 nm size. Analysis results corresponds formula Fe_0.88-0.9As_0.95-0.98S. Allanite is in form of inclusions of 1-3 mkm inside gold, and also larger grains (~15 mkm) are determined on gold surface in accretion with biotite. Calculated formula is (Ca_1.32-1.6Ce_0.2-0.24La_0.1-0.1Nd_0.06-0.11)_1.73-2.05(Al_1.66-1.9Fe_0.95-1.17Cr_0.19-0.34Mn_0.00-0.08)_3.05(OH)O[Si_1.1-1.48O₄][Si₂O₇]. Admixtures of Fe, Cr, Mn are identified. On the surface of gold such inclusions as sulfides (chalcopyrite), nonmetal minerals (albite, micas, peridot (?), garnet (?), quartz) and epigenetic minerals (scorodite, iron hydroxide, bismuth and lead arsenates).

This research showed that gold from milonitized rocks of pyritization zones has ore outlook, characterized by fine gradation class, various morphology with predomination of platy and lump gold, mainly relatively high fineness, common impurity elements are Ag и Hg, rarely Cu. Explored gold from pyritization zone is similar to placer gold of Izyavojski district by a range of features, therefore these mineralization zones could be considered as primary source of placer gold.

Ore mineral association compound of micro inclusions inside gold and in accretion (chalcopyrite, arsenopyrite) and accessory epigenetic minerals (scorodite, iron hydroxide, bismuth and lead arsenates) lead us to suppose of resemblance between gold-sulfide mineralization in milonitized rocks of pyritization zones on Izyavojski district and same mineralization in gold occurrences of Manitanyrd ore unit [4].

The work is supported by UB & FEB RAS projects № 12-C-5-1006 and Presidium RAS project № 12-P-5-1027.

References:

1. Ustyugova K.S. Sulfide micro association in disseminated mineralization zone of the Enganepe Ridge (Polar Urals)//Materials of II International Research-to-Practice Early Career Scientists and Specialists in Commemoration of academician A.P.Karpinski. SPb: RGRI, 2011. P.183–187.

2. Kuznecov S. K., Spirin A. V. Alluvial gold of Enganepe region // Syktyvkar mineralogical digest. Syktyvkar, 2003. № 3. P. 70–80.

3. Petrovskaya N.V. Native gold. M.: Nauka, 1973. 347 p.

4. Noble metals from eastern flank of the North of the Urals and Timan/ Kuznecov S.K., Tarbaev M.B., Mayorova T.P. et al. Syktyvkar: Geoprint, 2004. 48 p.

Дата добавления: 2015-10-29