related structures linked to the emplacement of the dianfla granodiorite pluton (central cote...

8/10/2019 Related Structures Linked to the Emplacement of the Dianfla Granodiorite Pluton (Central Cote dIvoire): Contribution to the Understanding of Gold Mineraliz

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International Journal of Engineering Sciences, 2(8) August 2013, Pages: 404-414

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* Corresponding author.Email address: [email protected]

Related Structures Linked to the Emplacement of the DianflaGranodiorite Pluton (Central Cte dIvoire):

Contribution to the Understanding of Gold Mineralization in theBirimian Area of West Africa

Gbele Ouattara *1, Gnammytchet Barthelemy Koffi 2, Gnanzou Allou 3

1Institut National Polytechnique Flix Houphout-Boigny (INP-HB) / Dpartement Sciences de la Terre et des Ressources Minires (STeRMi) / Laboratoire

des Gosciences, Cadre de Vie, Environnement et Sciences Gographiques / Yamoussoukro (Cte dIvoire).2Institut National Polytechnique Flix Houphout-Boigny (INP-HB) / Dpartement Sciences de la Terre et des Ressources Minires (STeRMi) / Laboratoire

des Gosciences, Cadre de Vie, Environnement et Sciences Gographiques / Yamoussoukro (Cte dIvoire).3CNRS-UMR8148 IDES, Laboratoire Interactions et Dynamique des Environnements de Surface, Universit Paris-Sud 11, Btiment 504, 91405 Orsay,

France.

A R T I C L E I N F O A B S T R A C T

Keywords:

Pluton emplacementMetasedimentsLithostructural mappingSatellite imageryDianfla granodioriteGold mineralizationCte d 'Ivoire

The Dianfla granodiorite is intrusive in metasediments and metamorphosed volcano-sediments in thegold prospect of Bouafl-Centre (Cte d'Ivoire). Using field data, thin sections, satellite imagery andoriented drill cores data permit to determine the lithology of the different formations and geologicalstructures that affect them. It also helped provide some answers as the emplacement of the pluton, itscontribution to the structuration and the origin of the gold mineralization.

The Dianfla pluton is an undeformed granodiorite with two facies (fine-grained leucocratic and amedium-grained mesocratic facies). However, on its eastern border, in contact with its host rocks(chlorite schists, sericite schists, graphitic schists, sandstones and greywackes), the pluton is highlydistorted, with the appearance of a mylonitic facies.

Geological structures are cleavages and schistosity with average direction N35 to N40W, faults andfractures of varying directions (N-S, NE-SW, E-W and NW-SE), lithological contacts for most NNE-SSW to N-S, quartz veins and tension gashes filled with quartz and sulphides with N120 to N140, E-Wand NE-SW directions.

The model of mineralization that we propose is the following: Before the emplacement of the Dianflapluton, sinistral submeridional shear zones (Danangoro fault in the west and Okabo faults in the East) arecrosscuted by sinistral N50 to N60 faults. These allow openings for the rising of the magma. Theemplacement in the form of forced intrusion (diapiric pluton) causes several stresses on its borders. Thiscreates a mylonitic shear zone in the contact with the host rocks. The hydrothermal fluids were injectedinto the cleavages planes to give a N40 mineralization (M1). The final Eburnean NW-SE shorteningcauses the opening of N120 to N140 tension gashes. The latter will undergo a strong hydrothermalalteration accompanied by sulphides, what will be a second phase of mineralization (M2).

2013 Int. j. eng. sci. All rights reserved for TI Journals.

1. Introduction

The Birimian more generally refers to structured formations by the Eburnean orogeny (2500 to 1600 My, [1]. These formations aremetasedimentary basins, volcano-sediments and various granitoids generally oriented NNE-SSW [2]. In Cte d'Ivoire, they permit to thediscovery of several gold deposits. That is why they are predominant targets of mining exploration by many companies.

Works made by the BRGM in the Yaour mountains (East of the Bouafl gold province) helped to discover the Angovia gold deposit. In2007, after several studies, CLUFF GOLD CI mining company highlighted another gold deposit, currently exploited by AMARA Mining.Since July 2009, the Bouafl gold province is subject to high researches by NEWCREST CI SA Company. These works permit to thediscovery of a mineralized shear corridor in metasediments located in the Pakouabo-Okabo-Krigambo prospect at the East of the Dianfla

pluton. Knowing the structural control associated with the Dianfla pluton and its host rocks, as well as its contribution to the mineralizationcan open new perspectives of research. The general objective that emerges from this work is to identify geological structures, associatedwith the Dianfla pluton that could control the Bouafl mineralization and to expand the horizons of research. To do this we willspecifically: 1) Study the Dianfla pluton and direct host rocks on petrographic and structural plans, 2) Make a proposal for theemplacement of the pluton and 3) Determine, its possibly contribution to the mineralization of Bouafl gold district.

Bouafl region contains a wide variety of formations with a predominance of granitoids, volcano-sedimentary and metavolcanic rocks(Figure 1).


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granitoids are Trondhjemite-Tonalite-Granodiorite type (TTG), two-mica granites and amphiboles granitoids located in the north-west and south-east of the area;

volcano-sedimentary formations consist of conglomerates, schists, metagreywackes, quartzites and metapelites located on eitherside of the Dianfla pluton;

quartzic metavolcanics (metarhyolites) and non-quartzic (metabasalts, metaandesites) are located to the east of the area. basic volcanics are few in the area, but we can found some in the south of the region (Koupela).

On the structural level, the deformation is essentially subhorizontal linear in the Ferk batholith. In the granodiorite of Dianfla, the minerallineation is vertical while it is subhorizontal in granitoids and volcanosediments (with plano-linear distortion) bordering the batholith Ferk[3]. Many directions of faults and fractures were also found [4-7].

Figure 1.Geological map of the Bouafl region at 1/200000 scale (after Ouattara, 1998; [3]) 1. Metasiltites; 2. Meta-arenites; 3. Metagabbros; 4.Undifferentiated metavolcanic (basalts, andesites ...); 5. Ferk Batholith; 6. Tonalite; 7. Amphibolites;. 8. Biotite granites; two micas granites; 10.

Granodiorites; 11. Metagranodiorites; 12. Porphyritic granites; 13. Porphyritic granodiorites; 14. Micaschists; 15. Basic metavolcanic rocks; 16. Rhyolites.


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2. Materials and Methods

Our work is organized around four themes. First, a literature search, then works on satellite imagery, field works, oriented drill coresdescription, and finally the processing of data from any of the foregoing.

Concerning the satellite imagery, works were based on the processing and analysis of images. They lead us to observe geomorphologicobjects and to develop teleanalytic maps. Treatments are either analog way (on paper) or numerically (using software such as ENVI). Inaddition, data from the geological maps are compared to satellite images taken from linear structures to give their structural significance [8-

9]. When the anthropogenic of a linear structure has been proven (roads, railways, forest boundaries or cultivated areas, power lines ...),then it is deleted. Thus, the remaining ones are likely to correspond to the fracturation [9-15]. Teleanalytic maps obtained weresupplemented by field observations and compared to existing mapping data to ascertain the geological nature of the observed objects,geological structures in outcrop and relative chronology.

3. Results

3.1 Petrographic StudyDianfla Granodiorite

The Dianfla pluton is located in the western of the study area. It is bordered to the northeast and southwest by mafic rocks (basalts andandesites), and pyroclastic rocks particulary in the Marahou National Park [3]. It has been described as a non-deformed and posterior tometasediment rocks [16]. In fact, Dianfla granodiorite has a slightly elliptical shape with a NE-SW elongation axis. It is intrusive intometasediments located on its western and eastern border. It outcrops in the form of small isolated inselbergs or slabs. It is on its eastern

border metamorphosed in a mylonitized granodiorite. Some sites are described below.

Site 1:The outcrops are located in the west of the Dianfla village. They extend for several kilometers. The outcrops are massiverocks without preferential orientation, with two facies; one leucocratic fine grains and other mesocratic medium grains. Themineralogical composition reveals the presence of plagioclase, quartz, K-feldspar, amphiboles, biotite and sericite. Plagioclase iseuhedral, large and constitutes the essential mineral phase. Rare clusters of biotite are often altered in sericite. We can found manydark grained to microgranular enclaves, often rounded or angular. The mineralogical composition of these enclaves consists ofamphiboles, biotite, plagioclase, K-feldspar, quartz and are strongly magnetized. These enclaves are gabbro and diorite composition.The contact between these enclaves and granodiorite is rarely sinuous. Some sericite veins crosscut the pluton and are oriented N170.Quartz and epidotes veins are in the N85 to N90direction. The study of thin sections reveals a grainy texture (Figure 2). The mineralassemblage consists of quartz, plagioclase often saussuritized, albitized, damouritized with the appearance of a secondary formed

biotite (Bt2), with a sub-automorphic form of microcline, orthoclase, poikilitic amphiboles (actinolite) often destabilized in biotite,sometimes with included clinopyroxene (diopside).

Site 2:The site is located behind the K domain of Kouassi field between Pakouabo and Krigambo. The outcrop is oriented N40with 1 km longer with a width of up to 10 m. The rock is crushed with mylonitic texture, strongly magnetic, reflecting the presence ofsome magnetite minerals. It has a schistose structure with N45 direction and dip to 70W. Some veins rich in quartz and sericite,

often deformed and boudinated, intersect outcrops in several directions: N40, N85, N100 and N160. Microscopic study reveals amylonitic texture. Minerals are difficult to observe, but the paragenesis is composed of plagioclase, orthoclase, sericite whichconstitutes the bulk of the matrix, anhedral cordierite, quartz, mafic minerals not identifiable due to the alteration, and opaqueminerals. It is a mylonitized granodiorite.

Site 3:The study area is situated approximately 1 km at the north-east of the Pakouabo village. It is a massive rock orientedN40. It is grainy, rich in plagioclase which is an essential mineral phase and is often albitized and damouritized. The matrix consistsof green to dark green sericite and epidotes. It is a mylonitized granodiorite. A N45 contact between a felsic rock and the mylonitizedgranodiorite is visible on the outcrop (Figure 3). This rock is pink in the same direction of the mylonitized granodiorite in which it isin vein. The essential minerals are feldspar, quartz and some mafic minerals. Silicification is marked by many tensions gashes filledwith quartz oriented N135 to N140 and power up to 26 cm. This rock contains disseminated sulphides (2%). This is an aplite vein.

The metasedimentary host rocksMetasediments form the Marahou and Yaour basins (Figure 1). They represent the majority of the rocks in the study area. These rocksconsist of schists, micaschists, arkosic sandstones and para-amphibolites [3]; often similar to those of the Yaour mountains [17-18].Metasediments are altered and are sericite schists and chlorite schists. Other rocks were observed on oriented drill cores. Several sites have

also been described.

Site 4:The outcrop is located in the south of Krigambo village. It is small isolated inselbergs. The rock has a schistose structureoriented N60 and a subvertical dip. It is a dark green to black and is highly silicified. This silicification results in quartz veinsfolded between the cleavages planes. It is a chlorite schist (Figure 4A).

Site 5:The outcrop is located in the bed of a river in south-western village of Krigambo. The rock has a higher cleavage than thegranodiorite with which it is in contact, and oriented N40. The only recognizable minerals are oxides and iron hydroxides. This is arock with a slightly crushed schistose structure. Minerals are quartz, mafic minerals and chlorite. It is an almost chloritoschiste as atsite 4 (Figure 4A).


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Site 6:The outcrop is located after the Okabo village in the north. This is a schistosed rock yellow orange to yellow green. The rockis oriented N40 and the dip is 80 to the west. The structure is lepidoblastic. Recognizable minerals are oxides, iron hydroxides andquartz. The rock is altered and silicified. Indeed, we observe quartz veins associated with sericite and sulphides. We are also in the

presence of sericite schist (Figure 4B).

Site 7:The outcrop is located to the South-East of Senoufokro village on the road to Aka N'Guessankro. It is a schistose rockoriented N45 and dipping 75 to the west. The structure is lepidoblastic. Mineralogy consists of quartz, sericite, oxides andhydroxides of iron. This is a sericite schist as at site 6 (Figure 4B).

Figure 2. Overview of the Dianfla granodiorite (A: dark enclaves; B to E: thin sections; Bt = biotite, Am = amphiboles, Op = opaque minerals, Pl =plagioclase, Qz = quartz, Cpx = clinopyroxene, Opx = orthopyroxene, TNT = titanite).


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Figure 3.Aplitic facies showing many veins and tension gashes filled with quartz and sulphides.

Figure 4.Photographs of thin sections of chlorite schist (A) and sericite schist (B).

3.2 Structural StudySpot XS multispectral imaging

The visual interpretation of SPOT XS optical image, shows the Dianfla pluton smoother and bluish color. It contours are traceable withreasonable accuracy while the metasediments are characterized by a combination of colors from orange to dark brown. Several fracturesdirections can be identified (Figure 5): 1) submeridional fractures (F1) of Danangoro; 2) NE-SW to NNE-SSW fractures (F2) of Krigamboand Tibta; 3) NW-SE to NNW-SSE fractures (F3) of Dianfla and west of Tibta; 4) E-W fractures (F4) very rare in North Okabo and

South-East Krigambo.

Landsat 7 ETM +

The visual interpretation of the image in 432 composite color (Figure 6) permits to define the different lithologies and get several directionsof fractures. Depending on the textures and tints, four major lithologies are distinguished. These are: the tonalite of Bouafl (brown),granodiorite of Dianfla (orange) showing numerous circular structures, metabasalts in the North East and South West of the Dianfla pluton(green), metasediments (yellow), which often show a network river system as "Fishbone". On the structural level, several fractures weremapped: 1) N-S fractures: Danangoro, Tibta and Okabo East; 2) NE-SW to NNE-SSW fractures bordering or not the Dianfla pluton; 3)

NW-SE fractures; 4) some E-W fractures.

3.3 Field dataIn the granodiorite

At Site 1: The outcrops are weakly or no deformed. Plagioclase and hornblende sometimes present a subvertical stretching lineation. Theonly visible structures are fractures of different directions. Three directions of fractures were observed: N90, N175 and N05. Thesefractures show all sinistral displacement. In other outcrops, fractures with N75, N80, N135, N145, N165 directions were observed.At Site 2: Different markers of brittle and ductile deformation were observed.


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- an early schistosity characterized by alternating light and dark minerals with a subvertical dip;-

two plans of cleavages with N40 and N70 directions affect the formation;- quartz veins, sometimes boudinated, showing sinistral (N90 and N125) and dextral (N100, N90 and N70) shear zones;- deformed quartz veins;- joints with N140, N100, N105, N90, N80 and N45 directions;- tension gashes with N90 direction intersected by N140 and N-S fractures;- en-echelon tension gashes towards N100 and N140.

At Site 3: The aplite is a vein (1 to 1.5 m power) in the mylonite. Mylonitic granodiorite is affected by greenschist facies metamorphismdue to the presence of minerals such as sricite, chlorite and epidotes. A subhorizontal mineral lineation and the presence of two planes ofcleavage N65 and N100 is observed. Quartz veins intersect the mylonite following N135, N40, N30 directions. The aplite iscrosscuted by many rich quartz tensions gashes (Figure 3). These structures are oriented N165, N140 to N135, N110 and N80. Themylonite is not affected by these many structures. Chronologically, the N-S sinistral fractures are older and those of E-W are the latest.

Figure 5.Extract of SPOT XS image of the study area.

Figure 6.Extract from the Landsat 7 ETM+ in 432 colorful composition of the Bouafl region and its lithostructural interpretation.


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In the metasedimentary rocksAt Site 4: a contact zone is observed between the granodiorite and metasediments. This is a weakly fractured zone. The schists have aregular flow that reflects a N60cleavage and microfolds of quartz veins. It should be noted as a subhorizontal alignment of minerals in thegeneral direction of the cleavage. This area has been the scene of a compressive tectonic due to the flattening of minerals.At Site 5: Two types of rocks with two different orientations (N40 and N60). The contact area is marked by ductile deformation. We canobserve quartz veins with N120 direction and a dip of 45SE, and other towards N140-80SW. The schist is crosscuted by fractures with

N100 direction.

At Site 6: The rock is oriented N40 -80 SE. Observable structures are: N40 cleavage dominant and a second E-W crenulation cleavage(Figure 7); a quartz vein with N85 direction and another N20; N40 and N120 fractures and 35 NE dip.

Figure 7.Crenulation cleavage S2 (vertical) in sericite schists.

3.4 Oriented drill cores dataThe cored holes were drilled (Diamond Drill) on the eastern border of the Dianfla pluton, precisely in the Pakouabo-Okabo-Krigambo

prospect. The holes were drilled on the supervision of the NEWCREST CI SA company and had intercepted the mineralization. Structuraldata were pooled and interpreted by types of s tructures (quartz veins, schistosity/cleavages, faults and lithological contacts).

Quartz veins:Of all measurements, quartz veins are oriented NE-SW with a mean direction of N40. The average dip is 82 to theNW (Figure 8A). Some veins have E-W and NW-SE directions.

Schistosity and cleavages: They are mostly oriented NE-SW, with a mean direction of N35. Dips are very variables(subhorizontal to subvertical). However, the average dip is 88 NW (Figure 8B).

Faults:They have variable directions: N-S, NE-SW, E-W, NW-SE. But the majority of these faults is in the NE-SW direction. Dipsare also variables, but the average dip is 40 to the southeast (Figure 8C).

Lithological contacts:They show two major directions (NNE-SSW and N-S). Dips of these contacts are very high and subvertical(average 85SE, Figure 8D).


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Figure 8.Wulff stereograms and roses diagrams of quartz veins (A), schistosity and cleavages (B), faults (C) and lithological contacts (D).


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4. Discussion

4.1 Emplacement of the Dianfla granodiorite plutonThe granodioritic pluton is plagioclase and amphiboles rich. We know that the granodiorites magmas are likely, in a shallow crust, togenerate a "diapir". The metamorphic grade is generally low in the rocks, as elsewhere in West Africa [19-24]. So we can think that theenvironment of the emplacement of the Dianfla pluton corresponds to the contexts of emplacement of diapirs [25-27].

In addition, in the border of the pluton, the cleavages are oriented N45 in the North West (South of Pakouabo) and become N-S to NNE-

SSW in Northern Okabo. This shows that the emplacement caused a reorientation of these geological structures.

4.2 How can we explain the mylonitic zones in contact areas?The contact areas are conducive to contact metamorphism. Host rocks heated and compressed by the magma will tend to deform ductily.But how to explain the sense of the sinistral displacement? It should then review the main fractures, their directions, their dips and theirorder of appearance.

Two submeridian faults have been clearly defined. These sinstral faults are the Danangoro faults in the West and the Okabo faults in theEast. These two faults, if they work together, in the same period, could not open the metasediments for the ascent of magma. We believethat other fractures with different directions could result in openings. The N50 to N60 sinistral fractures posterior and who are veryregular in the region, particulary in the tonalite of Bouafl (also containing sulphides), combined with those of submeridional directions canlead to openings for the emplacement of the Dianfla pluton (Figure 9), as a pull apart type. However, this configuration does not permitopenings in the triple points (zones A, B, C, D; Figure 9) which allow the recovery of mineralizing fluids.

It has been shown that the triple points near plutons are very favorables to gold mineralization zones. We also know that the area A (Figure

9) corresponds to the Bouafl-South and Bouafl-Centre prospects. These zones, especially that of Bouafl-Centre, shows significant goldmineralization. So, how to explain the mineralization?

Figure 9.Emplacement model of the Dianfla granodiorite pluton.

4.3 Origin of the mineralization

Mineralization has several directions, including NE-SW (according to cleavage) and NW-SE (according to tension gashes). We believe thatthe emplacement of the Dianfla pluton has generate a first mineralization in the direction of the regional cleavage in host rocks (S1: N40).The regional tectonic shortening known throughout the Birimian of West Africa, with NW-SE direction ending Eburnean orogenic cycle(causing a second crenulation cleavage S2, Figure 7) was able to generate the NW-SE (N140 to 120) tension gashes. These have enabledthe rise of late hydrothermal fluids. Chronologically we have:

1) Emplacement of the Dianfla pluton followed by mineralization in the host rocks (M1) subparallel to the cleavages.2) N40 to N45 fracturation of the granodiorite and emplacement of aplitic veins.3) Regional NW-SE shortening creating N120 to N140 tension gashes.

4) Rise of hydrothermal fluids to generate a second mineralization (M2).5) Further investigations should be conducted to better understand the paragenesis of the two phases of mineralization suspected

earlier.


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5. Conclusion

To achieve the objectives listed in the introduction, we used petrographic data (macroscopic and microscopic) and structural data (satelliteimagery SPOT XS and Landsat 7 ETM+, field data and oriented drill cores data).

At the end of our study, we can say that the Dianfla granodiorite has two facies (one mesocratic the other leucocratic) and has manyenclaves (gabbros and diorites). This pluton is weak or no deformed. However, in the contact areas, in the eastern border, we have a

mylonitized granodiorite. The host rocks consists of chlorite schists, sericite schistes, graphitic schists, politic schists, greywackes andmetavolcanics.

In structural terms, host rocks are affected by different structures. These are cleavages, schistosities, fractures, faults and quartz veins.Schistosities, cleavages measured on outcrops and and oriented drill cores have N35 to N40 average direction and a dip of 88NW. Faultsshow variable directions (N-S, NE-SW, E-W and NW-SE) with an average dip of 40SE. Lithological contacts are NNE-SSW and N-S(subvertical). Quartz veins intersect other structures and have NE-SW, E-W and NW-SE directions.

The low-grade metamorphic environment in the host rocks shows that the granodiorite would be emplaced in a diapiric form. Itsemplacement and its lateral swelling exerted strong stress on the rocks. In Pakouabo area, stress or iented NW-SE, eventually led to N120to N140 tension gashes.

The emplacement of the Dianfla pluton could cause a subparallel gold mineralization to the regional cleavage (M1). Then, a N40 to N45fracturation led to the emplacement of aplitic veins. The regional NW-SE shortening at the end of the Eburnean orogeny has created N120to N140 tension gashes which were subsequently mineralized (M2). In view of the available data, we can consider at least two phases of

mineralization (M1 and M2).

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related structures linked to the emplacement of the dianfla granodiorite pluton (central cote...

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