Geochemistry of Magmatic Rocks of the Syama Belt, Southern Mali, West African Craton ()
1. Introduction
The West African Craton (WAC) is the part of Africa which is stabilized around 1700 Ma [1] and is constituted by magmatic, metamorphic and sedimentary rocks (Figure 1). These geological formations were shaped during eburnean orogeny [2] [3] [4]. It is established within the WAC, the basic magmatic rocks belong to two series succeeding each other in time: tholeiitic and calc-alkaline, the latter presenting in addition to the basalts a differentiated series up to the dacites and rhyolites [2] [3]. However, the geodynamic context of the genesis of the tholeiitic basaltic rocks is always a subject of discussion. According to previous studies, the tholeiitic magmatism from the Birimian basement would be related to: an oceanic MORB-type [5], an oceanic shelf domain [2] [3] [6] [7] [8] and an island arc domain [9] - [17].
On the other hand, all of the authors admit a subduction context for calc-alkaline rocks [17] [18] [19] [20]. Concerning the magmatic rocks of the Syama belt, the geochemical studies to date focused only on major elements [18] [21]. Analyzed samples were taken from drill cores in order to be as free as possible from supergene alteration due to the tropical climate, a widespread alteration in the study area. The present study was initiated to complete the major elements data by adding traces and rare earth elements (REE) with the aim to: i) accurately identify the mafic rocks (basalts and lamprophyres) as well as the various felsic rock types encountered in the three deposits; ii) propose a geodynamic of these rocks.
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Figure 1. Geological map of Leo-Man Shield with gold showing distribution after [24].
Geological setting
In the WAC, two Precambrian Shields (Reguibat and Leo) and two Precambrian Inliers are noted [1] [22] [23]. This craton is composed of Precambrian formations [1] [24] which are modelled by Eburnean orogeny date to 2.1 Ga [4]. The Leo Shield contains our study area (Figure 1) and is situated in the south of Paleozoic basin of Taoudenit.
The eastern part of this shield is composed of Proterozoic formations, also is called the Boualé-Mossi domain. This domain is made up of plutonic, volcano-sedimentary and metamorphic formations that outcrop in Mali in the southern part.
The south of Mali contains five deposits (e.g., Morila, Komana, Kalana, Syama, Tabakoroni) and many gold showing such as Bananso, Syama-extension and Tellem [19] [25] [26] [27] distributed along main shear-zones (e.g., Yanfolila, Banifing, Syama). Along with the Syama shear, three gold deposits (Syama, Tabakoroni and Tellem) are located follow a direction globally N-S (Figure 2). This region is formed by a succession of greenstone belts oriented NNE-SSW and separated by sedimentary basins (Figure 1). These belts are composed of basic volcanic or sedimentary-dominated volcano-sedimentary rocks and are cross-cut by plutons and granitoid stocks [28]. Among these belts, one of Syama is presented as a narrow belt that extends 100 km within the Banifing shear zone. On the south, it connects to the Sassandra fault in Ivory Coast [29] [30] [31].
The Syama gold deposit as well as ones of Tabakoroni and Tellem is located on the Syama belt (Figure 2). These gold deposits are exploited for a long time and are majorly constituted of quartz veins hosted in volcano-sedimentary rocks.
2. Materials and Methods
The main characteristic of the study area is the absence of exploitable outcrops. Indeed, the bedrock is recovered on the surface by several meters thickness of ferruginous lateritic. Therefore, the analyzed samples have been mainly taken from drill cores in order to avoid supergene alteration, which is caused by the tropical climate in the region. Some other samples have been taken from the open pit of Syama. Whole rock geochemical analyses have been performed on 20 representative magmatic rock samples. The choice of these samples was made taking into account the freshness of the rock. The samples have been carefully cleaned and cleared of all surface elements and quartz veins. After, they have been pulverized to avoid any contamination through a long preparation process. Geochemical analysis was performed at Chemex Laboratories in Canada and ALS in Spain.
The ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) method with lithium tetraborate fusion technique for digestion has been used for samples analysis. This technique was used for geochemistry analysis, in particular the major elements. A prepared sample of 0.200 g is added to the lithium
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Figure 2. Local geology of the Syama deposit with location of associated deposits and showings, modified from [32].
metaborate/lithium tetraborate of 0.90 g. The whole is mixed well and melted in a furnace at 1000˚C. The resulting melt is then cooled and dissolved in 100 mL of 4% nitric acid or one 2% hydrochloric acid solution. This solution is then analyzed by ICP-AES and the results are corrected in the inter-element spectral interferences. The oxide concentration is calculated from the determined elemental concentration, and the result is reported in a table (Table 1).
3. Results
3.1. Classification
The different magmatic rocks of the Syama belt have been defined from the
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Table 1. Major and trace elements for 20 representative samples of igneous rocks from the Syama belt. Major elements in % by weight; Trace elements in ppm;
classification of [33] (Figure 3), sub-alkaline series classification (Figure 4) and FeOt/MgO vs SiO2 distribution (Figure 5). The classification is also based on the major elements SiO2, Na2O and K2O and that of Harker trace elements Nb, Y, Zr and Ti (Figure 6). These Harker’s diagrams are used to assess the degree of fractional crystallization. So, they allow us to understand mineralogical evolution and to highlight the genetic relations between the different magmatic rocks. These classifications complete the results of thin sections of the magmatic rocks with an optical microscope [26].
The two diagrams obtained show that the magmatic rocks in the belt consist of several types: i) a basic set combining basalt and lamprophyre of Syama and Tabakoroni; ii) an intermediate set corresponding to andesitic-dacitic rocks collected at Tabakoroni; iii) a differentiated set corresponding to microgranites at Tellem. In addition, the diagram of [33] shows that the different rocks belong to the sub-alkaline series (Figure 4). These rocks have been also plotted in Harker
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Figure 3. Classification diagram of magmatic rocks of the Syama-Tabakoroni-Tellem zone according to alumina saturation (A/NK as a function of A/CNK) from [34].
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Figure 4. Alkaline-Silica diagram of magmatic rocks of Syama, Tabakoroni and Tellem, modified after [35].
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Figure 5. Distribution of magmatic rocks of the Syama-Tabakoroni-Tellem zone according to [39].
diagrams (oxides vs SiO2) (Figure 6), which highlight: i) the dispersion of the data and the absence of correlation between oxides and SiO2; ii) the variability of the contents of certain oxides such as TiO2, Na2O and K2O within the same petrographic rock type (basalt or lamprophyres) This variability would be related to the differential of hydrothermal alteration.
3.2. Mobility of the Components
Syama belt rocks have undergone strong supergene and hydrothermal alteration. Thus, in order to define the primary characteristics of the magmatic rocks, we chose rocks less affected by silicification phenomena and those where vein networks were less dense.
Major elements analysis shows that supergene alteration is highly variable (Figures 7(a)-(l)). It results in a loss on ignition (LOI) of 5% to 17% in the Syama basalts, 1% to 2.2% in the Tabakoroni basalts, 12.7% to 21.8% in the Syama and Tabakoroni lamprophyres, 1.6% to 3.5% in the Tabakoroni andesites, and 1.7% to 3% in the Tellem microgranites. To be sure to characterize the igneous rocks despite the high loss on ignition values and a possible hydrothermal alteration in some facies, we plotted the incompatible element Nb and the trace element Zr (Figure 7(h)). The trace element Zr is known to be immobile in even altered mafic volcanic rocks [36] [37] [38]. This diagram shows a good correlation between
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Figure 6. Harker-type variation diagrams of major elements vs SiO2 of igneous rocks of the Syama-Tabakoroni-Tellem areas showing an absence of discontinuity in the evolution.
the two elements, following a straight line through the origin, indicating that the elements have not been affected by these secondary processes. They can therefore be used to recover the original characteristics of our rocks (Figure 7(h)).
3.3. Rares Earth Elements Spectra
In order to eliminate the variation in abundance between rare earth elements of odd-even atomic numbers, the values of the Syama-Tabakoroni-Tellem magmatic rocks have been normalized with the concentration of the early mantle [40]. This normalization allows us also to discern the extent of any fractionation among rare piles of earth in the studied samples (Figure 8). The rare earth spectra show a general decrease from light rare piles of earth to heavy rare piles of earth except for basalts where the spectra remain constant. These spectra show mostly a positive anomaly in Yb except the spectra of the lamprophyres. The spectra of the different types of rocks are generally cross. However, andesites-dacites and microgranites spectra show parallelism between them.
The spectra most often show a negative anomaly in Eu (Eu/Eu* < 1). This implies that Eu plays an important role in the fractionation of plagioclases by substituting Sr, Ba or Ca (Figure 9). The REE spectrum shows a negative absence of Eu anomaly for all the samples analyzed (Figure 8).
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Figure 7. Harker-type variation diagrams of trace elements vs SiO2 of igneous rocks of the Syama-Tabakoroni-Tellem areas.
The slope (La/Lu) of the spectra is negative, implying that magmatic rocks of Syama-Tabakoroni-Tellem present enrichment in LREE compared to HREE. Except for basalts where the slope is constant, so no enrichment or depletion of LRRE compared to HREE (Figure 8).
3.4. Rares Earth Elements Expanded Spider Diagram
The expanded REE values of the Syama-Tabakoroni-Tellem magmatic rocks have been normalized to the primitive mantle, after [41] to multi-element spider diagrams (expanded REEs). These spider diagrams (Figure 9) show positive anomalies which are characterized by peaks of enrichment. We are noted also negative anomalies marked by indentations of depletion.
As in the REE spectra, these normalized multi-element spider diagrams of the Syama-Tabakoroni-Tellem rocks (Figure 9) are cross, except for the spectra of andesites-dacites and microgranites which are parallel to each other. The lamprophyre spider diagrams are too intersecting and are difficult to interpret, so they are not taken to account in Figure 9.
All the spider diagrams of basalts, andesites-dacites and microgranites show positive anomalies in Cs, K, Sr, Yb and negative anomalies in Rb, Nb, La-Ce, P, Zr, Lu (Figure 9). In addition to these common anomalies, basalts present a
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Figure 8. Rare earth spectra of igneous rocks (9 basalts, 4 lamprophyres, 2 andesites-dacites and 5 microgranites) of the Syama-Tabakoroni-Tellem area of the study area normalized to the early mantle [40].
positive anomaly in U and negative anomalies in Ba, Th, Y. On the other hand, andesites-dacites and microgranites present positive anomalies in Ba, Pb, Nd, Sm and negative anomalies in U, Pr, Ti.
3.5. Basalts
Nine Basalt samples have been analyzed whose six at Syama and three at Tabakoroni. Their SiO2 contents vary from 38 wt% to 49 wt%, ones Fe2O3 from 9 to 18 wt% while Al2O3, MgO and CaO contents are relatively homogeneous and respectively ~ 12%, ~ 6% with an exception reaching 10%, 6.5% with an exception reaching 10.7%. Cr and Ni contents are high, reaching 730 ppm and 562 ppm respectively in some samples. The basalts are very weakly enriched in REE with a ∑REE varying between 25 and 60 ppm. An exceptional high value is recorded in Syama basalt where the ∑REE reaches 100 ppm related to higher La (19 ppm), Ce (42 ppm) and Nd (25 ppm) contents. In the chondrite-normalized diagram of [41], the basalts show nearly flat rare-earth (REE) spectra (La/Yb CN = 0.95 - 1.70 at Syama, 0.92 - 0.97 at Tabakoroni, devoid of anomaly or with low
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Figure 9. Expanded rare earth spider diagrams of igneous rocks of the Syama-Tabakoroni-Tellem zone normalized to the early mantle [41].
negative europium anomaly (Eu/Eu* = 0.86 - 1.04 at Syama, 0.95 - 0.97 at Tabakoroni). In a multi-element diagram of values normalized to the early mantle data [41], the basalts show some dispersion in alkali element concentrations (Cs, Ba, Rb) related to their mobility but relatively flat spectra, with no negative anomalies in Nb and Ta for the other elements, notably the HFSE (Figure 9(b)). These data indicate that crustal contamination did not play an important role in the evolution of the compositions of these magmas [42]. Significant crustal contamination would be indicated by strong negative anomalies in Nb and Ta. Finally, the Zr/Y ratio, lower than 4 (2.6 and 2.8 respectively for Syama and Tabakoroni), would indicate a tholeiitic affinity for these basalts [43]. Indeed this ratio is between 4 and 7 for transitional lavas, and higher than 7 for calc-alkaline lavas.
3.6. Lamprophyre
Five lamprophyre samples collected in Syama and in Tabakoroni have been analyzed. The result shows they present mafic lavas characteristics comparable to basalts (Figure 5), and ones of ultramafic rocks. These samples are characterized by a very high value of LOI up to 21 wt%, low contents of SiO2 ranging from 38 and 43 wt%. The Fe2O3 contents are variable (between 7 wt% and 17 wt%) while the ones of MgO are between 7.8 wt% and 10.5 wt%. The major characteristic is their very high content of Cr (up to 1910 ppm) and of Ni (up to 470 ppm). The lamprophyres are enriched in Rare Earth Elements with ∑REE ranging from 47 to 386 ppm. In the chondrite-normalized diagram, the REE shows a good fractionation with (La/Yb) CN between 4 and 61. The Eu anomalies are clearly negative with Eu/Eu* ratio from 0.76 to 0.94. The lamprophyres also show high alkaline concentrations (Cs, Ba, Rb) compatible with their mica richness [27], very high U and Th contents (100 times higher than those in the Primitive Mantle) and evident negative anomalies in Nb and Ta (Figure 8). These chemical characteristics are those of calc-alkaline lamprophyres of the spessartite type [44]. They are also similar to calc-alkaline lamprophyres from the Black Mountain [45], eastern Pontides in NE Turkey [46], NW Mexico City [47], and also to lamprophyres from the Daping gold deposit [48].
3.7. Andesite-Dacite
For andesite-dacite, two samples from the Tabakoroni deposit have been analyzed. These samples are characterized by high SiO2, Al2O3, Na2O contents (respectively equal to 67 wt%, 16 wt% and 6 wt%). They are highly enriched in REE with a ∑REE ranging from 95 to 101 ppm. So, they are highly fractionated with a very high enrichment in light REE (La/YbCN = 31 - 40). In multi-element diagram normalized to the Primitive Mantle (Figure 8 and Figure 9(b)), the andesite-dacite samples show a negative Nb-Ta anomaly. The Zr/Y ratio of the andesites is between 10 and 20, a value compatible with a calc-alkaline affinity. This result is confirmed by the diagram of [49] in which andesitic samples are placed in the calc-alkaline field (Figure 5).
3.8. Microgranites
The analyzed microgranitic vein of Tellem is characterized by high SiO2 (71 wt% to 74 wt%) and Al2O3 (~15 wt%) contents and relatively high K2O proportion (between 1.87% and 2.42%). It is enriched in REE with ∑REE varying between 47 and 73 ppm and its spectra are strongly fractionated with La/YbCN ranging from 18 to 23 ppm. It generally shows a negative anomaly in Eu (Eu/Eu* = 0.84) with an exceptional positive value (Eu/Eu* = 1.16). However, multi-element diagram shows very strong negative anomalies in Nb, Ta and Th.
4. Discussion
In the Syama belt, three populations of magmatic rocks have been identified: basalts with a tholeiitic affinity, and more advanced rocks (andesites with microgranites) and lamprophyres with a calc-alkaline affinity. The intersection between lamprophyric veins and basalts verify the chronology proposed for this birimian period, namely the calc-alkaline event is posterior to the one of tholeiitic.
In order to characterize the tectonic the emplacement context of these various magmas, we consider that the principle of uniformitarianism is verified and that the processes of magma genesis for the Precambrian period are similar to those of the present period [50]. Thus, if we consider current island arc basalts (IABs) we note that the concentrations in Nb (<2 ppm; [51] is lower than basalts from other tectonic settings. This translates into a negative Nb anomaly and also Ta (similar chemical properties to Nb) in diagrams normalized to the early mantle [52]. These chemical characteristics are found in many tholeiitic basalt suites of Precambrian greenstone belts that have then been interpreted as formed in an arc setting [50] [51] [53].
In Syama belt, the basalts do not show negative anomalies of Nb and Ta, but relatively REE flat spectra. This type of spectrum is characteristic of basaltic sequences in current oceanic shelves (e.g. Nauru).Thus, a contextual analogy has been considered for similar ancient oceanic shelf basaltic sequences [2] [54]. To discuss the composition of mantle sources, [37] instead favors elemental ratios using HFSEs, as these elements have similar incompatibilities in the mantle. Thus, the author’s diagram combining Zr/Nb ratios with Nb/Th applied to the Syama and Tabakoroni basalt samples would indicate an oceanic shelf signature for them.
For differentiated rocks (andesite and microgranite), data strongly fractionated spectra (Figure 8), and the negative anomalies in Nb, Ta, Zr/Nb on Nb/Th ratios (Figure 9(b)), indicate the arc magmatism type.
Within the WAC, the tholeiitic basalts, which have the same characteristics as those of the Syama belt [2] [11] have been interpreted differently. Thus, [5] considers the Birimian tholeiitic basalts of the WAC as MORB equivalents. However, many authors [9] - [17] propose an o island arc context for these basalts. Finally, an oceanic shelf tectonic environment in relation to a mantle plume has been considered by [2] for the tholeiitic basalts (2.1 Ga) of the WAC parts (e.g., Mauritania, Senegal, Burkina-Faso), by [6] for volcanics from northeastern Ivory Coast, by [7] for volcanics from the Kédougou-Kéniéba Inlier (eastern Senegal), and recently by [8], for metavolcanics from the neighboring Mana district in the Houndé belt of Burkina Faso. Similarly, [55] proposed for the iron-rich tholeiitic basalts of the Birimian-aged El Callao formation, located on the Guiana Shield (equivalent to the WAC for the South American craton), an oceanic shelf tectonic setting. However, in this last example, in addition to the large volume of basalt (maximum thickness estimated at 1200 m), the existence of komatiite-type formations underlying the basalts has been highlighted, formations that have not been recognized to date in the WAC. In these last two examples (Mana and El Callao), the tholeiitic basaltic formations are host to gold mineralization. The same remark is done in Syama deposit.
The absence of isotopic data, the state of alteration affecting rocks and the poor conditions of outcrop does not make it possible to estimate the real volume of these various formations. It is thus difficult in our case to define precisely the tectonic context of these tholeiitic basalts.
In the Syama region, calc-alkaline formations are represented by mafic plutonic (lamprophyres) and more differentiated rocks (andesites and microgranites). The existence of mafic rocks (ultrabasic to basic) belonging to the calc-alkaline series was shown by [13] in the Loraboué region (Burkina Faso) and by [56] in the Kadiolo belt, located to the east of Syama belt.
The differentiated rocks of the Syama belt show strong similarities with the rocks of the calc-alkaline series of the Kédougou buttonhole in Senegal [57], Guinea [58] or Kalana in the south-east of Syama in Mali [25]. For the emplacement of the calc-alkaline rocks of the WAC belts, an island arc setting has been very often proposed [2] [9] - [17]. For the Bagoé belt, [18] [21] also proposed an origin in a back-arc context, and the Kadiana Madinani domain terrains are interpreted as a back-arc basin. Similarly, [57] proposed an island arc context for the emplacement of a set of ultramafic to mafic rocks of the Kadiolo belt, located east of the Syama belt. In conclusion, the calc-alkaline series represented by the differentiated volcanic rocks would be developed in an island arc context.
As for the spessartite lamprophyres, it is important to note that they are frequently found in association with gold mineralization. This is the case in Archean terrains, such as in Matheson, in Ontario [58] [59] and the Yilgarn Craton, Australia [60], but also in Cenozoic formations such as in China’s Yunnan Province with the Daping gold deposit, located along the Ailao-Shan-Red-River gold shear-zone, the latter being related to the collision of the Indian and Eurasian plates [48]. These various authors consider that this type of magmatic rock would be set up late during the geodynamic evolution, during late transmission to post-collisional tectonic phase. These particular rocks, rich in compatible (Cr and Ni) and incompatible (LILE and HFSE) elements would derive from magmas generated from a metasomatized mantle source [44] [46]. So, this origin is generally the case for this type of highly potassic-rich magma. In the case of Syama, the lamprophyres, intersecting the basalts in the belt, would mark the first extensive post-collisional stages that would take place at the end of the Eburnian orogeny.
5. Conclusions
The coexistence of heterogeneous geochemical signatures of igneous rocks in the Syama Belt (tholeiitic and then calc-alkaline), suggests that the rocks were formed in different tectonic settings during the Eburnian orogeny.
The coexistence of tholeiitic and calc-alkaline volcanic rocks is a magmatic association that is not unusual in the WAC. In the Baoulé-Mossi domain, this association has been identified in the Boromo and Houndé belts [12] [13] and interpreted as an evolution of juvenile island arc or oceanic plateau type during the Birimian. This oceanic province is dominated by the basal tholeiitic formation and the magmatic evolution continues to a mature arc, dominated by the calc-alkaline formations. This evolution could be also considered as cause of the petrological diversity of magmatic formations of the Bagoé belt in Syama region.
Acknowledgements
This work is based on unpublished thesis data of Traoré (2017). Resolute Mining is thanked for making before documents and drill cores of the deposit available for study, with special thanks to Pro. Pierre Débat, Dr. Mark Jessell, Dr. Hugues Pobanouh Diarra, Pro. Badié Diourté, and Dr. Mamadou Diawara. Geology department colleagues of FST/USTTB are also thanked for constructive comments and suggestions. We sincerely thank the reviewers and assigned editor of this paper, Mr. Joanna Cheng.