Birimian Granitoids Research Articles

Petrographic and structural analysis of Paleoproterozoic Birimian granitoids and associated rocks of Boankra areas in the Kumasi Basin of Ghana

Petrographic and structural analysis of Paleoproterozoic Birimian granitoids and associated rocks of Boankra areas in the Kumasi Basin of Ghana

Geochemistry of Mafic Rocks from the Birimian Basement of Doropo (Northeast of Côte d’Ivoire): Petrogenetic and Geodynamic Implications

The northeastern region of Côte d’Ivoire is characterised by a granitic basement mainly composed of biotite granite rocks. According to mapping work in the Gbabédjou and Doropo areas, these Birimian granitoids are cut by gabbro dykes and amphibolite enclaves, which are the subject of this study. In order to better understand the role and the implication of mafic rocks in the Doropo basement emplacement, a multidisciplinary methodology integrating microscopic observations and geochemical analyses of major and trace elements was carried out on 4 samples considered representative of the outcrops studied. Green hornblende, clinopyroxene, and accessory sphene minerals are found in mafic mineral phases, according to petrographic research. Whole-rock analyses reveal that mafic samples with TiO2 contents 1.1 ratios giving them an orogenic granite nature (I-type). Their REE patterns are moderately fractionated (La/Sm)N = 2.66 - 6.13 and (La/Yb)N = 11.17 - 43.70) with a very negative Eu anomaly (Eu/Eu* = 0.75 - 0.97). The multi-element diagrams are characterized by negative Nb-Ta anomalies and geotectonic studies have identified them as volcanic arc formations. All these characteristics allowed us to distinguish the Doropo mafic rocks as formations originating from the juvenile continental crust, emplaced under the Archean tectonics model with significant crustal contamination in the source. Magma driven by mantle diapir has been injected at the base of the continental crust and the heat induces the partial melting of the overlying crust giving rise to mixed liquids. This magma now enriched in LILE was immediately drained to the upper crust to form the mafic rocks from the studied area.

Comparative Petro-Geochemistry of the Intrusive Granitoids of the Comoé Basin and the Granitoids of the Ferkessédougou Batholith (Côte D’Ivoire, Man-Leo Shield): Geodynamic Implications for the West African Craton (WAC)

The study of Birimian granitoids is of great importance because it allows us to understand the architecture of the West African crust and the processes that shaped it. In order to contribute to the improvement of knowledge on the geodynamic context of the emplacement of certain granitoids of the West African craton, this article addresses some essential problems of the Birimian, namely distinguishing the real nature of the magmas and the mechanisms that generated this Birimian crust. On the West African craton, there are intrusive granites in volcano-sedimentary furrows, in meta-sedimentary basins and granites that form batholiths separating these structures. To provide an answer to this scientific concern, we conducted a comparative study of the granitoids of the Comoé basin (Tiassalé region) and those of the large batholith of Ferkessédougou (Daloa region). From this study, it appears that these Birimian granitoids have been identified as granites, granodiorites and tonalites in the Tiassalé region while in Daloa, they are assimilated to anatexites and granites. They present very diverse aspects and contexts of emplacement: the granitoids of the Comoé basin have characteristics of type I granite, indicating direct crystallization of mantle magmas in a syntectonic emplacement, while in the Daloa region, some granitoids are magmatic, others migmatitic or metasomatic, reflecting a certain complexity relating to their genesis.

Petrology and geochemistry of TTG and K-rich Paleoproterozoic Birimian granitoids of the West African Craton (Ghana): Petrogenesis and tectonic implications

We conducted petrologic, geochemical and isotopic studies on Paleoproterozoic Birimian granitoids that intrude the Bole-Nangodi Belt in northern Ghana, in southeastern West African Craton (WAC), to constrain the geodynamic evolution of the Birimian Supergroup. The rocks are potassic (K)-rich granitoids (KRGs), tonalities, trondhjemites, granodiorites (TTGs), pegmatite and aplite. The K2O contents of the KRGs are up to 5 wt%, with generally high concentrations of Rb, Ba, Sr, Cr and Ni. They are highly fractionated calc-alkaline I-type granitoids, enriched in LILE and LREE relative to HREE and HFSE. Chondrite-normalized REE diagrams indicate fractionated LREE (average LaN/SmN = 5.05) and HREE (GdN/YbN = 4.56) patterns, with slight negative Eu anomalies (average Eu/Eu* = 0.75). This may indicate an evolved magma source with varying degrees of plagioclase fractionation. The KRGs share similar geochemical characteristics with the TTGs into which they intrude. Both rock types are characterized by Ba and Th enrichment and depletion in Nb‐Ta, Zr‐Hf, and Ti, which are typical characteristics of subduction-related magmas. The pegmatite-aplite association is slightly geochemically different and may have formed at a time later than the KRGs. The ԐNd(2.1 Ga) values range from + 0.90 to + 1.24 and −0.86 to + 1.37 respectively for the TTGs and KRGs, with TDM1 ages of 2.34–2.38 Ga and 2.36–2.53 Ga and TDM2 ages of 2.21–2.24 Ga and 2.22–2.39 Ga respectively. These indicate their juvenile character, possibly a depleted mantle source with minor contributions from a pre-Birimian (or Archean?) crustal material in their source material(s). The TTGs and KRGs are volcanic arc granites (VAG) and syn-collisional (Syn-COLG) granites, with formation temperatures and pressures of ~640–750 °C and ~2–6 kbar respectively. Based on the high temperatures, I-type signature, calc-alkaline nature and other geochemical characteristics, we infer that the TTGs, KRGs and pegmatite-aplite association developed under high oxygen fugacity conditions in an orogenic tectonic setting related to subduction. Thus, our geochemical and isotopic results are consistent with an island arc tectonic setting arising from subduction–accretion processes, which are typical for the Paleoproterozoic terranes of the WAC. These processes may have contributed to the amalgamation of the Columbia (Nuna) supercontinent during the Paleoproterozoic (2.1–1.8 Ga) orogeny.

Integrating gravity and magnetic field data to delineate structurally controlled gold mineralization in the Sefwi Belt of Ghana

Gravity and magnetic surveys were used to delineate potential gold mineralization zones in the Sefwi belt of Ghana. The study area is an intrusive dominated area that hosts pockets of small scale mining operations locally referred to as Galamsey. These Galamsey operations are not guided by a scientific approach to back the trend of gold mineralization which is conventionally mined. The study aimed at mapping lithological units, structural setting and relating Galamsey sites to delineate potential zones of gold mineralization. A Scintrex CG5 gravimeter and GEM's Overhauser magnetometer were used for gravity and magnetic data acquisition respectively. The magnetic data were corrected and enhancing filters such as reduction to the pole (RTP), analytical signal and first vertical derivative were applied using Oasis montaj 7.1. Gravity data were also reduced to the geoid using the Oasis montaj software to produce a complete Bouguer anomaly map. The regional/residual separation technique produced a residual gravity map. The RTP and analytical signal filters from the magnetic data and residual gravity anomaly map from the gravity data helped in mapping belt type (Dixcove) Birimian granitoids and mafic intrusive unit, interpreted as gabbro. The first vertical derivative filter was useful in mapping NE/SW minor faults and crosscutting dykes largely concentrated in the belt type Birimian granitoids. All the three mapped Galamsey sites fell on a minor fault and are associated with the belt type granitoids which were used in delineating four potential zones of gold mineralization.

Zircon Trace Element Constraints on the Evolution of the Paleoproterozoic Birimian Granitoids of the West African Craton (Ghana)

The Paleoproterozoic Birimian granitoids of the West African Craton (WAC) in the northwestern part of Ghana, have been studied for their zircon trace elements concentrations to infer the source characteristics, origin, and magmatic evolution. The zircons in the granitoids have Th/U ratios ranging from 0.03 to 1.55, and display depleted light rare earth elements (LREE) and enriched heavy rare earth elements (HREE) contents, characterized by pronounced positive to negative anomalies of Eu (Eu/Eu*=0.14–0.98 and 1.01–6.06, respectively) and Ce (Ce/Ce*=0.08–0.98 and 1.02–116, respectively), which may imply that they were derived from both magmatic and hydrothermal sources. The geochemical plots of U/Yb vs. Y and Hf, the positive correlation between Hf and the other high field strength elements (HFSE) and high rare earth elements (REE) contents, with enrichment in Ce and depletion in Eu, indicate that the granitoids possibly formed from partial melting of the crust. The trace elements characteristics (i.e., wide range of Hf, Ce/Ce*, Th/U and Zr/Hf values) of the zircons suggest that crystallization of the magma occurred under variable oxidation states, which spanned over a longer period, implying that our data corroborate interpretations from studies of whole-rock geochemistry and geochronology on the granitoids of northwestern Ghana. This further indicates that the evolution of the Birimian granitoids in this part of the WAC occurred earlier than what had been reported in the literature.

Extensive reworking of Archaean crust within the Birimian terrane in Ghana as revealed by combined zircon U-Pb and Lu-Hf isotopes

Located in the southeastern parts of the Baoulé Mossi domain of the West African Craton, the Birimian terrane in Ghana is known to consist of predominantly juvenile crust emplaced during the Eburnean orogeny. Here we present novel coupled zircon U-Pb and Lu-Hf isotope data from nine Birimian granitoids intruding different domains from northwestern, western and southeastern Ghana. Sub-chondritic ɛHf-values indicating reworking of Archaean crust are recorded in zircon from both northwestern and southeastern Ghana. As a conservative estimate 71% of all analysed zircon grains spanning in age from 2220 Ma to 2130 Ma require contribution from a reworked Archaean source, contradicting the common belief that the Birimian terrane consist of predominantly juvenile crust. A minimum ɛHf-value of −10.5 at 2139 Ma suggests a Palaeoarchaean to late Mseoarchaean component as the contributing ancient source. Combined with previously reported heavy zircon δ18O signatures from Birimian river zircon in Ghana, our new data suggests reworking of Archaean aged sediments during subduction initiated crustal growth in the vicinity of one, or possibly in between two Archaean aged cratons.

New zircon U–Pb ages for erratic emplacement of 2213–2130 Ma Paleoproterozoic calc-alkaline I-type granitoid rocks in the Lawra Volcanic Belt of Northwestern Ghana, West Africa

Paleoproterozoic Birimian granitoid rocks from the Lawra volcanic belt of northwestern Ghana have been analyzed for their major, trace, REE concentrations and their zircon U–Pb age. Based on the mineral assemblages, the calc-alkaline, metaluminous to weakly peraluminous I-type granitoids were classified as: (1) gneissic biotite granite, (2) hornblende granodiorite, (3) biotite granite, (4) two-mica granite and (5) pyroxene hornblende gneiss. The high LaN/YbN (∼4–146) values of the granitoids, characterized by enriched LREE pattern and flat to depleted HREE pattern ((Dy/Yb)N=0.95–3.47), is typical of rocks derived from an evolved magma or partial melting of the depleted mantle. Similarly, the slightly negative Sr- and Eu-anomalies indicate varying degrees of plagioclase fractionation. Their high but varying SiO2 contents (58.1–76.1wt.%), K2O/Na2O ratios (0.17–1.2) and (FeO+Fe2O3)/MgO ratios (0.98–4.02), and low Ce and Ti suggest that they are volcanic arc granites (VAG). The granitoids have zircon U–Pb ages between 2130.8±9.5Ma and 2213±76Ma, and preclude the contamination of the juvenile Birimian crust by notable amounts of reworked Archaean crustal material. However, these ages spanning ∼81Ma were recorded by all the rock types in the study area, with no distinct age difference among them. Accordingly, the area could be described as an undifferentiated terrain that experienced erratic emplacement of pulses of granitic magma during the period. Furthermore, the oldest ages of 2211Ma and 2213Ma recorded in this study suggest that the emplacement of Birimian granitoids in Ghana may have commenced much earlier than previously reported in the literature. The results demonstrate the juvenile character of the granitoid rocks. We therefore infer that the Birimian granitoids of the Lawra volcanic belt formed from large-scale crustal growth process with large amounts of juvenile crust formed from the depleted mantle in an island arc environment.

Chemical and Sr–Nd isotopic compositions and zircon U–Pb ages of the Birimian granitoids from NE Burkina Faso, West African Craton: Implications on the geodynamic setting and crustal evolution

The West African Craton was the site of massive juvenile crustal addition in the early Proterozoic during the Eburnean orogeny (2.1Ga). In this study, we determined chemical and isotopic compositions as well as zircon U–Pb ages of granitoid samples from NE Burkina Faso. These data were then used to constrain the chemical characteristics, emplacement age and tectonic environment of these rocks in the context of the West African Craton evolution. Birimian (Paleoproterozoic) granitoids of NE Burkina Faso are associated with volcanic and volcano-sedimentary sequences of the greenstone belts. Analyses indicate that the Na-rich Birimian granitoids of tonalite–trondhjemite–granodiorite (TTG) composition were generated by partial melting of a garnet-bearing amphibolite crust. Granite intrusions derived from partial re-melting of the deep basement of felsic calc-alkaline nature with contribution of metasediments. Zircon U–Pb isotopic analyses yielded ages between 2122±15Ma and 2181±7Ma for the Birimian Na-rich granitoids and 2151±10Ma for a biotite granite (IJ10). Sr–Nd isotopic data show very low initial Sr composition (ISr (T)≤0.7007), positive epsilon neodymium values (ɛNd (T)=+0.7 to +2.5) and Sm–Nd model ages (TDM) of 2363–2558Ma). These data are similar to those of mafic rocks of the Pissila greenstone belt: (ISr (T)=0.7003–0.7015, ɛNd (T)=+3.2 to +4.1, TDM=2336–2552Ma), and further suggest that the Na-rich granitoids are of juvenile nature. The present study together with published data on the Birimian volcanics and meta-sedimentary rocks of the West African Craton preclude contamination of the juvenile Birimian rocks of NE Burkina Faso by reworked Archean materials. The Birimian granitoids of NE Burkina Faso were mostly emplaced during the early stage (2.19–2.15Ga) of a large scale crustal growth event (2.2–2.0Ga) where large amount of juvenile materials were added to the crust from the depleted mantle in a tectonic environment comparable to modern volcanic arcs.

Fluid Characteristics of Granitoid-Hosted Gold Deposits in the Birimian Terrane of Ghana: A Fluid Inclusion Microthermometric and Raman Spectroscopic Study

Fluid inclusions in vein quartz from 10 granitoid-hosted gold deposits and prospects in the Birimian terrane of Ghana, as well as from the Sansu mine (Ashanti shear zone type) and quartz veins in the nonmineralized Princess Town granodiorite, were studied by microthermometry and Raman microspectrometry. Fluid inclusions from the granitoid-hosted gold deposits are dominated by aqueous H2O-CO2-NaCl type 1 and liquid CO2-N2 ± CH4 type 2, with minor (<10%) aqueous H2O-NaCl type 3. Type 1 inclusions show large variations in CO2 phase volume proportions (10–90%) at 25°C and have salinities commonly between 0 and 6 wt percent NaCl equiv. Their bulk densities fall in a major range from 0.62 to 1.11 g/cm3. Type 2 inclusions show no visible H2O phase at room temperature and have bulk densities between 0.30 and 0.92 g/cm3. Type 3 inclusions, containing 10 to 20 vol percent H2O vapor, have salinities of 1 to 8 wt percent NaCl equiv and bulk densities of 0.77 to 1.03 g/cm3. In most cases, the three types of fluid inclusions coexist as groups in individual quartz grains of the samples studied. These fluid inclusions are interpreted to be trapped during phase separation of an originally homogeneous H2O-CO2 fluid, with low salinity (<6 wt % NaCl equiv) and moderate to high density (0.65–0.95 g/cm3). The type 1 fluid inclusions are suggested to be heterogeneous mixtures of the two end members of type 2 and 3 inclusions. Fluid immiscibility is documented by petrographic characteristics and microthermometric results of the three types of inclusions. Trapping temperatures and pressures, estimated from microthermometry of type 3 inclusions, equation of state, and the P-T-X nature of the H2O-CO2-NaCl system, are mainly between 200° and 350°Cand 1 and 3 kbars for the gold deposits. By contrast, fluid inclusions in the Sansu mine at Ashanti mostly comprise the liquid CO2-N2 ± CH4 type 2. The inclusions are considered to represent postentrapment modifications of trapped fluids. In addition, fluid inclusions in barren vein quartz from the Princess Town granodiorite comprise the low-salinity (commonly <6 wt % NaCl equiv) H2O-NaCl type 3. Raman microspectrometry shows that gaseous compositions of fluid inclusions from both granitoid- and shear zone-hosted gold deposits are mainly composed of CO2 (80–95 mol %), with significant amounts of N2 (2–20 mol %) and CH4 (0–10 mol %). The distinct low-salinity H2O-CO2-rich fluids from the granitoid-hosted gold deposits are comparable in composition to those from the major Ashanti and Tarkwaian types of gold deposits in the Birimian terrane of Ghana. These fluids are most likely to be metamorphic in origin and associated with the waning stages of the regional Birimian orogeny. Gold deposition within the Birimian granitoids was related to fluid phase separation and sulfidization of host rocks during hydrothermal alteration and mineralization. The present study, together with previous publications, suggests that fluid inclusions are characterized by H2O-CO2-NaCl and/or CO2-N2 ± CH4 types in mineralized areas, whereas H2O-NaCl fluid compositions dominate in barren areas. Fluid inclusion characteristics may, therefore, be a useful tool for regional gold exploration in the Birimian terrane of Ghana.

Le paléovolcanisme de la bordure occidentale de la boutonnière de Kédougou, Paléoprotérozoïque du sénégal oriental: incidences géotectoniques

The main petrographical and geochemical patterns (major, trace and rare earth elements) of the epimetamorphosed volcanic rocks from the Mako Supergroup (Birimian) from the central and southern part of the western border of the Kedougou-Kenieba Inlier (Palæoproterozoic) are presented. The petrographic patterns indicate a predominance of tholeiites with the middle and upper parts of the lithological sequence containing scarce andesite, rhyodacite and rhyolitic end members of calc-alkaline affinity. The observed geochemical trend is compared with the patterns from other Palæoproterozoic areas of the West African Craton, as well as recent volcanic rocks, whose geotectonic setting is well constrained. The evolution of the Kedougou Inlier corresponds to an ocean island-arc environment, in agreement with previous research on epimetamorphic volcanodetrital and sedimentary formations, and Birimian granitoids. This research underlines the importance of island-arc accretions in crust generation during Palæoproterozoic times in West Africa.

Crustal growth in West Africa at 2.1 Ga

Birimian (∼2.1 Ga) terranes in the West African craton are a mixture of highly metamorphosed volcanic, sedimentary and plutonic rocks and low grade metavolcanics and metasediments. The volcanic units contain thick, commonly pillowed, tholeiitic basalts overlain by pelagic sediments cherts and black shales; The sedimentary units are characterized by an abundance of clastic turbiditic sediments. Andesites and calc‐alkaline felsic volcanics occur at uppermost stratigraphic levels and as dykes. Field relationships between the volcanic and sedimentary units remain a matter of debate. Calc‐alkaline and local alkaline granites, which intruded in distinct pulses and occasionally are related to transcurrent tectonics, represent almost half of the Birimian terranes. New isotopic work on the highly metamorphosed units greatly improved the chronology for the Birimian crust. The age of the early Dabakalian event is precisely defined by a U‐Pb zircon age at 2186 ± 19 Ma, while Rb‐Sr and Sm‐Nd methods give ages of 2162 ± 19 Ma and 2141 ± 24 Ma, respectively. A Sm‐Nd garnet‐whole rock age of 2153 ± 13 Ma suggests that metamorphism culminated at about the same time. In contrast, the most precise zircon U‐Pb and Sm‐Nd data for the more widespread Birimian terranes (sensu stricto), from this study and from the literature, cluster between 2.12 and 2.07 Ga. The major evolution of the Birimian crust apparently lasted less than 50 Ma. Isotopic evidence indicates that Birimian granitoids contain a negligible component of Archean crust: εNd(2.1‐Ga) values are positive and similar to those of Birimian basalts, crustal residence times are shorter than 200 Ma, U‐Pb ages for detrital zircons from clastic sediments range from 2098 ±11 Ma to 2125 ± 17 Ma, while granite chemistry and Nd isotopic characteristics are unrelated. Only very locally in Guinea is there isotopic evidence of interaction between Birimian felsic magmas and the Archean rocks from the Man craton. In accord with Abouchami et al.'s (1990) suggestion that Birimian basalts represent oceanic plateaus, the present data argue that the protolith of much of the West African continent was created around 2.1 Ga in an environment remote from Archean crust. Intrusion of calc‐alkaline magmas into the tholeiitic units suggests that island arcs formed on top of the assumed oceanic plateaus which then collided with the Man Archean craton. Taking the Birimian formations from the Guyana shield into account, the minimum crustal growth rate at 2.1 Ga is about 1.6 km3/a, some ∼60% higher than the present growth rate. Birimian crust growth at 2.1 Ga is reminiscent of Archean processes but contrasts with 1.7 – 1.9 Ga crust formation in the North Atlantic continent which generally involved significantly more interaction with older continental crust. A comparison of the Birimian crustal growth rate with the average crustal growth rate over the Earth history implies that a large part of the Birimian crust has been recycled into the mantle or incorporated into younger orogenic segments. This apparent deficit in the crustal budget is even more dramatic for the Archean crust.