The Samapleu mafic-ultramafic intrusion and its Ni-Cu-PGE mineralization: an Eburnean (2.09 Ga) feeder dyke to the Yacouba layered complex (Man Archean craton, western Ivory Coast)
Franck GouedjiChristian PicardYacouba CoulibalyMarc-Antoine AudetThierry AugéPhilippe GonçalvesJean-Louis PaquetteNaomi Ouattara
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Abstract The Yacouba layered complex intrudes the Archean (3.5–2.7 Ga) Kenema-Man craton in the Samapleu-Yorodougou area, western Ivory Coast. In Samapleu area, the complex was recognized in drill holes at three locations: Samapleu Main (SM); Samapleu Extension 1 (E1) and Yorodougou (Yo). It comprises websterites, peridotites and gabbro-norites arranged symmetrically with mafic layers at the center and ultramafic layers at both margins. The complex is inclined at 70–80° to the SE. The thickness of individual layers varies from 2 to 60 m and the total thickness is 120 to 200 m. At the E1 site, the complex extends to depths > 500 m. Contacts with the country rock gneiss are characterized by a hybrid zone that is a few meters thick and composed of plagioclase-orthopyroxene bearing metabasites, and locally (E1 site) a metamorphic assemblage of sapphirine-cordierite-sillimanite-spinel ± rutile. This assemblage is attributed to contact metamorphism during intrusion of the complex in the lower crust at a depth of about 25 km. Zircons in country rock gneisses and granulites, as well as in the hybrid facies, yield Archean ages of ~ 2.78 Ga, similar to ages reported in the Man craton. Rutiles in the hybrid zone give a U-Pb age of 2.09 Ga, which is interpreted as the age of contact metamorphism and emplacement of the intrusion. The Samapleu Main and Samapleu Extension 1 sites contain Ni and Cu sulfide deposit with reserves estimated as more than 40 million tons grading 0.25% Ni and 0.22% Cu (Sama Nickel-CI, August 2013). The Ni-Cu mineralization is composed of pentlandite, chalcopyrite, pyrrhotite and rare pyrite, which is disseminated mainly in pyroxenite or occurs as subvertical and semi-massive to massive sulfide veins. The sulfide textures range from matrix ore, net-textured, droplets or breccia textures. Zones enriched in PGM, particularly Pd, are associated with the sulfides and several chromite bands are also present. These observations suggest that an immiscible sulfide liquid formed from a parental silicate liquid and percolated through the crystal pile. The parental melt composition, determined using the Chai and Naldrett [1992] method, has a SiO2-rich mafic composition with 53% SiO2 and 10% MgO. This result, the presence of the hybrid zone, and the trace-element signature determined using the Bedard [1994] method, suggest a mantle-derived basaltic parental magma that had assimilated abundant continental crust. These observations indicate that Samapleu intrusion corresponds to a magmatic conduit of the Yacouba complex as at Jinchuan (China), Voisey’s bay (Canada), Kabanga (Tanzania) or Nkomati (South Africa).Keywords:
Ultramafic rock
Layered intrusion
Greenstone belt
Peridotite
Abstract This work presents the results of a fluid inclusion study of an amphibolite‐granulite facies transition in West Uusimaa, S.W. Finland. Early fluid‐inclusions in the granulite facies area are characteristically carbonic (CO 2 ), in contrast to predominantly aqueous early inclusions in the amphibolite facies area. These early inclusions can be related to peak metamorphic conditions (750‐820°C and 3‐5 kbar for peak granulite facies metamorphism). Relatively young CO 2 inclusions with low densities (<0.8g/cm 3 ) indicate that the first part of the cooling history of the rocks was characterized by a near isothermal uplift. N 2 ‐CH 4 inclusions, with compositions ranging between pure CH 4 and pure N 2 (Raman spectral analysis), were found in the whole area. They are probably syn‐ or even pre‐early inclusions. Only nearly critical homogenizing inclusions have been found (low density). Pressure estimates, based on densities of early fluid inclusions, show that the rapid transition of amphibolite towards granulite facies metamorphism is virtually isobaric. Granulite facies metamorphism in West Uusimaa is a thermal event, probably induced by the influx of hot, CO 2 ‐bearing fluids.
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Metadolerites occurring in the vicinity of Agto (within the Isortoq Complex of central West Greenland) have been metamorphosed in upper amphibolite to garnet granulite facies. Detailed sampling and petrographic study of these demonstrates that the entire sequence of assemblages may occur in individual dykes, with the amphibolite assemblages normally restricted to dyke margins, garnet granulite facies rocks occurring in the dyke cores, and pyroxene granulite facies rocks intervening between these two. Bulk rock chemical analyses demonstrate that the dykes are chernically homogeneous with respect to all oxides except Al2O3 and Na2O. Nevertheless, variation in the latter two oxides is small, and demonstrably is not a parameter which significantly controls assemblage development.
Pyroxene
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Greenstone belt
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A Thesis Submitted to the Faculty of Science
University of the Witwatersrand, Johannesburg
for the Degree of Doctor of Philosophy.
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The southernmost Hidaka metamorphic belt consists mainly of cordierite tonalite intrusions and pelitic metamorphic rocks ranging from the greenschist to the granulite facies. Anatectic migmatites are common in the higher amphibolite and granulite facies zones. Compositional changes in major, rare earth elements and some other trace metals are so small that they are undetectable among the pelitic metamorphic rocks of zones A + B + C and D, but they are large enough to be detected in the higher amphibolite (zone D) to the granulite facies rocks (zone E). The enrichment of Fe, Mg, Na, Eu, and Sc, and the depletion of K, P, La, Ce, Nd, Cs and Rb are statistically significant in pelitic granulites, while heavy REEs are very variable. The chemical variation of pelitic granulite was derived from the accumulation of plagioclase + garnet. This suggests that more than 50-60% of the total volume of pelitic granulite was melted to produce a large amount of tonalitic magma, leaving pelitic granulite as a restite. Migmatites of the higher amphibolite facies are anatexites, and their K, P, Cs, Rb and light REE content is the same as that of lower grade metamorphic rocks. Migmatites of the higher amphibolite facies melted incipiently to segregate only a small amount of melt, and could not produce a large magmatic mass such as the cordierite tonalites. Cordierite tonalites are S-type granites, and their major elements, Cs, Rb and light REE concentrations are similar to those of lower grade metamorphic rocks. The chemical variation of cordierite tonalites is explained by the extraction of plagioclase + garnet from a tonalitic magma and the variation of original sedimentary rocks. The small chemical difference between the cordierite tonalites and the lower grade metamorphic rocks suggests that the former was derived from a massive melting of metapelites or that much of the restite is retained. The material migration among higher amphibolite facies rocks, pelitic granulites, migmatites and cordierite tonalites took place through mineral/melt interaction in the lower crust.
Migmatite
Anatexis
Pelite
Cordierite
Greenschist
Sillimanite
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1961)
In this paper is presented a detailed subdivision of socalled gabbro-amphibolite zone of the Hidaka metamorphic belt, the axis of island of Hokkaido. It is composed of four tectonic units which are arranged in the following order from west to east, as is shown in figure I. 1) green schist zone, 2) epidote amphibolite zone, 3) green hornblende schistose amphibolite zone and 4) saussurite gabbro zone. Concerning zones 1), 2) and 3), their original rock facies are not clearly determinable. The zone of saussurite gabbro is composed of three varieties of amphibolite, a) hornblende schist facies, b) coarse grained amphibolite (=metasomatic) facies and c) compact amphibolite facies, all of which are derived from saussurite gabbro. In such a diverse metamorphic process, the different characters of tectonic forces and combined metasomatic effects have been the chief determinants.
Hornblende
Metasomatism
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Trace element
Rare-earth element
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Abstract The Eulogie Park Gabbro comprises approximately 65 saucer‐shaped layers consisting of olivine gabbros, ferrigabbros, troctolites, leucogabbros, gabbros and magnetite‐rich rocks. The intrusion is divisible into three main parts: a lower portion consisting mainly of ferrigabbros and magnetite‐rich layers, a middle portion composed of troctolites and olivine gabbros, and an upper portion consisting mainly of olivine gabbros and leucogabbros. The minimum stratigraphic thickness is 3,000 ft (900 m). Modal compositions and textures exhibit marked changes across the boundaries of the layers. Mineral compositions vary irregularly with height in the layered sequence, and small oscillations are common. Compositional data suggest that temperature increased during the first half of the crystallisation period, and then remained fairly constant. Chemical and petrographie studies of the iron‐titanium oxides indicate that the primary phase was an ulvöspinel‐magnetite solid solution. Subsolidus oxidation resulted in magnetite‐ilmenite intergrowths, which also contain exsolved pleonaste. The oxygen fugacity, which controlled the crystallisation and exsolution behaviour of the iron‐titanium oxides, is thought to have beeen affected by the fugacity of sulphur. Many features of this unusual intrusion are not obviously related to the conditions of crystallisation commonly supposed to exist in basic intrusions, and most of the anomalies have not been satisfactorily resolved. Undercooling and rhythmic concentration changes are believed to have been responsible for both the small‐scale rhythmic layering and the larger scale layering in the intrusion.
Layered intrusion
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Layering
Mineral redox buffer
Ilmenite
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Migmatite
Basement
Muscovite
Cordierite
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