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1.
The Taparko gold deposit, located in the eastern branch of the Proterozoic Birimian Bouroum-Yalogo greenstone belt (Burkina
Faso) consists of a network of quartz veins developed in a N 170° trending shear zone (250 m wide, 4 km long) superimposed
on the regional Birimian structural pattern. The quartz vein network is composed of: (a) a dominant array of quartz veins
(type 1), parallel to the shear zone and comprising strongly deformed dark quartz exhibiting foliation, layering, ribbon,
tension gashes, etc.; (b) oblique and subparallel related veins (type 2) of gray to white weakly deformed quartz crosscutting
the dominant quartz veins resulting in breccia structures; and (c) shallow dipping veins (type 3), cross-cutting veins types
1 and 2 and filled by undeformed white buck structure quartz. Cross-cutting relationships and different quartz types in different
veins and within individual veins imply a concomitant filling of the veins during the progressive deformation. Initial sinistral
transcurrent shearing evolved with time to sinistral reverse shearing. Metallic minerals occur only in type 1 and 2 veins
and were deposited in two stages, with native gold being related to second stage sulfides. Gold (and chalcopyrite) precipitated
preferentially upon the surfaces of fractured pyrite grains in low-pressure sites (pressure shadow zones) around and/or within
the sulfide grains (along subsequently annealed fractures). The formation of the South Taparko deposit can be divided into
a succession of events: (a) during the first event, N 170°-directed sinistral transcurrent shearing resulted in a N 20° mylonitic
foliation and fractured rock which allowed H2O-, CO2- and SiO2-rich fluids to circulate and deposit quartz with buck texture; (b) during the second event, type 1 quartz was strongly deformed
and type 2 veins formed with sigmoidal shapes as viewed on a horizontal plane; and (c) during the third event, the sinistral
transcurrent shearing evolved to sinistral reverse shearing and the deformation style evolved correspondingly from ductile
to brittle-ductile. During the last phase of deformation gold nucleated and deposited in low-pressure zones.
Received: 9 July 1997 / Accepted: 23 March 1998 相似文献
2.
The major Ghanaian lode gold deposits are preferentially aligned along the western and eastern contacts of the Kumasi Basin with the Ashanti and Sefwi Belts, respectively. The investigated area of the Abawso small-scale concession, covering the workings of the old Ettadom mine, is situated 3 km west of the lithological contact of the Birimian metavolcanic rocks of the Akropong Belt in the east with the Birimian metasedimentary rocks of the Kumasi Basin in the west. The rocks of the Abawso concession represent a steeply NW-dipping limb of a SE-verging anticline with an axis plunging to the SW. Quartz veining occurs predominantly in the form of en échelon dilatational veins along NNE–SSW-striking shear zones of a few metres width and shows evidence of brittle and ductile deformation. Also stockwork-style quartz veining occurs in the vicinity of the main shaft of the old Ettadom mine. Hydrothermal alteration includes sericitisation, sulphidation and locally carbonatisation. The auriferous quartz veins mainly follow the trend of brittle to ductile deformed quartz veins; however, some occur in stockwork. Fluid inclusion studies reveal a large number of H2O inclusions along intragranular trails in auriferous quartz vein samples, as well as an overall dominance of H2O and H2O-CO2 inclusions over CO2 inclusions. Textural observations and physico-chemical fluid inclusion properties indicate post-entrapment modifications for all quartz vein samples due to grain boundary migration recrystallisation. This process is interpreted to be responsible for the generation of the CO2 inclusions from a H2O-CO2 parent fluid. In comparison with mineralisation at the Ashanti and Prestea deposits, which are characterised by CO2±N2 inclusions, the observed inclusion assemblage may be due to a shallower crustal level of mineralisation, or different degrees and styles of recrystallisation, or a less pronounced development of laminated quartz veins due to comparably restricted pressure fluctuations. Furthermore, the microthermometric observations allow the reconstruction of a possible retrograde P-T path, depicting near-isothermal decompression in the P-T range of the brittle/ductile transition.Editorial handling: E. Frimmel 相似文献
3.
Kwabina Ibrahim Frank K. Nyame Thomas Armah Johnson Manu Jeff Baah‐Acheamfour Jacob Tigme 《Island Arc》2020,29(1)
Manganiferous rocks in the Mankwadzi area in the southernmost portion of the Kibi‐Winneba metavolcanic belt, one of several Mn occurrences in the Paleoproterozoic Birimian of Ghana, are hosted in hornblende schist and amphibolite. These rocks are, in places, intruded by hornblende dyke. In outcrop, the manganiferous rocks appear to be conformable with the host schist and amphibolite, are macroscopically dark, fine‐grained and structurally massive to distinctly banded. Observed alternating light and dark occasionally macro‐folded bands suggest post‐depositional deformation of both light and dark bands. Microscopic observations revealed that the light bands are dominantly Si‐rich and the dark bands mainly of opaque minerals. Whole rock analyses of the manganiferous rocks show high contents of MnO (16.75–27.4 wt%) suggesting that the opaque minerals are likely rich in Mn. The analyzed rock samples show moderate to strong enrichments in light rare earth elements compared to heavy rare earth elements. Whereas the manganiferous rocks show perceptibly negative Eu anomaly, host hornblende schist and hornblende dyke do not. Eu anomaly in amphibolite samples is, however, uncertain as the three samples analyzed gave positive, negative and no Eu anomalies. Based on the field characteristics, microscopic and geochemical features, we suggest that the Mn occurrence in the Mankwadzi area originated via sedimentary deposition and was later modified by metamorphism, hydrothermal and/or supergene processes similar to manganiferous occurrences at Nsuta and Tambao in the Birimian of West Africa. 相似文献
4.
Located in the southeastern parts of the BaouléMossi 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-chondriticε_(Hf)-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 from2220 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 minimumε_(Hf)-value of-10.5 at2139 Ma suggests a Palaeoarchaean to late Mseoarchaean component as the contributing ancient source.Combined with previously reported heavy zirconδ~(18)O 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. 相似文献
5.
Sandow Mark Yidana Bruce Banoeng-Yakubo Abdul-Samed Aliou Thomas M. Akabzaa 《水文科学杂志》2013,58(6):1168-1183
Abstract Groundwater is a priceless resource in the economies of the rural populations in northern Ghana. A combination of multivariate statistical and spatial analytical techniques was applied to groundwater data from the Voltaian and Birimian aquifers in parts of northern Ghana. The objective was to classify the groundwater quality control parameters and determine whether the aquifers deliver groundwater of acceptable quality for domestic and commercial irrigation purposes. It was found that groundwater quality is dominated by the weathering of accessory minerals that are predominant in the Obosum and Oti beds of the Middle Voltaian in the north, and incongruent silicate mineral weathering ranks second among the major causes of variation. The two processes account for over 70% of the total variance in the hydrochemistry and interpolation maps generated for these two major factors are discussed. The mineral weathering and dissolution processes are less apparent within the Birimian aquifers. Four spatial groundwater types were distinguished by differences in EC and pH, which are high among the water types within the Obosum and Oti beds, and generally low within the Birimian aquifers. Mineral stability diagrams suggest that montmorillonite is the most stable clay mineral phase in the system, a significant finding in terms of cation exchange processes. This, in turn, has affected the irrigation quality of groundwater from the aquifers in the area. Our findings suggest that the Birimian aquifers are more suitable for irrigation, due to generally low salinities and sodicities, and those associated with the Obosum and Oti beds of the Middle Voltaian are less suitable. Editor D. Koutsoyiannis; Associate editor Y. Guttman Citation Yidana, S.M., Banoeng-Yakubo, B., Aliou, A.-S., and Akabzaa, T., 2012. Groundwater quality in some Voltaian and Birimian aquifers in northern Ghana—application of mulitvariate statistical methods and geographic information systems. Hydrological Sciences Journal, 57 (6), 1168–1183. 相似文献
6.
《International Geology Review》2012,54(9):1073-1086
Field studies in the Palaeoproterozoïc Daléma basin, Kédougou-Kéniéba Inlier, reveal that the main tectonic feature comprises alternating large shear zones relatively well-separated by weakly deformed surrounding rock domains. Analysis of the various structures in relation to this major D2 phase of Eburnean deformation indicates partitioning of sinistral transpressive deformation between domains of dominant transcurrent and dominant compressive deformation. Foliation is mostly oblique to subvertical and trending 0–30° N, but locally is subhorizontal in some thrust-motion shear zones. Foliation planes of shear zones contain a superimposed subhorizontal stretching lineation which in places cross-cuts a steeply plunging stretching lineation which is clearly expressed in the metasedimentary rocks of weakly deformed surrounding domains. In the weakly deformed domains, the subhorizontal lineation is absent, whereas the oblique to subvertical lineation is more fully developed. Finite strain analyses of samples from surrounding both weakly deformed and shearing domains, using finite strain ratio and the Fry method, indicate flattened ellipsoid fabrics. However, the orientation of the long axis (X) of the finite strain ellipsoid is horizontal in the shear zones and oblique within the weakly deformed domains. Exceptionally, samples from some thrust zones indicate a finite strain ellipsoid in triaxial constriction fabrics with a subhorizontal long axis (X). In addition, the analysis of the strain orientation starting from semi-ductile and brittle structures indicates that a WNE–ESE (130° N to 110° N) orientation of strain shortening axis occurred during the Eburnean D2 deformation. 相似文献
7.
Asinne Tshibubudze Kim A.A. Hein Pascal Marquis 《Journal of African Earth Sciences》2009,55(5):245-256
Birimian supracrustal sequences in NE Burkina Faso are dominated by meta-volcaniclastic greywacke, intercalated meta-conglomerate, siltstone and shale. The sequences where subjected to two phases of deformation and contact metamorphosed to hornblende–hornfels facies during emplacement of pyroxenite–gabbro–norite (Yacouba Mafic complex), granodiorite–tonalite (Tin Taradat granodiorite–tonalite) and dolerite dykes.Structural studies indicated that the NE-trending, first-order crustal-scale Markoye Shear Zone (MSZ; Markoye Fault of [Jeambrun, M., Delfour, J., Gravost, M., 1970. Carte géologique de L’Oudalan. Bureau De Recherches Geologiques et Miniéres, Burkina Faso.]) has undergone at least two phases of reactivation concomitant to two phases of regional deformation. The first phase of deformation, D1, resulted in the formation of NNW-NW trending folds and thrusts during dextral-reverse displacement on the MSZ. The deformation is termed the Tangaean Event and predates the Eburnean Orogeny. D2 phase involved a period of SE–NW crustal shortening and sinistral-reverse displacement on the MSZ, and is correlated to the Eburnean Orogeny 2.1 Ga. Deformation in D2 is characterised by NE-trending regional folds (F2) and a pervasive NE-trending foliation (S2-C to S2). Within the MSZ, deformation is characterised by NNE-trending zones of mylonite that are bordered in the hangingwall and footwall by pseudotachylite veins. Buck quartz-carbonate veins and quartz cataclasite veins crosscut the mylonite zones and are, in turn, crosscut by quartz–chlorite–(muscovite) shears that formed during reactivation of the MSZ late in D2. Several generations of veins are recognised at the Essakane main deposit (EMZ): Arsenopyrite–pyrite–gold mineralization in quartz veins formed in D1 during metasomatic alteration of the host rocks; Vein-stockwork gold mineralization is interpreted to have formed late in D2. 相似文献
8.
Abstract: The gold deposit at Ashanti occurs in the Proterozoic Birimian formation of Ghana. Two main ore types mined from the deposit are gold-bearing quartz veins, and gold-sulfide disseminations in metasediments and metavolcanics. The main sulfide minerals in the gold-sulfide disseminated ores are arsenopyrite, pyrite and pyrrhotite, and to a very minor extent, sphalerite and tetrahedrite. Carbonate alteration and sericitization are prominent in the metavolcanics and the metasediments, respectively. In the quartz veins, pyrite and arsenopyrite commonly occur in small amounts, but gold mostly occurs in contact with tetrahedrite, chalcopyrite, galena, aurostibite, and sphalerite. Pyrrhotite is absent in the quartz veins.
Microprobe studies indicate that As content of homogeneous arsenopyrite grains ranges from 27. 0 to 31. 7 atm%, and gives mineralization temperatures from 170 to 430°C, although mostly from 300 to 400°C. Chlorite geothermometry using temperature dependence of substitution of Al for Si in the tetrahedral site gives formation temeratures of 330 to 400°C, comparable to the arsenopyrite temperatures. Applying sphalerite–pyrite–pyrrhotite geobarometry to sphalerite with FeS contents from 13. 6 to 12. 5 mol%, the pressure was estimated to be in a range from 5. 9 to 7. 0 kb at the stage of elevated temperatures.
Mineralogical observations, especially absence of pyrrhotite in the quartz veins, together with microprobe data for gold and associated minerals suggest that the fluids having ascended through fissures in the Ashanti deposit were reduced by the reaction with carbonaceous materials in the metasediments during the declining stage of the regional metamorphism. 相似文献
Microprobe studies indicate that As content of homogeneous arsenopyrite grains ranges from 27. 0 to 31. 7 atm%, and gives mineralization temperatures from 170 to 430°C, although mostly from 300 to 400°C. Chlorite geothermometry using temperature dependence of substitution of Al for Si in the tetrahedral site gives formation temeratures of 330 to 400°C, comparable to the arsenopyrite temperatures. Applying sphalerite–pyrite–pyrrhotite geobarometry to sphalerite with FeS contents from 13. 6 to 12. 5 mol%, the pressure was estimated to be in a range from 5. 9 to 7. 0 kb at the stage of elevated temperatures.
Mineralogical observations, especially absence of pyrrhotite in the quartz veins, together with microprobe data for gold and associated minerals suggest that the fluids having ascended through fissures in the Ashanti deposit were reduced by the reaction with carbonaceous materials in the metasediments during the declining stage of the regional metamorphism. 相似文献
9.
The Diagorou-Darbani Birimian greenstone belt includes (i) tholeiitic metabasalts with flat to slightly depleted LREE patterns, (ii) amphibolites having high contents in both compatible and incompatible elements with strongly fractionated spidergrams showing large HFSE negative anomalies, and (iii) metabasalts and amphibolites with intermediate features between the two previous groups. The chemical compositions of these rocks are not those of oceanic plateau basalts, but suggest a magmatic evolution from an arc environment to a back-arc oceanic basin. Source of the highly enriched amphibolites could be a depleted mantle metasomatized by slab-derived siliceous melts. To cite this article: A. Soumaila et al., C. R. Geoscience 336 (2004). 相似文献
10.
Abstract: Manganese carbonate ore beds and host rock manganiferous phyllites at the Nsuta mine, western Ghana, contain well developed garnet crystals. Individual crystals are idioblastic, sometimes porphyroblastic, and homogeneous, and are associated with rhodochrosite (with or without kutnahorite), quartz and muscovite. The conspicuous absence of chlorite in garnet-rich assemblages, and of garnet in chlorite-rich rocks, suggest chemical constraints may have been important in the formation of the two minerals. Gondite bands within carbonate ores are interpreted to have resulted from localised processes in which manganese carbonates, in environments rich in alumino-silicate minerals, may have been completely exhausted during metamorphic reactions. 相似文献