The role of evaporites in the genesis of base metal sulphide mineralisation in the Northern Platform of the Pan-African Damara Belt, Namibia: geochemical and fluid inclusion evidence from carbonate wa, 2000, Chetty D, Frimmel He,

The Otavi Mountain Land is a base metal sulphide ore province in northern Namibia where deposits are hosted by platform carbonates of the Otavi Group in a foreland fold-and-thrust belt on the northern edge of the Pan-African Damara Belt. Deposits have been classified as the Berg Aukas- or Tsumeb-types, based on differences in ore association? stratigraphic position and geochemistry of ores and gangue carbonates. Mineralisation at these deposits is accompanied by carbonate alteration in the form of dolomite and calcite veins, carbonate recrystallisation, calcitisation and carbonate silicification. Based on cathodoluminescence imaging, trace and rare earth element (REE), O and C isotope, and fluid inclusion data, a series of carbonate generations, constituting wall rock alteration around the Tsumeb and Kombat (Tsumeb-type) and Berg Aukas (Berg Aukas-type) deposits, was established. Similar data obtained on the recently discovered Khusib Springs deposit indicate a strong affinity to Tsumeb-type deposits. Tsumeb-type deposits are distinguished from Berg Aukas-type deposits by having trace element and REE concentrations that are significantly higher in the alteration products compared to the carbonate host rocks. Only around Tsumeb-type deposits a relative enrichment in light REE is noted for the hydrothermal carbonate generations that are cogenetic with the main stage of mineralisation. Microthermometric results from fluid inclusions in carbonate alteration phases and associated quartz indicate relatively high salinity (17-33 wt% NaCl equivalent) for the main mineralising and subsequent sulphide remobilisation stages at the deposits investigated. Estimated mineralisation temperatures are significantly higher for Tsumeb-type deposits (370-405 degrees C) with early sulphide remobilisation in Tsumeb at 275 degrees C, whereas they are lower at Berg Aukas (up to 255 degrees C). Fluid inclusion leachate analysis suggests that most of the observed salinity can be ascribed to dissolved, predominantly Ca- and Mg-carbonates and chlorides with subordinate NaCl. Na-Cl-Br leachate systematics indicate a derivation of the fluid salinity from the interaction with evaporitic rocks en route. Tsumeb-type mineralisation is interpreted to be derived from fluids expelled during Pan-African orogeny in the more intensely deformed internal zones of the Damara Belt further south. When the high salinity fluids reached the carbonate platform after having scavenged high concentrations of base metals, base metal sulphide precipitation occurred in zones of high porosity, provided by karst features in the carbonate sequence. Results obtained for the Berg Aukas-type deposits emphasise their derivation from basinal brines, similar to Mississippi Valley-type deposits, and confirm that mineralisation of the Berg Aukas- and Tsumeb-types are both spatially and temporally distinct

Existence of karsts into silicated non-carbonated crystalline rocks in Sahelian and Equatorial Africa, hydrogeological implications, 2002, Willems Luc, Pouclet Andre, Vicat Jean Paul,

Various cavities studied in western Niger and South Cameroon show the existence of important karstic phenomena into metagabbros and gneisses. These large-sized caves resulted from generalized dissolution of silicate formations in spite of their low solubility. Karstification is produced by deep hydrous transfer along lithological discontinuities and fracture net works. The existence of such caves has major implications in geomorphology, under either Sahelian and Equatorial climate, and in hydrogeology and water supply, particularly in the Sahel area. Introduction. - Since a few decades, several karst-like morphologies are described in non-carbonated rocks (sandstones, quartzites, schistes, gneisses...) [Wray, 1997 ; Vicat and Willems, 1998 ; Willems, 2000]. The cave of Guessedoundou in West Niger seems to be due to a large dissolution of metagabbros. The cave of Mfoula, South Cameroon, attests for the same process in gneisses. This forms proof that big holes may exist deeper in the substratum even of non-carbonated silicate rocks. Their size and number could mainly influence the landscape and the hydrogeology, especially in the Sahelian areas. Guessedoundou, a cave into metagabbros in West Niger. - The site of Guessedoundou is located 70 km south-west of Niamey (fig. 1). The cave is opened at the top of a small hill, inside in NNE-SSW elongated pit (fig. 2 ; pl. I A). The hole, 3 to 4 m deep and 20 m large, has vertical walls and contains numerous sub-metric angular blocks. A cave, a few meters deep, comes out the south wall. Bedrocks consist of metagabbros of the Makalondi greenstone belt, a belt of the Palaeoproterozoic Birimian Formations of the West Africa craton [Pouclet et al., 1990]. The rock has a common granular texture with plagioclases, partly converted in albite and clinozoisite, and pyroxenes pseudomorphosed in actinote and chlorite. It is rather fairly altered. Chemical composition is mafic and poorly alkaline (tabl. I). A weak E-W schistosity generated with the epizonal thermometamorphism. The site depression was created along a N010o shear zone where rocks suffered important fracturation and fluid transfers, as shown by its silification and ferruginisation. The absence of human activity traces and the disposition of the angular blocks attest that the pit is natural and was due to the collapse of the roof of a vast cavity whose current cave is only the residual prolongation. To the vertical walls of the depression and at the cave entry, pluridecimetric hemispheric hollows are observed (pl. I B). Smooth morphology and position of these hollows sheltered within the depression dismiss the assumptions of formation by mechanical erosion. In return, these features are typical shape of dissolution processes observed into limestone karstic caves. That kind of process must be invoked to explain the opening of the Guessedoundou cave, in the total lack of desagregation materials. Dissolution of metagabbro occurred during hydrous transfer, which was probably guided by numerous fractures of the shear zone. Additional observations have been done in the Sirba Valley, where similar metabasite rocks constitute the substratum, with sudden sinking of doline-like depressions and evidence of deep cavities by core logging [Willems et al., 1993, 1996]. It is concluded that karstic phenomena may exist even in silica-aluminous rocks of crystalline terrains, such as the greenstones of a Precambrian craton. Mfoula a cave into gneisses in South Cameroon. - The cave of Mfoula is located 80 km north-east of Yaounde (fig. 3). It is the second largest cave of Cameroon, more than 5,000 m3, with a large opening in the lower flank of a deep valley (pl. I C). The cavity is about 60 m long, 30 m large and 5 to 12 m high (fig. 4; pl. I D). It is hollowed in orthogneisses belonging to the Pan-African Yaounde nappe. Rocks exhibit subhorizontal foliation in two superposed lithological facies: the lower part is made of amphibole- and garnet-bearing layered gneisses, and the upper part, of more massive granulitic gneisses. Average composition is silico-aluminous and moderately alkaline (tabl. I). The cave is made of different chambers separated by sub-cylindrical pillars. The ceiling of the main chamber, 6 m in diameter, is dome-shaped with a smooth surface (D, fig. 4). The walls have also a smooth aspect decorated with many hemispherical hollows. The floor is flat according to the rock foliation. They are very few rock debris and detrital fragments and no traces of mechanical erosion and transport. The general inner morphology is amazingly similar to that of a limestone cave. The only way to generate such a cavity is to dissolve the rock by water transfer. To test the effect of the dissolution process, we analysed a clayey residual sampled in an horizontal fracture of the floor (tabl. I). Alteration begins by plagioclases in producing clay minerals and in disagregating the rock. However, there is no more clay and sand material. That means all the silicate minerals must have been eliminated. Dissolution of silicates is a known process in sandstone and quartzite caves. It may work as well in gneisses. To fasten the chemical action, we may consider an additional microbial chemolitotrophe activity. The activity of bacteria colonies is known in various rocks and depths, mainly in the aquifer [Sinclair and Ghiorse, 1989 ; Stevens and McKinley, 1995]. The formation of the Mfoula cave is summarized as follow (fig. 5). Meteoric water is drained down along sub-vertical fractures and then along horizontal discontinuities of the foliation, particularly in case of lithological variations. Chemical and biological dissolution is working. Lateral transfers linked to the aquifer oscillations caused widening of the caves. Dissolved products are transported by the vertical drains. Regressive erosion of the valley, linked to the epeirogenic upwelling due to the volcano-tectonic activity of the Cameroon Line, makes the cavities come into sight at the valley flanks. Discussion and conclusion. - The two examples of the Guessedoundou and Mfoula caves evidence the reality of the karsts in non-carbonated silicated rocks. The karst term is used to design >> any features of the classical karst morphology (caves, dolines, lapies...) where dissolution plays the main genetical action >> [Willems, 2000]. Our observations indicate that (i) the karst genesis may have occurred into any kind of rocks, and (ii) the cave formation is not directly dependent of the present climate. These facts have major consequences to hydrogeological investigations, especially for water supply in Sahelian and sub-desertic countries. Some measurements of water transfer speed across either sedimentary pelitic strata of the Continental terminal or igneous rocks of the substratum in West Niger [Esteves and Lenoir, 1996 ; Ousmane et al., 1984] proved that supplying of aquifers in these silico-aluminous rocks may be as fast as in a karstic limestone. That means the West Niger substratum is highly invaded by a karstic net and may hidden a lot of discontinuous aquifers. The existence of this karst system can be easily shown by morphological observations, the same that are done in karstic limestone regions (abnormally suspended dry valleys, collapses, dolines...). Clearly, this must be the guide for any search of water, even in desertic areas where limestones are absent

Phenomenes karstiques fossiles et actuels au sein des formations metamorphiques silico-alumineuses de la nappe pan-africaine de Yaounde (Sud-Cameroun): Fossil and present-time karstic phenomena in sil, 2002, Vicat Jean Paul, Mvondo Hubert, Willems Luc, Pouclet Andre,

ResumeDiverses formes karstiques ont ete mises en evidence au sein des micaschistes et des gneiss non carbonates de la nappe de Yaounde. Leur formation est fortement controlee par les caracteres structuraux des roches. Les puits et les conduits de paroi resultent d'une karstification actuelle, initiee par la dissolution des plagioclases sous l'effet des eaux pluviales acides. Les alveoles de paroi et les grottes sont des formes exhumees ante-Miocene, dues a une dissolution en milieu noye. Pour citer cet article : J.-P. Vicat et al., C. R. Geoscience 334 (2002) 545-550

PRELIMINARY CONSIDERATIONS ON HYPOGENE MORPHOLOGY IN TOCA DA BOA VISTA E TOCA DA BARRIGUDA CAVES, NORTHEASTERN BRAZIL, 2014, Borges S. , Casarin C. , Menezes C. , Srivastava N. , Silva R. , Bezerra F. , Auler A.

The Toca da Boa Vista and Barriguda caves are located in Northeastern Brazil. They occur in the Neoproterozoic carbonates (limestones and dolomites) of the Salitre Formation, located at Irecê Basin. This set of rocks occurs within the São Francisco Craton, a region that was not affected by the Brasiliano-Pan-African orogeny (Pedreira et al., 1987). The caves occur at a dis­tance of approximately 300 m apart and there is a possibility of a link between them, but so far this has not been proven. Toca da Boa Vista has about 108 km of mapped passages and is therefore the largest cave in South America. Toca da Barriguda is smaller and has about 32 km of mapped galleries.

The architecture of the Toca da Boa Vista and Barriguda caves present both a 2D network and spongework type (Auler, 2009). The control of the conduits is related to faults, fractures and axial planes of antiforms. The general configuration of the caves seems to follow the Pacuí riverbed that has its channel located about 1km southeast. The origin of these hypogenic caves was first postulated by Auler & Smart (2004), who described some hypogenic features and reported a acid source (H2S) coming from existing pyrite in carbonates to explain the corrosion and dissolution of carbonate rocks. Klimchouk (2009) wrote about the need to investigate deeper this issue. He drew attention to the apparent feeders presence coming from the lower aquifer as well as to the importance of determi­nation of the source of acidity, since the amount of pyrite present doesn’t seem to be significant for the origin and development of the caves by hypogenic speleogenesis.

Although the origin and development of the caves are still under discussion, abundant hypogenic forms are present. Feeders, rising wall channels, half ceiling tubes, half wall tubes, ceiling cupolas, convection cupolas and wall niches are the major forms found. The linear geometry of caves suggests that they have a structural control. In addition, cavities generated at Toca da Boa Vista and Barriguda caves seem to follow the same stratigraphic level, as well as existing permeable structures such as fractures, faults and axial planes of antiforms. The process of ascending flow through these structures has resulted in the opening of the cavities by hypogenic dissolution as well as the collapse of blocks caused by the lack of sustainability of the layers generated by the voids left by the dis­solution. Outlets that would flow to levels above were not found. The origin and evolution of the cave system, however, needs further investigation.