Geologica Belgica

The evolution of the mineralogical and petrophysical properties of a weathered limestone in southern Belgium

Tue, 21 May 2013 00:00:00 +0200

The weathering of limestone results in the partial dissolution of calcium carbonate and leaves a soft porous material called alterite. The properties of the weathered rock differ significantly from those of its parent due to the changes in composition and the removal of soluble materials. The resulting increase in porosity modifies the hydrological proprieties of the rock such as its permeability, hydraulic conductivity, and reservoir capacity. The loss of material also weakens the structure of the rock and decreases its mechanical resistance. This study quantifies the progressive changes in the mineralogical, petrographic and mechanical properties that occur during the weathering of the Carboniferous Petit Granit, a limestone found around Soignies in southern Belgium. The rock samples, representing various stages of weathering, were collected within the same stratum and analysed in terms of their porosity, permeability, density, carbonate content, and mechanical resistance. The mineralogical and petrophysical changes were documented through X-ray diffraction and thin sections analysis. It is clearly seen that weathering exerts a great influence on the rock properties: the widespread increase in porosity is associated with a decrease in density, a large increase in permeability, and a fall in the mechanical strength to zero. We show how these properties change with the intensity of the weathering.

Altération de type fantôme de roche dans les calcaires oligocènes du Médoc (France) : étude pétrographique et micromorphologique

Tue, 21 May 2013 00:00:00 +0200

En Gironde, les calcaires oligocènes du Stampien (Rupélien) présentent des faciès altérés à l’origine de grottes et d’effondrements, notamment dans les vignobles. Dans le Médoc, le calcaire affleure rarement en raison du relief faible (0-25 m) et de l’ancienne couverture fluviatile de la Garonne. Il existe des émergences, mais aucune grotte n’est connue. La carrière souterraine de Larrivaux (commune de Cissac) a permis d’observer le phénomène de fantômisation sous plusieurs formes : (i) des zones altérées verticales et horizontales présentant des éléments non altérés flottant dans l’altérite ; (ii) des petits conduits vidés en partie de leur altérite ; (iii) des coupoles de plafond ; (iv) des cheminées de soutirage (sols). Des lames minces de grandes dimensions (70 x 140 mm) ont permis d’analyser en continu le passage de la roche non altérée à la roche altérée. La fantômisation se présente sous forme d’une porosité importante et d’une première ferruginisation. On observe localement un soutirage d’éléments provenant des sols (quartz, cutanes de goethite, argile massive) et surtout des cristaux aciculaires de lublinite dans les pores, indicateur d’un processus particulier de pédogenèse souterraine.

Ghost rock alteration of Oligocene limestones in Medoc, Gironde (France), petrographic and micromorphologic study. In Gironde, the Oligocene limestone facies of Stampian are altered by ghost-rock process giving rise to caves and collapses, especially in the vineyards. In Medoc peninsula, limestone outcrops are scarce because of low relief (0-25 m) and the Pleistocene alluvial deposits of Garonne river. There are several springs an a large polje (Reysson) covered by marine deposits, but no cave is known. The underground quarry of Larrivaux (Cissac) allowed to observe the phenomenon of ghost-rock in different forms: (i) vertical and horizontal alterated areas with floating rock elements in the alterite ; (ii) small passages emptied of their alterite ; (iii) some roof cupolas ; (iv) collapse-chimneys. Large thin section (70-140 mm) were used to analyze the transition between unaltered limestone and altered rock, and also the mixture ghost rock / pedologic contamination. The ghost-rock alteration is in the form of a high dissolution porosity: small voids of 50-150 µm especially in micrite of foraminifera and large voids of 0.3-2 mm. The penetration of iron is proportional to the degree of alteration with the probable activity of ferro-bacteria and mangano-bacteria (filaments). Because of the proximity to the surface, we observe soils infillings (clay, quartz) and iron coatings. Typical lublinite fine crystals in the small and large voids are an indicator of a peculiar underground pedogenesis in ghost-rock alterite.

Antithetic-dominated relics of the immature stage of the North-Armorican shear zone (Brittany, France)

Tue, 23 Apr 2013 00:00:00 +0200

Reliques antithétiques de la phase primitive du cisaillement Nord Armoricaine (Bretagne, France). La zone de cisaillement nord armoricaine (NASZ) (Bretagne, France) orientée E-W et active au Paléozoïque est reconnue comme une zone de cisaillement dextre intracontinentale. La zone cataclasique de Queffleuth (QCZ) a été définie dans la vallée de Queffleuth au sein de la NASZ. La QCZ contient des protocataclasites à des cataclasites. Dans la QCZ, quatre classes morphologiques de failles ont pu être pétrographiquement définies. Perpendiculairement au plan de failles de la première classe, le pourcentage de matrice augmente tandis que le pourcentage de clastes diminue, en s'éloignant du plan de faille. De ce fait, cette classe est interprétée comme pene-contemporaine des cataclasites. Les trois autres classes recoupent les cataclasites et sont ainsi datées post-cataclase. Il a été remarqué que l’orientation privilégiée des failles était N-S. Une analyse en termes de tenseurs de contrainte a permis d’identifier cinq événements cohérents. La prédominance de la direction N-S a été interprétée comme une expression antithétique par rapport au mouvement général de la NASZ. Il en résulte la récurrente de mouvements dextres et senestres de la QCZ pendant et après la phase de cataclase. Ainsi on propose une alternance de plusieurs tenseurs de contraintes pendant la phase primitive du cisaillement nord armoricain, comme dans la zone sismique du sud de l’Islande.

The Palaeozoic North-Armorican shear zone (NASZ) (Brittany, France) is described as an E-W trending dextral, intracontinental shear zone. In the valley of the Queffleuth river, we outline a well-exposed zone of protocataclasite to cataclasite as the Queffleuth cataclastic zone (QCZ). Four morphological fault classes are defined in the QCZ based on the petrographic characteristics of the fault planes. The first fault class is clearly related to cataclasis, as the matrix/clast ratio increases and the clast size decreases towards the individual fault planes. The other three morphological classes cross-cut the cataclasite and are therefore considered to postdate cataclasis. A palaeostress analysis allowed to distinguish five distinct stress states, which are dominated by oblique to pure strike-slip N-S trending faults, that are oriented antithetic with respect to the overall trend of the NASZ. Both dextral and sinistral E-W kinematics are inferred for the QCZ. Because all morphological fault classes are identified in each of the stress states, dextral and sinistral kinematics must have occurred during and after cataclasis. Based on the kinematic similarities with the active South Iceland seismic zone, it is suggested that the defined stress states reflect local stress permutations during the early, immature stage of the NASZ.

Antithetische restanten van de primitieve fase van de Noord-Armoricaanse schuifvervormingszone (Bretagne, Frankrijk). De Palaeozoische Noord-Armoricaanse schuifvervormingszone (NASZ) (Bretagne, Frankrijk) wordt beschreven als een O-W georiënteerde dextrale intracontinentale schuifvervormingszone. In de vallei van de Queffleuth is een goed ontsloten zone met protocataclasiet en cataclasiet gedefinieerd als de Queffleuth cataclastische zone (QCZ). In de QCZ zijn vier morfologische breukklasses omlijnd op basis van de petrografische kenmerken van de breukvlakken. De eerste morfologische breukklasse is duidelijk gerelateerd met cataclase, doordat zowel de matrix/klast ratio stijgt, als de klastgrootte daalt naar de individuele breukvlakken toe. De breukvlakken van de andere drie morphologische breukklasses doorsnijden de cataclasiet en worden daarom geïnterpreteerd als zijnde postcataclase. Met behulp van een palaeospanning analyse zijn vijf palaeospanning toestanden afgeleid van de meestal N-Z georiënteerde oblique tot laterale breuken. Deze N-Z breuken zijn antithetisch ten opzichte van de NASZ. Voor de QCZ zijn zowel dextrale als sinistrale O-W georiënteerde schuifvervorming afgeleid. Omdat alle palaeospanning toestanden gebaseerd zijn op een mengsel van breuken uit de verschillende morfologische breukklasses, hebben de dextrale en de sinistrale schuifvervorming plaatsgevonden tijdens en na cataclase. Er wordt voorgesteld dat de palaeospanning toestand lokaal wisselde tussen dextraal en sinistraal tijdens een primitieve fase van de NASZ, zoals in de actieve Zuid-Ijsland seismische zone.

Petrographic and mineralogical characterisation of fractionated pegmatites culminating in the Nb-Ta-Sn pegmatites of the Gatumba area (western Rwanda)

Tue, 12 Mar 2013 00:00:00 +0100

The Karagwe-Ankole belt in Central Africa hosts numerous rare-metal pegmatites and Sn–W mineralised quartz veins, which are related to the granite generation that formed at 986 ± 10 Ma, i.e. the G4-granites in Rwanda. This early Neoproterozoic granite generation features in the Gatumba area (western Rwanda) a linkage with a zoned cluster of barren and Nb-Ta-Sn mineralised pegmatites. The most distal pegmatite bodies in this area suffered from intense alkali metasomatism, i.e. widespread growth of albite and white mica. A petrographic and geochemical study has been carried out on the pegmatite bodies in order to determine the petrogenetic evolution of the pegmatite zonation. The compositional variation of schorl and elbaite, sampled along the regional zonation sequence, implies that the zonation can be expressed by 4 zones: a biotite, a two-mica, a muscovite and a mineralised pegmatite zone. Alkali element variations and enrichments in muscovite and K-feldspar along the zonation sequence indicate that the origin and the formation of the different pegmatite zones can be explained by a single path of fractional crystallisation. This substantial chemical differentiation accompanies in the Gatumba pegmatite field the evolution from the G4-granite generation to common pegmatites and eventually rare-element pegmatites.

JONGERENDAG - JOURNEE DES JEUNES – 2012Conchological differentiation in an ongoing radiation of Lanistes gastropods from ancient Lake Malawi: how adaptive is shell morphology?

Tue, 12 Mar 2013 00:00:00 +0100

Essai de description d’algues nouvelles paléozoïques

Mon, 11 Mar 2013 00:00:00 +0100

Description d’une dizaine de nouveaux genres (Chegatellina, Emsiella, Flexitubulla, Kamaenina, Koninckoporella, Masloviporella, Paraumbellina, Pseudoepiphytella, Sphaerinviella, Vintonella, Wapitella) et d’une quarantaine de nouvelles espèces. La microflore, associée à quelques Microproblématiques, provient de bassins à l’échelle mondiale. L’échantillonnage couvre le Dévonien, le Carbonifère et le Permien.

Description of a dozen new genera (Cheggatellina, Emsiella, Flexitubulla, Kamaenina, Koninckoporella, Masloviporella, Paraumbellina, Pseudoepiphytella, Sphaerinviella, Vintonella, Wapitella) associated with about forty new species. The microflora, including a few Microproblematica, are derived from many basins on a worldwide scale. The samples range from the Devonian, Carboniferous and Permian.

Petrographic and mineralogical study of the sediment-hosted Cu-Co ore deposit at Kambove West in the central part of the Katanga Copperbelt (DRC)

Mon, 11 Mar 2013 00:00:00 +0100

Kambove West is a sediment-hosted Cu-Co deposit in the Neoproterozoic Mines Subgroup, which is mainly composed of dolomitic shales and dolostones. Key factors and processes responsible for high-grade mineralization were identified by studying multiple cores. The stratigraphic position of the mineralized zones adjacent to the Roches Siliceuses Cellulaires and brecciated zones are the most important factor controlling the formation of a high-grade mineralization as they most likely acted as conduits for the mineralizing fluids. The presence of organic matter, stromatolite fragments and anhydrite pseudomorphs promoted mineralization. The organic matter caused a reducing environments required for the precipitation of the sulfides. Sulfate-reducing microorganisms, which were main components of Precambrian stromatolite communities, could also have caused the necessary reduction reactions. In addition to sulfate from the pores of the sediment, anhydrite formed a sulfate source. A paragenesis with six stages was established based on microscopic observations and cold cathodoluminescence petrography. Two major hypogene mineralization phases, which consist of pyrite, chalcopyrite, bornite, chalcocite and carrollite, formed during diagenesis and low-grade metamorphism, and were succeeded by supergene remobilization, of which chalcocite forms the dominant Cu-phase in the cementation zone. The distinction between hypo- and supergene chalcocite is based on the identification of multiple microtextures, chalcocite polymorphism, and the association with iron and other (hydro)oxides or late generations of dolomite. However, none of the features are conclusive. The combination of them is highly suggestive for a supergene remobilization.

Mineralogical characterization of cassiterite concentrates from quartz vein and pegmatite mineralization of the Karagwe-Ankole and Kibara Belts, Central Africa

Wed, 06 Mar 2013 00:00:00 +0100

The Mesoproterozoic Kibara belt (KIB) and the Karagwe-Ankole Belt (KAB) in Central Africa are characterized by the presence of numerous rare metal mineralized Sn-(Nb-Ta) pegmatites and Sn-W mineralised quartz veins that are related to S-type granite generation that formed at 986  10 Ma. Cassiterite concentrates have been studied by different petrographic and mineralogical techniques. The concentrates have been collected from the rock and mineral collection of the Royal Museum for Central Africa (RMCA) and originate from historical exploitations of eluvial and alluvial cassiterite deposits. No quantitative study of the concentrates has been envisaged since no information is available about the history of the samples prior to sampling. Microscopic investigation revealed the presence of cassiterite crystals with metallic and non-metallic luster, of which the latter show growth zoning. The colour from the cassiterite crystals can vary from transparent colourless to black non-transparent. The variation in color in a single grain can be as varied as the color variation between grains for an entire concentrate. The mineralogical composition of the cassiterite concentrates contains minerals that are typical gangue and accessory minerals in the primary mineralization and that were liberated during weathering. In addition, minerals can be found that result from the weathering from the metasedimentary and doleritic host-rocks of the primary mineralization. Except the occasional presence of a certain mineral or a special colour for a certain cassiterite, no systematic variation can be observed between the concentrates from the different locations. Often, the mineral and color variation in one concentrate can be as large as for concentrates found from different locations.

Stratigraphy of the Lower Palaeozoic of the Brabant Massif, Belgium.Part I: The Cambro-Ordovician from the Halle and Ottignies groups.

Wed, 06 Mar 2013 00:00:00 +0100

Multidisciplinary research in the last 25 years and recent geological mapping in the Brabant Massif, have completely changed our knowledge about one of the poorly known part of Belgian geology. The sedimentary succession is surprisingly complete by comparison with the literature before the 1970s, from the lower Cambrian to the top of the Silurian, and very thick (>13 km), thus highlighting the need to produce an up-to-date stratigraphic nomenclature. In this first paper about the Cambrian and the lowest Ordovician, we describe in detail the formations, which are classified into two new groups, how the description of the units evolved through time, their lithology, sedimentology, boundaries and contacts, thickness, fossil content and type sections or most typical outcrop areas. The lower Halle Group comprises the Blanmont, Tubize and Oisquercq formations that consist of sandstone, siltstone and pale coloured slate. The overlying Ottignies Group comprises the Jodoigne, Mousty and Chevlipont formations formed of a more argillaceous and notably darker lithology. The two groups constitute a very thick (> 9 km) lower Cambrian to lowermost Ordovician siliciclastic succession, mostly pelagic and turbiditic. The cumulative thickness curve is concave-upwards which shows an extensional rift basin. A comparison with the Condroz Inlier shows that only the uppermost part of the sedimentary pile was observed in boreholes (Chevlipont Formation).

Rare earth element and yttrium characteristics of carbonates within the sediment-hosted Luiswishi and Kamoto Cu-Co deposits, Katanga Copperbelt (Democratic Republic of Congo ‒ DRC).

Wed, 06 Mar 2013 00:00:00 +0100

The Neoproterozoic Central African Copperbelt is a world-class metallogenic province characterized by sediment-hosted stratiform Cu-Co ore deposits and polymetallic vein-type deposits. Two main mineralization phases have been recognized in the Katanga Copperbelt (Democratic Republic of Congo). The first phase occurred during early to intermediate diagenesis and the second during deep burial and the Lufilian orogeny. The Rare Earth Element and Yttrium (REY) concentrations of gangue dolomites associated with the second mineralization phase at the Kamoto and Luiswishi ore deposits were determined and compared to those of their respective host rocks. Kamoto samples exhibit convex Upper Continental Crust-normalized patterns with a pronounced light REE depletion without significant Eu anomalies and with positive La anomalies. The Luiswishi samples exhibit either progressively increasing REY patterns or ramp-shaped patterns with minor mid REE-heavy REE fractionation, accompanied by pronounced negative Ce and Eu anomalies. These differences suggest that a more intense metamorphic overprint at Luiswishi contributed to the differences with the Kamoto deposit.