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Nader F.H.,French Institute of Petroleum | Lopez-Horgue M.A.,University of the Basque Country | Shah M.M.,French Institute of Petroleum | Shah M.M.,Geodynamics and Geofluids Research Group | And 7 more authors.
Oil and Gas Science and Technology | Year: 2012

Field characteristics, petrographic and geochemical signatures, as well as some petrophysical aspects of fault-related dolomite bodies in the Ranero area (Karrantza Valley, NW Spain) are presented in this paper. These dolomite bodies are hosted by Albian slope to platform carbonates, which were deposited in the Basque-Cantabrian Basin. Replacive and void-filling dolomite phases - postdating palaeo- and hypogene karstification - are interpreted to have originated from hydrothermal fluid pulses, and are spatially related with faults and fractures. Hydrothermal calcite cements pre- and postdate dolomitization. Mineralogical and geochemical investigations (XRD, ICP-MS/OES, XRF, stable and Sr isotopes) helped in distinguishing various dolomite and calcite phases. Dolomite phases can be grouped into ferroan (early) and non-ferroan (late). Dolomites are generally stoichiometric and exhibit a broad range of depleted δ 18O values (-18.7 to -10.5‰ V-PDB), which advocate for multiphase dolomitization and/or recrystallization at relatively high temperatures (150-200°C). The observation that bed-parallel stylolites pre- and post-date dolomites suggests that dolomitization occurred during the Late Albian regional tectonic activity and related fluid expulsions. Based on carbonate chemistry, authigenic silicate chemistry and replacement relationships, two contrasting types of dolomitizing fluids are inferred. Both arguably may have initiated as sulphatedominated brines and/or basin compactional fluids, but they seemingly undergo sulphate reduction in contact with host rocks of contrasting compositions (Fe-rich silicate vs Fe-poor carbonate) thus evolving either to acidic and ferroan (limestone replacive) or to neutral, Fe-poor and sulfidic (Fe-dolomite replacive). Fluid drives are not well constrained by our data, but both fluid types are focused along major faults that cross cut the platform edge and are associated with diapir tectonics. © 2012, IFP Energies nouvelles.

De Boever E.,Geodynamics and Geofluids Research Group | De Boever E.,French Institute of Petroleum | Muchez P.,Geodynamics and Geofluids Research Group | Swennen R.,Geodynamics and Geofluids Research Group | Dimitrov L.,Institute of Oceanology IO BAS
Geofluids | Year: 2011

Faults are often important fuelling methane seep systems; however, little is known on how different components fault zones control subsurface fluid circulation paths and how they evolve through time. This study provides insight into fault-related fluid flow systems that operated the shallow subsurface of an ancient methane seep system. The Pobiti Kamani area (NE Bulgaria) encloses a well-exposed, fault-related seep system unconsolidated Lower Eocene sandy deposits of the Dikilitash Formation. The Beloslav quarry and Beloslav N faults displace the Dikilitash Formation and are typified by broad, up to 80m wide, preferentially lithified hanging wall damage zones, crosscut by deformation bands and deformation band zones, smaller slip planes and fault-related joints. The formation of a shallow plumbing system and chimney-like concretions the Dikilitash Formation was followed by at least two phases of fault-related methane fluid migration. Widespread fluid circulation through the Dikilitash sands caused massive cementation of the entire damage zones the fault hanging walls. During this phase, paths of ascending methane fluids were locally obstructed by decimetre-thick, continuous deformation band zones that developed the partly lithified sands upon the onset of deformation. Once the entire damage zone was pervasively cemented, deformation proceeded through the formation of slip planes and joints. This created a new network of more localized conduits close vicinity to the mafault plane and around through-going slip planes. 13C-depleted crustiform calcite cements several joints record the last phase of focused methane fluid ascent. Their formation predated Neogene uplift and later meteoric water infiltration along the joint network. This illustrates how fault-related fluid pathways evolved, over time, from 'plumes' unconsolidated sediments above damage zones, leading to chimney fields, over widespread fluid paths, deflected by early deformation structures, to localized paths along fracture networks near the mafault. © 2011 Blackwell Publishing Ltd.

El Desouky H.A.,Geodynamics and Geofluids Research Group | Muchez P.,Geodynamics and Geofluids Research Group | Boyce A.J.,Isotope Geoscience Unit | Schneider J.,Geodynamics and Geofluids Research Group | And 3 more authors.
Mineralium Deposita | Year: 2010

The sediment-hosted stratiform Cu-Co mineralization of the Luiswishi and Kamoto deposits in the Katangan Copperbelt is hosted by the Neoproterozoic Mines Subgroup. Two main hypogene Cu-Co sulfide mineralization stages and associated gangue minerals (dolomite and quartz) are distinguished. The first is an early diagenetic, typical stratiform mineralization with fine-grained minerals, whereas the second is a multistage syn-orogenic stratiform to stratabound mineralization with coarse-grained minerals. For both stages, the main hypogene Cu-Co sulfide minerals are chalcopyrite, bornite, carrollite, and chalcocite. These minerals are in many places replaced by supergene sulfides (e. g., digenite and covellite), especially near the surface, and are completely oxidized in the weathered superficial zone and in surface outcrops, with malachite, heterogenite, chrysocolla, and azurite as the main oxidation products. The hypogene sulfides of the first Cu-Co stage display δ34S values (-10. 3‰ to +3. 1‰ Vienna Canyon Diablo Troilite (V-CDT)), which partly overlap with the δ34S signature of framboidal pyrites (-28. 7‰ to 4. 2‰ V-CDT) and have {increment}34SSO4-Sulfides in the range of 14. 4‰ to 27. 8‰. This fractionation is consistent with bacterial sulfate reduction (BSR). The hypogene sulfides of the second Cu-Co stage display δ34S signatures that are either similar (-13. 1‰ to +5. 2‰ V-CDT) to the δ34S values of the sulfides of the first Cu-Co stage or comparable (+18. 6‰ to +21. 0‰ V-CDT) to the δ34S of Neoproterozoic seawater. This indicates that the sulfides of the second stage obtained their sulfur by both remobilization from early diagenetic sulfides and from thermochemical sulfate reduction (TSR). The carbon (-9.9‰ to -1.4‰ Vienna Pee Dee Belemnite (V-PDB)) and oxygen (-14.3‰ to -7.7‰ V-PDB) isotope signatures of dolomites associated with the first Cu-Co stage are in agreement with the interpretation that these dolomites are by-products of BSR. The carbon (-8.6‰ to +0.3‰ V-PDB) and oxygen (-24.0‰ to -10.3‰ V-PDB) isotope signatures of dolomites associated with the second Cu-Co stage are mostly similar to the δ13C (-7.1‰ to +1.3‰ V-PDB) and δ18O (-14.5‰ to -7.2‰ V-PDB) of the host rock and of the dolomites of the first Cu-Co stage. This indicates that the dolomites of the second Cu-Co stage precipitated from a high-temperature, host rock-buffered fluid, possibly under the influence of TSR. The dolomites associated with the first Cu-Co stage are characterized by significantly radiogenic Sr isotope signatures (0.70987 to 0.73576) that show a good correspondence with the Sr isotope signatures of the granitic basement rocks at an age of ca. 816 Ma. This indicates that the mineralizing fluid of the first Cu-Co stage has most likely leached radiogenic Sr and Cu-Co metals by interaction with the underlying basement rocks and/or with arenitic sedimentary rocks derived from such a basement. In contrast, the Sr isotope signatures (0.70883 to 0.71215) of the dolomites associated with the second stage show a good correspondence with the 87Sr/86Sr ratios (0.70723 to 0.70927) of poorly mineralized/barren host rocks at ca. 590 Ma. This indicates that the fluid of the second Cu-Co stage was likely a remobilizing fluid that significantly interacted with the country rocks and possibly did not mobilize additional metals from the basement rocks. © 2010 Springer-Verlag.

Kipata M.L.,Geodynamics and Geofluids Research Group | Kipata M.L.,Royal Museum for Central Africa | Delvaux D.,Royal Museum for Central Africa | Delvaux D.,University of Witwatersrand | And 3 more authors.
Geologica Belgica | Year: 2013

Since the first and paroxysmal deformation stages of the Lufilian orogeny at ~ 550 Ma and the late Neogene to Quaternary development of the south-western branch of the East African rift system, the tectonic evolution of the Lufilian arc and Kundelungu foreland in the Katanga region of the Democratic Republic of Congo remains poorly known although it caused important Cu-dominated mineral remobilizations leading to world-class ore deposits. This long period is essentially characterized by brittle tectonic deformations that have been investigated by field studies in open mines spread over the entire arc and foreland. Paleostress tensors were computed for a database of 1889 fault-slip data by interactive stress tensor inversion and data subset separation. They have been assembled and correlated into 8 major brittle events, their relative succession established primarily from field-based criteria and interpreted in function of the regional tectonic context. The first brittle structures observed were formed during the Lufilian compressional climax, after the transition from ductile to brittle deformation (stage 1). They have been re-oriented during the orogenic bending that led to the arcuate shape of the belt (stage 2). Unfolding the stress directions allows to reconstruct a well-defined N-S to NNE-SSW direction of compression, consistent with the stress directions recorded outside the belt. Constrictional deformation occurred in the central part of the arc, probably during orogenic bending. After the bending, the Lufilian arc was affected by a NE-SW transpression of regional significance (stage 3), inducing strike-slip reactivations dominantly sinistral in the Lufilian arc and dextral in the Kundelungu foreland. The next two stages were recorded only in the Lufilian arc. Late-orogenic extension was induced by σ1-σ3 stress axis permutation in a more trans-tensional regime (stages 4). Arc-parallel extension (stage 5) marks the final extensional collapse of the Lufilian orogeny. In early Mesozoic, NW-SE transpressional inversion felt regionally (stage 6) was induced by far-field stresses generated at the southern active margin of Gondwana. Since then, this region was affected by rift-related extension, successively in a NE-SW direction (stage 7, Tanganyika trend) and NW-SE direction (stage 8, Moero-Upemba trend).

Haest M.,Geodynamics and Geofluids Research Group | Haest M.,CSIRO | Muchez P.,Geodynamics and Geofluids Research Group
Geologica Belgica | Year: 2011

Stratiform deposits in the Pan-African Orogen are of the Cu-Co type and are restricted to carbonates and siliciclastic sediments that are stratigraphically close above the basement. The stratiform Cu-Co deposits formed during early diagenesis (possibly around 820 Ma) and during late diagenesis/metamorphism and the Pan-African Orogeny (~580 to ~520 Ma). The early diagenetic Cu-Co sulphides were partly remobilised into the second stratabound Cu-Co mineralisation, with precipitation of Cu-Co sulphides in nodules, veins and as breccia cements. Vein-type Cu-Pb-Zn mineralisation occurs at two distinct levels, higher in the stratigraphy. The lower level vein-type deposits occur in dolomite and are dominantly of the Zn-Cu type. The higher level vein-type deposits occur at the contact between dolomite and sandstone and are dominated by Cu. The Cu-dominated deposits that have been dated, developed during the waning stage of the Pan-African Orogeny (~530 to ~500 Ma). The Zn-dominated deposits for which a mineralisation age has been established, formed after the Pan-African Orogeny. Some of these vein-type mineralization have been remobilised after their formation, with the precipitation of massive Cu(-Ag) sulphides.

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