Francis Tollon

A Paleoproterozoic ultramafic-mafic assemblage and associated volcanic rocks of the Boromo greenstone belt: fractionates originating from island-arc volcanic activity in the West African craton

Abstract The Loraboue Birimian ultramafic-mafic assemblage, located in the Boromo greenstone belt (Burkina Faso), is interpreted as the remains of a magma chamber that crystallized at the base of an island arc. The ultramafic rocks exhibit an heteradcumulate texture, being generally made up of wehrlites and more rarely dunites. The crystallization sequence inferred from the cumulates is olivine+chromite followed by clinopyroxene+amphibole±orthopyroxene±biotite. The gabbroic rocks are mainly layered and grade into more differentiated facies with sub-pegmatitic texture, containing up to 70% modal plagioclase including zircon and, more commonly, apatite crystals. Textural relationships and mineral phase chemistry are indicative of crystallization at elevated pressures (>8 kbar), the parental magma being generated by a moderate to high degree of partial melting of a mantle source affected by previous metasomatic events above a subducted oceanic slab. The Loraboue volcanic formations exhibit a range of geochemical features. They consist dominantly of calc-alkaline basalts, pyroclastites and rhyolite and, more rarely, of basalt, dolerite and gabbro of tholeiitic affinity. These different types of basalt, as well as the dolerite and the isolated massive gabbro, show the classic features of arc magmatic suites, namely LILE and Pb enrichment, depleted HFSE patterns and high Ce/Nb and Th/Nb ratios. Thus, the calc-alkaline plutonic and volcanic assemblages of Loraboue could represent the roots of an island arc and the associated coeval volcanic rocks. The Paleoproterozoic crust of the West African craton was heterogeneous and was not the consequence of a single process of genesis. As some modern igneous province, the Birimian crust was generated by both volcanic arc accretion and oceanic plateau accretion.

Multi-stage fluid incursion in the Palaeozoic basement-hosted Saint-Salvy ore deposit (NW Montagne Noire, southern France)

Abstract The Saint-Salvy vein-hosted Zn (+Ge) deposit occurs in an E–W fault system which flanks the southern margin of the late Variscan Sidobre granite, and cross-cuts Cambrian black shales of the Palaeozoic basement. Comprehensive mineralogical and geochemical studies of vein samples have revealed four mineralizing events (M1–M4) related to late and post-Variscan tectonic events. A further late-stage event may be related to weathering. M1 (=skarn deposits) and M2 (=patchily mineralized quartz veinlets) are associated with granite emplacement. Quartz contains low salinity, H2O CO2 (NaCl)-dominated fluids(⩽6wt% NaCl equiv.) of relatively high temperature (300–580°C), trapped under moderate to high pressure. Estimated M1 fluid δD and calculated fluidδ18O plot within the metamorphic water field. There appears to be no involvement of magmatic fluids. By contrast, M3 (= barren quartz) and M4 (= zinciferous economic mineralization) stages have H2O CO2 NaCl CaCl2 fluid inclusions with high salinities (23–25 wt% NaCl equiv.) and low temperatures(∼ 80–140°C), which were trapped under low-pressure conditions. The high salinity and NaCl + CaCl2 content of both M3 and M4 indicates that their parent fluids leached evaporitic salts. M3 fluids are meteoric water dominated, falling close to the meteoric water line (δD andδ18O averaging −64 and −8‰, respectively). M4 fluids have highly distinctive δD averaging −101‰, and calculated fluidδ18O varying from−1.2to+7.1‰. The unusually low δD composition of M4 suggests the involvement of “organic” fluids, in which H is derived directly or indirectly from organic matter. The relatively highδ18O of M4 fluids indicates that considerable isotopic exchange with sedimentary material took place, displacing theδ18O from the meteoric water line. The data imply interaction of meteoric waters with evaporite and hydrocarbon-bearing sedimentary sequences, most probably the adjacent Aquitain Basin. The main economic mineralization (M4 stage) took place during a tensional event, probably coincident with the Lias-Dogger transition. Calculatedδ34SH2S of M4 sulphide(+5.4to+8.2‰) is almost identical toδ34S of local Cambrian sulphides(+4.7to+9.4‰) suggesting a genetic link. Abundant siderite associated with M4 sphalerite hasδ13C ranging from−2.6to−4.4‰ indicating that carbon was sourced from sedimentary carbonate mobilized by, or equilibrated with the hydrothermal fluid. Late-stage sulphides exhibit extraordinary and highly distinctiveδ34S. Sphalerite has extremely low δ34S(−42.5to−50.5‰), whereas pyrite has an extraordinary large range from−33.2‰to+74.3‰. Closed system sulphate reduction is held to be responsible for the extremely highδ34S: whereas more open system reduction produces the very low values. The coincidence of isotopically lowδ13C(−7.6to−11.9‰) for co-genetic calcite suggests the involvement of organic matter in the reduction process.

Continental basinal origin of ore fluids from southwestern Massif central fluorite veins (Albigeois, France): evidence from fluid inclusion and stable isotope analyses

The most important fluorspar mining district in France is located in the Palaeozoic basement of the Albigeois in southwestern French Massif Central. The massive fluorite is hosted within large E–W striking fractures, crosscutting Cambro–Ordovician clastics, associated with large zones of hypersilicified tectonic breccia which form the wall of the mined deposits. Fluid inclusion data for pre-fluorite and fluorite stage fluids have salinities between 20–26 wt% NaCl equiv., with homogenisation temperatures between 85–170°C. Furthermore, low first ice melting temperatures (around −50°C) indicates the presence of significant CaCl2 and possibly MgCl2 together with NaCl. Calculated fluid δ18O for pre-fluorite quartz ranges from −9.1‰ to −5.2‰, with δD between −55‰ to −64‰, placing the data directly on the present day meteoric water line. Fluorite stage fluids have δ18O between +0.1‰ to +3.2‰, and δD ranging from −53‰ to −75‰, indicating an interacted meteoric fluid origin. Combining the fluid inclusion and stable isotope data illustrates that the main fluorite depositing fluid has characteristics typical of a basinal brine. The authors have no evidence that a magmatic system was involved in the deposit genesis. The proposed model highlights that mineralisation was related to major Mesozoic extensional events coinciding with the gradual opening of the Atlantic and Tethys oceans. In order to account for the chemistry of the fluids, and the siting of the deposits, the authors postulate a genetic relationship with local, continental, evaporite-bearing basins coincident with, and controlled by the E–W fractures.

Le filon (Zn, F) de Peyrebrune (SW Massif central, France): caractérisation géochimique des fluides au cours du Mésozoïque à la bordure orientale du bassin d'Aquitaine

Fluid inclusion and stable isotope data on the Palaeozoic basement-hosted Peyrebrune (Zn, F) Iodedeposit (Albigeois region), indicate that mineralization originated from circulation of basinal-derived brines during distinct successive Mesozoic extensional events. These new data, set in a regional context, suggest the occurrence of two distinct evaporite-bearing sedimentary basins, now concealed by the Tertiary cover: to the south, an organic matter-rich basin leading to economic Zn-sulfide deposition, and to the north an organic matter-free basin, leading to a small Cu-sulfide deposition. During the fluorite episode, the two basins differentiate by the development of the E-W fracturation: minor in the southern basin, major in the northern one.

The Economic Gold Deposits of Southeast Asia, the Caraibes and the Alpine-Himalayan Fold Belts

This report surveys Tethyan gold metallogeny by reviewing the principal deposits of the Caraibes, Alps—Himalaya, and southeastern Asia segments. The objective was not to draw up an exhaustive inventory of the gold deposits during the long-lived (≈250 million years) evolution of the Tethys; only those in which gold is, or has been, industrially extracted have been considered. In these cases, the tonnage is usually greater than 5 tons, inclusive of estimated reserves when concentration still is being mined. The resulting inventory is composed of about 150 deposits belonging to the four main Tethyan segments and represents (reserves + input) about 15% (e.g., ≈13,000 tons) of world production (Bache, 1982).