Kris Piessens

Fluid flow, alteration and polysulphide mineralisation associated with a low-angle reverse shear zone in the Lower Palaeozoic of the Anglo-Brabant fold belt, Belgium

Abstract In the Lower Palaeozoic rocks of the Brabant Massif (Belgium), a recently discovered polysulphide mineralisation is related to a low-angle reverse shear zone. This shear zone has been attributed to the main early Devonian deformation event. Data from boreholes and outcrops allow a detailed investigation of the alteration pattern and palaeofluid flow along this shear zone. Macroscopic observations of the mineralogy and quantitative changes in the phyllosilicate mineralogy indicate that this shear zone is characterised by an envelope of intense sericitisation and silicification. In addition, chloritisation is associated with this alteration. The alteration zone may reach a thickness of 250 m. Ore mineralisation occurred synkinematically and is spatially related to the shear zone. The mineralisation consists of pyrite, marcasite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, stibnite and smaller amounts of tetrahedrite and other sulphosalts. It is concentrated in quartz–sulphide veins or occurs diffusely in the host rock. The mineralising fluids have a low-salinity H 2 O–CO 2 –CH 4 –NaCl–(KCl) composition and a minimum temperature of 250–320 °C. The δ 18 O values of quartz vary between +12.3‰ and +14.5‰ SMOW, and δ D compositions of the fluid inclusions in the quartz crystals range from −65‰ to −35‰ V-SMOW. The δ D and the calculated δ 18 O values of the mineralising fluids fall in the range typical for metamorphic fluids and partly overlap with that for primary magmatic fluids. The δ 34 S values, between +4.7‰ and +10.6‰ CDT, fall outside the interval typical for I-type magmas. Important migration of likely metamorphic fluids, causing a widespread alteration and a polysulphide mineralisation along a low-angle shear zone, has, thus, been identified for the first time in the Caledonian Anglo-Brabant fold belt.

Genesis of the vein-type tungsten mineralization at Nyakabingo (Rwanda) in the Karagwe?Ankole belt, Central Africa

The vein-type tungsten deposit at Nyakabingo in the central Tungsten belt of Rwanda is located in the eastern flank of the complex Bumbogo anticlinal structure. The host rock is composed of alternating sequences of sandstones, quartzites, and black pyritiferous metapelitic rocks. Two types of W-mineralized quartz veins have been observed: bedding-parallel and quartz veins that are at high angle to the bedding, which are termed crosscutting veins. Both vein types have been interpreted to have been formed in a late stage of a compressional deformation event. Both vein types are associated with small alteration zones, comprising silicification, tourmalinization, and muscovitization. Dating of muscovite crystals at the border of the veins resulted in a maximum age of 992.4u2009±u20091.5xa0Ma. This age is within error similar to the ages obtained for the specialized G4 granites (i.e., 986u2009±u200910xa0Ma). The W-bearing minerals formed during two different phases. The first phase is characterized by scheelite and massive wolframite, while the second phase is formed by ferberite pseudomorphs after scheelite. These minerals occur late in the evolution of the massive quartz veins, sometimes even in fractures that crosscut the veins. The ore minerals precipitated from a H2O–CO2–CH4–N2–NaCl–(KCl) fluid with low to moderate salinity (0.6–13.8xa0eq. wt% NaCl), and minimal trapping temperatures between 247 and 344xa0°C. The quartz veins have been crosscut by sulfide-rich veins. Based on the similar setting, mineralogy, stable isotope, and fluid composition, it is considered that both types of W-mineralized quartz veins formed during the same mineralizing event. Given the overlap in age between the G4 granites and the mineralized quartz veins, and the typical association of the W deposits in Rwanda, but also worldwide, with granite intrusions, W originated from the geochemically specialized G4 granites. Intense water–rock interaction and mixing with metamorphic fluids largely overprinted the original magmatic-hydrothermal signature.

Alteration and fluid characteristics of a mineralised shear zone in the Lower Palaeozoic of the Anglo-Brabant belt, Belgium

Abstract In the Lower Palaeozoic rocks of the Brabant Massif (Belgium), a recently discovered polysulphide mineralisation is intimately related to a high strain zone. Data from drillings, completed with outcrop data allow a detailed investigation of mineralisation, alteration and fluid characteristics of this high strain zone, currently interpreted as a low-angle reverse shear zone and attributed to the main Early to early Middle Devonian Acadian deformation event. Ore mineralisation occurred synkinematically and was closely associated with the shear zone. Low saline H2O–CO2(–CH4)–NaCl fluids with temperatures >260°C were involved in the hydrothermal circulation, which caused alteration of the host rock and extensive sericitisation in the shear zone. Isotope data and the general setting indicate a metamorphic-driven system.