Stijn Dewaele

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.

Interplay between sulphur sources in polysulphidemineralisation in the Lower Palaeozoic of the Anglo-Brabant Fold belt, Belgium

Abstract Polysulphide mineralisation in the Lower Palaeozoic of the Anglo-Brabant Fold belt is the subject of a detailed sulphur isotopic investigation to constrain possible sulphur sources. A distinction can be made between sedimentary pyrite, with low δ34S values and identified as pyrite with a bacteriogenic origin, and pyrite with high δ34S, formed during hydrothermal circulation along strain zones. Both kinds of pyrites are present in the important orogenic mesozonal polysulphide mineralisation in the Marcq area. The hydrothermal sulphur is presumed to be H2S released through thermal decomposition of kerogen present in Lower Palaeozoic strata. Pyrite with intermediate δ34S values occurs in less important strain zones. These values are explained by mixing of hydrothermal sulphur and sulphur with a sedimentary origin.