Author:

Keywords:

fluid flow, Brittany, France, quartz vein, oxygen isotopes, fluid inclusions, chlorite, Ar-Ar dating, salinity, P-T conditions

Abstract:

Mineral deposits are of great importance for our society. They are the source of many metals (e.g. iron, nickel, cobalt, copper) and industrial minerals (e.g. apatite, barite, kaolin), used in daily life. Research into the formation and the occurrence of mineral deposits provides a better understanding and therefore leads to a more sustainable exploitation of these natural resources. A mineral deposit is often formed by the precipitation from a fluid (i.e. water and dissolved salts and minerals) in a crack or fracture (vein) or in the pores of a rock. This mineral deposit is the final and usually the only vestige of the responsible fluid system. A fluid system consists of a discharge zone, commonly materialised by the mineral deposit, a fluid source (e.g. rainwater infiltration or formation water) and a migration path, linking source and sink (e.g. a fault or pore network). By determining the properties of the mineral deposit (e.g. composition, size and geometry), the fluid system can be reconstructed. This fluid system reconstruction can be performed on economic valuable mineral deposits, but besides, numerous mineral deposits can be studied, which are of less value (e.g. quartz). The aim of this study includes the reconstruction of fluid systems in Central Armorica, exposed in western Brittany, mainly based on mineral deposits of low economic value. This includes both the characterisation of the fluid systems (fluid composition, spatial organisation, water-rock interaction, etc.), but also studying the interactions between fluid systems and how they evolved in space and time. During this study various mineral deposits are examined in the western part of the Armorican Massif (Brittany, France), part of the European Variscides. This eroded mountain range consists of metasedimentary and igneous rocks of Neoproterozoic to Carboniferous age, which deformed during the Late Devonian to Late Carboniferous Variscan orogeny by wrench tectonics. The reconstruction of the fluid systems in this area is based on two research topics. Firstly, the characterisation and regional organisation of mineral deposits in siliciclastic metasediments of the Plougastel Formation and secondly, the study of possible fluid systems related to the regional North Armorican shear zone (NASZ). Six areas were selected to study both topics. The Monts d'Arrée, the Saint-Rivoal, Elorn and the Queffleuth areas expose the Plougastel Formation. The Queffleuth area is situated along the NASZ, as well as the Douron and Guic area. The techniques used to study the mineral deposits and their host rocks are lithological and structural field observations, petrography with incident light and hot cathode luminescence microscopy, microthermometry and Raman microspectrometry of fluid inclusions, stable isotope analysis of quartz, geochemical analysis of chlorite and dating of sericite. The starting point of the research is situated in the Monts d'Arrée, where (van Noorden, 2007) investigated the mineral deposits in the Plougastel Formation. This study showed that different veintypes occur in the metapelites and quartzites of the Plougastel Formation. The geometry and intersecting relationship with other structural elements (bedding, cleavage, shear zones), indicate that these veins formed during the burial and entire deformation history. These vein types are subsequently identified during fieldwork in the Saint-Rivoal, Elorn and Queffleuth research areas. The mineralogical composition of all vein types in the Plougastel Formation consists of quartz and a small amount of chlorite and muscovite. A microstructural study of thin sections of the veins indicates that the quartz underwent bulging recrystallisation. This deformation mechanism is not found in the youngest quartz vein type (CN type), suggesting that they formed at retrograde conditions, during or after the last phase of the Variscan orogeny. A microthermometry and Raman microspectrometry study on the fluid inclusions shows that the fluid from which these minerals precipitated, consisted of water with salts (NaCl) and small amounts of CO2, CH4 and N2 (van Noorden, 2007; Berwouts et al., 2008. The salt content is rather low and the highest salinity is observed in veins in which fluorapatite occurs. Research on the stable oxygen isotope ratios of veins and rocks in the Monts d'Arrée and Saint-Rivoal area indicates that the veins are buffered by their surrounding rock.The stable isotope ratio of the quartz veins remains alike over the different types, suggesting a similar fluid source of the vein types over time. The temperature during the formation of chlorite varies between 320 and 420°C. The Ar-Ar dating of sericite in the wall of a vein type that is sampled both in the Monts d'Arrée and in the Queffleuth areas, gives an age of 315.5 +/- 0.5 Ma. This date probably indicates the moment at which the closure temperature of sericite (350°C) is reached. Since veins in the Plougastel Formation formed regionally, their geochemistry is relatively stable and the fluids are buffered by the host rock, one can assume that they all belong to one fluid system. This system will be called the 'Plougastel fluid system'. The source of the fluids is the formation water in the pores of the metasediments, which is buffered by the host rock. The redistribution of these fluids occurred mainly along pore networks. The final precipitation takes place in fractures with limited dimensions (less than several meters long). The formation of this fracture network developed during burial and deformation of the Plougastel Formation, resulting in a variety of geometries between the different vein types.The second research topic in this study is the influence of the NASZ on fluid systems. Such vertical fault system, which is typical of wrench tectonic settings, is known for its control on fluid flow between different structural levels (Oliver, 2001; Lawrence & Cornford, 1995; Carter et al., 1990; Sibson, 2001). The NASZ is a late Palaeozoic fault system that is exposed in the Armorican Massif at various structural levels (middle and upper crust) and that plays an important role in the global geodynamics of Central Armorica (cf. Gumiaux et al., 2004). The geometry and deformation of lens-shaped veins (LFP type) found in foliated rocks around the NASZ, suggest that these veins were deformed by the NASZ. This vein type consists of quartz and a small amount of muscovite and chlorite. The quartz in the veins are deformed by subgrain rotation recrystallisation (SGR), indicating temperatures above 380°C during deformation. Besides this vein type, which was formed before or during the activity of the NASZ, no other mineral deposits are found in the research areas along the NASZ, suggesting the absence of large-scale fluid migration during the fault activity. This suggests that the NASZ rather acted as a fluid barrier, instead of a migration path. The Guic research area is besides the LFP vein type, also characterised by contact metamorphism around the Plouaret granite (andalusite and garnet mineral assemblages) and the occurrence of less deformed vein types (FPGuic and CCGuic) in strongly foliated rocks. The latter two vein types developed after the formation of the NASZ. One of these vein types (FPGuic) contains fluid inclusions with an H2O-NaCl-KCl composition, formed at a temperature between 210 and 300°C. Additionally, some quartz crystals in this vein type contain static recrystallisation, indicating high temperatures. The sequence of quartz generations indicates that after the NASZ activity, metamorphic fluids migrated through the area, comparable to those in the Plougastel fluid system, followed by hot fluids, possibly related to a magmatic fluid source. The migration paths of the different fluids are a joint system, that formed parallel to (FPGuic) and crosscutting the main foliation (CCGuic). The FPGuic and CCGuic vein types in the Guic research area belong to the 'Plouaret-Quintin fluid system', named after the granite intrusions in this area. The LFP vein type is placed in the Plougastel fluid system, based on the occurrence in a lateral equivalent of the Plougastel Formation and the similar compositional range of the fluids with fluids from the Plougastel fluid system.In all research areas a vein type is observed that crosscuts several structural elements, among which the NASZ. This implies that this vein type is younger than the NASZ. These veins have a strike, ranging from N30W to N30E and are probably vertical. They are named 'NS veins' in this study. The filling of these veins varies over time from blocky quartz with recrystallised crystals over broken quartz, crosscut by small veinlets, to large prismatic purple and clear quartz crystals. The observed composition of the fluids from which these veins precipitated, changes with the type of quartz filling. In the deformed quartz, fluid inclusions contain a H2O-NaCl+/-CO2-CH4 fluid, the broken quartz contains H2O-NaCl fluids with a very low salinity and the prismatic quartz has fluid inclusions with both a H2O-NaCl composition and a low salinity (lessthan 3% mass fraction) and a H2O-NaCl-CaCl2 composition with high salinity (up to 25% mass fraction). A stable isotope study shows that the fluid that formed the veins, is not in equilibrium with the host rock, i.e. an open fluid system. The temperatures at which fluids were trapped in the inclusions, varies between 340 and 390°C for the deformed crystals, between 170 and 260°C for the broken quartz and between 70 and 230°C for the prismatic quartz. The decreasing temperature of the quartz fillings indicates a cooling during the formation of the large NS veins. The typical characteristics of the NS veins, suggests that one can speak of a 'NS fluid system'. This system has a regional occurrence and includes the youngest mineral deposits of quartz and kaolin in the area. The source of fluids in this system is likely to vary over time and changes from metamorphic fluids towards possibly meteoric and hydrothermal fluids in contact with Ca-rich minerals (e.g. plagioclase, pyroxene and amphibole). The migration paths of these fluids is a fracture network that is found all across the Armorican Massif and which could be related to the break up of the supercontinent Pangaea (Vigneresse, 1988).This study demonstrates that fluid systems are active over a long period and that there is little interaction between the different fluid systems. The trigger for the fluid redistribution is usually deformation, metamorphism and/or magmatism. In each particular context, and on any scale (local and regional) in Central Armorica, a fluid system is active, but they not always form economically exploitable deposits.