Mineral, fluid and thermal evolution in veins from late orogenic coal basins of the Cantabrian Zone (Variscan, NW Spain)

The detailed study of syn-tectonic mineral veins and textural features of fluid inclusions is a useful approach for reconstructing the fluid and thermal evolution in diagenetic and very low-grade metamorphism environments. Here, mineralised fissures within the clastic sedimentary rocks of the Cinera-Matallana and Sabero Stephanian pull-apart coal basins (Cantabrian Zone, Variscan of NW Spain) are investigated. These bedding perpendicular fissures (cm-wide) formed in sandstones synchronous with folding and faulting during bedding-parallel slip. Three stages of infilling and mineral growth are recognised within the veins, reflecting different fracture-opening events under different fluid, thermal and tectonic regimes at hydrostatic pressure conditions: Stage 1 was related to the temperature rise during burial before thermal peak conditions were attained (during Permian times). Euhedral quartz crystallised from a heterogeneously trapped, immiscible H2O-CH4 -CO2 (± NaCl) fluid mixture. P-T conditions evolved from 73 °- 110 °C and 15-56 MPa to 87 °-129 °C and 10-27 MPa. At a late stage of quartz growth saddle dolomite precipitated. Materials for mineral precipitation were derived locally from the detrital fraction within the Stephanian rocks,and transported into solution by advection of large volumes of gas (CH4 -CO2) and water formed in response to progressive maturation of the coal. Stage 2 also occurred under burial conditions, with lower temperature diagenetic dissolution-precipitation reactions controlling mineral growth. Brittle reactivation of veins caused microcracking of the large quartz crystals and extensive dedolomitisation. Ferroan blocky calcite precipitated at 60 ° to 100 °C from a H2O-NaCl-CaCl2 -CH4 fluid with total salinities from 20.5 to 31.9 eq.wt%, also preserved as secondary inclusions in the vein quartz. This fracture re-opening and fluid flow event was related to the infiltration of marine water during Cretaceous tectonic subsidence, subsequently modified and expelled during late tectonic reactivation (Alpine Orogeny). Stage 3 was linked with uplift and exhumation of the basin fill. Late calcite and authigenic kaolinite formed in response to low-temperature diagenetic and weathering reactions of the Stephanian rocks. Mineral precipitation was controlled by a constant supply of meteoric water (T <50 °C), that circulated preferentially along fault structures reaching the deeper parts of the stratigraphic section. Shortly after the Stage 1 of vein evolution,diorite sills were emplaced into the basinal sequences and small veins containing quartz and kaolinite formed within some of these intrusives.The hydrothermal quartz also crystallised from an immiscible H2O-CH4 -CO2 (± NaCl) fluid at 164 °-296 °C and 3-48 MPa,and is considered to be the product of extensive mineral alteration as the cooling magma intruded along coal seams. A variety of thermally re-equilibrated fluid inclusions are reported in fissure quartz within the clastic rocks. Five textural types of re-equilibration were recognised in progressive order of inclusion modification: scalloped, hairy, annular-ring shaped, haloes and decrepitation clusters. These textures resulted from a combination of brittle fracturing and dissolution and re-precipitation of quartz, with preferential loss of water. Decrepitation of fluid inclusions in vein quartz throughout the entire basin reflects the wide extent of the thermal peak. Short-lived, high temperatures are related to the intrusion and alteration of the diorites in the basin, which further enhanced degassing and dehydration of the coal. The generated fluid probably circulated by advection through the already formed fissure network. Keywords: fluid-rock interaction, coal basins, veins, quartz, fluid inclusions, re-equilibration textures, thermal peak and heat transmission.