oa Hydrothermal Dolomites along Faults/Fractures in the Aptian-Albian Platform Carbonates (Karrantza Valley, N.W Spain): Petrographic and Geochemical Results

Aptian-Albian carbonates were deposited in the Basque-Cantabrian Basin (NW Spain) as a result of an<br>intense rift-related subsidence. This rock succession is affected by N-S, E-W, NW-SE and NE-SW<br>oriented normal and strike-slip faults, which acted as fluid flow conduits to selectively dolomitize the<br>Aptian-Albian carbonates in the study area (Karrantza valley, NW Spain).<br>Field studies exhibit excellent beige coloured dolomite exposures along the faults surrounded by light<br>grey coloured host limestone. Linear fault restricted dolomite and irregular dolomite bodies, indicate<br>that lithology mechanically controls the geometry of the dolomites.<br>Different dolomite facies exhibit systematic variation away from the fault/fracture. Paragenesis of these<br>dolomites indicate their development over different dolomitising events. Several phases of calcite<br>cement pre- and post-date the dolomitisation events. Pyrite (2-phases), galena and sphalerite are also<br>locally observed. Zebra dolomite shows the most depleted values with δ18O varying between -18.6 to<br>-15.2 ‰V-PDB and δ13C between -0.7 to +1.6‰ V-PDB compared to non-planar and planar dolomite<br>which show overlapping stable isotopic ratios ranging from -18.7 to -11.5‰ δ18O V-PDB and -2.6 to<br>+1.9‰ δ13C V-PDB. The broad range and highly depleted values of δ18O are interpreted to reflect<br>multiphase dolomitization and dolomite recrystallization. Limestones close to the dolomites show<br>depleted δ18O values (similar to those of the dolomites), implying isotopic resetting during<br>dolomitization and large scale fluid migration. Microprobe analysis indicate Fe-rich central part of<br>dolomite (1.6 to 1.9 wt.%) as compared to their external zones and the dolomite cement (Fe: 0.0 to<br>0.6 wt.%). The Mg content in the void-filling, non-luminescent calcite is high (0.9 to 1.3 wt.%) as<br>compared to luminescent, replacive calcite (0.1 to 0.8 wt.%). Fluid inclusion analyses show Th values<br>range from 120 to 200°C and estimated salinities between 10 and 24 eq. wt. % NaCl for zebra and<br>nonplanar dolomites, which confirms very hot dolomitizing fluids (hydrothermal origin)of the investigated dolomites.<br>Above stated studies helped in defining the geometry of dolomite bodies (reservoir heterogenity<br>characterization), constraining the genesis of the dolomites and superposition of diagenetic events,<br>documenting the characteristics of hot, saline dolomitizing fluids and related fluid flow history.