The chemistry of brines from an Alpine thrust system in the Central Pyrenees: An application of fluid inclusion analysis to the study of fluid behaviour in orogenesis

Quartz filled veins and fractures which formed late in the Alpine thrusting of the Central Pyrenees contain inclusions of hypersaline Na-Ca-Cl brines with total dissolved salts of up to 25 wt%. The total salinity is similar in all samples, irrespective of the vein or the wall rocks, but there are large variations (particularly in the Na Ca ratio) in the chemistry of the fluids between samples. With one exception, each sample contains only a single dominant fluid population. Crush-leach extraction and chemical analysis of the inclusion electrolytes for Na, K, Ca, Mg, Ba, B, Li, Sr, Rb, Fe, Mn, Zn, Pb, F, Cl, Br, and Sr and Pb isotopes reveals that the fluid chemistry is strongly influenced by the local rocks. Of the four different lithologies in the thrust stack sampled, the Triassic mudstones and Cretaceous limestones or Silurian phyllites acted as sources for the vein fluids during the late thrusting. The composition of the fluid in the veins was dependent on the proximity to these lithologies. For example, fluids from the Trias were dolomite saturated, whilst those close to limestone were calcite saturated. Strontium and lead isotopic analysis of inclusions and host rocks confirm that the more Narich fluids were in equilibrium with Triassic redbeds while Ca-rich fluids have been in isotopic equilibrium with either Cretaceous limestones or Silurian phyllites. Some samples have intermediate compositions due to mixing of the two endmember fluids prior to trapping as inclusions. The similarity of the Br Cl ratio (approximately twice seawater) and the consistent high salinity of all the inclusion fluids in the thrust stack indicate that they were all originally derived from a single source but progressively changed their cation and isotope chemistry through interaction with different host rocks. This ultimate source is likely to have been Triassic connate waters. We conclude that a local increase in permeability occurred when the veins formed and that fluid movement was over short distances. No evidence was found for a significant input of either surface or metamorphic fluids during thrusting.