The latest Post-Variscan fluids in the Spanish Central System: evidence from fluid inclusion and stable isotope data

Abstract

The Spanish Central System has been subjected to repeated fluid incursions, which were responsible for a variety of mineralizing episodes including W-Sn, Cu-Zn-Pb-As-(Ag), F-Ba and barren quartz veins. These hydrothermal fluids occurred over a 200 Ma time period and the latest hydrothermal event is recorded in barren quartz veins. This study is a multidisciplinary approach leading to the characterization of the hydrothermal fluids preserved in barren quartz veins, which are spatially but not temporally related to Hercynian upper crustal granites. The veins were dated by the 39 Ar/4o Ar method, and the fluids were examined using petrographic, microthermometric, chemical and isotopic methods. Fluid inclusions in barren quartz veins indicate that two fluids were related to this hydrothermal event. The main part of the quartz veins were formed from an early low salinity « 1 wt% N aCI) H20-NaCl fluid. This fluid was trapped at around 270 ± 25 QC and 0.1-1 kbar under sublithostatic to hydrostatic conditions. a180 (-9 to 2%0) and aD (-70 to - 34.5%0) values indicate a meteoric origin for water, with significant water/rock interactions. The latest H20-NaCI-CaCI2 fluid is found in two types of fluid inclusions: a primary liquid-vapour type (16-24 wt% NaCl and 1-12 wt% CaClz) and secondary hypersaline type (7-15 wt% NaCI and 21-27 wt% CaCI2). Significant Li concentrations in this fluid were confirmed. This late Ca-bearing fluid formed quartz crystals in the central part of the veins, and was trapped at 70- 140 QC, at a maximum pressure of 0.5 kbar. The low al80 (-20 to -6%0) and aD (-137 to -116%0) values suggest a meteoric origin for this fluid, however its high salinity probably requires a source from Triassic evaporite basins located in the NE tip of the Spanish Central System. Anomalously low isotopic values have been previously reported from kaolinites of Lower Cretaceous age. Anomalous climatic conditions during the Cretaceous appear to be the main reason to explain this very negative meteoric water. Strong isotopic depletion in meteoric water has been observed in modern areas with monsoonal climates. The hydrothermal evolution of barren quartz veins in the Spanish Central System is comparable to other hydrothermal Post-Variscan events in central and south-western Europe related to the opening of the NorthAtlantic during Cretaceous time.