Geology and geochemistry of Linares-La Carolina Pb-ore field (southeastern border of the Hesperian Massif)
Base-metal late Hercynian vein and stratabound-type deposits occur in the Linares-La Carolina mining district (Sierra Morena, Spain). No other ore deposits than iron beds and a supergene iron-manganese karst-related deposit were found in the studied area. However, Upper Ordovician-Lower Silurian rocks show Ni, Zn, Pb content above the average values of sandstones. Upper Ordovician-Lower Silurian country rocks are grouped into two stratigraphical sequences that reflect sea-level rise-fall cycles in a basin developed in a passive margin. The duration of these cycles indicates that they were caused by a combination of eustatic processes and intraplate extensional tectonics. The Late Hercynian hydrothermal deposits hosted by metasediments and granitoids, comprise (Ba)-(Pb-Zn-Cu- [Ag]) vein and stratabound type deposits that were emplaced before the Ladinian, during an extensional stage starting in the Upper Permian. Three stages of ore deposition have been recognised, the first stage is characterised by the formation of pyrrhotite, arsenopyrite, Ag sulphosalts and minor arsenopyrite and Bi sulphides. In the second stage, pyrite, Cu sulphides, barite and Ni-Co mineral s precipitated. Main deposition of galena and sphalerite took place at the end of the first stage and the beginning of the second stage. There is no evidence of selective Ag enrichment. Hydrothermal alteration of granitic host rocks is characterised by argillic-phyllic, argillic-propylitic assemblages overprinted by a late alkai metasomatism. There is a strong relation between composition of secondary minerals, mineralogy of the alteration zone and whole-rock composition. Mass transfers of alumina appear to be important. There is a significant introduction of Si, AI, K, Na, Rb and Pb, together with a depletion in Ca and Sr. Chlorite and illite geothermometers indicate temperatures of 276.7-321.5 ·C. Three events of fluid entrapment at hydrostatic pressures have been recognised, with fluids evolved from low to moderate, locally carbonaceous hot fluids to highmoderately polysaline cooler fluids. Ore-lead was derived from Palaeozoic U-'enriched' country rocks, although some participation from the Hercynian granitoids can not be ruled out. The distribution of sulphur isotopes is not homogeneous in the scale of the district. Differences exist between those deposits at the El Centenillo-Santa Elena sector (where the dominant process was the mixing of 'magmatic'-derived sulphur and sulphur derived from the metasediments) and those occurrences at the Linares-La Carolina sector (where the dominant sulphur was 'magmatic'-derived).