The geological and metallogenic setting of stratabound carbonate-hosted Zn-Pb mineralizations in the West Asturian Leonese Zone, NW Spain

Abstract

Several carbonate-hosted stratabound zinc-lead ores in the Ponferrada-Caurel area (NW Spain) are hosted by the Lower to Middle Cambrian Vegadeo Formation. Two clearly distinct groups of mineralizations occur in different stratigraphic positions. The stratiform disseminated ore is located in the Lower Member as irregular and millimetre-thick layers of sphalerite and galena replacing earlier pyrite. The lack of hydrothermal alteration and the heavy C., O and S isotopic signatures suggest that this ore is of premetamorphic origin, the sulphur and fluids being derived from the host carbonates. The more likely source of the sulphide is the abiogenic thermal reduction of sulphate derived from sulphate beds intercalated with the carbonates. The second group of mineralizations is located at the top of the Vegadeo Fm, always along its contact with the overlaying shales and sandstones of the Cabos Series. This group is economically more important and include three styles of strata-bound mineralizations. The more common one is the ‘silica ore’, a hydrothermal rock that traces the contact between the carbonate and the detrital rocks along more than 50 km. Locally, a ‘carbonate-rich ore’ is found along the contact between the silica ore and the Vegadeo Fm. Laterally to these rocks, there are large bodies of the ≪breccia ore≫, made up of sulphides and calcite in a matrix of chlorite. The ore assemblage is composed of sphalerite and galena with minor amounts of chalcopyrite and pyrite. Co-Ni-As sulphides, bismuthinite, tetrahedrite and Pb-Bi sulphosalts are also found as trace minerals. The geological relationships and the isotopic signatures suggest that the three ores are synchronous and of late Hercynian age. They are interpreted as linked with a tectonically driven fluid flow along the stratigraphic contact between the carbonate and the detrital rocks.

The model of ore genesis involves the circulation of fluids in likely equilibrium with the detrital rocks that react with the Vegadeo Fm leading to the metasomatic replacement of limestones by quartz with synchronous precipitation of sulphides. The genesis of breccias is probably due to the formation of overpressured zones. The hydrothermal alteration results in a systematic depletion in both δ ¹⁸O and δ ¹³C of the carbonates due to the infiltration of fluids, of likely mixed metamorphic and surface origin. Fluid inclusions in the chloritic breccia suggest that the ore formation took place at temperatures higher than 200 °C in relationship with low salinity (up to 1.2% wt. NaCl eq.) water-rich (H₂O>99%) fluids. Sulphur isotopes suggest that most of the sulphur has a common origin with the stratiform ores, but here there is a significant but variable input from the detrital rocks. Lead isotopes of the different ores are within the ‘Cambrian signature’ of the southern Hercynian Belt, with a long crustal history. However, mixing with a minor juvenile component cannot be ruled out. The geographic and stratigraphic proximity and the similar lead signatures between the premetamorphic and the Hercynian mineralizations suggest that the latter was derived from the remobilization, in a ‘lead frozen system’, of the stratiform-disseminated ones. The premetamorphic mineralizations can be interpreted as similar to the widespread Mississippi Valley-type deposits found in the southern Hercynian Belt. The second group of deposits can be defined as synto postmetamorphic stratabound, carbonate-hosted Zn-Pb deposits, broadly similar to MVT but formed in an orogenic setting. Specific features such as the presence of chlorite, the fluid composition (low saline H₂O-NaCl fluids) and the temperatures of formation (above at 200 °C) are interpreted as characteristic of this tectonic setting.