Fluid inclusions in pre-ore minerals from the carbonate-hosted mercury deposits in the Espadلn Ranges (eastern Spain)

The fluids associated with pre-ore minerals in the carbonate-hosted mercury veins from the Espadán Ranges (eastern Spain) have been studied using microthermometry, Cryo-SEM-EDS, Raman spectroscopy and crush-leach analyses, in order to determine their origin and the processes responsible for mineralization. Three primary fluid inclusion types have been found in quartz and dolomite: (1) type S, composed of liquid + vapour (mainly CO2) + halite; (2) type S-V, composed of liquid + high-density CO2 double bubble + halite; and (3) type V, composed of high-density CO2N2 mixtures. Type-S fluid inclusions are preferentially trapped in Betxí-type veins, characterized by the presence of hydrothermal dolomite, whereas type-V fluid inclusions are trapped almost exclusively in Eslida-type veins, which were without dolomite. From these data two different fluids can be inferred to occur during the period of formation of the veins: (1) a complex polysaline brine of NaClKClCaCl2(MgCl2, SO42−)H2O composition with a salinity of up to 33 eq wt% NaCl; and a (2) CO2N2-rich fluid. The brine halogen composition points to its secondary origin caused by the dissolution of underlying marine evaporites. thought that these deposits were formed during Jurassic or Cretaceous rifting periods, as a consequence of fluid mixing between a high-salinity secondary brine and connate waters. The generation of a CO2N2-rich phase caused the pressure increase of the fluids that favoured the hydraulic fracturing of the enclosing rock (dolostone). The origin of this volatile-rich phase probably occurred due to the oxidation and thermochemical breakdown of the organic matter contained in the host rock. This event is recorded by the coetaneous trapping of both aqueous and carbonic fluids in quartz crystals. The formation conditions for the hypogene sulphide paragenesis are obscure but the coincidence between the preferential trapping of type-V fluid inclusions, the absence of hydrothermal dolomite and the major abundance of mercury in Eslida-type veins is thought to be linked to a mercury fractionation event to the vapour phase during the release of the volatiles.