Sulfur and lead isotope systematics: Implications for the genesis of the Riópar Zn-(Fe-Pb) carbonate-hosted deposit (Prebetic Zone, SE Spain)

Abstract

The Zn-(Fe-Pb) deposits of the Riópar area (Prebetic Zone, SE Spain) are hosted by dolostones that replace Berriasian to Valanginian (Upper Jurassic-Lower Cretaceous) limestones. Mineralization consists of hypogene sphalerite, marcasite and galena, and supergene calamine zones. The hypogene ores are associated with a saddle dolomite gangue. The ore bodies occur as discordant and stratiform lenses, ore-cemented breccias, cm- to mm-wide veins and veinlets, disseminations and stylolite porosity filling within the host dolomites. The main ore controls include stratigraphy and/or lithology, tectonics (faults, fractures and breccias) and availability of metals and sulfur. The morphologies and epigenetic character of the hypogene ore bodies are consistent with the classification of this mineralization as a Mississippi Valley-type (MVT) deposit. The Ga/Ge geothermometer in sphalerite yielded a temperature range of 194-252ºC, which represents the temperature of the source region of the ore solution. This value is comparable to the temperature obtained in the ore deposition site, 159±15ºC from the Δ34S geothermometer in sphalerite galena pairs. This similitude points to a hydrothermal fluid that did not cool down significantly during flow from the fluid reservoir area to the precipitation site. δ34S values of base-metal sulfides (-7.5 to +3.5 ¿) are consistent with thermochemical reduction of Triassic sulfate (seawater and/or derived from dissolution of evaporites) by interaction with organic compounds (e.g., hydrocarbons, methane), which reduced sulfate to sulfide in the deposition site. The lead isotope ratios (206Pb/204Pb = 18.736-18.762; 207Pb/204Pb = 15.629-15.660; 208Pb/204Pb = 38.496-38.595) of galena suggest that Pb, and probably other metals as Zn, is derived from continental crustal rocks. On the other hand, these relations points to an unique metal source probably derived from the Paleozoic basement rocks. The relationship between bedding-parallel stylolites, dolomitization, sulfide precipitation and Alpine tectonic affecting the MVT ore, suggests a relative timing range for the mineralization in the Riópar area of 95 to 20 Ma (Upper Cretaceous-Tertiary). The sulfide mineralization and the associated dolomitization are thus explained by the contribution of two fluids that mixed in different proportions during dolomitization and mineralization: i) a fluid probably derived from Cretaceous seawater saturating Mesozoic sediments (Fluid A), characterized by being dilute and initially low temperature, which should have contained organic rich compounds in the ore deposition site (e.g., hydrocarbons and CH4 dissolved gas); and ii) a high salinity hydrothermal brine (Fluid B) rich in both metals and sulfate, circulated through the Paleozoic basement. During the pre-ore dolomitizing stage the fluid phase was dominated by the diluted fluid (Fluid A > Fluid B), whereas in a later fluid pulse, the proportion of the high salinity fluid increased (Fluid A < Fluid B) which allowed sulfide precipitation. MVT exploration in the Prebetic Zone should focus towards the SW of the Riópar mines, in the vicinity of the Alto Guadalquivir-San Jorge fault.