| Resum: |
An exploration guide for Mississippi Valley-type (MVT) deposits is proposed and illustrated in the Riópar area (SE Spain), where extensive dolostone geobodies hosting Zn-(Fe-Pb) MVT mineralization occur. These base metal deposits are found within stratabound and patchy dolostones replacing Upper Jurassic to Lower Cretaceous limestones. However widespread stratabound dolostones, with no Zn-(Fe-Pb) mineralization associated, are also found in the same area replacing Lower Jurassic to Upper Cretaceous carbonates. A detailed sampling of both, Zn-(Fe-Pb) mineralized dolostones (fertile), as well as unmineralized ones (barren), has been performed in an area of ∼106 km2 to characterize their rare earth element (REE) and isotopic (C, O, Sr) compositions. Barren Lower and Middle Jurassic and Upper Cretaceous dolostones show C, O and Sr isotopic characteristics (δ13C = +2. 1 to +3. 8‰ VPDB, δ18O = +27. 6 to +29. 8 ‰ VSMOW, 87Sr/86Sr = 0. 70736 to 0. 70764) like the Jurassic-Cretaceous undolomitized limestones and marine carbonates, and consistent with dolostones produced by low temperature seawater. On the contrary, barren and fertile Upper Jurassic to Lower Cretaceous stratabound and patchy dolomitized limestones are depleted in δ13C and δ18O and relatively enriched in 87Sr/86Sr compared to the host limestones. This δ13C and δ18O composition can be explained by an interaction of warm fluids with regional carbonates (2-3% of fluid/rock interaction at 190-230 °C). The hydrothermal character for the dolomitizing fluid is supported by REE geochemical data (e. g. , (Pr/Pr*)PAASN and (Ce/Ce*)PAASN anomalies). Thus, C and O isotopes, as well as (Ce/Ce*)PAASN and (Pr/Pr*)PAASN ratios, can be used to discriminate between low temperature dolostones, formed from cold seawater that do not contain Zn-(Fe-Pb) mineralization (barren), from hydrothermal dolomites (HTDs), which may host Zn-(Fe-Pb) mineralization. Furthermore, most barren HTDs show more restricted and higher δ13C (+0. 4 to +0. 9 ‰ VPDB) and δ18O values (+26. 4 to +27. 1 ‰ VSMOW), as well as lower ∑REE contents (5. 41 to 7. 38 mg kg−1), compared to Zn-(Fe-Pb) mineralized HTDs (δ13C: −2. 3 to +0. 6 ‰ VPDB; δ18O: +25. 1 to +27. 1 ‰ VSMOW; ∑REE 14. 10 to 54. 79 mg kg−1). Therefore, the obtained REE contents, and partially the δ13C values, can be used to discriminate between HTDs associated with Zn-(Fe-Pb) mineralization (fertile dolostones) and barren HTDs, revealing their potential as a new geochemical tool for MVT exploration in dolomitized terrains. |
visitant ::
identificació
(Universitat Autònoma de Barcelona. Departament de Geologia)
