Abstract

The Marquesado area of the Betic Cordillera, southeastern Spain, contains two iron deposits (Alquife and Las Piletas) located within a metamorphosed Permian to Triassic carbonate sequence. The main mineralization at Las Piletas forms stratiform lenses of specular hematite, whereas the mineralization at Alquife consists of irregular bodies of sideritic marble, transformed to goethite and hematite by weathering processes.

REE patterns of the host rocks show trends either typical of marine carbonates or similar to those of the specular hematite and siderite, the latter two characterized by upward-convex REE trends and a positive Eu anomaly. Specular hematite and siderite have 87Sr/86Sr values from 0.7095 to 0.7104 and from 0.7103 to 0.7117, respectively, whereas the host carbonates are less radiogenic (87Sr/86Sr values from 0.7077–0.7085). Siderite has δ13C and δ18O values of − 7.6 to − 7.9 per mil (PDB) and 18.68 to 20.00 per mil (SMOW), respectively, whereas the host carbonates have δ13C and δ18O values of −1.40 to +2.02 per mil (PDB) and +19.32 to +25.11 per mil (SMOW). Both the mineralization and host rocks show a broad negative correlation between δ13C and 87Sr/86Sr and between δ18O and 87Sr/86Sr.

Field and petrographic data, together with geochemical and isotopic data, indicate that formation of the deposits resulted from both exhalative sedimentary and replacement processes at temperatures between 150° and 200°C from originally hot (> 200°–250°C) crustal hydrothermal fluids enriched in radiogenic Sr and isotopically light C and O. The hydrothermal system acquired abundant Fe by leaching graphite-bearing metapelites under acid conditions within the underlying Paleozoic basement. A stratiform iron oxide facies (precursor of the specular hematite mineralization) formed by precipitation in a Triassic depositional basin under oxidizing conditions at Las Piletas. In contrast, irregular siderite bodies formed by metasomatic replacement of the Triassic carbonates under reducing conditions in the Alquife area.

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