Unusual sedimentary geodes filled by gold-bearing hematite laths

Unusual mineralized and silicified carbonate-rich geodes were found hosted in Lower Triassic red mudstones in Central Spain. From their borders to the ir centres, the geodes display (a) a millimetric carbonate crust, (b) a qua rtz rim, (c) massive dolomite, (d) gold-bearing iron-rich infills (hematite laths and goethite with up to 7000 ppm of gold) and (e) calcite cements th at sometimes seal the central hollow. Textural evidence indicates that the geodes were originally anhydrite, which was subsequently replaced by quartz and dolomite. The resultant porosity from this replacement, or by later di ssolution, has been filled by epithermal gold-bearing iron-oxide hydroxides , romanechite and calcite. delta(13)C values indicate the participation of meteoric waters in an envir onment which was characterized by both a sub-desert climate and a temperate -tropical climate. Oxygen signatures reflect very variable temperatures for all minerals, with the exception of calcite, which appears to have precipi tated at <38 degrees C. Iron-oxide temperature values can reach up to 85 de grees C (epithermal stage). The mineralogical assemblages of these unusual geodes denote early diagenetic replacement followed by the epithermal activ ity. This mineralization is linked to the Late Hercynian, calc-alkaline vol canism of Central Spain (the Hiendelaencina mining district).