Unusual mineralized and silicified carbonate-rich geodes were found hosted in Lower
Triassic red mudstones in Central Spain. From their borders to their centres, the geodes display (a) a
millimetric carbonate crust, (b) a quartz 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 that 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 dissolution, has been filled by epithermal gold-bearing iron-oxide hydroxides, romanechite and
calcite.
δ13C values indicate the participation of meteoric waters in an environment 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 precipitated at
<38°C. Iron-oxide temperature values can reach up to 85°C (epithermal stage). The mineralogical
assemblages of these unusual geodes denote early diagenetic replacement followed by the epithermal
activity. This mineralization is linked to the Late Hercynian, calc-alkaline volcanism of Central Spain
(the Hiendelaencina mining district).