Genesis and evolution of strontium deposits of the granada basin (Southeastern Spain): Evidence of diagenetic replacement of a stromatolite belt
There are important strontium deposits in the Granada Basin, the most noteable of which (Montevives) has an annual production of approximately 50,000 tons of almost pure celestite. Two types of mineralization can be recognized: a primary variety consisting of stromatolitic carbonate that has been partially replaced by celestite, and a secondary variety consisting of celestite-pebble karst deposits. Both are included in an evaporitic Messinian succession. The primary variety of mineralization is within carbonates that interfinger with, and prograde across, gypsum deposits. The development of these deposits can be interpreted in the context of the general evolution of the Granada Basin during Late Tortonian and Messinian times. Open marine conditions prevailed during the late Tortonian. During the transition from Tortonian to Messinian a restriction of the basin resulted in evaporite sedimentation, with stromatolites thriving at the basin's margin. The stromatolites were distributed along a coastal belt that was limited on the east by the tectonically active Sierra Nevada with its local alluvial fans. Runoff from the Sierra Nevada produced a freshwater lens and surface salinities that permitted the development of stromatolites, rather than the accumulation of gypsum. The replacement of stromatolitic carbonate by celestite occurred within the mixing zone of the coastal aquifers during sedimentation and/or early diagenesis. An essentially marine origin is considered for the strontium. Supplementary influxes from continental weathering are also thought to have been produced. Further restriction of the Granada Basin led to complete desiccation and the deposition of a 20 m thick halite layer. Later, gypsum deposits were exposed, and resulting cavities ("dolinas") were filled with celestite pebbles. The return of sediment accumulation within lakes buried and preserved these deposits.