THE ORIGIN OF BARREN BODIES IN THE SUBIZA POTASH DEPOSIT, NAVARRA, SPAIN - IMPLICATIONS FOR SYLVITE FORMATION

''Barren bodies'' are meter-size lens-like features in sylvite beds wh ere sylvite is absent or depleted, They are commonly found when mining potash deposits, Most barren bodies are believed to be formed by sele ctive dissolution of sylvite, due to the circulation of diagenetic or later fluids, Contrary to this commonly favored postdepositional inter pretation, barren bodies of the Subiza mine, Navarra, Spain are though t to be the result of synsedimentary processes, This potash deposit co ntains a 100 m thick Upper Eocene succession of alternating claystone and evaporites (sulfate, halite, and sylvite), The evaporites accumula ted in an elongated basin forming one of the depocenters of the 250 km long South Pyrenean foreland basin, Along the margin of the basin, sl ope instability, promoted probably by tectonism, created mass wasting, forming mounds 0.5-2 m high and tens of meters in extension, As evapo ration progressed, two stratified brines formed, Halite precipitated a t the air-brine interface and sank to the bottom of the basin with ter rigenous clays, Sylvite, however, precipitated from the lower brine, T he mounds extended into the upper brine, thus sylvite did not precipit ate over these upper zones, With progressive deposition the lower brin e covered the mounds, the sylvite beds overlapping the mounds, The min eralogical and petrographical features of the barren bodies and their surroundings are continuous, Analyses of primary fluid inclusions from the halites of the barren bodies show a Mg-K-Cl rich composition, Thi s corresponds to a primary brine and is unlike the Na-CI rich brines e xpected from replacement processes, The thickness of the sylvite-formi ng brine (< 1 m) inferred from the mounds is inconsistent with the thi ckness required to form centimeter-thick beds of sylvite in a closed b asin, Alternatively, the existence of a more restricted subbasin, open to a halite-forming basin similar to that observed in saline ponds, i s proposed, Numerical simulations of the evaporation processes under t hese conditions give results that match the mineral associations obser ved and the solute content of the fluid inclusions.