A model for the diagenetic formation of sandstone-hosted copper deposits in Tertiary sedimentary rocks, Aragón (NE Spain): S/C ratios and sulphur isotope systematics

Abstract

Numerous small, noneconomic copper deposits are associated with syntectonic sandstones of Tertiary age (Campodarbe and Uncastillo Formations) in the South Pyrenean foreland basin (SPFB). Sulphides, native metal, and their oxidation products occur within elongate lenses in fluvial sandstones and conglomerates. In all cases, the geometry of the ore is highly consistent with the primary depositional and biogenic structures of the host sandstones. Copper ores replace wood debris and diagenetic pyrite or, more frequently, appear as intergranular cement replacing previous diagenetic calcite cement. Sulphide sulphur and organic carbon distribution shows that pyrite-rich sandstones are the copper ore precursor, and that mineralizing processes provoked the depletion of both reduced S and organic C as a consequence of interaction with an oxidized Cu-bearing fluid. On the other hand, low δ 34S values are consistent with bacteriogenic derivation of sulphur after Pyrenean evaporites. Petrographic studies, sulphide sulphur and organic carbon concentrations, and δ 34S values suggest that Cu-rich fluids were introduced into reduced sediments during diagenesis. Mineralizing processes took place in two different periods: the first one related to the filling of the Jaca Basin during erosion of the Pyrenean basement and the second one related to the emplacement of a thrust ramp which initiated the erosion of the previously formed cupriferous sandstones.