The high-grade Las Cruces copper deposit, Spain: a product of secondary enrichment in an evolving basin

The Las Cruces deposit (Iberian Pyrite Belt) includes a large, high-grade cementation zone capped by unusual rocks that contain carbonates, galena, iron sulphides, and quartz. Between the Late Cretaceous(?) and Tortonian, the volcanogenic massive sulphides were exhumed and affected by subaerial oxidation that formed paired cementation and gossan zones. Onset of Alpine extension produced accelerated growth of the cementation zone along extensional faults, leading to formation of the high-grade copper ore at ca. 11 Ma. Later, replacement of the overlying gossan by sulphide- and carbonate-rich rocks beneath sealing marl sediments is thought to have involved microbial processes, occurring between the Messinian (ca. 7.2 Ma) and today. Isotope data show that the cementation zone formed by the mixing of descending acidic waters derived from oxidation of the massive sulphides, with upwelling geothermal waters flowing at temperatures above 100 A degrees C. The C, O, and Sr isotope values of the mineralization (Sr-87/Sr-86 0.7101-0.7104) and of the local groundwater (0.7102-0.7104) reflect equilibration with basement rocks, and indicate that influence on the ore-forming process by marl-equilibrated water (0.7091-0.7093) or Miocene seawater (0.7086-0.7092) was negligible. The high sulphur isotope values of the sulphides in the biogenic zone (most +19 to +24 aEuro degrees) are well above those of the primary sulphides (delta S-34 ca. -6.8 to +10.3 aEuro degrees) and likely reflect formation of the biogenic sulphides by reduction of aqueous sulphate in the groundwaters. Sulphur isotope values of the cementation zone (delta S-34 ca. -2.4 to +21.7 aEuro degrees) are also consistent with some contribution of sulphur from the biogenic reduction of aqueous sulphate.