Replacive IOCG systems in the Ossa Morena Zone (SW Iberia): The role of pre-existing ironstones as a geochemical trap

• The Las Herrerías deposit consists of a copper-rich mineralization dated at ca. 333–330 Ma that replaces an Early-Middle Cambrian ironstone formed in a third order oxic basin related to the Cambrian rifting event in SW Iberia. • The copper mineralization seems to be related with E-W tensional structures that focused hydrothermal fluids probably derived from distal peraluminous granite dated at ca. 332 Ma. Sr-Nd isotope geochemistry suggests that fluids are of crustal derivation. • Numerical modeling shows that ironstone is an excellent trap for Cu mineralization due to the control that it exerts of the pH and redox conditions of fluids at intermediate to low fluid/rock ratios. However, the precipitation of copper sulfides needs an extra source of sulfur that cannot be transported by the same hydrothermal fluid. • At Las Herrerías, the most likely source of the reduced sulfur is the thermogenic reduction of barite present in the ironstone. The amount of reduced sulfur is perhaps the major control to the grade and size of the mineralization. The central Ossa Morena Zone (SW Iberia) hosts a regionally extensive ironstone level interbedded with bimodal volcanic rocks, limestone and shale of Lower-Middle Cambrian age. The stratabound ironstone includes dominant magnetite and hematite with locally abundant chert and barite. It is interpreted as being (sub-)exhalative at or near the seafloor and formed during a rifting event that postdated the Cadomian orogeny. In some places, such as in the Las Herrerías deposit, the ironstone is irregularly replaced by a chalcopyrite-rich ore; the Cu-rich mineralization is accompanied by the pervasive phyllic alteration of the hosting siliciclastic sediments. The highest copper grades are found when the ironstone is crosscut by WNW-ESE-trending late-Variscan extensional brittle-ductile structures that are interpreted as the feeder channels for deep hydrothermal fluids. A similar nearby copper-rich mineralization (Pallares) is is likely controlled by the tectonic contact between limestone and pyrite-rich black shale. Sr-Nd whole-rock isotope geochemistry data suggests that the Sr in the ironstone ( 87 Sr/ 86 Sr i ≈ 0.7088) is close to isotopic equilibrium with the local exhalative barite (0.7084–0.7086) and Cambrian seawater. The ironstone has a significantly more crustal εNd initial signature (<-1.8) than the coeval volcanic rocks (+5.2 to + 7.9). The younger sulfide mineralization inherited the Nd isotope composition of the ironstone but shows a significant enrichment in 87 Sr ( 87 Sr/ 86 Sr > 0.7091) that is interpreted as related with the input of genetically different and more crustally-derived hydrothermal fluids. 39 Ar- 40 Ar dating of the phyllic alteration suggest that the copper mineralization was formed at ca. 332–330 My. These ages are coeval with those of small peraluminous granite intrusions that host Cu-Au vein-like mineralization and dated at 331.8 ± 1.6 Ma (LA ICPMS U-Pb zircon). Our interpretation is that the copper-rich mineralization at the Las Herrerías area is the distal expression of an intrusion-related hydrothermal system. Numerical modelling shows that ironstone is an effective trap for copper precipitation due to the large changes in pH and f O 2 that take place when copper-bearing acid and reduced fluids react with the brittle ironstone. The precipitation of chalcopyrite, however, is controlled by the amount of available reduced sulfur in the ore trap. The δ 34 S values of the sulfides (+12.6 to + 21.6 ‰) suggest that the most likely source for the reduced sulfur is the thermogenic reduction of aqueous sulfate equilibrated with the exhalative barite (δ 34 S, +31.4 to + 35 ‰) with some minor input of reduced sulfur leached from the metasediments. This system could be considered as a variant of the IOCG clan. The formation of the ironstone and the copper mineralization, however, are separated by more than 200 My. Probably, many IOCG systems have a similar origin as Las Herrerías, with an ironstone being just a passive geochemical trap for the copper–gold mineralization.