From deep to shallow fluid reservoirs: evolution of fluid sources during exhumation of the Sierra Almagrera, Betic Cordillera, Spain

Abstract

Palaeo-fluids trapped in quartz and siderite-barite veins hosted by graphitic schists recorded the fluid and metal transfers during the Neogene exhumation of the Sierra Almagrera Metamorphic Core Complex. First quartz veins registered the ductile then brittle-ductile extensional shearing. The reservoir at that time was wetted by high-salinity fluids with a low density volatile phase resulting from the dissolved Triassic evaporites. Low salinity fluids occurred during the exhumation within the brittle domain as revealed by transgranular fluid inclusion planes affecting previous veins. This suggests an opening of the system and the penetration of surficial fluids from uplifted ranges during Serravalian to early Tortonian times. Transcurrent tectonics generated marine basins since late Tortonian. At depth quartz veins discordant to the foliation were associated to hematite indicating oxydizing conditions. A stop of the low-saline record is revealed by high-salinity Fe-rich fluids issued from the underlying metamorphic reservoir. The Messinian ongoing activity of the sinistral Trans-Alboran tectono-volcanic trend led to the formation of ore deposits. Reducing conditions and Fe-rich fluid led to the formation of siderite and pyrite. The subsequent formation of galena and barite under oxydizing conditions has been related to a probable increase of temperature. A higher salinity and the Cl/Br ratio indicate another source of secondary brine issued from dissolved Messinian evaporites as confirmed by δ34S signature of barite. Transcurrent tectonics clearly modified the hydrogeology of the upper reservoir. The faulting connected the deeper and upper reservoir generally separated by the brittle/ductile transition rheological and hydrogeological barrier under extensional tectonic regimes as confirmed by the stage of low-saline fluids.