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Location: 16 km NW from Sevilla, Spain
Carretera SE-3410 – Km. 4,100 Gerena – SevillaSpain41860
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The Cobre Las Cruces (CLC) deposit is overlain by 150 to 200 m of Cenozoic Guadalquivir Basin sediments which, towards the base, hosts a confined porous aquifer (1 to 3 m thick) known as the Niebla-Posadas Aquifer. The aquifer has a ph of ~ 8.0, low salinity and a temperature of 18-31 °C. The host Paleozoic sequence underlies these basin sediments and has an east-west strike, dipping between 20° to 45° to the north. The Paleozoic rocks also contain pressurised water which is more saline and with temperatures between 22 to 43 °C. No methane has been intersected in either of the aquifers.Deposit TypesThe CLC deposit is a Volcanogenic Massive Sulphide (VMS) deposit which has resulted from precipitation of metals from hydrothermal fluids in a volcanically active sub-marine area. The metals precipitate as sulphides inter-layered with the volcanic sediments. Consequently, CLC’s mineralization is hosted by black shales, sedimentary and felsic volcaniclastic rocks.The deposit is characterized by a tabular dipping massive sulphide body underlain by a stockwork vein system. The deposit has been folded and faulted, resulting in dislocated mineralised volumes. Hydrothermal alteration is typically chloritic/quartz sericitic and is restricted to the stockwork zones. CLC deposit shows zonation where copper rich volumes are flanked by zinc and lead mineralization. Lead, zinc and silver tend to concentrate in unevenly distributed polymetallic zones. Minor gold is noted in both polymetallic and copper rich areas. Pyrite is dispersed throughout.At CLC, post-deposition tectonism uplifted the deposit, exposing it to erosion and weathering with partial oxidation of some of the primary sulphides. An uppermost copper-leached gossan resulted with some immobile gold and silver and overlies the cementation zone of secondary sulphide mineralization.Primary Polymetallic Sulphide (PPS) MineralizationPPS mineralization comprises two mineralization domains, primary massive sulphides and semi-massive primary sulphides or stockwork.The primary massive sulphide occurs as a single 30 to 130 m thick lens with around 1,000 m of strike and a 500 m down dip length. Primary mineralization underlies secondary mineralization and is open at depth and to the northeast. Mineralization thickens to the west but is truncated by a north-south striking fault. The PPS massive sulphide mineralisation is underlain by a zone of mineralised stockworks.The primary massive sulphides have an upper Zn-rich zone and a lower Cu-rich zone interbedded with barren massive pyrite. Sulphides are dominated by pyrite with chalcopyrite, sphalerite, and galena. The polymetallic zones contain centimetre-thick bands of alternating pyrite + chalcopyrite and sphalerite + chalcopyrite + galena. Deformation is responsible for the formation of alternating layers of sphalerite-chalcopyrite-galena and pyrite. The sulphides other than pyrite show widespread recrystallization to a granoblastic assemblage hosting abundant minute subhedral pyrite.The massive sulphides show conspicuous layering with few clear sedimentary features. Features include banding and graded bedding, especially in the interbedded shale and chert. However, the widespread late recrystallization has masked most primary features.While the uppermost shale-hosted massive sulphides are likely to be exhalative, the lowermost orebody formed mainly by replacement. Evidence of replacement includes the presence of relicts of variably altered dacite within the massive sulphides and a gradual contact with the underlying stockwork mineralised zone. No oxidation has been observed in the hanging wall of the massive sulphides, thus indicating that seafloor oxidation processes were minimal and that the massive sulphides were originally isolated from an oxidising open ocean.The PPS massive sulphide mineralization is underlain by a mineralised stockwork and related hydrothermal zone which extends some 200 m beneath the massive sulphides and tends to be copper rich with localized zinc. The stockwork is hosted in hyaloclastic dacite and shale having chlorite ± quartz alteration. The stockwork veins are between 0.5 to 2 cm thick and are folded or sub-parallel to the dominant foliation.
