Summary:
Mantos Blancos is a copper stratabound deposit with subordinate content of silver mineralization hosted by Jurassic volcanic sequences. Mantos Blancos has the typical rock, mineralization, alteration and structural setting of the Jurassic deposits, in spite of its bigger size in comparison with other Chilean stratabound copper deposits. The area has been the subject of geological studies and interpretation since 1917 and the deposit geology has been extensively studied by Ramirez (2006) and Maksaev (2002).
Mantos Blancos is associated with the La Negra Formation. A volcanic arc is recognized in the La Negra Formation in the Coastal Range of northern Chile. This formation comprises mainly thin flows of calc-alkaline basalts, basaltic andesites and high-K basaltic andesites, as well as some volcaniclastic intercalations with a total thickness of 7 km to 10 km. The beginning of the volcanism of the La Negra Formation has been determined to be of Lower Jurassic age. A comparable volcanic belt is also recognized in the Lower Cretaceous in several areas of northern Chile.
The deposit is mainly characterized by pyroclastic and intrusive host rock units. The pyroclastic and intrusive units are from the Permo-Triassic and Early Jurassic age. Andesite belonging to the La Negra Formation (Middle-Late Jurassic), tonalite, granodiorite, aplite and dacite porphyry belonging to the El Ancla and Alibaud plutons of Middle to Late Jurassic age also outcrop along the deposit. Rock types in Mantos Blancos consist of a rhyolitic dome and magmatic–hydrothermal breccias which are intruded by dioritic and granodioritic stocks and sills. The dioritic and granodioritic stockwork locally grade upwards into magmatic–hydrothermal breccias that are mineralized to different degrees. Late mafic dikes cross all the previously mentioned rock units and are essentially of no economic value.
The sulphide mineralization consists of chalcocite (and/or digenite), covellite, bornite, chalcopyrite, pyrite, specularite, magnetite, galena and low sphalerite, occurring in disseminated form, with varying thicknesses. The oxidized copper minerals are atacamite, chrysocolla and minor malachite, antlerite, tenorite, cuprite and almagres occurring as dissemination and fracture filling. Silver occurs in the crystal structure of the copper sulphides and occasionally as native silver. The geometries of the mineralized bodies are irregular lenses and oxidized copper sulphides are arranged in tabular form with a 100 m to 200 m thick interval (Boric et al., 1990) that is strongly controlled by structures.
The mineralization has a distinct vertical zonation (Infanta, 2002), with specularite at the top (porphyritic andesite and upper andesite), followed by an oxidized copper (atacamite with minor chrysocolla) that migrates in depth to a high-grade sulphide (chalcocite-bornite). The latter corresponds to irregular chalcocite rich centre lenses which decrease towards the edge to predominant bornite. Surrounding these lenses is a zone of lower grade with chalcopyrite and bornite, ending at depth with a pyritic zone, occasionally in some sectors associated with chalcopyrite. The areas with secondary enrichment are of small extent, predominantly with covellite over chalcocite and located near major faults. The oxidized copper would have developed by the in situ oxidation of primary sulphides.