Chanarcillo has been the most important of a group of silver-mining districts lying along a narrow belt paralleling the coast and corresponding approximately to the Andean Mesozoic geosynclinal belt. The host rocks are interbedded limestones and tuffs of Neocomian (Cretaceous) age. The mineralized veins occur mainly along north-to northeast-striking radial fractures associated with a domed structure, and to a lesser degree in northwest-trending faults transecting the structure. Primary mineralization was due to activity of hydrothermal solutions in a late stage of the cycle of magmatism responsible for emplacement of the Andean granodiorite batholith and related intrusions. Subsequent oxidation and supergene sulfide enrichment have been extensive and resulted in formation of the well-developed zone from which most of the mined ore has been obtained.