Ricardo Boric

El Soldado is the largest (>200 Mt @ 1.4 % Cu) of the known Cu mantotype deposits in central Chile, hosted by the submarine volcanic unit of the Lower Cretaceous Lo Prado Formation. Petrographic and lithochemical evidence indicate that the host rocks are a bimodal sequence of calcalkaline basalt and rhyodacite, which have been regionally affected by very low grade metamorphism and extensive albitization in the mine camp. The rocks have the characteristic alteration of spilite and keratophyre, explaining the previous misidentification as alkaline andesites and trachytes.

Although stratigraphically restricted (stratabound), the clustered orebodies are mostly veinlike and discordant, controlled by a system of NS to NNW faults formed within a dilational jog within a sinistral, strikeslip brittle shear system. Individual orebodies are zoned, with an external a zone of barren pyrite, followed inward by concentric zones with chalcopyritepyrite, chalcopyrite, chalcopyritebornite, and a central zone of bornitechalcocite (±covellite) and abundant hematite. At the deposit scale, the bornitechalcocite association is more abundant along the western branch of the deposit, and to the south. Pyrite is more abundant at depth and to the east away from the main shears faults. Specular hematite is recognized for the first time as an important constituent of the ores in the northern part of the deposit. Gangue minerals are calcite, albite, kfeldspar and chlorite. The hydrothermal Cu mineralization is associated with an increase in Na and depletion in K in host rocks, although there are localized zones of K increase in bornitechalcocite assemblages near structures.

The deposit was formed in two main phases: 1) a lowtemperature, diagenetic phase during which framboidal pyrite developed in association with migrated petroleum, at ca. 130 to 120 Ma; 2) a hightemperature (> 300oC from fluid inclusions) hydrothermal phase at ca. 103 Ma, (coinciding with the main phase of batholith emplacement), that deposited chalcopyrite, bornite, chalcocite, mostly replacing preexisting pyrite, with the excess Fe developing hematite. It is suggested that the Cu was extracted from oxidized subaerial lavas of the overlying Veta Negra Formation, and was precipitated when, channeled by faults, the oxidized hydrothermal brines reacted with reduced diagenetic pyrite and pyrobitumen. The high salinity of fluid inclusions (21-26% NaCl equivalent) suggests a basinal connate brine was responsible for Cu transport. The Cretaceous batholith probably provided heat, and a (distal) magmatic contribution to the mineralizing fluids is possible.

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Pages: 392
Supervisor: Marcos Zentilli