The geology of the El Soldado manto type Cu-(Ag) deposit, central Chile
by
Ricardo Boric, Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada, Carmen Holmgren, Consultant, Santiago, Chile, Nicholas S. F. Wilson Energy and Environment, Geological Survey of Canada, Calgary, Alberta, Canada, Marcos Zentilli, Department of Earth Sciences, Dalhousie University Halifax, Nova Scotia, Canada.
in Porter, T.M. (Ed), 2002 - Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide, v. 2, pp 163-184.
ABSTRACT
El Soldado is the largest (>200 Mt @ 1.4% Cu) of the known Cu manto-type deposits in central Chile. It is strata-bound within a submarine, bimodal calc-alkaline basalt - rhyodacite unit of the Lower Cretaceous Lo Prado Formation, which also contains marine carbonaceous shales and volcaniclastic sandstones. Although stratigraphically restricted, the clustered orebodies are mostly vein-like and discordant, controlled by a system of north-south to NNW faults formed within a transtensional zone (cymoid loop) of a sinistral, strike-slip brittle shear system. Individual orebodies are zoned, with an external and deeper zone of barren pyrite, followed inward by concentric zones with chalcopyrite-pyrite, chalcopyrite bornite, bornite-chalcocite, and a central zone of chalcocite (±digenite ±covellite) and abundant hematite. The deposit was formed in two main phases: 1) a low-temperature, diagenetic phase during which framboidal pyrite developed in association with migrated petroleum, at ca. 130 to 120 Ma; 2) a high-temperature (>300°C from fluid inclusions) hydrothermal phase at ca. 103 Ma, (coinciding with batholith emplacement), that deposited chalcopyrite, bornite and chalcocite, mostly replacing pre-existing pyrite, with the excess Fe forming hematite. Gangue minerals are calcite, albite, k-feldspar and chlorite. The hydrothermal Cu mineralization is associated with an increase in Na and depletion in K in host rocks, although there are localised zones of K increase in bornite-chalcocite assemblages near structures. Isotopic studies indicate that: a) the sulphur in diagenetic pyrite provided the bulk of the sulphur for Cu sulphides; b) petroleum was the source of carbon in bitumen and part of the carbonate; c) osmium in diagenetic pyrite was derived from the black shales; d) strontium in calcites was inherited from the Cretaceous arc lavas; e) oxygen isotopes in carbonates, and K-feldspar and atmospheric argon in K-feldspar plus the high salinity of fluid inclusions (21-26% NaCl Equiv.) suggest a basinal connate-metamorphic brine was responsible for Cu transport, yet a (distal) magmatic component to the fluids cannot be ruled out.
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