Hydrocarbon Involvement in the Genesis of Ore Deposits: An Example in Cretaceous Stratabound (Manto-Type) Copper Deposits of Central Chile

The analysis of the empirical association of organic carbon in its various forms (e.g., oil, gas, coal) with a variety of ore-deposit types can be a rewarding exercise in the formulation of genetic models and exploration. In many ore provinces, organic matter, residing in its source rocks, migrating in the form of hydrocarbons, or in transient, permanent, leaking, or degrading reservoirs, is likely to have interacted with ground waters, with hydrothermal solutions of diverse origin, or with magmas. After a brief review of these associations, we describe the ubiquitous occurrence of remnant petroleum in two representative “manto-type” stratabound copper deposits in Chile and suggest its possible genetic role and exploration significance. Overmature petroleum (solid bitumen, pyrobitumen) is found in association with copper ores in the southern orebodies of the El Soldado copper deposit in central Chile. Textures indicate that petroleum occupied the host-rock porosity before copper mineralization. Isotopic data for C and O in total organic carbon and carbonate confirm the affiliation with organic carbon and suggest that gangue carbonates are in part the result of the oxidation of hydrocarbons. Reflectance of the solid bitumen is highest near ore (BR o ∼ 4%), and its geochemistry (Rock-Eval) indicates that the residual hydrogen (i.e., hydrocarbon) content is very low. It is proposed that mineralizing solutions interacted with a pre-existing degraded hydrocarbon reservoir. The hydrocarbons physically prevented low-grade metamorphic minerals from sealing the porosity, were oxidized to form carbonate gangue and alteration, and possibly acted as a reductant at the site of sulfide precipitation. Copper sulfides (bornite, chalcocite, chalcopyrite) filled spaces among bitumen, carbonate, and pre-existing sulfides (arsenopyrite, pyrite) only after significant burial and faulting of the host volcanic strata and trachytic subvolcanic intrusives had occurred, after migration of liquid petroleum into the porosity and during or after its solidification there; this makes syngenetic and early diagenetic models previously proposed for the deposit untenable. Similar occurrences of bitumen in the ores of the Talcuna copper/manganese district and in several other stratabound copper deposits in central and north-central Chile suggest that organic matter may have been involved in the genesis of many Andean manto-type deposits. The identification of source-trap systems that could have generated oil or gas reservoirs, which in turn could have interacted later with hydrothermal solutions from various sources, may be a useful exploration model for metallic ore deposits in the back-arc basins of the Andes.