Pyrobitumen related to silver-copper deposits in a Cretaceous volcanic-sedimentary sequence: Talcuna district, Coquimbo, Chile

In recent years, hydrocarbons have been observed in some stratabound copper deposits inthe Cretaceous volcanic-sedimentary sequences in north and central Chile. At the “21 de Mayo” and“Socavon 2001” mines in the Talcuna district in north-central Chile (2930’–2945’ S/7045’–7100’ W),pyrobitumen is associated with ore minerals in Lower Cretaceous rocks. In this district, recent investigationsdemonstrate that the ore mineralization, traditionally regarded to be of stratabound (manto) type, has bothstructural and lithological controls. Paragenetic relations reflect a hydrothermal emplacement of boththe pyrobitumen and the ore and gangue minerals, while isotopic analyses suggest an organic marineprovenance of the former. Reflectivity 2.0–2.4% indicates the maturity of the pyrobitumen. Its occurrence isspatially related to sulphide mineralization (chalcopyrite, bornite, chalcocite) and to barren zones, especiallyin the walls of fractures and faults that were channels for post-ore diorite dikes. Organic matter was probablyhydrothermally mobilized from limestones of the Arqueros Formation and precipitated first in the ore zonesand then in fractured post-ore dikes. Hydrothermal fluids sealed in calcite and barite related to thepyrobitumen reveal homogenization temperatures ranging from 180 to 320C and salinities from 6.5 to21 wt.% NaCl equivalent. These data characterize the mineralization as a moderate- to low-temperatureevent involving fluids of considerable salinity.