The Role of Organic Matter in the Genesis of the

Solid bitumen (residual petroleum) occurs intimately associated with sulfides in several manto-type Cu de-posits of central Chile. E1 Soldado (32ø38 • S; 71o04 ' W) is one of the largest manto-type deposits (>130 million metric tons @ 1.5 % Cu) and is hosted by rhyolites and andesites ofthe Lower Cretaceous Lo Prado Formation. As a result of basinal burial in the Early Cretaceous back-arc basin, petroleum was generated from organic-rich shales in the underlying lower Lo Prado Formation and migrated into primary and structural porosity in the upper Lo Prado Formation. Migration was controlled by extensional basinal faults. Low-temperature (<90øC) biodegradation f petroleum developed an early stage I assemblage of (mostly framboidal) pyrite (+_ sphalerite + _ chalcopyrite). Geopetal structures related to pressure degassing of semi-solid petroleum suggest that migration occurred while the strata were horizontal, at ca. 130 Ma. Continued basinal burial ed to ther-mal degassing, increased maturation, and solidification f the petroleum. Low-grade regional (pumpellyite-prehnite) metamorphism of the sequence was followed, at ca. 110 to 103 Ma, by the influx of high-temperature (ca. 300øC), Cu-rich fluids concurrently with emplacement of the Cre-taceous batholith into the basin. Basinal f uids were focused into a now tilted sequence by a brittle shear (north-south to north-northwest) fault system. Copper sulfides (stage II assemblage) r placed preexisting pyrite an