Solid pyrobitumen (residual petroleum) is found intimately associated with sulfides in several manto-type (strata-bound) Cu deposits of central Chile. El Soldado (lat 32 degrees 38.8＇S, long 71 degrees 06.7＇W) is one of the largest deposits of this type (> 130 Mt @ 1.5% Cu) and is heated by volcanic and subvolcanic rocks of the Cretaceous Lo Prado Fm. Detailed organic petrology of authigenic sulfide inclusions, degassing vesicles, and anisotropic pyrobitumen and graphitic carbon allowed the elucidation of a complex diagenetic history of a biodegraded reservoir in faulted volcanic and subvolcanic rocks. Due to the overmaturity of the host sequence, biomarker techniques (aliphatic and aromatic) were unsuitable to investigate the composition of the pyrobitumen. However, microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analyses, Rock-Eval pyrolysis, and C-13 NMR techniques were used to ascertain the composition of the pyrobitumen. Spatial variations in the maturity of the pyrobitumen were assessed using both random reflectance (R-o) and rotational reflectance (R-omax and R-omin). The data are consistent with the following evolution for El Soldado: As a result of burial in the Cretaceous back-arc basin, petroleum was generated from organic-rich shales in the underlying lower Lo Prado Fm. and migrated into primary and structural porosity in the host rocks, the predominantly volcanic upper Lo Prado Fm. Low-temperature (< 90 degrees C) biodegradation of petroleum developed an early assemblage (Stage I assemblage) of pyrite (+/- sphalerite +/- chalcopyrite). Geopetal structures related to pressure degassing of semisolid petroleum suggest that migration occurred while the strata were horizontal, at ca. 130 Ma. Continued basinal burial led to thermal degassing, increased maturation and contraction of the petroleum into pyrobitumen. Around 20 Ma later an influx of high-temperature (ca. 250-350 degrees C) Cu-rich fluids induced by regional granitoid intrusion (Stage II assemblage) replaced preexisting pyrite and pyrobitumen. The mineralizing fluids altered the pyrobitumen physically and chemically: (a) increasing R-o to greater than ca. 5% in the core of the orebodies against a background level of ca. 3%; (b) developing anisotropy (nongranular, fine to coarse mosaic, and coarse flow mosaic) within the pyrobitumen and locally graphitized the pyrobitumen, developing graphitic carbon; and (c) incorporating Cl (+/- Cu +/- Fe) into the pyrobitumen during the development of anisotropy. (C) 2000 Elsevier Science B.V. All rights reserved.