Organic petrography and elemental analyses were used to study alteration of organic matter in the Bonneterre Formation dolostones. Reflectance measurements and petrographic characteristics of organic particles define an alteration zone about 20 km wide, centered on the Viburnum Trend ore deposits. Within this zone, bitumen has been generated from kerogen. In contrast, no evidence of local bitumen generation was observed in samples located more than 10 km from the Viburnum Trend. This could indicate a level of thermal maturity significantly greater than the regional background level.The organic matter in the Bonneterre Formation consists primarily of amorphous marine kerogen, algal remains (alginite), and solid bitumen. In the absence of vitrinite, relative levels of thermal maturity were determined from the reflectance, fluorescence, and state of preservation of fossil unicellular algae. Within the alteration zone the alginite has been impregnated with bitumen, and there is convincing petrographic evidence that the bitumen was locally generated. Outside of the alteration zone, the algae are well preserved and exhibit the bright yellow fluorescence characteristic of immaturity. There is also petrographic evidence of increased solid bitumen reflectance due to interaction with pore fluids, both within and outside the alteration zone. Elemental analyses suggest that organic matter has been oxidized throughout the study area.Although there is no direct evidence linking the organic maturation and alteration with ore deposition, the association of the alteration zone with a major Mississippi Valley-type deposit permits the hypothesis that the regional flow of hot brines associated with the deposit was the cause of the anomalous heating, and that the evolution of methane and other gases during this process played a role in the ore deposition.