Dispersed organic matter (OM) is present in dolostones of the Beekmantown Group (Lower Ordovician) in the St. Lawrence Lowlands. OM is altered in the vicinity of mineral showings and near Lower Cretaceous alkaline intrusions, and each alteration assemblage is uniquely characterized by its petrographic properties. OM near mineral showings is characterized by mosaic textures,alteration rims, abnormal anisotropy, and retlectances (Ro) higher than background levels in the Beekmantown Group. OM near alkaline intrusions is characterized by rare mosaic textures, absence of alteration rims, and normal anisotropy. Intrusions affect Ro values to distances twice the diameter of the intrusions, that is, less than 5 km. Aureoles of OM alteration around mineral showings, on the other hand, exceed 10 km.

The petrographic properties of OM allow outlining distinctive aureoles around mineral showings. As these aureoles are different from those around intrusions, their recognition can be considered as potentially useful in mineral exploration. The gradients of Ro reflect types of thermal alteration that are quite distinct for mineralization and intrusions. Despite the numerous alkaline bodies within a 15-km-wide belt extending for 140 km, and contact aureoles of hornfels, the extent of thermal alteration around the intrusions is such that their emplacement had little effect on regional trends of maturation in the sedimentary basin. It is suggested that the differences in OM alteration between mineral showings and intrusions is a result of the mode of heat propagation involved in each case. The mineral showings are no doubt of hydrothermal origin, and the fluids involved could have both altered the OM and efficiently transported the heat required to increase Ro. The intrusions, by contrast, must have been relatively "dry," and emplaced in rocks with little extant porosity and permeability, so that heat transfer could not have been enhanced by movement of pore fluids.

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