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We present and discuss the petrography and carbon stable isotope data on carbonaceous materials from tertiary and precambrian rocks from the Iron Quadrangle. Carbonaceous material in the Tertiary sediments presents a narrow range of δ 13 C (28.3 to -25‰) and larger concentrations of total organic carbon if compared to the precambian rocks (-29.8 to -5.9‰). This narrow range of δ 13 C indicates that the carbon isotopic compositions of these rocks have not been significantly affected by diagenetic and post-diagenetic processes. The precambrian samples have, in general, less than 1 wt% total carbon. The wide range of δ 13 C of these rocks was related to the original isotopic composition of the rocks, as well as to sedimentary and metamorphic processes. We suggest that these processes led to volatilization of carbon compounds, such as CO, CO 2 and CH 4 . Even though we have not characterized the conditions of volatilization, we further suggest that they may be related to the presence of volatiles during the metamorphism. The carbonaceous material shows a broad peak in the same region of the main peak of graphite, indicating a low degree of crystallization.