Coal metamorphism by igneous intrusion in the Raton Basin, CO and NM: Implications for generation of volatiles

Abstract

Abstract The Raton Basin contains a series of Tertiary mafic dikes and sills that have altered millions of tons of coal to natural coke and may have played a minor role in generating some of the large coalbed methane resources currently being exploited in this region. Four outcrops within the Raton Basin were selected to investigate local coal rank elevation and volatile generation due to intrusive activity. Coal and organic shale samples were collected on traverses across meter-scale dikes and sills, and were analyzed for vitrinite reflectance, total organic carbon, bulk carbon isotopic ratios, and coke petrography. Reflectance patterns in the contact aureoles of the dikes display very consistent concave-up patterns, with reflectance values returning to background within one intrusion width from the contact. Bulk coal samples collected across dikes display an increase in δ13C of approximately 1‰ approaching all contacts. Reflectance patterns within the contact aureoles of sills do not decrease significantly until one half of an intrusion width away from the contact, but then drop steeply to background values by one intrusion width away. Bulk coal samples display complex isotopic patterns approaching sills, with an initial increase in δ13C followed by a steep decrease of approximately 1.5‰ approaching the top contacts. The distinct isotopic and reflectance patterns for dikes and sills may indicate that the coal in the contact aureoles of the sills experienced a longer heating duration than the coal near the dike contacts. This longer heating duration was due to the low thermal conductivity of coal, which acts as a thermal insulator to the sills that intrude coal beds. This style of intrusion is commonly observed throughout the central portions of the Raton Basin. By contrast, dikes intrude a variety of country rock types, and are not insulated by coal. Heat may escape the contact zone through nearby sandstones or shales. Based on reflectance patterns, hydrothermal convection played a role in heat transfer away from dikes. Data from both the dike and sill contacts suggest that time plays a role in controlling peak vitrinite reflectance values, at least over the short heating durations (tens of years) expected for meter-scale intrusions. Decreasing total organic carbon values approaching all contacts suggests the generation and escape of volatile species such as methane. Isotopic values within the contacts of dikes are consistent with the loss of 12C-enriched methane. The isotopic pattern observed at the contacts of sills may be due to accumulation of 12C-rich pyrolytic carbon close to the contacts, which is supported by petrographic observations. Even though multiple, thin, sheet intrusions are insignificant for conductive heating on a basin-scale, methane generation by sheet intrusions was volumetrically important in the Raton Basin because sills preferentially intrude coal beds in this region.