Low temperature diagenetic–metamorphic and magmatic contributions of external CO 2 gas to a shallow ground water system

Abstract

A small (<15 km 2), low temperature (<20°C), CO 2 gas overpressured, gently dipping Paleozoic carbonate aquifer rests on Precambrian granite in a narrow canyon along the eastern edge of the Rocky Mountain Front Range, Colorado. The carbonate aquifer is bounded on three sides by granite and in the down dip direction by the Front Range fault. The fault, in a major continental intraplate zone of weakness along which magmatic fluids and gases could migrate to the surface and near surface from great depths. The fault has also overthrusted slices of carbonate and clastic rocks several kilometres below the granitic basement. Stable isotopic ( δ 2H and δ 18O) and discharge temperature data suggests that carbonate aquifer ground waters are of meteoric origin and have not circulated to depths greater than the base of the carbonate aquifer (≈650 m). Elevated CO 2 and the δ 13C of HCO 3 − in the carbonate aquifer suggest an external crustal source of CO 2 gas. 3He/ 4He, O 2/N 2 and Ar/N 2 gas ratios indicate gas contributions from both magmatic and atmospheric sources. Atmosphere contributions account for about 25% of the exsolving gas, whereas magmatic CO 2 accounts for 7 to 14%. Possible external CO 2 sources, which are consistent with the mean HCO 3 − δ 13C=−2.4‰ (PDB), are clay–carbonate mineral diagenesis or low temperature metamorphism of siliceous–carbonate rocks that have been overthrust by 3 to 6 km of granite. Diagenetic or metamorphic CO 2, mixed with some magmatic gas, appears to have migrated from the source rock area upward along the Ute Pass thrust fault until it encountered the shallow carbonate aquifer ground water system where it was further diluted with atmospheric gas.