LOW-TEMPERATURE DIAGENETIC-METAMORPHIC AND MAGMATIC CONTRIBUTIONS OF EXTERNAL CO2 GAS TO A SHALLOW GROUND-WATER SYSTEM

A small (<15 km(2)), low temperature (<20 degrees C), CO2 gas overpres sured, gently dipping Paleozoic carbonate aquifer rests on Precambrian granite in a narrow canyon along the eastern edge of the Rocky Mounta in Front Range, Colorado. The carbonate aquifer is bounded on three si des by granite and in the down dip direction by the Front Range fault. The fault, in a major continental intraplate zone of weakness along w hich magmatic fluids and gases could migrate to the surface and near s urface from great depths. The fault has also overthrusted slices of ca rbonate and elastic rocks several kilometres below the granitic baseme nt. Stable isotopic (delta(2)H and delta(18)O) and discharge temperatu re 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 (approximate to 650 m). Elevated CO2 and the delta(1 3)C Of HCO3- in the carbonate aquifer suggest an external crustal sour ce of CO2 gas. He-3/He-4, O-2/N-2 and Ar/N-2 gas ratios indicate gas c ontributions from both magmatic and atmospheric sources. Atmosphere co ntributions account for about 25% of the exsolving gas, whereas magmat ic CO2 accounts for 7 to 14%. Possible external CO2 sources, which are consistent with the mean HCO3- delta(13)C = -2.4 parts per thousand ( PDB), are clay-carbonate mineral diagenesis or low temperature metamor phism of siliceous-carbonate rocks that have been overthrust by 3 to 6 km of granite. Diagenetic or metamorphic CO2, mixed with some magmati c 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 atmosphe ric gas. (C) 1997 Elsevier Science B.V.