Geology and hydrogeochemistry of the Jungapeo CO_2-rich thermal springs, State of Michoacan, Mexico

摘要

We present the first geothermal assessment of the Jungapeo CO_2-rich mineral springs, which are located in the eastern part of Michoacan State (central Mexico) at the southern limit of the Trans-Mexican Volcanic Belt. All but one of the > 10 springs occur at the lower contact of the distal olivine-bearing basaltic andesite lavas of the Tuxpan shield, a 0.49- to 0.60-Ma-old cluster of monogenetic scoria cones and lava flows. The Tuxpan shield has a maximum radius of 6 km and was constructed on top of a folded and faulted Cretaceous basement consisting largely of marine limestones, marls, and shales. The mineral waters are characterized by moderate temperatures (28 to 32℃), mild acidity (pH from 5.5 to 6.5), relatively high discharge rates, effervescence of CO_2 gas, clarity at emergence and abundant subsequent precipitation of hydrous iron, silica oxides, and carbonates around pool margins and issuing streamlets. Chemical and isotopic (deuterium, oxygen, and tritium) analyses of water and gas samples obtained during the period 1991-1997 indicate that the springs are largely composed of meteoric water from a local source with relatively short residence times (water ages of 7 to 25 years). Spring waters are chemically characterized by moderate SiO_2, Ca+Mg nearly equal to Na+K, high HCO_3, moderate to low Cl, low F and SO_4, high B, moderate Li, while Br and As are low. In contrast, Fe+Mn is exceptionally high. Thus, the Jungapeo waters cannot be regarded as high-temperature geothermal fluids. Instead, they resemble soda spring waters similar to other low-to-medium temperature soda waters in the world. Gas samples are extremely rich in CO_2 with no detectable geothermal H_2S or H_2 and very low contents of CH_4 and NH_3, indicating the gases are not derived from a high-temperature resource. Carbon-13 analyses of CO_2 show a narrow range (-6.7 per thousand and -7.2 per thousand) that falls within the range for MORB CO_2. Thus, most CO_2 seems to originate from the mantle but some CO_2 could originate from thermal degradation of organic remains in underlying Cretaceous rocks. ~(3/4)He ratios range from about 2 to 3 Rc/Ra, indicating that a small mantle/magmatic He component is present in the gases. In conclusion, the mineral waters are the surface expression of a low-temperature geothermal system of limited size that originates from the combined effects of a high regional heat flow and (possibly) the remnant heat released from subjacent basaltic andesite magma bodies that constitute the root zone of the Tuxpan shield.