GEOTHERMAL EXPLORATION OF THE WINSTON GRABEN, CENTRAL NEW MEXICO, USA

The geothermal potential of the Winston graben (WG) is evaluated using well temperature logs, gravity and magnetic data, water chemistry, and water table measurements. The WG is a late Cenozoic rift basin, part of the Rio Grande rift, which is bordered by the Black Range (BR) to the west and is separated from the Rio Grande valley by the Sierra Cuchillo (SC), a horst block composed of Proterozoic-Paleozoic rocks intruded by an Oligocene laccolith. Two low-temperature warm springs, Ojo Caliente and Chise, indicate that hot water may be present at depth.

Geothermal gradients from wells range from 20°C/km to 60°C/km, with higher gradients located on the eastern side of the SC horst, coincident with both warm springs. The Chise warm spring occurs at the intersection of the eastern boundary fault of the SC and a NE-striking accommodation zone called the Chise lineament. Gravity highs and magnetic anomalies coincide with the Morenci and Chise volcanic lineaments along the northern and southern ends of the graben.

Water chemistry data from recharge (BR) and discharge areas (Chise warm spring) show a trend of calcium to sodium and carbonate to chloride-dominated waters. Water table data suggest both along axis and cross axis flow direction within the basin. This chemical trend may be the result of ion exchange along a deep, long-lived west to east flowpath. Carbon 14 dating at Ojo Caliente yielded groundwater ages in excess of 13 ka. Mineral solubility simulations (carbonate and silicate) indicate that chemistry from the Chise and Ojo Caliente warm springs were in thermal equilibrium at temperatures between 70 and 100°C.

Because the temperature anomalies trend east-west and water table gradients trend both east-west and north-south, a two component hydrogeologic system is proposed. The deeper system consists of groundwater flowing from the BR eastward below the WG, where the water is heated by high background heat flow. The warm water continues to flow eastward through the SC, emerging within and to the east of the horst.