Circular 124—Ground-water characteristics in a recharge area, Magdalena Mountains, Socorro County, New Mexico

By W. K. Summers, G. E. Schwab, and L. A. Brandvold, 1972, 18 pp., 6 tables, 13 figs., 1 appendix.

Water samples yield new information on chemistry and distribution. The Magdalena Mountains are a horst block in the Basin and Range Province. Silicic Tertiary volcanics overlie sedimentary rocks of Mississippian, Pennsylvanian, and Permian ages and metamorphic rocks of Precambrian age. Although many arroyos drain the mountains, none contains a permanent stream.

Springs occur at local permeability barriers in the canyons and arroyos, and my be intermittent. Temperatures of discharging ground water, after discounting variations created by ambient air temperatures and solar radiation, increase with diminishing altitude at a rate of 0.46ºC/100 ft. Field pH ranged from 6.4 to 7.8, with one sample collected from a stock tank having a pH of 8.7. Laboratory pH ranged from 7.2 to 8.8. Calcium, bicarbonate, and total dissolved solids concentrations diminished with time in several samples. Over a period of about 100 days the bicarbonate concentration in each of nine samples diminished more than 100 ppm.

Analyses were made for fluorite, nitrate, lithium, rubidium, strontium, barium, chromium, copper, zinc, iron, and manganese. Except for fluoride and strontium, most concentrations were near or below the limit of detectability. The distribution of sample locations and the low solubility of metallic ions somewhat limit the efficacy of trace element analysis to delineate areas for subsequent metals exploration. However, one anomalous copper value occurred in a sample taken from a spring near a known copper deposit, and anomalous zinc values suggest a new area to prospect. Lithium concentration varied with lithology. Concentrations of the other trace elements were not particularly significant.

The chemical character of ground water depends upon where the water is in the ground-water flow system and on the chemical composition of the rocks through which the water has passed. Relatively few studies of ground-water chemistry have been made in regions where the recharge area could be delineated with precision or in areas where silicic igneous rock predominates.

Mineral explorationists in New Mexico have indicated that chemical analyses of ground-water samples have become a geochemical prospecting tool. Yet very little information is available about trace element distribution in ground water in New Mexico. This circular reports the procedures and results of a project initiated (1) to learn more about the general chemistry of ground water in a mountainous recharge area and (2) to determine whether trace element concentrations in the ground water would correlate with known ore deposits, or would suggest other areas to prospect.

The Magdalena Mountains in south-central New Mexico cover an area of approximately 250 mi2 and range in altitude from about 6,000 ft to 10,783 ft. Annual precipitation on the mountains ranges from 13.4 inches at 7,000 ft to 17.7 inches at 10,630 ft. Vegetation is sparse below 6,500 ft and above 10,000 ft. But from about 7,000 to about 9,000 ft juniper, piñon, and ponderosa pine are plentiful.