Nelia W. Dunbar, PhD., Charles E. Chapin, PhD., New Mexico Bureau of Geology and Mineral Resources

The As content of rocks in many geological settings can be enriched by secondary processes, most commonly by alteration related to a hydrothermal fluid. These fluids can either related to magmatic systems, or to some types of ore deposition common in the western US. The enrichment factor for arsenic in a magmatic vapor phase (Asvapor/reference elementvapor)/(Asmelt/reference elementmelt) is typically between 103 and 106, and As is consistently among the most enriched elements in these systems, resulting in As contents of altered rocks of up to 100's or 1000's of ppm. Lower temperature fluids from playa alkaline-saline brines also can result in As enrichment in rocks, but to a lesser extent than higher-temperature fluids. Arsenic can be remobilized by groundwater passing through the altered rock.

Arsenic may provide a natural tracer in surface and groundwater hydrology. The As content of the Rio Grande in northern New Mexico is low (averaging 2 ppb). But, dissolved As increases as the river flows along the eastern edge of the Datil- Mogollon volcanic field (averages between 4 to 5.3 ppb). The high As values, plus the lack of perennial tributaries in this reach, suggest substantial augmentation by groundwater. Arsenic concentrations in the Snake River are also elevated by flowing through silicic volcanic terrains in southwestern Idaho. In contrast, arsenic concentrations in the Colorado River remain low where the Colorado flows through volcanic rocks between Las Vegas, Nevada, and Yuma, Arizona, because of the lack of groundwater input in this reach.

CORRESPONDING AUTHOR: Nelia W. Dunbar, PhD., New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, NM, 87801, USA. (575) 835-5783