DUNBAR, N.W., CHAPIN, C.E., and ENNIS, D.J., N.M. Bureau of Mines and Mineral Resources, New Mexico Tech, Socorro, NM, 87801 (

The As content of rocks in a large area around Socorro, New Mexico, is elevated as compared to normal crustal rocks. A set of more than 100 samples analysed for As reveals a large area with an average level of As abundance of 5-10 ppm in silicic and mafic rocks as compared to 1-2 ppm in the same rock units further from the Socorro area. This area of As enrichment coincides with the area of marked K enrichment in rocks by the process of K- metasomatism, which is interpreted to be the result of alteration by alkaline-saline brines that were associated with a playa system in the Socorro area from approximately 15 to 7 Ma. Superimposed on this low level of As enrichment due to K-metasomatism are localized areas of very high As enrichment (up to 220 ppm As in whole rock). These areas coincide with localities of hydrothermal mineralization, either Mn or precious metals; samples also show strong enrichments of other element such as Sb, Pb, and Zn. The local enrichment of this set of elements suggests that the high level of As enrichment is related to a high-temperature fluid associated with a magmatically related hydrothermal system.

Chemical analysis of mineral separates from altered rocks indicates that the As content of bulk rock is, in general, higher than that in quartz, potassic feldspar and/or clay minerals. Arsenic is therefore interpreted to be largely adsorbed to mineral phases in the rock, rather than being bound into a mineral structure, and the higher As content of bulk rock as compared to quartz/feldspar/clay mineral separates suggest that the As is preferentially adsorbed onto mafic oxide phases. The adsorbed state of the As would allow mobilization of As by a secondary fluid phase passing through an altered rock. The high As content (47 ppb) of the warm springs at Socorro is an example of groundwater leaching As as it circulates through K- metasomatized and hydrothermally altered rocks.

Arsenic may provide a natural tracer of use in both surface and groundwater hydrology. The As content of the Rio Grande and its tributaries in northern New Mexico is low (averaging 2 ppb). But, dissolved As in the Rio Grande 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 that the Rio Grande must be substantially augmented by groundwater. Arsenic concentrations in the Snake River are elevated in a similar fashion where the Snake flows through rhyolitic terrains in southwestern Idaho. In contrast, arsenic concentrations in the Colorado River remain at low levels where the Colorado flows through rhyolitic and K-metasomatised rocks between Las Vegas, Nevada, and Yuma, Arizona, because of the lack of groundwater input in this reach.

The sorption properties of As onto Fe and Mn oxide phases in oxidizing surface waters provide a mechanism for removing dissolved As from river water. The dissolved As in large rivers seldom exceeds 6 ppb, and dissolved As decreases after passing through a rhyolitic terrain. Dissolved As contents of small tributary streams, however, may be very high, such as Jemez Creek (28-66 ppb), which drains the Jemez volcanic field. A plume of higher temperature and high As groundwater extends southward from the Jemez Mountains to beyond Albuquerque, and helps to delineate groundwater flow and mixing paths in the northern Albuquerque Basin.

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