Arsenic as an indicator of groundwater flow

Arsenic is an element that occurs naturally to a level of several parts per million in average crustal rocks (Krauskopf, 1979; Emsley, 1991). However, local concentrations of As to much higher levels in rocks on the earth's surface can be brought about by a number of processes, particularly mobilization and redeposition of As by hydrothermal fluids.

Although As emissions from active volcanoes are not likely to be a problem in New Mexico, igneous rock bodies that cool and crystallize at depth in the crust can also emit As- rich fluids that can cause near-by host rocks to be strongly enriched in As (Papike et al., 1991; Papike, 1992). For example, dacite host rock near the hydrothermal system at the Valley of Ten Thousand Smokes in Alaska contain up to 730 ppm As (Papike et al., 1991). Arsenic deposited as a result of high-temperature hydrothermal alteration can be remobilized by a later fluid phase, and can result in local waters being As-enriched. In New Mexico, there are many areas where igneous activity may have resulted in local As-enrichments of host rocks due to circulation of As-rich fluids in hydrothermal systems that have some input from the igneous sources. For example, around the area of Socorro, NM, there are two separate areas of high- As rocks due to fossil hydrothermal systems that result in As contents of rocks between 20 and 220 ppm, and some probably considerably higher. These are associated with Mn mineralization in the Luis Lopez mining district, and also in the southern part of the Magdalena Mountains, where hydrothermal Mn mineralization has also occurred.

REFERENCES

1. CHAPIN, C.E. and DUNBAR, N.W., 1995. A regional perspective on arsenic in waters of the middle Rio Grande Basin, New Mexico. Proceedings of the 39th Annual New Mexico Water Conference. WRRI Report No. 290, p. 257-276 ABSTRACT.
2.  DUNBAR, N.W. and CHAPIN, C.E., 1995. Arsenic mobility during geological processes: Implications for tracing groundwater flow. SEGH Second International Conference on Arsenic Exposure and Health Effects, p. 50. ABSTRACT
3. DUNBAR, N.W., CHAPIN, C.E., and ENNIS, D.J., 1995. Arsenic enrichment during potassium metasomatism and hydrothermal processes in the Socorro, NM, area- Implications for tracing groundwater flow. N.M. Geol., 17, 26. ABSTRACT
4. DUNBAR, N. W., C. E. CHAPIN, D. J. ENNIS, and A. R. CAMPBELL, 1994, Trace element and mineralogical alteration associated with moderate and advanced degrees of K-metasomatism in a rift basin at Socorro, New Mexico: New Mexico Geological Society Guidebook, v. 45, p. 225-232.

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