Alkaline, saline brines in a late Cenozoic rift basin near Socorro, New Mexico, caused pervasive potassium metasomatism of volcanic rocks beneath the basin floor as well as of the synrift sediments. The objectives of this project are: 1) to determine the mobility of potential ore-forming elements during K-metasomatism; 2) to determine where elements enriched in the altered rocks occur in the alteration assemblage of adularia, hematite, -_ quartz, _ clay; 3) to determine the temperature and composition of the fluids by analysis of fluid inclusions; 4) to determine the stable isotope composition of the metasomatic fluid; and 5) to determine the age and duration of the metasomatic event by 40Ar/39Ar dating of secondary K-feldspar. Preliminary analyses indicate that arsenic may be a mobile element in the oxidizing hydrologic systems of rift basins and may be a more common problem than is realized.
