1. New Mexico Bureau of Geology and Mineral Resources
2. Geoscience Department
New Mexico Institute of Mining and Technology
Socorro, NM, 87801
Silicic ignimbrites (ash-flow tuffs) and interbedded mafic lavas of Oligocene age were altered by alkaline, saline brines beneath the Popotosa basin of the Rio Grande rift near Socorro, New Mexico. This alteration caused a variety of chemical and mineralogical changes in the affected rocks. The characteristic alteration mineralogy in highly metasomatized rock is adularia + hematite quartz clay minerals (Lindley, 1985), but we have determined, based on detailed mineralogical analysis of altered plagioclase, that the ratio of phases is highly variable, possibly as a function of degree of metasomatism. The relative proportion of adularia compared to clay minerals, and the composition of clay minerals is variable within the metasomatized rocks, possibly resulting from variations in fluid composition and temperature. During metasomatism, elements such as K, As, Rb, Sb, Ag, Cs, Au, Pb, Th, and U were enriched in the altered rocks, whereas Na, Mg, Ca, and Sr were depleted. Some trace elements, rather than varying smoothly with degree of rock alteration (estimated by K/Na ratio), change dramatically during either low or high degrees of alteration. These data suggest that the trace-element signature of alteration may be dependent on the mineralogical alteration assemblage, which may, in turn, be dependent on the composition of fluids which cause alteration. Dating of the metasomatic event by 40Ar/39Ar suggests that the alteration was active between 7.4 and 8.9 Ma. The estimated age range for deposition of playa clays suggests that metasomatism may have begun as early as 15 Ma ago, hence the metasomatism may have been active for many millions of years. The long duration of the metasomatic event and implied long time available for elements to be progressively scavenged from surrounding rocks may explain the lack of a distinctly depleted source area for elements enriched by the metasomatic alteration.
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