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. The characteristic alteration mineralogy in highly metasomatized rock is adularia + hematite quartz clay whereas in rocks which may not be strongly metasomatized the assemblage is dominated by clays and zeolites. Dating by 40Ar/39Ar suggests that the alteration occurred between 8 and 10 Ma. Elements such as K, As, Rb, Sb, Ag, Cs, Au, Pb, Th, and U are enriched in the altered rocks, whereas Na, Mg, Ca, and Sr are depleted. Some trace elements, rather than varying smoothly with degree of rock alterations (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.
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