The effects of K-metasomatism on the mineralogy and geochemistry of silicic ignimbrites near Socorro, New Mexico

K-metasomatism of the upper Lemitar and Wells Mesa silicic ignimbrites near Socorro, New Mexico is thought to be the result of downward percolation of alkaline, saline brines in a hydrologically closed basin [Chapin, C.E., Li ndley, J.I., 1986. Potassium metasomatism of igneous and sedimentary rocks in detachment terranes and other sedimentary basins: economic implications. Arizona Geological Society Digest, XVI, 118-126.]. During the chemical cha nges associated with metasomatism, Na-rich phases, primarily plagioclase, a re replaced by secondary mineral phases. Adularia, a low-temperature K-feld spar, is the dominant mineral formed during K-metasomatic alteration, but m ixed-layer I/S, discrete smectite, and kaolinite can also be present in the assemblage. The formation of discrete smectite within the assemblage durin g K-metasomatism may have occurred during periods of low cation/H+ ratios i n the alkaline, saline brine. Upon an increase in the cation/H+ ratio, such as could occur during evaporation of the alkaline lake, the solution may h ave become sufficiently concentrated to cause illitization of smectite resu lting in the formation of mixed-layer I/S within the assemblage. Distributi on of phases in the alteration assemblage strongly suggests a dissolution-p recipitation reaction for K-metasomatism in the Socorro area as indicated b y the presence of dissolution embayments in plagioclase crystals, the prese nce of euhedral adularia, and the common occurrence of authigenic clay mine rals in the assemblage. K-metasomatism causes significant chemical modifica tion of the silicic ignimbrite, particularly increases in K2O and Rb and de pletion of Na2O and Sr with increasing adularia abundance. The correlation between Rb and K2O suggests that Rb is enriched during alteration due to su bstitution for K in adularia. The effect of hydrothermal activity, either p rior to, or following metasomatism, is also observed in some samples, as sh own by high concentrations of elements such as Ba, As, Sb, Pb and Cs. Enrichment of middle and HREE in the upper Lemitar Tuff samples and depleti on of middle and HREE in Wells Mesa Tuff samples suggests attempted re-equi libration between the secondary alteration assemblage and the metasomatizin g fluid. Preliminary data indicate clay minerals within the secondary assem blage may have played an important role in the incorporation of REE during redistribution. (C) 2000 Elsevier Science B.V. All rights reserved.