Abstract

Zeolites (analcime, clinoptilolite, and phillipsite) and K-feldspar of authigenic origin form a stratigraphically zoned succession in the 9065-foot sequence of the Esmeralda Formation (Miocene and Pliocene) in southern Big Smoky Valley, Esmeralda County, Nevada. Analcime and K-feldspar are confined to a 500-foot zone near the base of the sequence. Clinoptilolite, usually associated with opal, occurs within a 7700-foot zone above the analcime- K-feldspar bearing beds, and phillipsite is present in a thin zone at the top of the sequence. Clinoptilolite and phillipsite occur primarily as pseudomorphic replacements of vitric material, whereas analcime generally occurs as subhedral crystals that are thought to have formed directly from vitric material. K-feldspar is present predominantly as overgrowths on detrital grains of orthoclase and plagioclase.

The zeolites and K-feldspar formed after burial under the influence of groundwater solutions, and the different zones are thought to reflect primarily differences in the composition of the subsurface water in contact with the rocks at the time of alteration. The alkalinities and salinities necessary for the formation of the zeolites and K-feldspar are attributed to the solution and hydrolysis of vitric material in the altered rocks.

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