Abstract

Heulandite, clinoptilolite, mordenite, analcime, thomsonite(?), erionite, and chabazite(?) occur with other authigenic minerals in the thick Miocene volcaniclastic sequence of the southern Desatoya Mountains, Nevada. The mineral assemblages formed diagenetically in an open system probably at depths of 2,500 m or less and in a variety of rock types, including rhyolite, quartz latite, dacite, latite, and andesite. The authigenic minerals developed mainly through the breakdown of the inherently unstable glassy components, and the original porosity and permeability of the rocks were major factors controlling the degree of alteration. Determination of the temperatures of diagenesis depends greatly on ascertaining the time of diagenesis and, thus, the geothermal gradient at that time.

The paragenetic sequence of events between primary and secondary materials and among authigenic mineral species is (1) partial replacement of glass by clay minerals, (2) solution of remaining glass, (3) precipitation of heulandite and/or clinoptilolite (± silica) and/or other authigenic minerals, and (4) deposition of opal or chalcedony in cavities or veinlets.

Twenty-seven electron microprobe analyses are given for single crystals and crystal clusters of heulandite, clinoptilolite, and thomsonite(?).

Chemical comparisons between (1) petrographically identified glass and its adjacent alteration products and (2) unaltered and/or partially altered and completely altered whole rocks from the same unit indicate that Si, Ca, Na, K, and H2O were mobile components. It is inferred from petrographic observations that Fe and Mg also were mobile. Na, and possibly some K, apparently were lost during diagenesis. Mineral development and chemical changes seem to have followed the general pattern for leaching in an open system.

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