Volcanic sandstones of the Fremouw and Falla formations, like many volcanic sandstones in productive and nonproductive circum-Pacific basins, contain abundant zeolite-facies authigenic minerals. Mineral and chemical patterns in Fremouw and Falla sandstone suggest that porosity and authigenic mineralogy were controlled by parent material composition, fluid chemistry, permeability, and temperature. Mineral patterns suggest simple rock-fluid reactions in which unstable volcanic rock fragments and plagioclase grains were altered to clay, heulandite, albite, laumontite, and/or prehnite. Chemical patterns suggest that significant mass transfer of Na⁺, Ca⁺², and Si⁺⁴ occurred in the sandstone during alteration, whereas Al⁺³ mobility wa restricted to migration from reaction sites to nearby pore space where Al⁺³ was incorporated into clay and/or zeolite cements. These cements typically reduced primary porosity to a few percent; however, some secondary porosity was created by the dissolution of detrital plagioclase grains. An examination of albitization and laumontization reactions involving detrital plagioclase suggests that porosity loss resulting from these reactions is directly proportional to the anorthite content of the grains undergoing alteration. Chemical and mineral patterns also suggest that clay mineral diagenesis in mudstone units supplied at least some of the ions required by reactions in neighboring sandstone.

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