Abstract

Sandstone, mudstone and tuff of the Triassic Fremouw and Falla formations display diagenetic and zeolite-facies mineral assemblages that were controlled by parent material composition, fluid chemistry, permeability and temperature. Diagenetic reactions between sediments and ground water at near-surface temperature and pressure resulted in formation of smectite, chlorite, quartz, heulandite, mordenite and analcime cements and grain replacements. Zeolite-facies alterations, which resulted from simple rock-fluid reactions driven by heat from Jurassic diabase intrusions, converted smectite to illite-rich ordered mixed-layer illite/smectite and produced albite, laumontite, prehnite, epidote and chlorite cements and grain replacements. Organic maturation data suggest that, away from local effects of the intrusions, temperatures may have been as low as 130 degrees to 160 degrees C, whereas stratigraphic evidence suggests maximum pressures of about 750 bars. Chemical patterns in sandstone near intrusions indicate that, although diabase supplied heat to drive rock-fluid reactions, it did not contribute a significant amount of cations to the hydrochemical system. Mineral and chemical patterns, together with hypothesized mineral reactions, suggest that significant mass transfer of Na (super +) , Ca (super +2) and Si (super +4) occurred during alteration, whereas Al (super +3) mobility was restricted to migration from the reaction site to nearby pore space where Al (super +3) was incorporated into clay and/or zeolite cements.

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