Mineralogy and K-Ar geochronology of mixed-layered illite/smectite from The Geysers coring project, California, USA

Clay fractions for K Ar dating, separated from Franciscan-Assemblage (late Mesozoic) metagraywacke and argillite from The Greysers Coring Project core hole SB-15-D, and a "background" outcrop consist of illite. mixed-layer ill ite/smectite. mixed-layer chlorite/smectite, and chlorite with minor calcit e and quartz. Except for the chlorite/smectite, these phases were formed du ring both subduction-related Franciscan regional metamorphism and late Ceno zoic (post-1.1 Ma) hydrothermal alteration and mineralization related to Th e Geysers hydrothermal system. The chlorite/smectite is exclusively hydroth ermal in origin. In spite of careful efforts to physically separate the met amorphic and layer silicates: prior to analysis. all samples contained vari able contributions from both sources. The proportion of illite plus illite/ smectite clays in the < 5 <mu>m fractions of the samples ranges widely from about 41 to 97 wt.%. K2O contents of the dated clay fractions (less than o r equal to 3 mum) also ranged widely, from 1.60 to 8.0 wt.%. Despite having been heated to at least 300 C. matrix illite and illite;smectite do not sh ow depth- and temperature-dependent trends of downward-increasing illite in terlayers as is the case in many old sedimentary basins (ef Texas Gulf Coas t and North Sea oil fields) and other geothermal systems (e.g. Valles calde ra and Salton Sea). Fourteen mixed-layer illite. smectites of different siz e fractions (0.1-0.35. 0.35 1 and 1-3 mum) from the SB-15-D meta-graywackes and argillites land a nearby "background" sample of the same rock type) yi elded K-Ar dates ranging widely from 105.5 Ma (the outcrop sample) to 1.5 M a. Although there is: a general decrease in apparent ape with depth, most o f the clays yielded middle Tertiary dates (35.4-19.4 Ma). The outcrop age i s consistent with published data for Franciscan regional metamorphism. The youngest of the core dates, 2.3 and 1.5 Ma, are slightly older than the ear liest Geysers felsite intrusions (1.1 Ma), and substantially older than vei n aduluria from the corehole (0.57 Ma). Thus, even these youngest illite/sm ectite dates for the SB-15-D cores likely reflect minor contamination from metamorphic phases. These K-Ar dates do, however, indicate that pre-existin g detrital and metamorphic clays have not been totally degassed even though the host rocks have resided in a high-temperature geothermal system for mo re than a million years. We conclude that the partially reset metamorphic-h ydrothermal illite/smectite dates probably reflect the effects of low host- rock permeabilities. of sluggish reaction kinetics of the metamorphic layer silicates, and of "fines migration" of Franciscan Complex clays, along fra ctures, induced by hydrothermal fluid circulation. Published by Elsevier Sc ience Ltd on behalf of CNR.