| Abstract |
The understanding of the structures, thermal stability and identificationof mixed-layer clays has advanced considerably in years. studies of mixed-layer and, to a extent, illitechlorites, show well-defined sequences of increasing order towards illite and/or chlorite end member structures with increasing temperatures in hydrothermal systems or increasing depth of burial in sedimentarybasins. The and methodologiesof mixed layer clays arereviewed and applied to samples two drillholes and in the Te Mihi Sector of the Wairakei geothermal system which penetrate a sequence of tuffs, ignimbrites, rhyolite lava, breccias and lacustrine sediments. The sedimentsand breccias are over m thick in well but only comprise about m in well Within the sedimentary sequence in drillhole diffuse fluid flow occurs and there is a well-developed Sequenceof mixed-layer clays. The shallowest (146 m) downwell appearance of interlayered clay has 60%illite layers and occurs where the temperature is only Interlayered illite-chlorite occursat m depth in association with highly ordered where the prevailing temperatures are 170?C. Discrete illite is present in the finest (less than0.2 fraction only below 297 m depth (above in the finest size fraction Chlorite appears in association with illite-smectiteat 177m depth (1 The lithology in drillhole is predominantly ignimbritesand rhyolites. The fluid flow in these rocks is dominated by conduit flow. The sequence of interlayered clays is not well developed, although illite occurs at relatively shallow depths. Temperatures range 140to 226?C. The permeability within the units is quite variable. Differences between the clay minerals present in the two cases demonstrate the strong control that the type of fluid flow can have on clay formation. Under conditionsof lowpermeability and diffuse fluid flow a well defined sequence of mixedlayer claysmay develop.These are absentwhere conduitflow |