Clay-mineral authigenesis in the Late Permian coal measures, Bowen Basin, Queensland, Australia

Mineralogical studies were performed on authigenic clay minerals of mudrock s, sandstones, and bentonites from 38 boreholes in the Late Permian coal me asures of the Bowen Basin. Clay-mineral separations of samples from the nor thern Bowen Basin consist mainly of (Reichweite, R) R = 1 and R greater tha n or equal to 3 interstratified illite-smectite (I-S), chlorite, and kaolin ite. In the southern Bowen Basin, samples from higher stratigraphic section s are characterized by randomly ordered (R = 0) I-S mixed layers, and kaoli nite and chlorite in smaller amounts. Samples from the lower sections consi st of (R greater than or equal to 3) I-S, chlorite, chlorite-rich chlorite- smectite (C-S), and laumontite. Examination of the mineralogy and distribution of authigenic clay minerals from the Late Permian coal measures in the northern part of the Bowen Basin indicated that the presence of clay minerals is not systematically related to depth and clay occurrences do not occur regularly. These mineralogical variations of clay in volcaniclastic sediments are incompatible with therma l control. Variations in the rate of fluid flow and potassium supply owing to permeability exert major influences on clay-mineral paragenesis and the reaction of illitization. In more permeable zones (possibly faults or fract ure zones), highly illitic clays with lath-shaped morphologies may have pre cipitated directly from potassium-rich fluids migrating from deeper parts o f the basin. In addition, abundant chlorite precipitated contemporaneously with illitic clays, which may have resulted from sufficient magnesium and i ron occurring in the fluids as a result of dissolution of intermediate or m afic-rock fragments. At the same time, clay paragenesis with less illitic I -S, kaolinite, and minor chlorite occurs outside the channelized zones of h igh fluid flow, where a diffusive-flow regime may have predominated with lo wer ratios of the activities of K+ and H+ (i.e., alpha(K+)/alpha(H+)) in th e solutions. In the southern Bowen Basin, depth-related changes in the distribution of c lay minerals are evident and may be indicative of thermal control on clay-m ineral reactions. Zeolites are present locally in the Late Permian volcanic lastic rocks in the southern Bowen Basin, but not in the north. This is att ributed to a low ratio of alpha(CO2)/alpha(H2O) (where alpha = activity) an d/or more saline and alkaline solutions.