GEOCHEMISTRY OF THE HEPWORTH CARBONIFEROUS SEDIMENT SEQUENCE AND ORIGIN OF THE DIAGENETIC IRON MINERALS AND CONCRETIONS

The major element geochemistry of a mudstone sequence of Westphalian age has been studied in detail and mineralogical distributions calculated. Quartz and zircon trends indicate the energy of the depositional environment and this is closely reflected in the clay mineralogical composition. Of the detrital clays kaolinite is concentrated in the finer-grained sediments relative to illite. Chlorite and interstratified illite-smectite probably underwent diagenetic modification and small amounts of authigenic clays may have formed. Siderite, which occurs in both concretionary and disseminated form, and pyrite developed during diagenesis under reducing conditions and account for most of the iron present. Pyrite grew first deriving its iron partly from detrital clays. Carbon isotopic evidence suggests that early siderite precipitated in pore space as nodules and later in dispersed form. A second concretionary phase, pistomesite, grew during late dewatering. Siderite iron was derived mainly from Mn-rich oxidate phases. Titanium occurs as anatase uniformly distributed through the sequence. Phosphorus is mainly in diagenetic apatite and the sodium potassium ratio is controlled by the micas. Adsorbed water is closely related to grain size and clay content. © 1979.