Potash deposits and their associates in the area of the Boulby Mine, Cleveland

Abstract

Part I of, this thesis describes the results of a petrological and geochemical examination of the upper part of the Boulby Halite, Potash and Shale at Boulby aimed at contributing to our knowledge of the geological history of these rocks.

These beds may have been deposited in a basinal/ lagoonal (as defined in Chapter 8), supratidal or possibly both of these types of environments. The chemical character of the strata favours the first hypothesis.

Detrital material, of windblown or fluvial origin, was deposited on the halitel plain which evolved during the final stages of desiccation of either of the above postulations. A marked dilution of the brines occurred after deposition of the lower beds of the Boulby Shale.

Numerous diagenetic reactions occurred in these beds. In the lower rocks of the Boulby Shale carnallite was replaced br halite, leading to reprecipitation of secondary sylvite; and borate minerals grew and formed a borate nodule bed. In the upper beds of this unit anhydrite has replaced or displaced clays. The sylvite of the Boulby Potash may be secondary.

Fibrous sylvinite veins developed in the rocks from the Boulby Shale to the Upper Anhydrite during burial. The Boulby Potash was overfolded and overthrust (axes N. N. W. -S. S. E.) when it was buried at greatest depth (2200-3200 metres) towards the end of the Cretaceous and early Tertiary. Subsequently small scale swells (axes at 0240+1100) developed in the top of the Boulby Potash and monoclines (E-W axes) formed at its base.

Where relevant, these beds are compared with other evaporite deposits.

Part II describes an experimental study of the system NaCl-H₂0, at 25°C, and pressures up to 2.5 kilobars. The solubility of NaCl in water increases from 26.42 weight percent at atmospheric pressure to 27.23 weight percent at 2.5 kilobars.