OXIDATION-REDUCTION BEHAVIOUR PARAMETERS OF SEDIMENTARY ROCKS AND THEIR APPLICATIONS TO SEDIMENTARY FACIES ANALYSIS

The oxidation-reduction behaviour parameters of sedimentary rocks formed in different sedimentary environments have important implications for the restoration and identification of sedimentary facies. The systematic analysis of the parameters plays an important role in elucidating sedimentary environments and the controls of different oxidation-reduction rock zones on some of the sedimentary ore deposits.
Up to date, no generally recognized standards have been established for the oxidation-reduction behaviour parameters of sedimentary rocks. This paper suggests, referenced to the domestic and foreign literatures, that the oxidation-reduction behaviouss depend largely upon:(1)low-valence iron (Fe²⁺) contents and Fe³⁺/Fe²⁺ ratioes; reduced sulfur (S^2-) and S^2-/Fe²⁺ ratioes, organic carbon (Cy) and ether valence-varying elements.
In the evaluation of the reducting capacity of sedimentary rocks, only when all the reduction components are measured can the correct concepts on the reduction bchaviours of the rocks be obtained. In the 1960's, the former Russian scholars and (1965) gained the △Eh values for the oxidized sedimentary rock powder by using strong oxygenant KMnO₄ under alkaline conditions (pH=11-13). The values are considered as primary parameters for evaluating the reducting capacity of sedimentary rocks.
The Eh₇ values are another important parameters for evaluating the oxidation-reduction features of sedimentary rocks. The former Russian scholar (1975) once put forward a method to determine the Eh and pH values of sedimentary rocks by putting sedimentary rock powder into a cup, making the powder paste-like with distilled water, covering the cup with a rubber cork with determining electrodes inserted in it so as to determine Eh and pH values under sealed conditions. In order to compare these values with the unified standard (pH=7), the author proposed the formula
Eh=K(pH-7)+Eh_c (K=59 my)
to correct the determined Eh values of the rocks. In reference to the criteria for distinguishing the oxidation and reduction of ground water and the parameters cited above, new criteria for distinguishing the oxidation and reduction of sedimentary rocks are proposed in this paper.
Taking one carbonate rock-type uranium deposit(in the Carboniferous stata) and one pyrite deposit (in the Devonian strata) in southern China as examples, the present paper deals in detail with the analytical methods and significance of sedimentary facies (environments) with the aid of the study of the oxidation-reduction features of sedimentary rocks in the analysis of sedimentary facies of the two mining districts.