Depositional and Diagenetic C-S-Fe Signatures and the Potential of Shales to Generate Metal-Rich Fluids

Abstract

The potential of shales to react with dissolved sulfides introduced during late diagenesis or catagenesis is constrained initially by source area effects (which dictate total iron content) and is subsequently diminished by the removal of iron to form pyrite in early diagenetic sulfate reduction. The efficiency of iron removal during sulfate reduction can be measured by the degree of pyritization (DOP), which represents the ratio of pyrite Fe to pyrite Fe plus HCl-soluble Fe. Acid extraction conditions are chosen such that iron mineral reactivity towards HC1 is similar to that towards H2S during early diagenesis. DOP values so defined demonstrate that the efficiency of reactive iron removal is mainly dependent on depositional environment. Reactive iron removal in marine shales is minimized where deposition occurs from fully oxygenated bottom waters. Here, low DOP values (less than 0.4) result because organic carbon, remaining after benthic activity, is poorly metabolized by sulfatereducing bacteria, and hence pyrite formation is limited. Still lower DOP values may occur where shales have been rapidly deposited and reworked. In freshwater environments low DOP values also result because the availability of dissolved sulfate limits sulfate reduction and hence also pyrite formation.