The late diagenetic conversion of pyrite to magnetite by organically complexed ferric iron

Abstract

The geochemistry of late stage diagenetic replacement of pyrite by magnetite was investigated by laboratory experiments of pyrite dissolution and magnetite precipitation involving organically complexed ferric iron. The design of the experiments is based on the hypothesized geochemical conditions for in situ diagenesis in the Pennsylvanian age Belden Formation, an organic-rich carbonate in northwestern Colorado with pyrite grains rimmed with magnetite that contains a Cretaceous paleopole position. The geochemistry is applicable to both deep and near surface reactions involving pyrite, magnetite and organic matter and bears upon the geologic interpretation of secondary chemical remanent magnetizations (CRM). Aqueous solutions of ferric-ligand complexes and pyrite were heated (60°C) for 60 days in the absence of light and at low partial pressures of oxygen. Additional experiments with each ligand were performed both in the presence of bentonite and without ferric iron (ligand only). Raising the pH to ⪖ 8.5 produced a ferrous hydroxide precipitate, and subsequent heating (90°C) resulted in the formation of magnetite. Ferric complexes of oxalate, salicylate and acetylacetonate resulted in pyrite dissolution and magnetite formation. The activity of strongly reducing ligands (e.g., catechol) or unsaturated Fe(III)-exchange capacity (particularly in experiments with bentonite or with solutions of uncomplexed ligands) can result in an effective reduction of ferric iron concentration and prevent pyrite dissolution and/or interfere with the crystallization of magnetite. The bentonite acted to adsorb dissolved species and also underwent direct dissolution, especially by the oxalate ligands. Although these reactions could occur at any depth, a continuing supply of molecular oxygen (more available in near surface settings) would result in the formation of hematite or the production of acidic sulfate conditions, whereas at greater depths (or restricted porosity), the redox gradation is halted at magnetite. The results of this study thus provide a likely geochemical mechanism for the replacement of pyrite by magnetite during the latter stages of diagenesis.