Intrusion of diabase into, and alteration of, magnetic taconite (chert+magnetite±silicate±carbonate iron-formation) negatively impacts iron-ore quality at a Minnesota mine. Ore contamination by diabase results in elevated concentrate silica content, while altered ore is characterized by elevated silica liberation indices and concentrate Fe+3/Fe+2. Paradoxically, ore adjacent to the dikes also exhibits enhanced Fe-oxide weight recovery. Here, I address the mineralogical, geochemical, and textural characteristics of alteration- and diabase contamination-induced ore degradation. In addition to direct contamination by diabase, three sequential alteration events affect ore quality.

The diabase is a highly evolved Fe-tholeiite containing abundant acicular Fe-Ti oxides. These oxides are sufficiently magnetic to be retained by magnetic separators during ore processing; however, associated bound silica is not satisfactorily removed until ground to a much finer size than iron-formation magnetite. Consequently, ore contamination by diabase results in elevated concentrate silica.

The first alteration event to affect ore quality was thermal metamorphism associated with dike emplacement, which formed secondary Fe-oxides in adjacent iron-formation. Background values of ~5 vol% Fe-oxide content in granular layers of cherty iron-formation increase to ~30 vol% adjacent to the diabase, causing observed high weight recovery and silica liberation indices of altered ore. The second alteration event was a post-intrusion hydrothermal fluid focused on dike margins that altered both diabase and adjacent iron-formation. Diabase alteration is characterized by extreme leaching of Na+Ca+Mg+Ba+Sr, producing the assemblage chlorite+quartz+residual Fe-Ti oxides. Iron-formation wallrock was similarly leached, resulting in isovolumetric destruction of Fe-silicates and carbonates, leaving residual chert+magnetite+void space. The final alteration event affecting the iron-formation was supergene oxidation by meteoric fluids. Fe-silicates and carbonates were altered to goethite, and magnetite to hematite. High-porosity hydrothermally altered iron-formation allowed oxidizing fluids to penetrate deeper adjacent to the dikes, extensively altering magentite to hematite, producing elevated Fe+3/Fe+2 ratios.