2008 Joint Annual Meeting (5-9 Oct. 2008): Mineralogy, Textures and Origin of the Pilot Knob Magnetite Deposit, St. Francois Mountains Proterozoic Terrane, Missouri

Study of thin sections and polished sections from eighteen core holes through the Pilot Knob magnetite deposit are providing textural and mineralogical details for this deposit. Previous workers have suggested that the deposit originated by either igneous intrusion, igneous extrusion, or hydrothermal replacement. Our work suggests that the deposit is dominantly of igneous intrusive origin with a minor, late hydrothermal overprint. Textural and mineralogical indications of the deposit having resulted from an ore magma intrusion are as follows:

•Albite plagioclase (An2-An5) is the dominant matrix mineral, with quartz and K-feldspar next most abundant.

•Albite plagioclase being the dominant matrix mineral, plus the fine-grained nature of the ores, makes magnetite microsyenite the most common petrologic name for the ore.

•Magnetite shows porphyritic textures and some phenocrysts are zoned.

•Most of the magnetite and matrix minerals have typical igneous textures.

•Some of the magnetite ores are finely layered which is interpreted to be of rhythmic gravitational origin.

•Some of the plagioclase has an intercumulus poikilitic texture, containing included magnetite.

•Calcite is a common matrix mineral and where it is dominant in small segregations within the magnetite microsyenites, the rock is considered to be a magnetite carbonatite. Where calcite is an important but not dominant matrix constituent, the rock is considered to have carbonatite affinities. Calcite is also a common constituent of sharply-bounded, late cross-cutting veins.

•Hematite is present within the deposit, usually as minor late-stage alteration of magnetite or as specular hematite within late-stage hydrothermal veins or breccia fill. However, a minor part of the ore is composed dominantly of hematite, with textures indicating that it is primary igneous hematite.

•Brecciation is also an important ore feature, with fragments both of host rock volcanics, and pieces of the magnetite ore itself. It is suggested that most of the breccias are intrusion-related autobreccias.