TEXTURAL AND MINERALOGICAL INDICATIONS OF AN IGNEOUS ORIGIN FOR THE PILOT KNOB MAGNETITE DEPOSIT, ST. FRANCOIS MOUNTAINS TERRANE, IRON COUNTY, MISSOURI

The Pilot Knob magnetite deposit lies under the town of Pilot Knob and was mined from 1966 to 1982. Study of thin sections and polished sections from ten newly acquired core holes through the deposit are providing textural and mineralogical details in this poorly understood deposit. Various workers have suggested that the deposit may have 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: The dominant matrix mineral is albite plagioclase with a composition of about An2-An5. Some previous workers have said that the main matrix mineral is quartz; Albite plagioclase being the dominant matrix mineral, plus the fine-grained nature of the ores, makes magnetite microsyenite the most common petrologic name for this igneous ore; Magnetite shows porphyritic textures where the phenocrysts are zoned-cores contain silicate inclusions and rims are clean. Phenocrysts themselves indicate two stages of cooling, which is corroborated by zoning; Most of the magnetite and matrix minerals have a hypidiographic granular igneous texture; Some of the magnetite ores are layered which is considered to be of rhythmic gravitational origin-the layering is on a millimeter to centimeter scale; Some of the plagioclase has an intercumulus poikilitic texture and contains 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; Brecciation is also an important ore feature. Fragments are composed both of host rock volcanics, and pieces of the magnetite ore itself. It is suggested that fragments of magnetite ore within the magnetite ore intrusion originated by autobrecciation during intrusion; Contacts between magnetite ore and host rock volcanics are very sharp.