TEXTURAL AND MINERALOGICAL INDICATIONS OF A MAGMATIC ORIGIN OF THE PILOT KNOB MAGNETITE DEPOSIT, MISSOURI

Thin & polished sections from eighteen core holes through the Pilot Knob magnetite deposit are providing textural & mineralogical indications that the deposit is dominantly of igneous intrusive origin with a hydrothermal overprint of late veins and breccia fillings plus an envelope of adjacent host rocks impregnated with magnetite. Indications that bulk of the deposit resulted from an ore magma intrusion are: Albite (An2-An5) is the dominant matrix mineral, with quartz and K-feldspar next abundant. Some of the albite is trachytic. Magnetite syenite is the most common petrologic name for the ore. Magnetite shows porphyritic textures with some phenocrysts zoned. Magnetite and matrix minerals have typical igneous textures, including triple-junction texture, indicating equilibrium between magnetite, hematite, and silicates. Some of the magnetite ores are finely layered, interpreted to be of rhythmic gravitational origin. Some of the plagioclase has an intercumulus poikilitic texture, containing included magnetite. Dendritic chains of magnetite crystals exist. The orebody is finer grained near its contact with the host rocks, a magmatic chill zone. Calcite is a common matrix mineral; where it is dominant in small segregations within the magnetite syenites, the rock is considered to be a magnetite carbonatite. Where calcite is present as a minor matrix constituent, the rock is considered to have carbonatite affinities. Calcite is also common within sharply-bounded, late cross-cutting veins. Hematite is present as minor late alteration of magnetite & as specular hematite within late hydrothermal veins or breccia fill. However, a minor part of the ore is composed mainly of hematite, with textures indicating that it is magmatic in origin. 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. New microprobe data is adding new information about the orebody. Ferriallanite is present with the main REE's being cerium and lanthanum. The garnet has been found to be two species, andradite and almandine/spessartite. Calcite inclusions within the cores of zoned magnetite crystals prove calcite in the original melt. Tennantite and galena have been added to a list of sulfides.