Volcanic stratigraphy of the Pilot Knob iron deposits, Iron County, Missouri.

The Pilot Knob area of SE Missouri contains two iron deposits, a subsurface magnetite deposit and a surface deposit of hematite. Mapping and petrographic study has led to the construction of a volcanic stratigraphy for the area and the placement of these two iron deposits within the stratigraphic sequence. The bulk of the subsurface magnetite is a replacement of what originally was a simple cooling unit of an ash-flow tuff. Mineralization was controlled by the porosity and permeability within the cooling unit. A different type of magnetite ore lying at the base of the replacement ore zone is interpreted as possibly being a magmatic injection of this material. About 240 m of volcanic material separates the magnetite from the surface hematite. The hematite is itself of replacement origin, and is hosted by a laminated tuff which retains sedimentary features suggestive of a shallow water depositional environment. Knowledge of the volcanic stratigraphy has also clarified some of the structural features in the area.-J.E.S.