Alteration of magnetite-ulvoespinel to turbid sphene, and olivine to Ia chlorite in differentiated Precambrian basic intrusive rocks of southeastern Missouri involves significant mobilization of Fe as well as other elements. This type of alteration also causes mineralogic and chemical changes in the rocks that affect the density, magnetic susceptibility and possibly the thermo-remanent magnetic properties of basic rocks. Because the sphene that replaced magnetite-ulvoespinel may be virtually opaque in thin sections, it may be overlooked. This paper describes fresh and altered basic rocks and suggests the possibility of large scale Fe mobilization and concentration to form ore deposits of magnetite and/or hematite.

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