A thermochemical study of the ore deposits of the Milliken mine, New Lead Belt, Missouri

The ore body at the Milliken mine is a typical Mississippi Valleytype ore deposit. Mineralization is in a dolostone and structures are subtle, but very important ore controls. The major mineralogy is simple, predominately galena, luminescent hydrothermal dolomite, sphalerite and chalcopyrite. Twenty-three additional minerals have been identified in the ores, including barite and Pb-aragonite, bot h of which have not previously been recognized in the district. Paragenetically, the ores are very complex, with four major, distinct periods of mineralization plus numerous periods of dissolution. The paragenesis provides evidence for the dynamic, evolving nature of the ore system. Galena textures, euhedral crystals to amorphous spheres, are interpreted to indicate that a wide range of supersaturation existed during mineralization. Textural evidence indicates that the reaction responsible for galena precipitation must simultaneously dissolve an equal volume of dolostone. Two other totally independent reactions cause the dissolution of dolostone and formation of hydrothermal dolomite. Two of the dissolution textures are experimentally shown to be caused by electrolytic leaching. Several important thermochemical parameters of the ore solutions indicate sulfur activity varied during mineralization from 10-22 to 10-18 and the ore fluids had a pH of at least 7. The ore is interpreted to have formed by the mixing of a hot metal-bearing brine with cooler, more dilute, sulfurous formation waters. Oil in the formation is postulated to be the original source for the sulfur --Abstract p.i