SP-02 Geology and Origin of the Iron Ore Deposits of the Zenith Mine Vermilion District, Minnesota

Abstract

The Zenith mine, at Ely in the Vermilion iron district of Minnesota, yielded about 21 million tons of high grade iron ore before closing in 1962. The ore that was mined consisted of massive crystalline hematite, brecciated and cemented by a later generation of crystalline hematite. The deposits are steeply dipping tabular bodies enclosed within walls of low-rank metabasalts of the Precambrian Ely Greenstone; they occupy the stratigraphic position of and grade upward into a jaspilitic iron-formation. Geologic and petrographic relations indicate that the deposits are post-metamorphic in age. One of the ore bodies has a mineralogic zoning characterized by a magnetite zone incorporated within and surrounded by a hausmannite zone. Near the bottom of the deposit both the cementing hematite and the early brecciated hematite give way to carbonate minerals. Two zones of alteration can be recognized in the greenstone wall rocks: an outer zone composed entirely of chlorite and an inner zone in which the rocks are stained by hematite. The inner zone has a mineralogic zoning; dominant kaolinite grades outward into dominant 2Ml muscovite. The altered zones contain substantially more iron than the unaltered country rock. The composition of some of the carbonate minerals indicates that they were deposited at a temperature near 400°C. The mineralogic relations indicate that during much of the period of ore deposition the fugacities of oxygen and sulphur fluctuated around the equilibrium values for the coexistence of hematite, magnetite, and pyrite, and that the fugacity of CO2 was on the order of 103 atmospheres. The kaolinitic alteration adjacent to the deposits indicates that the altering fluid may have been acid, and it is postulated that the acidity resulted from a relatively high concentration of CO2 in the fluid. It is concluded (1) that the deposits were formed by the replacement of the silica in the iron-formation by hematite, (2) that the iron probably was transported in large part as free ferrous ions in an acid hydrothermal fluid, (3) that deposition and replacement was largely restricted to the jaspilite because of its brecciation, and (4) that two of the principal causes of ore deposition were an increase in pH of the fluid resulting from the escape of CO2 in the brecciated zone and oxidation of the iron to the ferric state. The oxygen may have been derived either from the dissociation of unstable oxidized agents introduced with the ore fluid or by the downward diffusion of atmospheric oxygen through ground water. The source of the metals and the fluid is unknown; gross spatial relations suggest that possibly both the metals and the fluid were derived from depth, in an environment of more intense metamorphism.