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.
Machamer, J.F., Geology and Origin of the Iron Ore Deposits of the Zenith Mine Vermilion District, Minnesota. Special Publications Series 2. 56 p.
56 p., 3 pls.
Machamer, Jerome F..
SP-02 Geology and Origin of the Iron Ore Deposits of the Zenith Mine Vermilion District, Minnesota.
Minnesota Geological Survey.
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