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Clay-Mineral Alteration in the Upper Mississippi Valley Zinc-Lead District

Published online by Cambridge University Press:  01 January 2024

A. V. Heyl ,
J. W. Hosterman  and
M. R. Brock
A. V. Heyl
Affiliation:
United States Geological Survey, Beltsville, Md.; Denver, Colo.
J. W. Hosterman
Affiliation:
United States Geological Survey, Beltsville, Md.; Denver, Colo.
M. R. Brock
Affiliation:
United States Geological Survey, Beltsville, Md.; Denver, Colo.
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Abstract

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Clay-mineral alteration in a carbonaceous shale bed seems to be related to hydrothermal zinc-lead depositions in the Upper Mississippi Valley district in southwestern Wisconsin, northwestern Illinois, and northeastern Iowa. A 3-in. thick, dark-brown, carbonaceous shale at the base of the Quimbys Mill Member of the Platteville Formation can be traced accurately in the Thompson-Temperly mine near New Diggings, Wis., through the ore bodies into alteration aureoles around the ore bodies and out into areas barren of mineralization or apparent alteration.

X-ray diffraction studies indicate a progressive alteration of the clay-mineral and, in part, nonclay-mineral components of the shale from unaltered rock into ore-zone rock. Illite of md polymorph in unaltered rock is altered to 1 m and 2 m illite in the altered rock, and 2 m illite polymorph increases markedly within the ore bodies. Accompanying this alteration, calcite decreases and dolomite and microcline increase in amount toward the ore zone; quartz remains unchanged. The conclusion that the alteration was effected by low-temperature and low-pressure environment over a geologically long period of ore deposition is supported by fluid-inclusion studies of the associated ore minerals, which indicate a maximum temperature of 120°C to 130°C, and the known stability of synthesized 1 m and 2 m mica polymorphs at 200°C and 15,000 psi water pressure.

Type
General
Information
Clays and Clay Minerals (National Conference on Clays and Clay Minerals) , Volume 12 , February 1963 , pp. 445 - 453
Copyright
Copyright © The Clay Minerals Society 1963

Footnotes

Published by permission of the Director, U.S. Geological Survey.

References

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