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Document Type

Research

Keywords

paleokarst, sulfide mineralization, eastern Iowa

Abstract

Operations at the Linwood Mine in Scott County, Iowa, have exposed, on two working levels, extensive pre-Pennsylvanian paleokarst and associated sediment fillings and mineralization in Middle Devonian limestones. Cavities in Davenport-Spring Grove host rocks range up to 270 meters in length and are virtually all filled with fluviatile sediment. Cavities in Cedar Rapids host rocks, on average, are smaller and some are unfilled. Limestone dissolution was controlled by high-angle fractures with a wide variety of orientations and by bedding plane and stylolitic partings. Dissolutional features on both levels of the mine provide evidence for phreatic and vadose processes. Most of the unfilled cavities in the Cedar Rapids Limestone contain abundant mineralization, which occurs as cavity linings, fracture linings and fillings, breccia cements and disseminations in sediment fillings and cavity walls. Mineralogically, calcite is dominant, marcasite, barite and pyrite are widespread and locally abundant, sphalerite is uncommon, and chalcopyrite and quartz are rare. Calcite appears in several generations involving alternations between acute and obtuse crystal forms. Barite occurs in a wide variety of habits; multictystalline barite differs from monoctystalline barite in color and distribution within cavities. Iron sulfide precipitation began early, and continued intermittently throughout the period of the mineralization. Calcite is intermediate to late and barite is late. Distinctive features of mineralization include minerals contained wholly within mudstone, boxwork, and iron sulfide which appears to be stalactitic. Vertically, most of the mineralization occurs in the well-bedded lower part of the Cedar Rapids Limestone. Presence or absence of specific minerals, mineral distribution and paragenesis suggest that conditions during karst-filling sedimentation and mineralization were alternately reducing and oxidizing, but most of the mineralization formed during reducing (phreatic) conditions. Mineralization at Linwood shows similarities and differences when compared to other paleokarst-associated deposits in eastern Iowa. Similarities with main district upper Mississippi Valley zinc-lead deposits include broadly similar paragenesis and similarities in multiple generations of calcite.

Publication Date

March-June 1995

Journal Title

Journal of the Iowa Academy of Science

Volume

102

Issue

1-2

First Page

1

Last Page

21

Copyright

© Copyright 1995 by the Iowa Academy of Science, Inc.

Language

EN

File Format

application/pdf