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Abstract

Stratiform ore deposits may comprise geologically similar end members formed by different complex paths of genesis. Low-temperature stratiform deposits of the Mississippi Valley type are found in three regions in the United States: (1) Appalachian Valley and Ridge province, (2) greater Mississippi Valley, and (3) scattered districts in the mountains of the West. The deposits in the three major regions show similar geologic features, but in each region variations in lead-isotope composition, mineralogy, trace elements, geochemistry of the ore-forming fluids, and other geologic features indicate diverse origins.

The Appalachian Valley deposits of Tennessee may have been deposited by heated early Paleozoic metamorphic hydrothermal solutions formed in the volcanic and tectonically active belt that is now the Piedmont. Possibly the heated metamorphic solutions migrated northwestward into sediments filled with connate brines, mingled with them, and deposited sphalerite and dolomite.

In the Mississippi Valley, the available geologic and geochemical data suggest that the main solutions depositing the ores were connate brines heated to temperatures of about 100º C and enriched by smaller potassium-rich fractions carrying a distinctive assemblage of minor elements; probably the heat, potassium, and minor elements were derived from deep-seated alkalic batholiths and stocks. In addition to the main deposits, beds of siderite nodules containing lead and zinc sulfides probably were deposited during diagenesis, and sulfide-bearing geodes may be of meteoric origin. Syngenetic pyrite occurs in black shale beds in microspherules.

Most stratiform deposits of the Mississippi Valley type in the West are probably low-temperature hydrothermal deposits in which the ore-forming fluids were mainly circulated ground water, magmatically heated in a granitic province, plus a small magmatic fraction of solutions.

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