--> ABSTRACT: A Critical Review of Theories on the Origin of Mississippi Valley-Type Lead-Zinc Deposits in the North American Midcontinent, by Jeffrey A. Nunn, David Deming; #91020 (1995).

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A Critical Review of Theories on the Origin of Mississippi Valley-Type Lead-Zinc Deposits in the North American Midcontinent

Jeffrey A. Nunn, David Deming

We review the most widely discussed hypotheses proposed to explain the origin and migration of Mississippi Valley-type (MVT) brines: flow driven by water released from compaction or metamorphism following overthrusting, and topographically driven flow.

Although sediment compaction and fluid expulsion by overthrusting is intuitively appealing, numerical simulations have shown fluid flow rates (~cm/yr) are inadequate by at least an order of magnitude to efficiently transport heat to the sites of ore deposition unless flow is spatially or temporally focused. However, field studies have shown that abnormally high paleotemperatures were not confined to the ore deposits themselves.

While additional fluid released by metamorphic reactions has not been examined quantitatively, the amount of fluid available is still limited and would probably have to be temporally focused.

Topographically driven flow is a physical process that is observed in several sedimentary basins today. Flow velocities can be comparatively high (~1-10 m/yr) if aquifers of sufficient permeability arc present. High flow velocities imply that heat can be efficiently transported over hundreds of kilometers. In the North American midcontinent, the Cambrian Lamotte sandstone may have functioned to channel flow to sites of MVT ore deposition on the craton. However, there are several observations that the topographically driven flow hypothesis cannot by itself satisfactorily explain. The very high homogenization temperatures (~80-120°C) associated with fluid inclusions at shallow depths (500 - 1500 m) in the southeast Missouri ore district cannot be duplicated by any model of topograp ically driven flow known to us, including models with thermal transients, which has realistic values of permeability and background heat flow. Other problems include the apparent presence of multiple fluids during ore deposition, a severe mass balance problem, and periodicity in ore deposition as revealed by banding

Although considerable progress has been made in understanding the physical limitations of different hypotheses for formation of MVT deposits, the mechanism which created these ores is still an open question.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995