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Life Cycles of Mississippi Valley-Type (MVT) Deposits

A fundamental goal of the project was to understand the key genetic links between Mississippi Valley-type (MVT) mineralization — which accounts for 35 percent of the world's lead and zinc resources, respectively — and regional- and global-scale tectonic processes. Such connections will substantially improve our ability to assess seemingly permissive tracts for MVT and similar sediment-hosted deposits on a worldwide basis, because it is far easier to figure out the tectonic history of a frontier region than to predict its endowment of undiscovered MVT deposits. Current models for MVT ore-forming systems relate ore fluid migration to compressive tectonic regimes. This project evaluated the role of continental extension to the migration of MVT-ore fluids. Research on MVT deposits within the Alpine tectonic system of Europe and North Africa provided a test of the role of continental extension on these ores that have historically been attributed to the Mesozoic disassembly of the Pangea supercontinent. Results from this effort provided essential constraints for the development of a global tectonic model. Research was also directed at establishing geoenvironmental and process-oriented deposit models for Mississippi Valley-type (MVT) lead-zinc ores. Detailed geochemical studies in the Polish MVT district provided an analog for assessing geoenvironmental issues related to past and future mining of MVT ores in the mid-continent of the U.S. The ore deposit and environmental models provided unbiased scientific information for assessing future mineral resources for this important class of lead-zinc ores in a context of identified environmental concerns.

Project Objectives and Strategy:

This project contained three research objectives:

A) Development of a Geoenvironmental Model:

Geochemical studies were conducted in the Upper Silesia MVT district (Poland) as an analog for MVT deposits in the U.S. The geochemistry of soils, surface water, and ground water in the district were used to quantitatively assess the migration of ore-related components. This study considered the effects of ore mineralogy (i.e., iron sulfide content), trace element distribution in different mineral assemblages, hostrock mineralogy, pre-mining background levels, and hydrology on the migration of ore-related components. New geochemical data for surface and ground waters from the Ozark region (U.S. mid-continent) were used together with published geochemical data to evaluate various aspects of ore mineralogy, geology, and hydrology that may influence the environmental response to mining MVT ores. These studies, together with the genetic components of the MVT ore deposit model, were used to make a general geoenvironmental model that can be broadly applied to any MVT deposits in the U.S.

B) Development of a MVT Ore deposit Model:

MVT deposits in North America have been attributed to large-scale migration of basin fluids during convergent orogenic processes whereas many other districts (e.g., Alpine deposits of Europe and North Africa) have been attributed to Late Paleozoic to Mesozoic continental extension of Pangea carbonate platforms. An important component of the research was directed at establishing age for ore formation for important deposits believed to have formed during continental extension. This work involved key collaborative research with universities in Europe, Canada, and the U.S. Various methods were used to date the ores that include radiometric and paleomagnetic methods. These dating efforts placed ore formation within a context of crustal tectonics and fluid migration. The nature of the ore fluids (temperature and fluid compositions) were determined from fluid inclusion studies to quantify the rock-water processes that controlled the ore-forming capacity of the fluids. Hydrological modeling was conducted by French collaborators to test fluid migration from basins during large-scale extensional tectonic events. MVT deposits in the Cevennes region of southern France, the Reocin deposit in Northern Spain, and the Touisit district of Morocco were used as classic examples of MVT deposits believed to have formed during continental extension. Results of these studies provided essential attributes to the global tectonic model.

C) Development of a Global Tectonic Model:

Results from this project demonstrated that MVT deposits formed during two main episodes in Earth history: Late Paleozoic (associated with the Pangea collisions), and Tertiary (associated with the Alpine and Laramide collisions). In order for this information to be of much predictive value in global mineral assessments, we needed to address these points: Why do MVTs form in some, but not all carbonate platforms in collisional forelands? 2) What is it about the late stage of some collisions that induces regional-scale fluid migrations? (3) What is the role of orographic climate during mineralization? (4) Do MVT deposits ever form in Andean-type (as opposed to collisional) forelands?

Project Contact

David Leach
Phone: 303-236-5522
Email: dleach@usgs.gov

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