MISSISSIPPI VALLEY-TYPE LEAD-ZINC DEPOSITS THROUGH EARTH HISTORY

Mississippi Valley-type (MVT) deposits are a family of epigenetic ores occurring in carbonate-dominated platform successions. Formation of MVT deposits requires a series of critical features, including dolostone host rocks, basinal brines, a regional fluid-flow system, and mechanisms for focusing and mixing metal- and sulfur-bearing fluids. Most MVTs occur in Phanerozoic host rocks; a few are in Proterozoic rocks; none are known from rocks older than latest Archean. When viewed over the span of earth history, the age distribution of MVT mineralization is similar to that of host rocks, but shifted slightly to younger modes. Most Phanerozoic MVTs formed on the cratonic flanks of collisional foreland basins. Peak times of mineralization are due to the chance confluence of plate geometry and paleolatitude. The biggest pulse coincided with the Paleozoic assembly of Pangea, when arcs collided with passive margins on all sides of Laurentia. These margins had accumulated carbonates and evaporites at low latitudes. Had Laurentia instead lain at high latitudes, favorable host rocks and brines would be lacking and collisions like the Ouachita would not have produced MVTs. The patchy distribution of the Proterozoic MVTs is not well understood and results, in part, from lack of preservation. None are demonstrably related to collisional orogeny; different fluid-drive regimes thus may have prevailed. Suitable conditions did exist somewhere on Earth by 2300 Ma, the age of the oldest MVT in S. Africa. Explanations for the absence of Archean MVTs are being sought in secular variations in atmospheric oxygen, seawater sulfate, and tectonic style.