THE SECULAR DISTRIBUTION OF MISSISSIPPI VALLEY-TYPE PB-ZN DEPOSITS

Mississippi Valley-type (MVT) Pb-Zn ores formed mainly during two periods in geologic time: the Devonian to Late Permian that corresponds to the assembly of the supercontinent Pangea, and the Cretaceous-Tertiary that corresponds to the Laramide and Alpine orogenic cycles. These periods are characterized by optimum conditions for MVT ore deposition which include the presence of extensive carbonate platform sequences, abundant evaporative environments, and intense orogenic activity that is required to drive potential ore-forming brines into favorable depositional environments.

There are remarkably few MVT deposits that formed in the Precambrian or are hosted by Precambrian rocks. The low number of MVT deposits in Proterozoic rocks is unlikely due to their erosion or destruction, because related SEDEX (sedimentary exhalative) Pb-Zn deposits are at least, equally susceptible to these processes but they are abundant in the Proterozoic. An important control may have been the permeability of the carbonate rocks, which changed in lithology from dominantly stromatolites and seafloor precipitates in the Precambrian to more porous clastic carbonates in the Phanerozoic.

The oldest known deposits of possible MVT affinity are some carbonate hosted lead-zinc ores in the Neoarchean Transvaal Supergroup (2.5-2.65 Ga) that have been dated at 2.0 to 2.1 Ga. However, recent studies suggest these deposits may be related to the intrusion of the Bushveld Igneous Complex, and therefore, should not be classified as MVT deposits. Possibly the oldest MVT deposits, as well as SEDEX ores, are about 1.8 to 2.1 Ga. Some important changes occurred in Earth evolution during this period that could have contributed to the emergence of these ores. These include changes in the composition of the hydrosphere and atmosphere (i.e., reduced to oxidized) that allowed for the transport of metals in brines and the evolution of sulfate reducing bacteria that provided local reduced sulfur traps for the ores. By about 1.8 Ga, most of the Earth's crust had formed and the first cycle of supercontinent breakup and reassembly was nearing completion. Thus, by about 1.8 to 2.1 Ga, extensive shallow marine carbonate platforms would have been exposed to oxidized brines capable of transporting and depositing Pb and Zn ores.