Abstract

We have evaluated the temporal distribution of Mississippi Valley-type (MVT) Zn-Pb deposits with special attention to the nature and number of deposits of Pre-cambrian age. Our evaluation is based on the widely used model for MVT mineralization involving metal-bearing brines that lack reduced S and that deposit sulfides only where they encounter a reservoir of sulfide or where sulfate in the metal-bearing brine is reduced to sulfide. For MVT systems of this type, basins with abundant sulfate would be most favorable for development of MVT mineralization because these would allow transport of metals in sulfate-rich brines and deposition of metals in areas where the sulfate was reduced. Because abundant sulfate requires abundant atmospheric oxygen, the distribution of MVT deposits through time might reflect compositional changes in Earth's atmosphere, especially the suggested Great Oxidation Event (GOE). A compilation of new data for the Bushy Park-Pering district in the Transvaal Supergroup of South Africa, the world's oldest known MVT province, and published information on other Precambrian MVT deposits in the Ediacara, Berg Aukas/Abenab, Gayna River, Warrabarty, Nanisivik, Kamarga (Century), McArthur River (Coxco), Ramah, and Esker districts shows that they are generally similar in geologic setting and mineralogy to those in Phanerozoic rocks. Fluid inclusions in some Neoproterozoic deposits, including Berg Aukas/Abenab, Gayna River, Warrabarty, and Nanisivik, record higher temperatures and salinities than found in most Phanerozoic deposits, possibly reflecting igneous activity or a more proximal basinal setting during Precambrian time. Fluid inclusion leachate data for several Precambrian MVT deposits suggest that their parent brines formed by evaporation of seawater, and S isotope compositions indicate that the S was derived largely from coeval seawater sulfate. Comparisons of data from all deposits show no evidence for a gradual increase in temperature or salinity backward through time, such as might be caused by higher heat flow during early stages of Earth history, although the magnitude of this effect might be lost in the uncertainty of most fluid inclusion measurements. These observations confirm that MVT deposits reflect the chemistry of their source basins, which are as old as 2.6 Ga. No MVT deposits or suitable host rocks of an older age are known. Precambrian MVT deposits do differ from their Phanerozoic analogues in the magnitude of mineralization. Precambrian deposits and districts formed at an estimated rate of 5.5 per billion years versus a significantly larger rate of similar to 60 per billion years for Phanerozoic deposits, and the Phanerozoic deposits are considerably larger. Furthermore, the transition from low-magnitude, Precambrian-type to high-magnitude, Phanerozoic-type MVT mineralization took place at the beginning of Cambrian time rather than at the 2.3 Ga GOE. This appearance of widespread MVT mineralization is closer to the time at which sulfate concentrations in the world ocean are estimated to have reached present-day levels. Although these conclusions are subject to considerable uncertainty because of the limited number of Precambrian deposits, the lack of an increase in the frequency of MVT mineralization at the GOE suggests that widespread MVT mineralization requires higher levels of sulfate than could have been provided by this event, or that the appearance of sulfate in the ocean was considerably delayed. Finally, the presence of MVT deposits in basins that formed considerably before the GOE suggests that local sulfate concentrations were available at even early points in Earth's history.