The Geology and Metallogeny of Volcanic-Hosted Massive Sulfide Deposits:Variations through Geologic Time and with Tectonic Setting

摘要

Analysis of metallogenic data, including grade and tonnage, host-rock succession, ore and alteration mineralogy, and lead and sulfur isotope data, indicates significant secular changes in the character of volcanic-hosted massive sulfide (VHMS) deposits, which appear to be related to changes in tectonic processes, tectonic cycles, and changes in the composition of the hydrosphere and atmosphere. The distribution of these deposits, whether measured in number of deposits, tons of ore, or tons of metal, is episodic, with major peaks at 2740 to 2690, 1910 to 1840, 510 to 460, and 390 to 355 Ma. These peaks correspond to the assembly of major continental land masses, including Kenorland, Nuna, Gondwana, and Pangea, respectively. Periods when fewer VHMS deposits formed correspond to periods of supercontinent and/or supercraton stability. The VHMSdeposits do not form during supercontinent and/or supercraton breakup; rather, these intervals are associated with deposition of elastic-dominated sediment-hosted zinc-lead deposits. The main exception to these generalizations is the amalgamation of Rodinia, which was not accompanied by significant VHMS formation. Rodinian amalgamation may have been dominated by advancing accretionary orogenesis, whereby the overriding plate did not go into extension. In this case, slab rollback and the associated extension to form back-arc basins would not have been common, a setting typically conducive to the formation and preservation of VHMS deposits. Large ranges in source ~(238)U/~(204)Pb (μ) that characterized VHMS deposits in the Archean and Proterozoic indicate early (Hadean to Paleoarchean) differentiation of the Earth. A progressive decrease in u variability may indicate homogenization with time of these differentiated sources. Secular increases in the amount of lead and decreases in 100Zn/(Zn+Pb) relate to an increase in felsic rock-dominated successions as hosts to deposits and to an apparent absolute increase in the abundance of lead in the crust with time. The increase in the abundance of barite and other sulfate minerals in VHMS deposits, from virtually absent in the Mesoarchean and Neoarchean to relatively common in the Phanerozoic, relates to the progressive oxidation of the atmosphere and hydrosphere. The total sulfur content of the oceans also increased, resulting in the enhanced importance of seawater sulfur in VHMS ore fluids with time. In Archean to Paleoproterozoic deposits, the bulk of the sulfur was derived by leaching rocks underlying the deposits, with little contribution from seawater, resulting in uniform, near-zero per mil values of δ~(34)S_(sulfide) In contrast, the more variable δ~(34) S_(sulfide) values of younger deposits reflect the increasing importance of seawater sulfur in the hydrothermal systems. Unlike Mesoarchean and Neoarchean deposits, Paleoarchean deposits contain abundant barite. This sulfate is inferred to have been derived from photolytic decomposition of atmospheric SO_2 and does not reflect overall oxidized oceans. Archean and Proterozoic seawater was significantly more saline than that in the Phanerozoic, particularly upper Phanerozoic seawater. The VHMS ore fluids reflect this, being on average more saline in Archean and Proterozoic deposits. This variability introduces uncertainty into genetic models advocating brine pools or mag-matic-hydrothcrmal contributions babsed on high-salinity ore fluids.