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General Topics and Reviews |
Institute of Materials, University College, Cardiff, CF2 1TA, UK
The Proterozoic contrasts with the Archaean and Phanerozoic in the apparent relative lack of volcanogenic mineral deposits. However, analogues of the three distinct volcanogenic mineralization styles recognized in the Recent record can be traced back into the Proterozoic and probably into the Archaean. These are: (1) massive sulphide deposits, (2) iron ores and (3) epithermal deposits. The Proterozoic is characterized by sedimentary basins and stable cratons, and Proterozoic ore styles tend to reflect these trends. However, volcanogenic mineralization appears to be at least as abundant in suitable successions in the Proterozoic as in other eras.
Although analogues can be found in the Proterozoic for the Phanerozoic volcanogenic ore styles, no satisfactory Recent analogue of the Broken Hill-type has been identified. Similar processes in the mid-Proterozoic and Phanerozoic record appear to have resulted in SEDEX Zn-Pb deposits of Mt Isa and Irish types. The Broken Hill-type was formed in a shallow aqueous/sub-aerial environment and, as with the coeval sedimentary ores, may have been modified by non-actualistic, early Proterozoic surface environmental conditions. In contrast, the major metal geochemistry of Proterozoic massive sulphide ores differs little from their Archaean or Phanerozoic counterparts, possibly reflecting their deep ocean genesis.
Formation of Proterozoic volcanogenic massive sulphide deposits was concentrated around 1.9 Ga. The ore districts are restricted to a linear zone, about 1000 km wide and some 8000 km long, on the eastern margin of the reconstructed Proterozoic supercontinent. It is suggested that the massive sulphide ore districts trace the scar of an early Proterozoic Wilson cycle. In contrast, Proterozoic Broken Hill-type ores and their sub-varieties display a wide geographic spread, consistent with their essentially continental character.
