『Abstract
Manganese metallogenesis was initiated in a modest scale during
the Late Archean time selectively in India and Brazil. Sedimentary
deposits occur in both greenstone belts and high grade (granulite)
terranes in shallow-water shelf regimes only and are often intimately
associated with stromatolites. The changes in the rates of continental
crustal growth, mantle heat flux and outgassing along with the
evolution of tectonic regimes, sedimentary environments, the state
of the biosphere and finally the compositions of interacting atmosphere-hydrosphere
system from the Early to the Late Archean time have ben traced
as interdependent parameters and interpreted to explain the first
appearance of sedimentary manganese deposits promoted by possibly
localized oxygenated environments. Compared to the spectacular
‘manganese event’ represented by the giant Paleoproterozoic Kalahari
deposit, South Africa wuich is interpreted to have been formed
as a consequence of the rapid increase in atmospheric oxygen level
ca. 2.5-2.4 Ga ago, the modest initiation of the Late Archean
manganese depositions was triggered by localized photosynthetic
oxygen supply forming basin-margin oxygen oases as a prologue
for the secceeding stratified ocean system. The dissolved manganese
in the basin water that was oxidized to form shelf-type shallow-water
deposits is suspected (by analogy with contemporary banded iron
formations (BIFs)), but not proved, to be of hydrothermal ancestry.
The initiation of concentrated manganese sedimentation during
the Late Archean by itself, provides an evidence for an increase
in the oxygen content in the contemporary atmosphere-hydrosphere
system compared to that in the early period, supporting the geochemical
indications from paleoweathering profiles.』
1. Introduction
2. The Archean scenario
2.1. Crustal evolution
2.2. Heat flow, tectonic style, and outgassing
2.3. Sedimentary environments and the rock record
2.4. Emergence and development of the biosphere
2.5. Compositional evolution of the atmosphere and hydrosphere
2.6. Lessons from BIF
2.7. Summary
3. Characteristics features of Archean manganese deposits
3.1. Deposits in India
3.2. Deposits in Brazil
4. Early paleoproterozoic explosion of manganese deposition
- a continuum
5. Summary and discussion
Acknowledgements
References