Late Archean initiation of manganese metallogenesis: its significance and environmental controls

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 been 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 which 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 succeeding 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 earlier period, supporting the geochemical indications from paleoweathering profiles.