Radical change in tectonic regime and paleo-environment at the end of Neoarchean evidenced by a unique metal co-existing deposit

Abstract

Banded iron formation and Cu-Zn sulfide deposits within volcanic-argillaceous sequences (as volcanogenic massive sulphide deposits (VMS)-like type) occur together in the Qingyuan greenstone belt of the North China Craton, recording the first appearance of oxidized ores and sulfide ores co-existing in the early Earth. The unique metal co-existing deposits should meet two requirements: tectonic setting and sedimentary environment. As regards to tectonic setting, plate-like tectonics might have started since the end of the Neoarchean because continents had grown large enough and there occurred volcanic arcs and backarc basins similar to modern ones in a way. Partial melting of subducted continental crust is conductive to providing ore-forming elements. As for sedimentary environment, late Neoarchean seawater was rich in Fe2+ and anoxic. Instantaneous oxidation of the seawater resulted possibly from frequent submarine volcanic eruptions and facilitated precipitation of the banded iron formation. At this point, it is also favorable for the enrichment of Cu and Zn ions in seawater. The VMS-like deposits tended to form when the seawater was reduced again. Studies of isotopic elements like sulfur, oxygen, iron and silicon support the above geological processes. It is shown that the geologic conditions only existed in the late Neoarchean and Paleoproterozoic for a short period of time. The banded iron formations disappeared around 1.85 Ga, and the associated sulfide metal deposits also became dominant sedimentary exhalative deposits in the meso-Neoproterozoic Boring Billion, as a result of increasing oxidation of the oceans and the increasing maturity of the continental crust. This study is significant not only for decoding metallogenic genesis but also helping understand rapid change in Precambrian tectonic regimes and Earth’s environments.