Marine-sedimentary manganese metallogenesis through geologic time and its coupling with major geoenvironmental events

Abstract

Marine-sedimentary manganese (Mn) deposits, which boast significant reserves compared with other Mn deposit types, are vital sources of Mn for steel manufacturing and the global chemical industry. Recent discoveries of new marine-sedimentary Mn deposits and the application of advanced analytical methods have led to fresh insights into marine-sedimentary Mn metallogenesis. Mn is a redox-sensitive element. However, Mn cycling, which encompasses transport, deposition, and mineralization, is not a straightforward redox chemical process. Over the past two decades, studies on the biogeochemical behaviors of Mn and geological case studies of sedimentary Mn deposits have highlighted the crucial role of microorganisms in Mn cycling, which exceeds that of inorganic reactions alone. Nevertheless, the specific geoenvironmental conditions that facilitate microbially mediated Mn metallogenesis and the interrelationships among these events remain unclear. In this study, we review significant marine-sedimentary Mn deposits throughout geological history and their metallogenic geoenvironmental contexts. Our findings suggest that, despite the extensive temporal gaps between sedimentary Mn deposits, they likely share a similar metallogenesis mechanism. Pre-accumulated Mn(II) is converted to Mn oxides through the activation of multicopper oxidase enzymes under obligatory oxic conditions. In deposition sites with sufficient organic matter, heterotrophic microbes subsequently reduce Mn oxides to Mn carbonates by coupling with organic matter decomposition under suboxic or anoxic conditions. We propose four essential geoenvironmental prerequisites for large-scale Mn metallogenesis: the Mn source, high availability of molecular oxygen in water, a redox-stratified restricted setting, and the exchange of anoxic with oxic water columns. These conditions fundamentally arise from the breakup and assembly of supercontinents throughout Earth's history. Rifting has created a series of restricted basins, where hydrothermal activity has provided a substantial source of Mn(II). Continental inputs from weathering-derived Mn, including lateritic crusts, may also play a crucial role in the genesis of large sedimentary Mn accumulations and should not be overlooked. The sustained increase in atmospheric and hydrospheric oxygen levels has primarily been driven by photosynthesis, mantle overturning, and the expansion of continental regions. The anoxic depths of redox-stratified restricted rift basins have served as effective reservoirs for pre-accumulated Mn(II) and have provided refuge for calm biomat formation by limiting debris input. The exchange of oxygenated surface water with anoxic, metal-rich water in deep basins, facilitated by currents or fluctuations in sea level, creates the oxic conditions required for the enzymatic processes that promote microbial Mn(II) oxidation. We also identify several scientific issues that warrant further research, including the effects of the duration of oxic conditions in bottom water on the preservation of Mn oxides and subsequent Mn‑carbonate precipitation, the metallogenic potential of Mn-dependent anaerobic oxidation of methane, the superimposition of various biomineralization stages and later geological processes that may obscure biosignatures used to investigate microbially mediated Mn ores. We hope this study draws attention to and expands upon the original conclusions of previous studies that overlooked microbially mediated Mn metallogenesis, while also paving the way for future exploration of hidden Mn resources.