Insights into hydrothermal controls and processes leading to the formation of the Late Ediacaran Gaoyan stratiform manganese-carbonate deposit, Southwest China

Abstract

The stratiform Mn-carbonate ore from the Gaoyan deposit occurs within terrigenous-sedimentary rocks of the upper part of the Ediacaran Doushantuo Formation and is characterized by rhodochrosite spherulites. Here we examine the effect of depositional environment and hydrothermal contribution based on new petrographical, mineralogical and geochemical data to shed light on major controls on formation of sedimentary Mn mineralization. The significant geochemical characteristics such as high Mn/Fe ratios (average 32.07), high Ba content (average 1940.9 ppm), positive Eu anomalies (average 1.38) and low REE contents (average 51 ppm) reveal a hydrothermal source. Meanwhile, 87Sr/86Sr ratios of Mn ores are substantially higher than those of carbonate wall rocks and contemporaneous seawater, indicating mixing of radiogenic Sr from the Mn-rich hydrothermal extraction from underlying sediments. This explanation is supported by a positive linear correlation between 87Sr/86Sr ratios and Eu anomalies. The slightly positive Ce anomaly of most Mn carbonate ore samples is mirrored in the negative Ce anomaly of black shale, while different Ce anomalies provide evidence that Mn redox cycling took place across a sub-oxic to anoxic/euxinic water interface. The euxinic conditions, as evidenced by the co-occurrence of fine-grained disseminated pyrite to framboidal aggregates (average diameters <5 μm) with rhodochrosite, can account for the high Mn/Fe ratios via ongoing Fe-sulfide precipitation. Calcite introduction to metal-rich hydrothermal waters with a lower pH relative to surface waters, together with Mn-oxide reduction and sulfate reduction, triggers substantial increases in rhodochrosite saturation and precipitation near the water–sediment interface. Subsequently, increasing Ca2+ in early diagenesis resulted in the precipitation of kutnohorite rims. Altogether, we develop a hydrothermal sedimentary model of the Gaoyan Mn mineralization with rhodochrosite spherulites as the hydrothermal sedimentary mineral phases, and kutnohorite forming during early diagenesis. Slightly light carbon isotope compositions (∼–6.1‰) of the manganese carbonate ores than those of normal dolostones (∼–4.8‰) suggest a limited carbon source from oxidation of organic material by methanogenesis processes during burial diagenesis. This model supports the model for direct Mn-carbonate precipitation from redox-stratified water columns and means that sedimentary Mn mineralization may be mainly controlled by hydrothermal contribution, rather than seawater redox conditions. This means exploration strategies that focus on oxic zone alone as an indication of prospectivity may be ineffective.