Molybdenum contents of sulfides in ancient glacial diamictites: Implications for molybdenum delivery to the oceans prior to the Great Oxidation Event

Abstract

In order to determine whether sulfides are major reservoirs for Mo in the upper continental crust (UCC), we determined the composition and mode of occurrence of sulfides and evaluated their contribution to the molybdenum budget in twelve glacial diamictites with ages ranging from 2900 to 300 Ma. The diamictites provide a snapshot of UCC mineralogy and composition at the time of their deposition and show systematic depletion in bulk rock Mo concentrations after the Great Oxidation Event (GOE), reflecting the effects of oxidative weathering in their provenance (Gaschnig et al., 2014; Li et al., 2016). Sulfides are generally confined to Archean and Paleoproterozoic diamictites, although they also have been found in one Phanerozoic sample with an ancient provenance. We classify the sulfides based on their compositions and morphologies. Detrital sulfides are generally rounded, may be a single mineral, or an assemblage of minerals, and show a very wide range in mineralogy, including a single molybdenite grain. Sedimentary sulfides are pyrites, generally with framboidal-like textures. Pyrites also include non-framboidal textured authigenic pyrites. Epigenetic sulfides consist of irregular pyrrhotite aggregates (sometimes pyrrhotite intergrown with chalcopyrite and cobaltite), late-stage euhedral pyrites and pyrite aggregates in veins. High Mo concentrations (up to ∼230 ppm) are found in some sedimentary framboidal-like pyrites from the Mesoarchean Coronation and Paleoproterozoic Makganyene Formations, epigenetic pyrrhotite aggregates and chalcopyrite in the Ramsay Lake diamictite, and in detrital sulfides in Timeball Hill diamictites that may have originated from hydrothermal fluids in the sedimentary basins. Other detrital sulfides have widely variable Mo concentrations (0.5–36 ppm). Mass balance calculations show that sulfides can account for only <8% of the whole rock Mo contents in all glacial diamictites except for the Makganyene Formation, and thus sulfides (including molybdenite) are unlikely to be a significant host of Mo within the UCC before the GOE. In the Makganyene diamictites, ∼37% of whole rock Mo is contained within sulfides, which are predominantly framboidal-like pyrites interpreted to have grown in the matrix at the time of deposition. In these, the Mo contents correlate with the size of the microcrystals: smaller-sized crystals contain significantly higher concentrations than larger-sized crystals, suggesting that rapid nucleation and growth of framboidal pyrites leads to incorporation of large amounts of Mo available in the pore water or Mo expulsion during recrystallization. The presence of Mo in sedimentary pyrites from a pre-GOE deposit – the 2900 Ma Coronation Formation – points to the availability of Mo and S in pre-GOE seawater (and associated pore fluids), and is consistent with previously reported evidence for isolated pulses of atmospheric oxygen and oxidative continental weathering prior to the GOE. However, the lack of significant whole rock Mo depletion in pre-GOE diamictites limits the amount of Mo released from the continents prior to the GOE.