https://reader.elsevier.com/reader/s...20211226054153

However, Kashiwabara et al. (2014) determined experimentally that Te(VI) is incorporated into ferrihydrite by co-precipitation, as the Te(VI) octahedron is similar to Fe(III) but unlike the Se(VI) tetrahedron, and Te(IV) is not oxidized by ferrihydrite, indicating that the variations in the Te/Se ratio are yet to be fully understood. From another perspective, at lower oxygenation levels, Te might be more readily sequestered into reducing environments than at higher oxygenation levels. This is consistent with the occurrence of Te enrichments in Proterozoic reduction spheroids reported here. Notably, Fe(III) reducing bacteria that are implicated in the leaching of iron oxide grain coatings, can concentrate both Se and Te (Klonowska et al., 2005, Kim et al., 2013).

There remains the question of why such a rare element as Te should be concentrated to the degree observed. The high concentration could reflect anomalous availability. Tellurium and gold are present at relatively high levels in Archean and Palaeoproterozoic rocks, often together as gold telluride minerals (e.g. Bierlein et al., 2006, Helt et al., 2014, Large et al., 2015, Rezeau et al., 2017). As these older rocks were being eroded into Mesoproterozoic sediments, as evidenced from age data for detrital zircons (Hawkesworth and Kemp, 2006, Parnell and Lindgren, 2016), Te and Au would have been recycled into the surface environment. Although they do not have distinctive habits, the mineral grains are, however, clearly neoformed from groundwaters during burial diagenesis, not simply concentrations of placer minerals eroded from the source rocks. A placer origin for the tellurides can be excluded for multiple reasons, including (i) an absence of the most common heavy minerals such as zircon and garnet; (ii) a selective distribution of tellurides in the cores of reduction spheroids, although the rest of the host rock has the same grain size; (iii) a size range for the telluride grains less than one tenth of the detrital grains in the host rock; and (iv) grains of several different metallic tellurides in close proximity within a single spheroid in the Belt Supergroup samples.

Our focus on Te and Se shows that:
i. The spheroids are markedly and consistently enriched in Te.

ii. Discrete telluride minerals were precipitated, including tellurides of mercury, copper, lead and gold.

iii. The abundance of Te relative to Se is greater than in younger spheroids.


We speculate that the occurrence ofTe in Mesoproterozoic continental sediments reflects erosion of older basement sources relatively enriched in Te.