Sm–Nd and Sr isotope fingerprinting of iron mining tailing deposits spilled from the failed SAMARCO Fundão dam 2015 accident at Mariana, SE-Brazil

Abstract

This work presents the first Sm–Nd and Sr isotope TIMS data on the fine-grained fraction (<74 μm) of iron-rich deposits related to the November 5th, 2015 Mariana accident, caused by the rupture of the Fundão dam, an iron tailings pond operated by SAMARCO Mining. The failure of the Fundão dam provoked the gravitational flow of approximately 62 Mi m3 of a quartz and iron oxide-rich slurry composed of water, fine sand and silt, that flowed into the Gualaxo do Norte and Carmo rivers, and eventually along the whole Doce River. Samples were collected between 5 and 80 km downstream from the Fundão Dam, along the Santarém creek and the Gualaxo do Norte and Carmo rivers. XRD analyses show that samples are >90% composed of quartz and iron oxides (hematite, goethite and magnetite), with minor phyllosilicates (kaolinite and mica). Owing to sediment transport processes, heavy (hematite, goethite, magnetite) to light (quartz, phyllosilicates) mineral fractionation is observed. As with XRD data, major and trace element compositions also closely match those of the original Paleoproterozoic Cauê Formation BIF iron ore. The studied samples yielded δNd between −17.0 and −19.7 pptt (part per ten thousand), with the exception of one sample, with δNd of −23.2 pptt. Strontium 87Sr/86Sr ratios show variability between 0.7351 and 0.7729. The TDM ages scatter between 1.94 Ga and 2.75 Ga, with a negative correlation with phyllosilicate contents, indicating that the latter has preferential provenance from younger, more juvenile sources than the Cauê Fm itabirites. This interpretation is corroborated by two component mixing models of Nd and Sr isotope ratios, where the Nd–Sr isotopic signatures of the studied samples are compatible with >98% of the Cauê Fm itabirite component, mixed with a Neoproterozoic juvenile mafic rocks component, such as those from the ophiolitic assemblages of the distal Macaúbas Group, or tholeiitic mafic rocks of the Pedro Lessa suite. The reported Sm–Nd and Sr isotope compositions closely match those of the original iron ore and contrast with either those of the upriver or downriver Precambrian to Ordovician bedrock units. The firstly defined isotope signature can thus be potentially used as a proxy for ongoing suspended sediment dispersion studies farther downstream along the Doce River and in adjacent marine environment.