Comparison of Lower and Middle Riphean sparry magnesite deposits of the Southern Urals province

Sparry magnesite deposits (SMD) of the Lower Riphean (LR) and Middle Riphean (MR) sequences in the western slope of the South Urals have some distinguished peculiarities, which allows to compare them and speculate about the mechanism of ore formation and Mg-source origin. LR magnesites are located in some stratigraphic levels and lie within widespread dolostone horizons. MR magnesites that occur in the lower carbonate member of Avzyan fm. are represented by dolomitized limestones. The shape of LR magnesite body is very often layer-like with sharp contacts. The shape of MR one is sometimes lens like, contacts of ore body are complecated with zones of impregnable magnesites near the hosting dolomites. The crystalline structures of LR magnesites are typically corse-grained with average size of crystalls 10 mm. The magnesite grains of the MR one have 1-3 mm of average size. The chemical composition of MR magnesite ore shows increase of SiO 2 , Al 2 O 3 , FeO and decrease of LREE fractionation in comparison with the LR magnesites. As we assume, metasomatic magnesites of the Lower Riphean time are connected with sedimentation/early diagenesis stage. The source of Mg 2+ came from a crust of weathering of basic and ultramafic rocks in humid climate. Magnesite occurrences in Middle Riphean time are connected with high Fe-contents metasomatic fluids, which were resulted of evaporite Mg-enriched solutions transformation during kathagenesis

Comparison of Lower and Middle Riphean sparry magnesite deposits of the Southern Urals province

Sparry magnesite deposits (SMD) of the Lower Riphean (LR) and Middle Riphean (MR) sequences in the western slope of the South Urals have some distinguished peculiarities, which allows to compare them and speculate about the mechanism of ore formation and Mg-source origin. LR magnesites are located in some stratigraphic levels and lie within widespread dolostone horizons. MR magnesites that occur in the lower carbonate member of Avzyan fm. are represented by dolomitized limestones. The shape of LR magnesite body is very often layer-like with sharp contacts. The shape of MR one is sometimes lens like, contacts of ore body are complecated with zones of impregnable magnesites near the hosting dolomites. The crystalline structures of LR magnesites are typically corse-grained with average size of crystalls 10 mm. The magnesite grains of the MR one have 1-3 mm of average size. The chemical composition of MR magnesite ore shows increase of SiO 2 , Al 2 O 3 , FeO and decrease of LREE fractionation in comparison with the LR magnesites. As we assume, metasomatic magnesites of the Lower Riphean time are connected with sedimentation/early diagenesis stage. The source of Mg 2+ came from a crust of weathering of basic and ultramafic rocks in humid climate. Magnesite occurrences in Middle Riphean time are connected with high Fe-contents metasomatic fluids, which were resulted of evaporite Mg-enriched solutions transformation during kathagenesis

Transient deep-water oxygenation recorded by rare Mesoproterozoic phosphorites, South Urals

EES acknowledges start-up funds from the School of Earth & Environmental Sciences at the University of St Andrews. The isotope work was supported by RFBR project 19-05-00623. This contribution is dedicated to Galina Ovchinnikova, a pioneer in U-Pb dating of sedimentary apatite at IPGG RAS (St. Petersburg, Russia).The Mesoproterozoic deep ocean is thought to have been largely anoxic, dominated by ferruginous conditions. Phosphate was scavenged from the water column by iron oxyhydroxides, and dissolved nitrate was rapidly reduced along the redoxcline. Primary productivity was therefore generally low, and eukaryotic life was restricted to oxic marine margins. This model is supported by new nitrogen isotope data from the Lower Riphean (ca. 1.55 Ga) strata of the South Ural Mountains, where we find evidence for strong redox stratification, coupled with low organic carbon content. In contrast, unconformably overlying siliciclastic sedimentary rocks of Middle Riphean age (ca. 1.33 Ga) preserve a rare occurrence of apatite concretions, associated with relatively high concentrations of organic carbon (up to 4 wt%). The Pb-Pb isochron age for the apatite concretions of the Zigazino-Komarovo Formation is 1330 ± 20 Ma (MSWD = 3.7). We measured nitrogen isotope ratios across this interval and found δ15N values up to + 7.6‰, which are similar to those in modern upwelling zones and indicative of a significant nitrate reservoir. These observations are most parsimoniously explained by upwelling of phosphate- and nitrate-bearing waters, suggesting that the deep ocean was at least regionally and temporarily oxygenated. The lack of redox-sensitive trace element enrichment in these strata and the general scarcity of apatite accumulations in the Mesoproterozoic sedimentary record suggest that oxic deep-waters were not a global and persistent phenomenon, but our results confirm that oxic, nutrient-rich refugia existed in Mesoproterozoic oceans and were perhaps important for the radiation of early eukaryotes.PostprintPeer reviewe