Vol 54, No 4 (2019)

This work considers for the first time mineralogical features of the Lower Cambrian (Tommotian) glauconite collected from terrigenous–carbonate rocks of the Kessyusa Group (upper part of the Mattaia Formation and lower part of the Chuskuna Formation), as well as from basal beds of the overlying limestones of the Erkeket Formation. Samples were taken from three sections on the northwestern slope of the Olenek Uplift, North Siberia. Their stratigraphic assignment is given on the basis of recent data (Nagovitsin et al., 2015 and others). Grains of the layer silicates are made up of the mixed layer mica–smectite phases with relatively low (<10%) and higher (10–20%) contents of expandable layers (unit cell parameter b = 9.06–9.12 Å). Micaceous minerals form a series from glauconite to Al-glauconite (Al index K_Al = ^VIAl/(^VIFe³⁺ + ^VIAl) is 0.11–0.47 and 0.60, respectively), with the K₂O content varying from 6.80 to 8.54%. Detailed lithological-mineralogical characteristics is given for the first time for the glauconite-bearing rocks, the primary sediments of which were accumulated in the prefrontal beach zone and transitional beach–shelf zone in the Siberian epicontinental marine paleobasin (Marusin, 2016). Origin of the studied grains (authigenic, allothigenic) is discussed and their secondary alterations at different stages of lithogenesis are considered (rewashing, phosphatization, pyritization, calcitization, ferrugination, and others). It is shown that the obtained preliminary Rb–Sr dates (450–320 Ma) are “rejuvenated” and do not correspond to the age value of 541.0 Ma accepted for the Vendian–Lower Cambrian boundary (Gradstein et al., 2012). This can be related to diverse secondary alterations of the glauconite grains during the rewashing, transportation, and different stages of diagenesis of primary sediments after their redeposition, as well as at stages of catagenesis and hypergenesis of the glauconite-bearing rocks.

The brine-saturated leaching zone discovered in the Verkhnyaya Kama salt deposit is scrutinized. It comprises a cavernous core (40 × 70 m) composed of aposylvinite syngenite (halite rock with gypsum and kalistrontite) and a recrystallization aureole (60–80 m wide) without secondary sulfates. The leaching zone is overlain by a linear zone of the weathering and decarbonatizaton of marls in the suprasalt sequence. This zone is controlled by a nearly latitudinal anticlinal ridge extending parallel to the Durinsky trough related to synsedimentary normal faulting. Injection of suprasalt waters into the salt body is attributed to deformations of the salt sequence as a result of its articulated bending during the formation of the Durinsky trough in the Early Permian. Brines in the studied leaching zone are linked with the infiltrational suprasalt calcium sulfate waters. Their position in the general genetic typification of natural brines and waters circulating in the salt body of the Verkhnyaya Kama deposit is defined.

Manganese ores of the Lower Proterozoic Hotazel Formation (Transvaal Supergroup) associated with the banded Fe-silicites are marked by high concentrations of several rare elements (B, Ge, W, Mo, Cr, Ni, Zn, Cd, Pb, Ag, Bi, As, Sb, Te, Se). High boron contents in the oxide‒carbonate ores (Mn-lutites) are attributed to the chemosorptional concentration of this element on Mn-carbonates. Owing to hydrothermal transformations, a wide range of the ore-forming (mainly Fe and Mn) and rare elements (REE included) was removed from the underlying hyaloclastic basaltic andesites of the Ongeluk Formation. Manganese ores and Fe-silicites are characterized by the typical values of the cerium (Ce/Ce* 0.28–1.72) and europium (Eu/Eu* 0.57–16.31) anomalies that can suggest that primary sediments were deposited in a marginal shallow-marine basin with a prominent oxic surficial water layer and subanoxic conditions near the floor. At different stages of lithogenesis, metalliferous (Mn, Fe) sediments of the shallow-water basin were enriched in Eu (positive Eu/Eu*) and subjected to metasomatism (with the redistribution of Mn and the formation of manganese carbonates) and the consequent regional metamorphism (up to the stage of green sericite schists).