Isotopic Composition (δ13C and δ18O) and Genesis of Mn-bearing Sediments in the Ushkatyn-III Deposit, Central Kazakhstan

Abstract

The Ushkatyn-III deposit is located 300 km west of Karaganda (Central Kazakhstan). It is classified as a weakly metamorphosed Atasu-type hydrothermal-sedimentary ore deposit. Stratiform iron, manganese, and barite–lead orebodies occur in the Upper Devonian (D3fm2) carbonate sequence of this deposit. The Fe- and Mn-bearing rocks were studied. Iron ores are composed of hematite, calcite, and quartz. Manganese ores are divided into two (braunite and hausmannite) types. Braunite ores are composed of braunite, calcite, quartz, and albite; hausmannite ores are composed of hausmannite, rhodochrosite, Mn-calcite, tephroite (sometimes with sonolite and alleghanyite), and friedelite. The mineral composition of ores was formed during the burial of metal-bearing sediments containing oxides of Fe3+ and Mn3+/Mn4+. The nature and intensity of the postdepositional processes was controlled by the content of organic matter (OM) in initial rocks. Braunite ores formed under oxidizing conditions; hausmannite ores, under reducing conditions. The initial presence of OM is indicated by the carbon isotope composition in carbonates. High δ13Ccarb values (from –0.3 to 2.1‰) in hematite ores, braunite ores, and host limestones are close to those in the seawater bicarbonate precipitated as a part of biogenic calcite (e.g., shell fragments). At the same time, low δ13Ccarb values (average –12.3‰) in rhodochrosite and Mn-calcite from hausmannite ores indicate the influence of carbon from the OM buried in sediments. The general oxygen isotope composition in iron ores, manganese ores, and host rocks was determined by variations of the isotope ratio in the initial precipitates of 18O-rich calcite and 16O-rich Fe/Mn oxides. Conversion of the primary sedimentary Fe3+ and Mn3+/Mn4+ oxides to hematite, braunite, and hausmannite slightly changes the oxygen isotope composition of Fe–Mn minerals. Average values of δ18Otot determined in the hematite (1.7‰), braunite (14.8‰), and hausmannite (12.7‰) ores are close to those in the sedimentogenic iron (0‰) and manganese oxides (11.5‰). The oxygen isotope composition suggests that initial manganese oxides did not participate in the formation of carbonates in braunite ores. Average oxygen isotope composition of carbonates in these ores (δ18Ocarb 19.1‰) is close to that in host limestones (20.8‰). During the formation of hausmannite ores, in contrast, manganese oxides reacted with OM leading to the formation of rhodochrosite and silicates. Consequently, the light isotope oxygen in the starting oxides was fixed in carbonates (δ18Ocarb 15.9‰) and rocks in general (δ18Ototal 12.7‰). The oxygen isotope composition was not averaged during the formation of hausmannite ores even in the mineralogically similar aggregates in the adjacent rock layers.