Petrography and Geochemistry of the Carboniferous Ortokarnash Manganese Deposit in the Western Kunlun Mountains, Xinjiang Province, China: Implications for the Depositional Environment and the Origin of Mineralization

Abstract

The Upper Carboniferous Ortokarnash manganese ore deposit in the West Kunlun orogenic belt of the Xinjiang province in China is hosted in the Kalaatehe Formation. The latter is composed of three members: (1) the 1st Member is a volcanic breccia limestone, (2) the 2nd Member is a sandy limestone, and (3) the 3rd Member is a dark gray to black marlstone containing the manganese carbonate mineralization, which, in turn, is overlain by sandy and micritic limestone. This sequence represents a single transgression-regression cycle, with the manganese deposition occurring during the highstand systems tract. Geochemical features of the rare earth elements (REE+Y) in the Kalaatehe Formation suggest that both the manganese ore and associated rocks were generally deposited under an oxic water column with Post-Archean Australian Shale (PAAS)-normalized REE+Y patterns displaying characteristics of modern seawater (e.g., light REE depletion and negative Ce anomalies). The manganese ore is dominated by fine-grained rhodochrosite (MnCO3), dispersed in Mn-rich silicates (e.g., friedelite and chlorite), and trace quantities of alabandite (MnS) and pyrolusite (MnO2). The replacement of pyrolusite by rhodochrosite suggests that the initial manganese precipitates were Mn(IV)-oxides. Precipitation within an oxic water column is supported by shale-normalized REE+Y patterns from the carbonate ores that are characterized by large positive Ce (>3.0) anomalies, negative Y (~0.7) anomalies, low Y/Ho ratios (~20), and a lack of fractionation between the light and heavy rare earth elements ((Nd/Yb)PAAS ~0.9). The manganese carbonate ores are also 13C-depleted, further suggesting that the Mn(II) carbonates formed as a result of Mn(III/IV)-oxide reduction during burial diagenesis.

Source Characteristics of the Carboniferous Ortokarnash Manganese Deposit in the Western Kunlun Mountains

“…Geochemical analysis for major and trace elements was already performed by Zhang et al [44]; thus, this study focused on the isotope geochemistry. The strontium (Sr) and neodymium (Nd) isotopic determination of whole-rock samples followed procedures similar to those described by Li et al [74,75].…”

“…The Mn metallogenic zone is roughly 60 km (west to east) in length by 2 km (north to south) in width and encompasses one active (Ortokarnash) and two prospecting (Muhu and Malkantu) mines as well as several manganese mineralization spots (Figure 1B). The exposed strata include: [44]). (B) Simplified geological map of the giant Malkansu manganese carbonate zone, northwestern portion of the West Kunlun orogenic belt (modified after [44]).…”

“…Stratigraphic succession for the Carboniferous to Permian volcano-sedimentary sequence of the Malkansu back-arc basin, highlighting the Kalaatehe Formation composed of three members and the interval of manganese mineralization (modified after [44]). Previously published age constraints are also listed, and these data are from [40,72,73].…”

“…Previous studies have provided significant information about the geological and geochemical characteristics of the Mn-bearing sequences of the Kalaatehe Formation [40][41][42][43]. Based on detailed lithological observations combined with rare earth elements and C isotope studies, Zhang et al [44] suggested that both the Mn ores and the associated carbonate rocks at Ortokarnash were possibly deposited under an oxic water column, and the Mn(II) carbonates were the result of dissimilatory Mn(III/IV) oxide reduction during sedimentary diagenesis. However, the source of Mn ore intervals in the Kalaatehe Formation is still unclear.…”

“…These data, combined with elemental data published previously, served to more fully characterize the geochemical composition of the ore deposit in order to better understand both the geochemical background of the depositional basin and the source of Mn, as well as the mechanism of the Mn mineralization. (A) Simplified tectonic units of the West Kunlun orogenic belt (modified after [44]). (B) Simplified geological map of the giant Malkansu manganese carbonate zone, northwestern portion of the West Kunlun orogenic belt (modified after [44]).…”