Origin and metal source of the Carboniferous Ortokarnash manganese deposit in the Western Kunlun Orogen, Northwest China

Abstract

The formation mechanisms of Mn(II) carbonate mineralization and the precise sources of metals in ancient sedimentary Mn ore deposits remain subjects of debate. The Ortokarnash Mn ore deposit occurred within a mixed carbonate-siliciclastic sedimentary sequence of the Upper Carboniferous Kalaatehe Formation consisting of three lithological members. In this study, detailed component-specific solution analyses were performed on same sample powders from the Ortokarnash Mn(II) carbonate ores and wall rocks. The acetic acid-soluble fractions (i.e., leachates) in Mn(II) carbonate ores show a distinct PAAS-normalized positive Ce anomalies (3.73 ± 0.21), negative Y anomalies (0.90 ± 0.03), and low Y/Ho ratios (22.42–24.84). These are typical features of modern marine hydrogenetic Mn(III/IV) oxide precipitates, indicating that the Mn(II) carbonate mineralization likely the diagenetic product of precursor Mn(III/IV) oxide reduction. The remarkable positive δ⁵³Cr values (1.04 ± 0.11 ‰) in leachates of the Mn ores further confirm that Mn(II) carbonates formed during diagenesis through the reduction of Mn(III/IV) oxides originally deposited from an oxygenated water column. Moreover, the PAAS-normalized REE + Y patterns for leachates in associated wall rocks from the 3rd Member are characterized by no meaningful or a slight positive Ce anomalies (1.03–1.49, mean = 1.17), which is indicative of an active Mn(III/IV) oxide shuttle across a redox-stratified basin water column.

The δ⁵³Cr values (−0.17 to 0.04 ‰) in leachates of the associated wall rocks from the 3rd Member lie within or close to the Bulk Silicate Earth, and thus likely indicate discharge of Cr(III) of submarine hydrothermal origin in the local basin. The acetic acid-insoluble fractions (i.e., residues) in wall rocks are characterized by low Th/Sc (0.02–0.24) ratios, slight chondrite-normalized light REE enrichment (Nd/Yb_N = 2.08 ± 0.77), primitive mantle-normalized LILEs enrichment, positive U-Pb and negative Nb-Ta-Ti anomalies, and high radiogenic Nd (–3.63 ≤ εNd(t) ≤ 3.94) and low radiogenic Sr (0.704854 ≤ (⁸⁷Sr/⁸⁶Sr)_i ≤ 0.707710) isotope compositions, indicating that siliciclastic debris in wall rocks were likely derived from a depleted mantle source (e.g., WKO mafic volcanic rocks). By contrast, the residues in Mn(II) carbonate ores display low Al/Ti (13.63–21.85) and high Th/Sc (1.04–2.43) ratios, marked chondrite-normalized light REE enrichment (Nd/Yb_N = 7.52 ± 2.23), primitive mantle-normalized LILEs enrichment, positive U-Th-Pb and negative Ta-Ti anomalies, and low radiogenic Nd (εNd(t) = –6.07 ± 0.40) and high radiogenic Sr ((⁸⁷Sr/⁸⁶Sr)_i = 0.708064 ± 0.000612) isotope compositions. These element and isotope geochemical features are consistent with that of felsic/intermediate rocks, suggesting that siliciclastic component in Mn(II) carbonate ores were likely sourced from old crustal units (e.g., Tarim felsic/intermediate basement rocks). Thus, it has been suggested that the Mn metal was likely derived from submarine hydrothermal vent fluids (δ⁵³Cr =  − 0.03 ± 0.07 ‰), while this hydrothermal vent system likely occurred on an ancient continental crust (εNd(t) = –6.07 ± 0.40).