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 1^st Member is a volcanic breccia limestone, (2) the 2^nd Member is a sandy limestone, and (3) the 3^rd 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.

It has been proposed that anoxic and iron-rich (ferruginous) marine conditions were common through most of Earth history. This view represents a major shift in our understanding of the evolution of marine chemistry. However, thus far, evidence for ferruginous conditions comes predominantly from Fe-speciation data. Given debate over these records, new evidence for Fe-rich marine conditions is a requisite if we are to shift our view regarding evolution of the marine redox landscape. Here we present strong evidence for ferruginous conditions by describing a suite of Fe-rich chemical sedimentary rocks—banded iron formation (BIF)—-deposited during the Early Cambrian in western China. Specifically, we provide new U-Pb geochronological data that confirm a depositional age of ca. 527 Ma for this unit, as well as rare earth element (REE) data are consistent with anoxic deposition. Similar to many Algoma-type Precambrian iron formations, these Early Cambrian sediments precipitated in a back-arc rift basin setting, where hydrothermally sourced iron drove the deposition of a BIF-like protolith, the youngest ever reported of regional extent without direct links to volcanogenic massive sulphide (VMS) deposits. Their presence indicates that marine environments were still characterized by chemical- and redox-stratification, thus supporting the view that—despite a dearth of modern marine analogues—ferruginous conditions continued to locally be a feature of early Phanerozoic seawater.