Textural, mineralogical, and geochemical characteristics of non-volcanogenic Fe–Mn deposit of Motuosala, western Tianshan, northwestern China

Modern and ancient volcanogenic massive sulfide (VMS) systems commonly have associated Fe–Si–(Mn) occurrences that typically are hosted by volcanic suites. However, the Motuosala Fe–Mn deposit in western Tianshan, northwestern China, which shows similar geological features with the Fe–Si–(Mn) occurrences in VMS systems, is hosted by volcanic-free sedimentary sequences and has no clear relationship in space with VMS mineralization. In this paper, textural, mineralogical, and geochemical studies were conducted to unravel the genesis of the Motuosala Fe–Mn deposit. The hematite ores from the Motuosala deposit consist predominantly of fine-grained hematite, barite, and quartz, with minor amounts of galena and chalcopyrite. The jasper samples are mainly composed of amorphous silica with little micro-sized hematite, barite, and galena. Geochemically, the hematite ore and jasper samples all are characterized by enrichment of Cu, Pb, Zn, and Ba, positive Eu anomaly, slightly light rare earth elements enrichment, and negligible Ce anomalies. Based on these results, we proposed that the Fe-, Si-, and Mn-rich layers at Motuosala are the diagenetic products of ancient seafloor hydrothermal sediments which have not suffered significantly replacement by post-precipitation hydrothermal alteration and deformation. This study implies that Fe–Si–(Mn) sediments in volcanic rock-free areas also can be the distal product of submarine hydrothermal system, which can be considered as the proxy for identifying associated submarine volcanogenic iron oxide or VMS mineralization.