Field and zircon U–Pb geochronological evidence for the occurrence of Cambrian banded iron formations in the West Kunlun Orogenic Belt, China

Abstract

Abundant bedded iron deposits have been discovered recently in the Taxkorgan Terrane, West Kunlun Orogenic Belt, China. Magnetite is the dominant iron mineral, which is interbedded with amphibolite-facies metamorphic silicate minerals, quartz, and calcite. Field observations revealed that the iron orebodies are hosted in the Bulunkuole Complex, which consists mainly of submarine volcanic and sedimentary sequences. The iron orebodies are typically in conformable contact with bimodal volcanic rocks. The nature of iron mineralization in this region is unclear, mainly because it appears to be similar to typical banded iron formations (BIFs), but is of inferred Cambrian age. We undertook detailed field investigations of this area and zircon U–Pb dating of meta-rhyolites from three iron deposits (i.e., the Zankan, Taaxi, and Ziluoyi deposits). Meta-rhyolites from the deposits have ages of 540.2 ± 2.4, 540.2 ± 2.4 to 515.2 ± 2.7, and 526.4 ± 2.5 to 508.8 ± 3.6 Ma, respectively, indicating that the iron orebodies can be subdivided into four groups, based on their formation ages (i.e., 540, 526, 515, and 508 Ma). These ages suggest that the iron orebodies are unique Cambrian BIFs, although the gangue minerals in these orebodies have been mainly converted into amphibolite-facies minerals, owing to the regional high-grade metamorphism. The close relationship between iron orebodies and bimodal volcanic rocks indicates the iron deposits are Algoma-type BIFs. Abundant pyrite in the iron orebodies from the Zankan deposit suggests that the basin in which these rocks precipitated was sulfidic and in a relatively shallow shelf environment. The lack of pyrite in ores from the Taaxi and Ziluoyi deposits indicates they precipitated in ferruginous basins with relatively deep seawater. We propose that intense submarine magmatism and local anoxic basins were the key factors that controlled the formation of these unique Cambrian BIFs in the Taxkorgan Terrane.