Age and geochemistry of the Naxiguole banded iron formation (BIF), NW China: recurrence of superior-type BIF in the Neoproterozoic

The Naxiguole Banded Iron Formation (BIF), located in the Eastern Kunlun Orogenic Belt, Northwest China, is hosted by the Jinshuikou Group and has a Fe of 75 Mt at a grade of 15–28.83 wt.% Fe. In this paper, we use integrated petrological, geochemical, and geochronological data to limit the age and origin of the Naxiguole BIF. Country rocks mainly comprise marble, felsic paragneiss, plagioclase amphibolite schist, and biotite plagioclase gneiss. The features of sedimentary structure (blastopsammitic texture and blastobedding structure) and trace elements (e.g. La, Ni, Zr) from the country rock indicate that their protoliths are sedimentary rock. The 141 detrital zircons from the Jinshuikou Group yield a minimum age of 908 Ma, which can be considered the upper limit for the Naxiguole BIF. According to the age (602 Ma) of the mafic rocks intruding into the Jinshuikou Group, it is reasonable to assume that the formation age of BIF can be limited at 908–602 Ma (i.e. Neoproterozoic). The high contents of Al2O3 + TiO2 (3–6 wt.%), ΣREE (36–135 ppm) for BIF samples suggest that terrestrial clastic material contributed to BIF formation. The BIF samples show seawater-like features, such as depletion in LREEs, positive Y anomaly and high Y/Ho ratios (30.7–57.5, average = 42.5), implying that the Naxiguole BIF precipitated from seawater. The positive Eu/Eu*PAAS anomalies (average 1.52) suggest the influence of high-T hydrothermal fluids. Based on positive Eu anomalies and high terrestrial clastic components and no obvious Ce anomalies, the Naxiguole BIF was formed in an anoxic or suboxic sedimentary basin within active continental island arc, and is similar to Superior-type BIFs worldwide.

中国西北东昆仑造山带内的那西果勒条带状铁建造（Banded Iron Formation，简称BIF）赋存于金水口群地层中，铁矿石资源量达75 Mt，铁品位为15–28.83 wt.%。本文依托综合岩石学、地球化学与地质年代学数据，约束了该BIF的形成时代与成因机制。该BIF的围岩主要由大理岩、长英质副片麻岩、斜长角闪片岩以及黑云母斜长片麻岩构成。围岩的沉积构造特征（变砂状结构与变层理构造）以及微量元素（如镧La、镍Ni、锆Zr）特征均表明，其原岩为沉积岩。对金水口群中141颗碎屑锆石的测年结果显示其最小年龄为908 Ma，这可作为那西果勒BIF形成时代的上限。根据侵入金水口群的镁铁质岩的形成年龄（602 Ma），可合理推断该BIF的形成时代限定为908–602 Ma，即新元古代。BIF样品的Al₂O₃+TiO₂含量（3–6 wt.%）与总稀土元素（Total Rare Earth Elements，ΣREE）含量（36–135 ppm）较高，表明陆源碎屑物质对该BIF的形成存在贡献。该BIF样品表现出类海水地球化学特征：轻稀土元素（Light Rare Earth Elements，LREEs）亏损、正钇异常以及较高的Y/Ho比值（30.7–57.5，平均值为42.5），这指示那西果勒BIF系从海水中沉积形成。相对于后太古代澳大利亚页岩（Post-Archean Australian Shale，PAAS）标准的正Eu/Eu*异常（平均值为1.52），暗示了高温热液流体的影响。综合正铕异常、较高的陆源碎屑组分以及无明显铈异常等特征，那西果勒BIF形成于活动大陆岛弧内的缺氧或次氧化沉积盆地中，与全球范围内的苏必利尔型条带状铁建造（Superior-type BIF）特征相似。