Geology and geochemistry of the high-grade Zankan magnetite ore, Western Kunlun Mountains, NW China

Abstract

Over the past decade, several magnetite-rich iron deposits have been discovered in the Taxkorgan region of the western Kunlun Mountains of northwestern China. These include the Zankan, Jiertieke, Laobing, Yelike, and Taaxi iron deposits. Prospective iron ore reserves are estimated at over 1 billion tons, with an average magnetite grade of 28% (Feng et al., 2018). The largest of the deposits, the Zankan Iron Deposit (ZID), is estimated to contain 628 million tons of iron ore, with at least 10% of the ores being high-grade (TFe > 50 wt%). However, the geological processes by which the high-grade ores in the ZID were formed remains poorly constrained. In this study, we combine whole rock and ore geochemistry data, oxygen and sulfur stable isotopes, and uranium-lead and hafnium isotopes from zircons to elucidate the plausible mechanisms underpinning the formation of the ZID ores. Based on mineralogy, we define two ore types. Type 1 is essentially comprised of quartz and magnetite, interlayered with schist, and it retains the sedimentary fabrics of the precursor iron-rich sediments which were an iron formation (IF). Type 2 formed by the intrusion of a dacite-porphyry into the IF which led to magnetite remobilization and recrystallization as manifest in magnetite-ferro actinolite veining. Rare earth element (REE) characteristics of the high-grade ores differs from those of the precursor IF, with chondrite normalized REE patterns showing enrichment of LREE and significant negative europium anomalies close to those of the dacite-porphyry. The oxygen and sulfur isotopic composition of magnetite and pyrite in the high-grade ores suggests a further meteoric influence on the ZID with silica enrichment driven by leaching of the precursor IF from elsewhere in the sequence. Circulation of the ore forming fluid is attributable to the development of faults and fracture zones that resulted from the intrusion of the dacite-porphyry. Laser ablation uranium-lead analyses of zircons from the dacite-porphyry yield two groups with ages of 551.25 ± 3.32 Ma and 497.82 ± 3.81 Ma, representing captured detrital zircons and the intrusive age of dacite porphyry, respectively. Given the intimate geological relationship between the dacite-porphyry and T2 ore, we suggest that the high-grade ores are formed after 497.82 Ma. Collectively our field observations and geochemical data reveal the detailed alteration processes that led to the development of the high-grade iron ores in the ZID, identifying distinctive features that may help to guide future exploration for additional high-grade ore deposits in the Taxkorgan region.