Petrological and geochemical features of the early Paleozoic granitic gneisses and iron ores in the Tianhu iron deposit, Eastern Tianshan, NW China: Implications for ore genesis

作者：Zheng, Jiahao^*; Mao, Jingwen; Yang, Fuquan; Chai, Fengmei; Shen, Ping

来源：Lithos, 2017, 286: 426-439.

DOI：10.1016/j.lithos.2017.06.021

• Tianhu iron deposit
• Granitic gneisses
• Eastem Tianshan
• Early Paleozoic
• Ore genesis

This paper reports whole-rock geochemical, zircon U-Pb and Hf isotopic data for ore-hosted granitic gneisses, mineral compositions of oxides, and sulfur isotopic data for sulfides in iron ores from the Tianhu deposit, central part of the Eastern Tianshan. Our results can provide crucial constraints on the genesis of granitic gneisses and early Paleozoic tectonic setting of the Eastern Tianshan. LA-ICP-MS U-Pb dating on magmatic zircons yielded weighted mean Pb-206/U-238 ages of 463 to 438 Ma, interpreted as the crystallization ages of the granitic protoliths and the formation ages of the Tianhu Group. Zircon U-Pb age of ore-hosted granitic gneiss (ca. 459 Ma) can provide reliable constrains on upper limit for iron mineralization age in the Tianhu deposit. Geochemical characteristics suggest that the protoliths of the Tianhu granitic gneisses are metaluminous to weakly peraluminous high-K calc-alkaline granitic rocks, exhibiting typical subduction-related features such as strong enrichment in LREE and LILE and depletion in HFSE. Zircon Hf isotopic compositions show a positive trend from similar to 463 to similar to 438 Ma, indicating that similar to 460 Ma magmas came from both ancient and juvenile sources, whereas similar to 438 Ma magmas involved more juvenile material. Some early Paleozoic granitoids were recently identified in the Eastern Tianshan with the ages between ca. 475 and ca. 425 Ma. The formation of these early Paleozoic granitoids was in response to subduction processes, suggesting that subduction of Junggar Ocean probably began in the Early Ordovician and lasted until Late Silurian. @@@ Pyrite and pyrrhotite in iron ores have delta S-34(CDT) values from +4.6 to +15.7 parts per thousand which are consistent with the marine source, but inconsistent with the magmatic source or those involved evaporites in skarn iron deposit. Geological, geochemical, and isotopic data suggest that the Tianhu iron ores were formed by volcano-sedimentary processes in a subduction environment during the early Paleozoic time, and Tianhu is a metamorphosed volcano sedimentary iron deposit, rather than a skarn iron deposit as previously proposed.

• 出版日期2017-8
• 单位新疆大学; 北京大学; 中国科学院地质与地球物理研究所兰州油气资源研究中心; 中国地质科学院矿产资源研究所; 中国科学院地质与地球物理研究所