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

Abstract

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 206Pb/238U ages of 463 to 438Ma, 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. 459Ma) 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 ~463 to ~438Ma, indicating that ~460Ma magmas came from both ancient and juvenile sources, whereas ~438Ma magmas involved more juvenile material. Some early Paleozoic granitoids were recently identified in the Eastern Tianshan with the ages between ca. 475 and ca. 425Ma. 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 δ34SCDT values from +4.6 to +15.7‰, 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.