Zircon and garnet U-Pb dating and in situ trace element of magnetite from the Hongyuntan Fe deposit, Eastern Tianshan, NW China

Abstract

The relationship between the timing of volcanism and hydrothermal alteration and the geochemistry of magnetite are studied to better understand the metallogeny of the Hongyuntan deposit. The orebodies are hosted in the volcanic rocks with banded or lenticular forms, and main massive, disseminated, and veined ores and subordinate mineralized andesite and dacite host rocks are identified at Hongyuntan. SHRIMP zircon U-Pb dating of dacite and LA-ICP-MS garnet U-Pb dating of skarn yield nearly the same ages of 333.8 ± 5.3 Ma and 331.0 ± 6.4 Ma, respectively, indicating an isochronous relationship between volcanic rock and skarn. The age of the diabase dike that crosscuts the orebodies and volcanic strata indicates that the mineralization occurred earlier than 307.6 Ma. The dataset of trace element compositions shows that magnetite grains from stratiform mineralized andesite have high Ti, V, Cr, Ni, Mg, and Sn contents and formed under high-temperature and high-oxygen-fugacity conditions, whereas disseminated magnetite grains from mineralized dacite and chlorite skarn have medium above-element contents, suggesting they were hydrothermally altered under relatively high-oxygen-fugacity and fluid-rock interaction conditions. The fluid mixing and reduced temperature and oxygen fugacity might result in the deposition of massive ores. The increasing Cr, Ni, Mg, and Sn contents in magnetite grains from veined ores and their associated mineral assemblages indicate that overprint mineralization occurred in the Hongyuntan deposit, which was probably caused by multiple periods of volcanic activity or intrusion emplacement. Discrimination plots show that Hongyuntan different magnetite samples have compositions similar to different deposit types and likely formed in magmatic and hydrothermal processes.