In-situ LA-ICP-MS trace element analysis of magnetite from Mesozoic iron oxide apatite (IOA) deposits in the Luzong volcanic basin, eastern China

Abstract

The iron-copper-gold belt in the Middle and Lower Yangtze River region is among the most important metallogenic provinces in eastern China. The Luzong (Lujiang—Zongyang) ore district hosts many significant iron deposits in the Luzong Basin and is an important part of this belt. The Yangshan deposit is hosted in the Cretaceous volcanic rocks of the middle Luzong Basin. The Makou deposit in the southern-central Luzong Basin has orebodies at the contact between gabbro-diorite and volcanic rocks of the Zhuanqiao Formation. There are three types of magnetite in these two deposits: (1) magmatic magnetite from the Makou gabbro-diorite, (2) hydrothermal magnetite from the Makou deposit, and (3) hydrothermal magnetite in the volcanic rocks from the Yangshan deposit. Magmatic magnetite from gabbro-diorite in the Makou deposit has concentrations of elements, Mg, Al, K, Sc, Ti, V, Cr, Mn, Co, Ni and Zn, higher than hydrothermal magnetite. Magmatic magnetite from the Makou deposit are the highest concentrations of Cr, Sc, Ni, Al, and Zn, but hydrothermal magnetite from this deposit has the lowest concentrations of these elements, whereas hydrothermal magnetite from the Yangshan deposit contains intermediate concentrations of these elements. Overall, there are positive correlations between Mg and Al, and Ti and Mn. The Makou and Yangshan hydrothermal magnetite show highly variable Mg values, apparently influenced by clinopyroxene (diopside) crystallisation from hydrothermal fluids. The Ti, Ga and Sn contents of the Makou hydrothermal magnetite are generally higher than those from the Yangshan deposit, suggesting the Makou magnetite formed under higher temperatures. The formation of IVAl-rich clinopyroxene in the Yangshan deposit may have resulted in the low V content of magnetite, whereas the high Cr content may have been derived from the associated trachyandesite. Leaching of thick volcanic sequences via magmatic fluids may have led to the incorporation of certain elements into the ore-forming fluids, resulting in higher Al, Mn, and Zn concentrations in the Yangshan magnetite. The Makou hydrothermal magnetite has Na and K contents similar but Ca content higher than that of the magmatic magnetite, indicating that Na and K were retained in the melt, but Ca was enriched in the fluid. Both the Makou and Yangshan hydrothermal magnetites contain high V and low Cr concentrations and resemble magnetite in IOA deposits elsewhere. Combined with the wall rocks and mineral assemblages the Makou and Yangshan deposits are best classified as magnetite-apatite deposits. Intensive diopside alteration in these two deposits may be related to the marine sedimentary strata at the depth.