Hydrothermal fluid characteristics and implications of the Makou IOA deposit in Luzong Basin, eastern China

Abstract

A large number of iron oxide–apatite (IOA) deposits occur in the Middle and Lower Yangtze River metallogenic belt (MLYB), eastern China. Proterozoic phosphorus-rich strata are also widely exposed in this belt, providing a good opportunity to research the relationship between phosphorus-rich layers and IOA deposits. The Makou IOA deposit is located in the central part of the volcanic Luzong Basin. The ore body is developed in the contact zone between gabbro-diorite and Cretaceous trachyandesite, and it is the only IOA deposit in the basin that spatially associated with a dioritic intrusion. The main hydrothermal alteration minerals are albite, diopside, apatite, chlorite, and carbonate. In this study, we employed SHRIMP SI to analyse in situ O isotope ratios of magmatic and hydrothermal apatite and in situ S isotope ratios of pyrite from the Makou deposit. Additionally, LA-ICP-MS was used for trace element analysis. The results show that the sulfur in the pyrite of the Makou deposit comes from different sulfur phases (e.g. H2S and SO2) in the hydrothermal fluid, and ΣS34total is similar to that of magmatic sulfur, indicating that the hydrothermal fluid was weakly affected by evaporite, which is substantially different from other IOA/skarn iron deposits in the MLYB. Both analysed apatite samples are classified as fluorapatite, in which the δ18Oap of the Makou magmatic/hydrothermal apatite is similar to that of the initial mantle (3.7–5.5‰) and much lower than that of the Susong sedimentary-metasomatic apatite (13.7–14.2‰). The LREE/HREE ratio increase from magmatic to hydrothermal apatite, whereas the Sr/Y decreases, indicating that the hydrothermal fluid metasomatised the dioritic intrusive rock, resulting in a large amount of albite crystallisation and release Fe from pyroxene and biotite to hydrothermal fluid, indicates that Fe-rich fluid in IOA deposit could be formed by albitization of the consolidated intrusive rock. By combining this finding with the oxygen isotope data, we propose that the mineralization of the Makou deposit was mainly controlled by a magmatic-hydrothermal process. The low CO2 and SO42− contents in the volcanic rock are a key factor leading to apatite precipitation. The addition of a phosphorus-rich layer or evaporites is not necessary for the formation of IOA-type deposits in the MLYB.