Re-distribution and upgrade of metals induced by the superimposition of later magmatic fluids in the Archean Hongtoushan VMS deposit, NE China

Abstract

The Hongtoushan copper-zinc deposit is a highly metamorphosed Archean volcanogenic massive sulfide deposit (VMS) in China. After metamorphism, the deposit underwent later fluid superimposition, which is of significance for the re-distribution of metals and upgrading ores. However, the hydrothermal modification mechanism and fluid source remain unclear due to the difficulties in accurately recognizing the mineral textures and assemblages formed by hydrothermal modification. In this study, the mineral texture was investigated to identify the modification mechanism of later fluid and in situ sulfur isotope and trace elements of chalcopyrite were tested to track the source of later fluid and metals. The dominant assemblage formed by superimposed fluid consisted of pyrite, chalcopyrite, sphalerite, minor galena, magnetite, calcite, anhydrite, and hydrothermal alteration minerals, i.e. sericite and chlorite. These minerals infilled the fractures in early pyrite porphyroblasts or precipitated along the grain boundary. Additionally, the pre-existing sulfides were corroded by these minerals. They exhibited replacement and porous textures, indicating that the superimposed fluids resulted in a dissolution-reprecipitation process and formed new sulfides. The hydrothermal metasomatism produced the sericitization of plagioclase and chlorization of biotite, which concurrently liberated silica to form silicification. These interactions suggested that the hydrothermal fluid is acid fluid. The chlorite geothermometer defined the formation temperature of 226–279°C, implying that the superimposed fluid has a moderate temperature. The chalcopyrite precipitated from superimposed fluid either coexisted with pyrite (Ccp 2a) or magnetite (Ccp 2b). The sulfur isotope of Ccp 2a defined a closing zero range (0.75–2.41‰), demonstrating the superimposed fluids were derived from magmatic fluids. In contrast, the Ccp 2b enriched heavy sulfur with a δ34S range of 4.10–8.63‰, which was attributed to the oxidation of fluids. Partial ferrous ions were oxidized to ferric iron, and ferrous and ferric iron precipitated as magnetite. The mineral assemblage changed from chalcopyrite + pyrite, followed by magnetite + chalcopyrite + pyrite, to magnetite + pyrrhotite, representing the local increase of the oxidation state of hydrothermal systems. During the oxidation process, the oxygen fugacity of the superimposed fluid decreased, whereas the pH increased. These results provide crucial evidence on the re-distribution pattern of metals and the post-hydrothermal modification process in the Hongtoushan Cu-Zn deposit.