Comparative study of ore-forming fluids of hydrothermal copper–gold deposits in the lower Yangtze River Valley, China

Abstract

Fluid-inclusion and stable isotope studies were carried out on five types of Mesozoic (Yanshanian) hydrothermal copper–gold deposits in the lower Yangtze River Valley. Deposits include (1) copper in cryptoexplosive breccia pipes, (2) skarn copper, (3) porphyry copper, (4) high-temperature quartz vein-type copper and gold, and (5) medium–lower temperature fracture zone gold. This research has allowed a comparison between various types of ore-forming fluids. Melt-fluid inclusions in garnet from the matrix of the breccia pipe at the Shizishan copper deposit reveal the existence of a water-rich magma. In all deposit types, fluid temperatures and salinities were higher at early stages and generally decreased with time. Magmatic water is dominant in the high-temperature ore-forming fluids, whereas meteoric water was involved only in the medium–lower temperature Xiaomiaoshan gold deposit and in the post-mineralization stage of the Shaxi porphyry copper deposit. Fluid boiling played an important role in the mineralization of most deposits, particularly at Shizishan, where multi-stage boiling was associated with the formation of cryptoexplosive breccia, skarn, quartz-sulphide, and quartz-carbonate-sulphide stages. Boiling of an aqueous magmatic fluid system at high temperatures reflects the release of crystallization heat and increase of total volume of the magma–fluid system, and hence it can be referred to as active boiling. On the contrary, boiling of a fluid at lower temperatures is typically triggered by pressure release due to fracturing or dilation in the surrounding rocks, and is thus referred to as passive boiling. In general, passive boiling occurs more commonly at the higher levels of a hydrothermal mineral system and at later stages of the ore-forming process.