Fluid inclusion and H–O isotope evidence for immiscibility during mineralization of the Yinan Au–Cu–Fe deposit, Shandong, China

Abstract

The fluid inclusion and H–O isotope studies have provided the evidences for the source of ore-forming fluids, and helped to recognize two types of immiscibility and their relationships with mineralization. Hydrogen and oxygen isotopic geochemistry shows that the earlier ore-forming fluids during the anhydrous skarn stage (I) and the hydrous skarn-magnetite stage (II) were mainly derived from magmatic water, while the later fluids during the quartz-sulfide stage (III) and the carbonate stage (IV) were mainly from magmatic water mixed with small amounts of meteoric water. Various types of fluid inclusions, including abundant vapor- or liquid-rich two-phase aqueous inclusions, daughter minerals-bearing multiphase inclusions, CO 2–H 2O inclusions, and less abundant liquid inclusions, vapor inclusions and melt inclusions, are present in hydrothermal minerals of different stages. The liquid–vapor fluid inclusions are mainly composed of H 2O, with significant amounts of CO 2 and a small amount of CH 4. In the opaque-bearing fluid inclusions, the hematite and fahlore (tetrahedrite) were identified. The homogenization temperature of the aqueous fluid inclusions decreases from Stage I (520–410 °C), through Stage II (430–340 °C) and III (250–190 °C), to Stage IV (190–130 °C). The coexistence of melt inclusions with simultaneously trapped vapor- or liquid-rich two-phase aqueous inclusions and daughter minerals-bearing multiphase inclusions in garnet, diopside and epidote of Stages I and II suggests an immiscibility between silicate melt and hydrothermal fluid. It is an effective mechanism on scavenging and transporting ore-forming components from magmas. The aqueous fluid inclusions with various vapor/liquid ratios (from <10% to >65%) commonly coexist with simultaneously trapped liquid inclusions, vapor inclusions, daughter minerals-bearing multiphase inclusions and CO 2–H 2O inclusions in the quartz of Stage III, and the different kinds of the fluid inclusions have similar homogenization temperatures. This indicates that the boiling – another kind of immiscibility, widely took place during Stage III. It resulted in the precipitation and enrichment of gold, copper and iron.