A CO2-rich porphyry ore-forming fluid system constrained from a combined cathodoluminescence imaging and fluid inclusion studies of quartz veins from the Tongcun Mo deposit, South China

Abstract

The Tongcun deposit is a medium-sized porphyry Mo deposit in the eastern section of the Qin-Hang metallogenic belt, South China. Fluid inclusions and scanning electron microscope -cathodoluminescence (SEM-CL) of different generations of quartz from multi-stage veins were studied to reconstruct the evolution of the porphyry hydrothermal system at the Tongcun Mo deposit. Four stages of hydrothermal veins were recognized at Tongcun, including the early barren quartz vein, the main ore-stage quartz-molybdenite vein and quartz-polymetallic vein, and the late quartz-calcite (±pyrite) vein. SEM-CL imaging revealed that these veins were composed of at least four successive quartz generations (A, B, C and D) in which only the quartz B and C are in close association with Mo mineralization. Fluid inclusions from the Tongcun deposit can be classified into four types, namely three- or four-phase CO2-rich (C), two-phase vapor-rich (V), two-phase liquid-rich (L) and three-phase liquid-rich solid-bearing (S) type. The C type fluid inclusions were further divided into C1 subtype with higher carbonic proportion (50–70% vol%), C2 subtype with lower carbonic proportion (25–40% vol%) and molybdenite daughter mineral, and C3 subtype with lower carbonic proportion (25–40% vol%) but no daughter mineral. The V type fluid inclusions were further divided into V1 subtype with higher filling degree (0.2 to 0.3) and V2 subtype with lower filling degree (<0.1). The C1 and V1 subtypes were secondary inclusions in quartz phenocrysts (P) and primary inclusions in quartz A, and show homogenization temperatures (Th) of 257–378°C, with salinities of 0.6–9.6wt% NaCl equivalent. The immiscibility assemblage of coexisting C2 and V2 subtype inclusions in quartz B exhibit a Th of 237–285°C and salinities of 1.2–4.4wt% NaCl equivalent. C3 subtype inclusions in quartz C display Th of 191–267°C and salinities of 0.2–4.6wt% NaCl equivalent. Finally, the L and S type inclusions in quartz D display Th of 133–245°C and salinities of 0.4–7.0wt% NaCl equivalent. The fluid immiscibility is restricted in quartz B and only generated low salinity end member inclusions, similar to the immiscible behavior commonly observed in orogenic gold deposits. The likely molybdenite daughter minerals were only observed in C2 subtype inclusions in quartz B, indicating enrichment of Mo had occurred during fluid immiscibility. Combined with SEM-CL texture and Raman analysis, we proposed that a rapid Mo precipitation occurred in the narrow interval during or immediately after fluid immiscibility and thus formed the Tongcun Mo deposit. The CO2-rich ore-forming fluid and continental arc setting of the Tongcun deposit provide a special case of CO2-rich porphyry Mo system in non-collision settings.