The CO 2 -rich magmatic-hydrothermal fluid of the Qiyugou breccia pipe, Henan Province, China: implication for breccia genesis and gold mineralization

Abstract

The Qiyugou deposit, in the Xiong’er terrane, Qinling Orogen, is an auriferous breccia pipe developed in continental collision setting. The breccia pipe exhibits variable-sized clasts and clast mixing, which are typical of fluidized breccias. Brecciation, alteration, and gold mineralization are related to granite porphyry emplaced at c . 134 Ma. However, the relationships of the CO 2 -rich ore-forming fluids with the tectonic setting, structural control, granite magma evolution, and hydrothermal brecciation have not been clearly discussed. New fluid inclusion data presented in this paper indicate that the formation of the Qiyugou deposit includes: (1) an early stage defined by K-feldspar-epidote-quartz-pyrite assemblage; (2) a middle stage of quartz-polymetallic sulphides; and (3) a late stage typified by quartz-carbonate±adularia. Alteration and mineralization resulted from escape of hot (>320 °C), high-salinity (>40 wt% NaCl eq.) magmatic fluid exsolved during the final stage of crystallization of the granite porphyry. Significant pressure drop from 85–90 to 20–38 MPa resulted in breccia formation and precipitation of quartz, sulphides, and gold. A vapour phase was produced at 355–403 °C due to fluid boiling, and then was cooled and condensed under near-critical conditions to a moderately saline, warm liquid. Aqueous-carbonic inclusions were formed by trapping of heterogeneous fluids unmixed from the original H 2 O-CO 2 -NaCl fluid. Late magmatic-hydrothermal fluid mixed with meteoric fluid, and was cooled and diluted to produce lukewarm, low-salinity fluid that precipitated clear quartz and calcite with little or no mineralization. Our review of the literature suggests that the brecciation and mineralization likely coincided with sinistral transtensive faulting during 140–120 Ma. Therefore, synthesis of the foregoing geochemical information with pertinent geological information from the literature lead us to propose that fluidization and seismic pumping were important in the breccia formation and associated gold mineralization at Qiyugou.