A fluid inclusion study of the Suicide Ridge Breccia Pipe, Cloncurry district, Australia: implication for breccia genesis and IOCG mineralization

The Suicide Ridge Barren Breccia Pipe (not, vert, similar1527 ± 4 Ma) is one of numerous breccia occurrences that characterize the Cloncurry district, Australia. Interest in breccia systems in the Cloncurry district is related to their possible link with iron oxide copper–gold deposits. The Suicide Ridge Barren Breccia Pipe is temporally and spatially associated with a granite that outcrops at its southwestern boundary and which is ascribed to the Williams and Naraku magmatic event (ca. 1550–1500 Ma). The breccia is characterized by clasts of variable dimensions, degree of rounding and composition that includes dominant calc-silicate clasts of the Corella Formation that underlie rocks of the Soldiers Cap Group that host the pipe, indicating a dominant upward transport of the fragments. Other breccia fragments include granite bodies characterized by albite–quartz mineralogy and textures typical of the magmatic-hydrothermal transition. These rocks are interpreted to have formed in a fractionated granite carapace during the exsolution and accumulation of volatiles associated with the cooling magma. Two dominant types of inclusions have been identified at the Suicide Ridge Barren Breccia Pipe. These include early primary and pseudosecondary carbonic fluid inclusions, observed only in magmatic-hydrothermal transitional textures of granite breccia clasts, and secondary sodic (-calcic) brine inclusions that occur in all the collected samples. Variable homogenization temperatures in the primary and pseudosecondary pure CO2 fluid inclusions indicate that they were trapped under different pressure conditions. Pressures estimated from the primary CO2 fluid inclusions suggest that they were trapped at 4.2 kbar, at least 1 kbar above the inferred lithostatic pressure. This likely represents the overpressuring produced by release of CO2-rich volatiles during the crystallization of magma and which ultimately caused the subsequent brecciation. Estimated pressures for pseudosecondary CO2 fluid inclusions indicate pressure entrapment conditions around 2.7 kbar, similar to the lithostatic pressure and 1.5 kbar lower than pressures recorded by type Ia fluid inclusions. This significant drop of pressure likely represents the sudden pressure depression occurring in response to brecciation. Evidence from field relationships, breccia characteristics, mineralogy and textures of granite clasts within the pipe and fluid inclusion studies support an origin of the Suicide Ridge Barren Breccia Pipe in response to overpressuring generated by the release, expansion and possibly phase separation of fluids of dominant carbonic composition from a crystallizing magma beneath and adjacent to the pipe. Sodic (-calcic) fluid inclusions represent a later influx of fluids in the breccia system that produced widespread albitization. Significantly, high temperature, ultrasaline multisolid fluid inclusions abundant in IOCG deposits and considered to be samples of the mineralizing fluid were not observed at the Suicide Ridge Breccia Pipe. Their absence in the barren systems suggests that they were critical for the formation of IOCG deposits