Fluid inclusions in crack-seal veins at Dugald River, Mount Isa Inlier: implications for palaeostress states and deformation conditions during orogenesis

An integrated geometrical investigation and microthermometric analysis of fluid inclusion trails in fibrous crack-seal veins at Dugald River, Mount Isa Inlier, allowed distinction of two general types of fluid inclusions: a CO2 ± CH4-rich variety and a H2O-rich variety. At least three stages of fluid percolation are identified to be related to the vein formation. Trails of pseudosecondary inclusions have a constant orientation relative to the direction of the regional maximum principal shortening direction (λ3). This relationship allows the trails to be regarded as potential structural markers. However, trails of secondary inclusions generally show a deviation from the established bulk shortening direction and possibly reflect the overprinting of a later deformation event. Microthermometric data combined with knowledge of metamorphic reactions suggest that the P-T path of crackseal deformation started at about 450 °C and 2.8 kbar, then crossed 340 °C and 1.2 kbar, and possibly terminated around 130 °C and at near-surface pressure, corresponding to the regional crustal uplift. This study shows the importance of combined geometrical and microthermometric studies of fluid inclusions for understanding the physical conditions of crack-seal deformation, a common phenomenon in low-grade metamorphic terrains.