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

Abstract

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 CO 2 ± CH 4-rich variety and a H 2O-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 ( λ 3). 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.