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Abstract\nKEY POINTSThe Browns Creek Au–Cu skarn is restricted to a 50 m-wide fault zone developed in an interbedded sequence of Upper Ordovician mafic Blayney Volcanics and Cowriga Limestone Member adjacent to the mid-Silurian (434–425 Ma) Carcoar Granodiorite. The mineralised skarns are hosted within and controlled by the Mount David/4000E Fault zone, a dextral transtensional strike-slip fault that in cross-section has the form of a negative flower structure. This is consistent with components of dextral strike-slip motion recorded on regional faults likely guiding magma emplacement during the Benambran Orogeny. The skarns comprise a barren Stage 1 prograde pyroxene–garnet–wollastonite–anorthite assemblage followed by Stage 2 hornblende–biotite–epidote–magnetite–Fe–Cu–As sulfide skarn. These are overprinted by the strongly mineralised Stage 3 retrograde epidote–andradite garnet–chlorite Cu–Au skarn containing the bornite–chalcopyrite–chalcocite–gold ore, when probable magmatic-dominated hydrothermal fluids were channelled through the previously skarned strike-slip fault system that had become a permeable fault zone. Stage 4 sheeted vein-hosted prehnite–epidote–chlorite Au–Cu skarn formed when the strike-slip fault subsequently failed in tension to form gold-rich vertical extensional veins. Conditions for extensional fault reactivation and vein formation are discussed in terms of reshear criteria and fluid overpressure. The age of the mineralisation is constrained by late-stage monzonitic dykes that both crosscut and are themselves skarned by stages 3 and 4 assemblages. These co-magmatic dykes were dated at ca 435–425 Ma, which closely corresponds with the age of the Carcoar Granodiorite itself. Stable and radiogenic isotopes indicate that hydrothermal fluids associated with prograde and retrograde skarns are within the range for magmatic fluids with some mixing with fluids of meteoric origin during later stages of mineralisation. The Browns Creek Au–Cu skarn is typical of Au-skarns worldwide and is linked to mid-Silurian magmatism at a late stage in the Benambran Orogeny.The Browns Creek Au–Cu skarn formed in Upper Ordovician mafic volcanics and interbedded limestone adjacent to a mid-Silurian granodiorite intrusion.The mineralised skarns are contained within a fault zone that developed at the granodiorite margin during magma emplacement at a late stage in the Benambran Orogeny.Retrograde skarns contain Au–Cu mineralisation characterised by bornite; chalcopyrite; chalcocite and gold with grades typically in the range 1–10 g/t Au and 0.3–0.4 wt% Cu.Geological relationships, age dating, stable and radiogenic isotopes all indicate that hydrothermal fluids derived from the granodiorite intrusion were the cause of skarning and Au–Cu mineralisation when channelled through the fault zone. [ABSTRACT FROM AUTHOR]

Sedimentary basins within fold-and-thrust belts preserve the structural record of complex deformations and are essential to many orogenic systems worldwide. One of the component of such deformations is strike-slip tectonics. This research uses high-quality, three-dimensional seismic reflection data to investigate the geometry and kinematics of strike-slip faults developed within a geologically complex region in the Upper Assam foreland basin, NE India. Faults in the basin have a dominant NE-SW trend, with most faults in the SE and SW parts exhibiting sinistral strike-slip. These lateral movements are evidenced by extensional horsetail splay faults, negative flower structures, and minor transfer faults. The displacement profiles of the strike-slip faults also reveal complex segmentation, linkage, and mechanical interactions at different structural levels, indicating their prolonged histories and development. Tectonic tilts of the basin, along with sediment loading from the bordering fold-and-thrust belts over the geologic time, structurally changed the basin configuration giving rise to strike-slip deformations in the study region. These findings contribute to our knowledge of sedimentary basins within fold-and-thrust belts and their structural records of complex deformations. [Display omitted] • Unravels strike-slip tectonics within the Upper Assam foreland basin. • Elucidates structural indicators associated with strike-slip fault movement. • Deciphers the kinematics of strike-slip faults in the Dibrugarh filed. [ABSTRACT FROM AUTHOR]