Formation of oreshoots in mesothermal gold-quartz vein deposits: examples from Queensland, Australia

Abstract

Mesothermal gold/quartz vein deposits in granitoid-hosted and melange/sediment-hosted goldfields, contain oreshoots in areas of structural dilation. The environment of formation is localized at depths of 2–5 km in brittle to brittle-ductile shear zones which have stick-slip seismic movement. Lithologic contacts and fissure intersections affect the shape and geometry of the oreshoots. Mineralizing fluids in these shear zones range between 3 and 10 wt% NaCl equiv. with δ 18O values of about 5–8%. Fluids are locally CO 2-bearing with formation temperatures of between 220° and 350°C. Fluid source is either of deep magmatic or metamorphic origin and can rarely be correlated directly with nearby plutonism. Fluid transport is via single pass flow in high permeability conduits, such as mylonites in granitoids or melange cleavage in sediments. Complex mixtures of mineralized quartz, gouge, fault rocks and altered wall-rock reflect the environment of formation. Comb, ribbon, buck and breccia quartz and microscopic textures indicate different processes and stages of oreshoot formation. Quartz deposition is due to changes in silica solubility resulting from temperature and pressure fluctuations. Local pressure changes are due to reduced velocity of the fluid in dilated zones according to Bernoulli's equation. At restricted or dilated portions of the fissure, throttling and adiabatic cooling are common and result in quartz deposition and channel choking which lead to pressure and temperature build-ups and faulting. The long dip-lengths in many mesothermal oreshoots may also account for substantial pressure and temperature reductions in the fluid from bottom to top. The venturi effect in connecting fissures may also affect quartz deposition. Microscopic textures indicate that multiple generations of quartz have been involved in cracking, stress corrosion and dissolution due to porosity changes and dilation prior to and during faulting. Gouge and clay seams stabilize fault movement and may have acted as impermeable barriers and pressure seals which channelled fluid flow, and if hydrated, may have expelled concentrated brines when compressed by faulting. Hydrothermal alteration indicates early diffusive transport and chemical and pH gradients away from the oreshoots. Four main stages of oreshoot formation occur from the outside to the inside of oreshoots as: (1) ground preparation and nucleation, (2) reinjection and sheeting, (3) major fault movement, channeling of fluid flow, local dissolution and cracking, and (4) consolidation and oreshoot growth, including fluid stagnation and ponding.