Abstract Orogenic gold systems are open, flow-controlled thermodynamic systems and generally occur in mid- to upper crustal environments where there is strong coupling between fluid flow and dilatant plastic deformation. This paper considers the principles involved in such coupling, with an emphasis on the elastic and plastic volume changes and their influence on the fluid, mechanical and thermodynamic pressures. Some misconceptions regarding the magnitudes of these three distinctly different pressures and their influences on fluid flow and chemical equilibrium are addressed, with examples at both the tens of metres scale and the crustal scale. We show that the mean stress is less than twice the lithostatic stress for Mohr–Coulomb materials with cohesion and the thermodynamic pressure only has meaning under isentropic conditions and hence is less than many previously published estimates based on high mean stresses. At the crustal scale, we also include the role of critical behaviour in influencing the geometry and magnitudes of fluid pressure gradients and fluid flow velocities in open, flow-controlled systems.

Abstract Episodic fluctuations in fluid pressure and temperature are characteristic of the behaviour of orogenic gold systems and are commonly attributed to processes external to the system, such as seismic events and associated adiabatic fault valve or suction pump/piston behaviour; any temperature changes are attributed to the adiabatic nature of the process. Such processes are commonly associated with fluctuations in fluid pressure with little, if any, changes in temperature. We describe aseismic, non-adiabatic processes internal to the hydrothermal system that lead to episodic modes of temperature and fluid pressure behaviour and the deposition of gold. These gold deposition processes are essentially controlled by localized changes in temperature; such temperature-dependent gold deposition processes are normally thought of as inconsequential. We propose that internal episodic behaviour is fundamental in hydrothermal mineralizing systems associated with orogenic gold deposits. Importantly, the time period for these hydrothermal events appears to be small relative to metamorphic systems, with 1–2 myr as an upper limit. This has important ramifications for rates of heat production and for the resultant kinetics of mineral reactions during alteration and mineralization. We explore these systems as non-linear, non-equilibrium dynamic, open flow systems.

Abstract: Deccan volcanism was synchronous with rifting along the west coast of India. Pre- and synmagmatic rifting has been widely reported in the Deccan Volcanic Province, but extension post-dating magmatism, and predating India–Eurasia collision, is less well known. A recent study in the Kachchh area of western India documented weathering of basalts to kaolinite at the base of Cenozoic rift basins, with rift flanks relatively less altered to smectites, and this was attributed to post-magmatic rifting. This study models basalt weathering under open- and closed-system conditions to simulate rainwater interacting with basalts either on topographical slopes (within rifts) or on flat-topped hills (flow tops). Both systems were modelled under p CO 2 conditions ranging from low, present-day values to higher values more appropriate for the end-Cretaceous–early Paleocene time, after basalt emplacement. The results show that if p CO 2 exceeded values of 10 −2.5 , basalts would be altered to kaolinite in both open and closed systems. Existing p CO 2 estimates in the aftermath of Deccan volcanism fall below this value, implying that the differential basalt weathering was more likely to have been caused by terminal to post-magmatic rifting. This indicates that extensional tectonics along the Indian west coast in the Kachchh region continued even after cessation of Deccan volcanism.