A fracture-loop thermal balance model of black smoker circulation

Abstract

This time-dependent model examines the thermal balance between hot rocks and hydrothermal circulation during the formation of volcanogenic ore deposits, in order to establish whether the major heat supply to black smoker systems comes from the rock or from an underlying magma chamber. Flow is modelled as an open thermosyphon in a single fault, which is parallel and very close to the mid-ocean ridge axis. This configuration is based on seafloor and ophiolite evidence of black smoker systems and their structural location. The fault is represented in the model by a series of pipes carrying turbulent flow, which is driven by the buoyancy difference between upflow and downflow. Hydraulic resistance to flow is predominantly in the discharge zone. The fault is allowed to propagate downwards. The horizontal conductive heat flux from the rock is computed analytically, with the water flow treated digitally, so that the heat exchange at the rock-water interface at every position in the loop can be calculated for each time step. The model thus provides the exit water temperature and mass flow rate throughout the lifespan of a hot spring, enabling the resulting ore mass to be estimated. Results show that this system cannot obtain sufficient heat from a thin layer of rock (such as the 1–2 km thick layer of lavas and dykes overlying an axial magma chamber) to sustain long-lived, ore-forming black smokers. If the circulation penetrates through a greater volume of hot rock to the base of a recently-solidified magma chamber, ore masses of a few million tonnes can be formed if conditions are particularly favourable. We conclude that systems producing larger ore masses must tap a magmatic heat supply.