The long-term (hydro-)thermal evolution around laccolith-like intrusions with implications for tin deposit formation

Water-rich magmatic systems are important for the formation of many magmatic hydrothermal ore deposits because degassing intrusions can provide both the heat and the chemical compounds needed for their formation. However, for different types of ore deposits, the processes and their complex interplay leading to mineral precipitation vary. Here, we use numerical simulations to investigate the thermal evolution of silicic magmatic systems similar to those hosting some of the largest Sn deposits. Detailed field and oxygen isotope studies suggest that these deposits can form due to convective mixing of hot magmatic fluids and meteoric water (e.g., Fekete et al., 2016). The timing and location of this fluid mixing as well as the long-term thermal evolution of large magmatic hydrothermal systems, however, has attracted surprisingly little attention (e.g., Large et al., 2021) and has mostly been investigated by either using purely conductive heat transfer approaches or by neglecting magmatic fluid production and degassing during crystallisation.

Here, we present 2D fluid-flow simulations for laccolith-shaped intrusions emplaced at 3 and 5 km depth to (1) explore the long-term hydrothermal evolution related to silicic magmatic systems, and (2) identify preferred conditions to form tin deposits (i.e., mixing of meteoric and magmatic water). The intrusion has dimensions of 20 km width and 1 km thickness and releases aqueous fluids during crystallisation.

We observe a significant temporal delay between the crystallisation of the magma chamber and the emergence of the hydrothermal system. While the intrusion solidifies completely within a few 10³ to 10⁴ years, the hydrothermal convection reaches its maximum extent after full solidification of the magma chamber. Importantly, the temporal delay is larger for intrusions emplaced at 5 km depth. This is because the lower rock permeability at greater depths limits fluid flow velocities and thus prevents significant advection. Fluid convection starts to establish once heat conduction provides sufficient heat to higher-permeability rocks at shallower depths. Due to the low rock permeability, a considerable amount of the magmatic fluids remains trapped in the crystallised intrusion.

For an emplacement depth of 3 km, significant mixing of magmatic and meteoric fluids can occur already during the early stages of degassing (< 10,000 years). Here, higher rock permeability allows for mixing of hot ascending magmatic fluids into meteoric water. In the case of deeper intrusions, the most significant mixing occurs at later stages of the convecting hydrothermal system (> 60,000 years), when downwards-flowing meteoric fluid slowly infiltrates into the crystallised intrusion and mixes with the trapped magmatic fluid.

Overall, we suggest that emplacement depth is a critical parameter controlling the location and timing of fluid mixing around laccolith-like intrusions and therefore potential tin precipitation mechanisms.

 

REFERENCES 

Fekete, Sz. et al. 2016: Contrasting hydrological processes of meteoric water incursion during magmatic–hydrothermal ore deposition: An oxygen isotope study by ion microprobe, EPSL, 451, 263‒271.

Large, S. et al. 2021: Copper-mineralised porphyries sample the evolution of a large-volume silicic magma reservoir from rapid assembly to solidification, EPSL, 563, 116877.