VOLATILE FLUID OVERPRESSURE IN LAYERED INTRUSIONS AND THE FORMATION OF POTHOLES

Potholes that develop in layered intrusions show morphologic similarities with sedimentary features known as pockmarks. By analogy with pockmark formation theory, it is suggested that the cumulate section below the pothole region develops significant overpressures as solidifying intercumulus liquid separates a volatile-rich fluid phase. A fluid overpressure develops as fluid separates from the intercumulus liquid faster than the fluid can escape from the crystal pile. The increase in pore pressure expresses itself as a dome-shaped swelling on the chamber floor and in the uppermost part of the crystal pile, which eventually fractures and results in the violent escape of fluid. The combination of fluidization of the cumulates and incongruent melting, caused by the depression of liquidus temperature as a result of volatile elements added to less evolved liquid and crystal assemblages, leads to the formation of the pothole. Excess sulphide precipitation should accompany the fluid-liquid mixing event as S-bearing fluids dissolve in fluid-undersaturated liquids, producing the associated platiniferous sulphide-mineralized zones.