Skarns usually develop in carbonate country rocks at the contact with intrusive plutons, as shown above. They form as a result of metasomatism by silica, aluminium, iron and magnesium bearing fluids originating from the intrusive.
A common pattern has been defined in the formation of skarns and involves three stages:
1) The intrusion of hot magma forces out ground, formation and metamorphic waters producing a metamorphic aureole, recrystallising limestone to marble, shale to hornfel and sandstone to quartzite.
2) With continued infiltration into the contact rocks by hydrothermal-magmatic fluids, pure and impure marbles are converted into skarns. Initial ore deposition occurs as the pluton cools and some sulphide replacement mineralisation may take place.
3) This is the retrograde (destructive) stage, where cooling of the intrusion allows meteoric fluid to infiltrate the intrusion and the skarns, producing sericitisation in the pluton and hydrous alteration of early skarn minerals. Sulphide replacement bodies also develop at this stage.