Some intrusions specialise not in copper (porphyry) but in tin and tungsten, plus a related style of gold. These form in and around a cooling granite, from its late, fluid-rich final stage. This is the deposit family the word "intrusion" points to beyond porphyry.

Greisen & vein tin-tungsten

As a granite crystallises, its last hot fluids alter the top of the granite into a rock called greisen and fill cracks with veins of cassiterite (tin) and wolframite / scheelite (tungsten). (Recall Unit 2: cassiterite is the tin ore.) These cluster in the dome-like top — the cupola — of the granite body, also called a pluton (an intrusion that cooled underground).

Intrusion-related gold systems (IRGS)

A gold cousin of porphyries, IRGS are tied to certain granite intrusions but are lower in sulfide and copper, often forming sheeted gold veins in and around the pluton.

Where: Cornwall (England), mined for tin since ancient times; Panasqueira (Portugal) for tungsten; the Bolivian and SE Asian tin belts; and Fort Knox (Alaska) in the intrusion-related gold belt.