The Basics
- Intrusion centered
- Spatially associated with porphyritic intrusions
- Dominantly related to a volcanic edifice
- Geometry reflects the intrusive system, structures, and host rock form/composition
- Disseminated Cu+/-Mo, Au, Ag mineralization
- Concentric alteration zones – 2->5 times the size of the deposit; vertical zoning ~500- 3000m
The Basics: Field features
- K-silicate, later sericite alteration
- Qtz-vein stockwork, zoned Cu sulfides
- Size: 100 to >5000 million tonnes; 0.5 to 1.5% Cu;
- Vertical cylinder ~500 x 500 x 1000m
- Regional alteration – hornfels/metamorphism?
- Some associated with epithermal Au deposits
Largest deposits:
Gold (-Rich) Porphyry Deposits
- Grasberg: 2,480 Mt @ 1.13 wt % Cu, 1.05 g/t Au, (2,480 t Au)
- Bingham Canyon: 3,228 Mt @ 0.88 wt % Cu, 0.02 wt % Mo, 0.50 g/t Au, (1,603 t Au)
- Kal’makyr: 2,700 Mt @ 0.40 wt % Cu, 0.51 g/t Au, (1,374 t Au)
Porphyry Copper-Gold Deposits
- Oyu Tolgoi: 2,467 Mt @ 0.68 wt % Cu, 0.32 g/t Au, (790 t Au)
- Batu Hijau: 1,644 Mt @ 0.44 wt % Cu, 0.35 g/t Au, (572 t Au)
- Frieda River: 1,103 Mt @ 0.61 wt % Cu, 0.32 g/t Au, (354 t Au)
Porphyry Cu-Mo Deposits
- El Teniente: 12,482 Mt @ 0.63 wt % Cu, 0.02 wt % Mo, 0.035 g/t Au, (437 t Au)
- Chuquicamata: 7,521 Mt @ 0.55 wt % Cu, 0.024 wt % Mo, 0.04 g/t Au, (301 t Au)
Tectonic Setting

PORPHYRY MODEL
Porphyry Cu Systems Granitic cupola at 3-10 km depth Hydrothermal alteration & ores at 1 to >6 km depth Central high sulfide & metals Increasing low pH, high fS2 alteration upward in system Transition from deep Ppy Cu to shallow epithermal environm’t Role of non-magmatic fluids traditionally restricted to dilute groundwater (meteoric)

Hypogene Mineralisation
Main ore minerals: chalcopyrite, bornite, gold, molybdenite Hypogene Mineralisation 11,000E 9,600RL 9,450RL 9,800RL 10,000RL 10,200RL E26 – Cu & Au Grades 10,600E 10,800E 11,200E 200 m>2 g/t Au 1 – 2 g/t Au 0.5 – 1 g/t Au >2 % Cu 1- 2 % Cu 0.5 – 1 % Cu Gangue: qz, or, anh, mt, bt ± ser ± py Within deposit zonation: • low pyrite, Cu-rich core, • outer pyrite-rich halo Some deposits have Cu-Au rich cores surrounded by intermediate Mo-rich annulus & outer pyrite halo House, 1994 Free gold in qz-mt
Genesis
B: Volatile Exsolution
- Magma ascends to neutral buoyancy level
- Shutdown of volcanism?
- Volatiles exsolved during fractional crystallisation (mafic magma involvement?)
A: Fertile Magma Production
- Partial melting in migmatitic zone at base of crust
- Melts transferred to upper crust along dykes in shear zones
C1: Ore Formation
- Cooling is a symptom, not a cause
- The sulfur conundrum: H2Spredominant fluids, but sulfur transported in the melt as SO2
- Sulfate reduction mechanism? (water-rock interaction, fluid mixing)
C2: Hydrothermal Alteration
- Early K-metasomatism (brines) – lithostatic load
- Late H-ion metasomatism (gases) – hydrostatic load