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Porphyry Deposits: General characteristics and modeling

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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 copper deposits are typically formed in a continental arc and island arc settings along the Pacific Ring of Fire – the west coast of North and South America to the Eastern portions of Asia.

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)

Omer Hag, Sami & El Khidir, Sami & Yahya, Mohammed & Galil, Abdel & Eltom, Abdalla & Elsheikh, Abdalla & Awad, Musab & Eljah, Hassan & Ali, Mohammed. (2015). Remote Sensing And Gis Investigations For Geological And Alteration Zones Related To Hydrothermal Mineralization Mapping, Maman Area, Eastern Sudan. Journal of Remote Sensing and GIS. 3. 2052-5583.

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
Cite this article as: Geology Science. (2019). Porphyry Deposits: General characteristics and modeling. [online] Available at: http://geologyscience.com/applied-geology/mining-geology/porphyry-deposits-general-characteristics-and-modeling/ [11th November 2019 ]
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