Porphyry Deposits

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Porphyry Deposits
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General Description

• Petrography
• Hosted by acidic to intermediate intrusives such
  as granite, granodiorite, tonalite, diorite.
• Porphyry Cu deposits are generally associated
  with I-type granitoids magma derived from upper
  mantle melt
• Multiple intrusive events are common in areas of
  porphyry Cu minzn, with the host intrusions being
  the youngest and most differentiated plutons
• Usually emplaced passively rather than forcefully
  ie an extensional rather than compressive regime

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Porphyry Hand Specimen
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Hydrothermal Alteration

• Four alteration zones assoc with porphyries
• Potassic zone development of secondary
  orthoclase-biotite-chlorite and minor sericite
  which replace primary orthoclase, plagioclase and
  mafic minerals
• Phyllic zone characterized by development of
  quartz-sericite-pyrite assemblage with minor
  chlorite, illite rutile. Sericitization affects
  feldspars and biotite. This rean releases silica
  resulting in silicification and quartz production
• Argillic zone clay minerals dominate. Kaolin
  dominates near the orebody and montmorillonite
  further away
• Propylitic zone always present! Chlorite is the
  predominant mineral along with pyrite, calcite
  and epidote. Mafic minerals are partially
  replaced by chlorite and carbonate, plagioclase
  may be unaffected. Gradually grades into
  surrounding rocks up to 1km.

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Alteration Halos
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Alteration Shell
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Hydrothermal Circulation
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Multiple Intrusives
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Structural Settings
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Youngest Phase of Intrusion
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Hypogene Mineralization

• Ore can be found in 3 situations
• Totally within the host stock (veins)
• Partially in stock and partially in country rock
• Totally within the country rock
• Orebodies are usually surrounded by a pyrite-rich
  shell, which occur in concentric zones.
• Usually a central barren core passing outwards
  firstly to Mo-rich minzn, then Cu-rich minzn as
  the main ore shell is encountered. Pyrite
  gradually increases to form a pyrite-rich halo
  (10-15) but with minor chalcopy and Mo.
• The highest Cu values often occur at the boundary
  between the potassic and phyllic zones with weak
  minzn in the propolyitic zones

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The Diorite Model

• Some porphyries assoc with low silicaalkali
  ratios and referred to as the diorite model
• Diorite model deposits have low sulfur conc. in
  the mineralizing fluids. Consequently not all the
  iron oxides are converted to pyrite and much
  remains in chlorite and biotite while excess iron
  tends to occur as magnetite
• The phyllic and argillic alteration zones are
  usually absent so that the potassic zone is
  surrounded by the propylitic zone
• Significant amounts of gold occur and Mo and Cu
  is usually low
• Gangue material may be devoid of quartz but
  contain abundant chlorite, epidote and albite

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Distribution

• Most porphyry deposits occur within Mesozoic and
  Cenozoic orogenic belts associated with either
  island-arcs and convergent continental margins
• Some porphyries occur in Paleozoic orogenic belts
  in Central Asia, Australia and US
• Few are found in Precambrian rocks due to their
  low preservation potential (erosion)

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Porphyry Cu Locations
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Genesis of Porphyry Cu Deposits

• A question of magmatic versus meteoric derivation
  for the mineralizing fluids and the origin of the
  metals and sulfur
• Crackle brecciation indicates that at least some
  of the fluids originated from the pluton.
  Consists of fractures (brecciation) that have
  been healed with veinlets to form the stockwork
  mineralization
• Brecciation is a result of volume increase in the
  magma chamber within 0.5-2km of surface caused by
  continual fractionation of anhydrous minerals and
  the generation of volatiles and an increase in
  vapour pressure.
• If vapour pressure rises above confining
  pressure, retrograde boiling occurs which may
  overcome the tensile strength of the rock
  resulting in expansion and extensive and rapid
  brecciation
• Retrograde boiling produces an aqueous phase
  (hydrothermal fluid) rich in chloride and
  bisulfide ions which act as an important
  transport mechanism for base metals and gold
• Stable isotope data indicate that the potassium
  silica alteration occurred at 550-700C and
  derived from primary magmatic fluids. However,
  fluids from sericites in the phyllic zone are a
  mix of meteoric and magmatic fluids

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Cadia/Ridgeway

• Shoshonite association

Tholeiite trend
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Where does Cadia fit into the typical porphyry
model? Island-arc? Continental arc? Or no arc at
all intracontinental? What environment are
highly alkaline shoshonites most likely to form?
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References/Links

• Porphyry deposits
• http//www.geo.arizona.edu/geos256/azgeology/porph
  yry.html
• http//geology.csupomona.edu/drjessey/class/GSC433
  /Porphyry.htm
• http//geology.csupomona.edu/drjessey/class/GSC433
  /Moly.htm