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Porphyry Deposits

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Porphyry Cu deposits are generally associated with I-type ... Breccia. May Occur. Rare. Stockwork. Important. Important. Characteristics of S & I Type Granites ... – PowerPoint PPT presentation

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Title: Porphyry Deposits


1
Porphyry Deposits
2
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

3
Porphyry Hand Specimen
4
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.

5
Alteration Halos
6
Alteration Shell
7
Hydrothermal Circulation
8
Multiple Intrusives
9
Structural Settings
10
Youngest Phase of Intrusion
11
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

12
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

13
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14
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15
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)

16
Porphyry Cu Locations
17
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

18
Cadia/Ridgeway
  • Shoshonite association

Tholeiite trend
19
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20
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?
21
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
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