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EOSC 331:EOSC 331: Porphyry depositsPorphyry deposits
Bingham, Utah (USA)
Stefan Wallier
EOS-South 058
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What is going to be covered
Definition of porphyry deposits
Occurrence of porphyry deposits
Classification Metal-based classification Cu, Mo, Au, Sn, W
Systematic relationships to magma types Alkalinity and silica content
Related deposit types
How are they formed
Styles of alteration and mineralization
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What is a porphyry Cu ( other metals) deposit?
Large tonnage and low hypogene
grade (
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Defining characteristics of
porphyry deposits Genetic association with
porphyries, igneous rocks that
both Are porphyritic
Have a sugary (aplitic), fine-grainedgroundmass
Large volumes of uniform,low to moderate grademineralization Systems with spatial dimensions of
kilometers, yet Processes that formed deposits
occurred on scales of veins
Multiple commodities
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Porphyry deposits are major sources of metals
Copper > Mo ~ Au > Pb, Zn, Rh, W, etc
Copper used in construction, currency, electronics
Molybdenum used in high strength alloy and high-T steel;
aircraft parts, paints and lubricants
chalcopyrite CuFeS2
digenite Cu9S5 Chalcocite Cu2S
bornite Cu5FeS4 enargite Cu3AsS4
molybdenite MoS2
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Distribution in time
All classes strongly skewed to Cenozoic
Function of preservation
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Global
distributionof porphyries
Convergent margins Circum-Pacific
Alpine-Himalayan Altaides
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Tectonic settings Only in settings that generate large and moderately hydrous
magma chambers
Variety of settings; copper deposits mostly in arcs
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Size and
grade Sizes
10 Mt to 10 Gt Cu > Mo ~ Au > W ~ Sn
Grades
Typical ore grades:
0.4 - 1.0 % Cu
0.001 - 0.1 % Mo
0.001 - 1 g/t Au
100 to 10,000 x crustal
abundance
Seedorff et al. (2005)
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Classes
Based on the principal contained metals
Classes Porphyry copper
Porphyry molybdenum
Porphyry gold
Porphyry tungsten
Porphyry tin
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Subdivision based
on igneous rocks Consistent broad patterns in
Metals Setting
Alteration types
Distinctive features
Seedorff et al.
(2005)
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Modif ied from
Blevin, 2003
Metal Endowment & Magma Chemistry
Cu - Au
Sn W
Mo
WW - Mo
Cu - Mo
Sn
Increasing
fractionation
Increasing
oxidation
Rb/Sr fractionation
Fe2O3 /
FeO
101
100
10-1
oxidation
state
10-110-210-3 102101100 103
Metal endowment of
intrusion-related
deposits controlled
by magmatic: oxidation state
compositional
evolution silica content
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What is a porphyry Cu ( other metals) deposit?
Large tonnage and low hypogene
grade (
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Source of metals and fluid
- Metals from magma
- Fluid from magmatic volatile phase which are exsolved from a
crystallizing magma body.
Surface (meteoric) water (and intergranular fluids
within country rock)
Transport of metals in
- Magma
- Magmatic derived fluid- Meteoric fluid
Trap
- Boiling- Fluid mixing (?)
- Changing physio-chemical conditions (P,T,Xxi,XO2)
- Structural traps greater fluid-rock interaction
- Magmatic breccia traps greater fluid-rock interaction
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Upper crustal magma chambers
Cuernos del Paine, Chile
Goodale pluton, California
Chita pluon, Argentina
Yoshinubo et al.,
Mushroom shaped with flat
tops and bottoms
Wider (10-20 km) than thick
(
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Yerington - Advanced argillic to pluton
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Formation of mid-crustal magma chamber and exsolution and trapping of
hydrothermal fluid in apical zones of chamber is a critical first step in a
magmatic-hydrothermal system
169.5 Ma
168.5 Ma
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Luhr Hill granodiorite
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Yerington Batholith Plan (1-3 km depth) with Luhr Hill
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Yerington Batholith Plan (1 3 km depth) with Luhr Hillcupolas & ppy dikes, alteration zones, & ore deposits
Lyon Cu
Fe-oxide
Na-Ca altn
Na-Ca altn
Dilles & Proffett, 1995; Dilles, 2000
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Related deposit types
With genetic ties
Lodes
Skarn
Replacement
Epithermal
Coeval in some areas Iron-oxide-copper-gold (IOCG)
Volcanogenic massive sulfides
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What is a porphyry Cu ( other metals) deposit?
Large tonnage and low hypogene
grade (
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Variety of porphyry
Cu deposits reflects
igneous association
and style of
hydrothermal system
normal PCD
diorite/alkalic
breccia
Porphyry Mo: Similar story
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Porphyry Mo: Similar story
Bingham
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Veins in porphyry Cudeposits
Bingham
Referred to as stockworks, which
implies random arrangement ofveins.
Silver Bell
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Goonumbla, AustraliaRidgeway, Australia
Courtesy of David Cooke
Courtesy of Alan Wilson & David Cooke
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Veins in porphyry deposits
Widths
Typically less thana few cm
Most only a few
millimeters
Implications
Narrow fractures fractures easily filled and
sealed within a short period of time Generally straightforward to relate formation of
alteration envelopes to filling of veinlets
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Alteration envelopes Wall-rock alteration envelopes
Envelopes = selvages = halos
Generally symmetrical about vein
Commonlyproduces specific,and usuallycharacteristic,observed mineralassociations
Stable equilibriumnot necessarilyimplied
Relict minerals mayremain unreacted
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Vein sequences:
Reflects fluid evolutionTemperature
Water rock interaction
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Grouping of vein types
High-temperature Biotitic veinlets associated
with potassic assemblages
Veins dominated bymagnetite, amphibole, andplagioclase
Sugary quartz veinletsassociated with potassic
assemblages Veins with sodic-calcic
envelopes
Calc-potassic veins
Veins with silicic and potassicenvelopes
Moderately high temperature Quartz veins that commonly
lack alteration envelopes
Quartz-bearing veins withcomplex mineralogy
Banded quartz veinlets
Moderate temperature
Pyritic veins with feldspar-destructive envelopes
Greisen veins
Veins with propylitic envelopes
Low temperature
Base-metal veins Generally barren veins without
alteration envelopes
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Seedorff et al. (2005)
High-temperature potassic assemblages
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Fish Lake, BritishColumbia (Caira et al.,
1995, Fig. 15A)
g p p g
El Salvador, Chile
Gustafson and Hunt (1975)
High temperature sugary quartz
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High-temperature sugary quartz
veinlets associated with potassicassemblages
Mineralogy of veins Quartz (50-90 vol%)
K-feldspar
Anhydrite
Bornite and chalcopyrite
Rare biotite
Potassic (K-silicate)
alteration of wall rocks K-feldspar replaces
plagioclase
Biotite replaces amphibole
Commonly Cu mineralized(disseminated)
X
Seedorff et al. (2005)
Moderately high temperature
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Moderately high temperature
quartz veins that commonly lackalteration envelopes
Orientation and morphology Continuous along strike for
meters to tens of meters
Texture
Coarse grained Mineralogy
Quartz
Molybdenite
Chalcopyrite Anhydrite (vugs if leached)
Minor pyrite
Lesser tourmalineSteeply dipping B veinlet at El
Salvador (Gustafson and Hunt,
1975, Fig. 15A)
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Moderately high T veins
Sulfides
Molybdenite
Chalcopyrite pyrite
Morphology
Continuous planar
structures Parallel walls
Internal banding, including
centerlines
Alteration envelopes
veins generally lack
alteration halos
X
Seedorff et al. (2005)
Moderately high temperature
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Moderately high temperature
banded quartz veinlets Orientation and morphology
Randomly oriented
Discontinuous and wispy
Mineralogy Quartz
Magnetite
Texture Distinctly banded
Dark color of bands due to
Abundant vapor-rich fluidinclusions
Micrometer-sized grains ofmagnetite
Dark, banded quartz veinlet fromPancho deposit, Refugio district, Chile
(Muntean and Einaudi, 2001, Fig. 5A)
Moderate temperature pyritic veins
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Moderate-temperature pyritic veins
with feldspar-destructive envelopes
Orientation and morphology
Continuous, systematicallyoriented fracturessheeted sets
Commonly steeply dipping,imperfectly radial pattern
Vein filling Dominated by pyrite Lesser amounts of other sulfides
Minor quartz, with anhydrite andminor dolomite
Alteration envelope Feldspar-destructive alteration
halos are characteristic
Sericite or sericite + chlorite
Also pyrite, quartz, anhydrite,
other sulfide minerals, and rutile
Two D veins with pyrite and bright
sericite halo, cutting B veins with
purple quartz at Rosia Poieni,
Romania.
2 cm
Moderate temperature pyritic veins
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Moderate-temperature pyritic veins
with feldspar-destructive envelopes Synonyms
D veins
Quartz + sericite + pyrite(QSP) veins
Phyllic veins
X
Seedorff et al. (2005)
L t t b t l i
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Low-temperature base-metal veins
Alteration envelopes Intermediate argillic
alteration envelopes aremost common
Sericitic alterationenvelopes also observed
Certain veins lack wall-rock
alteration Of exploration interest
Commonly characterize theregion above and beyond
the bulk-tonnage target Mineralogy and metal
ratios can give clues as tothe class and subclass ofthe underlying porphyry
systemLowell, 1991, Fig. 1
X
Low-temperature generally barren
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Low-temperature, generally barren
veins without alteration envelopes Sulfide-poor veins
that commonly lackalteration envelopes
Mineralogy
Carbonate silicaminerals are common
Prehnite and zeolites
may occur in moremafic wall rocks
May contain precious
metals
X
Seedorff et al. (submitted)
Main points Alteration mineralization
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Main pointsAlteration-mineralization
Porphyry deposits exhibit diverse types of veins andalteration envelopes Phase equilibria of mineral assemblages provide a geochemical
context for understanding porphyry systems
Diversity of alteration-mineralization features attributed toinfluence of numerous geologic variables
Certain vein types are common and widely recognized High-temperature sugary quartz veinlets associated with
potassic alteration Pyritic veins with feldspar-destructive envelopes
Other types are not widely recognized but may beindicators of certain geologic environments High-temperature green mica veinlets Banded quartz veinlets
Greisen veins
P h l
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Porphyry systems are complex
The real world of geology is complicated
Models rarely can capture all the detail Valuable for generating insight but not for
recreating reality
Interpretations of origin, distribution of grade,
and many other factors depend onunderstanding time-space relationships From deposit-scale to regional
Analytical and theoretical tools aidunderstanding, but Understanding the sources of diversity revealed
by high-quality geology is the key to scientific and
practical breakthroughs
OXIDATION & TRANSPORT during weathering:
S i h t
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Supergene enrichment
OXIDATION & TRANSPORT during weathering:
METALS REMOVAL
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METALS REMOVAL
Removed: Cu, Zn, Fe, Mn, K, +/- As, AuRemaining: Mo, Pb, Ag, Mn, Fe
METALS ACCUMULATION as oxides, sulfides
PROTOLITH: metals, reduced sulfur are metastable
DEVELOPMENT OF GEOCHEMICAL STRATIGRAPHY:
OXIDATION LEACHING PROCESSES
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OXIDATION-LEACHING PROCESSES
(applied to a protolith comprising Py >> CuSx and withvariable neutralizing capacity )
Oxidation and Transport:lose Cu, Zn, ~Fe, +/- As, Au;
sulfur (as sulfate); ~Al, ~Mn
Accumulation of Cu, S=, Fe,
Zn, Au, Al, +/- As
Lateral transport and
formation of CuOx
(FeOx, MnOx)
Development of oxidation profiles: Cu + Fe + (Mn,Al)
mobility during oxidative destruction of sulf ides
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Fe+++ + H2O Fe(OH)3 + 3H+(aq) Fe(OH)3 FeOOH(s) + H2O
Santa Rita, New Mxico
goethite
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(post- mineral
sediments)
pyrite, chalcopyrite
red hematite + goethite
pyrite goethite + jarosite
Cuajone, Per
Sulfide accumulation zonewith
pyrite, chalcopyrite + Cu
chalcosite, covelli te,
bornite
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