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EOSC 331:EOSC 331: Porphyry depositsPorphyry deposits

Bingham, Utah (USA)

Stefan Wallier

EOS-South 058

[email protected]


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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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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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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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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


Moderately high temperature


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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




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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


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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