GEGN 401 - Mineral Deposits

Lecture 22 - Iron Oxide-Cu-Au (IOCG) Deposits

Hematite Breccia with chalcopyrite

Olympic Dam, Australia

This is a “new” class of deposits only recognized in the early

1990’s. They are one of the “hottest” exploration targets today.

Murray Hitzman

Iron Oxide-Cu-Au (IOCG) Deposits

• Class of deposits includes:

– Magnetite-apatite deposits (low to no Cu-Au)
– IOCG deposits

• These two types of deposits appear to be genetically related

(similar metal suites, alteration types, tectonic settings, etc.).

• Differentiating between the two is a current focus of research

and economic importance since only IOCGs are financially
attractive.

Murray Hitzman

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 Characteristics of Iron Oxide-Cu-Au (IOCG) Deposits
• Age - late Archean to Pliocene
• Tectonic setting - variable, 3 end-members
Intra-cratonic orogenic collapse
Intra-cratonic anorogenic magmatic province
Extension along a subducting margin
• Association with igneous activity - vast majority spatially and temporally related
to magmatic events. No specific magma composition related to deposits.
Mgt-apatite - commonly show a direct relationship to igneous activity
Iron oxide-Cu-Au - commonly show an indirect relationship to igneous activity
• Apparent association with evaporites and/or brines
• Structural control
Mgt-apatite - structural control not always present
Iron oxide-Cu-Au - localized in highly permeable zones along high-angle or low-angle faults
which are commonly splays off crustal-scale structures.
• Morphology of deposits is highly variable
• Mineralogy
Iron oxide minerals (magnetite, hematite) dominant, minor sulfides (generally chalcopyrite,
lesser pyrite), gangue carbonates, calc-silicate minerals, quartz, barite
• Elemental Association
Mgt-apatite - Fe, LREE, (Ba)
Iron oxide-Cu-Au - Fe, Cu, Au, LREE, Ba (Ag, Co, Ni, Zn, F)
Murray Hitzman

Distribution of Magnetite-apatite & IOCG Districts

Wernecke Kiruna

Grenville 01

SE Missouri Carajas
Cloncurry
N. Chile Lufilian
Gawler
CIB
Magnetite-apatite districts
Iron Oxide-Cu-Au districts
Murray Hitzman

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 Alteration in IOCG Deposits

• These deposits display

vertically zoned alteration.
– Deep sodic (albite,
actinolite, chlorite,
magnetite)
– Intermediate potassic (Ksp,
biotite, quartz,
magnetite/hemtatite) or
sodic-calcic (diopside,
actinolite, magnetite)
– Shallow hydrolytic (sericitic)
alteration (“sericite”,
carbonate, chlorite, quartz,
hematite)
• Alteration zones in these
systems typically large (km
vertical and horizontal
extent).
from Hitzman et al., 1992

Murray Hitzman

Magnetite-apatite & IOCG Deposits -

Alteration Mineralogy Depends of Host Lithology
“Hydrolytic” alteration
Basalt Rhyolite basalt: chl - carb - hm
rhyolite: qzt- ser - chl - hm
“Potassic” alteration
basalt: alb - chl - cal - hm
Limestone
rhyolite: Ksp - bio - mgt -hm

Sodic alteration
alb - scap - act - hbl

Sandstone Skarn
andradite - pyrox - act
Evaporites
Hematite /
Intrusive Magnetite
(modified from Barton & Johnson, 1996)
Murray Hitzman

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 Magnetite- Apatite & IOGC Deposit
Alteration and Metal Zonation

Depth Fe Cu LREE U Au

Sericitic

Potassic

Sodic

Murray Hitzman

Magnetite- Apatite vs. Iron Oxide-Cu-Au (IOCG) Deposits

• Magnetite-apatite deposits are:

– Generally directly related to igneous activity.
– Contain well-developed sodic (and sodic-calcic)
alteration zones.
– Are dominated by magnetite; mineralization can occur at
top of sodic zone or in potassic zone.
– Temporally early relative to IOGCs

• Iron oxide-Cu-Au deposits are:

– Not always obviously directly related to magmatism
– Invariably structurally controlled
– Generally occur in potassic alteration zones
– Are temporarily late.

Murray Hitzman

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 Tectonic Setting of
Magnetite- Apatite &
Iron Oxide-Cu-Au (IOCG) Deposits

Three dominant settings are recognized:

– Anorogenic magmatism

– Orogenic basin collapse

– Subduction related

Murray Hitzman

Magnetite-apatite & Iron Oxide-Cu-Au Deposits:

Anorogenic Magmatism
Volcanic rocks Sedimentary
(dominantly felsic) rocks
Magnetite-apatite deposit

Iron oxide-Cu-Au deposit

Intrusives
Mantle underplating
Basement
from Hitzman, 2000

• Felsic volcanic rocks dominant. Produced by incipient rifting, hot spots (mantle underplating)
• Relatively minor sedimentary rocks - presence of evaporites unclear
• Typical large (regional) alteration resulting in formation of sodic-calcic zones and “syenites” (brick-
red volcanic and intrusive rocks with enriched K + hm).
• Large alteration zones probably leached of base and precious metals (?)
• Deposits along major fractures and above and adjacent to intrusions.

Murray Hitzman

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 Olympic Dam Cu-U-Au-Ag-LREE Deposit
• Deposit found in 1975 while drilling
unconformably overlying sediments for
stratabound Cu target. Olympic Dam
represented a coincident gravity and magnetic Original Cross Section (1983)
anomaly near the intersection of several
continent-scale lineaments. The first hole
intersection 38m of 1.05%Cu in the basement
beneath the sediments at a depth of 353m.
• The deposit was initially explored by drilling.
Geology based on the drilling lead the explorers
to believe that they had found a “new type of
strata-bound sediment-hosted ore deposit” in
which mineralization occurred in relatively flat
lying coarse sedimentary breccias.
• In the mid-1980’s when underground exposure
was available it became clear that the deposit is
hosted in a vertically oriented breccia pipe.
• The deposit contains ore reserves in excess of
600 Mt averaging 1.8% Cu, 0.5 kg/t U3O8, 0.5 g/t
Au, and 3/6 g/t Ag (+ significant Ce, La). The
reserves are included within a resource of >2.3 (from Roberts and Hudson, 1983, Econ Geol.)
Bt. The deposit has an average Fe grade of 26%.
• The deposit is important not only because of its
large size and high grade, but because it spurred
work on the genesis of IOCG deposits.

Murray Hitzman

Olympic Dam IOCG Deposit

Gawler Craton - Stuart Shelf, Australia
Prominent Hill Late Proterozoic -
Olympic Cambrian sediments
Dam
Torrens Hinge Zone

Acropolis Pandurra Fm.

(modified from Reeve et al., 1990)

Sediments

Gawler Volcanics
Indian
Ocean Hiltaba granitoids

100 km
Basement

• 1600 - 1585 - Hiltaba granitoids and Gawler Range volcanic rocks

• 1590 - Formation of Olympic Dam iron oxide - Cu - Au breccia complex

Magnetite-apatite (Acropolis) and iron oxide - Cu - Au mineralization (Olympic

Dam)appear to be contemporaneous with volcanism. Murray Hitzman

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 Olympic Dam Cu-U-Au-Ag-LREE Deposit

• The deposit occurs within an

elongate breccia body sitting
within granite.

• The breccia body is zoned from

an outer zone of veined and
weakly brecciated granite into
hematite matrix granite
breccias to hematite-rich
breccias.

• The core of the pipe contains

massive hematite breccia,
zones of silicification, and
down dropped blocks of
volcaniclastic rocks (Gawler
volcanics).

(from Reynolds, 2001, MESA Journal)

Murray Hitzman

Olympic Dam Cu-U-Au-Ag-LREE Deposit

• The deposit occurs

within an elongate
breccia body sitting
within granite.

• The breccia units

flair out upwards.

• The pipe is cut by a

number of vertically-
oriented mafic and
felsic dikes.

(from Reynolds, 2001, MESA Journal)

Murray Hitzman

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 Olympic Dam Cu-U-Au-Ag-LREE Deposit
Granite cut by
hematite veins.

Granite-rich
breccia.

Heterolithic
hematite breccia
with granite and
hematite clasts.

Hematite-quartz
breccia.
Murray Hitzman
(from Reynolds, 2001, MESA Journal)

Olympic Dam Cu-U-Au-Ag-LREE Deposit

(from Reynolds, 2001, MESA Journal)
(from Oreskes and Einaudi, 1990, Econ Geol.)

Granite-rich breccia. Massive hematite preserving

plagioclase textures from granite

The hematite breccias at Olympic Dam often preserve original granite textures.
This textural evidence indicates that some hematite formed by replacement of
granite. Other hematite formed as a breccia matrix or as hematitic sediments.

Murray Hitzman

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 Olympic Dam Cu-U-Au-Ag-LREE Deposit -
Mineralization

Bornite-chalcocite mineralization in a Disseminated chalcopyrite mineralization

hematite granite breccia. in both clasts and matrix of a hematite-
rich breccia.

Mineralization at Olympic Dam occurs as disseminated grains and veinlets of

sulfides (cpy, bn, cc) and uraninite (pitchblende), with lesser coffinite and
brannerite. Gold and silver occur with copper sulfides. Most mineralization is
within the matrix of hematitic breccias.

(from Reynolds, 2001, MESA Journal)

Murray Hitzman

Olympic Dam Cu-U-Au-Ag-LREE Deposit -

Mineralization

The deposit is zoned from an upper and core zone of bn-cc, outward and
downward to cpy. Pyrite (and magnetite) appear at depth.

(from Reynolds, 2001, MESA Journal)

Murray Hitzman

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 Olympic Dam Cu-U-Au-Ag-LREE Deposit -
Mineralization

Mineralization is
concentrated along the
axis of the breccia
pipe, coincident with
the location of
hematite-rich breccias.

(from Reynolds, 2001, MESA Journal)

Murray Hitzman

Fluid Mixing Model

Olympic Dam, Australia

Maar crater

Hematite Volcanics
breccia
Groundwater
(+ Cu, Au, S ?)
Granite

Deeply circulating
or magmatic fluid

(modified from Haynes et al., 1995)

Murray Hitzman

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SW Heterolithic
Magnetite-apatite Hematite breccia
body

Pilot Knob, SE Missouri —

Magnetite-apatite System
gabbro

1 km Felsic ash flow

tuffs

(modified from Panno & Hood, 1983)

• Deposit contains:
– magnetite - apatite body (subsurface deposit) with typical sodic-calcic alteration
– Heterolithic hematite breccia which is texturally extremely similar to the Olympic Dam breccia
but contains no Cu - Au

• The two deposits are approximately 800 m apart vertically and offset laterally from
one another by approximately 1 km. There is no known physical connection between
the deposits.

• The absence of Cu - Au in this system may be the result of an absence of a second

fluid.
Murray Hitzman

Anorogenic Magmatic Setting —

Summary & Questions

• Alteration and mineralization take place within a large mass of dominantly

subaerial volcanic rocks (relatively oxidized).

• May be a problem of generating high salinity fluids (many of these

districts do not obviously contain evaporites).

• Energy for hydrothermal circulation probably dominantly from

magmatism (batholiths and mantle underplating).

• Many magnetite-apatite deposits have local permeability controls (more

porous and permeable volcanic units). Only known major iron oxide - Cu
- Au deposit (Olympic Dam) is in highly permeable maar breccia whose
location may be controlled by deep-seated faults.

• Problem with source of large volumes of aqueous fluids - how is system

recharged?

Murray Hitzman

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 Magnetite-apatite & Iron Oxide-Cu-Au Deposits:
Intra-Cratonic Orogenic Collapse
Sedimentary rocks volcanics
• Basins characterized by overall high
oxidation state (relatively minor black
shales).
• Basins appear to have contained
Mantle underplating significant evaporites.
Basement

Magnetite-apatite deposit
Extension - Basin Formation
Iron oxide-Cu-Au deposit
• Deposits hosted by volcanic, intrusive, and/or
sedimentary rocks.
• Mineralization appears to occur during late
stages of metamorphism and is often temporally
(and spatially) related to intrusions.
• Typical large (regional) sodic-calcic alteration
zones associated with magnetite - apatite
mineralization (pre-iron oxide - Cu - Au Intrusives
mineralization).
• Iron oxide - Cu - Au mineralization associated Mantle underplating
with potassic alteration.
• Iron oxide - Cu - Au deposits along fault zones
which appear to be splays off crustal-scale
structures.
Basement

Compression - Basin collapse & magmatism

Murray Hitzman

Mt. Isa - Cloncurry Region, Australia

N Century basement
• 1780 - 1720 - Deposition of Cover Sequence 2
• Post 1670 - Deposition of Cover Sequence 3
Eastern Fold • 1590 - Damantinian Orogeny
• 1550-1500 - Isan Orogeny
Belt
Oxidized • 1540 -1500
Rock Package
50 km – Williams and Naraku batholiths
– Regional sodic-calcic alteration
– Magnetite-apatite mineralization
Ernest • 1510 - Iron Oxide-Cu-Au mineralization

Henry
Mt. Isa
Eloise
Cover
Western Fold sequence 2
Belt
Reduced Willams - Naraku
Rock Package
Starra Granites
Cannington
Iron oxide-Cu-Au
Cover sequence 3 Osborne Isa-type Zn-Pb-Ag
BHT Pb-Zn-Ag
Murray Hitzman

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 Ernest Henry Deposit, Australia
NNW SSE
Overburden
Felsic volcanic rocks

Metasiltstone

Mafic volcanic rocks

Ore breccia (in
potassically altered
zone)

300 m

(modified from Selmon, 2000 & Craske, 1995)

Potassically altered felsic volcanic

rock brecciated by magnetite. Minor
late chalcopyrite.
Murray Hitzman

Magnetite-apatite & Iron Oxide-Cu-Au Deposits:

Extension along a Subducting Margin

Magnetite-apatite deposit
Iron oxide-Cu-Au deposit

• Intermediate, subaerial volcanics dominant. Numerous felsic to mafic intrusive rocks.

Produced by incipient rifting, hot spots (mantle underplating).
• Thick sedimentary sequence may be present and may contain evaporites.
• Alteration and mineralization may be temporally equivalent to burial metamorphism.
• Typical large (regional) alteration resulting in formation of sodic-calcic zones and
“syenites” (brick-red volcanic and intrusive rocks with enriched K + hm).
• Large alteration zones probably leached of base and precious metals (?)
• Deposits along major fractures and above and adjacent to intrusions.

Murray Hitzman

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22 ° 72 ° 70 ° Extension along a Subducting Margin:
Northern Chile
24 ° AFZ
Northern Chile contains three
overlapping belts of ore deposits:
26 ° Magnetite - apatite deposits

Manto copper deposits

28 ° Iron oxide - Cu - Au deposits

AFZ
30 °
The mineralized belts are spatially coincident
with the trace of the Atacama Fault Zone (AFZ)
which underwent strike-slip movement during
the Jurassic and shifted to dip-slip (down to
32 ° 100 km east) in mid-Cretaceous.

Murray Hitzman

22 ° 72 ° 70 ° Northern Chile - Chronology

• 196 - 90 Ma La Negra and Bandurrias (subaerial basalts
and andesites) emplaced in low relief arc. Back-arc basin
developed with mixed lacustrine/marine
24 ° AFZ sedimentation.

• 195 - 150 Left lateral strike-slip movment on Atacama fault

zone.
26 °
• 170 Initiation of burial metamorphism.

• 170 - 145 Manto copper deposits formed (extending to 100

28 ° Ma) .

• 135 - 110 CIB magnetite-apatite deposits formed.

AFZ
30 ° • 120 Change to dip-slip movement on Atacma Fault zone.

• 115 - 90 Iron oxide - Cu - Au deposits formed.

• 55 - 38 Incaic deformation
32 ° 100 km
• 35 Major Oligocene porphyries formed.

Murray Hitzman

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 Extension along a Subducting Margin:
Northern Chile - 190 Ma
La Negra volcanics
backarc basin

+ -

AFZ

• Initiation of oblique subduction and formation of the Atacama left lateral strike-
slip fault zone.

• Eruption of subaerial, oxidized La Negra basalts and andesites. Minor

sediments at base of La Negra Formation.

• Initiation of back-arc extension and formation of sedimentary basin.

Murray Hitzman

Extension along a Subducting Margin:

Northern Chile - 150 Ma
Manto Cu deposits
basin dewatering

AFZ
Burial metamorphic fluids

• Continued eruption of oxidized subaerial basalts and andesites results in a thick volcanic
pile which begins to undergo burial metamorphism.

• Continued extension and sedimentation in the back arc with deposition of shallow marine
sediments including evaporites.

• Formation of manto-type copper (hematite) deposits through interaction of burial

metamorphic fluids (+ igneous fluids?) with reductants or sulfur sources (+ fluid
mixing?), primarily adjacent to AFZ.

Murray Hitzman

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Extension along a Subducting Margin:
Northern Chile - 130 Ma
Mgt-apatite Manto Cu deposits
deposit

AFZ

• Continued eruption of oxidized subaerial basalts and andesites and

continuing burial metamorphism.

• Continued extension and sedimentation in the back arc.

• Formation of magnetite-apatite deposits (CIB) along AFZ, most associated

with diorites.

• Continued formation of manto-type copper deposits generally to east of AFZ.

Murray Hitzman

Extension along a Subducting Margin:

Northern Chile - 100 Ma
Iron oxide-Cu-Au Manto Cu deposits
deposits

Future
Domeyko
Fault System

• Waning eruption of oxidized subaerial basalts and andesites and continuing

burial metamorphism. Intrusion of intermediate to felsic plutons and
batholiths.
• Back arc spreading is minimal.
• Atacama Fault changes to dip-slip movement.
• Formation of iron oxide - Cu - Au systems along the AFZ and other major
fault zones splaying off of deep-seated faults.
• Continued formation of small manto-type copper deposits to east of AFZ.

Murray Hitzman

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22 ° 72 ° 70 ° Chilean Iron Belt (CIB)

• Deposits are commonly spatially associated

24 ° with pyroxene diorites.
AFZ
• Alteration and mineralization cut the associated
intrusions.
26 °
• Alteration characterized by sodic-calcic
assemblage: actinolite/tremolite - albite -
scapolite - (tourmaline).
28 °
• Iron oxides dominantly magnetite. Associated
apatite. Trace pyrite.
AFZ Alteration and mineralization probably due to inflow of
30 ° saline brines (Na-, Ca-rich; S-poor) due to dewatering
of evaporitic sediments within La Negra volcanic pile
or inflow from ocean to W. or basinal fluids to E.

32 ° 100 km Magnetite - apatite deposits

Murray Hitzman

Chilean Iron Oxide-Cu-Au Deposits

• A number of IOCG deposits are recognized in coastal Chile.

• Candelaria-Punta Del Cobre

– Candelaria contains 470 Mt of 0.95% Cu, 0.2 g/t Au, 3.1 g/tAg. The Punta del
Cobre deposits consist of small (<5 Mt), higher grade (1.2 - 2% Cu) deposits.
– Deposits are hosted by Early Cretaceous volcanic and volcaniclastic rocks
which are intruded by the Copiapo granitic batholith.
– Mineralization occurs with magnetite in potassically altered rocks (Candelaria)
and with hematite in potassically and sodic-calcic alteration (Punta del Cobre).
– Deposits in the Punta del Cobre district are overlain by calcite veins with Ag,
Co, Zn

• Manto Verde
– Highly structurally controlled district which appears to be tilted with magnetite-
rich deposits to south and hematite-rich deposits to north.
– Both magnetite and hematite-Cu mineralization associated with potassic
alteration.

Murray Hitzman

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 Chilean IO!CG Deposits:
Candelaria - Punta del Cobre district, Chile

W Calcite
siltstones
(+Ag, Co)
skarn lms.

hornfels andesite

Hm-sulfide
Mgt-sulfide
Sodic-calcic
alteration
intrusive
Potassic alteration
1 km 5 km

Murray Hitzman

Geology of the Manto Verde Area,

Manto
Chile
N Ruso
• Small scale workings since 19th century.
Santa
Clara
Central Atacama Flt.

• Mined from 1906-1939 (0.4 Mt @ 3%Cu).

Eastern Atacama Flt.

Manto • EMMB S.A. (Anglo-American) optioned in 1988 and

drilled to 1991 proving reserves of 120 Mt@0.72% Cu.
Verde
Norte • EMMB put mine into production in 1995.

• Exploration since 1995 has added approximately 180

Mt@0.5% Cu, 0.2 g/t Au.

Altavista • Deposit is still open.

Sur
alluvium
Diorite (124 - 127 m.a.)
Montecristo Granite (125 - 131 m.a.)

1 km Jurassic - lower Cretaceous andesite (La

Negra Fm.)
Cu-Au mineralization with iron oxides
(modified from Zamora and Castillo, 2001)
Murray Hitzman

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 Alteration and Mineralization
Manto Verde Area, Chile
• Vertical zonation from deeper magnetite-rich system to
shallower hematite-rich system. Hematite-rich system
apparently overlain by calcite-(sulfide) system.
– Manto Verde Sur and Altavista is a magnetite - sulfide (cpy-py)
breccia. Zone is associated with calcite - alteration which is
spatially related to potassic (Ksp) alteration.
– Manto Verde Norte is a specular hematite - sulfide (cpy- py) -
calcite breccia. Zone is associated with calcite alteration and
weak argillic alteration.
• Alteration ranges from deep potassic (Ksp) zone to
shallow argillic zone. Peripheral sodic-calcic alteration.
Entire Manto Verde system enveloped by chlorite - silica
Norte alteration.
• Alteration took place 117+3 Ma. (K/Ar) - post-dates
emplacement of nearby intrusive rocks.
A’
A
Argillic alteration

Chlorite - silica alteration

N Sur
Potassic (Ksp-biot) alteration

Calcite veins and breccias

Specular hematite + Cu-Au

1 km Magnetite + Cu-Au

(modified from Zamora and Castillo, 2001) Intrusive rocks

Andesitic volcanic rocks Murray Hitzman

Cross Section Manto

Verde Area, Chile

A A’
Chlorite - silica alteration
117 m.a.
Potassic (Ksp-biot)
124 - alteration
127 m.a.
Calcite veins and
breccias
121 m.a. Specular hematite +
Cu-Au
Magnetite + Cu-Au

200 m Intrusive rocks

Andesitic volcanic
(modified from Zamora and Castillo, 2001)
rocks

Murray Hitzman

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 Structural Control
Manto Verde Area, Chile
• Manto Verde deposit consists of a number of spatially
related orebodies.

• Manto Verde Norte and Sur occur along the Manto Verde
fault - high angle structure connecting central and
eastern Atacama fault systems. Manto Verde fault itself
cuts ore.

• Other ore systems occur along faults subparallel to

Manto Verde fault (Manto Ruso) and Atacama fault (Santa
Clara).
Norte
• Actual faults controlling alteration and mineralization not
A’ always evident.
A
Argillic alteration

Chlorite - silica alteration

N Sur
Potassic (Ksp-biot) alteration

Calcite veins and breccias

Specular hematite + Cu-Au

1 km Magnetite + Cu-Au

(modified from Zamora and Castillo, 2001) Intrusive rocks

Andesitic volcanic rocks Murray Hitzman

Evaporite Brine Model

for Fe oxide (Cu-REE-U-Au) Deposits
Si, Al, K, (Ca, Mg) (out of system)

+H, +Fe, + S
+Na, +Fe, + K

Basinal
water

+Na, +Fe
-Si, -Al, -K
Evaporites
-Na Magmatic
-Fe water
Large magma body important primarily as heat pump
Murray Hitzman

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