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Aspects of Lachlan Orogen

Magmatic Arc Cu-Au-Ag Exploration

with lessons from the Pacific rim

Greg Corbett

www.corbettgeology.com

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Terry LeachSymposium

2008

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Model for Pacific rim magmatic arc Cu-Au

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Mines & Wines Lachlan Orogen

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Porphyry Cu-Au with time

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Magnetite destruction

by phyllic (sericite)alteration overprint

(of potassic alteration)

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Overprinting alteration and magnetics

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Overprinting

intrusions and veins

Ridgeway, Straya

Rio Grande, Argentina

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Overprinting intrusions

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Fluid evolution and levels in

Porphyry Cu-Au

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Wall rock porphyry

Gaby, Ecuador

Cadia Australia

Maricunga Belt, Chile

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Porphyry - wall rock porphyry

Exploration implications Highest value targets

Complex overprinting relationships increase metal grade

Position in the anatomy is critical (late barren intrusions)

Structural control to sheeted veins important

Oyu TolgoiMaricunga

b b l

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Carbonate-base metal Au

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Carbonate-base metal Au Leach and Corbett, 1993, 1994, 1995; Corbett and Leach, 1998

Porgera

Kelian

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Polymetallic Ag-Au

carbonate base metal Au

sphalerite zonation

Kelian, Indonesia

Bowdens,

Australia

Kumrock,

Kamchatka,

E. Russia

San Martin

Peru

C b t b t l A

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Carbonate base metal Au

carbonate types

Upper Ridges, Wau, PNG

Mt Kare, PNG

C l

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Cowal-

Carbonate -base metal Au

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Cowal pull-apart basin fractures

Ch i i

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Changes in convergence as a trigger

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Carbonate-base metal Au

Exploration implications Most prolific Au producers in the SW Pacific rim

Au with zoned Zn>Pb and carbonate

Irregular Au distribution & variable metallurgy

Local overprinting bonanza Au

Structural control to bleed fluids from magmatic source

Porgera

Polymetallic Ag Au

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Polymetallic Ag-Au

A d P l t lli A A Caylloma Peru

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Andean Polymetallic Au-Ag Caylloma, Peru

Arcata, Peru

1000 oz/t Ag

Polymetallic Ag Au Mexico

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Polymetallic Ag-Au Mexico

FresnilloPalmarejo

P l lli A Z Pb

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Polymetallic Ag-Zn-Pb

Corani, Peru San Cristobal, Bolivia

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Zonation in polymetallic Ag-Au

Tops white (Zn-rich) sphalerite, siderite,

elevated precious metal grades, Ag>Au

Bottoms black (Zn-poor) sphalerite,chalcopyrite

Huevos Verde

Caylloma

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Mungana

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Polymetallic Ag-Au (Pb-Zn-Cu)

Overlooked by the majors or many years but company

makers for juniors

Ag>Au Spatially zoned with bonanzas in lower temperatures

Multiple control to ore shoot development

Corani, Peru Palmarejo, Mexico

High sulphidation epithermal Au + Cu+ Ag

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High sulphidation epithermal Au + Cu+ Ag

High sulphidation Cu-Au

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High sulphidation Cu Au

A d hi h l hid ti A A

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Andean high sulphidation Au-Ag

La Coipa, Chile Veladero, Argentina

Yanacocha, PeruPascua-Lama, Chile-Argentina

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High vs Low Sulphidation

CENTRAL

HOT ACID

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High sulphidation

Zoned acid alteration

HOT ACID

OUTER

LESS ACID

Alunite

Pyrophyllite-diaspore

Dickite-kaolinite

Alt ti ti

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Alteration cross section

- Nena, PNG

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Two phase fluid flow model

Hi h l hid ti i l

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High sulphidation mineralogy

El Indio, Chile

Nena, Papua New Guinea

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Permeability

Breccia

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Breccia

Lepanto, Phillipines

Veladero, Argentina

Yanacocha, Peru

Pascua, Chile

Lith l i l C t l

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Lithological Control

Pierina, Peru

VughyVughy silica altered ignimbritesilica altered ignimbrite

LongLong Section.VolkertSection.Volkert et al 1998et al 1998

Structural controlStructural control -- El Indio ChileEl Indio Chile

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Structural controlStructural control - El Indio, ChileEl Indio, Chile

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Mt Carlton

Polybasite

9Ag2S.Sb2S3

El Indio

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El Indio

Chile

HighSulphidation

Low

Sulphidation

Wafi, Papua New Guinea

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Transition to low sulphidation

from Leach, 1999

High sulphidation

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High sulphidation

Exploration Implications Zoned alteration & permeability controls

Often poor metallurgy & low Au grade

Better Au grade & metallurgy with temporal & spatial

transition to lower sulphidation

Caution with barren advanced argillic alteration

Model for Pacific rim magmatic arc Au

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Good prospecting!!

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