INTRUSION RELATED GOLD DEPOSITS:CLASSIFICATION, CHARACTERISTICS

AND EXPLORATION

Society of Economic Geologists Regional VP Lecturer 2003

Dr. T. BakerEconomic Geology Research Unit, School of Earth Sciences, James Cook University

Townsville, QLD 4811, AustraliaPh: 61-7-47814756, Email: Timothy.Baker@jcu.edu.au

INTRUSION-RELATED GOLD DEPOSITS

• Definition & Classification

• Geological & Exploration Model

• Examples

– Tintina Gold Belt, Alaska & Yukon

– Australian Examples

• Genesis

• Exploration Summary & Potential

INTRUSION RELATED GOLD DEPOSITSCHARACTERISTICS

• Metals

– Au, Bi, Te, W, Mo, As (Sb, Sn, Pb, Cu)

• Magmas

– Felsic (granodiorite to granite)

– I-type (crustal input, transitional S-type)

– Ilm>Mag

– W-Sn-Mo association

(Thompson et al., 1999)

INTRUSION RELATED GOLD DEPOSITSCHARACTERISTICS

• Tectonic environment– Continental setting, inboard, commonly late

• Age– Phanerozoic (Precambrian – Archean?)– Intrusions = mineralization

• Ore– Au, Bi, Te, W, Mo, As (Sb, Sn, Pb, Cu)– Reduced (no Mag-Hem), low sulfide (Po-Py-Apy)

• Style– Sheeted, breccia, stockwork, flat-vein, disseminated

• Alteration– Feldspathic (Na>K), phyllic, carbonate

(Thompson et al., 1999)

NOMENCLATURE

• Porphyry Au (Hollister, 1992)

• Fort Knox-style Au (Bakke, 1995)

• Intrinsic Au (Newberry et al., 1995)

• Plutonic Au (McCoy et al., 1997)

• Intrusion-related Au (Thompson et al., 1999)

• Granitoid Au (Goldfarb et al., 1999)

MAGMA-METAL ASSOCIATION

Reduced Oxidized-30 -20 -10

Log fO2

Reduced Au association

Oxidized Au association

I M

10

0

Fewt %

Sn

W

Mo

Cu/Mo

Cu

(Thompson et al., 1999)

LOCATION OF MAJOR INTRUSION RELATED GOLD DEPOSITS

(Lang & Baker, 2001)

TWO IMPORTANT QUESTIONS

• Are IRGS worth exploring for?

– Fort Knox ~ 4 M.oz., Pogo ~ 4.8 M.oz., Kidston ~ 4.5 M.oz. Donlin

Creek >10 M.oz.

• Are there coherent, useable exploration strategies that can

be applied to IRGS?

– Can define explorable characteristics

EXPLORABLE CHARACTERISTICS

• Association with moderately reduced felsic igneous suites

• Au, Bi, Te, W, Mo, As (Sb, Sn, Pb, Cu)

• Chemistry & deposit styles zoned vertically & laterally

about intrusive-centred hydrothermal system

• Target styles within the broader framework of the model

IRGS MODEL (modified from Lang et al., 1999)

TINTINA GOLD BELT0 500

kilometres

Alaska

PogoDublinGulch

BreweryCreek

Yukon

Tintina Gold Belt

Tombstone Plutonic Suite

Fort Knox, Ryan Lode& True North

Gold occurrences

Shotgun

DonlinCreek

Farallon(100Ma)

Kula(85Ma)

Kula(56Ma)

Major gold deposits

Tungsten P lutonic Suite

(Flanigan et al., 2000)

TPS WESTERN YUKON

FORT KNOX (4 M.oz.)

• Exploration & mining

– Placer gold 1902 in creeks down stream

– Au-W veins & skarns 1913 peripheral to FK

– Bismuthinite with Au 1980 proximal creeks

– Visible Au in granite 1984

– Advanced exploration 1987 to 1994

– Construction 1995; bulk tonnage open pit

– Production 1996; 169 Mt @ 0.93 g/t

FORT KNOX

(Bakke, 1994)

FORT KNOX

FORT KNOX

Sch ist Ve in

Shear Zone

Pegmat it e

Med.Grani t e

Coarse Grani t e

5 0 m

N S

FORT KNOX (looking W)

(Bakke, 1994)

Intrusion Characteristics

Granodiorite to granite

Ilmenite series, I-type

Aplites/pegmatites

Locally UST

Age

U-Pb 92 Ma - Intrusion

Ar-Ar ~88-86 Ma – Ms. Re-Os 92.5 Ma -Moly

• Vein Characteristics

Pegmatites & sheeted veins (min’l)

Overprinted quartz filled faults (min’l)

Au-Bi-Te-As-Sb-W-Mo (inc. deeper)

Sulfide <1% - Py, Po, Apy, Mo, Sch

• Ore Characteristics

Bi, Bi2S3, Bi2Te3

Free Au, ~111microns, >960 fineness

Au:Bi 0.86

FORT KNOX

(Bakke, 1994; McCoy et al., 1997)

• Alteration

Early Albite > K-feldspar

Quartz-Sericite-Carbonate

Regional propylitic & pyrite halo

• Fluids

Low salinity aqueous-carbonic

250-500°C @ >1.5kbar, >5km

Oxygen isotopes fluid 5 to 10 per mil

Sulphur isotopes 0±5 per mil

FORT KNOX

(Bakke, 1994; McCoy et al., 1997)

DUBLIN GULCH (~2 M.oz.)

N

1.6 km

Eagle Zone

RayGulch

Peso-Ag

Rex-Ag

Biotite hornfels and calc-silicate skarn aureoleGranite and aplite

Granodiorite

Grit Unit

Upper Schist Central QuartziteLower Schist

(Maloof et al, 2001)

EAGLE ZONE VEIN

2.0cm

Ser +Carb + Qtz Ksp > Ab Qtz + Ksp + Sch

Ore Minerals + Carb + Ms + Qtz

EAGLE ZONE PARAGENESIS

(Maloof et al, 2001)

qtz

carb

py

carb-ser

1.8mm

Bi/Pbsulfosalt

Au

0.2mm

EAGLE ZONE PARAGENESIS

(Maloof et al, 2001)

EAGLE ZONE GEOCHEMISTRY

Au Bi As Sb Ag Mo W Zn Pb

Cu 0.14 0.15 0.59 0.56 0.63 0.23 0.00 0.59 0.64Pb 0.08 0.07 0.68 0.89 0.89 0.08 0.00 0.93Zn 0.03 0.02 0.58 0.74 0.77 0.10 0.00W 0.03 0.00 0.00 0.00 0.00 0.55Mo 0.15 0.17 0.07 0.04 0.07Ag 0.24 0.21 0.79 0.85Sb 0.07 0.05 0.61As 0.28 0.24Bi 0.90

(Maloof et al, 2001)

RAY GULCH GEOCHEMISTRY

W Mo Sn Au (ppb)

Bi Sb As Zn Ag

Wol-Qtz Skarn (n=2)

<334 <6 bd bd <1 <0.5 1-5 bd bd

Pyx Skarn (n=9)

200 to 50000

<180 bd bd (10, 38,

13)

<1 <5 <5 <300 bd

Vein (n=14)

0 to >100000

<90 bd bd (148)

<1 <5 <14 <150 bd

(Brown et al, 2001)

IRGS MODEL (modified from Lang et al., 1999)

TINTINA GOLD BELT0 500

kilometres

Alaska

PogoDublinGulch

BreweryCreek

Yukon

Tintina Gold Belt

Tombstone Plutonic Suite

Fort Knox, Ryan Lode& True North

Gold occurrences

Shotgun

DonlinCreek

Farallon(100Ma)

Kula(85Ma)

Kula(56Ma)

Major gold deposits

Tungsten P lutonic Suite

(Flanigan et al., 2000)

DONLIN CREEK (> 10 M.oz.)

(Ebert et al., 2000)

• Geology & mineralization

– Rhyolite dykes hosted in

reduced sediments

– Magmatism & mineralization 74

to 65 Ma

– Narrow Au-As-Sb-Hg veins in

dykes>sediments

– Ore within NNE extensional

fracture zone

– Epithermal/epizonal

characteristics

DONLIN CREEK VEINS

SHOTGUN (~1 M.oz.)

(Rombach & Newberry, 2001)

SHOTGUN TEXTURES

A - Stockwork

C - Breccia

D - UST/brain rock

Au-Bi – 0.73(Rombach & Newberry, 2001)

KIDSTON (4.5 M.oz)

(Baker & Andrew, 1991)

KIDSTON TEXTURES

TIMBARRA (0.4 M.oz.)

(Mustard, 2001)

TIMBARRA GRANITE FACIES & Au(Au-Bi = 0.69)

(Mustard, 2001)

TIMBARRA PARAGENESIS & FLUIDS

MineralizationStyle

Transitional

Magmatic HydrothermalHydrothermal

Miarolitic cavities

Melt InclusionsHigh XCO2Mod-Low XCO2Mod-Low Salinity H2O

Aplite dykesPegmatite veinsVein-dikesQuartz-moly veinsFractures (Au)Comb veinsChalcedonic veinsHematite staining

Late Magmatic-

Stage 1 Stage 2a Stage 2b Stage 3 Stage 4

Process

(Mustard, 2001; Mustard, 2000)

IRGS MODEL (modified from Lang et al., 1999)

TINTINA GOLD BELT0 500

kilometres

Alaska

PogoDublinGulch

BreweryCreek

Yukon

Tintina Gold Belt

Tombstone Plutonic Suite

Fort Knox, Ryan Lode& True North

Gold occurrences

Shotgun

DonlinCreek

Farallon(100Ma)

Kula(85Ma)

Kula(56Ma)

Major gold deposits

Tungsten P lutonic Suite

(Flanigan et al., 2000)

POGO REGIONAL SETTING

Delta Junction

BigDelta

Shaw C

reek Fau

lt

PogoSillimanite isograd

147o 146o145o 144o

64o

65o

Surficial deposits

Surficial deposits

Post Metamorphic Igneous Rocks

Cretacous granite

Tertiary granite

Volcanic & sedimetary rocks

Prehnite-pumpellyite and greenschist-facies quartzite, argillite, phyllite, and minor metalimestone.

Greenschist-facies schist, semischist, quartzite,marble, greenstone and phyllite.

Amphibolite-facies schist, gneiss, augen gneiss, amphibolite, quartzite, and minor marble.

High and Intermediate-pressure amphibolite-facies gneiss, schist, amphibolite, and marble.

Ultramafic rocks.

Strongly metamorphosed pluton. Amphibolite facies.

Metamorphic rocks

0 25 50

Km

N

(Smith et al., 1999)

POGO (4.8 M.oz.)• Exploration

– Geochemical sampling Goodpaster River 1981

– Au, As, W anomalies in Pogo & Liese Creeks

– Soil sampling & surface-exploration 1993

– >100ppb Au in soils anomaly 2km2

– Drilling soil anomaly 1994 – Liese zone

– Drive developed 1999-2000

(Smith et al., 1999)

POGO

POGO

(Smith et al., 1999)

POGO• Host rocks

– L. Proterozoic - M. Paleozoic gneiss

• Amphibolite facies

– M. Cretaceous granite dykes, aplites & pegmatites

• Reduced I-type, 15% vol.

– Post-mineralization dolerites

• Age

– U-Pb 107 to 93 Ma – Intrusions

– Ar-Ar ~91-92 Ma - Mica alt

– Re-Os ~104 Ma - Molybdenite

(Smith et al., 1999; Hart et al., 2002)

POGO• OreSulphide ~3%Reduced assemblagePo-Lo-Apy-Py-CcpAu-Bi-Pb-Te-Ag-S phases

(Smith et al., 1999)

0.89

POGO

POGO

POGO

Grade vs Tonnage Plot

SUMMARY CHARACTERISTICS

• Early brine, CO2-vapour

• Later low salinityH2O-CO2

As, Sb, Hg,

Bi, Te, Cu,Mo , W, Pb,Zn

Clays, carb, fsp

• veinlets, stockwork, breccia

• dikes, stocks, sills

Shallow

(<3km, <1 kbar)

FluidsMetalsAlterationStyle

Mesothermal styles, is there a magmatic-hydrothermal explanation for the varying styles & fluid characteristics?

• CO2-H2O

• Minor late brine

Bi, Te, W,Mo, As, Sb

Fsp, carb• sheeted, disseminated

• plutons

Deep

(>3km, >1 kbar)

FluidsMetalsAlterationStyle

Evidence for magmatic fluids - epithermal/porphyry style characteristics

(Baker, 2002)

• CO2 solubility in felsic melts

– CO2 ~ 10 times less soluble than H2O in melt

– Exsolve higher pressures & earlier than H2O & Cl

– Devolatisation will occur over a wider range of

pressures

– CO2 more abundant at depth

Magmas & H2O-NaCl-CO2

(Giggenbach, 1997)

(Lowenstern, 2001)

Rhyolite -H2O-CO2

Brewery CreekDonlin Creek

KidstonShotgun

Fort KnoxDublin Gulch

Timbarra

Style ExampleEpithermalDyke, sill, domeVeinsDis’minated

PorphyryStocks, plugsBrecciasStockwork

MesothermalPlutonsSheetedDis’minated

CO2-H2O

+

v vv vvvv

++

+

++

+

++

+

++

++

++

+++

+

++

+

BrineMeteoric

H2O

0

5

10

km

MagmaticDEPTH-FLUIDS MODEL FOR IRGS

(Baker, 2002)

MAGMATIC-HYDROTHERMAL TRANSITION

CONCLUSIONS I• IRGS have a coherent, useable set of empirical

exploration characteristics

• Critical features include

– Vertical & lateral zonation about mod-reduced granitic

intrusions

– Set of pathfinder elements including Au, Bi, Te, As, W,

(Mo, Sn, Sb)

– Variety of target types within IRGS

CONCLUSIONS II• Belts known for magmatic related W-Mo-Sn

systems are high priority target areas– Au-(Bi-W) placer occurrences provide good indicator

– Such regions commonly lack thorough testing of IRGS model

– Commonly not sampled for Au & Bi

• Many regions can be regionally evaluated quickly through database searches & GIS approaches

N QLD HODGKINSON PROVINCE

(modified from Garrad & Bultitude, 1999)