Camp- to deposit-scale spatial zonation and temporal ...Kalg. Group, Komatiite, Lunnon Kalg. Group, mafic rocks Kapai Slate M erougil B ds Gravity Lows Oxidized – Hematite – Magnetite

pmd CRCCampCamp-- to depositto deposit--scale spatial scale spatial zonationzonation and and

temporal succession of temporal succession of redoxredox indicator indicator sulphidesulphide--oxide minerals oxide minerals -- vectors to vectors to ArchaeanArchaean orogenicorogenic gold gold deposits? An example from the St. Ives gold camp, deposits? An example from the St. Ives gold camp,

YilgarnYilgarn CratonCraton, Western Australia, Western Australia

Peter Neumayr1, Steffen G. Hagemann1, John Walshe2, Leo Horn2, Robert Morrison3

1 Centre for Global Metallogeny, UWA2 CSIRO, Division of Exploration and Mining3 St. Ives Gold Mining Company Pty Ltd

AcknowledgementsAcknowledgements

pmd CRCThe project is funded by the pmd*CRC and St. Ives Gold Mining Company Pty Ltd. (Gold Fields). Permission to publish is gratefully acknowledged.

pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery

pmd CRCOutline of presentationOutline of presentation

• Camp-scale geological overview– Stratigraphy– Structural controls on mineralization

• Camp-scale redox variations• Deposit-scale alteration variation

– Paragenetic studies• Temporal and spatial integration• Conclusions

3800

00 m

E

3700

00 m

E

6525000 mN

6545000 mN

6535000 mN

Fault, movement

Fault, inferred

Thrust Fault

Anticline, Syncline

BeddingFault, foliationFault, listricl

ll

l

1.5 3

kilometres0

Orchin

Victory/Defiance

North OrchinBritannia/Sirius

Delta

Revenge

Mars

Minetor

IntrepideBahama/Santa Ana

Redoutable

Junction

Argo

30

45

70

70

2070

Basalt undifferentiatedBlack Flag BedsGranitoidKalg. Group, Komatiite, LunnonKalg. Group, mafic rocks, KSLMerougil Beds

PerthKalgoorlieKambalda

Geological map Geological map of the St. Ives of the St. Ives

gold campgold camp

Cross-section


pmd CRCStratigraphyStratigraphy

• Metamorphosed to upper greenschist- to lower amphibolite-facies conditions


pmd CRCStructure Structure –– control on Aucontrol on Au

• First-order structure: Lefroy Fault• Au controlled by contractional jog along

second-order Playa Fault• Au hosted in third- and fourth-order

oblique- to reverse faults (Repulse, NO1, W66)


pmd CRCCrossCross--section through section through IntrepideIntrepide

After Karen Connors (2002)

Geological OverviewGeological Overview


pmd CRC

Lefroy Fault Zone

Victory-Defiance

Conqueror Fault

Playa Fault

Revenge

Foster Thrust

Merougil Fault

North Orchin

Delta Island

3880

00 m

E

6538000 mN

3880

00 m

E

6534000 mN

3840

00 m

E38

4000

mE

3800

00 m

E38

0000

mE

3760

00 m

E

6534000 mN

3760

00 m

E

6538000 mN

kilometres

1 2

Thrust Fault

Fault, movement

Anticline

Fault, inferred

0

Detailed GravityDetailed Gravity


pmd CRCFelsic PorphyriesFelsic Porphyries

3880

00 m

E

3800

00 m

E

6534000 mN

6538000 mN

Lefroy Fault Zone

Victory-Defiance

Conqueror Fault

Playa Fault

Revenge

Foster Thrust

North Orchin

Merougil Fault

Delta Island

1 2

kilometres

0

Gravity LowsGravity Lows


pmd CRCRelative timing of alterationRelative timing of alteration

• Early epidote-magnetite-carbonate• Magnetite predating and early syn-gold• Gold-related chlorite,biotite, feldspar-

carbonate-pyrite / +magnetite-hematite


pmd CRCRedox Range of AlterationRedox Range of Alteration

-28

-26

-24

-22

-3 -2.6 -2.2 -1.8 -1.4 -1 -0.6 -0.2

Sum S { log(H2S+sulfate) }

Red

ox(lo

g fO

2)

Oxidized MagmaticVolatiles

Reduced Fluids

High V/Al in magnetite

Low V/Al in magnetite

Pyrrhotite PyriteMagnetite Po

CO2g-CH4g

SO2g - H2Sg

HSO4 -H2S

HematiteMagnetite

Hematite

Pyrite

OxidizedFluids

Volatiles

Reduced Po-Py

Oxidized: Mag-Py, He

Wallrock approx.

400ºC, 500 bars


pmd CRC

Oxidized– Hematite– Magnetite– Pyrite– Arsenopyrite– Pyrrhotite– Loellingite

Reduced

Redox variations in the St. Ives Redox variations in the St. Ives Gold CampGold Camp

CampCamp--scale scale redoxredox variationsvariations


pmd CRC

Lefroy Fault Zone

Victory-Defiance

Conqueror Fault

Playa Fault

Revenge

Foster Thrust

North Orchin

Merougil Fault

Delta Island

pymagmag-pypo-pypo

1.50.75

kilometres0

Fault, movement

Fault, inferred

Fault,dip

l

Fault, thrust

Bedding

Fault listric

ll

l

3880

00 m

E

3800

00 m

E

6534000 mN

6538000 mN

70

45

20 70

70

30

Basalt undifferentiatedBlack Flag BedsGranitoidKalg. Group, Komatiite, LunnonKalg. Group, mafic rocksKapai SlateMerougil Beds

Gravity LowsGravity Lows

Oxidized– Hematite– Magnetite– Pyrite– Arsenopyrite– Pyrrhotite– Loellingite

Reduced

Camp Scale Camp Scale RedoxRedox Variations Variations –– Interflow Interflow sedimentssediments


pmd CRC

Lefroy Fault Zone

Victory-Defiance

Conqueror Fault

Playa Fault

Revenge

Foster Thrust

North Orchin

Merougil Fault

Delta Island

reduced zone (po-py, po)

oxidized zone (mag-py)

high grade gold (>100 ppm)

1.50.75

kilometres0

Fault, movement

Fault, inferred

Fault,dip

l

Fault, thrust

Bedding

Fault listric

ll

l

3880

00 m

E

3800

00 m

E

6534000 mN

6538000 mN

30

70

7045

2070

Basalt undifferentiated (3)Black Flag Beds (2)Granitoid (25)Kalg. Group, Komatiite, Lunnon (2)Kalg. Group, mafic rocks, KSL (1)Merougil Beds (1)

Camp Scale Camp Scale RedoxRedox Variations Variations –– Interflow Interflow sedimentssediments


pmd CRC

DCB

KSL

DFDDFD

PBS

PBS

W E

DCB

KSL

KSL?

FP

FP

SC

X-SECTION

A B

Geological OverviewGeological Overview


pmd CRC

Different generations of Different generations of magnetite:magnetite:•• epidote-magnetite

• magnetite

EpMag

Magnetite pre-dates gold

Mag

Ep bands, qtz-mag-

chpy veins

Hbl-btporphyry

Geochemical traverseGeochemical traverse


pmd CRCRevenge Gold Mine Revenge Gold Mine –– Ore PyriteOre Pyrite

Redox switches are important for gold mineralization LD7114 181.6 m

Magnetite

Hematite

10 um10 um

Geochemical traverseGeochemical traverse


pmd CRCRedox Redox -- gradient spatialgradient spatial

Distance ►

Reduced (po-py)

Oxidized (mag)

Very oxidized (hem)

GoldGold

Magnetite

Haematite

fO2


pmd CRC

Project Review 3-4 Dec. 2002, Canberra

ConclusionsConclusions

• Camp is zoned with respect to redox conditions of hydrothermal fluids• Hydrothermal alteration forms at different times• Hydrothermal magnetite (oxidized fluid) rims gravity lows on camp scale

and has a strong spatial association with abundant porphyries• Hydrothermal hematite occurs typically within the gold lodes and late• Oxidized fluid are highly focussed (camp and deposit scale)• Pyrrhotite (reduced fluid) occurs in areas subjacent to magnetite

alteration• Reduced fluids are diffused (poorly focussed)• Redox state of hydrothermal alteration changes during Au precipitation• Redox shift occurred either through fluid mixing or through interaction

of ore fluid with pre-existing alteration


pmd CRCImplicationsImplications

• Mapping out redox variations on a camp scale has potential to locate gold

• Several different types of fluids involved in the make-up of camp scale hydrothermal cells

• Alteration observations across scale are the key

• Alteration features can be recognized at all scales


pmd CRCOngoing/Future ResearchOngoing/Future Research

• Determine extent of hydrothermal magnetite in other lithologies; specified structures

• Distinguish different magnetite types using microanalysis• Identify reduced and oxidized fluid conduits along fault zones

using mineralogy and whole-rock geochemistry• Fingerprint the geochemical characteristics of reduced and

oxidized fluids in the different structural corridors• Integrated geochemical/structural models with real fluid

compositions• Construct 3D/4D geochemical and structural model (Pathfinder

Elements)

Documents

Camp- to deposit-scale spatial zonation and temporal ...Kalg. Group, Komatiite, Lunnon Kalg. Group, mafic rocks Kapai Slate M erougil B ds Gravity Lows Oxidized – Hematite – Magnetite