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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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
pmd CRCStratigraphyStratigraphy
• Metamorphosed to upper greenschist- to lower amphibolite-facies conditions
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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)
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
pmd CRCCrossCross--section through section through IntrepideIntrepide
After Karen Connors (2002)
Geological OverviewGeological Overview
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
pmd CRC
DCB
KSL
DFDDFD
PBS
PBS
W E
DCB
KSL
KSL?
FP
FP
SC
X-SECTION
A B
Geological OverviewGeological Overview
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
pmd CRCRedox Redox -- gradient spatialgradient spatial
Distance ►
Reduced (po-py)
Oxidized (mag)
Very oxidized (hem)
GoldGold
Magnetite
Haematite
fO2
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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
pppredictiveredictiveredictive mmmineralineralineral dddiscoveryiscoveryiscovery
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)