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OROGENIC GOLD TARGETING
What is important and why?
Using the five questions approach
This approach identifies the processes that form
the gold deposit (it does not describe the deposit)
across the full range of scales
OROGENIC GOLD TARGETING
Criteria 1
Fractionated basalt sequence often hosting nickel
sulphide deposits in a back arc basin setting
Basalts are derived from magma crystallising magnetite
in the source region
Fractionation of magnetite adds oxygen (O) to the SubContinental Lithospheric Mantle (SCLM)
This event can pre-date the gold event by tens to
hundreds of millions of years
Kalgoorlie Geochronology
2720
2700
2680
2660
2640
Thol. Basalt +/- UM volcanism
Komatiite
TTG volcaniclastic (Black Flag)
Bimodal (Teutonic)
dolerite
Coarse clastic
High-Ca Granite
Low-Ca Granite
Mafic Granite
De
De
Ni
D2
VMS
Au
Laverton Geochronology
2820
2800
2780
2760
2740
2720
2700
2680
2660
2640
Thol. Basalt +/- UM volcanism
Calc-alk andesite
Komatiite/Thol. Basalt
Volcaniclastic
Coarse clastic
High-Ca Granite
Low-Ca Granite
Mafic Granite
Ni
Au
WINDARRA
ADMIRAL
HILL
Syenite
Ni
AGNEW
Au
GRANNY SMITH, SUNRISE, WALLABY
OROGENIC GOLD TARGETING
Criteria 2
Boundaries of cratonic blocks are faults that extend to
crust mantle boundary
Large scale continental collision
Subduction adds water (H2O) back into the SCLM that
was lost during mafic magma extraction
S-O-H2O are necessary pre-cursors to create thiosulphate
complexes that will carry gold from the mantle
Backarc/Peri-cratonic:
VMS & CD base metal, Ni sulphide
VMS Base Metal:
Melting of metasomatised CLM (metal source)
Most favored if spreading ridge migrates into CLM
Moderate preservation potential if peri-cratonic basin narrow
Graham Begg, 2011
Back-arc/Peri-cratonic : Orogenic Au
Orogenic Au:
Flanked by metasomatised CLM (metal source)
Closure and suturing leads to selective melting of source
Reduced sediments are a good host rock
High deposit preservation potential
Graham Begg, 2011
OROGENIC GOLD TARGETING
Criteria 3
Late basin develops after first collisional event
Basin inversion during second compressive deformation
creates large scale anticline in the hanging wall of mantle
tapping fault
Source of reductant CH4 (methane) is black shale within
the late basins and methane drives gold deposition by
destabilising the thiosulphate complex
REGIONAL
SEISMIC CROSS SECTION KALGOORLIE 100moz
GINDALBIE
TERRANE
KALGOORLIE TERRANE
Kunanalling
Shear
Bullabulling
Dunnsville
Shear
Anticline
Avoca Fault
Scotia-Kanowna
Anticline
Mt Monger
Fault
Emu
Fault
{does not outcrop in this
section because it is intruded
by the Arcoona Granites}
Arcoona Granite
5
Basal detachment ?
10
V =1
H
10km
Upper basalt
Undivided basalt
Early granite
Felsic gneiss
Felsic volcanic rocks
Lower basalt
Komatiite
Late granite
Basal felsic schist
Felsic volcanic unit
CDP
7200
0
9200
11200
13200
Greenstone sequence
(in Bardoc Shear Zone)
GINDALBIE
TERRANE
KALGOORLIE TERRANE
15200
KURNALPI
TERRANE
17200
5
Two-way time (s)
Depth (km)
IDA
FAULT
Kurrawang
Syncline Mt Pleasant BARDOC
Zuleika
Anticline Deformation
Zone
Shear
KURNALPI
TERRANE
10
15
0
20
10 km
MID CRUST
ANOMALY
V/H = 1 (approx) for a Velocity of 6.0km/s
OROGENIC GOLD TARGETING
Criteria 4
Mafic granites are High Calcium suite granites that have a
mantle component added to the magma chamber at the
base of the crust
Granites are emplaced along mantle tapping faults to
high crustal levels (basement cover sequence
boundaries) and are emplaced into large anticlines
These granites carry gold rich fluids from base of the
crust to upper crustal levels then exsolve fluids at fluid
saturation at lower pressure
Non-reflective zone; intrusive complex
that produces the gravity-low.
Seismic section courtesy of Ned Stolz, Gold Fields, St Ives
Interpretation of Seismic Section
Paringa Basalt
Paringa Basalt
Porphyry Complex
Tripod Hill Komatiite
Lunnon Basalt
Lower Felsic Complex
KD3011 607m, 1.23g/t
Anhydrite vein
Disseminated
pyrite
Significant Au and pyrite content in porphyries that are not notably altered;
=> Fluid is in equilibrium with the porphyry.
Some of the St Ives porphyries are inherently enriched in Au and S
3D Model of the Beta Porphyry
1billion tonnes
mean gold grade = 0.37g/t
~ 12 million ounces
Footwall porphyries commonly have 100mt to 1,000mt of porphyry @ 0.05 to 0.5 g/t Au, e.g.
Victory, Revenge, New Celebration, Binduli, Mt Pleasant, Kundana, Granny Smith, Lancefield, Mt
Morgans, Agnew, etc
All gold-rich porphyries have the same enriched trace element signature!!!
FI 7, Beta Porphyry
Interplay of fluids-architecture
Revenge
through
Victory-Defiance
After
Ned Stolz &
Janet Tunjicja
OROGENIC GOLD TARGETING
Next Steps
Testing of regional targets with multi-element
geochemistry at low density (1 sample per 16 km2) will
identify mineralised systems
Darlot, 4m oz
Agnew, 5m oz
Thunderbox, 2m oz
Sons of Gwalia, 10m oz
Tarmoola, 2m oz
Scale
330km by 150km
3000 samples
OROGENIC GOLD TARGETING
Mineral systems have characteristic spacing of gold
deposits within the system
Archaean gold systems in the Yilgarn are defined by 60km
by 60km boxes within which deposits are spaced at 30km
The crust is 30km thick at Kalgoorlie and Laverton
The gold systems are spaced at 130km
OROGENIC GOLD TARGETING
What is important and why?
Using the five questions approach
OROGENIC GOLD TARGETING