Transcript – A Variation The Cobar Goldfield on a Theme Rex Berthelsen, Chief Geologist
The Cobar Goldfield – A Variation on a Theme
Rex Berthelsen, Chief Geologist Mines and Wines, Cessnock 2006
Where is Peak?
2
Cobar The Peak
Lachl an Fol d Bel t
200km Newcastle Sydney The Peak N e w S o u t h W a l e s
S Y D N E Y
3 Modern Production History • 1981 – Peak deposit discovered by CRA.
• 1992 – Production commenced at Peak.
• 1994 – Perseverance discovered.
• June 2000 – New Cobar Open pit commenced.
• Nov 2001 – New Occidental commenced production from 92 level (approx 920m below surface) • Nov 2003 – Perseverance commenced production from 9305 level (960 m below surface).
• Feb 2004 – New Cobar open pit finished.
• June 2004 – New Cobar U/G portal established.
• Sept 2005 – New Cobar Production commenced • 3 Moz Au produced from the field to Dec 2005
4 Cobar Gold Field Deposits • Common Features – Short strike lengths 250 -> 350m – Shear / Fault hosted (GCF/PK-PER shear) – Steep northerly plunging ore shoots, but lithologically controlled – Metal associations • Au – Bi • Cu – Ag • Pb – Zn • Au - Zn
Cobar Gold Field Deposits 5 • Significant magnetic anomalies • Magnetic source differs between deposits – Primary Magnetite – Chesney, New Cobar, Great Cobar and New Occidental – Peak and Perseverance – spatial relationship between pyrrhotite and gold – copper ores New Cobar Chesney New Occidental Peak Perseverance
Deposit types • Group 1 – Peak and Perseverance • Group 2 –New Occidental, Chesney and New Cobar •Group 3 – Gladstone and Great Cobar 6
CGF Long Section Perseverance Peak 900m Bi Au Cu Ag Zn Pb 3100m New Occidental 1800m Chesney New Cobar Great Cobar 800m 1900m 7
8 Group 1 – Peak and Perseverance • Hosted by variety of litho-stratigraphic packages.
– Great Cobar Slate siltstones – Chesney Formation sandstone and siltstones – Rhyolite and rhyolite breccias • Shear Zones – Steep, grossly west dip by with locally east dipping zones
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10 650 N 10 550 N 10 350 N C OP P ER CO PP TH ER NOR NOR CO R EP R EP U LS TH E E N OR U LS TH N OR E C L A H H U T
N o D ata
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After Stegman and Pocock,1996 INC R N LE A D Z W E S T E 10 450 N E E P U L S H O U T R S 10 750 N
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E E P U L S R S O U T H 600 Level After Stegman and Pocock,1996 R EPU LSE NO RTH 10 750 N
C
10 650 N 10 550 N 10 450 N 10 350 N R W E S T E 10 450 N E P U L S E R S O U T H 10 350 N INC R N LE A D Z W E S T E ZIN C S T E R N L E A D W E C OP P ER
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11 Perseverance – Sediment Hosted
12 Perseverance – Rhyolite Hosted
13 Perseverance Long Section
m RL m RL m RL m RL
• North Plunging Shoots • Rhyolite controlling “bonanza” grades • “Hulk Lens” – Better intercepts include • 3m @ 38.7g/t Au, 0.45% Cu • 11.5m @ 12.3g/t Au, 0.85% Cu.
• 4.8m @ 7.2 g/t Au, 0.78% Cu 500
14 Peak Rhyolite
15 Rhyolite Breccia (Hyaloclastite)
16 Rhyolite Hosted Au – Zn • Flow banded rhyolite • Cross cutting qtz-sph-gal gold vein
17 Group 2 – New Occidental to New Cobar • Controlled and hosted by Great Chesney Fault and related splays • Relatively simple geometries compared to Peak – Perseverance • Short strike lengths <300m, most continuous vertically (New Occ > 1200m, 5-30m wide • Cryptocrystalline or chalcedonic silica is most characteristic closely associated with Au – Bi mineralisation
18 New Occidental N
19 Alteration • Early silicification and pervasive Fe chlorite • Coliform banded qtz-magnetite veins most common at New Cobar, less common at Chesney and New Occidental • 2 styles of magnetite veining – Coliform and porphyroblastic veins • Stilpnomelane associated with fibrous and coxcomb textured quartz veins (syntectonic)
New Occidental 20 Silicified Sediment Matrix Breccia NO1D, 874m
21 New Occidental • Coliform qtz – magnetite • Chalcedonic silica • Overprinting bucky white quartz
22 New Cobar Magnetite Chalcedonic silica Overprinting Cu
New Cobar 23 1cm • Magnetite • Chalcedonic silica
24 New Cobar • Northerly plunging gold and copper shoots within multiple lenses.
Chesney 800m • North plunging gold shoots within main copper zone • Eastern gold lens similar to New Ox 25
Group 3 – Gladstone – Great Cobar 26 • 400 to 900m west of GCF • Great Cobar Slate (siltstone hosted) • Sandstone units likely to have some control • Gladstone ore – Cpy + Po + Qtz bx • Great Cobar has very wide mineralised halo (~80m 1.2% Cu)
Great Cobar 83.5
64.2
37.6
27.1
23.4
19.5
18.5
17.4
9900 mN 16.4
15.4
14.8
9800 mN 14.3
13.8
12.8
8.2
GC0002F, 15m @ 1.4%Cu, 0.16g/t Au GC0002E, 9m @ 0.84%Cu, 0.16g/t Au CM0016, 8.9m @ 1.12%Cu, 0.15g/t Au CM0015C, 10m @ 1.29%Cu, 0.05g/t Au GC0002A, 27.6m @ 0.94%Cu, 0.06g/t Au GC0002D, 11.8m @ 1.3%Cu, 0.07g/t Au GC0002B, 17.6m @ 4.4%Cu, 0.63g/t Au 200 m NG0256, 11.2m @ 1.9%Cu, 0.51g/t Au NG0257, 4.3m @ 4%Cu, 0.83g/t Au NG0259, 10.7m @ 1.02%Cu, 0.26g/t Au NG0259D, 22m @ 1.18%Cu, 0.26g/tAu CM0001, 65.8m @ 1.27%Cu, 0.05g/tAu 27 • Historic workings to 450m (early 1900’s) • Fe rich system • Copper late and occurs in four lenses • Generally broad low grade Cu ,very low grade Au in main zone.
• Best intercept, [email protected]%Cu
Great Cobar 28 • At least 4 lenses identified – East, Central (historically mined) West (Pb-Zn) and North • Early silicification -> Fe Chlorite alt -> • Syntectonic – extensional quartz veins with intergrowths of stilpnomelane, biotite and chlorite.
• Early pyrite -> replacement by magnetite, overprinted by chalcopyrite, pyrrhotite and sphalerite.
• Sulphides intergrown in equilibrium with chlorite, quartz, albite, stilpnomelane and biotite.
29 Great Cobar Intergrowths of stilpnomelane and quartz Chalcopyrite overprinting
30 Great Cobar • Intergrowths of quartz, stilpnomelane, chlorite • Sphalerite overprinting
31 Great Cobar Quartz, stilpnomelane, magnetite Chalcopyrite overprint
32 Great Cobar • Porphyroblastic magnetite within massive sulphide (chalcopyrite and pyrrhotite) • Best intercept – 17.6m @ 4.5% Cu, 19g/t Ag, 0.6g/t Au
Great Cobar 33 Extensional veins of chalcopyrite and pyrrhotite in strongly chlorite altered siltstone
Conclusions 34 • Ore shoots have steep northerly plunges • Early silicification - > Fe chlorite • Competency contrasts integral to ore deposition (SS / SI or SED / RHY) • Early Au – Bi hosted by chalcedonic silica veins • Overprint by Cu - Ag and later Pb – Zn • Syntectonic ore deposition
35