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AlkalicAlkalic porphyry Cuporphyry Cu--Au Au deposits: a BC deposits: a BC specialityspeciality
Thomas Bissig, MDRUThomas Bissig, MDRU
Acknowledgements: Acknowledgements: Dick Tosdal, Dave Cooke, Kirstie Simpson, Claire Chamberlain, CrDick Tosdal, Dave Cooke, Kirstie Simpson, Claire Chamberlain, Craig aig Hart, Janina Micko, Kevin Byrne, Heidi Pass, Meghan Jackson, PauHart, Janina Micko, Kevin Byrne, Heidi Pass, Meghan Jackson, Paul l Jago, Adam Bath.Jago, Adam Bath.Geoscience BC and sponsors of the MDRU Geoscience BC and sponsors of the MDRU alkalicalkalic projectproject
Outline
• What are alkalic porphyry deposits and how are they different from calc-alkalicones?
• BC examples: Galore Creek (silica undersaturated) and Mount Milligan (silica saturated)
• Regional volcanic setting and characteristics of host rocks
• Conclusions
alkalic
sub-alkalic
40 50 60 70
4
8
12
16
SiO2 (wt%)
Na 2
O +
K2O
(wt%
)
High-K calc-alkalic
Cu-Au(-Mo)
Silica-undersaturatedalkalic
Cu-Au
Silica-saturated alkalic
Cu-Au
Modified from Lang et al., 1995
• Porphyry Cu-Au-Mo deposit classification
Arizona calc-alkalic
Cu-MoCu-Au
SW Pacificcalc-alkalic
Free gold in quartz-bornitevein, Ridgeway, NSW
Igneous Associations
General alteration model(calc alkalic type)
BAJO DE LA ALUMBRERA (1975, pre-mining)
QzQz--mtmt +/+/-- KK--sfpsfp, barren, barren
BiBi--kspksp +/+/-- mtmt ((potassicpotassic))
QzQz--ser +/ser +/-- clay overprint (clay overprint (phyllicphyllic +/+/-- argillicargillic))
EpEp--chlchl ((propyliticpropylitic))
Se also Ulrich and Heinrich, 2002)
Not shown: Advanced argillic/silicic lithocap
Alkalic porphyry Cu-Au:N-Parkes, NSW, Australia
MZ
Biotite altered mineralized rock
Alteration footprint is narrow, less evident than calc-alkalic cousins
Alkalic porphyry Cu-Au deposits: alteration model
Jensen and Barton, 2000
Distal Propylitic (chlorite sub-zone: chl-carb hm )± ±epi
Skarn (py-hm-mt-chl-carb-gt)
Skarn propylitic (epi-py)
Alkalic lithocap (ab-kf-ser-carb--py-tm)
Sodic (ab-qz-hm)
Outer Propylitic (albite-actinolite subzone: ab-act-qz-carb-py)Inner Propylitic (actinolite-hematite-epidote subzone: ab-chl-act-epi -qz)-hm
Legend - Alteration
Outer calc-potassic(Kf-chl-bt-ab-act-qtz-cp)Inner calc-potassic(bt-act-mt-kf-ab-qtz-bn)
Reddened propylitic halo(hematite dusting of feldspars, negative S isotopes in sulfides, increasing magnetic susceptibilitytowards ore, Zn and Pb geochemical anomaly)
Skarn (mt skarn may form in limestone or reactive volcanic / volcaniclastic rocks,magnetic high)
Alkalic lithocap (albite - K-feldspar - sericite- quartz - carbonate tourmaline) - chargeability high, magnetic low±
Calc-potassic or potassic core(magnetic high, Cu-Au-Mo geochemical anomaly)
Wilson et al. 2003
• Cu-Au association• Includes some of the highest gold grades in
porphyry systems (Ridgeway, Cadia Far East)• PGE enrichment in some systems (Copper
Mountain, Afton, Lorraine) • Magnetite locally very abundant (sold as Fe ore)• Environmentally benign (low pyrite, high
neutralization capacity of some host rocks)• Untapped potential in BC and elsewhere
Why target alkalic porphyry Cu-Au systems?
Mt MilliganMt Milligan
1 10 100 1000 10000.01
.1
1
10
Au
ppm
Mt
SulphuretsMt Milligan
Total CadiaDistrict
Red Chris
Mount Polley
Kerr
Copper Mountain
Total Galore Creek
AftonAjax
Lorraine
Ladolam
Epithermal
Porphyry
0.1 t Au
1 t Au
10 t Au
100 t Au
Grade and Tonnage-Alkalic systems
BC porphyriesOther locations
Au in alk. Epith. Deposit, LkCowal
Alkalic vs Calc-Alkalic
Alkalic districts typically have…
Several small deposits
N-Parkes
Complicated and apparently small alteration footprint.
Potassic, calcic, sodic altn, abundant mt, hem dusting typical. Phyllic and argillic altn scarce and qz veins only in deposits related to silica saturated rocks. Generally low py
Breccias important in some deposits particularly in silica undersaturated types. May host high grade
Myarolitic cavity filled with Bn-cpy, N-parkes
Ab-ep-act-chl +/- carb alteration, Mt Milligan
Mt Polley NE zone
MagnetiteMagnetite--Apatite veins, Afton, BCApatite veins, Afton, BC
Pegmatite style KPegmatite style K--fspfsp, coarse bi veins , coarse bi veins with late with late cpycpy, Copper Mountain, BC, Copper Mountain, BC
Gnt-diop-biot-cemented breccia, Galore Creek Megacrystic Or-plag-phyric monzonite, Galore Creek
Garnet
Features reminding of IOCG’s, Skarns
StuhiniGroup
Takla &Nicola Groups
StikiniaTerrane
QuesnelliaTerrane
Cache CreekOcean
Slide Mountain Ocean
HazeltonGroup
StikiniaTerrane
TaklaGroup
QuesnelliaTerrane
800 kmCache Creek
Ocean
208-193 Ma: Mihalynuk et al., (1994)
Structural and Tectonic Environment of BC AlkalicCu-Au Porphyry Deposits
Highland Valley
Red ChrisGalore Creek
Lorraine
Mt. Milligan
Mount Polley
Afton
Gibraltar
KemessSulphurets
Kerr
Copper MountainVancouver
Dease Lake
Prince Rupert
Prince George
Stewart
Pacific Ocean
USA
Yukon
Alberta
Alkalic Cu-Au porphyries
Calc-alkalic Cu-Au porphyries
-Si+Si
The Galore Creek ProjectJ. Micko, PhD candidate, K. Byrne MSc
The Galore Creek district contains 5 deposits and 7 prospects.
Silica undersaturated class, age roughly between 210-203 Ma
The overall measured and indicated resource is estimated at 785.2 million tonnes grading at 0.52% Cu, 0.29g/t Au and 4.87 g/t Ag(NovaGold Resources Inc; press release June, 2008).
Currently owned by “Galore Creek Mining Company” (GCMC), a 50:50 joint venture between NovaGold Resources Inc. and Teck Cominco Ltd. The Galore Creek district -
(after NovaGold Resources Inc., 2007)
Hydrothermal alteration and mineralization (NGL)
W E W E
From Micko, in Prep
Potassic (orthoclase, biotite, magnetite ± hem)
Calcic (Garnet, diopside ± anhydrite )
Propylitic (chlorite, epidote, carb. ± muscovite)
Sulfur isotopes distribution (NGL)
W E W E
Sulfide sample locations δ34S isotopic zonation
From Micko, in Prep
Mt Milligan
• Resource figures, pictures590 Mt at 0.193 % Cu and 0.35 g/t Au
Silica saturated Alkalic porphyry, Age: ~185 Ma
Panoramic view over pre-mining deposit
Mount MilliganMount MilliganP. Jago, P. Jago, MScMSc
Plan provided by Placer Dome Inc.
DWBX zoneDWBX zone = Cu= Cu--rich, Aurich, Au--moderatemoderate
WBX zoneWBX zone = Cu= Cu--rich, Aurich, Au--poorpoor
MBX zoneMBX zone = Cu= Cu-- and Auand Au-- rich rich
““6666”” zonezone = Cu= Cu--poor, Aupoor, Au--richrich
Mount Milligan rocks
Volcanic host rocks, strong bi +/-mt altn, augite phenos replaced by sulfides
Flow-banded rainbow dyke. Sulfides in flow bands. K-spar +/- bi alteration
Ab-ep-act-chl +/- carb alteration (inner propylitic)
Thin fracture controlling sericitizationof feldspars, upper trachyte
~500 m
Silica saturated monzonite
Fine grained basinal facies
Carbonate rocks
2
3
1
Magnetite richProximal Basalts
Silica undersaturated monzonite
4
5
6
Nicola, Takla, Stuhini groups Intrusive rocks
General volcanic setting for BC porphyry Cu-Au deposits
1: Mt. Polley2: Galore Creek3. Cu Mountain4: Mount Milligan5: Red Chris6: Lorraine
Subaqueous pyroxene phyric basalts
Nicola Group, Tackla Group, Stuhini Group
Medial facies basalts
0 2 4 6 80
50
100
150
200
Mag
netic
sus
cept
ibilit
y *1
0 S
I uni
ts-3
Fe O/Fe3+2+
Increasing oxidation state
group 2 group 1
Highly oxidized parts of the arc are more likely to host a Mt. PolleyOxidized rocks tend to be more alkalic
Mt P
olle
y, L
ac L
a H
ache
Titanaugite with magnetite inclusions
Augite without magnetite inclusions
Mt. Milligan and NE of Bridge Lake
Magnetic Susceptibility as Proxy for Oxidation State
Ongoing MDRU research; Bissig et al. 2010
0 50 100 150 2000
1
2
3
4
5
6
7
8
Magnetic susceptibility * 10 SI units-3
LT NavLT Npt
LT NpvLT Nvl
0 50 100 150 2000
1
2
3
4
5
6
7
8
Magnetic susceptibility * 10 SI units-3
Magnetic Susceptibility, Host rocks
Mt. Polley
Lac La Hache
Magnetic susceptibility generally high (>20*10-3 SI)
Mt. Milligan
Magnetic susceptibility * 10 SI units-30 50 100 150 2000123456789
10111213141516
Potassic alteration
Least altered basalts
0 50 100 150 2000
1
2
3
4
5
6
7
8
Magnetic susceptibility * 10 SI units-3
Magnetic susceptibility generally low (>10*10-3 SI)
Bridge Lk
COEVAL Cu-A
u
COEVAL Cu-A
u
20 M
a you
nger
Cu-Au
No Cu-A
u
Ongoing MDRU research; Bissig et al. 2010
QUEST Aeromagnetic Signatures
!
570000.000000
570000.000000
580000.000000
580000.000000
590000.000000
590000.000000
600000.000000
600000.000000
610000.000000
610000.000000
5810
000.0
0000
0
5810
000.0
0000
0
5820
000.0
0000
0
5820
000.0
0000
0
5830
000.0
0000
0
5830
000.0
0000
0
5840
000.0
0000
0
5840
000.0
0000
0
5850
000.0
0000
0
5850
000.0
0000
0
5860
000.0
0000
0
5860
000.0
0000
0
LegendMag2_Main.grdValue
High : 62759.687500
Low : 55648.089844
Mount Polley: Relatively high magnetic intensity in Nicola Gr. volcanic rocks
MP
Logan et al., 2010
Ongoing MDRU research; Bissig et al. 2010
QUEST Aeromagnetic Signatures
!
420000.000000
420000.000000
430000.000000
430000.000000
440000.000000
440000.000000
450000.000000
450000.000000
460000.000000
460000.000000
470000.000000
470000.000000
6080
000.0
0000
0
6080
000.0
0000
0
6090
000.0
0000
0
6090
000.0
0000
0
6100
000.0
0000
0
6100
000.0
0000
0
6110
000.0
0000
0
6110
000.0
0000
0
6120
000.0
0000
0
6120
000.0
0000
0
6130
000.0
0000
0
LegendMag2_Main.grdValue
High : 62759.687500
Low : 55648.089844
Mount Milligan: Generally subdued magnetic intensity in volcanic units. Mag high corresponds to mid Jurassic (169 Ma) intrusion
MM
Ongoing MDRU research; Bissig et al. 2010
Logan et al., 2010
Summary
- Most BC porphyry deposits have been tilted - Negative sulfur isotopes in sulfides vector to
mineralization- Magmatic-hydrothermal breccias are important
hosts- Porphyries represent a variety of volcanic and
igneous settings but host rocks to all are subaqueous basaltic rocks and K-spar megacrystic monzonite porphyries are commonly found in proximity to mineralization
- In most districts several discrete mineralized centers with varied Cu:Au ratios are present
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