Alkalic porphyry Cu-Au deposits

28 Pages·2010·5.84 MB·English

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Outline
• What are alkalic porphyry deposits and
how are they different from calc-alkalic
ones?
• BC examples: Galore Creek (silica
undersaturated) and Mount Milligan (silica
saturated)
• Regional volcanic setting and
characteristics of host rocks
• Conclusions
Igneous
Associations
• Porphyry Cu-Au-Mo deposit classification
Silica-
undersaturated
16
Silica-saturated
alkalic c
) a li
% k
alkalic a l
c
t a li
k
w 12 a l
b -
u
( s
Cu-Au
O
2
K 8
Cu-Au(-Mo)
Cu-Mo

+
u
A
C u - Cu-Au
O
4
2 Arizona calc-
a
SW Pacific High-K calc-
N alkalic
calc-alkalic alkalic
40 50 60 70
Free gold in
SiO (wt%)
quartz-bornite
2
vein, Ridgeway, Modified from Lang et al., 1995
NSW
General alteration model
(calc alkalic type)
QQzz--mmtt ++//-- KK--ssffpp,, bbaarrrreenn
EEpp--cchhll ((pprrooppyylliittiicc))
BBii--kksspp ++//-- mmtt ((ppoottaassssiicc))
QQzz--sseerr ++//-- ccllaayy oovveerrpprriinntt ((pphhyylllliicc ++//-- aarrggiilllliicc))
Not shown: Advanced argillic/silicic lithocap
BAJO DE LA ALUMBRERA (1975, pre-mining)
Se also Ulrich and Heinrich, 2002)
Alkalic porphyry Cu-Au:
N-Parkes, NSW, Australia
Biotite altered
mineralized rock
Alteration footprint
MZ
is narrow, less
evident than calc-
alkalic cousins
Alkalic porphyry Cu-Au
deposits: alteration model
Alkalic lithocap
(albite - K-feldspar - sericite- quartz - carbonate ± tourmaline) - chargeability high, magnetic low
Skarn
(mt skarn may form
in limestone or reactive volcanic
/ volcaniclastic rocks,
magnetic high)
Reddened propylitic halo
(hematite dusting of feldspars, negative S isotopes
in sulfides, increasing magnetic susceptibility
towards ore, Zn and Pb geochemical anomaly)
Legend - Alteration
Distal Propylitic (chlorite sub-zone:
chl-carb±hm±epi)
Skarn (py-hm-mt-chl-carb-gt)
Jensen and Barton, 2000 Skarn propylitic (epi-py)
Alkalic lithocap (ab-kf-ser-
carb--py-tm)
Sodic (ab-qz-hm)
Calc-potassic Outer Propylitic (albite-actinolite
subzone: ab-act-qz-carb-py)
or potassic core
Inner Propylitic (actinolite-hematite-
(magnetic high,
epidote subzone: ab-chl-act-epi-hm-qz)
Cu-Au-Mo geochemical
Outer calc-potassic
anomaly)
(Kf-chl-bt-ab-act-qtz-cp)
Inner calc-potassic Wilson et al. 2003
(bt-act-mt-kf-ab-qtz-bn)
Why target alkalic porphyry
Cu-Au systems?
• 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
MMtt MMiilllliiggaann
Grade and Tonnage-
Alkalic systems
10
BC porphyries
Other locations
Ladolam
Epithermal
1
Sulphurets Total Cadia
Porphyry
m Mt Milligan District
p
p Mount Polley AftonAjax Total Galore

u Creek
A Kerr
Red Chris
Lorraine
Copper Mountain
.1
1
0
0

1 t
0
. 0
1 t 1 t t A Au
A A u
u u
.01
1 10 100 1000 10000
Mt Au in alk. Epith. Deposit, Lk
Cowal
Alkalic vs Calc-Alkalic
Alkalic districts typically have…
Myarolitic cavity filled with Bn-cpy, N-parkes
Several small deposits
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
N-Parkes
Mt Polley NE zone
Ab-ep-act-chl +/- carb alteration, Mt Milligan
Breccias important in some
deposits particularly in silica
undersaturated types. May host
Complicated and apparently small alteration
high grade
footprint.
Features reminding of
IOCG’s, Skarns
PPeeggmmaattiittee ssttyyllee KK--ffsspp,, ccooaarrssee bbii vveeiinnss
wwiitthh llaattee ccppyy,, CCooppppeerr MMoouunnttaaiinn,, BBCC
MMaaggnneettiittee--AAppaattiittee vveeiinnss,, AAffttoonn,, BBCC
Gnt-diop-biot-cemented breccia, Galore Creek Megacrystic Or-plag-phyric monzonite, Galore Creek
Garnet