GEOL 5320
ADVANCED IGNEOUS AND
METAMORPHIC PETROLOGY
Modeling the Petrology
and PGE Reef Mineralization
of the Sonju Lake Intrusion
December 7, 2009
Sonju Lake
Intrusion
The
Sonju
Lake
Intrusion
From MGS Map
M-71 (Miller
et al., 1989)
Modal Variation and Cumulus Texture
Modal Variations
Modal Rock Names
Leucogranite
Cumulus
Mineral Cumulus Code
Mode
Quartz ferromonzonite
Apatitic olivine
ferromonzodiorite
Apatite olivine ferrodiorite
PCFOAg
PCFOAh
Ol-bearing oxide gabbro
PCFoi
Ol-bearing gabbro
PC(O)fi
Troctoliteaugite troctolite
POcfi
Dunite
Melatroctolite
O
OP
Cryptic Variation
Sheet-like Geometry of the Sonju Lake Intrusion
Estimation of Bulk Composition
Becomes a 1-dimensional problem
>10 km
1 km
>20 km
Bulk Intrusion Composition
= Parent Magma
SiO2
TiO2
Al2O3
FeOt
MnO
MgO
CaO
Na2O
K2O
P2O5
Volatiles
Total
mg#
47.6
2.28
14.0
14.7
0.21
8.3
9.4
2.47
0.55
0.30
0.20
100.0
50.2
Sc
V
Cr
Co
Ni
Rb
Sr
Ba
Y
Zr
Nb
Hf
La
Ce
Sm
Eu
Tb
Yb
Lu
34
192
111
75
185
20
233
171
20
114
17
3.1
14.7
33.4
4.1
1.6
0.8
2.1
.32
Liquid Line of Descent
Calculated by summing composition of
rock column above a specific horizon
= moderately evolved olivine tholeiitic basalt
From Miller and Chandler (1998)
and Miller and Ripley (1997)
Fractional Crystallization Modelling
CHAOS 2 (NIELSEN, 1990)
Model Parameters : fO2 = -2 log QFM; trapped liquid = 20%
Discovery of Stratiform
PGE Mineralization
Skaergaard Intrusion
0
100
200
300
400
Cu (ppm)
500 600
Feb. 1999
Outcrop Sampling
PGE
Reef
SLI Chemostratigraphy
From Miller (1999)
Evolution of
Sulfide in the
Sonju Lake
Intrusion
Exploration
Drilling by
Franconia
Minerals
July 2002
Core Logging and Sampling
Plagioclase
65-73%
Sampling Regime
Phase 1 – 1’ sample every
10’
Phase 1- continuous 1’
across PMZ
423 Total Whole Rock
Analyses
1 cm
Detailed Geochemical Profiling of the PMZ
Cu-Pd Ratios
Cu/Pd
SL02-1
Meters above
Cu-Au break
SL02-1
Precious
Metals Zone
(PMZ)
Pd (ppb)
after Barnes et al. (1993)
PMZ Metallogenesis
• What was the PGE mineralizing agent?
Cu-Fe Sulfide - close physical association of PGM and “cumulus” chalcopyrite
• Why the paucity of sulfide in the PMZ?
Sulfide dissolution by deuteric and low-T hydrothermal fluids – dissolution and
replacement textures in sulfide associated with silicate alteration;
secondary pyrite above PMZ
• Are the stratigraphic variations in grade primary?
Yes for Pd & Pt, not for Cu and Au – Pd and Pt concentrations correlate to
subtle silicate layering; textural evidence of unreactive PGM;
experimental evidence of Au and Cu mobility in oxidizing fluids
PGE Mineralizing Agent?
Cu-Fe Sulfide?
Fe-Ti Oxide???
Data from SL02-1 PMZ
0 to 105m below Cu-Au Break
“Cumulus” Sulfide in the PMZ
Meters above
Cu-Au break SL02-1
SL02-2
Bn
SL02-3
Cp
100 um
Cp
Precious Metals Zone
(PMZ)
Paucity of Sulfide in PMZ?
PGM
Chl-Act
Uralite
Uralitized
Augite
Mobility of Sulfide
Desulfurization and Oxidation of Cu Sulfide
Are Metal Offsets Primary?
Kinetic Model
Preservation of variable
degrees of disequilibrium
during sulfide liquation
Controls on Equilibrium
•Sulf/Sil distribution coefficient
•Diffusivity of metals in silicate
melt
•Nucleation density of sulfide
droplet
•Size/Growth rates of sulfide
droplets
•Settling rate of sulfide (strain
rate of silicate melt)
Relationship of PGE to Modal Variations in the PMZ
Plagioclase Augite
Fe-Ti Oxide
Correlation of Leucocratic Gabbro to Skaergaard Platinova Reefs
From Andersen et al. (1998)
Restite PGM / Resorbed Sulfide
50 um
50 um
Augite
Augite
Cp
Actinolite
Augite
Augite
Cp
Pd-Sb
ChloriteActinolite
Pd-Sb
Plagioclase
50 um
Pt-Pd-As
Cp
Correlation by Secondary Pd Peaks
Mineralization Model
“Downer” Stage
PGE scavenging of
magma column
complete
Initial sulfide
saturation
Upgrading sulfide in
intercumulus magma
Dsulf/sil~104-108
Rapid Diffusion
Dsulf/sil~102
Slow Diffusion
Mineralization Model
“Upper” Stage
Comparing the SLI to other PGE reefs
Meters above sulfide increase
Skaergaard-type
Offset
Classic
IF ONLY.....
PGE
reefs
PELE – MELTS-based Modeling Program
developed by Alan Boudreau
Oxygen Buffers
MH - magnetite-hematite
4 Fe3O4 + O2 = 6 Fe2O3
Common fO2 range for
magmatic conditions
NiNiO nickel-nickel oxide
2 Ni + O2 = 2 NiO
FMQ fayalite-magnetite-qtz
3 Fe2SiO4 +O2 = 2 Fe3O4 + 3
SiO2
WM wustite -magnetite
3 Fe1-xO + O2 ~ Fe3O4
IW iron - wustite
2(1-x) Fe + O2 = 2 Fe1-xO
QIF quartz-iron-fayalite
2 Fe + SiO2 + O2 = Fe2SiO4