GEOL 5320 GEOL 5320 ADVANCED IGNEOUS AND ADVANCED IGNEOUS AND METAMORPHIC PETROLOGYMETAMORPHIC PETROLOGY

Modeling the Petrology Modeling the Petrology

and PGE Reef Mineralization and PGE Reef Mineralization

of the Sonju Lake Intrusionof the Sonju Lake Intrusion

December 7, 2009

Sonju LakeIntrusion

The The Sonju Sonju Lake Lake IntrusionIntrusion

From MGS Map M-71 (Miller et al., 1989)

Leucogranite

Quartz ferromonzonite

Apatitic olivine ferromonzodiorite

Apatite olivine ferrodiorite

Ol-bearing oxide gabbro

Ol-bearing gabbro

Troctolite-augite troctolite

DuniteMelatroctolite

PCFOAg

PCFOAh

PCFoi

PC(O)fi

POcfi

OOP

Modal Rock NamesCumulus Code

Cumulus Mineral Mode

Modal Variations

Modal Variation and Cumulus TextureModal Variation and Cumulus Texture

Cryptic VariationCryptic Variation

1 km>20 km

>10 km

Sheet-like Geometry of the Sonju Lake IntrusionSheet-like Geometry of the Sonju Lake Intrusion

Estimation of Bulk Composition Becomes a 1-dimensional problem

SiO2 47.6TiO2 2.28Al2O3 14.0FeOt 14.7MnO 0.21MgO 8.3CaO 9.4Na2O 2.47K2O 0.55P2O5 0.30Volatiles 0.20Total 100.0mg# 50.2

Sc 34V 192Cr 111Co 75Ni 185Rb 20Sr 233Ba 171Y 20Zr 114Nb 17Hf 3.1La 14.7Ce 33.4Sm 4.1Eu 1.6Tb 0.8Yb 2.1 Lu .32

Bulk Intrusion CompositionBulk Intrusion Composition= Parent Magma= Parent Magma

= moderately evolved olivine tholeiitic basalt= moderately evolved olivine tholeiitic basalt

From Miller and Chandler (1998)and Miller and Ripley (1997)

Liquid Line of DescentLiquid Line of Descent

Calculated by summing composition of rock column above a specific horizon

Fractional Crystallization ModellingFractional Crystallization ModellingCHAOS 2 (NIELSEN, 1990)CHAOS 2 (NIELSEN, 1990)

Model Parameters : fO2 = -2 log QFM; trapped liquid = 20%

0 100 200 300 400 500 600

Cu (ppm)

Discovery of Stratiform Discovery of Stratiform PGE MineralizationPGE Mineralization

Skaergaard Intrusion

Feb. 1999

PGEPGEReefReef

Outcrop SamplingOutcrop Sampling

From Miller (1999)

SLI ChemostratigraphySLI Chemostratigraphy

Evolution of Evolution of Sulfide in theSulfide in theSonju LakeSonju LakeIntrusionIntrusion

Exploration Exploration Drilling by Drilling by Franconia Franconia MineralsMinerals

July 2002

1 cm

Core Logging and SamplingCore Logging and Sampling

Sampling RegimeSampling RegimePhase 1 – 1’ sample every 10’Phase 1- continuous 1’ across PMZ423 Total Whole Rock Analyses

Plagioclase

65-73%

Detailed Geochemical Profiling of the PMZDetailed Geochemical Profiling of the PMZ

SL02-1

Cu-Pd RatiosCu-Pd Ratios

Precious Metals Zone (PMZ)

Meters above Cu-Au break SL02-1

Pd (ppb)

Cu

/Pd

after Barnes et al. (1993)

PMZ MetallogenesisPMZ 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?PGE Mineralizing Agent?

Cu-Fe Sulfide? Fe-Ti Oxide???

Data from SL02-1 PMZ 0 to 105m below Cu-Au Break

Meters above Cu-Au break

Precious Metals Zone (PMZ)

““Cumulus” Sulfide in the PMZCumulus” Sulfide in the PMZ

SL02-1

SL02-2 SL02-3

100 um

Bn

Cp

Cp

Paucity of Sulfide in PMZ?Paucity of Sulfide in PMZ?

UralitizedUralitizedAugiteAugite

PGMPGM

Chl-ActChl-ActUraliteUralite

Mobility of SulfideMobility of Sulfide

Desulfurization and Oxidation of Cu SulfideDesulfurization and Oxidation of Cu Sulfide

Are Metal Offsets Primary?Are Metal Offsets Primary?

Kinetic ModelPreservation of variable degrees of disequilibriumduring 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 PMZPlagioclase Augite Fe-Ti Oxide

From Andersen et al. (1998)

Correlation of Leucocratic Gabbro to Skaergaard Platinova ReefsCorrelation of Leucocratic Gabbro to Skaergaard Platinova Reefs

Restite PGM / Resorbed Sulfide

Chlorite- Actinolite

Cp

Augite

Augite

Pd-Sb

50 um

Plagioclase

Pd-Sb

CpActinolite

Augite

Augite

50 um

Cp

Pt-Pd-As

50 um

Correlation by Secondary Pd PeaksCorrelation by Secondary Pd Peaks

Mineralization ModelMineralization Model ““Downer” StageDowner” Stage

Upgrading sulfide in intercumulus magma

Initial sulfidesaturation

PGE scavenging of magma columncomplete

Dsulf/sil~104-108

Rapid Diffusion

Dsulf/sil~102

Slow Diffusion

Mineralization ModelMineralization Model ““Upper” StageUpper” Stage

ClassicOffsetOffsetSkaergaard-typeSkaergaard-type

Met

ers

abo

ve s

ulf

ide

incr

ease

Met

ers

abo

ve s

ulf

ide

incr

ease

Comparing the SLI to other PGE reefsComparing the SLI to other PGE reefs

IF ONLY.....IF ONLY.....

PGEreefs

PELE – MELTS-based Modeling Programdeveloped by Alan Boudreau

MH - magnetite-hematite4 Fe3O4 + O2 = 6 Fe2O3

NiNiO nickel-nickel oxide2 Ni + O2 = 2 NiO

FMQ fayalite-magnetite-qtz3 Fe2SiO4 +O2 = 2 Fe3O4 + 3 SiO2

WM wustite -magnetite3 Fe1-xO + O2 ~ Fe3O4

IW iron - wustite2(1-x) Fe + O2 = 2 Fe1-xO

QIF quartz-iron-fayalite2 Fe + SiO2 + O2 = Fe2SiO4

Common fO2 range for magmatic conditions

arc

non-arc

Oxygen Buffers