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