Pegmatitic and other felsic rock
pockets and dike-like intrusions are abundant in the South Kawishiwi Intrusion of the Duluth Complex, including the basal, Cu-Ni-PGE mineralized units. These occurrences are found as
pockets, pods or as veins and contain abundant accessory apatite and quartz. Quartz hosts primary fluid inclusions as well as silicate
melt inclusions. Combined microthermometry and Raman spectroscopy helped to determine the bulk composition of primary fluid inclusions that are CO
2-rich (95 mol % ) and contain small amounts of H
2O (4.5 mol % ), CH
4 (0.4 mol % ) and trace N
2, respectively. This combined technique also made it possible to measure total homogenization temperatures of the inclusions (Th
tot = ~ 225 ¡À 10 ¡ãC), otherwise not detectable during microthermometry. Silicate
melt inclusions have been quenched to produce homogeneous glasses corresponding to the original
melt. Composition of the entrapped
melt is granitoid, peraluminous and is very poor in mafic components. We interpret the
melt as a product of partial
melting of the footwall rocks due to the contact effect of the South Kawishiwi Intrusion. The presence of CO
2 in the vapor bubbles of the quenched
melt inclusions and petrographic evidence suggest that the fluid and
melt inclusion assemblages are coeval. The composition of the fluid and
melt phase implies that the fluid originates from the mafic magma of the South Kawishiwi Intrusion and the fluid and
melt phases coexisted as a heterogeneous
melt-fluid system until entrapment of the inclusions.
Coexistence of primary fluid and melt inclusions makes it possible to calculate a minimum entrapment pressure (~ 1.7 kbar) and thus estimate formation depth (~ 5.8 km) for the inclusions. Chlorine is suggested to behave compatibly in the silicate melt phase in the fluid-melt system represented by the inclusions, indicated by the high (up to 0.3 % ) Cl-concentrations of the silicate melt and CO2-rich nature of the fluid.
Apatite halogen-contents provide further details on the behavior of Cl. Apatite in pegmatitic pockets often has elevated Cl-concentrations compared to troctolitic rocks, suggesting enrichment of Cl with progressive crystallization. Systematic trends of Cl-loss at some differentiated melt pockets suggest that in some places Cl exsolved into a fluid phase and migrated away from its source. The segregation of Cl from the melt is probably inhibited by the presence of CO2-rich fluids until the last stages of crystallization, increasing the potential for the development of late-stage saline brines.
Platinum-group minerals are often present in microcracks in silicate minerals, in late-stage differentiated sulfide veinlets and in association with chlorapatite, indicating the potential role of Cl-bearing fluids in the final distribution of PGEs.