SEGREGATION OF MAGMATIC FLUIDS DURING THE CRYSTALLIZATION OF THE CU-NI-PGE-MINERALIZED SOUTH KAWISHIWI INTRUSION OF THE DULUTH COMPLEX, MINNESOTA, USA

The South Kawishiwi Intrusion is one of the intrusions that host Cu-Ni-PGE mineralization of economic interest in the 1.1 Ga Duluth Complex. Remobilization of primary magmatic sulfide ore by hydrothermal fluids has been suggested by several authors. Our aim was to provide detailed information on the significance of footwall-melt interaction and the evolution of magmatic fluids in metal mobilization.

Halogen composition of apatite from various rock units and rock types suggests that in the last stages of the crystallization of the originally troctolitic, but later fractionated silicate magma, a magmatic fluid separated. Chlorine likely partitioned into the fluid phase which left the residual silicate melt and thus crystallizing apatite relatively F-enriched. Extremely chlorine-rich apatite crystals of X^Cl>0.8, previously unreported from the Duluth Complex, are however found in late-stage, microscopic sulfide veins crosscutting all rock-forming silicates with associated secondary textured PGMs.

Microthermometric and combined Raman-spectroscopic investigation of coexisting, primary fluid and silicate melt inclusions, entrapped in quartz in a felsic, sulfide-bearing contaminated rock sample suggests that 1) the primary magmatic fluid was CO₂-dominated (X_CO2=~0.94) with minor amounts of methane, N₂ and an aqueous phase, 2) fluid-melt immiscibility likely occurred on relatively high, magmatic (>1000°C) temperatures and 3) felsic composition of the silicate melt can be the result of the partial melting and assimilation of granitoid footwall rocks. The segregated primary magmatic fluid has the potential to remobilize some of the ore metals. This is recorded by chalcopyrite-bearing miarolitic cavities (and associated CO₂-CH₄-bearing fluid inclusions in K-feldspar) in the mineralized unit and the above mentioned Cl-rich, PGM-bearing sulfide veinlets.

Chalcopyrite-filled microcracks with CH₄-dominated, H₂O- and N₂-bearing fluid inclusions suggest that there has been a later remobilization event, not directly associated with the primary magmatic fluid. Moreover surface mapping and investigation revealed Cu-anomalies related to amphibole-chlorite-prehnite-carbonate alteration along hundred-meter-scale fractures in the otherwise barred anorthositic hangingwall of the intrusion.