title: Hydrothermal alteration and Cu-Ni-PGE mobilization in the charnockitic rocks of the footwall of the South Kawishiwi intrusion, Duluth Complex, USA.
abstract: In the Neoarchean (~ 2.7 Ga) contact metamorphosed charnockitic footwall of the Mesoproterosoic (1.1 Ga) South Kawishiwi intrusion of the Duluth Complex, the primary metamorphic mineral assemblage and Cu-Ni-PGE sulfide mineralization is overprinted by an actinolite + chlorite + cummingtonite + prehnite + pumpellyite + quartz + calcite hydrothermal mineral assemblage along 2-3 cm thick veins. In calcite, hosted by the hydrothermal alteration zones and in a single recrystallized quartz porphyroblast, four different fluid inclusion assemblages are documented; the composition of these fluid inclusions provide p-T conditions of the fluid flow, and helps to define the origin of the fluids and evaluate their role in the remobilization and reprecipitation of the primary metamorphic sulfide assemblage. Pure CO2fluid inclusions were found as early inclusions in recrystallized quartz porphyroblast. These inclusions may have been trapped during the recrystallization of the quartz during the contact metamorphism of the footwall charnockite in the footwall of the SKI. The estimated trapping pressure (1.6-2.0 kbar) and temperature (810-920 °C) conditions correspond to estimates based on felsic veins in the basal zones of the South Kawishiwi intrusion. Fluid inclusion assemblages with CO2-H2O-NaCl and CH4-N2-H2O-NaCl compositions found in this study along healed microfractures in the recrystallized quartz porphyroblast establish the heterogeneous state of the fluids during entrapment. The estimated trapping pressure and temperature conditions (240-650 bar and 120-150 °C for CO2-H2O-NaCl inclusions and 315-360 bar and 145-165 °C for CH4-N2-H2O-NaCl inclusions) are significantly lower than the p-T conditions (> 700 °C and 1.6-2 kbar) during the contact metamorphism, indicating that this fluid flow might not be related to the cooling of the Duluth Complex and its contact aureole. The presence of chalcopyrite inclusions in these fluid inclusions and in the trails of these fluid inclusion assemblages confirms that at least on local scale these fluids played a role in base metal remobilization. No evidences have been observed for PGE remobilization and transport in the samples. The source of the carbonic phase in the carbonic assemblages (CO2; CH4) could be the graphite, present in the metasedimentary hornfelsed inclusions in the basal zones of the South Kawishiwi intrusion. The hydrothermal veins in the charnockite can be characterized by an actinolite + cummingtonite + chlorite + prehnite + pumpellyite + calcite (I-II) + quartz mineral assemblage. Chlorite thermometry yields temperatures around 276-308 °C during the earliest phase of the fluid flow. In the late calcite (II) phase, high salinity (21.6-28.8 NaCl + CaCl2equiv. wt.%), low temperature (90-160 °C), primary aqueous inclusions were found. Chalcopyrite (± sphalerite ± millerite), replacing and intersecting the early hydrothermal phases, are associated to the late calcite (II) phase. The composition of the formational fluids in the Canadian Shield is comparable with the composition of the studied fluid inclusions. This suggests that the composition of the fluids did not change in the past 2 Ga and base metal remobilization by formational fluids could have taken place any time after the formation of the South Kawishiwi intrusion. Sulfur isotope studies carried out on the primary metamorphic (δ34S = 7.4-8.9‰) and the hydrothermal sulfide mineral assemblage (δ34S = 5.5-5.7‰) proves, that during the hydrothermal fluid flow the primary metamorphic ores were remobilized.
authors.affiliation: Institute of Earth Sciences, University of Graz, Universitätsplatz 2, 8010 Graz, Austria.
authors.forename: Zsolt
authors.initials: Z
authors.lastname: Benkó
authors.affiliation: Institute of Earth Sciences, University of Graz, Universitätsplatz 2, 8010 Graz, Austria.
authors.forename: Aberra
authors.initials: A
authors.lastname: Mogessie
authors.affiliation: Geological Survey of Finland, Betonimiehenkuja 4, 02151 Espoo, Finland.
authors.forename: Ferenc
authors.initials: F
authors.lastname: Molnár
authors.affiliation: Institute of Earth Sciences, University of Graz, Universitätsplatz 2, 8010 Graz, Austria.
authors.forename: Kurt
authors.initials: K
authors.lastname: Krenn
authors.affiliation: Department of Geological Sciences & Engineering, University of Nevada - Reno, 1664 N. Virginia St., Reno, NV 89557-0138, USA.
authors.forename: Simon R
authors.initials: SR
authors.lastname: Poulson
authors.affiliation: Natural Resources Research Institute, University of Minnesota, 5013 Miller Trunk Highway, Duluth, MN 55811-1442, USA.
authors.forename: Steven
authors.initials: S
authors.lastname: Hauck
authors.affiliation: Natural Resources Research Institute, University of Minnesota, 5013 Miller Trunk Highway, Duluth, MN 55811-1442, USA.
authors.forename: Mark
authors.initials: M
authors.lastname: Severson
authors.affiliation: Department of Geological Sciences & Engineering, University of Nevada - Reno, 1664 N. Virginia St., Reno, NV 89557-0138, USA.
authors.forename: Greg B
authors.initials: GB
authors.lastname: Arehart
keywords: Charnockite; Duluth Complex; Fluid inclusions; Hydrothermal alteration; South Kawishiwi intrusion; Sulfur isotopes