Interactions of mixed volatile-brine fluids in rocks of the southwestern footwall of the Duluth Complex, Minnesota: evidence from aqueous fluid inclusions

Abstract

Drillcore samples of troctolites and gabbros were studied along the southwest footwall of the Duluth Complex, Minnesota. The rocks were intruded into an active rifting environment, which enhanced melt-wallrock interaction. The samples analyzed contain variable, sub-economic amounts of Fe-Ti oxides and Cu-Fe-Ni sulfides and some uneconomic enrichments of platinum-group elements (PGE). The goal of the study was to investigate the nature of the aqueous fluids that interacted with the igneous rocks and possibly affected the concentration and distribution of ore minerals. Microthermometric and Raman microprobe analyses were done on about 150 fluid inclusions hosted by quartz and apatite in hydrously altered mafic rocks. The inclusion data suggest that the Complex experienced significant interactions with mixed aqueous (NaCl + CaCl2 + KCl)-volatile (CH4 + N2 ± CO2) fluids over a temperature range from above 700° to below 200°C at about 2 kb pressure. -from Authors