Mineralogy and geochemical controls on the Easter-Duffy five-element vein system, NWT, Canada

The East Arm of the Great Slave Lake is a region where multiple occurrences of cobalt and nickel mineralization, including the Easter Duffy occurrence, have been reported, explored, and mined. A combination of reflected light microscopy and EPMA analyses at Easter Duffy shows a system with high mineralogical diversity (over 30 opaque minerals identified) evolving from native metals, to arsenides, to sulfarsenides, and then to sulfides. This paragenetic sequence is repeated after an episode of brecciation, suggesting a minimum of two fluid pulses with comparable geochemical characteristics. The mineralogy, texture, and paragenesis identified at Easter-Duffy and other deposits in the district fall within the definition of five-element vein (also known as native metal-arsenide) deposits and provides new supporting evidence for the East Arm Region as a district in which this type of system is common. Similar to other five-element vein deposits, progressive reduction of an oxidized fluid best explains the paragenetic sequence at Easter Duffy. The regional context for Easter Duffy, which is hosted in granites and gneisses locally adjacent to gabbro dikes, suggests that a probable reducing agent in this system may be the serpentinization of local gabbros.