THE DISCOVERY OF EAGLE EAST: AN EXAMPLE OF MODEL-DRIVEN EXPLORATION

In 2016, Lundin Mining Corporation announced the new Eagle East magmatic nickel-copper deposit, located near its Eagle Mine in Michigan’s Upper Peninsula (Lundin, 2016). Both deposits are hosted by 1 Ga Midcontinent Rift-related intrusions that intrude the Paleoproterozoic Baraga Basin. Both intrusions modally classify as feldspathic lherzolites to feldspathic olivine websterites (Mulcahy et al., 2016). Massive sulfides (MSU) consist of greater than 80% pentlandite, chalcopyrite, and pyrrhotite. Semi-massive sulfides (SMSU) consist of net-textured sulfides and silicates. Also present are magmatic breccias and disseminated sulfides. The Eagle deposit is a sub-vertical blowout. In contrast, the Eagle East zone is horizontal.

Early indications that Eagle East (aka Yellow Dog Peridotite) could host economic mineralization were based on trace copper and sulfur (Klasner et al., 1977). Kennecott explored for magmatic nickel-copper in the region from the early 1990’s, eventually discovering Eagle in 2002. Lundin acquired the project in 2013 and renewed exploration focusing on near mine targets. Eagle East hosted low grade mineralization and was open at depth. Nickel and copper tenors were higher at Eagle East than at Eagle, so the potential remained for higher grades. Directional drilling was used to trace the thin, deep feeder dike. Early intercepts in 2013 and 2014 yielded SMSU and magmatic breccias not found in the upper part of Eagle East and spurred more drilling (Lundin, 2014). Further tracing found that the dike changed from a 50 degree plunge to horizontal. It was in this flat section where high grade MSU and SMSU were discovered. Eagle East is thus an excellent example of the chonolith model of magmatic mineralization where accumulations of sulfides occur at a change in the orientation of a dynamic conduit.

In recent studies, Ding (2010) interpreted incompatible trace element ratios at Eagle to indicate the involvement of two to four distinct parental magmas. Mulcahy et al. (2016) concluded that this range can be attributed to a single parental magma type. A two magma source was also asserted in Ding et al. (2012) based on sulfur isotope studies. Hinks et al. (2016) suggested that mixing of crustal-sourced sulfur from host sediments and Archean basement could have averaged the observed δ³⁴S with a single magma source.