GEOCHEMICAL AND PETROLOGICAL INVESTIGATION OF THE PROSPECTIVE NI-CU-PGE MINERALIZATION AT THE ECHO LAKE INTRUSION IN THE UPPER PENINSULA OF MICHIGAN, USA

The Echo Lake intrusion is located in Houghton and Ontonagon counties in the Upper Peninsula (UP) of Michigan. The intrusion is approximately 200 m below the surface and covers about 18 km². The Echo Lake intrusion is associated with a large geologic feature called the Midcontinent Rift which is mostly exposed throughout the Lake Superior Region. It is responsible for the formation of Ni-Cu ± PGE deposits in Minnesota, Wisconsin, and UP Michigan. Several Rift-related ultramafic-mafic intrusions have been identified and studied in the UP, most notably the Eagle and Eagle East Deposits near Marquette, MI and 65 km east of Echo Lake.

Echo Lake is an ultramafic-mafic intrusion that is dominated by coarse-grained troctolites, with interlayered peridotites, mafic pegmatoid, olivine gabbro, magnetite gabbro, anorthosite, leuco-gabbro, and augite gabbro. The troctolite is coarse-grained and contains plagioclase, olivine and minor pyroxene, with thin zones of serpentinization along widely spaced fractures. The mafic pegmatoid is composed of coarse-grained plagioclase, pyroxene, olivine, iron oxides, and biotite. The peridotite is a black fine-grained olivine-pyroxene (augite) ± plagioclase rock. The magnetite gabbro is composed of medium to coarse-grained plagioclase, pyroxene-olivine-magnetite. Previous drilling has identified several PGE enriched zones with grades ranging up to 1.01 g/t Pt+Pd+Au in the magnetite gabbro unit.

Owing to the proposed association with the 1.1 Ga Midcontinent Rift system, the observed occurrences of sulfide minerals and localized high PGE concentrations, the Echo Lake intrusion is identified as a prospect for an economic magmatic sulfide deposit like Eagle and Tamarack intrusions. The studied sulfides are largely disseminated in the Echo Lake intrusion, where-as in the other mineralized intrusions in the region there are Ni-Cu-PGE-rich semi-massive to massive sulfide units. Preliminary thin section analysis indicates disseminated chalcopyrite crystals intergrown with magnetite in the PGE-rich horizon which is interlayered with olivine gabbro units. Preliminary whole rock geochemical analysis shows 8-12 wt% MgO in the PGE-rich horizon. In addition, Pt and Pd concentrations increase with depth in the 45 m magnetite gabbro section, however, diminish abruptly in the lowermost 8 m.