GEOCHEMISTRY OF THE NICKEL LAKE MACRODIKE AND IMPLICATIONS FOR CU-NI SULFIDE EXPLORATION

1.1 billion years ago, the Nickel Lake Macrodike(NLM) intruded various Archean and Paleoproterozoic country rock units in association with failed Midcontinent Rift magmatism. The NLM is a member of the Layered Series of the Duluth Complex and was the conduit to the sulfide-bearing South Kawishiwi Intrusion (SKI). The SKI and other related intrusions are currently the focus of fevered exploration and as the feeder dike to the SKI, the NLM is an area of important study. The NLM intruded in three main phases: a heterogeneous and locally sulfide-bearing troctolite phase, an oxide-gabbro phase, and a layered troctolite phase. The dike is roughly 1 km wide and extends northeast to southwest for 6 km through swamp and dense forest straddling the border of the Boundary Waters Canoe Area in northern Minnesota.

Crustal contamination is the major source of sulfur in the sulfide deposits of the various intrusive suites in the Duluth Complex. The Paleoproterozoic Biwabik Iron Formation and the Virginia Formation (1.8 Ga) are incorporated as the main sources of sulfur. The goal of this study is to conceive a plausible model of country rock incorporation. Do trace element (specifically REE) values differ significantly between the three main phases of the Nickel Lake Macrodike? Assuming the parental magmas had the same initial values (North Shore Volcanic Group) can country rock incorporation be qualified? What are the consequences of these findings in aiding exploration for magmatic Ni-Cu ore deposits?

Rock chemistry, particularly trace elements, indicates that the NLM became increasingly more evolved with each phase of intrusion. The heterogeneous troctolite exhibits greater crustal contamination than the earlier phases. Because this is the sulfide-bearing phase of the NLM, this indicates that REE analysis may be an effective tool in aiding exploration.