DISSEMINATED CU-NI-PGE DEPOSITS IN THE SOUTH KAWISHIWI INTRUSION OF THE DULUTH COMPLEX; END RESULT OF AN INTRUSION OF SULFIDE-BEARING MAGMATIC SLURRIES

The most essential part of analyzing and understanding the magmatic processes that formed the Cu-Ni-PGE deposits of the South Kawishiwi Intrusion (SKI) of the Duluth Complex involves the clear and concise knowledge of the nature of the SKI magmatic system at its onset: The Initial Conditions. The fundamental initial conditions that must be addressed include the: (1) Geometry of the system; (2) Composition and initial state of crystallinity of the magma; (3) Sequence of emplacement of the magma; and (4) Location of channelized magma flow and its resultant variable ores.

Regional-scale detailed geologic mapping (~20,000 outcrops mapped) in the area integrated with over 1,000,000 feet of core drilling has shown that the SKI is a large (>2,000km³), shallowly-dipping, 4km-thick, sill-like intrusion. The system was delivered batches of magma subhorizontally from the northeast through the Nickel Lake Macrodike (NLM) at the regional-scale Archean/Paleoproterozoic - Mesoproterozoic unconformity. The intrusion has a footwall of Archean granitoids and/or Paleoproterozoic metasedimentary rocks and a hangingwall of Mesoproterozoic anorthosite.

The composition and initial state of crystallinity of the SKI magmas has been determined through detailed drill core logging of igneous textures, detailed mapping within the NLM, mapping of an analog system in the Dry Valleys of Antarctica (the Ferrar Dolerites), geochemical modeling, and literature review. This work has shown that the magma that eventually formed SKI Cu-NI-PGE deposits was initially a sulfur-supersaturated basaltic silicate melt carrying an extensive array of phenocrysts of plagioclase and olivine as well as droplets of immiscible sulfide liquid. In addition, the sulfide droplets were enriched in base and precious metals (average sulfide tenor recalculated to 100% sulfide are 25% Cu, 8% Ni, and 30 grams per tonne TPM).

Detailed geologic mapping has led to a robust knowledge of the linked intrusive history of the NLM and the SKI. In the end, hard work and intellectual geologic thought has been used to identify one of the world’s largest resources of Cu-Ni-PGEs.