ORE GENESIS RELATED TO THE DEVELOPMENT OF THE 1.1 GA MIDCONTINENT RIFT SYSTEM

The emplacement of large volumes of mafic magma into the crust during the formation of the Midcontinent Rift System at ~1.1 Ga resulted in the formation of several types of mineral deposits. Early stage magmas of the Duluth Complex were contaminated by Archean and Proterozoic country rocks. Sulfur, in particular, was derived largely from the Paleoproterozoic Animikie Group sedimentary rocks of the Virginia Formation and lower portion of the Biwabik Iron Formation. High positive d³⁴S values (5 to 15‰) of Cu-Ni sulfide mineralization within the earliest intrusive rocks of the Complex indicate that up to 60% of the sulfur was derived from country rocks. In addition, isotopic studies of Pb, Nd, and Os all point to strong interaction between magmas and country rocks, and a significant external contribution of metals, perhaps including Cu and Ni, as well as platinum group elements (PGE).

Some PGE-rich occurrences in the troctolitic to gabbroic rocks of the Complex appear to be linked to sulfide liquid immiscibility caused either by the enrichment of S and metals during fractional crystallization, or to the exsolution of late magmatic S- and PGE-bearing fluids. Neither the host rocks or PGE-rich ores show the isotopic effects of contamination processes that controlled sulfide genesis in the early stage magmas.

The emplacement of mafic magmas into the crust also generated hydrothermal flow systems in overlying rocks. Oxygen and hydrogen isotopic studies suggest that native Cu-deposits of the Keweenaw Peninsula of Michigan and much lower-grade occurrences in the North Shore Volcanic Group of Minnesota involved both heated meteoric water and seawater. Flow systems may have persisted for durations up to 40,000 years, and appear to have been episodic in nature.