Setting Mafic rocks appear to have been intruded into subsiding, fault-bounded linear basins. Unlike the LMI s, the gabbroic ore hosts appear to have undergone little or no differentiation. The sill is usually massive, to only weakly layered, but does often grade upward into diorite. Some elemental zoning has been noted. Iron tends to increase upward at the expense of Mg.
Overlying basalts have roughly the same composition as the gabbros, as do diabasic feeder dikes.
There is little doubt the basalt, diabase and gabbro are cogenetic. The similarity of these districts to the more heavily studied, but unmineralized Columbia River Plateau has led to the suggestion that these are flood basalts complexes emplaced during episodes of continental rifting. Ore Deposits Little is known about the actual ore occurrences in this group of deposits since the Soviets have released no information, nor have American companies involved in extensive drilling of the Duluth Gabbro. It is known that the ore bodies occur at the base of gabbroic sills as rather thin layers. Also pentlandite and pyrrhotite predominate, but chalcopyrite is present in appreciable amounts.