Magnetite taconite ores - 6. The chief gangue minerals are quartz, clay minerals, and minor carbonate species. Deposits of soft ores occur in all the ranges. In Michigan, deposits of hard ore occur in the Marquette Range where they consist of fine-grained, dense aggregates of hematite, magnetite, or martite. The nearly pure masses of these iron oxides occur near the unconformable contact of the Negaunee Iron Formation with the overlying Goodrich Quartzite.
Conglomerate ore consists of pebbles and pieces of iron formation in a limonitic matrix.
A small amount of such ore has been mined from conglomerates in the Goodrich Quartzite in the Marquette Range. Siliceous ore, mined chiefly in the Marquette Range and more recently in the Menominee Range, contains chiefly hematite and magnetite with about 40 percent cherty quartz. It consists of magnetite, chert, siderite, and iron silicates. Jasper ore, mined on the Menominee Range at the Groveland mine at Iron Mountain, is a banded specular hematite-chert rock with some magnetite and iron silicates. Under low-grade metamorphism, characteristic recrystallization products include stilpnomelane, chlorite, minnesotaite, siderite, hematite, and magnetite.