Hematite deposits at Steep Rock Lake, Ontario, in the northern part of the Lake Superior Region, are classed as hydrothermal replacements. A comparison is made with deposits of the Vermilion and Marquette Iron Ranges and current hypotheses as to origin reviewed.In contrast to the generally accepted theories that these latter deposits were formed essentially by leaching, either surficial or hydrothermal, followed by concentration and compaction of iron minerals present in the iron formations so leached, the hypothesis is set forth that later, additional iron from deeper magmatic sources has been introduced and that replacement of brecciated iron carbonates and oxidized iron formations has taken place on a large scale. In this manner, the presence of clean, massive hematite at great depth is explained.

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