Abstract

Hard iron ore on the Marquette Range consists of nearly pure masses of specularite, magnetite, or martite and occurs near the unconformable contact of the Negaunee Iron-Formation with the overlying Goodrich Quartzite. Most ore is in an upper jaspilite unit of the Negaunee, but some orebodies are entirely within the basal part of the Good-rich Quartzite or span the contact of the two formations.In the past, hard ore has been considered a single ore class having a single mode of origin. It is argued here, however, that it is three distinct types of ore, each having a different mode of origin.Specularite ore varies from virtually pure specularite to ore containing as much as about 10 percent magnetite. It was formed by the leaching of silica from the Negaunee Iron-Formation and the precipitation of iron oxide in its place during the period of erosion immediately after deposition of the iron-formation, and it was later deformed and metamorphosed to its present hard specular form.Magnetite ore is nearly pure magnetite, containing in places as much as 10 percent specularite. It was formed during regional metamorphism and localized in the upper jaspilite unit of the Negaunee because of oxidation by the jaspilite of iron-bearing metamorphic fluids generated in lower parts of the iron-formation. Flow of the fluids was concentrated along structurally or stratigraphically controlled channelways, and magnetite was precipitated as iron solubility was lowered by oxidation of the fluid.Clastic ore contains specularite and lesser amounts of magnetite. It is a placer concentrate of iron oxides in the basal Goodrich Quartzite. The iron oxides in the ore were derived from the Negaunee Iron-Formation and were concentrated during the transgression marked by the onset of Goodrich deposition.Hard ore accounted for about 20 percent of high-grade ore production from the Marquette Range; about 67 million tons were produced. It is no longer mined, except incidentally at the Republic open pit where hard-ore bodies are within the taconite presently being mined as concentrating-grade ore.

First Page Preview

First page PDF preview
You do not currently have access to this article.