The Geology and Metamorphism of the Ironwood Iron-formation, Gogebic Range, Wisconsin

A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Dennis Patrick Laybourn in partial fulfillment of the requirements for the degree of Master of Science, January 1979.The Ironwood Iron-formation, of the Gogebic Range of northern Wisconsin and Michigan, extends approximately 80 miles in a generally east-northeasterly direction from near Mineral Lake, Wisconsin to Lake Gogebic, Michigan. The iron-formation is considered part of the Menomiee Group of the Marquette Range Supergroup. Stratigraphically, the iron-formation has been divided into five members on the basis of the predominance of ferruginous cherts or ferruginous slates in the stratigraphic section. Total thickness of the Ironwood Formation in Wisconsin seems to show a progressive thickening from around 500 feet near the Wisconsin-Michigan border to about 650 feet near its westernmost extent. The westernmost 20 miles of the iron-formation has been contact metamorphose by the Mineral Lake Intrusive, a Middle Keweenawan gabbroic body. From data of the present study, four metamorphic zones may be distinguished in the Ironwood Formation by changes in mineralogy along strike length of the iron-formation toward the intrusive contact. Zone 1 extends to within about 7500 feet (2300 m.) of the intrusive contact. It is recognized by the presence of low-grade metamorphic assemblages containing iron-bearing phyllosilicates (minnesotaite, stilpnomelane, chlorite) and iron-beating carbonate phases (siderite, dolomite-ankerite). Primary textural features, such as granules and oolites, are generally well preserved, although they have been slightly modified by recrystallization and replacement. Zone 2 is characterized by a moderate grade of metamorphism and the development of grunerite, talc, and garnet. Iron-bearing carbonates and iron-bearing phyllosilicates are absent. The degree of metamorphic recrvstallization has increased from Zone 1, such that primary textural features are partially to totally obliterated. Zone 3 iron-formation occurs within about 4500 feet (1370 m.) of the intrusive contact and is characterized by high-grade metamorphic assemblages containing orthopyroxene (ferrohypersthene), clinopyroxene (ferroaugite), hornblende, actinolite and tremolite. The iron-formation has usually undergone a complete recrystallization with primary textural features being completely erased. Zone 4 occurs within about 100 meters of the intrusive contact and is characterized by the presence of pigeonite and fayalite. Pressures of metamorphism are believed to be in excess of 3 kb. Temperatures attained in Zone 1 were probably in the range 150-350C. Maximum temperatures of Zone 2 could not have exceeded about 450C., the upper temperature limit of talc in calc-silicate rocks. Maximum temperatures along the intrusive contact could not have greatly exceeded about 700 C., the approximate decomposition temperature of grunerite. Metamorphism of the iron-formation is believed to be largely isochemical and the result of a progressive dehvdration and decarbonation of the iron- formation as the intrusive contact is neared. Oxygen fugacity is internally buffered within the confines of various quartz-ferrous silicate-magnetite buffer assemblages. Evidence from Zone 4, however, suggests the decomposition of magnetite in response to the external reducing influence of the intrusive contact