Hydrothermal Alteration and Metamorphism of a Metadiabase Sill, Lake Michigamme, Northern Michigan
1985-06
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Hydrothermal Alteration and Metamorphism of a Metadiabase Sill, Lake Michigamme, Northern Michigan
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1985-06
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The study area is at the western end of the Marquette synclinorium which developed during the Early Proterozoic. In the area, the stratigraphy of the northern limb of the synclinorium from north to south is Siamo Slate, Negaunee Iron- Formation, and Goodrich Quartzite. The sill intruded the iron- formation and separates the iron- formation from the slate in the northern part of the area whereas it separates the iron- formation from the quartzite in the southern part of the area. Large euhedral crystals of varying mineralogy especially almandine and grunerite and abundant chlorite are developed between the Negaunee Iron-Formation and a metadiabase sill at the Michigamme Mine (at the center of the area) in a chlorite schist. This study was undertaken to determine what caused chlorite to develop between the lithologies and whether it occurred before or after the regional metamorphism (1.8 to 1.9 b.y.) which produced the large crystals at the mine. Petrographic examination of the metadiabase sill and Negaunee Iron-Formation indicates that the chlorite was developed before the large crystals of almandine and grunerite. The Negaunee Iron- Formation contains an assemblage of quartz- magnetite- almandine- chlorite- grunerite stilpnomelane. It is commonly brecciated and separated from the chlorite schist by a layer of almandine. The metadiabase is divided into four zones, from the center of the sill to the margins, dependent on the amount of chlorite in the rock and the presence of relict poikilitic texture. A relict poikilitic texture of labradorite and augite located in Zone 1 can be correlated to a relict poikilitic texture of plagioclase and hornblende which is present from Zone 2 through Zone 3A of the sill. Almandine, in Zone 3B, developed after chorlitized hornblende. Zone 4 contains the most-altered sill which is predominantly a chlorite schist. The chlorite schist contains an ellipsoidal texture which can be traced from Zone 3B of the metadiabase and the same mineralogy as the iron-formation but in very different proportions. The ellipsoidal texture developed prior to the growth of almandine and grunerite. Whole rock chemical analysis was carried out on eleven samples across the metadiabase sill towards its upper margin which is in contact with the Negaunee Iron- Formation. These samples document the major element changes occurring in the sill through its four zones. The behavior of the major elements across the sill is comparable to the bahvior of major elements in several seawater alteration (hydrothermal) suites (Mottl, 1983; Miyashiro, 1979). FeO (total iron as FeO) and MgO increase while SiO2, CaO, and Na2O all decrease in abundance with increasing degree of alteration. The hydrothermal fluids probably percolated through the Negaunee Iron- Formationi where iron was leached. This aided in a major chemical exchange between calcium from the diabase and iron (or magnesium) from the altering fluid which may have been similar to seawater. Chlorite was probably developed as a result of this exchange at 300°C in a strongly fluid-dominated system (Mottl, 1983). The metadiabase, iron-formation and chlorite schist are related on a CFM diagram (Abbott, 1982). This diagram shows that the equilibrium assemblages in the sill are dependent on the bulk composition of the sill which varies from Zone 1 to Zone 4. Assemblages containing almandine are restricted to calcium-depleted and iron-enriched areas of the sill. The depletion of calcium and enrichment of iron moves the bulk compositions of the sill closer to the bulk compositions of the iron- formation and the link between the compositions of these two lithologies is the most-altered metadiabase (the chlorite schist). The compositional dependence of the assemblages suggests that hydrothermal alteration preceded regional metamorphism.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Ingrid Joyce Verhagen in partial fulfillment of the requirements for the degree of Master of Science, June 1985. There are 6 supplementary files also attached to this record, which contain Plate 1 and Plate 2 (parts 1-5) referenced in the thesis.
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