Structure and rock alteration at the Elizabeth mine, Vermont; part II, rock alteration at the Elizabeth mine

The Elizabeth mine orebodies are concordant within the Gile Mountain formation at the contact of amphibolite and mica schist. The ore consists of massive and disseminated sulfides comprised principally of pyrrhotite and chalcopyrite. The wallrocks of the orebodies show considerable metasomatic reaction involving the addition of potassium, copper, sulfur, and water, and the removal of sodium, calcium, magnesium, carbonaceous matter and CO,. The principal mineralogic changes that take place farthest from the ore are the pseudomorphic replacements of kyanite by muscovite, hornblende and garnet by biotite, calcite, pyrrhotite, and minor quartz, and the disappearance of carbonaceous matter within schist. Adjacent to the ore all silicate minerals including plagioclase are replaced by sericite. Apart from the above-mentioned alteration and later regional retrograde metamorphic effects, two other types of alteration are present within the mine: one predating, and the other postdating the alteration associated with the ore. It is believed that all three types of alteration were formed as a continuous process during decreasing temperature conditions following the thermal peak of metamorphism. Channelling of solutions from their initial passageways along permeable zones within amphibolite to more permeable zones associated with structural features led to the metamorphism of the earlier alteration zones and resulted in the formation of the metamorphic index minerals, garnet and staurolite within altered amphibolite. The various assemblages of ferromagnesian minerals suggest that alteration commenced shortly after the thermal peak of metamorphism and persisted through staurolite and garnet and possibly to chlorite grade of metamorphism during decreasing temperature. Although some weak mineralization is associated with the earliest and latest alteration, mineralization in the form of ore only accompanies alteration corresponding to metamorphic temperature and pressure intermediate between these two extremes.