Abstract

Intrusive contacts of several granodiorite bodies, with calcitic, dolomitic and shaly limestones are described in detail. Isochemical metamorphism included recrystallization and formation of minerals of the pyroxene hornfels facies; it was succeeded by several phases of alteration with addition of material. In the shaly and silicic limestones, addition of iron and smaller quantities of silica and magnesia altered biotite-calcite marble, through several phases, to a banded andradite-diopside tactite. Fluo-hydrothermal introduction of much iron subsequently altered this tactite to one composed predominantly of magnetite and pargasite; simultaneously magnetite and forsterite replaced the pure carbonate formations. Coarse, concentrically zoned garnet was deposited at some intrusive contacts. In the succeeding phase, soda-rich hydrothermal solutions locally replaced nearly all types of rock with scapolite or calcite; the iron was re-concentrated as magnetite at some places in the scapolite. In a final phase, epidote, andradite, and other minerals, derived from the surrounding rock, filled the joints. Pyrite, then other sulfides and gold, also locally followed the Scapolitization.

Fractures controlled all phases of the metamorphism, except probably the early reconstitution, although metamorphism was generally more pronounced near intrusive contacts. Most of the controlling structures are older than the intrusives. I n the shaly limestone at Philipsburg, thin aplites were emplaced along the bedding by a combination of replacement and sloping.

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