Abstract

New mineralogic and petrographic data, derived from study of a number of garnet-rich deposits in the southeastern Adirondacks, are presented, with particular emphasis on the Barton deposit at Gore Mountain. This deposit, long the world's largest producer of abrasive garnet and widely known for its giant hornblende-sheathed garnets, has been ascribed to various primary and secondary modes of genesis. The new data indicate that the Barton ore is an extreme metagabbro facies whose large garnets were derived by progressive recrystallization growth from microscopic corona garnet during the peripheral metamorphism of the gabbro by intrusive syenite magma. The progressive changes from corona garnet (C type) to very small poikiloblasts (P type), to megascopic non-poikilitic porphyroblasts (X type), and finally to very large hornblende-sheathed crystals (XH type) are traced petrographically and compositionally. These basic garnet types, representing different degrees of recrystallization growth of the original, probably deuteric, corona garnet, are recognized in many gabbroic and anorthositic facies as well as in hybrid facies of syenite-gabbro and syenite-anorthosite derivation. Because of this genetic relationship, garnet composition and textural type facilitate the petrogenetic interpretation of a number of garnetiferous facies and deposits, and throw new light on certain old petrologic problems of the Adirondack province.

Chemical analyses, specific gravities, optical and x-ray diffraction data, micrometric modes, micrographs, maps, and graphs are presented.

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