Magnetite ore formed during regional metamorphism, Ausable magnetite district, New York

Abstract

This investigation is a geological and geochemical study of several magnetite deposits in the Ausable magnetite district of New York. Geologic maps were prepared of part of the district and of these deposits, and laboratory investigations were carried out on the host rock gneiss and its constituent minerals.The Lyon Mountain granite gneiss appears to have been the source rock from which the Palmer Hill and Jackson Hill ore bodies were derived. Modal analyses indicate that accessory magnetite in the gneiss decreases near the ore bodies. Here, also, clinopyroxene has been recrystallized to garnet plus silica and iron. During regional metamorphism, accessory magnetite, and iron from fractured accessory magnetite and from recrystallized clinopyroxene, migrated as a dispersed phase of ions, molecules, or atoms along grain boundaries to shear and crush zones and were concentrated into magnetite bodies. The ore zones were regions of low pressure and high oxygen activity.In skarn areas in the northern part of the map area, small magnetite bodies apparently formed when clinopyroxene and hornblende gneisses rich in iron were recrystallized to yield pyroxene skarn relatively poor in iron plus magnetite.