Study of the pyroxene amphibolite host rock at the Scott magnetite mine, Sterling Lake District, New York, indicates that it could have been the source of the magnetite ore. Titrimetric, optical emission spectrographic, and fluorescent X-ray analyses show that the percentage of Fe in the mafic silicates and total rock decreases with proximity to the ore zone. In addition, much of the amphibolite has been replaced by gneiss and pegmatite and some of the Fe thus removed is believed to have migrated to the ore zone to form magnetite. The percentage of mafic silicates and magnetite in the amphibolite shows no systematic increase or decrease toward the ore zone. All of the constituents found in the ore bodies were available in the host rock. Highly dispersed material was released from amphibolite and concentrated into ore bodies in favorable structures. Magnetite ore has replaced pyroxene amphibolite along a low-pressure zone where the structure of the host rock changes markedly. Movement of material was largely along foliation, lineation, and grain boundaries, and, to some extent, through crystals by diffusion. The energy required to activate and move the elements from the mafic silicates of the host rock is believed to have been supplied by temperature and pressure changes during metamorphism, and by the introduction of replacement gneiss and pegmatite. The data presented are explained best as a result of metasomatism accompanying regional metamorphism and granitization.