Most of the magnetite deposits of the eastern Grenville Province have been described as precipitates from hydrothermal solutions derived from metamorphic processes, from early, late, or post-Grenville granites, or as metasediments. Granites host many of these deposits, but others, including the historic Edison iron mines of New Jersey, are hosted by potassium-feldspar gneiss, commonly interpreted as meta-arkose. Our new geochemical data indicate, however, that the protolith of the potassium-feldspar gneiss is rhyolite, not arkose. Supporting evidence includes (i) the absence of an underlying potassic provenance for arkosic sediment, (ii) potassium and sodium contents among host rocks that exceed the range of arkose but are consistent with A-type rhyolite, and (iii) a close chemical resemblance of the potassium-feldspar gneiss to an A-type granite (Byram granite) that is closely associated in time and space. As the ore zone through the Edison mines is approached, the K/Na ratio of the host rocks undergoes a distinct increase, consistent with extensive diagenetic alteration of rhyolitic pyroclastics in a hypersaline environment. This alteration provided a local ligand source for subsequent hydrothermal iron mineralization derived from the nearby pre-orogenic Byram granite. These iron concentrations were then remobilized and recrystallized during subsequent Grenville metamorphism. Although some of the magnetite deposits of the New Jersey Highlands display evidence of post-orogenic replacement and an association with undeformed pegmatites, the banded magnetite ore and related pegmatites of the Edison mines are conformable to the foliation of the host rock and are interpreted as metamorphic products of pre-orogenic, granite-derived, hydrothermal iron concentrates.