A prominent metamorphic complex composed mainly of K-feldspar gneiss, with many quartz–feldspar layers and veins, occurs in the Otter Lake terrane of the southern Grenville Province of the Canadian Shield. Intense deformation of the complex is indicated by folded and disjointed amphibolite dikes; many veins are also folded. The K-feldspar gneiss consists of quartz, plagioclase, K-feldspar, biotite, local garnet, and rare sillimanite. With few exceptions, the mineral assemblage of each vein is the same as that of the enclosing gneiss, including garnet and sillimanite, where present, and minor minerals, magnetite, zircon, apatite, and allanite. The volume fraction K-feldspar/(K-feldspar + plagioclase) ranges widely (from 0.05 to 0.90), but compared with adjacent gneiss, K-feldspar is always higher and biotite is lower. The chemical composition of veins is similar to that of enclosing gneiss, but K and Ba are higher and Mg, Fe, Mn, Ti, Zr, Rb, and P are lower. The bulk composition of veined gneiss where veins are numerous and that of adjoining gneiss where veins are scarce are virtually identical. Vein–gneiss differences in plagioclase and garnet composition are small or imperceptible.These results lead to the conclusion that many of the veins were locally derived. The rearrangement of atoms needed to produce a vein is considered in terms of Orville–Fisher exchange reactions, e.g., the transport of K, Al, and Si from gneiss to sites of vein growth, in exchange for Mg, Fe, and H. The Ramberg–Robin model of metamorphic differentiation, in which transport occurs by crystal-boundary diffusion, driven by pressure gradients, is proposed as the principal process of vein formation.