The Phinncy-Dash Lakes Complex (PDLC) of the western Wabigoon Subprovince, northwestern Ontario consists of subvolcanic and pyroclastic rhyodacite, subordinate rhyolite pyroclastic rocks and lava flows, minor intercalated basalt lava flows and related clastic and chemical sediments which have been metamorphosed to low greenschist grade. The basalt lying stratigraphically below the complex is intruded by tonalite that is coeval with the felsic volcanic rocks. Rhyolite in PDLC differs from the associated rhyodacite by containing K-feldspar, and having lower La/Yb ratios, negative Eu anomalies and generally lower Fe, Mg, Ca, Ti, Al, P, Zr, Sc and Cr, but higher Si, K, Rb, Ba and Nb. Major and trace element calculations using estimates of rhyodacite and rhyolite primary phenocryst compositions and estimates of partition coefficients were done to determine if the rhyolite may have been generated by fractional crystallization from the rhyodacite. It was found that fractionation of plagioclase, clinopyroxene, quartz, and small amounts of apatite, zircon, allanite and Fe-Ti oxides from the rhyodacite would produce a composition similar to the rhyolite only if the abundances of the mobile elements Si, Ca, Na, Sr, Ba and Mg in the rhyodacite initial composition were adjusted from the measured values. If the rhyolite fractionated from the rhyodacite, then there must have been (a) net addition of Si, (2) net depletion of Ca, Na, Sr and Ba, and (3) net depletion of Mg but conservation of Fe, in the rhyodacite during synvolcanic alteration or regional metamorphism. These inferences are supported by thin section evidence of (1) addition of quartz in microfractures and groundmass, (2) breakdown of primary plagioclase, and, (3) breakdown of primary mafic silicates. Plots of MgO versus TiO 2 and FeO versus TiO 2 also demonstrate that by comparison with the nearby and coeval Kishkutena tonalite, Mg is depleted and Fe is conserved in the PDLC rocks. The secondary mineral assemblages combined with the apparent chemical reconstitution suggest that during alteration of the PDLC rocks: (1) The water-to-rock ratio was small. (2) The fluid temperature was low, probably below about 300°C. (3) If the temperature and water-to-rock ratio were large, however, then the fluid source was probably not seawater because of the limited precipitation of Mg to the rocks.