Porphyry dikes from the Sigma and Lamaque mines, Val d'Or, Quebec. Two types of alteration: cryptic alteration (ubiquitous to all quartz-tourmaline veins) involves a 1- to 2-m region of sericitization and incipient carbonatization; visible alteration involves destruction of all mafic mineral phases and precipitation of calcite, apatite, and Ti oxides in addition to pyritization. In all examples visible alteration is adjacent to tourmaline-rich zones in the quartz-tourmaline veins. Considerable enrichments and fractionation of the light, middle, and heavy rare earth elements and Ti, P, Zr, Y, Hf, U, and Ta abundances are observed, while Rb, Sr, Sc, and Al are depleted. The cryptic alteration zone is characterized by rare earth element leaching and probably represents a low pH environment of incipient sericitic alteration.--Modified journal abstract.