Biotite and apatite occur with copper and iron sulfides, quartz, and traces of other minerals in a vesicular quartz latite dike which intrudes the quartz monzonite stock in the Bingham mine, Utah. Within each of three distinct phases of the dike, biotite in the rock adjacent to vesicles, and in secondary aggregates, is richer in Mg and F and depleted in Ba, Fe, Ti, and Cl relative to magmatic biotite. Both magmatic biotite and hydrothermal biotite increase in Mg and F content from the border phase of the dike to a central phase to a late aplitic phase. Apatite compositions show closely analogous variations involving F enrichment and Fe and Cl depletion. The trends in biotite composition within this single small intrusion are markedly similar to those documented by more extensive studies of the potassic zones at Bingham and at Santa Rita.The hydrothermal solutions that formed or exchanged with the secondary biotite were probably not markedly fluorine rich. Log f (sub H 2 O) /f HF during crystallization of the vesicle filling was about 5. Fluorine distribution between adjacent biotite and apatite grains does not represent high temperature equilibrium partitioning and cannot be used to determine the temperature of vesicle filling.Biotite compositions within the different phases of the dike and of the Bingham stock appear to indicate separate evolution from magmatic to hydrothermal conditions within the intrusive phases of the dike. However, the observed compositional variation could be due to different degrees of exchange with a common, chemically evolving hydrothermal fluid.