The partition of Cu, Mn, and Zn between coexisting hornblende and biotite was investigated in two Laramide intrusions in the Arizona porphyry copper province. Roozeboom diagrams suggest that an equilibrium distribution was attained and that changes in the partition coefficient are closely related to textural variations within a single rock type and to differences in rock types. Equilibrium partitioning of Cu was well developed, even though Cu concentrations in biotite varied from 15 to 3,658 ppm. Analyses of chlorite show that its Cu content is more variable than its Zn content and suggest differences between the magmatic and hydrothermal behavior of Cu and Zn. It is concluded that the Cu, Mn, and Zn concentration of hornblende and biotite were magmatically controlled and that high concentrations did not result from later introduction of material during subsolidus hydrothermal activity.The Cu content of biotite progressively increases during the early stages of magmatic crystallization. The decrease of Cu in biotite during the later stages is correlated with the appearance of a hydrothermal phase into which Cu was fractionated from the melt. Escape of this phase removed Cu from the system causing biotites growing in the residual molten portions of the intrusions to be impoverished in Cu. Metal balance calculations indicate that Cu lost from biotite can account for all the Cu in the genetically associated sulfide deposits.A practical evaluation of the data indicates that the variation of Cu in biotite can be a useful exploration tool. Copper analyses of biotite yield larger anomalous haloes and greater contrast between background and anomaly than other geochemical techniques.