Chemical composition of biotite from the Casino porphyry Cu–Au–Mo mineralization, Yukon, Canada: evaluation of magmatic and hydrothermal fluid chemistry

The chemical composition of biotite has been determined for the Dawson Range batholith and the Casino, porphyry-style Cu–Au–Mo occurrence, Yukon Territory, Canada. Biotite from the propylitic, phyllic and potassic alteration zones of the Casino occurrence possesses higher XMg, Al2O3, SiO2, F and Cl, and lower TiO2, BaO and MnO concentrations than biotite outside these alteration zones. This phenomenon is similar to that recorded for biotite of other porphyry Cu deposits. However, the absolute concentrations of the oxides and halogens vary significantly among deposits. Calculated log (fH2O)/(fHF), (fH2O)/(fHCl), (fHCl)/(fHF) ratios, determined from biotite microprobe data, indicate that hydrothermal fluids associated with potassic alteration at Casino were similar to those associated with phyllic alteration. In contrast, fluids responsible for propylitic alteration had higher log (fH2O)/(fHF) and less negative log (fHF)/(fHCl) values. The log (fH2O)/(fHCl) and (fHF)/(fHCl) ratios established for Casino are similar to those for other porphyry Cu deposits. However, log (fH2O)/(fHF) values vary greatly among porphyry deposits. This variation in log (fH2O)/(fHF) values may be related to the source of the magmas and/or magmatic processes (assimilation/fractional crystallization) associated with melt on its ascent through the crust prior to the exsolution of magmatic fluids that are responsible for porphyry mineralization. The biotite compositions from the Dawson Range batholith are either not consistent with the Fe–F and Mg–Cl avoidance principles or record changes in the F/Cl ratio of the fluid during crystallization of the granitic body.