The Niaz porphyry Cu-Mo deposit in the Arasbaran metallogenic belt of NW Iran exhibits extensive hydrothermal alteration developed in three temporally and spatially overlapping zones: early potassic, transitional phyllic and intermediate argillic, and late advanced argillic. The early and transitional zones contain biotite, either of magmatic (re-equilibrated) or hydrothermal (replacement and/or neoformed) origin. This study aims to understand the petrography and chemistry of the hydrothermal biotite for evaluating the fluid compositional changes during alteration processes. Selected samples from the different alteration zones were studied for petrography crossing from inner to outer parts of the Niaz deposit. Electron microprobe analyses (Cameca SX100) including halogens (F and Cl) were performed on the hydrothermal micas at the Centre CAMPARIS, Institut des Sciences de la Terre de Paris (ISTeP), Sorbonne University, France. The biotite composition displays an increase in Al2O3, FeO and Cl, but a decrease in TiO2, MgO and F, from the potassic to the transitional phyllic and intermediate argillic alteration zones. The hydrothermal biotite with high Mg (XMg = 0.61-0.72) inside potassic zone tends to incorporate more F and less Cl compared to the biotite with lower Mg; a crystal-chemical effect referred to as “Fe-F and Mg-Cl avoidance rules”. The biotite from the potassic zone possesses a moderate range of F content (0.24 to 0.91wt. %) that is significantly higher than in the phyllic (0.45 to 0.62 wt. %) and argillic (0.19 to 0.37 wt. %) zones, exhibiting a positive correlation with XMg and a negative correlation with Cl. However, the biotite from transitional phyllic as well as intermediate argillic alteration zones shows a scattered relationship.The biotite from the central potassic to transitional phyllic and intermediate argillic alteration zones have average log (XF/XOH) values of 1.16, 1.19, and 1.44, respectively. The log (XCl/XOH) values are 2.10, 1.97, and 1.98, whereas log (XCl/XF) val¬ues vary from 0.95, 0.78 to 0.54. The systematic variation of the logarithmic ratios reflects a sys¬tematic variation of the F content in biotite associated with these alteration zones.Microthermometric data of fluid inclusions show a decrease in temperature from potassic through phyllic to intermediate argillic zones (420, 360 and 280 °C, respectively). The log (fH2O/fHF) and log (fH2O/fHCl) values calculated for fluids equilibrated with biotite increase progressively outward in these alteration zones (6.04, 6.42 and 7.39, respectively). The decrease in halogen content of hydrothermal fluids toward outer parts of the deposits reflects an increase in the degree of mixing between magmatic fluid and meteoric water.The F content of biotite decreases systematically toward the outer part of the deposit, while the Cl content shows unsystematic variations crossing the alteration zones. This finding suggests that the Cl content cannot be used as exploration tool for vectoring the mineralization. However, the positive correlation between the F content in biotite and bulk concentration of Cu in the different alteration zones may provide a possible geochemical tool to vectoring the Cu mineralization in porphyry deposits.