The concentrations of fluorine and chlorine in biotite, apatite, hornblende, sphene, and chlorite from plutons of intermediate composition in the Basin and Range province have been determined by microprobe methods. Maximum and minimum concentrations in weight percent are 0.4 to 3.8 F and 0.0 to 0.26 Cl in biotite and 0.7 to 3.6 F and 0.0 to 0.8 Cl in apatite. Apatite is generally richer in Cl and F than associated biotite.Chlorine and fluorine concentrations differ significantly between individual plutons, but no differences which might prove useful for exploration could be recognized between mineralized and nonmineralized plutons. Hydrothermal biotite and apatite have equilibrated with saline, hydrothermal fluid and may contain either more or less chlorine and fluorine than magmatic minerals which reflect the relative abundance of HCl, HF, and H 2 O in the magma. The highest Cl content in biotites occurs in plutons containing pyroxene, whether mineralized or not.

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