PETROGRAPHY AND MINERAL CHEMISTRY OF THE COMPOSITE DEBOULLIE PLUTON, NORTHERN MAINE, USA - IMPLICATIONS FOR THE GENESIS OF CU-MO MINERALIZATION

Biotite and apatite mineral chemistry, particularly halogen abundances and ratios, are used to investigate the relation of the two contrasti ng parts of the Deboullie composite pluton (syenite-granodiorite) loca ted in northern Maine. Biotite mineral chemistry helps to classify the weakly developed porphyry-style mineralization (Cu-Mo) associated wit h syenitic rocks of the Deboullie pluton. Unmineralized syenite consis ts of K-feldspar + plagioclase + quartz + biotite + homblende +/- clin opyroxene and minor amounts of apatite, titanite, magnetite, zircon an d allanite. Biotite and apatite occur within the matrix of the rocks a nd within small multiphase inclusions hosted by clinopyroxene. The inc lusions are interpreted to be crystallized melt inclusions rather than solid inclusions, that were trapped by clinopyroxene during growth. T he multiphase inclusions consist of K-feldspar + quartz + biotite + ap atite + magnetite. The F contents of biotite display a strong positive correlation with the phlogopite component, or Xphl [Xphl = Mg/(sum oc tahedral cations)]. Biotites from the multiphase inclusions have highe r F contents and higher values of F/ (F + Cl + OH) and Xphl than bioti tes in the matrix that presumably formed later during crystallization and/or re-equilibrated with late-stage fluids. Contents of Ti02, large ly overlap for the inclusion and matrix biotite. Apatite compositions plot within the fluorapatite field [F/ (F + Cl + OH) = 0.57-0.99]. Apa tites in the multiphase inclusions have higher Cl contents than apatit es in the matrix or those enclosed in biotite. Biotite-apatite geother mometry for the multiphase inclusions yields magmatic or near-magmatic temperatures (725-850 degrees C); biotite-apatite pairs outside the i nclusions yield temperatures that range from near-magmatic (600-775 de grees C) to much lower ( < 450 degrees C), again indicating low-temper ature re-equilibration or disequilibrium alteration. The relation betw een (XF/XCl) and Xphl was used to infer HF/HC1 fugacity ratios in asso ciated hydrothermal fluids, and indicates that biotite from the Deboul lie pluton is strikingly similar to biotite from the porphyry-Cu depos it at Santa Rita, New Mexico, but distinctly different from biotite fr om the porphyry-Mo deposit at Henderson, Colorado. The assemblage quar tz + magnetite + titanite implies an oxidation state for the Deboullie pluton well above the quartz-magnetite-fayalite buffer, which is also consistent with magmas associated with porphyry-Cu deposits. On a reg ional scale, biotite compositions from,granitic plutons in Maine do no t vary in a systematic manner. This implies that biotite compositions no longer directly reflect source compositions, Finally, biotite and a patite compositions of the syenite and granodiorite are different; com positional ranges in the granodiorite are more restricted than those i n the syenite. This provides evidence that the two parts of this compo site pluton had different origins and are not related by crystal fract ionation.