Petrography and mineral chemistry of the composite Deboullie pluton, northern Maine, U.S.A.: Implications for the genesis of CuMo mineralization

Abstract

Biotite and apatite mineral chemistry, particularly halogen abundances and ratios, are used to investigate the relation of the two contrasting parts of the Deboullie composite pluton (syenite-granodiorite) located in northern Maine. Biotite mineral chemistry helps to classify the weakly developed porphyry-style mineralization (CuMo) associated with syenitic rocks of the Deboullie pluton. Unmineralized syenite consists of K-feldspar+plagioclase+quartz+biotite+hornblende±clinopyroxene and minor amounts of apatite, titanite, magnetite, zircon and allanite. Biotite and apatite occur within the matrix of the rocks and within small multiphase inclusions hosted by clinopyroxene. The inclusions are interpreted to be crystallized melt inclusions rather than solid inclusions, that were trapped by clinopyroxene during growth. The multiphase inclusions consist of K-feldspar+quartz+biotite+apatite+magnetite. The F contents of biotite display a strong positive correlation with the phlogopite component, or X phl [ X phl=Mg/(sum octahedral cations)]. Biotites from the multiphase inclusions have higher F contents and higher values of F ( F+ Cl+ OH) and X phl than biotites in the matrix that presumably formed later during crystallization and/or re-equilibrated with late-stage fluids. Contents of TiO 2 largely overlap for the inclusion and matrix biotite. Apatite compositions plot within the fluorapatite field [ F ( F+ Cl+ OH) = 0.57−0.99] . Apatites in the multiphase inclusions have higher Cl contents than apatites in the matrix or those enclosed in biotite. Biotite-apatite geothermometry for the multiphase inclusions yields magmatic or near-magmatic temperatures (725–850°C); biotite-apatite pairs outside the inclusions yield temperatures that range from near-magmatic (600–775°C) to much lower (< 450°C), again indicating low-temperature re-equilibration or disequilibrium alteration. The relation between ( X F X Cl ) and X phl was used to infer HF/HCl fugacity ratios in associated hydrothermal fluids, and indicates that biotite from the Deboullie pluton is strikingly similar to biotite from the porphyry-Cu deposit at Santa Rita, New Mexico, but distinctly different from biotite from the porphyry-Mo deposit at Henderson, Colorado. The assemblage quartz+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 regional scale, biotite compositions from granitic plutons in Maine do not vary in a systematic manner. This implies that biotite compositions no longer directly reflect source compositions. Finally, biotite and apatite compositions of the syenite and granodiorite are different; compositional ranges in the granodiorite are more restricted than those in the syenite. This provides evidence that the two parts of this composite pluton had different origins and are not related by crystal fractionation.