Abstract

Mineralogical, textural, and chemical analyses (EPMA and PIXE) of hydrothermal rutile in the El Teniente porphyry Cu-Mo deposit help to better constrain ore formation processes. Rutile formed from igneous Ti-rich phases (sphene, biotite, Ti-magnetite, and ilmenite) by re-equilibration and/or breakdown under hydrothermal conditions at temperatures ranging between 400°C and 700°C. Most rutile nucleate and grow at the original textural position of its Ti-rich igneous parent mineral phase. The distribution of Mo content in rutile indicates that low-temperature (<400-550°C), Mo-poor rutile (5.4± 1.1 ppm) is dominantly in the Mo-rich mafic wallrocks (high-grade ore), while high-temperature (<550-700°C), Mo-rich rutile (186?±?20 ppm) is found in the Mo-poor felsic porphyries (low-grade ore). Rutile from late dacite ring dikes is a notable exception to this distribution pattern. The Sb content in rutile from the high-temperature potassic core of the deposit to its low-temperature propylitic fringe remains relatively constant (35?±?3 ppm). Temperature and Mo content of the hydrothermal fluids in addition to Mo/Ti ratio, modal abundance and stability of Ti-rich parental phases are key factors constraining Mo content and provenance in high-temperature (=550°C) rutile. The initial Mo content of parent mineral phases is controlled by melt composition and oxygen fugacity as well as timing and efficiency of fluid-melt separation. Enhanced reduction of SO 2-rich fluids and sulfide deposition in the Fe-rich mafic wallrocks influences the low-temperature (=550°C) rutile chemistry. The data are consistent with a model of fluid circulation of hot (>550°C), oxidized (O 2=NNO+1.3), SO 2-rich and Mo-bearing fluids, likely exsolved from deeper crystallizing parts of the porphyry system and fluxed through the upper dacite porphyries and related structures, with metal deposition dominantly in the Fe-rich mafic wallrocks. © 2009 Springer-Verlag.