Molybdenite Mineralization and Re-Os Geochronology of the Escondida and Escondida Norte Porphyry Deposits, Northern Chile

Molybdenum is an economically important subproduct of North Chilean porphyry-type deposits, and thus spatial and temporal distribution of molybdenite as the primary Mo-bearing mineral in the Escondida and Escondida Norte deposits were characterized using several mineralogical and chemical techniques and the Re-Os dating method. Molybdenum is distributed extensively in the two deposits, and high molybdenum concentrations (>500 ppm) are recognized particularly in the chlorite-sericite transitional zone between the potassic and sericitic zones. Two modes of occurrence of molybdenite are observed in the Escondida deposit: aggregates with Cu-Fe-sulfide minerals in fine veinlets (sulfide-veinlet type), and monomineralic microveinlets associated with NE-trending faults. The former and the latter yielded ages of 36.1 ± 0.2 Ma and 35.2 ± 0.2 Ma, respectively. Re-Os dating of Escondida Norte molybdenites also show two distinct episodes, at 37.7 ± 0.3 Ma and a younger episode at 36.6 ± 0.2 Ma. These data indicate that the Escondida Norte is older than the main Escondida deposit. The Re-Os age data combined with those of the porphyry emplacement suggest that the molybdenite mineralization in the Escondida district occurred as several short episodic pulses during the late-magmatic to hydrothermal transition, and that the Cu-Mo deposits were formed in a variable overall period spanning 1 to 5 m.y.