Timing and formation of porphyry Cu-Mo mineralization in the Chuquicamata district, northern Chile: new constraints from the Toki cluster

The recently discovered Toki cluster, which includes the Toki, Quetena, Genoveva, Miranda, and Opache porphyry Cu-Mo prospects, is located 15 km south-southwest of the Chuquicamata-Radomiro Tomic mines in northern Chile. These prospects occur in an area of 5 x 6 km and are completely covered with Neogene alluvial deposits. Inferred resources for the cluster are estimated at about 20 Mt of fine copper, with Toki and Quetena contributing similar to 88 % of these resources. Mineralization in these deposits is associated with tonalite porphyries that intruded andesites and dacites of the Collahuasi Group and intrusions of the Fortuna-Los Picos Granodioritic Complex. Hypogene mineralization in the Toki cluster consists mainly of chalcopyrite-bornite with minor molybdenite with mineralization grading outward to a chalcopyrite-pyrite zone and ultimately to a pyrite halo. Alteration is dominantly of the potassic type with K-feldspar and hydrothermal biotite. Sericitic alteration is relatively restricted to late quartz-pyrite veins (D-type veins). Previous K-Ar geochronology for the cluster yielded ages within a range of 34 to 40 Ma. Four new Re-Os ages for Toki indicate that molybdenite mineralization occurred in a single pulse at similar to 38 Ma. Re-Os ages for three different molybdenite samples from Quetena are within error of the Toki mineralization ages. These ages are concordant with a new zircon U-Pb age of 38.6 +/- 0.7 Ma from the tonalite porphyry in Quetena. Two Re-Os ages for Genoveva (38.1 +/- 0.2 and 38.0 +/- 0.2 Ma) are also within error of the Toki and Quetena molybdenite ages. Four Re-Os molybdenite ages for Opache range between 36.4 and 37.6 Ma. The Miranda prospect is the youngest with an age of similar to 36 Ma. Four new Re-Os ages for the Chuquicamata deposit range between 33 and 32 Ma, whereas nine new 40Ar/39Ar ages of biotite, muscovite, and K-feldspar range between 32 and 31 Ma. Analyzed molybdenites have Re and Os concentrations that vary between 21-3,099 ppm and 8-1,231 ppb, respectively. The highest Re and Os concentrations are found in the Toki prospect. Three new 40Ar/39Ar ages for the Toki cluster are younger than the Re-Os mineralization ages. The age spectra for these three samples show evidence of excess argon and have similar inverse isochron ages of 35 Ma that probably reflect a late hydrothermal phyllic event. The new geochronological data presented here for the Toki cluster indicate that molybdenite mineralization occurred within a very short period, probably within 2 Ma, and synchronously (at similar to 38 Ma) in three mineralization centers (Toki, Quetena, and Genoveva). Furthermore, mineralization at the Toki cluster preceded the emplacement of the Chuquicamata deposit (35-31 Ma) and indicates that porphyry Cu-Mo mineralization occurred episodically over a period of several million years in the Chuquicamata district.