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

Abstract

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 × 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 ∼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 ∼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 ∼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 ∼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.