GEOLOGY AND GEOCHRONOLOGY OF THE TOKI CLUSTER: CHUQUICAMATA DISTRICT NORTHERN CHILE

The Toki cluster, which includes the Toki, Quetena and Genoveva prospects, is 15 km SSW of the Chuquicamata-Radomiro Tomic deposits. The prospects occur in an area of 10 x 12 km and are completely covered by Neogene alluvial deposits. Inferred resources for the cluster (including the southernmost Opache prospect) are estimated at about 17 million metric tons of fine copper, with Toki contributing with ca. 11.2 Mt and Quetena with ~3.8 Mt.

Mineralization in these deposits is associated to tonalite porphyries that intruded andesites and tonalites of the Fortuna–Los Picos Granodioritic Complex. Hypogene mineralization consists of areas of chalcopyrite-bornite with minor molybdenite centered on each prospect with mineralization grading outward to a chalcopyrite-pyrite zone and ultimately to a pyrite halo. Alteration is dominantly potassic with K-feldspar and hydrothermal biotite. Sericitic alteration is restricted to late quartz-pyrite veins (D-veins). Previous K-Ar geochronology for the cluster yielded ages of 34 to 40 Ma. Four new Re-Os ages for Toki indicate that molybdenite mineralization occurred in a single pulse at ~38 Ma (weighted average of 37.9 ± 0.3 Ma). Re-Os ages for three different molybdenite samples from Quetena (38.3 ± 0.2 Ma, 38.4 ± 0.2 Ma and 38.0 ± 0.2 Ma) are within error of the Toki mineralization ages. These ages are concordant with a new zircon U-Pb ICPMS-MC age of 38.6 ± 0.7 Ma from the tonalite porphyry in Quetena. Two Re-Os ages for Genoveva (38.2 ± 0.2 Ma and 38.0 ± 0.2 Ma) are also within error of the Toki and Quetena molybdenite ages. Analyzed molybdenites are high in Re and Os with concentrations of 613 - 3099 ppm and 243 - 1231 ppb, respectively. The highest Re and Os concentrations are found in the Toki prospect.

Three new ⁴⁰Ar/³⁹Ar ages are younger than the mineralization ages (~36 Ma) and show evidence of excess argon.

The new geochronological data presented here indicates that molybdenite mineralization occurred synchronously in all mineralization centers (at ~38 Ma) and probably within a very short period of time (< 1 m.y.). Furthermore, mineralization in the Toki cluster preceded the emplacement of the Chuquicamata deposit (35-31 Ma) and indicates that porphyry Cu-Mo mineralization occurred episodically in the Chuquicamata district within a period of several million years.