Refinement of the time-space evolution of the giant Mio-Pliocene Río Blanco-Los Bronces porphyry Cu–Mo cluster, Central Chile: new U–Pb (SHRIMP II) and Re–Os geochronology and 40Ar/39Ar thermochronology data

Abstract

Representing one of the largest known (estimated >5 Gt at 1 % Cu and 0.02 % Mo) porphyry system, the Río Blanco-Los Bronces deposit incorporates at least five hypabyssal intrusive and hydrothermal centres, extending for about 5 km from the Río Blanco and Los Bronces mines in the north, through the Don Luis mine, to the Sur Sur mine, La Americana and Los Sulfatos in the south. The new geochronology data, which now include data on differentmolybdenite vein types, confirm the U–Pb ages of the pre-mineralisation intrusions but slightly increase their age range from 8.8 to 8.2 Ma. The distinct magmatic pulses of the mineralisationassociated porphyritic intrusives (Late Porphyries) indicate an age interval instead of the previously suggested individual ages: the quartz monzonite porphyry ranges from 7.7 to 6.1Ma (Sur Sur 5.74±0.13Ma), the feldspar porphyry shows an interval from 5.8 to 5.2 Ma and the Don Luis porphyry from 5.2 to 5.0 Ma. The new Re–Os data on distinct molybdenite vein types confirm the protracted history of Cu(–Mo) mineralisation, inferred previously. The vein development occurred at least from 5.94 to 4.50 Ma, indicating a timespan of 1.5 Ma for the hydrothermal activity. Hydrothermal minerals dated by the 40Ar/39Ar method are generally too young to record the age of early, high-temperature mineralisation. The majority of the 40Ar/39Ar data in the Río Blanco porphyry cluster record reheating by either the youngestmember of the Late Porphyry suite or the post-mineralisation dacite or rhyolite plug formations at around 4.9–4.7 Ma.