Application of apatite (U-Th)/He thermochronometry to the determination of the sense and amount of vertical fault displacement at the Chuquicamata porphyry copper deposit, Chile

Abstract

Chuquicamata is the world's largest porphyry copper deposit, notwithstanding the fact that a portion of the orebody has been faulted off by postmineralization movement along the West fault. In order to locate the missing portion of the orebody in the vertical dimension, a study was designed to estimate the sense and amount of vertical displacement along this major structure by measuring the (U-Th)/He and fission-track ages of vertically distributed apatite samples from each of the crustal blocks (Fortuna and Chuquicamata Intrusive Complexes) bordering the fault. Apatite (U-Th)/He ages range from 32 to 16 Ma, whereas apatite fission-track ages range from 33 to 28 Ma, reflecting the lower closure temperature of the (U-Th)/He thermochronology method ( approximately 75 degrees vs. approximately 125 degrees C for cooling rates of approximately 10 degrees C/m.y.). The (U-Th)/He ages decrease systematically with depth in both blocks, however, the age-elevation curve for the western Fortuna block is shifted vertically with respect to the eastern Chuquicamata block, indicating that the postmineralization denudation was significantly greater to the west. The minimum vertical displacement along the West fault is estimated to be 600 + or - 100 m, implying that the missing portion of the Chuquicamata deposit should be located at a present-day elevation of at least 3,600 m. The new apatite ages, combined with previous thermochronometric data (Rb-Sr, U-Pb, Ar-Ar), reveal rapid cooling rates ( approximately 100 degrees C/m.y.) for the Chuquicamata deposit following emplacement at about 35 Ma, thereby indicating that the Cu mineralization took place at a depth of less than 4 km.