ARTICLE New thermochronologic constraints on the evolution of the Zaldívar porphyry copper deposit, Northern Chile Eduardo Campos & Jan Wijbrans& Paul A. M. Andriessen Received: 12 November 2008 / Accepted: 30 November 2008 / Published online: 23 December 2008 # Springer-Verlag 2008 AbstractLife spans and thermal evolution of hydrother- malsystems are of fundamental metallogenic importance. We wereableto establish the chronology and cooling history of the Zaldívar porphyry copper deposit (Northern Chile) by applying acombination of different isotopic dating methods in minerals with different closure temper- atures,including 40 Ar/ 39 Ar geochronology and zircon fission track thermochronology, together with fluid inclu- sion thermometry and previous published U–Pb zircon geochronology. The hydrothermal mineralization in the Zaldívardepositis geneticallyrelatedto the Llamo Porphyry unit.Samplesof igneousbiotitesfrom this intrusion yielded 40 Ar/ 39 Ar plateau ages between 35.5± 0.7 and 37.7±0.4 Ma defining a weighted average of 36.6± 0.5 Ma (2σ).In contrast, one sample from the Zaldívar porphyry, one from the andesites, and two from the Llamo porphyry yielded considerably younger fission track ages of approximately 29 Ma with a weighted mean for all ages of 29.1±1.7 Ma (2σ). Thermal and compositional constraints for the hydrothermal system in the Zaldívar deposit from fluid inclusions thermometry show that atleastthree fluid types broadly characterize two main hydrothermal episodes during the evolution of the deposit. The main mineraliza- tion and alteration event is characterized by high temper- ature (above 320°C) hypersaline fluids (salinity between 3 and 56 wt.% NaCl equivalents) coexisting with low-density gas-richinclusions(salinityless than17 wt.% NaCl equivalents) that homogenizing into the gas phaseat temperatures above350°C.The second episode corre- sponds to a low-temperature event which is characterized by liquid-rich fluid inclusions thathomogenize into the liquid phase at temperatures ranging from 200°C to 300°C with salinities lower than 10 wt.% NaCl equivalents. The 40 Ar/ 39 Ar data (36.6 ±0.5 Ma, weighted average) obtained from igneous biotites represent the minimum age for the lasthigh-temperature (above 300°C) hydrothermal pulse. When compared with previously published U–Pb ages (38.7 ±1.3 Ma) in zircons from the Llamo porphyry, a close temporal relationship between crystallization of the paren intrusionand the thermalcollapseof the last high- temperature hydrothermal eventis evident. Cooling took placefrom approximately 800°C (crystallization of the intrusive complex defined by zircon U–Pb ages) to below 300 ±50°C (biotite 40 Ar/ 39 Ar closure temperature) within approximately 1.5 m.y. Because the thermal annealing of fission tracks in zircons occurs at temperatures of 240 ± 3 the zircon fission track (ZFT) ages of29.1±1.7 Ma (2σ) mark the end of the thermal activity in the Zaldívar area, specifically the time when the whole area cooled below th temperature, wellafter the collapse of the main hydrother- maleventin the Zaldívar porphyry copper deposit. This cooling age roughly coincides with the age defined for the emplacement of dacitic dikes at 31±2.8 Ma (2σ) (publishe K–Ar whole rock), 5 km south of the Zaldívar deposit, in the Escondida area. This late magmatic pulse probably is responsible for high heat flow in the Zaldívar deposit as la as 29 Ma.There is no evidence that the low temperature hydrothermal pulse recognized by fluid inclusion studies is related to thisthermal event.The zircon fission track Miner Deposita (2009) 44:329–342 DOI 10.1007/s00126-008-0226-y Editorial handling: P. Williams E. Campos (*) Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile e-mail: edcampos@ucn.cl J. Wijbrans : P. A. M. Andriessen Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands