EUROPIUM ANOMALIES IN ZIRCON FROM GRANODIORITE PORPHYRY INTRUSIONS AT THE EL SALVADOR PORPHYRY COPPER DEPOSIT, CHILE

While zircon is often used as a geochronologic tool due to high U and Th concentrations, recent advances in analytical techniques and equipment now allow for quantification of other trace elements. Zircon preferentially incorporate heavy rare earth elements (HREE) relative to light (LREE) and often display negative europium anomalies (Eu/Eu*). Because the concentration of REE in zircon is dependent upon magma properties (composition, temperature) during crystallization and other crystallizing phases that incorporate REE, coupling trace element abundances with U/Pb age provides temporal constraints on magma evolution. Fifteen granodiorite porphyry intrusions related to porphyry copper mineralization were sampled from the El Salvador deposit in northern Chile. Ion microprobe (SHRIMP-RG) analysis of zircon provided both U/Pb ages as well as trace element abundances (including Ti). Temperature estimates using Ti concentrations in zircon ranged from ~890º to ~620º C. High temperature zircon contains higher concentrations of U, Th, REE, and lower concentrations of Hf relative to the lower temperature zircon.

Early pre-mineralization porphyries have a higher range of Eu/Eu* values with increasing Hf content compared with later porphyries. During magma crystallization, voluminous plagioclase growth removes significant Eu²⁺ from the system and this reduction of total Eu limits the Eu³⁺ available to zircon, resulting in progressively more negative Eu/Eu* in later grown zircon. Changes in the oxidation state of the magma also affect the Eu³⁺/Eu²⁺ ratio, independent of the total Eu in the system. The Eu/Eu* trend seen in the zircons from among the late stage porphyries suggest that even with continuing plagioclase crystallization, the later magmas were relatively more oxidized. This trend in the late porphyries coincides with the main hydrothermal Cu-Mo sulfide event that produced the ore deposit. Zircon trace element abundances suggest that multiple pulses of hot, deep, relatively mafic and hydrous magmas were emplaced into a heterogeneous upper crustal silicic magma chamber. The crystallization of the upper crustal chamber allowed for the extraction of magmatic-hydrothermal aqueous solutions rich in Cu, Mo, and S to form the ore deposit contemporaneous with emplacement of the late-stage porphyries.