Porphyry indicator zircons (PIZs): Application to exploration of porphyry copper deposits

Abstract

Magmas with a high oxidation state (FMQ + 1 to + 2), a high-water content (≥4 wt% H2O) and a high degree of fractionation are necessary to form porphyry copper deposits (PCDs). Different proxies, such as Fe3+/Fe2+ ratio of whole-rock, minerals indicative of high oxygen fugacity (fO2) and water content as well as geochemical signatures of whole-rock (e.g. high Sr/Y and V/Sc ratios) are commonly used to trace these magmatic conditions. However, these proxies are of limited use for rocks hosting porphyry copper mineralization because the hydrothermal fluids associated with ore deposits obliterate the primary minerals and their initial geochemical signatures. Zircon is a common accessory mineral of igneous rocks that retains its original chemical composition from the time of crystallization, therefore provides information related to parental magmas. Several studies on PCDs worldwide have investigated the geochemical characteristics of zircons (e.g., Hf concentration, Ce4+/Ce3+, Eu/Eu*) as indicators of a high oxidation state, high magmatic water content and high degree of magma fractionation. Here, the petrographic and geochemical characteristics of zircons collected from Chuquicamata, Mirador, Esperanza and Escondida PCDs, located in the upper Eocene to lower Oligocene metallogenic belt of northern Chile, were determined in order to validate the use of zircon as a tracer for porphyry fertility. Our results show that zircon collected from all studied PCDs crystallized from monzogranitic to granodioritic magmas at temperatures below 750 °C and show euhedral morphologies mainly characterized by prismatic forms of type {110} and pyramidal forms of type {101}. Most zircons from PCDs have Hf concentrations > 8,750 (ppm) and Ce/Nd > 1, Eu/Eu* > 0.4, 10,000x(Eu/Eu*)/Y > 1, (Ce/Nd)/Y > 0.01, Dy/Yb < 0.3, Th/U 〈1−〉 0.1. From this group of geochemical values measured on zircon, we propose the Porphyry Indicator Zircon concept (PIZ), where each zircon from PCDs must comply with the previously mentioned geochemical framework. Our results show that studied PCDs have abundant zircons with PIZ characteristic (>54%); at Mirador and Esperanza porphyries the PIZ proportion exceeding 70%. On the other hand, the non-mineralized intrusive bodies have no zircons with PIZ characteristics. Therefore, these results combined with previously published studies from other porphyry belts worldwide, validate zircons as an indicator of metallogenic fertility and its use as a porphyry indicator mineral and a potential exploration tool in search for PCDs.