Porphyry Indicator Zircons (PIZ) and Geochronology of Magmatic Rocks from the Malmyzh and Pony Cu-Au Porphyry Ore Fields (Russian Far East)

Abstract New data obtained by petrographical, geochemical and U-Pb geochronological studies of magmatic whole rock and zircon samples from the Malmyzh and Pony gold-copper porphyry ore fields are discussed. The trace elements distribution patterns (REE+Y, Hf, Ti, U, Th, Pb) in zircons have been measured by SIMS SHRIMP-IIe instrument in the same spots as for local U-Pb age dating. Geochronological data show a matching Cenomanian crystallization age of the Malmyzh and Pony magmatic rocks: 99-97 and 93 Ma, respectively. ∑REE in zircons from the Malmyzh granitoids varies from 217 to 1,158 ppm, Y – from 718 to 732 ppm, Hf – from 4,524 to 4,928 ppm, Ti – from 4.1 to 16.6 ppm, Pb – from 1.2 to 1.6 ppm, U – from 89 to 112 ppm, Th – from 58 to 76 ppm. Geochemical characteristics of zircons from the Pony monzodiorite-porphyry differ significantly from the Malmyzh granitoid zircons. Concentrations of most trace elements in the Pony zircons, especially LREE and MREE, as well as Y, Pb, U, Th, and Ti are two-three times higher than in zircons from the Malmyzh granitoids. Temperature of zircon crystallization of the Malmyzh granitoids revealed by «Ti in zircon» thermometer is estimated at 680-700°C, whereas temperature of zircon crystallization of the Pony monzodiorite-porphyry is approximately 784±15°C. Fertility assessment of magmatic rocks from the Malmyzh and Pony ore fields is based on geochemical characteristics of zircons like Eu/Eu* vs. Dy/Nd and Eu/Eu* vs. (Ce/Nd)/Y plots. These characteristics show that zircons from the Malmyzh granitoids fall into the field of fertile porphyry systems, whereas zircons from the Pony monzodiorite-porphyry plot within the field of barren suites. Comparative analysis of studied samples from the Malmyzh and Pony ore fields with magmatic rocks from porphyry deposits worldwide aids our understanding of the geodynamic framework of their formation and establishes links to adakitic granitoids generated under similar conditions. This result can be utilized during prognostic and metallogenic considerations not only within the limits of Sikhote-Alin territory but also within the whole Russian Far East region.