Uranium-Pb zircon ages, whole-rock and zircon mineral geochemistry as indicators for magmatic fertility and porphyry Cu-Mo-Au mineralization at the Bystrinsky and Shakhtama deposits, Eastern Transbaikalia, Russia

The Bystrinsky Cu-Au-Fe porphyry-skarn and Shakhtama Mo +/- Cu porphyry deposits in Eastern Transbaikalia, Russia, are spatially associated with a series of fertile and non-fertile calc-alkaline to shoshonitic I-type granitoid suites, ranging in composition from porphyritic monzonites and granodiorites to granodiorite- and graniteporphyries. The obtained zircon U-Pb geochronological data reveal a narrow age interval of 1-3 Ma for the igneous suites within the Bystrinsky deposit (ca. 159-163 Ma), which is consistent with the short-magmatic activity at the Shakhtama deposit (ca. 164-161 Ma). Within both deposits, the fertile magmatic pulses comprise hybrid, high-silica and high- to moderate-magnesian, oxidized melts with relatively high magmatic water contents. These weakly fractionated melts represent the late-stage magmatic pulses of the Shakhtama Complex. By contrast, the barren magmatic pulses comprise a wide range of compositions, generally with lower magmatic water contents and oxidation states as well as significant plagioclase fractionation and crustal contamination. The Shakhtama Mo-related porphyries are more evolved when compared to the Bystrinsky Cu-Au intrusions reflecting significant crustal assimilation in addition to amphibole-plagioclase fractionation. Both whole-rock geochemistry and zircon mineral chemistry from the Bystrinsky and Shakhtama deposits may be used as fertility indicators in target generation and regional mineral exploration in Eastern Transbaikalia. More specifically, we propose critical thresholds of whole-rock Sr/Y and 10,000*(Eu/Eu*)/Y ratios of > 65 and > 850, respectively, and zircon mineral Eu/Eu*, Ce/Ce* and Yb/Dy ratios of > 0.4, >750 and > 4, respectively. Additionally, we propose an alternative method to calculating the Ce anomalies of zircons using MS Excel-based software which is provided in the Supplementary data of this paper.