New whole-rock skarn and porphyry fertility indicators: Insights from Cu-Au-Fe skarn and Cu-Mo-Au porphyry deposits in Eastern Transbaikalia, Russia

Abstract

Identifying new whole-rock geochemical fertility indicators for porphyry and skarn productive systems is a geological challenge due to a large variety of tectonic settings, magmatic source regions and petrological evolution of the parental intrusions. This is especially true for non-porphyry-related Cu-Au-Fe skarns for which no relevant whole rock or chemical mineral indicators are currently recorded. Our study provides high‐precision whole‐rock data of granitoids from the Kultuma Cu-Au-Fe skarn deposit in Eastern Transbaikalia, Russia, in order to define productive skarn suites.The Kultuma Cu-Au-Fe skarn deposit consists of two contemporaneous (dated at ca. 159–161 Ma and 157–158 Ma using U-Pb methods) pulses of mineralized high-calc-alkaline to shoshonitic I-type granites that were emplaced during the final stage of collision following the closure of the Mongol–Okhotsk ocean. Igneous suites of both skarn-related pulses are high-K (3–5 wt% K2O) and range in composition from low-silica and high-magnesian monzonite porphyries to high-silica and high- to moderate-magnesian quartz‐syenite and granite porphyries. They originated from moderately oxidized (zircon Eu/Eu* = avg. 0.24 and ΔFMQ = avg. + 0.4 to + 0.6), moderately hydrous (whole-rock Sr/Y = 40–64; zircon Yb/Dy = avg. 3.6), low to moderately fractionated (whole-rock Eu/Eu* = avg. 0.86) magmas that evolved through substantial interaction with the lower crust (whole-rock La/Yb = 24–54, (Dy/Yb)CN = 1.4–2.0, Yb = 0.5–1.2 ppm) followed by deep-level amphibole and shallow-level plagioclase fractionation, respectively.Based on a comprehensive set of new whole‐rock data, we propose the use of (Sr/Y)/Y and 100[(Rb/Sr)/FeO*] ratios to discriminate between fertile non-porphyry-related skarn and porphyry and porphyry-related skarn suites and unmineralized granite intrusions. Our proposed new discrimination thresholds of (Sr/Y)/Y and 100[(Rb/Sr)/FeO*] are effective in Eastern Transbaikalia, but they could potentially be useful for the geochemical discrimination between intrusions prospective for Cu-Mo-Au-Fe skarn and porphyry mineralization and barren calc‐alkaline granitic suites in other arc‐related mineral belts worldwide.