Geochronology and geochemistry of the intrusive suite associated with the Khatsavch porphyry Cu-Au (Mo) deposit, South Mongolia

Abstract

The Khatsavch porphyry Cu-Au (Mo) deposit is located in the Gurvansayhan island arc terrane of South Mongolia, approximately 45 km southwest of the Oyu Tolgoi Cu-Au district, one of the largest porphyry-type Cu-Au deposits in the world. The mineralization of the Khatsavch deposit is spatially and temporally associated with the second stage of the Baga Khachig intrusive complex, which intruded the Early Devonian Ekhiin Gol formation and is overlain by Quaternary sediments. Based on geological relationships, the age of the intrusive complex has been determined as Carboniferous; however, the precise age remains to be constrained.Here we present new geochronological, mineralogical, and geochemical data for the intrusive event that facilitated the formation of the Khatsavch deposit. Laser ablation ICP-MS analysis of U-Pb isotopes in zircons yields an age of 366 ± 6.4 Ma, which is within the uncertainty of ages of the granitoids that host the Oyu Tolgoi deposits (∼372 Ma and ∼366 Ma). Mineralogically, the granitoids of the Khatsavch and Oyu Tolgoi porphyry deposits are quartz monzonites, quartz monzodiorites, and granodiorites. Disseminated Cu-Au (Mo) mineralization occurs in quartz veins as well as in the granitoids of the Khatsavch deposit and is dominated by chalcopyrite, pyrite, molybdenite, and rare gold. Geochemically, rocks from the Khatsavch deposit are I-type granitoids with high K calc-alkaline affinity emplaced in a volcanogenic arc setting. Chondrite-normalized bulk rock rare earth element data show listric patterns similar to that of the Oyu Tolgoi, which is interpreted to reflect amphibole fractionation in arc settings. Granitoids from both deposits display smoothly decreasing Y, Yb, and Sr concentrations with differentiation, and increasing Sr/Y and La/Yb ratios with decreasing Y and Yb, which are consistent with hornblende, titanite, and plagioclase fractionation. Moreover, these granitoids are enriched in LILE, Pb, and U and depleted in Ta, Nb, and Ti. Based on the mineralogical and geochemical similarities of the Khatsavch and Oyu Tolgoi deposits, and their spatial and temporal proximity, it is argued that both ore systems are associated with the same magma source and that undiscovered deposits derived from this source may exist. Therefore, we suggest that the Gurvansayhan island arc terrane is an excellent exploration target well beyond the existing deposits.