GEOCHRONOLOGY AND GEOCHEMISTRY OF THE KHATSAVCH PORPHYRY CU-AU (MO) DEPOSIT

The Khatsavch porphyry Cu-Au (Mo) deposit is located in the Gurvansaikhan island arc terrane of South Mongolia, approximately 40 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 estimated as Carboniferous; however, the precise age remains to be constrained.

Here we present new geochronological, mineralogical and geochemical data for the second stage intrusive event that facilitated formation of the Khatsavch deposit. Laser ablation ICP-MS analyses of U-Pb isotopes in zircons yield an age of 364±10 Ma, which is similar to ages of the granitoids which 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, monzogranites, 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 contents and a calc-alkaline affinity indicative of a volcanogenic arc origin. Chondrite-normalized bulk rock rare earth element data show listric patterns, similar to 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. Furthermore, the Khatsavch porphyry Cu-Au (Mo) deposit has likelihood of being a good exploration target.