New Sr-Nd, Hf and Pb isotope data are presented for a representative suite of 21 samples of plutonic and volcanic rocks (D (sub 3) to C (sub 1) ) from the giant Oyu Tolgoi porphyry Cu-Au district in the South Gobi, Mongolia. Sr-Nd isotopes (whole-rock) show a restricted range of initial compositions, with epsilon Nd varying from +2.1 to +7.4 and ( (super 87) Sr/ (super 86) Sr)t predominantly between 0.7035 and 0.7045 reflecting formation from a relatively uniform juvenile lithophile-depleted source. Hf isotopes (zircon) exhibit a range of -4.5 to +13.6 (Fig. 1). Felsic rocks show predominantly (apart from one sample) a juvenile mantle-derived signature whereas volcanic rocks exhibit some mixing with evolved crustal sources. Figure 1: Hf and Nd isotopes for the Oyu Tolgoi deposit. Pb isotopes (whole-rock) indicate a rather preserved array of isotopic compositions such as (super 206) Pb/ (super 204) Pb 17.773-19.058, (super 207) Pb/ (super 204) Pb 15.445-15.544 and (super 208) Pb/ (super 204) Pb 37.456-38.489. These are in full agreement with Sr-Nd-Hf isotopes indicating presence of a mantle component. Several samples show contributions from a MORB-related source. All four isotopic systems hint that magmas from which the large Oyu Tolgoi porphyry system was generated, originated predominantly from juvenile material within the subduction-related setting of the Gurvan-Saikhan terrane.