An ancient continental crustal source for Mo mineralisation in the eastern Central Asian Orogen: A case study of the Bilugangan Mo deposit

Abstract

The Bilugangan porphyry Mo deposit is located in the southern Great Xing'an Range, and the Bilugangan area near the Hegenshan-Heihe Fault in the eastern part of the Central Asian Orogen (CAO). The deposit is hosted by a NE-trending, partly concealed, composite granite consisting of highly altered porphyritic granite (G1), porphyritic monzogranite (G2), fine- to medium-grained monzogranite (G3), medium -grained monzogranite (G4), and monzogranitic porphyry dykes (G5). The Mo mineralisation forms hydrothermal stockworks or disseminated sulphides in the G2 and G3 phases and the adjoining hornfels. Magmatic zircons from the intrusions yield 206Pb/238U weighted mean ages of 295 ± 2 Ma for G1, 240 ± 1 Ma for G2, 235 ± 2 Ma for G3, 230 ± 1 Ma for G4, and 100 ± 2 Ma for G5,indicating that multiple magmatic events have been active during the Early Permian to Early Cretaceous emplacement of the Bilugangan Granite. The age of the Mo is constrained between the Middle and Late Triassic and the magma-related hydrothermal activity proceeded in at least two pulses over a period of around 5 Ma. The first pulse has a molybdenite Re-Os weighted mean age of 238 ± 2 Ma, and the second dates at 236 ± 1 Ma. These dates are coeval with those of G2 and G3, indicating a genetic association between them and the mineralisation.The Sr-Nd-Hf isotopic systematics shows that the Bilugangan Granite was probably formed by partial melting of at least two sources. The first source is a dominant Late Mesoproterozoic to Early Carboniferous juvenile lower crust indicated by the whole rock εNd(t) values of −1.9 to 5.1 and zircon εHf(t) values of 3.5 to 14.3 with two-stage depleted mantle Nd (TDM2) and Hf (tDM2) model ages of ca. 1086 to 350 Ma, which is possibly related to the evolution of the Palaeo-Asian Ocean. The second source includes Neoarchaean to Palaeoproterozoic crustal components and a subordinate Mesoproterozoic crustal component. The Neoarchaean to Palaeoproterozoic component is indicated by the presence of ca. 2700 to 2600 Ma xenocrystic zircons, negative εHf(t) values of −16.4 to −10.1 with tDM2 ages of ca. 2300 to 1900 Ma. The Mesoproterozoic component yields εHf(t) values of −4.5 to 1.1 and tDM2 ages of ca. 1600 to 1200 Ma. These data show that pre- Mesoproterozoic substrate is present beneath the southern Great Xing'an Range. Although the post-mineralisation G4 and G5 phases have comparable positive εHf(t) values with the earlier granitic phases, the G2 and G3 phases have a more complex source with distinctly negative εHf(t) values of −16.4 to –1.3, indicating that the older crustal components in the magma’s source are also probable sources for the Mo mineralisation.