Element Migration during Alteration and ⁴⁰Ar/³⁹Ar Dating of Sericite from the Dongwodong Deposit, Tibet and Its Geological Significance

The Dongwodong copper polymetallic deposit is located in the southern margin of Qiangtang terrane, east of the Duolong copper gold ore cluster. The study on the timing of alteration and element migration during the alteration has not yet been conducted. To determine the time of alteration, ⁴⁰Ar-³⁹Ar isotopic dating was carried out for altered sericites closely related to beresitizate mineralization. The results show that the plateau age is 122.20±0.84 Ma, which is consistent with the age (122 Ma) of ore-bearing porphyry. Thus, there is a close link between the mineralization and granodiorite porphyry. Meanwhile, the geochemical results of weakly-altered and mineralized granodiorite were compared, using the isocon equation and the derivation equation to judge the inclusion or extraction of each element and the element migration during the alteration. Results show that HFSEs are very immobile during the alteration, whereas REEs migrate insignificantly with more obvious migration of LREEs than HREEs. The ore-forming elements (Cu, Pb, Zn) are inclusion elements. The intrusive age of ore-bearing phophyries and the timing of hydrothermal alteration of Dongwodong deposit are consistent with those of other large-superlarge scale copper-gold deposits (Duobuza, Bolong, Tegelongnan) in Duolong copper gold ore cluster, indicating that they were controlled by the same tectonic-magmatic event and thus a great potential of ore-prospecting in Dongwodong mining district can be predicted.