Abstract:

Gangjiang coppermolybdenum deposit is located in the Nimu ore field in the middle section of the Gangdese porphyry copper belt. It is a typical porphyrytype deposit discovered in recent years, with more than half million tons of contained copper and molybdenum resources. The Gangjiang complex is composed of biotite quartz monzonite, quartz monzonite porphyry, rhyodacite and dacite. There are many types of hydrothermal alteration including patassic, silicific, phyllic, propylitic, chloritic and argillic alterations. There are obvious potassic and phyllic alteration zones from the central to the outward part of the complex. The copper and molybdenum mineralization occurs mainly in the overlapped part of the potassic and phyllic alteration zones. The supergene oxidation enrichment zone is developed in the top part of the ore bodies. In this paper, the zircon UPb dating method by secondary ion microprobe mass spectrometer (SIMS) is applied to study the intrusion ages of the main orebearing porphyries. The zircon UPb dating ages of the biotite quartz monzonite and rhyodacite are (14.73±0.13) Ma (MSWD=1.3, N=16), and (12.01±0.29) Ma (MSWD=2.3, N=8), respectively. They are in accordance with the ages of orebearing porphyry from other porphyry coppermolybdenum deposits in the Nimu ore field, so the Gangjiang porphyry coppermolybdenum deposit was formed in the extensional tectonic setting posterior to the collision between the Indian and Eurasian plates. On account of the missing of large propylitic alteration zone and the exposure of the main potassic zone, it is inferred that the depth of erosion in the Gangjiang area could be 23 km. This is similar to the estimation based on the published denudation rate (013023 mm/a) of the Tibetan Plateau.

Key words: ore deposit, SIMS zircon UPb age, denudation, Gangjiang porphyry coppermolybdenum deposit, Nimu, Tibet