SHRIMP U-Pb DATING ON HYDROTHERMAL ZIRCONS: EVIDENCE FOR AN EARLY CRETACEOUS EPITHERMAL EVENT IN THE MIDDLE JURASSIC DEXING PORPHYRY COPPER DEPOSIT, SOUTHEAST CHINA

Abstract

The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China (1,500 million tonnes [Mt] of ore at 0.43% Cu, 0.02% Mo, and 0.16 grams per tonne Au, approximately equivalent to 6.45 Mt Cu, 0.25 Mt Mo, and 24 tonnes Au). We report results of sensitive high resolution ion microprobe (SHRIMP) U-Pb dating on zircons separated from a thick (~20-cm-wide) pyrite-chalcopyrite-quartz vein in this deposit, which is mineralogically and texturally different from the porphyry-related ore-bearing thin quartz veins. The zircons from the thick quartz vein are unzoned and nonluminescent (black) in the cathodoluminescence images. They show relatively flat light rare-earth element patterns and have high La content and low (Sm/La)N and Ce/Ce* values. These features are typical of hydrothermal zircons. SHRIMP U-Pb dating on these hydrothermal zircons yielded weighted mean 206Pb/238U ages of 106.5 ± 0.9 and 100.9 ± 3.5 Ma, which are ca. 70 m.y. younger than the ages (172–171 Ma) of the ore-bearing porphyries and the molybdenite Re-Os ages (170.9 ± 1.1 Ma) of the porphyry-related disseminated and veinlet (usually <1 cm in width) ores. This suggests the occurrence of a younger, distinct Cu (Au?) hydrothermal event superimposed on the porphyry Cu-Mo-Au mineralization at Dexing. The mineralogical features and age of this hydrothermal event suggest that it could belong to the metallogenic event responsible for the formation of the late Early Cretaceous to early Late Cretaceous (110–95 Ma) epithermal Cu-Au-Ag deposits in southeast China.