Precise ages of gold mineralization and pre-gold hydrothermal activity in the Baiyun gold deposit, northeastern China: in situ U-Pb dating of hydrothermal xenotime and rutile

Timing of the major Baiyun gold deposit (similar to 70 t Au) in the northeastern North China Craton is controversial. A textural and geochronological study of high-grade gold ores identifies three generations of xenotime (Xtm-1, Xtm-2 and Xtm-3), even in single grains. The oldest Xtm-1, in grain cores, is of metamorphic origin, with very high U and U/Th ratios, and hump-shaped, HREE-enriched patterns with maximum normalized values at Yb and large negative Eu anomalies. It is dated at 1856 +/- 6 Ma, consistent with its crystallization during regional metamorphism related to cratonization. In contrast, 244 +/- 2 Ma Xtm-2 overgrowths are of hydrothermal origin, having low U and U/Th ratios and hump-shaped, MREE-enriched patterns with maximum normalized values at Ho and negligible Eu anomalies. Xtm-2 records a previously unrecognized Middle Triassic hydrothermal episode. Xtm-3, in equilibrium with native gold-hosting pyrite and rutile, is also hydrothermal. The Xtm-3 domains have similar U, U/Th ratios, and MREE-enriched patterns to Xtm-2, but a younger crystallization age of 231 +/- 1 Ma. Associated rutile grains that show close textural associations with auriferous pyrite and native gold have an in situ SIMS U-Pb an intercept age of 229 +/- 4 Ma, consistent with that of Xtm-3. Collectively, these reliable ages indicate that gold mineralization at Baiyun occurred at ca. 230 Ma, predating the emplacement of granite intrusions by more than 3 million years. Both the Middle Triassic hydrothermal event (similar to 244 Ma) and the Late Triassic gold mineralization event (similar to 230 Ma) lie within the temporal window of collisional tectonics in the host orogen. In combination, the new geochronology and fluid-inclusion and geological data support Bayun's classification as an orogenic gold deposit.

Automated summary

A textural and geochronological study of high-grade gold ores at the major Baiyun deposit in the northeastern North China Craton reveals three generations of xenotime and identifies the gold mineralization event at around 230 Ma, predating granite intrusions by 3 million years. This supports the classification of Baiyun as an orogenic gold deposit.