Abstract

Iron oxide minerals such as hematite and magnetite can provide valuable information on ore genesis. Uraniumbearing hematite has been proposed as a potential Pb-Pb chronometer for iron oxide copper-gold (IOCG) systems and allows for the direct dating of mineralization in oxide-rich hydrothermal deposits for which other suitable dating minerals are not available. For young samples, however, the reliability of hematite U-Pb geochronology is unclear due to the lack of matrix-matched reference materials and the compromise of high common Pb components. Here, we present the first application of hematite laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating integrated with monazite secondary ion mass spectrometry (SIMS) U-Th-Pb dating in the Cenozoic Yaoan gold deposit in southwest China.

The Yaoan deposit in the Jinshajiang-Red River metallogenic province is associated with a syenite porphyry and characterized by abundant specular hematite-rich veins. In situ SIMS U-Th-Pb dating on hydrothermal monazite, coprecipitated with the gold-hosting hematite, yielded a 208Pb/232Th age of 31.7 ± 0.7 Ma (2σ, n = 22), supporting a genetic link with the syenite porphyry (32.8 ± 0.3 Ma; 2σ, n = 25). The hematite in ore veinlets around the syenite porphyry is characterized by oscillatory/sectorial zoning, with variable U contents (up to 101 ppm). The use of LA-ICP-MS line-raster ablation minimized the matrix sensitivity during hematite U-Pb dating, although a nonmatrix-matched external standard was employed. Using a 207Pb-based correction, a weighted mean 206Pb/238U age of 30.2 ± 3.8 Ma (2σ, n = 46) was obtained. This date, within error of the monazite and zircon results, is robust and represents the age of gold mineralization at Yaoan.

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