The Big Gossan skarn (71 million tonnes of 2.4 wt % Cu and 0.9 ppm Au at a 1% Cu cutoff) is located in the prolific Ertsberg-Grasberg mining district, on the island of New Guinea in eastern Indonesia. The skarn formed within the steeply dipping southern limb of the Yellow Valley syncline near the conformable contact between the Cretaceous Ekmai limestone and Paleocene Waripi dolomitic limestone, adjacent to the western edge of the 3.1 to 2.8 Ma Ertsberg diorite. Andradite garnets from eight Big Gossan samples were directly dated using the laser ablation-inductively coupled plasma-mass spectrometry U-Pb method, and the age results show that Big Gossan formed between 2.9 and 2.7 Ma. To evaluate reproducibility, one sample was dated twice, four months apart, and gave overlapping ages of 2.75 ± 0.03 (n = 150 spots) and 2.73 ± 0.06 Ma (n = 50 spots) (lower intercept age, Tera-Wasserburg concordia). This precision was achievable due to the high U content (10–100 ppm) and single isotopic composition of common Pb in the Big Gossan garnets. Andradite garnet U-Pb ages are compatible with district-wide zircon U-Pb geochronology and a single 2.82 ± 0.04 Ma phlogopite 40Ar/39Ar age for the skarn. The new garnet ages confirm that Big Gossan was one of the last ore-forming events in the Ertsberg-Grasberg district. This study demonstrates that andradite garnet U-Pb chronometry can be a robust dating technique for constraining the timing of skarn-forming hydrothermal systems.

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