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Understanding the formation of economically important porphyry-Cu-Au deposits requires the knowledge of the magmaticto-hydrothermal processes that act within the much larger underlying magmatic system and the timescales on which they occur. We apply high-precision zircon geochronology (CA-ID-TIMS) and spatially resolved zircon geochemistry (LA-ICP-MS) to constrain the magmatic evolution of the underlying magma reservoir at the Pliocene Batu Hijau porphyry-Cu-Au deposit. We 15 then use this extensive dataset to assess the accuracy and precision of different U-Pb dating methods of the same zircon crystals. Emplacement of the oldest preto syn-ore tonalite (3.736 ± 0.023 Ma) and the youngest tonalite porphyry cutting economic Cu-Au mineralisation (3.646 ± 0.022 Ma) is determined by the youngest zircon grain from each sample, which constrains the duration of metal precipitation to less than 90 ± 32 kyr. Overlapping spectra of single zircon crystallisation ages and their trace element distributions from the pre-, syn and post-ore tonalite porphyries reveal protracted zircon crystallisation together with 20 apatite and plagioclase within the same magma reservoir over >300 kyr. The presented petrochronological data constrains a protracted early >200 kyr interval of melt differentiation and cooling within a large heterogeneous magma reservoir leading up to ore formation, followed by magma storage in a highly crystalline state and chemical and thermal stability over several 10s of kyr during which fluid expulsion formed the ore deposit. Irregular trace element systematics suggest magma recharge or underplating during this final short time interval. 25 The comparison of high precision CA-ID-TIMS results with in-situ LA-ICP-MS and SHRIMP U-Pb geochronology data from the same zircon grains allows a comparison of the applicability of each technique as a tool to constrain dates and rates on different geological timescales. All techniques provide accurate dates but with variable different precision. Highly precise dates derived by the calculation of the weighted mean and standard error of the mean of zircon dates obtained by in-situ techniques can lead to ages of unclear geological significance that are older than the maximum ages of emplacement given by 30 the CA-ID-TIMS ages of the youngest zircons in each sample. significantly older suggested emplacement ages than those determined by high-precision CA-ID-TIMS geochronology. This lack in accuracy of the weighted means is due to the protracted nature of zircon crystallisation in upper crustal magma reservoirs, suggesting that standard errors should not be used

Resolving the timescales of magmatic and hydrothermal processes associated with porphyry deposit formation using zircon U–Pb petrochronology