19 research outputs found
Zircon petrochronological evidence for a plutonic-volcanic connection in porphyry copper deposits
ISSN:0091-7613ISSN:1943-268
Resolving the timescales of magmatic and hydrothermal processes associated with porphyry deposit formation using zircon UāPb petrochronology
Understanding the formation of economically important porphyry CuāAu deposits requires knowledge of the magmatic-to-hydrothermal processes that act within the much larger magmatic system and the timescales on which they occur. We apply high-precision zircon geochronology (chemical abrasionāisotope dilutionāthermal ionisation mass spectrometry; CAāIDāTIMS) and spatially resolved zircon geochemistry (laser ablation inductively coupled plasma mass spectrometry; LA-ICP-MS) to constrain the magmatic evolution of the underlying magma reservoir at the Pliocene Batu Hijau porphyry CuāAu deposit. We then use this extensive dataset to assess the accuracy and precision of different UāPb dating methods of the same zircon crystals.
Emplacement of the oldest pre- to syn-ore tonalite (3.736Ā±0.023āMa) and the youngest tonalite porphyry to cross-cut economic CuāAu mineralisation (3.646Ā±0.022āMa) is determined by the youngest zircon grain from each sample, which constrains the duration of metal precipitation to fewer than 90Ā±32ākyr. 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 apatite and plagioclase within the same magma reservoir over >300ākyr. The presented petrochronological data constrain a protracted early >200ākyr interval of melt differentiation and cooling within a large heterogeneous magma reservoir, followed by magma storage in a highly crystalline state and chemical and thermal stability over several tens of thousands of years during which fluid expulsion formed the ore deposit. Irregular trace element systematics suggest magma recharge or underplating during this final short time interval.
The comparison of high-precision CAāIDāTIMS results with in situ LA-ICP-MS and a sensitive high-resolution ion microprobe (SHRIMP) UāPb 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 different precision. Highly precise dates derived by the calculation of the weighted mean and standard error of the mean of the 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 the CAāIDāTIMS ages of the youngest zircons in each sample. This lack of 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 as a means to describe the uncertainty in those circumstances. We conclude from this and similar published studies that the succession of magma and fluid pulses forming a single porphyry deposit and similarly rapid geological events are too fast to be reliably resolved by in situ UāPb geochronology and that assessing the tempo of ore formation requires CAāIDāTIMS geochronology