Resolving the timescales of magmatic and hydrothermal processes associated with porphyry deposit formation using zircon U–Pb petrochronology
Authors: Large Simon J. E. , Wotzlaw Jörn-Frederik , Guillong Marcel , von Quadt Albrecht , Heinrich Christoph A. 
Published: July 2020

Abstract. Understanding the formation of economically important porphyry Cu–Au
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 abrasion–isotope dilution–thermal ionisation mass spectrometry; CA–ID–TIMS) 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 Cu–Au deposit. We 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 pre- to syn-ore tonalite (3.736±0.023 Ma)
and the youngest tonalite porphyry to cross-cut 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 fewer
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
apatite and plagioclase within the same magma reservoir over >300 kyr. The presented petrochronological data constrain a protracted early
>200 kyr 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 CA–ID–TIMS results with in situ LA-ICP-MS
and a sensitive high-resolution ion microprobe (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 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 CA–ID–TIMS 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 U–Pb geochronology and that assessing the tempo of ore
formation requires CA–ID–TIMS geochronology.