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Geochronology Advances in geochronological science
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https://doi.org/10.5194/gchron-2020-5
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gchron-2020-5
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 25 Feb 2020

Submitted as: research article | 25 Feb 2020

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This preprint is currently under review for the journal GChron.

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

Simon J. E. Large1,a, Jörn F. Wotzlaw1, Marcel Guillong1, Albrecht von Quadt1, and Christoph A. Heinrich1,2 Simon J. E. Large et al.
  • 1Department of Earth Sciences, Eidgenössische Technische Hochschule (ETH) Zurich, 8092 Zürich, Switzerland
  • 2Faculty of Mathematics and Natural Sciences, University of Zurich, 8006 Zürich, Switzerland
  • acurrent address: Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK

Abstract. Understanding the formation of economically important porphyry-Cu-Au deposits requires the knowledge of the magmatic-to-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 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 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 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. 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 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 with variable 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 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 as a mean to describe the uncertainty in those circumstances. Thus, geologically rapid events or processes or the tempo of magma evolution are too fast to be reliably resolved by in-situ U-Pb geochronology and require ID-TIMS geochronology.

Simon J. E. Large et al.

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Simon J. E. Large et al.

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Short summary
The integration of zircon geochemistry and U-Pb geochronology (petrochronology) allows us to further our understanding of magmatic processes and here we could reconstruct the ~ 300 kyr evolution of the magma reservoir that sourced the magmas, fluids and metals to form the Batu Hijau porphyry Cu-Au deposit. The application of in-situ LA-ICP-MS and high-precision CA-ID-TIMS geochronology to the same zircons further allowed an assessment of the strengths and limitations of the different techniques.
The integration of zircon geochemistry and U-Pb geochronology (petrochronology) allows us to...
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