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Resolving the Timescales of Magmatic and Hydrothermal Processes Associated with Porphyry Deposit Formation Using Zircon U–Pb Petrochronology

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Resolving the Timescales of Magmatic and Hydrothermal Processes Associated with Porphyry Deposit Formation Using Zircon U–Pb Petrochronology Geochronology, 2, 209–230, 2020 https://doi.org/10.5194/gchron-2-209-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Resolving the timescales of magmatic and hydrothermal processes associated with porphyry deposit formation using zircon U–Pb petrochronology Simon J. E. Large1,a, Jörn-Frederik Wotzlaw1, Marcel Guillong1, Albrecht von Quadt1, and Christoph A. Heinrich1,2 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 Correspondence: Simon J. E. Large ([email protected]) Received: 7 February 2020 – Discussion started: 25 February 2020 Revised: 19 June 2020 – Accepted: 1 July 2020 – Published: 31 July 2020 Abstract. Understanding the formation of economically im- Irregular trace element systematics suggest magma recharge portant porphyry Cu–Au deposits requires knowledge of or underplating during this final short time interval. the magmatic-to-hydrothermal processes that act within the The comparison of high-precision CA–ID–TIMS results much larger magmatic system and the timescales on which with in situ LA-ICP-MS and a sensitive high-resolution ion they occur. We apply high-precision zircon geochronology microprobe (SHRIMP) U–Pb geochronology data from the (chemical abrasion–isotope dilution–thermal ionisation mass same zircon grains allows a comparison of the applicability spectrometry; CA–ID–TIMS) and spatially resolved zircon of each technique as a tool to constrain dates and rates on geochemistry (laser ablation inductively coupled plasma different geological timescales. All techniques provide accu- mass spectrometry; LA-ICP-MS) to constrain the magmatic rate dates but with different precision. Highly precise dates evolution of the underlying magma reservoir at the Pliocene derived by the calculation of the weighted mean and stan- Batu Hijau porphyry Cu–Au deposit. We then use this exten- dard error of the mean of the zircon dates obtained by in sive dataset to assess the accuracy and precision of different situ techniques can lead to ages of unclear geological signifi- U–Pb dating methods of the same zircon crystals. cance that are older than the maximum ages of emplacement Emplacement of the oldest pre- to syn-ore tonalite given by the CA–ID–TIMS ages of the youngest zircons in (3:736 ± 0:023 Ma) and the youngest tonalite porphyry each sample. This lack of accuracy of the weighted means is to cross-cut economic Cu–Au mineralisation (3:646 ± due to the protracted nature of zircon crystallisation in up- 0:022 Ma) is determined by the youngest zircon grain from per crustal magma reservoirs, suggesting that standard errors each sample, which constrains the duration of metal precipi- should not be used as a means to describe the uncertainty in tation to fewer than 90 ± 32 kyr. Overlapping spectra of sin- those circumstances. We conclude from this and similar pub- gle zircon crystallisation ages and their trace element distri- lished studies that the succession of magma and fluid pulses butions from the pre-, syn and post-ore tonalite porphyries forming a single porphyry deposit and similarly rapid geo- reveal protracted zircon crystallisation together with apatite logical events are too fast to be reliably resolved by in situ and plagioclase within the same magma reservoir over > U–Pb geochronology and that assessing the tempo of ore for- 300 kyr. The presented petrochronological data constrain a mation requires CA–ID–TIMS geochronology. protracted early > 200 kyr interval of melt differentiation and cooling within a large heterogeneous magma reservoir, fol- lowed 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. Published by Copernicus Publications on behalf of the European Geosciences Union. 210 S. J. E. Large et al.: Resolving processes associated with porphyry deposit formation 1 Introduction Recent advances in high-precision zircon geochronology by chemical abrasion–isotope dilution–thermal ionisation Zircon geochronology is widely applied to date geologi- mass spectrometry (CA–ID–TIMS; e.g. Mattinson, 2005; cal events and constrain timescales of geological processes. Bowring et al., 2011; McLean et al., 2011a, 2015; Condon et Combined with zircon geochemistry it has improved our un- al., 2015) now allow dating of the porphyritic intrusions asso- derstanding of crustal magmatic systems, such as those form- ciated with ore formation with unprecedented precision. The ing economically important magmatic–hydrothermal por- dramatically improved precision permits the constraint of phyry Cu–Au deposits. Advances in analytical techniques re- rapid events, such as individual porphyry emplacement and sulted in a shift from establishing the ages of magma em- hydrothermal mineralisation phases (< 100 kyr; von Quadt placement or crystallisation to resolving the durations of et al., 2011; Buret et al., 2016; Tapster et al., 2016), that magmatic and associated hydrothermal processes, such as typically occur at the end of a longer-term period of volcan- magma accumulation or recharge, fractional crystallisation, ism and intrusive magma emplacement extending over sev- or hydrothermal ore formation, and it has resulted in un- eral million years (e.g. Deino and Keith, 1997; Halter et al., precedented information about the mechanisms and scales of 2004; Maksaev et al., 2004; Rohrlach et al., 2005; Lee et al., magma ascent and storage in the earth’s crust (e.g. Vazquez 2017). The integration of the temporal and chemical informa- and Reid, 2004; Chamberlain et al., 2014; Barboni et al., tion gained from zircon is referred to as zircon petrochronol- 2016; Bucholz et al., 2017). ogy and can yield time-calibrated information about magma Porphyry copper deposits provide successively quenched chemistry, thermal evolution and crystallinity during zircon samples of magma extracted from large crustal-scale hydrous crystallisation in magmatic systems (e.g Schoene et al., 2012; magma systems. They are therefore a critical source of in- Chelle-Michou et al., 2014; Samperton et al., 2015; Buret et formation about the processes and rates of magma ascent, al., 2016; Szymanowski et al., 2017). magma storage and fluid generation, bridging those of vol- Timescales for magmatic and hydrothermal processes in- canism and pluton formation. The identification of the pro- volved in porphyry ore formation have been suggested based cesses that lead to porphyry deposit formation (e.g. Rohrlach on in situ U–Pb data (e.g. Garwin, 2000; Banik et al., 2017; et al., 2005; Audétat et al., 2008; Richards, 2013; Wilkinson, Lee et al., 2017) and CA–ID–TIMS geochronology, which 2013) and the timescales on which they operate can provide became increasingly precise (e.g. von Quadt et al., 2011; us with valuable information about arc magmatic processes Chelle-Michou et al., 2014; Buret et al., 2016; Tapster et al., but could also potentially help in discriminating possibly fer- 2016; Gilmer et al., 2017; Large et al., 2018). However, sev- tile magmatic systems from ubiquitous infertile systems re- eral studies applying multiple techniques to the same sam- sulting in barren intrusions or volcanic eruptions. ple sets have resulted in differing dates (von Quadt et al., Porphyry Cu–Au deposits commonly display clear field 2011; Chiaradia et al., 2013, 2014; Chelle-Michou et al., relationships of successive generations of porphyritic stocks 2014; Correa et al., 2016). The discrepancy demands a more or dikes, which were injected into sub-volcanic and other detailed understanding of the precision and accuracy of the upper-crustal rock sequences (Sillitoe, 2010). The injected techniques and statistical data treatment that are applied to porphyry magmas thus provide snapshots of the underly- derive a geological age. This is not only fundamental for re- ing, vertically and laterally extensive magma reservoirs (e.g. solving dates and rates of geological processes in ore deposit Dilles, 1987; Steinberger et al., 2013). Cross-cutting rela- research but provides more general insights into the geolog- tionships between veins and intrusive rocks suggest tem- ical meaning of magmatic dates and rates obtained by U–Pb poral overlap of hydrothermal alteration, ore mineralisation geochronology. and porphyry emplacement (Proffett, 2003; Seedorff and For the present paper, we obtained a large dataset of Einaudi, 2004; Redmond and Einaudi, 2010). Strong hy- zircon geochemistry and geochronology by laser abla- drothermal alteration of the intrusive rocks associated with tion inductively coupled plasma mass spectrometry (LA- ore formation severely disturbs the geochemical informa- ICP-MS) followed by high-precision geochronology of the tion of most minerals and whole-rock compositions. While same zircon crystals, utilising chemical abrasion–isotope providing important insights into the hydrothermal history dilution–thermal ionisation mass spectrometry (CA–ID– of a deposit (e.g. Roedder, 1971; Dilles and Einaudi, 1992; TIMS). These coupled data from the world-class Batu Hi- Landtwing et al., 2005; Cathles and Shannon, 2007; Seedorff jau porphyry Cu–Au deposit allow us to resolve the chemi- et al., 2008; Large et al., 2016), it limits the investigation of cal evolution and the changing
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