AbstractArc magmas have higher water contents (2-6 wt.% H2O) than magmas generated in other tectonic environments, with a growing body of evidence suggesting that some deep arc magmas may be ‘super-wet’ (>6 wt.% H2O). Here, we use thermodynamic modelling to show that the behaviour of zirconium during magmatic differentiation is strongly sensitive to melt water contents. We demonstrate that super-wet magmas crystallise zircon with low, homogeneous titanium concentrations (75th percentile <10 ppm) due to a decrease in zircon saturation temperatures with increasing melt H2O. We find that zircon titanium concentrations record a transition to super-wet magmatism in Central Chile immediately before the formation of the world’s largest porphyry copper deposit cluster at Río Blanco-Los Bronces. Broader analysis shows that low, homogeneous zircon titanium concentrations are present in many magmatic systems. Our study suggests that super-wet magmas are more common than previously envisaged and are fundamental to porphyry copper deposit mineralisation.

Subduction zone magmatism is a major control of volcanism, the generation of modern continental crust and the formation of economically important porphyry Cu-(Mo-Au) deposits. Reading the magmatic record of individual arc segments and constraining the rates of magmatic changes are critical in order to fully understand and quantify the processes that drive magma evolution in subduction settings during arc growth. This study focuses on the San Francisco Batholith and the Rio Blanco-Los Bronces porphyry deposit cluster in central Chile, which provides an igneous rock record over similar to 13.5 Myr of arc evolution. We use whole-rock geochemistry, zircon geochronology and Hf isotope geochemistry to track changes in the crustal magmatic system of this arc segment during crustal thickening and porphyry Cu deposit formation. By combining the analytical dataset with Monte Carlo fractional crystallisation and assimilation fractional crystallisation modelling, we test a model for significant crustal involvement during magma evolution. Systematic and continuous increases in Dy/Yb, La/Yb, V/Sc and Sr/Y in the magmas over time indicate a transition in the main fractionation assemblage from plagioclase-dominated to amphibole-dominated that reflects deeper crystallisation and/or a higher meltwater content. Concomitant decreases in epsilon Hf and Th/La as well as increasing Ba/Th are best explained by assimilation of progressively deeper crustal lithologies from low (Chilenia) to high Ba/Th (Cuyania) basement terranes. Our study highlights that an increasingly hydrous magma and a deepening locus of crustal magma differentiation and assimilation, driven by crustal thickening contemporaneous with increased tectonic convergence and ingression of the aseismic Juan Fernandez ridge, can account for all investigated aspects of the multi-Myr magmatic evolution leading up to the formation of the Rio Blanco-Los Bronces porphyry Cu deposits. Our findings corroborate the importance of high-pressure differentiation of hydrous magma for the formation of Andean-style porphyry deposits. Once magmas favourable for porphyry Cu mineralisation were generated in the lower crust, multiple episodes of efficient magma migration into the upper crust fed several, discrete, shallow magmatic-hydrothermal systems over similar to 3.5 Myr to form the world's largest known Cu resource at Rio Blanco-Los Bronces.

Volatile saturation influences the physicochemical behavior of magmas and is essential for the sequestration of metals in porphyry copper deposits. Tracking the evolution of volatile components (F, Cl, H2O, S) in arc systems is complicated by their mobility and tendency to rapidly re-equilibrate with late-stage melts. We demonstrate that accurate measurements of volatile concentrations in apatite offer a reliable method for identifying the occurrence of volatile saturation. Fluorine, Cl, S, and calculated OH concentrations in apatite obtained by scanning electron microscope-energy-dispersive X-ray spectroscopy and electron microprobe analysis were used to compare two end-member volcanic systems in the West Luzon Arc (Philippines): Pinatubo (a fluid-saturated analogue for porphyry copper deposits) and Taal (a barren and fluid-undersaturated comparator). Apatites from Pinatubo are S-rich (0.04-0.64 wt%) and show a progressive decrease in X-Cl/X-OH (0.6-0.25) and an increase in X-F/X-Cl (1.5-8) and X-F/X-OH (0.75-1.2) during crystallization. Modeling indicates that these changes result from efficient partitioning of Cl into a continuously saturated H2O-rich fluid, while high regions of S in apatite reflect episodic flushing by a separate S-rich flux. Little S is evident in apatites from Taal (<300 ppm), which show increasing X-Cl/X-OH and X-F/X-OH together with constant X-F/X-Cl during crystallization. This cannot be explained using an H2O-saturated model, and instead reflects fluid-undersaturated crystallization and cooling in a reduced and/or S-depleted system. Measured volatiles in apatite therefore effectively discriminate volatile-saturated and undersaturated magmatic systems, providing an important 'fertility' filter for porphyry exploration.

Although the evolution of residual melts in magmatic systems controls their eruptability and ore-forming potential, their compositions are obscured in plutonic rocks by a protracted near-solidus evolution and the absence of interstitial glass. Here, we trace the evolution of residual melt compositions in rocks from the Strontian Intrusive Complex, Scotland, using the trace element chemistry of amphiboles, and titanites which are intergrown with amphibole rims. Laser ablation mapping reveals an abrupt change in certain trace elements in the amphibole rims, with sharp increases in Eu/Eu* and Sr/Y, and decreases in rare earth elements, Ta, Nb, and Ta/Nb ratios. Core-rim variations in these elements in titanite show the same variations as in amphibole, but are more gradual. By reconstructing the crystallisation sequence of the Strontian magmas using textural observations and thermobarometric estimates, we determine that amphibole cores crystallised prior to titanite saturation, but amphibole rims crystallised simultaneously with titanite. Using the trace element composition of the mineral phases and their modal abundance in the rock, with comparison to the whole-rock chemistry, we determine that titanite hosts the majority of the rare earth and high field strength element budget of the rocks. We therefore propose that the onset of titanite crystallisation had a profound effect on the trace element composition of late-stage residual melts at Strontian, which were inherited by the amphibole rims and subsequent titanites. This is supported by Rayleigh fractional crystallisation modelling, which demonstrates that the composition of amphibole rims cannot be explained without the influence of titanite. We therefore show that the saturation of trace element-rich phases in magmas represents a significant geochemical event in the petrogenesis of intermediate to silicic magmas. This has implications for provenance studies that attempt to reconstruct bulk rock compositions from mineral compositions, as the residual melts from which those minerals crystallise can be driven to significantly different compositions from the host magma by late-stage accessory phase crystallisation.

EDITORIAL article Front. Earth Sci., 29 August 2023Sec. Petrology Volume 11 - 2023 | https://doi.org/10.3389/feart.2023.1279960

The key processes responsible for the genesis of world-class porphyry copper deposits remain ambiguous. A high water content of parental magmas evolved in the deep crust is posited to be one key control on their formation, yet remains controversial. Here, we report trace element compositions of whole-rocks and zircons that pre-, syn- and post-date mineralisation from several major porphyry copper districts. We find clear temporal trends of decreasing zircon Ti and whole-rock Zr. Across all studied deposits and literature data, we find low zircon Ti and whole-rock Zr concentrations are inherent to intrusive rocks associated with porphyry Cu deposits. According to the Ti-in-zircon thermometer and zircon solubility models, these concentrations are consistent with porphyry Cu-related magmas being cold relative to other intermediate-felsic magmas (<800 degrees C). We perform thermodynamic modelling to demonstrate that such low temperature magma differentiation can be readily explained by elevated water concentrations (>6 wt.%) of porphyry Cu magmas at depth which displaces the liquidus to lower temperatures. By integrating this with the latest zircon solubility models, we show that wet magma differentiation leads to zircon appearing as a relatively early crystallising phase at low temperature. Our study therefore provides new evidence that magmas related to porphyry Cu deposits contain characteristically high water contents (>6 wt.%).

Large volume, intermediate-felsic magma reservoirs are the source of melt and mineralising fluids which generate porphyry copper deposits. Cooling and crystallisation of hydrous magmas drives the exsolution and expulsion of a magmatic volatile phase—a process which remains challenging to constrain in porphyry Cu systems where the record of magma volatile compositions is rarely preserved. Here, we use the halogen compositions of apatite inclusions shielded as inclusions within zircon to constrain volatile evolution in magma reservoirs which pre-date and are synchronous with porphyry Cu mineralisation at Quellaveco, Southern Peru. Geochemical and textural data confirm that the zircon-included apatites escaped re-equilibration with hydrothermal fluids, unlike apatites found in the groundmass of the same rocks. We, therefore, recommend that future studies attempting to reconcile magmatic volatile budgets using apatite in porphyry Cu systems should focus on apatite inclusions in zircon. By combining the apatite inclusion data with numerical modelling, we find evidence that the magma reservoir sourcing porphyry Cu mineralisation remained fluid-saturated for the entire period recorded by apatite crystallisation. By contrast, the pre-mineralisation batholith shows more variable, potentially fluid-undersaturated behaviour. Our modelling suggests that in order to attain the porphyry melt volatile compositions inferred from apatite, the magma reservoir must have exsolved a large proportion of its volatile budget, consistent with having been held at high crystallinity (40–60% crystals). This crystallisation interval coincides with peak chlorine and copper extraction from intermediate-felsic magmas, and would have permitted efficient fluid migration and accumulation at the roof of the system. We suggest that the storage of large-volume, long-lived, crystal-rich magma reservoirs in magmatic arcs may be a critical step in generating world-class porphyry copper deposits.

Porphyry Cu ore deposits are a rare product of arc magmatism that often form spatiotemporal clusters in magmatic arcs. The petrogenetic evolution of igneous rocks that cover the temporal window prior to and during porphyry Cu deposit formation may provide critical insights into magmatic processes that are key in generating these systems. This study documents the magmatic evolution of the Palaeocene-Eocene Yarabamba Batholith, Southern Peru, that was incrementally assembled between ~67 and ~59 Ma and hosts three, nearly contemporaneous, giant porphyry Cu-Mo deposits that formed at 57-54 Ma (Quellaveco, Toquepala and Cuajone). Whole-rock geochemistry, U-Pb geochronology and zircon trace element chemistry are reported from Yarabamba rocks that span the duration of plutonic activity, and from six porphyry intrusions at Quellaveco that bracket mineralisation. A change in whole-rock chemistry in Yarabamba intrusive rocks to high Sr/Y, high La/Yb and high Eu/Eu* is observed at ~60 Ma which is broadly coincident with a change in vector of the converging Nazca plate and the onset of regional compression and crustal thickening during the first stage of the Incaic orogeny. The geochemical changes are interpreted to reflect a deepening of the locus of lower crustal magma evolution in which amphibole ± garnet are stabilised as early and abundant fractionating phases and plagioclase is suppressed. Zircons in these rocks show a marked change towards higher Eu/Eu* (>0.3) and lower Ti (<9 ppm) compositions after ~60 Ma. Numerical modelling of melt Eu systematics and zircon-melt partitioning indicates that the time series of zircon Eu/Eu* in these rocks can be explained by a transition from shallower, plagioclase-dominated fractionation to high-pressure amphibole-dominated fractionation at deep crustal levels from ~60 Ma. Our modelling suggests that any redox effects on zircon Eu/Eu* are subordinate compared to changes in melt composition controlled by the fractionating mineral assemblage. We suggest that growth and intermittent recharge of the lower crustal magma reservoir from ~60 Ma produced a significant volume of hydrous and metallogenically fertile residual melt which ascended to the upper crust and eventually generated the three giant porphyry Cu-Mo deposits at Quellaveco, Toquepala and Cuajone from ~57 Ma. Our study highlights the importance of high-pressure magma differentiation fostered by strongly compressive tectonic regimes in generating world-class porphyry Cu deposits.

Volatile-rich intermediate to silicic magmatic systems can feed devastating volcanic eruptions but also generate valuable magmatic-hydrothermal ore deposits that supply most of the world's copper. Understanding the geometry, dynamics and timescales of these magmatic systems is critical in developing models for predicting the occurrence of ore deposits and future large volcanic eruptions. Here, we use zircon petrochronology from an equigranular monzonite and successively emplaced porphyry dykes to reconstruct the time - temperature - composition evolution of the magma that sourced the giant Cu-MoAu deposit at Bingham Canyon (USA). Combining high-precision CA-ID-TIMS geochronology with in-situ trace element analyses by LA-ICP-MS shows the intra-grain, inter-sample and temporal geochemical changes recording the evolution of the magmatic system over 817 +/- 62 kyr. Systematic variation of zircon chemistry with time indicates crystallisation from a coherent magma reservoir. After reservoir assembly its thermal and chemical state was controlled by protracted monotonous crystallisation over similar to 650 kyr with rapid cooling over the first 200 kyr followed by a longer period approaching the granite solidus. Porphyry Cu-Au ore formation occurred after the early drop in magma temperature that resulted in large-scale fluid saturation and expulsion into the sub-volcanic environment but main Momineralisation occurred after protracted low-temperature magma storage and the emplacement of the last porphyry. Zircons do not quantify the depth of this reservoir but integrating independent geophysical evidence with 2-D thermal modelling indicates that the time - temperature evolution recorded by the zircons is consistent with rapid incremental assembly of this large pluton (magma emplacement rate greater than or similar to 0.0065km3/yr) by initially zircon-undersaturated monzonitic magma into pre-heated upper crust. Our results indicate that massive fluid expulsion from rapidly-formed, large magma reservoirs containing mobile but mushy magma (>40 wt.% melt) can occur in the upper crust, favouring the formation of giant porphyry copper deposits. (C) 2021 The Author(s). Published by Elsevier B.V.

The crustal-scale magmatic systems of Andean-style arcs produce thick volcanic deposits and abundant plutons that are emplaced into the crust. They can also generate spatially- and temporally-restricted, economically-important porphyry Cu deposits. These deposits are formed at the magmatic-hydrothermal transition and require significant amounts of volatiles and metals to be concentrated in the sub-volcanic environment. Thus, understanding the magmatic and tectonic processes acting within an arc segment and their effect on the volatile budgets of crustal magmas could be essential for identifying the constraining factors controlling the potential of a magmatic system to produce a porphyry deposit. In this study we examine the magmatic evolution of the Rio Blanco-Los Bronces district, ~30 km northeast of Santiago, Chile, which is host to the Earth’s largest resource of Cu. Eocene to Early Miocene volcanic rocks were intruded by the Miocene San Francisco Batholith that, in turn, partially hosts intrusions related to the Late Miocene to Early Pliocene Rio Blanco-Los Bronces porphyry deposit cluster. We apply a combination of whole-rock, apatite and zircon geochemistry and zircon geochronology to the intrusive rock suite of the district to provide temporally- constrained geochemical information over the entire duration of batholith assembly and ore formation. U-Pb geochronology reveals incremental assembly of the San Francisco Batholith by individual magma batches over >14Myr (~18 – 4 Ma), with ore formation occurring in discrete pulses in the last 3 Myr before cessation of intrusive activity within the district. Progressive changes in the trace element chemistry indicate crustal thickening and deeper magma evolution within the arc segment as a result of the subduction of the Juan Fernandez ridge. A temporal shift to elevated SO3 and Cl contents is recorded by zircon-hosted apatite inclusions from the intrusions with highest values occurring in porphyry intrusions directly associated with the ore forming events. These data suggest variable volatile budgets of magmas during zircon crystallisation and hint at crustal-scale controls on the porphyry ore-forming potential of an arc segment.

Porphyry Cu ore deposits are a rare product of arc magmatism that often form spatiotemporal clusters in magmatic arcs. The petrogenetic evolution of igneous rocks that cover the temporal window prior to and during porphyry Cu deposit formation may provide critical insights into magmatic processes that are key in generating these systems. This study documents the magmatic evolution of the Palaeocene–Eocene Yarabamba Batholith, Southern Peru, that was incrementally assembled between ~ 67 and ~ 59 Ma and hosts three, nearly contemporaneous, giant porphyry Cu–Mo deposits that formed at 57–54 Ma (Quellaveco, Toquepala and Cuajone). Whole-rock geochemistry, U–Pb geochronology and zircon trace element chemistry are reported from Yarabamba rocks that span the duration of plutonic activity, and from six porphyry intrusions at Quellaveco that bracket mineralisation. A change in whole-rock chemistry in Yarabamba intrusive rocks to high Sr/Y, high La/Yb and high Eu/Eu* is observed at ~ 60 Ma which is broadly coincident with a change in vector of the converging Nazca plate and the onset of regional compression and crustal thickening during the first stage of the Incaic orogeny. The geochemical changes are interpreted to reflect a deepening of the locus of lower crustal magma evolution in which amphibole ± garnet are stabilised as early and abundant fractionating phases and plagioclase is suppressed. Zircons in these rocks show a marked change towards higher Eu/Eu* (> 0.3) and lower Ti (< 9 ppm) compositions after ~ 60 Ma. Numerical modelling of melt Eu systematics and zircon-melt partitioning indicates that the time series of zircon Eu/Eu* in these rocks can be explained by a transition from shallower, plagioclase-dominated fractionation to high-pressure amphibole-dominated fractionation at deep crustal levels from ~ 60 Ma. Our modelling suggests that any redox effects on zircon Eu/Eu* are subordinate compared to changes in melt composition controlled by the fractionating mineral assemblage. We suggest that growth and intermittent recharge of the lower crustal magma reservoir from ~ 60 Ma produced a significant volume of hydrous and metallogenically fertile residual melt which ascended to the upper crust and eventually generated the three giant porphyry Cu–Mo deposits at Quellaveco, Toquepala and Cuajone from ~ 57 Ma. Our study highlights the importance of high-pressure magma differentiation fostered by strongly compressive tectonic regimes in generating world-class porphyry Cu deposits.

Andean-style subduction zones produce a high variety of magma compositions over time and space. The diversity of magmas in these settings can be driven by the flux from the subducting slab, the melting behaviour of the overlying mantle wedge and crustal differentiation but the dominant process remains debated

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.

The crustal-scale magmatic systems of Andean-style subduction zones produce thick volcanic deposits and abundant plutons emplaced into the upper crust. They can also result in the formation of spatially- and temporally-restricted, economically-important porphyry Cu deposits. Understanding the magmatic and tectonic processes acting within an arc segment, including changes in the fractionating assemblage, subduction angle, chemistry of slab-derived melts or water content, is essential to develop and refine quantitative models for the formation of these deposits. Specific geochemical signatures (e.g. elevated Sr/Y) are associated with magmas that source the metals and volatiles to form porphyry deposits based on empirical studies. However, it is unclear whether this geochemical signature is the result of geologically rapid processes resulting in sudden shifts in magma chemistry or whether they are the result of protracted changes within the crustal-scale magmatic system over extended timescales. In this study we examine the magmatic evolution of the Rio Blanco-Los Bronces district, ~30 km northeast of Santiago, Chile, which is host to the Earth’s largest resource of Cu. Eocene to Early Miocene volcanic rocks were intruded by the Miocene San Francisco Batholith that, in turn, partially hosts intrusions related to the Late Miocene to Early Pliocene Rio Blanco-Los Bronces porphyry deposit cluster. We apply a combination of whole-rock and zircon geochemistry, isotopic tracing and LA-ICP-MS U-Pb geochronology to the intrusive rock suite of the district to provide temporally- constrained geochemical information over the entire duration of batholith assembly and ore formation. U-Pb geochronology reveals incremental assembly of the San Francisco Batholith by individual magma batches over >13Myr (~17 – 4 Ma), with ore formation occurring in discrete pulses in the last 3 Myr before cessation of intrusive activity within the district. Temporally-resolved whole-rock major element chemistry shows that the progressively-emplaced magmas were not sourced from a common, continuously differentiating, lower crustal magma reservoir. Evolving trace element signatures over the recorded timescale indicate that magmas were sourced from progressively deeper fractional crystallisation reservoir(s) that exhibited increasing water contents. The geochemical evolution recorded over the entire investigated 13 Myr timescale could reflect geodynamic changes linked to the ingression of the subducting Juan Fernandez ridge from the north. However, within this continuous evolution, the most prominent geochemical shifts occur over a much shorter timescale of a few Myr, directly preceding economic ore-formation, implicating an additional mechanism for controlling the metallogenic potential of the magma source.

Enhancing our understanding of the processes that lead to the formation of fertile magmas is key to the identification of robust new fertility proxies indicative of porphyry Cu deposits. Once the essential magmatic and hydrothermal processes have been identified, their resulting characteristic geochemical signatures can provide a fast and cost-efficient mean to distinguish between potentially fertile or barren igneous suites. We aim to utilise whole rock and mineral chemistry data from the spatially extensive intrusive suite that hosts the Rio Blanco-Los Bronces deposit cluster to reconstruct the magmatic evolution leading up to the formation of Earth's largest known porphyry Cu system. The associated intrusive complex was emplaced over more than 10 My of barren and ultimately fertile magmatism, making it an ideal site to develop and test fertility indicators. Temporally resolved whole-rock data reveals the composite nature of the intrusive complex suggested incremental assembly from potentially different source magmas. Next steps include the application integrated accessory mineral petrochronology to reconstruct processes such as emplacement rates and the injection of mafic melts. The influence of these factors on the fertility of the system will be evaluated and potentially useful geochemical signatures in pristine magmatic accessory minerals will be identified.