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.

The prevailing view of the formation of porphyry copper deposits along convergent plate boundaries involves deep crustal differentiation of metal-bearing juvenile magmas derived from the mantle wedge above a subduction zone. However, many major porphyry districts formed during periods of flat-slab subduction when the mantle wedge would have been reduced or absent, leaving the source of the ore-forming magmas unclear. Here we use geochronology and thermobarometry to investigate deep crustal processes during the genesis of the Late Cretaceous-Palaeocene Laramide Porphyry Province in Arizona, which formed during flat-slab subduction of the Farallon Plate beneath North America. We show that the isotopic signatures of Laramide granitic rocks are consistent with a Proterozoic crustal source that was potentially pre-enriched in copper. This source underwent water-fluxed melting between 73 and 60 Ma, coincident with the peak of granitic magmatism (78-50 Ma), porphyry genesis (73-56 Ma) and flat-slab subduction (70-40 Ma). To explain the formation of the Laramide Porphyry Province, we propose that volatiles derived from the leading edge of the Farallon flat slab promoted melting of both mafic and felsic pre-enriched lower crust, without requiring extensive magmatic or metallogenic input from the mantle wedge. Other convergent plate boundaries with flat-slab regimes may undergo a similar mechanism of volatile-mediated lower-crustal melting. Laramide flat-slab subduction releases fluids into the overlying crust that mediate water-fluxed melting of precursor arc lower crust, ultimately forming porphyry copper deposits, according to a geochronology and thermobarometry study.

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.

Igneous and hydrothermal mineral chemistry is commonly utilized in the exploration for porphyry deposits. Despite its occurrence in igneous and alteration mineral assemblages, routine analysis of titanite in mineral exploration is rare. We present whole-rock and titanite chemistry from the giant Cobre Panama porphyry Cu district to distinguish between primary igneous and replacement titanite. We show that igneous titanite is characterized by high Fe/Al ratios, REE, Zr and Mn concentrations, whereas F and Sc are enriched in replacement titanite. These distinctions can be explained by differences in mineral-melt partitioning, mobility in hydrothermal fluids, or by inheritance from precursor igneous minerals. Furthermore, REE patterns of igneous titanite reflect whole-rock chemistry, with discrepancies in MREEs possibly indicating that significant fractionation of amphibole has occurred prior to titanite saturation.

Increases in the magnitude of positive Ce anomalies in zircons from igneous rocks are often interpreted to be controlled by progressive melt oxidation and have been used to provide evidence for the elevated redox state of magmas associated with porphyry Cu deposits. In this paper, we test this idea by comparing trace element compositions of zircons from the Resolution porphyry Cu-Mo deposit, Arizona, with numerical models of melt Ce systematics and zircon-melt trace element partitioning. We show that although Ce anomalies in Resolution zircons (estimated by the chondrite-normalised ratio of Ce and Nd) may increase by over an order of magnitude throughout the period of zircon crystallisation, oxybarometric estimates in fact indicate a constant melt redox during this time. We employ a Monte Carlo approach to model the evolution of the Ce anomaly in zircon as a function of temperature, fO2, and melt composition, and compare our model against literature zircon data from chemically well-constrained volcanic rocks. We find that large increases in the magnitude of the Ce anomaly can be reproduced by cooling at fixed oxidation state and that this effect is magnified by increasing the melt Ce/Nd ratio, which can be driven by the co-crystallisation of amphibole, apatite and especially titanite. Increases in melt oxidation state are not sufficient to explain high positive Ce anomalies in zircons from some hydrous, oxidised volcanic and hypabyssal rocks, which additionally require a combination of titanite co-crystallisation and low crystallisation temperature. We therefore caution against the interpretation of zircon Ce anomalies solely in the context of melt fO2 variation.

Apatite tracks melt evolution and metallogenic fertility in arc magmas Chetan L. Nathwani , Matthew A. Loader, Jamie J. Wilkinson , Yannick Buret, Robert H. Sievwright, and Pete Hollings 1 Department of Earth Science and Engineering, Imperial College London, Exhibition Road, South Kensington Campus, London, SW7 2AZ, UK 2 London Centre for Ore Deposits and Exploration (LODE), Department of Earth Sciences, Natural History Museum, Cromwell Road, South Kensington, London, SW7 5BD, UK Geology Department, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5EL, Canada

The chemical diversity observed in the rock record of volcanic arcs is determined by a multitude of processes operating between the magma source region and the surface. A fundamental step in producing this variability is fractional crystallisation, assimilation and melting in the lower crust which drives magmas to more evolved and hydrous compositions. During extensive fractionation of hydrous magmas in the lower crust, amphibole (± garnet) is stabilized in the fractionating assemblage and plagioclase is suppressed resulting in melts with elevated Sr, an absence of strong negative Eu anomalies (both elements being compatible in plagioclase), and depleted Y (compatible in amphibole and garnet). The high Sr/Y values that result can be used to provide insights into arc magma evolution, evaluate whether a magmatic system has the potential to form a porphyry-related ore deposit and track crustal thickness. However, this deep fractionation history may be obscured due to differentiation and mixing upon ascent to the shallow crust. Since arc rocks are a product of this multi-stage, polybaric process, unravelling the complete history of magmatic evolution using bulk-rock chemistry alone can be challenging. However, accessory minerals such as apatite, are capable of capturing discrete periods of melt evolution during differentiation [1]. For example, apatite has been shown to record the Sr content of the melt at the time of its crystallization which has been used to reconstruct host rock compositions in provenance studies [2, 3]. Here, we use a novel approach to track the petrogenesis of arc magmas using apatite trace element chemistry in volcanic formations from the Cenozoic arc of Central Chile. These rocks formed during magmatism that culminated in high Sr/Y magmas and porphyry ore deposit formation in the Miocene. We use Sr/Y, Eu/Eu* and Mg in apatite to demonstrate that apatite tracks the multi-stage differentiation of arc magmas. We apply fractional crystallization modelling to show that early crystallizing apatite inherits a high Sr/Y and Eu/Eu* melt chemistry signature that is predetermined by amphibole-dominated fractional crystallization in the lower crust. Our modelling shows that crystallisation of the in-situ host rock mineral assemblage in the shallow crust causes competition for trace elements in the melt that leads to apatite compositions diverging from bulk magma chemistry. Understanding this decoupling behaviour is important for the use of apatite as an indicator of metallogenic fertility in arcs and for interpretation of provenance in detrital studies. We suggest our approach is widely applicable for unravelling the composite evolution of arc magmas and studying magmatic processes conducive to porphyry ore deposit formation. References [1] Miles, A.J., Graham, C.M., Hawkesworth, C.J., Gillespie, M.R., and Hinton, R.W., 2013, Evidence for distinct stages of magma history recorded by the compositions of accessory apatite and zircon: Contributions to Mineralogy and Petrology. [2] Jennings, E.S., Marschall, H.R., Hawkesworth, C.J., and Storey, C.D., 2011, Characterization of magma from inclusions in zircon: Apatite and biotite work well, feldspar less so: Geology. [3] Bruand, E., Storey, C., and Fowler, M., 2016, An apatite for progress: Inclusions in zircon and titanite constrain petrogenesis and provenance: Geology, v. 44, p. 91–94.

We present LA-ICP-MS and EPMA data for hydrothermal apatites from the Oyu Tolgoi porphyry Cu-Au system in southern Mongolia. The Oyu Tolgoi cluster comprises several deposits, including the lower-grade Heruga deposit (0.48% Cu) and the higher-grade Hugo Dummett deposit (1.18% Cu). Hydrothermal apatites were readily distinguished texturally from igneous apatite as they are intergrown with alteration phases including sulphides. The main difference between hydrothermal apatites from Heruga and Hugo Dummett is the concentrations of Mn and Fe, which are significantly enriched at Hugo Dummett (up to 1.5 wt. % Mn, and 0.6 wt. % Fe). These divalent cations and others (Mg, Pb) show positive correlations with apatite Cl content, suggesting a link between apatite metal contents and high fluid salinity, which have been shown to transport higher concentrations of Mn, Fe, and also Cu. We therefore suggest that the locally high Cu contents in rocks from Oyu Tolgoi, in particular at Hugo Dummett, may be explained by deposition from particularly saline fluids, which also produced Mn- and Fe-rich apatite. We conclude that hydrothermal apatite from potassic alteration zones reflects the chemistry of fluids associated with porphyry mineralisation, and may be used to assess the mineralisation potential of such systems.