The prevailing paradigm for 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 unclear the source region of the ore-forming magmas. To resolve this paradox, we investigate deep crustal processes during the genesis of the Laramide Porphyry Province of Arizona, which formed between 80–50 Ma during flat-slab subduction of the Farallon Plate beneath North America. We show that: (1) Laramide granitic rocks have isotopic signatures implying a crustal origin suggesting that they were derived from Proterozoic aged crust; and (2) Proterozoic crustal rocks were pre-enriched in copper and underwent water fluxed anatexis between 73–60 Ma at a geothermal gradient of ~28°C/km, coincident with the zenith of granitic magmatism, porphyry genesis (73–56 Ma) and flat-slab subduction (75–65 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 anatexis of mafic (garnet pyroxenite and amphibolite) and felsic pre-enriched lower crust, without necessarily requiring significant juvenile mantle-wedge derived magmatism.

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.

Generation of silicic magmas leads to emplacement of granite plutons, huge explosive volcanic eruptions and physical and chemical zoning of continental and arc crust1–7. Whereas timescales for silicic magma generation in the deep and middle crust are prolonged8, magma transfer into the upper crust followed by eruption is episodic and can be rapid9–12. Ages of inherited zircons and sanidines from four Miocene ignimbrites in the Central Andes indicate a gap of 4.6 Myr between initiation of pluton emplacement and onset of super-eruptions, with a 1-Myr cyclicity. We show that inherited zircons and sanidine crystals were stored at temperatures <470 °C in these plutons before incorporation in ignimbrite magmas. Our observations can be explained by silicic melt segregation in a middle-crustal hot zone with episodic melt ascent from an unstable layer at the top of the zone with a timescale governed by the rheology of the upper crust. After thermal incubation of growing plutons, large upper-crustal magma chambers can form in a few thousand years or less by dike transport from the hot-zone melt layer. Instability and disruption of earlier plutonic rock occurred in a few decades or less just before or during super-eruptions. Analysis of inherited zircons and sanidines from Miocene ignimbrites in the Central Andes shows that plutons were emplaced for up to 4 million years prior to onset of volcanism and that disruption of plutonic rock occurs a few decades or less just before or during super-eruptions.

Hypogene mineralization in porphyry Cu deposits is typically associated with crustal thickening and rapid exhumation, whereas supergene enrichment requires slow exhumation to allow sufficient time for leaching and downward transport of Cu before it is lost to surface erosion. Therefore, spatial and temporal patterns of exhumation within a metallogenic belt can highlight favorable locations for hypogene mineralization, supergene enrichment, and preservation. Here, we determine average pluton exhumation rates along an similar to 730-km segment of the middle Eocene-early Oligocene metallogenic belt in northern Chile (17.8 degrees-24.2 degrees S). By combining zircon U-Pb geochronology with Al-in-hornblende geobarometry, we pinpoint the time and depth at which each pluton was emplaced and use the age of overlying cover units or supergene minerals to date its arrival at the surface (or near-surface) environment. Uranium-lead zircon ages for 49 samples from plutons and porphyries range from Carboniferous to Eocene (similar to 314-35 Ma). Al-in-hornblende emplacement depths for 19 plutons are similar to 4-7 km, with one Carboniferous pluton emplaced at similar to 12 km. Two phases of net exhumation are identified: early Permian-Middle Triassic and middle Eocene-late Oligocene, with an intervening period of net burial. The highest exhumation rates (>0.30 km/m.y.) derive from the second phase, coeval with the Incaic orogeny and the main phase of hypogene mineralization. Present-day preservation of plutons and porphyry Cu deposits required low post-Oligocene average exhumation rates of =similar to 4 km at El Abra), by burial beneath significant cover (e.g., Ministro Hales, Queen Elizabeth), or, in the Inti region of northernmost Chile, by a combination of the two. Hypogene mineralization in porphyry Cu deposits is typically associated with crustal thickening and rapid exhumation, whereas supergene enrichment requires slow exhumation to allow sufficient time for leaching and downward transport of Cu before it is lost to surface erosion. Therefore, spatial and temporal patterns of exhumation within a metallogenic belt can highlight favorable locations for hypogene mineralization, supergene enrichment, and preservation. Here, we determine average pluton exhumation rates along an similar to 730-km segment of the middle Eocene-early Oligocene metallogenic belt in northern Chile (17.8 degrees-24.2 degrees S). By combining zircon U-Pb geochronology with Al-in-hornblende geobarometry, we pinpoint the time and depth at which each pluton was emplaced and use the age of overlying cover units or supergene minerals to date its arrival at the surface (or near-surface) environment. Uranium-lead zircon ages for 49 samples from plutons and porphyries range from Carboniferous to Eocene (similar to 314-35 Ma). Al-in-hornblende emplacement depths for 19 plutons are similar to 4-7 km, with one Carboniferous pluton emplaced at similar to 12 km. Two phases of net exhumation are identified: early Permian-Middle Triassic and middle Eocene-late Oligocene, with an intervening period of net burial. The highest exhumation rates (>0.30 km/m.y.) derive from the second phase, coeval with the Incaic orogeny and the main phase of hypogene mineralization. Present-day preservation of plutons and porphyry Cu deposits required low post-Oligocene average exhumation rates of =similar to 4 km at El Abra), by burial beneath significant cover (e.g., Ministro Hales, Queen Elizabeth), or, in the Inti region of northernmost Chile, by a combination of the two.

The Atacama Desert, on the western margin of the Central Andes, hosts some of the world's largest porphyry copper deposits (PCDs). Despite a hyperarid climate, many of these PCDs have undergone secondary “supergene” enrichment, whereby copper has been concentrated via groundwater‐driven leaching and reprecipitation, yielding supergene profiles containing valuable records of weathering and landscape evolution. We combine hematite (U‐Th‐Sm)/He geochronology and oxygen isotope analysis to compare the weathering histories of two Andean PCDs and test the relative importance of climate and tectonics in controlling both enrichment and water table movement. At Cerro Colorado, in the Precordillera, hematite precipitation records prolonged weathering from ∼31 to ∼2 Ma, tracking water table descent following aridity‐induced canyon incision from the late Miocene onward. By contrast, hematite at Spence, within the Central Depression, is mostly younger than ∼10.5 Ma, suggesting exhumation ended much later. A heavy oxygen isotopic signature for Spence hematite suggests that upwelling formation water has been an important source of groundwater, accounting for a high modern water table despite persistent hyperaridity, whereas isotopically light hematite at Cerro Colorado formed in the presence of meteoric water. Compared with published paleo‐environmental and sedimentological records, our data show that weathering can persist beneath appreciable post‐exhumation cover, under hyperarid conditions unconducive to enrichment. The susceptibility of each deposit to aridity‐induced water table descent, canyon incision and deep weathering has been controlled by recharge characteristics and morphotectonic setting. Erosional exhumation, rather than aridity‐induced water table decay, appears to be more important for the development of supergene enrichment.

The Miocene Oxaya Formation, exposed along the western Andean slope in northern Chile, represents one of the largest ignimbrite provinces on earth. In this study, magnetic fabric data were acquired from a ~ 1-km-long core drilled vertically through a single cooling unit of the Oxaya Formation the ca.22 Ma Cardones ignimbrite. Samples for magnetic analysis were obtained every 20 m from the fine-grained matrix of the core. Detailed measurements of the variation in bulk magnetic properties, including natural remanent magnetization (NRM), susceptibility, and anisotropy of magnetic susceptibility (AMS), were used to monitor changes in magnetic mineralogy as well as changes in the strength and orientation of the magnetic fabric throughout the flow. AMS ellipsoid orientation and shape reflect rapid deposition from a concentrated granular fluidized flow and constrain both the location of the source caldera for this catastrophic eruption and processes of transport and deposition in this large-volume ignimbrite. After utilizing the magnetic remanence to correct for rotation about the core axis, well-grouped and imbricated petrofabric orientations reveal a well-defined SW (247°) transport direction down a proto-Western Andean slope indicating syn- or post-welding flow and confirming the deformed Lauca caldera as the likely source of the eruption. Systematic variations in fabric shape (T) and intensity (P) with depth reveal a predominately oblate fabric toward the top and base of the flow and predominately prolate fabrics in the center. These vertical changes in fabric reveal that this massive, apparently homogeneous, deposit has a systematic layering. This layering reflects depth-dependent temperature variations, temporal changes in the flow boundary zone during deposition and post-depositional processes.

The shallow submarine environment is known to be prospective for precious- and base-metal mineralization (e.g. Eskay Creek). Recently emerged volcanic-hydrothermal systems provide an ideal location to understand the influence of emergence on submarine systems. This study integrates hyperspectral airborne and handheld shortwave infrared (SWIR) spectroscopic techniques, with geological observations and mineralogy (XRD). Mineral mapping on Milos reveals a previously unrecognized alunite-kaolinite topographically controlled steam-heated horizon. We suggest hydrothermal activity was contemporaneous with emergence, and can potentially be used to track the paleo-shoreline. Downhole SWIR data from the shallow submarine Profitis Ilias low-sulfidation Au-Ag deposit displays pervasive alunite-kaolinite overprinting on the original argillic assemblage. Preliminary ratios between the brightness of the similar to 1.48 mu m feature and similar to 1.76 mu m in the SWIR may infer crystallographic order with increasing temperature. We conclude that the shallow submarine environment can be deleterious to ore preservation.

The Atacama Desert of northern Chile hosts many of the world's largest porphyry copper deposits (PCDs), many of which have been upgraded through supergene enrichment (Sillitoe and McKee 1996). Enrichment of exhumed PCDs occurs in the near-surface weathering environment, when copper is leached from primary sulphides and reprecipitated beneath the water table to form a concentrated blanket of secondary copper minerals (e.g. chalcocite). The Atacama is one of the driest regions on Earth, but supergene enrichment requires precipitation to drive the aqueous redistribution of metals. Previous alunite dating studies have suggested enrichment stopped in the middle Miocene, due to an increase in aridity. This conclusion is supported by the youngest supergene alunite ages from several PCDs clustering around 14 Ma (Sillitoe 2005). Hematite also forms in the leached caps of PCDs and hematite (U-Th)/He geochronology provides a tool to track the downward progression of weathering fronts (e.g. Cooper et al. 2016). This study will combine Ar-40/Ar-39 dating of supergene alunite (a proxy for the timing of copper enrichment) with hematite geochronology (as an indicator of the progression of weathering) to better understand the link between water table movement and supergene enrichment of Central Andean PCDs.

"Water table movement and supergene enrichment at Spence porphyry copper deposit, northern Chile." Applied Earth Science, 128(2), p. 70

The timing and duration of magmatism and ore formation in Porphyry Copper Deposit (PCD) systems offer valuable insight in to the geological processes leading to ore formation which may aid in future exploration efforts and can provide constraints upon the potential metal endowment of a deposit. The Spence PCD in Northern Chile is centered upon granodioritic porphyry stocks of Palaeocene age. Whilst Spence formed at similar to 57 Ma, spatio-temporal constraints on the evolution of magmatism are poorly defined and the deposit's geological model lacks the detail required for to investigate the evolution of the deposit at high resolution. We redefine the sequence of igneous units at the deposit based on key textural characteristics, these lithologies are then mapped using a novel and non-intrusive mapping strategy which utilises drill-core. We apply high precision CA-ID-TIMS U-Pb geochronology to the deposit using the lithologies defined in this study to reveal a complex magmatic history with at least four independent pulses of mineralising hydrothermal fluid. Our data illustrate that magmatism and hydrothermal activity initiated in the SSW of the deposit and gradually migrated NNE following the trend of the pre-existing Antofagasta-Calama Lineament.

The Helmand Basin in southern Afghanistan is a large (310,000 km(2)), structurally controlled, endorheically drained basin with a hyperarid climate. The basin hosts a high elevation (similar to 200 m) plateau (the Dasht-i Margo), 11 fluvial staircase terraces (T1l to Tl), 7 delta systems (D1 to D7), and 6 paleolake shorelines (SL1 to SL6) within the Sistan Depression on the western side of the basin. Mapping and surveying of these features by remote sensing is integrated with geological observations to reconstruct Quaternary landscape evolution of the basin. The fluvial systems, deltas, and paleolake shorelines are correlated with one another and with the younger terraces (T7 to T1). The shape of fluvial longitudinal profiles changes depending on whether they formed pre-, syn-, or post-growth of the Koh-i Khannesin volcano on the southern margin of the Helmand River. The age of the volcano (similar to 0.6 Ma) and correlation of the terraces with the global history of glacial-interglacial cycles constrain the age of the younger terraces to the late Pleistocene and indicates that the older terraces are middle Pleistocene (dating back to 800 ka). The Helmand Basin once hosted a large lake, called here the Sistan paleolake, which at SL6 times and before had a surface area >50,000 km(2). Since that time the lake elevation and area have decreased, evolving to the present-day dried out Sistan Depression with small ephemeral playa lakes. Episodic formation of terraces, deltas, and paleolake shorelines is attributed to changes in base level modulated by climate change related to Milankovitch cycles. Crown Copyright (C) 2018 Published by Elsevier B.V. All rights reserved.

The Maronia Magmatic Corridor is a NE-trending belt of Oligocene plutons that intrudes the Kechros Dome of the northern Rhodope Core Complex in northeastern Greece. The post-collisional magmatism transitions from early high-K calc-alkaline magmatism in the NE to a younger, shoshonitic phase in the SW. We use a full suite of whole-rock geochemical analyses, including rare earth elements, to show a shared metasomatized mantle source of the magmatism. Evidence of plagioclase saturation from the onset of crystallization and amphibole-pyroxene-controlled fractionation in the high-K calc-alkaline magmatism suggest a drier (<4.75 wt% H 2 O) parental magma than is typical of subduction-related magmatism. Continued H 2 O depletion of the metasomatized source mantle resulted in the transition to a shoshonitic trend where deep crustal fractionation of an H 2 O-poor (< ~2 wt% H 2 O) magma in the absence of major olivine resulted in incompatible enrichment over a small range of SiO 2 . High-precision U-Pb zircon geochronology is presented here for the first time to provide chronological markers for the transition in the magmatic evolution of the Kechros dome. A 2.2 Myr break in magmatism separates the intrusion of the shoshonitic Maronia pluton at 29.8 Ma from the emplacement of the rest of the high-K calc-alkaline Maronia Magmatic Corridor between 32.9–32.0 Ma. The Maronia pluton is the hottest, driest, and youngest episode of post-collisional magmatism in the Kechros dome; we suggest that the emplacement of Maronia marks the cessation of magmatism in the northern Rhodope Core Complex as asthenospheric mantle upwelling migrated southward.

"The Kassiteres porphyry – epithermal system, NE Greece: evidence for rapid exhumation leading to sub-economic mineralisation." Applied Earth Science, 126(2), pp. 84–85

"Regional controls on water table depth in the Northern Atacama Desert; implications for supergene enrichment." Applied Earth Science, 126(2), pp. 55–56

"Exhumation of Andean granites: implications for porphyry copper formation and enrichment." Applied Earth Science, 126(2), p. 52

"Textural, spatial and temporal variation within the igneous suites of the Spence porphyry copper deposit, northern Chile." Applied Earth Science, 126(2), pp. 47–48

"Shallow submarine barite mineralisation on Milos Island, Greece." Applied Earth Science, 126(2), pp. 80–81