Formation Of Porphyry Copper Deposits Research Articles

Porphyry copper formation driven by water-fluxed crustal melting during flat-slab subduction

AbstractThe 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.

Excess Alumina of Plagioclase Related to Copper-Gold Enrichment: Study Case at the Humpa Leu East (HLE) Porphyry CU-AU Prospect in HU’U District, Sumbawa Island, Indonesia

The formation of porphyry copper deposits is controlled by water available in magma to form a deposit since Cu ligands and S have finite solubilities. The high amount of water in magma allows the magma to reach saturation earlier and causes more Au to be drawn from the melt into volatile aqueous phases. High water content in magma is conducive to the migration and enrichment of metal elements, which are very important for mineralization. This study is aimed to examine trace element compositions of plagioclase related to early, intermediate and late causative intrusions of the Humpa Leu East (HLE) prospect by applying the mineral chemistry to assess magma fertility for copper and gold mineralization in term of magma water content. The presence of 'extra' alumina in plagioclase minerals has been linked to high water content in magma. Plagioclase may become enriched because 'excess' alumina excludes copper, enriching the remaining melts. Our findings show that the excess of alumina (Al*) in mineralized/fertile samples of the HLE porphyry Cu-Au prospect exhibits more pronounced (Al*) than the infertile samples of Mt. Kelud. The 'excess' alumina of plagioclase can be linked to high water content in the magma at the HLE porphyry copper prospect. Furthermore, Concentric zones in plagioclase phenocryst may represent fluid injections into the sub-porphyry magma chamber. Plagioclases in the intermediate intrusion shows a higher of 'excess' alumina than those in the early and late causative intrusive rocks, which indicates that the intrusion is genetically related to the high grade mineralization. This study proves that the chemical signatures of plagioclase could be an exploration indicator in term of magma fertility forming porphyry copper-gold deposits.

Identifying tectonic settings of porphyry copper deposits using zircon trace elements – A semi-supervised machine learning method

Porphyry copper deposits (PCDs), as main sources of Cu resources worldwide, have attracted great attention for both scientific research and mining purposes in the past decades. The formation of PCDs is closely related to magmatic-hydrothermal system formed in both (continental/oceanic) arc and non-arc environments. Many low-dimensional geochemistry discriminants were developed for identifying the tectonic environments where ore-related magmas formed. However, long-term geological processes may obscure the geochemistry information recorded in rocks and minerals, which inevitably introduces uncertainties for identifying tectonic environments when using the low-dimensional discriminant diagrams. Here, we proposed a semi-supervised machine learning (ML) approach based on zircon chemistry data (spanning 16 elements) to identify the tectonic environments of PCDs. The results show that the semi-supervised models perform better in identifying the tectonic environments compared to the conventional low-dimensional discriminants. Moreover, as another benefit of big data analysis, ML model can clarify the systematic geochemistry differences of igneous zircons formed in different tectonic environments. The results suggest that (U+Th)/Yb and LREE/HREE are the top two important zircon chemical features for distinguishing PCDs formed in arc or non-arc environments. Specifically, zircons from non-arc show higher (U+Th)/Yb compared to that of continental arc, while zircons from island arc show lowest (U+Th)/Yb and LREE/HREE. Finally, the semi-supervised learning model was applied to the Dexing porphyry copper deposit to approach the longstanding debate regarding the underlying metallogenic setting. The results support the viewpoint that the Dexing porphyry copper deposit was formed in an intracontinental (non-arc) environment related to asthenosphere upwelling.

Tectonic and Crustal Processes Drive Multi-Million Year Arc Magma Evolution Leading up to Porphyry Copper Deposit Formation in Central Chile

Abstract 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 ~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 εHf 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 ~3.5 Myr to form the world’s largest known Cu resource at Rio Blanco-Los Bronces.

The volatile record of volcanic apatite and its implications for the formation of porphyry copper deposits

Abstract 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 XCl/XOH (0.6–0.25) and an increase in XF/XCl (1.5–8) and XF/XOH (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 XCl/XOH and XF/XOH together with constant XF/XCl 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.

The origin of olivine basalts from Medvezhya Mount (Avachinsky group of volcanoes, Kamchatka): The evidence of assimilation of sulfide-bearing cumulates

The role and conditions of liquid immiscibility and crystallization of sulfide phase during evolution of subduction related magmas remains to be a debated topic, which bears relevance to mechanisms of porphyry copper deposit formation and evolution of the continental crust. We studied rare volcanic rocks with inclusions of magmatic sulfides in olivine – the basalts of Medvezhya Mount in the Avachinsky group of volcanoes. The rocks belong to primitive (Mg# = 66 mol. %) middle-K island arc olivine basalts. Olivine with normal zoning predominate (~98%) among phenocrysts. The olivine compositions are typical for Kamchatka basalts, except for an unusual trend of increase of MnO content from 0.20 to 0.55 wt. % and decrease of Fe/Mn from 60 to 35 with change of olivine composition from Fo87.8 to Fo78.2. Olivines of this group contain numerous inclusions of minerals of the spinel group varying in composition from chromium spinel to magnesian magnetite. Olivine phenocrysts with sulfide inclusions are characterized by the absence or weak reverse zoning and reduced contents of Ca, Ni, Mn, Cr, and Al. The estimated crystallization temperatures for olivines of the prevailing type are 1036–1241°C, for sulfide-bearing olivines – 1010–1062°C. The data suggest that crystallization of the main olivine population occurred under relatively shallow conditions and was accompanied by strong oxidation of the magmas. On the contrary, the zoning and composition features of sulfide-bearing olivine suggest its xenogenic origin and probable crystallization at conditions of deep crust from low temperature water-rich and/or low-Ca magmas. The results obtained confirm the possibility of saturation of oxidized island-arc magmas with sulfide phase at lower crustal conditions, but show that this process is rare and not typical for low-pressure crystallization stage.

PETROCHEMICAL AND STRUCTURAL CHARACTERISTICS OF PORPHYRY COPPER MINERALIZATION IN THE ASTANEH ORE DEPOSIT, MIDDLE PART OF THE URUMIEH-DOKHTAR MAGMATIC ARC (IRAN)

Within the Urumieh-Dokhtar Magmatic Arc in the central part of Iran, the formation of which is associated with the Neotethys closure, there are many porphyry copper deposits and ore occurrences. One of them is the Astaneh porphyry copper ore deposit, located in the central part of the Saveh-Ardestan ore region southeast of Ardestan city. The purpose of this study is to investigate the petrochemical characteristics of rocks and to determine the relationship between the distribution of porphyry copper mineralization and tectonic position of faults within the study area. To achieve the goal, there were used the structural and geological data obtained in the fieldwork, as well as the results of mineralogical and geochemical analyses. The obtained results show that rocks of different composition of the Astaneh ore deposit (andesite, andesite-basalt, basalt, trachybasalt) were formed in the suprasubduction zone, and probably in the environment prior to the collision of the of continental plates. Paragenetic relationships and mineralogical analysis show that the evolution of mineralization of the Astaneh ore deposit can be divided into three stages: pre-ore, hypogene and supergene mineralization. Geochemical research based on the study of the content of the major chemical elements in the rocks of the region shows that igneous rocks belong to calc-alkaline basalts and geodynamically can be attributed to the products of magmatism of the ensial island arc. The results concluded that the main stages of the formation of a porphyry copper ore deposit in the study area attain maximum spatio-temporal similarity with the tectonomagmatic phases of the development of the Neotethys Ocean. In addition, the Southern Ardestan fault, running through the study area and intersecting the basement structures, forms wide permeable zones favorable for the formation of porphyry copper deposits therein.

Quantifying Continental Crust Thickness Using the Machine Learning Method

AbstractCrustal thickness plays a key role in many geological processes. However, it remains challenging to quantify crustal thickness in the geological past. Here we propose an Extremely Randomized Trees algorithm‐based machine learning model to recover crustal thickness of old geological regions. The model is trained using major oxide and trace element compositions of 1,480 young intermediate to felsic rocks from global arcs and collisional orogens and geophysical measurements of crustal thickness. The model provides better estimations of crustal thickness than the commonly used methods based on Sr/Y and (La/Yb)N when applied to the testing data. The validity of this model is further demonstrated by its applications to the Kohistan–Ladakh, Gangdese and Talkeetna arcs, where paleocrustal thicknesses have been well constrained. We then use this model to construct the Mesozoic crustal thickness evolution of the Erguna Block in the southeast of the Mongol–Okhotsk suture belt. The closure time of the suture zone is still debated. Our results suggest that the crustal thickness of the Erguna Block increased from 43 ± 9 km at 210 Ma to 62 ± 7 km at 180 Ma, remained constant between 180 and 150 Ma, and then thinned to 36 ± 4 km at 120 Ma. These results suggest that the Mongol–Okhotsk Ocean closed in the Early–Middle Jurassic and the thickened crust was stretched during the Cretaceous. We show that the thick crust and compression‐extension transition seem to be favorable for the formation of porphyry copper deposits in the Erguna Block during the Late Jurassic.

Supergiant porphyry copper deposits are failed large eruptions

AbstractPorphyry copper deposits, the principal source of copper and molybdenum, form at convergent margins. Copper is precipitated from fluids associated with cooling magmas that have formed in the mantle and evolved at mid- to lower crustal levels, before rising toward the surface where they saturate and exsolve an aqueous fluid and copper. Despite advances in the understanding of their formation, there are still underexplored aspects of the genesis of porphyry copper deposits. Here we examine the role played by magma injection rates into the upper crust on the formation of porphyry copper deposits with different copper endowments. Mass balance calculations suggest that supergiant porphyry copper deposits (>10 million tonnes copper) require magma volumes (up to >2500 km3) and magma injection rates (>0.001 km3 year−1) typical of large volcanic eruptions from rift, hot spot, and subduction-related settings. Because large volcanic eruptions would destroy magmatic-hydrothermal systems or prevent their formation, the largest porphyry copper deposits can be considered as failed large eruptions and this may be one of the causes of their rarity.

Contributions of trans-magmatic fluid in the formation of porphyry copper deposits: A case study from the Baoshan deposit, South China

The classical mechanism “source-transport-storage” of the formation of porphyry copper deposit has been advanced in recent studies, as the “source” is not the main factor for the mineralization in some Cu deposits, and the metallogeny may be affected by other variables factors during the magma-fluid transportation or storage. We recommend the essential role of trans-magmatic fluid in the ore-forming process, this fluid is released from the melting of the sedimentary overlying the subducted plate with high water and volatiles concentrations and high oxygen fugacity. The Baoshan granodioritic cryptoexplosion breccia representing the influence of hydrothermal events as well as the unaltered Baoshan granodiorite porphyry are conducted by LA-MC-ICP-MS analysis, to identify the contribution of trans-magmatic fluid. In case of the εNd(t) of whole rock do not increase with the MgO increasing and SiO2 decreasing, the large variations of zircons εHf(t) values in Baoshan granodiorite porphyry (BGP, −14.24 to −6.38) and Baoshan granodioritic crypto-explosion breccia (BGCB, −25.24 to −6.62) were considered to be the interaction of partial melting of ancient mafic lower crust and Neoproterozoic juvenile crust. However, the copper mineralization requires high oxygen fugacity and a large amount of water, according to the tectonic settings of Baoshan, we recommend that it is the trans-magmatic fluid trapping and concentrating Cu from the whole pluton during the upwelling driven by magma convection. The initial magma was stalled by the ductile-brittle transition at shallow depths of upper-crust. The trans-magmatic fluid leads to the pressure increases at the top of the initial magma, then the overlying rock is ruptured and cryptoexplosion produced. Moreover, after the cryptoexplosion, the sudden reduction of circumference temperature and pressure leads to the decrease in the oxygen fugacity of the ore-forming system, which will change the valence state of sulfur from S6+ to S2−. Finally, sulfur precipitates with chalcophile elements like copper in the metallogenic system and forms porphyry copper deposits with the low Sr/Y ratio. This study highlights the use of trans-magmatic fluid and ductile-brittle transition in the formation of the Baoshan porphyry copper deposits.

Structural and petrophysical formation environments of porphyry copper stockworks

Structural and petrophysical environments of porphyry copper deposit formation are discussed. The paper analyzes deformation host environment effect on porphyry intrusion and breccia pipe morphology, form and structure of associated stockworks and metal ore grades. It is shown that major and giant deposit formation primarily depends on poorly permeable rocks, carbonate sequences suffering plastic deformation at high temperatures and pressure, present above ore-bearing intrusive, or basic and intermediate volcanogenic and intrusive lava facies and highly resilient and strong metamorphic rocks. The presence of such structural and petrophysical “screens” resulted in fluid plume emergence and extensive ore accumulation. Dolomites, limestones and Fe-rich basite assemblages hosted by these sequences also acted as geochemical barriers for hydrothermal solutions facilitating high-grade ore deposition.

Machine learning for geochemical exploration: classifying metallogenic fertility in arc magmas and insights into porphyry copper deposit formation

A current mineral exploration focus is the development of tools to identify magmatic districts predisposed to host porphyry copper deposits. In this paper, we train and test four, common, supervised machine learning algorithms: logistic regression, support vector machines, artificial neural networks (ANN) and Random Forest to classify metallogenic ‘fertility’ in arc magmas based on whole-rock geochemistry. We outline pre-processing steps that can be used to mitigate against the undesirable characteristics of geochemical data (high multicollinearity, sparsity, missing values, class imbalance and compositional data effects) and therefore produce more meaningful results. We evaluate the classification accuracy of each supervised machine learning technique using a tenfold cross-validation technique and by testing the models on deposits unseen during the training process. This yields 81–83% accuracy for all classifiers, and receiver operating characteristic (ROC) curves have mean area under curve (AUC) scores of 87–89% indicating the probability of ranking a ‘fertile’ rock higher than an ‘unfertile’ rock. By contrast, bivariate classification schemes show much lower performance, demonstrating the value of classifying geochemical data in high dimension space. Principal component analysis suggests that porphyry-fertile magmas fractionate deep in the arc crust, and that calc-alkaline magmas associated with Cu-rich porphyries evolve deeper in the crust than more alkaline magmas linked with Au-rich porphyries. Feature analysis of the machine learning classifiers suggests that the most important parameters associated with fertile magmas are low Mn, high Al, high Sr, high K and listric REE patterns. These signatures further highlight the association of porphyry Cu deposits with hydrous arc magmas that undergo amphibole fractionation in the deep arc crust.

Porphyry copper deposit formation in arcs: What are the odds?

AbstractArc magmas globally are H2O-Cl-S–rich and moderately oxidized (ΔFMQ = +1 to +2) relative to most other mantle-derived magmas (ΔFMQ ≤ 0). Their relatively high oxidation state limits the extent to which sulfide phases separate from the magma, which would otherwise tend to deplete the melt in chalcophile elements such as Cu (highly siderophile elements such as Au and especially platinum-group elements are depleted by even small amounts of sulfide segregation). As these magmas rise into the crust and begin to crystallize, they will reach volatile saturation, and a hydrous, saline, S-rich, moderately oxidized fluid is released, into which chalcophile and any remaining siderophile metals (as well as many other water-soluble elements) will strongly partition. This magmatic-hydrothermal fluid phase has the potential to form ore deposits (most commonly porphyry Cu ± Mo ± Au deposits) if its metal load is precipitated in economic concentrations, but there are many steps along the way that must be successfully negotiated before this can occur. This paper seeks to identify the main steps along the path from magma genesis to hydrothermal mineral precipitation that affect the chances of forming an ore deposit (defined as an economically mineable resource) and attempts to estimate the probability of achieving each step. The cumulative probability of forming a large porphyry Cu deposit at any given time in an arc magmatic system (i.e., a single batholith-linked volcanoplutonic complex) is estimated to be ~0.001%, and less than 1/10 of these deposits will be uplifted and exposed at shallow enough depths to mine economically (0.0001%). Continued uplift and erosion in active convergent tectonic regimes rapidly remove these upper-crustal deposits from the geological record, such that the probability of finding them in older arc systems decreases further with age, to the point that porphyry Cu deposits are almost nonexistent in Precambrian rocks.A key finding of this paper is that most volcanoplutonic arcs above subduction zones are prospective for porphyry ore formation, with probabilities only falling to low values at late stages of magmatic-hydrothermal fluid exsolution, focusing, and metal deposition. This is in part because of the high threshold required in terms of grade and tonnage for a deposit to be considered economic. Thus, the probability of forming a porphyry-type system in any given arc segment is relatively high, but the probability that it will be a large economic deposit is low, dictated to a large extent by mineral economics and metal prices.

Singularity analysis of igneous zircon U-Pb age and Hf isotopic record in the Zhongdian arc, northwest Yunnan, China: Implications for Indosinian magmatic flare-up and the formation of porphyry copper deposits

Situated in the southern part of Yidun arc in the southeastern Tibetan Plateau, the Zhongdian arc records magmatic activity that ranges from approximately 270 to 190 Ma, which is likely attributable to subduction of the Garze-Litang slab and involved steady low-volume magmatic lulls punctuated by short-lived high-volume flare-ups. Many porphyry copper deposits have been found in these magmatic rocks, but the temporal-spatial relationships among magmatism, porphyry copper deposits, and subduction processes around the Zhongdian arc have long been debated. Frequency distributions calculated from igneous zircon U-Pb ages are commonly utilized to interpret the duration of magmatic events. Recently, a new concept of fractal density and a local singularity analysis (LSA) method have been applied to analyze the geometric properties of zircon U-Pb age peak anomalies, which are linked to deeply rooted avalanches associated with short spurts of convection during the formation of supercontinents and continental crust growth. In this paper, the igneous zircon U-Pb dating data collected from the Zhongdian arc are analyzed by the LSA method from the perspective of fractal density to interpret the causational relationship between age peak and subduction process of the Garze-Litang oceanic crust. The results show that the age density around the age peak of 215 Ma can be well fitted and described by power law functions, and the strong singularity (k > 0.8) indicates the occurrence of extreme geological events. Meanwhile, systematic changes in the hafnium isotopic composition of igneous zircons with time reflect the upwelling of asthenospheric mantle materials, characterized by a rapid increase in εHf(t) values that reach the highest around the 215 Ma peak. Moreover, the log–log plot that shows the cumulative number-age distribution reveals another transitional age of 235 Ma. By comparison of the magmatic intensity between the western and eastern belts of the Zhongdian arc, it is indicated that the eastward magmatic migration might have begun at ca. 235 Ma. Combined with the previous work, we propose that the Indosinian magmatic flare-up in the Zhongdian arc might have been caused by superimposition of slab subduction, steepening of slab dip, and slab breakoff. This long-term magmatic hydrothermal evolution system and extreme singularity events also provide indispensable conditions for the formation of large-scale and high-grade porphyry copper deposits in the region. Our findings provide new insights into the subduction process of the Garze-Litang oceanic crust and Indosinian magmatic framework around the Zhongdian arc. It is believed that the LSA method has potential applications in depicting the variation of magmatic activity and predicting mineral resources.

Crustal magmatic controls on the formation of porphyry copper deposits

Crustal magmatic controls on the formation of porphyry copper deposits

Chemical feedbacks during magma degassing control chlorine partitioning and metal extraction in volcanic arcs

Hydrous fluids released from subducting oceanic lithosphere fuel arc magmatism and associated hydrothermal mineralization, including formation of porphyry copper deposits. Critical magma degassing parameters are the depth, chemistry and style of fluid release during magma ascent, notably the behaviour of chlorine, a key metal-transporting ligand. Currently, understanding is limited by restricted data on fluid-melt partitioning of chlorine as a function of pressure and magma chemistry, and the complex interplay between the two that occurs in polybaric magmatic systems. Here we present experimental determinations of chlorine partitioning as a function of fluid and melt composition at pressures from 50 to 800 MPa. We provide, for the first time, a quantitative understanding of chlorine and copper evolution that is valid for shallow, deep or transcrustal differentiation and degassing. Monte Carlo simulations using our new data reproduce the chemical evolution of melt inclusions from arc volcanoes and fluid inclusions from upper crustal intrusions and porphyry copper deposits. Our results not only provide a novel chemical framework for understanding magma degassing, but quantify the primacy of magmatic chlorine concentration at the point of fluid saturation in promoting efficient copper extraction from magmas.

From long-lived batholith construction to giant porphyry copper deposit formation: petrological and zircon chemical evolution of the Quellaveco District, Southern Peru

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.

A new model for the optimal structural context for giant porphyry copper deposit formation

AbstractPorphyry-type deposits are the main global source of copper and molybdenum. An improved understanding of the most favorable structural settings for the emplacement of these deposits is necessary for successful exploration, particularly considering that most future discoveries will be made under cover based on conceptual target generation. A common view is that porphyry deposits are preferentially emplaced in pull-apart basins within strike-slip fault systems that favor local extension within a regional compressive to transpressive tectonic regime. However, the role of such a structural context in magma storage and evolution in the upper crust remains unclear. In this work, we propose a new model based on the integration of structural data and the geometry of magmatic-hydrothermal systems from the main Andean porphyry Cu-Mo metallogenic belts and from the active volcanic arc of southern Chile. We suggest that the magma differentiation and volatile accumulation required for the formation of a porphyry deposit is best achieved when the fault system controlling magma ascent is strongly misoriented for reactivation with respect to the prevailing stress field. When magmas and fluids are channeled by faults favorably oriented for extension (approximately normal to σ3), they form sets of parallel, subvertical dikes and veins, which are common both during the late stages of the evolution of porphyry systems and in the epithermal environment. This new model has direct implications for conceptual mineral exploration.

Mechanisms and patterns of magmatic fluid transport in cooling hydrous intrusions

Volatile outgassing from hydrous magma intrusions emplaced and cooling in the Earth's upper crust is key to a number of geologic processes including volcanic eruptions and ore deposition, yet the physical interactions between production, storage and transport of a magmatic volatile phase within magmatic intrusions and their large-scale thermal evolution have remained elusive. We performed numerical simulations of aqueous volatile transport in generic magma chambers as they crystallize in response to conductive and convective cooling and thereby transition from a crystal - volatile - melt suspension to a mush and eventually to a rigid rock. We used simplified but realistic material properties based on published phase equilibrium experiments, published grain-scale modeling results and general geological constraints.We found that the intrusion depth and water content exert decisive control on the rates and mechanisms of intrusion outgassing. Above a critical volatile content, highly permeable capillary tubes develop in crystal mush regions with intermediate crystal volume fractions (∼ 0.5 to 0.7), and such grain-scale tubes provide rapid intrusion-scale degassing paths. These tubes are located in a ring-like mush region inside the intrusion, allowing strongly lateral flow and focusing towards the top of the intrusion. If the water content of the magma is close to saturation at the time of emplacement (5 to 6 wt% H2O) and the depth of the magma chamber roof is at least 4 km deep, initially distributed breakthrough points may coalesce to a single area of fluid release. Large-scale intrusion geometry determines this location of focused fluid release, and cooling rate and size of the intrusion determine overall fluid flux. Feedbacks between local hydrofracturing and heat advection break up the focused fluid expulsion into many short-lived pulses. These conditions for large-scale fluid focusing favor the formation of porphyry copper deposits, but might also trigger caldera-forming ignimbrite eruptions.

How to make porphyry copper deposits

Much of the world's economic copper resources are hosted in porphyry copper deposits (PCDs), shallow level magmatic intrusions associated mostly with thick (>45km) magmatic arcs, such as mature island arcs and continental arcs. However, a well-known, but unresolved paradox, is that arc magmas traversing thick crust, particularly in continental arcs, are generally depleted in Cu whereas in island arcs, where PCDs are less common, magmas become enriched in Cu. Here, we show that the formation of PCDs requires a complex sequence of intra-crustal magmatic processes, from the lower crust to the upper crust. PCDs form when the crust becomes thick (>45km) enough to crystallize garnet. Garnet fractionation depletes Fe from the magma, which drives sulfide segregation and removal of most of the magma's Cu into the lower crust, leaving only small amounts of Cu in the residual magma to make PCDs. However, because garnet is depleted in ferric iron, the remaining Fe in the magma becomes progressively oxidized, which eventually oxidizes sulfide to sulfate, thereby releasing sulfide bound Cu from the magma into solution. This auto-oxidation of the magma, made possible by deep-seated garnet fractionation, increases the ability of endogenic magmatic fluids to self-scavenge Cu from large volumes of otherwise Cu-poor magmas and then transport and concentrate Cu to the tops of magmatic bodies. Examination of the occurrence of PCDs in the central Andes shows that ore formation occurs when continental arcs reach their maximum thickness (>60km), just before the termination of magmatism.