Magmatic-hydrothermal Transition Research Articles

Magmatic to hydrothermal conditions in the transition from the A-type Pikes Peak granite (Colorado) to its related pegmatite

Fluid exsolution in magmas is a process that, in many silicic upper crustal reservoirs, starts at relatively low crystallinities (near liquidus), and precedes the precipitation of many ore bodies, including pegmatites. As any magmatic system approaches its solidus, the amount of the exsolved fluid phase increases and becomes progressively dominant over melt, allowing local over pressurization and the generation of pegmatitic pods/dykes. Such pegmatitic bodies show several features that point to both magmatic and hydrothermal environments, linking those realms and providing a unique opportunity to document and understand the magmatic-hydrothermal transition within silicic magmatic systems. We studied the 1.1 Ga classic A-type Pikes Peak granite (Colorado, United States) and one of its many internally-hosted pegmatites, the Wellington Lake pegmatite, to investigate the changes that occur within a granitic system as it crosses its theoretical water-saturated solidus and continues crystallizing beyond it. Textural and geochemical analyses of quartz, plagioclase, and K-feldspar minerals, as well as fluid inclusion studies, demonstrate this magmatic to hydrothermal transition in the granite and the pegmatite. Different thermometers (Ti-in-quartz, 2-feldspars, fluid inclusions) document the temperature evolution of the granitic system, from >850°C for the hottest magmatic minerals to <400°C for the pegmatite core. The magmatic-hydrothermal transition is recorded by plagioclase and quartz rims that yield temperatures well below the traditionally inferred haplogranite solidus. In the pegmatite, the magmatic-hydrothermal transition is observed between the graphic granite wall zone, which shows homogeneous quartz geochemical signatures at near-solidus conditions (700–670°C), and the intermediate zone, which crystallized at much colder temperatures (470–420°C). Although a significant process, our calculations suggest that subsolidus precipitation from exsolved, solute-rich magmatic fluids represent less than 20% of the total volume of the granite.

Convective Melting and Water Behavior around Magmatic-Hydrothermal Transition: Numerical Modeling with Application to Krafla Volcano, Iceland

Abstract Water is an essential component of rhyolitic magmas and nearly universally, silicic magmatism in the upper crust includes a transition from magma to water-saturated roof rocks. We have numerically simulated the effect of the addition of geothermal fluids to an intruded rhyolitic sill from the hydrothermal system contained within the porous felsitic roof rocks. Water uptake in the melt proceeds via its thermodynamically estimated saturation with a partial melt, corresponding to the fugacity of hydrothermal water in the melt-fluid zone at particular T–P-${X}_{H_2O}$ conditions. It is assumed that the exchange occurs until the melt fraction increases to the threshold melt fraction value εb ≈ 0.3–0.45. In this approximation, the amount of added water is the product of its solubility and the critical melt fraction εb. In two series of numerical experiments run at pressures of 200 and 50 MPa, the interaction of water-filled porous felsite with near liquidus rhyolite magma resulted in water absorption, induced partial melting creating a narrow several meters wide mushy zone, and sluggish convection below that distributed water across the intruded sill. At P = 200 MPa, the addition of about 1.5 wt% water results in stronger volume convection, causing the melting rate to increase to 20 m/year. However, the addition of <0.22 wt% water induced no melting on the magma/contact mush interface, and the intruded sill crystallizes without convection. We apply the results of these numerical experiments to hot and dry rhyolites of the Yellowstone hot spot track magmas and then to the 2009 AD rhyolite sampled by the IDDP-1 exploration well in Krafla (Iceland). An active contact between the hydrothermal system within felsite and hot 963°C rhyolite magma was accidentally crossed at the depth of 2100 m, with a very thin (<30 m) transition providing information for a partial verification of our theoretical model. With the parameters observed in 2009, including the water concentrations in the melt (1.8 wt%) and felsite (0.92 wt%) and the high temperature of the intruded magma (945°C–980°C), we obtained slow melting of the preheated felsite roof at a rate of about 1 m/year. This seems reasonable if the 2009 magma was intruded during the 1975–1984 Krafla Fires eruption. We additionally present new δD values (−118‰) and H2Otot (1.6–2.0 wt%) concentration and reinterpret published δD and H2Otot values for felsitic fragments from IDDP-1; we find these to be lower than the δD in the mantle-derived magmas or hydrothermal systems. We demonstrate that, in theory, the formation of a fluid with such a low δD can be provided by the addition of low-δD water from OH-bearing minerals in hydrothermally altered roof rock. This may happen during dehydration of epidote from the altered roof rocks, and, alternatively, may also proceed by the mechanism of thermal diffusion (the Soret effect) through the partially molten/hydrothermal transition zone controlled by fluid fugacity. The high-temperature gradient in the contact zone between magma and the geothermal system of about 15–17°/m with conditions at the cold end close to the critical point for the aqueous fluid further decreases the expected δD value at the hot end of the contact zone to less than −110‰.

Discrepant chemical differentiation and magmatic-hydrothermal evolution of high-silica magmatism associated with Pb–Zn and W mineralization in the Lhasa terrane

High-silica (SiO2 > 70 wt.%) granites (HSGs) are the main source of W, Sn, and rare metals. However, abundant HSGs, temporally, spatially, and genetically associated with Pb–Zn mineralization, in the Lhasa terrane (LT), provided an ideal opportunity to study the key factors responsible for Pb–Zn enrichment, instead of W–Sn enrichment. Here we contribute to this topic through U-Pb dating of zircon and garnet, and whole-rock and Sr–Nd–Hf isotopic geochemistry of ore-related quartz porphyries in the Bangbule deposit and compared these results with published data from large and giant Pb–Zn and W deposits in the LT. The magmatism-alteration-mineralization event in the Bangbule deposit was recorded by robust zircon U–Pb ages of 77.3 ± 0.9 Ma and hydrothermal garnet U–Pb ages of 75.7 ± 4.8 Ma, which is 10–15 Ma earlier than the main Paleocene metallogenic event and the first record of late Cretaceous Pb–Zn polymetallic mineralization in the LT. The late Cretaceous-Paleocene magmatism and mineralization events are a response to the subduction of Neotethyan oceanic lithosphere, which occurred as a result of the collision of the Indian and Asian plates. These HSGs related to Pb–Zn mineralization, with high total-alkalis and low magnesian contents, are enriched in Ba, Th, and Rb, but depleted in Ti, Eu, Sr, and P. They belong to either the S-type, or I-type granites. The Sr–Nd–Hf isotopic compositions of the Pb–Zn mineralized granites demonstrate that they were generated by the partial melting of Proterozoic basement with or without mantle-derived melt input. This was consistent with the postulated source of W enrichment in the LT. The Pb–Zn and W related granites have similar zircon-Ti-saturation temperatures, comparable low whole-rock Fe2O3/FeO ratios, and zircon oxygen fugacity. This indicated that the Pb–Zn–W enrichment in the high-silica magma system could be attributed to a relatively reduced magma. The Pb–Zn related HSGs, abundant quartz and feldspar phenocrysts, and weak fractionation of twin-elements in whole-rock analysis, can be used to reconstruct a model of the magma reservoir. We postulate that these features could be reproduced by silica-rich crystal accumulation in a magma reservoir, with a loss of magmatic fluids. The magma associated with W mineralization exhibited a higher level of differentiation compared to the Pb–Zn related magma; however, different groups of zircon texture with varying rare earth elements and concomitance of rare earth elements tetrad effect and high fractionation of twin-elements in whole-rock are formed by a magmatic-hydrothermal transition in highly evolved system. As the source and oxygen fugacities of the Pb–Zn and W related magmas are similar, the absence of a giant W–Sn deposit in the LT may indicate that parent magmas with a low degree of evolution and magmatic-hydrothermal transition are not conducive to their formation. This implies that the rocks that originated as highly evolved silicate-rich parent magmas, with a high degree of magmatic-hydrothermal alteration, would need to be targeted for W–Sn mineral exploration in the LT. In summary, our results emphasize that variations in chemical differentiation and the evolution of high-silica magmatic-hydrothermal systems can lead to differences in Pb–Zn and W enrichment. This has implications for the evaluation of the mineral potential of high-silica granites and hence their attractiveness as targets for mineral exploration.

Granitic magma evolution to magmatic-hydrothermal processes vital to the generation of HREEs ion-adsorption deposits: Constraints from zircon texture, U-Pb geochronology, and geochemistry

The key point for further prospecting of heavy rare earth elements (HREE) ion-adsorption deposit is to figure out the granites that could generate HREE ion-adsorption mineralization in weathering processes. In this study, we present a detailed study of zircons from granites associated with Zudong, Dabu, and Xinfeng HREE ion-adsorption deposits in South China. The zircons were studied with regards to their texture, crystallinity, U-Pb dating, and geochemistry. The zircons from these granites all can be subdivided into two types. The type-1 zircons show oscillatory zonation and have Th/U and Zr/Hf mass ratios of 0.4–1.0 and 30–50, respectively. These textural and geochemical features indicate crystallization in a fractionated magma. The type-2 zircons are unzoned, occasionally porous, and have a low crystallinity. They occasionally rim type-1 zircons. The type-2 zircons show significantly higher F, P, Hf, Th, U, and REE contents, but display lower ZrO2 and SiO2 contents and lower Th/U, Zr/Hf, La/Yb ratios than those of the type-1 zircons. These geochemical features are consistent with zircon formation in a volatile-HREE-rich magmatic-hydrothermal transition stage. Under these conditions, the HREEs were also hosted in volatile-rich REE mineral phases including synchysite-(Y), aeschynite-(Y), calcybeborosilite-(Y), and atelisite-(Y), which have been observed in these HREE deposits. These volatile-rich REE-mineral phases can easily be dissolved during weathering and release HREE3+ to generate ion-adsorption HREE deposits. Therefore, we conclude that the granitic magma progression to a volatile-HREE-rich magmatic-hydrothermal system is vital for the generation of HREE ion-adsorption deposits. The long-term Mesozoic extension of the South China favors the generation of highly fractionated granites and is thus important for the generation of HREE ion-adsorption deposits. Furthermore, the zircons generated in a volatile-rich environment could be used to determine the HREE ion-adsorption mineralization potential of granites.

Temporal Separation of W and Sn Mineralization by Temperature-Controlled Incongruent Melting of a Single Protolith: Evidence from the Wangxianling Area, Nanling Region, South China

Abstract Tungsten and Sn display similar behavior during magmatic processes and are commonly associated spatially and genetically with highly evolved granites. Nonetheless, they typically form separate deposits, even if their associated granites have the same protolith. This separation may be due to the fractionation of the metals at the magmatic-hydrothermal transition or their differential mobility during partial melting of the metasedimentary protolith. If this separation occurred at the magmatic-hydrothermal transition, the ages of the W and Sn deposits would be very similar, whereas if it occurred during partial melting, the deposits are likely to have different ages because of the concentration of the metals in different magma batches and, in extreme cases, during different magmatic events. New age data from the Wangxianling ore field in the western part of the world-class Nanling W-Sn metallogenic province demonstrate that the W and Sn mineralization took place at different times. The W mineralization (219.5 ± 3.4 Ma) is related to Triassic granites (224.9–217.8 Ma), whereas the Sn mineralization is related to granites of Late Jurassic age (154.7 ± 1.1 Ma). This difference in ages rules out fractionation at the magmatic-hydrothermal transition as an explanation for the spatial separation of the W and Sn deposits and implies that the separation was due to differences in the mobility of W and Sn during partial melting. Both suites of granite originated from the partial melting of the same metasedimentary rocks, and both are reduced and highly evolved. The W granites, however, have a lower zircon saturation temperature (~750°C) than the Sn granites (~800°C), which indicates that the magma forming the W granites was mainly the product of muscovite-dehydration melting, whereas that forming the Sn granites was largely the result of biotite-dehydration melting. The different melting paths indicate that W released during muscovite breakdown dissolved in the magma, whereas Sn was sequestered by restite biotite. At the higher melting temperature, the residual W and Sn, released during the subsequent breakdown of biotite, dissolved in the magma. Thus, the magma that generated at low temperature was enriched in W, leading to subsequent W mineralization, whereas the magma that generated at high temperature was enriched in Sn and produced an Sn-mineralized granite. The whole-rock Sr-Nd isotope data for the Triassic W granites plot in the compositional field of the regional basement rocks and are consistent with partial melting of an orogenically thickened crust by internal heating in a collisional setting. In contrast, the Sr-Nd isotope data for the Late Jurassic Sn(-W) granites are displaced toward a mantle composition, likely reflecting contributions from mantle-derived material. Given the emplacement of many of the Late Jurassic Sn(-W) granites close to the Chenzhou-Linwu fault, we propose that this structure was the focus of decompression melting of the mantle and the injection of mantle-derived melts into the crust during the Late Jurassic, which supplied the additional heat for the melting at higher temperature needed to generate magmas enriched in Sn. This model, which is based on differences in the behavior of Sn and W during crustal melting, is potentially applicable to other Sn-W metallogenic provinces where Sn and W deposits are temporally separated.

The Passa Três Granite Intrusion-Related/Hosted Neoproterozoic Gold Deposit (Paraná State, Brazil): Mineralogical, Geochemical, Fluid Inclusion and Sulphur Isotope Constraints

The Passa Três granite is a 5 km2 intrusion in southern Brazil with an NNE–SSW-elongated shape, hosting gold-bearing quartz veins with fluorite, carbonates, sulphides (pyrite, chalcopyrite, aikinite, molybdenite) and native gold. Orebodies are hosted by the pluton roof zone, which is marked by various textures indicating magmatic–hydrothermal transition processes. Mineralisation formed between 613 and 608 Ma in extensional pull-apart structures controlled by two conjugated N–S and E–W fault systems. We report results from petrography, quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), scanning electron microscopy (SEM), electron probe microanalyses (EPMA), X-ray fluorescence (XRF), trace element analyses of pyrite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), sulphur isotope (δ34S) analyses, and fluid inclusion microthermometry. Hydrothermal alteration is dominantly developed as phengite–quartz–carbonate and sericite–carbonate–chlorite assemblages along mineralised structures. Fluid inclusion study indicates mineralising fluids with H2O-CO2-NaCl composition, low to moderate salinity (0.2–12.84 wt % NaCl eq.), and temperatures from 400 to 150 °C. The sulphur isotopic composition of pyrite (−0.1 to 1.1‰) suggests magmatic origin. These data, in conjunction with structurally controlled mineralisation enriched in Au-Bi minerals shortly post-dating the granite emplacement, appoint towards similarities between the Passa Três deposit and intrusion-related gold systems. The specific location of the mineralisation in the core (and in the roof zone, regarding its vertical position) of the Passa Três granitic intrusion defines it as “granite-hosted” thus, it is representative of a specific model that can be used for exploration of other intrusion-related/hosted gold deposits near the studied area or in other locations.

Textural indicators of mineralisation potential in porphyry magmatic systems – A framework from the archetypal Yerington district, Nevada

Porphyry-type deposits are spatially and temporally associated with the relatively shallow and texturally complex parts of magmatic systems. Whilst certain textures offer snapshots into the physical processes which result in fluid exsolution and hydrothermal mineralisation, their documentation and interpretation remains disjointed. To address this, we describe a suite of magmatic and magmatic-hydrothermal textures from the classic Yerington Cu(-Mo-Au) porphyry district, Nevada, where Cenozoic extension and tilting has exposed a unique, ∼8 km palaeodepth, cross-section through the magmatic system. Within the granite cupolas that underlie the Ann Mason and Yerington porphyry deposits, these textures include pegmatitic pods and massive silica bodies. Emplaced through the cupolas, and genetically associated with ore formation, are aplite dykes that host mineralised unidirectional solidification textures (USTs), pegmatitic segregations, miarolitic cavities and early A-type quartz veins. Based on field relations, including associations with hypogene mineralisation, petrography and Ti-in-quartz crystallisation temperatures, we highlight how these textures may record the timing and location of the magmatic-hydrothermal transition and ore-formation. By doing so we provide a textural framework for exploration geologists to assess the likely 3D spatial and temporal architecture of porphyry mineralisation at the district-prospect scale before employing more invasive and expensive techniques.

Biotite as a recorder of an exsolved Li-rich volatile phase in upper-crustal silicic magma reservoirs

Abstract The magmatic-hydrothermal transition is key in controlling the fate of many economically important elements due to the change in partitioning when melt and magmatic fluid coexist. Despite its increasing economic importance, the behavior of lithium (Li) in such environments remains poorly known. We illustrate how compositionally unusual biotites from the rhyolitic Bishop Tuff (California, USA) and Kos Plateau Tuff (Greece) may contain a magmatic volatile phase trapped between layers of biotite crystals. Despite originating in pristine deposits and showing the expected X-ray diffraction spectra, these biotites return low (<95 wt%) analytical totals via electron microprobe (EMP) consistent with the presence of considerable amounts of light elements (non-measurable by EMP). Lithium contents and isotope ratios in these biotites are remarkable, with abundances reaching >2300 ppm, exceptionally light Li isotopic compositions (δ7Li as low as –27.6‰), and large isotopic fractionation between biotite and corresponding bulk samples (Δ7Libt–bulk as low as –36.5‰). Other mineral phases, groundmass glass, and melt inclusions from the same units do not support an extremely Li-rich melt prior to eruption. Biotites from phonolitic systems (Tenerife [Canary Islands] and Campi Flegrei [Italy]) do not show such extreme compositional differences, with biotite and melt showing roughly equivalent Li contents, underscored by significantly reduced Δ7Libt–bulk to a maximum of –10.9‰. We ascribe the difference in behavior to the near-liquidus appearance of biotite in alkaline magmatic suites, before widespread exsolution of a magmatic volatile phase in the magma reservoir, while in rhyolitic suites, biotite crystallizes at low temperature, trapping the coexisting exsolved fluid phase in the reservoir.

The Magmatic–Hydrothermal Transition in Lithium Pegmatites: Petrographic and Geochemical Characteristics of Pegmatites from the Kamativi Area, Zimbabwe

ABSTRACTLithium is a critical metal, vital for electrification of transport. Currently, around half the world's lithium is extracted from rare-metal pegmatites and understanding the genesis and evolution of these igneous rocks is therefore essential. This paper focuses on the pegmatites in the Kamativi region of Zimbabwe. A group of early pegmatites is distinguished from a late pegmatite suite which includes the ca. 1030 Ma Main Kamativi Pegmatite. Previously mined for tin, the mine tailings are now being investigated for lithium. Mineral-scale investigation of samples from the Main Kamativi Pegmatite has allowed recognition of a four-stage paragenesis: (1) an early magmatic assemblage dominated by quartz, alkali feldspar, spodumene (LiAlSi2O6) and montebrasite [LiAl(PO4)(OH, F)]; (2) partial alteration by widespread albitization, associated with growth of cassiterite and columbite group minerals; (3) irregular development of a quartz, muscovite, columbite group mineral assemblage; and (4) widespread low-temperature fluid-induced alteration of earlier phases to cookeite, sericite, analcime, and apatite. Whole-rock geochemistry indicates that the late pegmatites are enriched in Li, Cs, Ta, Sn, and Rb but depleted in Nb, Zr, Ba, Sr, and the rare earth elements relative to early pegmatites and country rock granitoids. A combination of field relationships and published dating indicates that the granitoids, and probably the early pegmatites, were emplaced toward the end of the ca. 2000 Ma Magondi Orogeny, whereas the late pegmatites are almost 1000 million years younger. The late pegmatites thus cannot be genetically related to the granitoids and are instead likely to have formed by partial melting of metasedimentary source rocks. The drivers for this melting may be related to crustal thickening along the northern margin of the Kalahari Craton during the assembly of Rodinia.

Geochemical characteristics and boron isotopes of tourmaline from the Baishaziling tin deposit, Nanling Range: Constraints on magmatic-hydrothermal processes

Greisenization is an important stage of Sn-W mineralization, its mechanism of Sn enrichment and deposition remains debated (e.g., boiling, cooling, fluid-rock interaction, and fluid mixing). In this study, we performed detailed petrography and geochemical studies of different types of tourmalines in the Baishaziling greisen-type tin deposit, aiming to investigate the magmatic-fluid evolution and ore-forming processes. Major elements show that almost all tourmalines have high Fe/(Fe + Mg) ratios with affinity of alkali group and schorl compositions. Disseminated tourmalines (Tur-G1) occurring in granite are primary magmatic origin with high Li, Zn, Sc, V, and Ga contents. In the late magmatic-hydrothermal transition stage, tourmalines occur as tourmaline-quartz nodules (Tur-G2) in the granitic body, which have similar geochemical characteristics to Tur-G1. The lower Sn content of Tur-G3 in early quartz-tourmaline veins indicates that the dissolution of fluid has not lead to the initial Sn enrichment in the Baishaziling deposit. In the greisen-type orebodies, Tur-O1 with highest content of Sn (up to > 1000 ppm) and HREE, result of the interaction between magmatic-hydrothermal fluid and host granite due to the silicate minerals alteration, especially biotite. On the other hand, both syn-ore Tur-O2 and Tur-O3 exhibit low Sn, Li, Ca, HREE contents, which are suggested to be related to the co-crystallization of cassiterite and accessory mineral. The inverse Fe–Al correlation, NaR(X□Al)-1 and NaR2(OH)(X□Al2O)-1 substitution vectors of tourmalines in ore bodies (Tur-O) imply the relatively oxide environment and high salinity during the precipitation of cassiterite. Based on calculation, the δ11B value of primary magma, initial hydrothermal fluids and ore-formation fluids are ranging from −15.5‰ to −14.5‰, −12.63‰ to −12.22‰, −11.90‰ to −10.01‰, respectively, indicating that they have the same boron source from magma.

Geochronology and tectonic setting of the giant Guobaoshan Rb deposit, Central Tianshan, NW China

Triassic magmatism and associated rare metal deposits are increasingly being identified within the Central Tianshan Massif in NW China. A lack of geochronological studies on these mineralized plutons, however, has limited our understanding of their origins and tectonic setting. The Guobaoshan pluton, situated in the eastern segment of Central Tianshan Massif, contains ca. 281,000 tons of Rb2O resource and may be the largest Rb deposit in the world. The pluton exhibits a gradual lithological zonation from muscovite granite to amazonite(-bearing) muscovite granite from west to east; pegmatite veins of amazonite occur within the pluton and around its periphery. In this contribution, LA–ICP–MS U–Pb age dating and trace-element analysis have been conducted on zircon, monazite, cassiterite, and columbite to address the timing of magmatism and Rb mineralization, and the processes that generated the mineralization. Internal textures and trace-element contents suggest that these minerals crystallized during the magmatic–hydrothermal transition stage over a period of ca. 10 million yeas from the Early to Middle Triassic (240–249 Ma). Combined with available data, it is suggested that the Triassic was an important period for granitic magmatism and rare metal metallogeny in the Central Tianshan Massif, which may be a response to this region being dominated by an intra-continental extensional setting during this time period.

Tin enrichment in a highly fractionated A-type granite: Origin and mineralization potential of the Dayishan granite batholith in the Shi–Hang magmatic zone, South China

A-type granite intrusions in the southern Shi–Hang magmatic zone, South China, contain several world-class W–Sn deposits. The Dayishan granite batholith is located in the north of the southern Shi–Hang magmatic zone, and contains potential Sn mineralization. The batholith contains three intrusive phases, which are the Guankou, Tangshipu, and Nibantian phases. The Guankou phase consists mainly of biotite–K-feldspar granite (G1). The Tangshipu phase consists of three secondary units, which are coarse-grained biotite monzogranite (G2) that cut monzogranite (G3) and fine-grained two-mica monzogranite (G4). The Nibantian phase consists of coarse-grained two-mica monzogranite (G5). Zircon U–Pb dating indicates that the G1 to G4 granites were emplaced at ca. 160 Ma and the G5 granites formed at ca. 144 Ma. All the granites are silica-rich (SiO2 = 69.24–77.02 wt%) A2 sub-type granites, among which G1 granites are metaluminous and G2 to G5 granites are peraluminous. High εNd(t) (−6.9 to − 5.2) and zircon εHf(t) (−6.2 to − 1.9) values suggest that the parental magma was probably derived by partial melting of lower crust, which was induced by input of mantle-derived basaltic magmas. Oxygen fugacity determinations show that the G2 to G5 granitic magmas were reduced, but the G1 granitic magmas were relatively oxidized.From the G1 to G4 granites, Ba, Th, Nd, Zr, Hf, and rare earth element contents decrease, while Rb and Ta contents increase, indicative of extensive fractional crystallization of K-feldspar, plagioclase, and accessory minerals. This suggests that the G1 granites represent the least fractionated magmas, while the G2 to G4 granites are the highly fractionated products of the G1 granitic magma, with the G4 granites being the most fractionated. The G5 granites were formed during a later magmatic activity, though they show similar geochemical features with the G2 and G3 granites.Due to this continuous fractionation process, Sn was efficiently and progressively enriched in the G2 to G4 granites, as indicated by the good correlations between Sn contents and TiO2 contents, Eu/Eu*, Rb/Sr, Rb/Ba, K/Rb, Zr/Hf, Nb/Ta, and Y/Ho ratios, and TE1,3 values. Furthermore, the G2 to G4 granites crystallized at the magmatic–hydrothermal transition. These data indicate that the G2 to G4 granites underwent Sn mineralization, but the G4 granites are the best target for exploration. The G5 granites might also have Sn mineralization potential considering the similar geochemical features with G2 and G3 granites. Given the reduced nature of the magmas, the potential economic mineralization types are greisen and skarn.

REE Tetrad Effect and Sr-Nd Isotope Systematics of A-Type Pirrit Hills Granite from West Antarctica

The Pirrit Hills are located in the Ellsworth–Whitmore Mountains of West Antarctica. The Pirrit Hills granite exhibits significant negative Eu anomalies (Eu/Eu* = 0.01~0.25) and a REE tetrad effect indicating intensive magmatic differentiation. Whole-rock Rb-Sr and Sm-Nd geochronologic analysis of the Pirrit Hills granite gave respective ages of 172.8 ± 2.4 Ma with initial 87Sr/86Sr = 0.7065 ± 0.0087 Ma and 169 ± 12 Ma with initial 144Nd/143Nd = 0.512207 ± 0.000017. The isotopic ratio data indicate that the Pirrit Hills granite formed by the remelting of Mesoproterozoic mantle-derived crustal materials. Both chondrite-normalized REE patterns and Sr-Nd isotopic data indicate that the Pirrit Hills granite has geochemical features of chondrite-normalized REE patterns indicating that REE tetrad effects and negative Eu anomalies in the highly fractionated granites were produced from magmatic differentiation under the magmatic-hydrothermal transition system.

Zircon Genesis and Geochronology for the Zhangbaoshan Super-Large Rubidium Deposit in the Eastern Tianshan, NW China: Implication to Magmatic-Hydrothermal Evolution and Mineralization Processes

The Zhangbaoshan (ZBS) super-large Rubidium deposit, located in the Eastern Tianshan, is a typical granite-type Rb deposit. The ZBS deposit is mainly hosted in the highly evolved Baishitouquan (BST) pluton enriched in F and Rb, which exhibits five lithological zones from the bottom to the top: leucogranite (zone-a), amazonite-bearing granite (zone-b), amazonite granite (zone-c), topaz-bearing amazonite granite (zone-d) and topaz albite granite (zone-e), as well as minor small lodes of amazonite pegmatite. Two types of zircon were identified from the BST pluton. Type-I zircons mainly occur in the zone–a, are characterized by obvious oscillatory zoning, high Zr contents (47.4–67.3 wt% ZrO2) and Zr/Hf ratios (21.72–58.23), low trace element concentrations, and heavy rare earth elements (HREE)–enriched patterns with prominent positive Ce anomalies (Ce/Ce* = 1.21–385) and strong negative Eu anomalies (Eu/Eu* = 0.008–0.551), indicative of early magmatic zircon. Type–II zircons mainly occur in the upper zones (zone-c to zone-e), exhibit porous and dark Cathodoluminescence images, inhomogeneous internal structure, plenty of mineral inclusions, low Zr (38.7–51.0 wt% ZrO2) and Zr/Hf ratios (3.35–11.00), high Hf (34,094–85,754 ppm), Th (718–4,980 ppm), U (3,540–32,901 ppm), Ta (86.7–398 ppm), Y (1,630–28,890 ppm) and rare earth elements (REEs) (3,910–30,165 ppm), as well as slightly HREE–enriched patterns and significant M–type tetrad patterns with t3 values (quantification factor of tetrad effect) of 1.51–1.69. It is suggested that the type–II zircons are crystallized from a deuteric F–rich fluid coexisted with the highly evolved residual magma during the transition from the magmatic to the F–rich hydrothermal stage of the BST pluton. The F–rich fluid exsolution during the magmatic–hydrothermal transition is one of the most important factors controlling the modification of highly evolved granite and related Rb enrichment and mineralization. The type–I zircon samples from zone–a yield concordant ages of 250 ± 2.5 Ma and 250.5 ± 1.7 Ma, respectively, indicating that the BST pluton was emplaced in the Early Triassic. The type–II zircons from zone–c to zone–e yield lower intercept U–Pb ages between 238 and 257 Ma, which may represent the age of F–rich fluid–melt interaction during the transition from the magmatic to the hydrothermal stage. The mineralization of the ZBS super–large Rb deposit should have occurred shortly after emplacement of the BST pluton in the Early Triassic. Combined with available data, it is suggested that the Triassic is an important period for granitic magmatism and rare metal metallogeny in the Eastern Tianshan.

Fluid mixing as primary trigger for cassiterite deposition: Evidence from in situ δ18O-δ11B analysis of tourmaline from the world-class San Rafael tin (-copper) deposit, Peru

We present a high-resolution in situ study of oxygen and boron isotopes measured in tourmaline from the world-class San Rafael Sn (–Cu) deposit (Central Andean tin belt, Peru) aiming to trace major fluid processes at the magmatic-hydrothermal transition leading to the precipitation of cassiterite. Our results show that late-magmatic and pre-ore hydrothermal tourmaline has similar values of δ18O (from 10.6‰ to 14.1‰) and δ11B (from −11.5‰ to −6.9‰). The observed δ18O and δ11B variations are dominantly driven by Rayleigh fractionation, reflecting tourmaline crystallization in a continuously evolving magmatic-hydrothermal system. In contrast, syn-ore hydrothermal tourmaline intergrown with cassiterite has lower δ18O values (from 4.9‰ to 10.2‰) and in part higher δ11B values (from −9.9‰ to −5.4‰) than late-magmatic and pre-ore hydrothermal tourmaline, indicating important contribution of meteoric groundwater to the hydrothermal system during ore deposition. Quantitative geochemical modeling demonstrates that the δ18O-δ11B composition of syn-ore tourmaline records variable degrees of mixing of a hot Sn-rich magmatic brine with meteoric waters that partially exchanged with the host rocks. These results provide thus direct in situ isotopic evidence of fluid mixing as a major mechanism triggering cassiterite deposition. Further, this work shows that combined in situ δ18O and δ11B analyses of tourmaline is a powerful approach for understanding fluid processes in dynamic magmatic-hydrothermal environments.

In-situ boron isotopic and geochemical compositions of tourmaline from the Shangbao Nb–Ta bearing monzogranite, Nanling Range: Implication for magmatic-hydrothermal evolution of Nb and Ta

The enrichment mechanism of tantalum and niobium are still in dispute due to complex textural features of Nb–Ta minerals. Tourmaline is an accurate passive geochemical monitor related to the W-Sn-Nb-Ta-Li mineralization, which can record Nb and Ta variations in melt/fluid. The newly discovered Shangbao Nb–Ta bearing monzogranite is located in Nanling Range, South China, which is characterized by tourmaline wide occurrence. In this paper, we systematically investigate the different types of tourmaline using in-situ major, trace elemental and boron isotope for studying the origin of tourmaline, boron source, and Nb–Ta enrichment process. Three types of tourmaline have been identified, namely, disseminated tourmaline (TD type) in monzogranite, nodular tourmaline (TN type), consisting of the yellow core (TNy type) and blue rim (TNb type), and veined tourmaline (TV type). Almost all tourmalines from the Shangbao Nb–Ta deposit exhibit very low Ca content, X-site occupancy and high ratio of Fe / (Fe + Mg), pertaining to the group of schorl and alkali. Trace elements primarily present the low concentrations of <10 ppm, while the contents of Sc, Li and Zn are higher.Based on the petrography and Al occupations in the Y-site of tourmaline, TD type tourmaline was precipitated straightly from the melt at the late magmatic stage, while TN type tourmaline crystallized during the magmatic-hydrothermal transition process, and TV type tourmaline was the origin of hydrothermal fluid. In-situ boron isotope analyses of TD and TN type tourmaline by MC-ICP-MS show the narrow δ11B values from −17.16‰ to −14.81‰, demonstrating the same boron source. TV type tourmaline shows the weak higher δ11B values of −14.48‰ to −13.46‰ likely resulted by Rayleigh fractionation. The concentration of Nb in TD and TV type tourmaline can reach up to 20 ppm, which is higher than that in TN type tourmaline. However, TN type tourmaline contains higher Ta content than other types, revealing magmatic-hydrothermal transitional stage is critical to Ta enrichment. Compared with tourmaline in other type deposits, tourmaline in Shangbao monzogranite contains high Nb and Ta contents, indicating that it is a suitable tool for exploration, and the tourmaline is available to targeting Nb and Ta mineralization.

Genetic relationship between greisenization and Sn–W mineralization in vein and greisen deposits: Insights from the Panasqueira deposit (Portugal)

The W–Sn Panasqueira ore deposit is a magmatic-hydrothermal system, which includes a high-grade quartz-vein type mineralization and a disseminated greisen-type mineralization occurring in the upper part of the Panasqueira two-mica granite. We investigated the genetic and chronological relationships between the greisenization of the Panasqueira granite and the formation of ore-bearing quartz veins by monitoring major and trace elements variations in quartz-white mica assemblages composing the two-mica granite, greisen and W–Sn-bearing quartz veins. The greisen is characterized by an overall depletion in Mg, Ti, Ca, Na, Ba, Sr, REE and enrichment in Fe, Li, Rb, Cs, Sn, W which reflect the breakdown of feldspars and fluid-rock interactions with W–Sn-bearing fluids. White-mica from greisen and mineralized quartz veins are enriched in granophile elements (F, Rb, Cs, Li, Sn, W and Zn) compared to magmatic muscovite from the two-mica granite. Trace elements contents in quartz depict trends which show the progressive enrichment in Ge and B and depletion in Al, Ti and Li from magmatic to hydrothermal quartz that emphasize the progressive evolution and cooling of the magmatic-hydrothermal system of Panasqueira. Geochemical similarities between quartz-white mica assemblages from greisen and wolframite-bearing veins suggest that greisenization and the formation of mineralized veins result from the same hydrothermal event and derived from the same source of hydrothermal fluids. Apatite from greisen and quartz vein yielded U–Pb ages of 292 ± 10 Ma and 295 ± 5 Ma respectively confirming that greisenization and the formation of mineralized veins occurred roughly at the same time. These ages also overlap with the emplacement age of the Panasqueira granite (296 ± 4 Ma), indicating a temporal link between greisenization, W–Sn mineralization and granite crystallization. Temperatures of the magmatic-hydrothermal system constrained by Ti-in quartz thermometry depicts a cooling trend from magmatic quartz of granite (700–600 °C) to hydrothermal quartz of greisen (500–400 °C) and veins (450–350 °C). These results suggest that greisenization and the formation of W–Sn bearing quartz veins occurred at the magmatic-hydrothermal transition, during which orthomagmatic fluids rich in volatils, incompatible elements and W–Sn were exsolved during the final solidification stage of the Panasqueira two-mica granite.

The tungsten-gold veins of Bonnac (French Massif central): new constraints for a Variscan granite-related genesis

In the Brioude-Massiac district (French Massif Central: FMC), a network of W-As-Bi-Au quartz veins constitutes the Bonnac deposit, where tungsten is the major economic element, together with high-grade gold (up to 15 g/t Au). The evolution of this mineralization has been divided into 3 stages: (i) an early deep-seated wolframite-löllingite stage formed between 12 to 9 km, at up to 400 °C; (ii) a ductile/brittle deformation stage associated with scheelite and arsenopyrite deposition, with an estimated temperature of 480–300 °C; (iii) a late stage controlled by fluid-overpressure potentially triggered by fault-valve mechanism, at a depth of 7 to 5 km, and a temperature estimated between 266 to 240 °C, is marked by micro-fracturing infilled by native bismuth, bismuthinite, hedleyite, electrum, pyrite and base-metals. Structural analysis and apatite LA-ICP-MS U/Pb dating demonstrate a spatial and temporal link between the emplacement of the peraluminous leucogranitic dykes and the Bonnac mineralization. In more details, the mineralization was deposited between 321–316 Ma, during, or just after, the emplacement of the peraluminous dykes estimated around 329–315 Ma, suggesting a magmatic-hydrothermal transition for the ore-forming process. In the proposed model, the cooling of a hidden two-mica granitic pluton could have generated a magmatic fluid, and acted as the heat source responsible for fluid flow towards inherited permeability zones. The magmatic fluid could have then re-equilibrated at high temperature by fluid-rocks interaction. The sharp changes in pressure, associated with the decrease of the temperature, and sulfide-fugacity generated by a late input of meteoric fluid were responsible for the deposition of the late gold-stage. At the regional scale, the tungsten-gold event is ascribed to an early hydrothermal stage, dissociated from the formation of the antimony event in the district. The leucogranitic dykes and Bonnac quartz veins are controlled by a NW-SE stretching direction, interpreted as an expression of the Serpukhovian-Bashkirian syn-orogenic extension (D4 event of the FMC). These new data provide evidence for an early tungsten and gold metallogenic event in the FMC, prior the “Or300” event. The genetic classification of the Bonnac mineralization is equivocal. The W-As-Bi-Au-quartz veins exhibit the features of both an “orogenic gold” deposit at a relatively deep emplacement level (mesozonal), and an Intrusion-Related-Gold-Deposit (IRGD) type with a spatial-temporal link with the peraluminous intrusion emplacement. We propose that the Bonnac deposits represent an intermediate type between a typical orogenic-gold deposit and an IRGD. We argue that the presence of economic high-grade gold content in tungsten vein-type, and more generally the IRGD deposits, have been underestimated in the Variscan French Massif Central.

Efficient enrichment of Rb during the magmatic-hydrothermal transition in a highly evolved granitic system: Implications from mica chemistry of the Tiantangshan Rb-Sn-W deposit

Rare metal mineralization is commonly associated with highly differentiated granitic systems, which evolve into pegmatite or hydrothermal veins. The newly-discovered large Tiantangshan Rb-Sn-W deposit provides an excellent opportunity to study Rb enrichment, as the Rb resources are mainly hosted in the hydrothermal micas (dominantly veins), rather than the K-feldspar or magmatic micas in the parent alkali feldspar granite and volcanic rocks, as is typical of other Rb-rich systems. Micas at Tiantangshan are found in the alkali feldspar granites, trachyandesites, layers with unidirectional solidification textures (USTs) and hydrothermal veins. The mica Rb2O concentrations in the alkali feldspar granites range from 0.19 to 0.81 wt% (mean 0.44 wt%), in the trachyandesites from 0.47 to 1.13 wt% (mean 0.65 wt%), in the USTs from 0.41 to 1.26 wt% (mean 0.81 wt%), and in the hydrothermal veins from 0.39 to 1.28 wt% (mean 1.02 wt%), and show a positive correlation with F and Li contents. The mica and K-feldspar in the alkali feldspar granites have low K/Rb values (13–29 and 25–103, respectively), indicating the highly evolved nature of the alkali feldspar granite. K/Rb and K/Cs values decrease gradually in the mica from the alkali feldspar granites to the USTs and then to the hydrothermal veins, whereas Li, F and Rb increase. Based on the textures and geochemistry of the micas, they are cogenetic with the alkali feldspar granites and represent the magmatic stage, whereas the USTs formed during the magmatic-hydrothermal transition stage and the hydrothermal veins correspond to the hydrothermal stage. The Tiantangshan micas record mineralogical and geochemical evidence of efficient Rb mineralization during magmatic-hydrothermal evolution in the alkali feldspar granitic system.

Magmatic fractionation and the magmatic-hydrothermal transition in rare metal granites: Evidence from Argemela (Central Portugal)

Rare Metal Granites (RMGs) are highly evolved intrusions with specific geochemical signatures that are the result of combined magmatic and hydrothermal processes. In most examples, the early evolution is overprinted by late subsolidus transformations and the role of magmatic and magmatic-hydrothermal fractionation processes remains unclear. The origin of fluids involved in magmatic-hydrothermal systems is also an open question. The Argemela RMG (Portugal) is a small subvolcanic intrusion, which exposes in remarkable continuity a sequence of magmatic, magmatic-hydrothermal and early hydrothermal processes. The intrusion comprises a main granitic facies and a border unit made of a complex alternation of aplitic and pegmatitic facies plus three generations of intragranitic veins. The granitic mineral assemblage includes quartz, K-feldspar, albite, muscovite and accessory montebrasite, cassiterite and columbite-tantalite. The whole-rock chemistry is strongly peraluminous, with very high P2O5, Li and F enrichments, elevated rare metal (Sn, Nb, Ta, W) concentrations and strong depletions in Ca, Fe, Ti, Mg. Trace element zonation in minerals, mainly muscovite and quartz, record crystallization/fractionation processes along the magmatic-hydrothermal transition. Three main stages are recognized, magmatic (crystal fractionation and accessory/minor mineral saturation), magmatic-hydrothermal (fluid/melt element partitioning) and early hydrothermal, corresponding to the segregation and collection of magmatic fluids in the intragranitic vein system. Those are followed by limited metasomatic transformations and a late sulphide stage. Stable isotope (O, H) data demonstrate a magmatic origin for the early fluids and a granite-buffered signature for the vein-forming fluids. The fractionation of rare elements (Sn, Nb, Ta, Li, W, Mn, B, Rb, Cs) from the magmatic to the early hydrothermal stage is quantitatively modelled from the combination of three mechanisms, crystal fractionation, magmatic crystallization of accessory/minor phase (MCMA) and fluid/melt partitioning. Sn and W show a markedly contrasted behaviour with Sn precipitated as disseminated mineralization within the intrusion and W partitioned toward the magmatic fluid and extracted from the intrusion. Geochemical trends along the magmatic-hydrothermal transition do not require phases with unusual properties, such as immiscible hydrous melts to be explained and they do not necessarily result in an amplification of initial rare metal melt enrichments. The Argemela RMG stresses the importance of MCMA to generate magmatic rare element pre-concentrations, which later can be hydrothermally redistributed to form large economic deposits.