Fluid Exsolution 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.

Metal source and hydrothermal evolution of the Jiaoxi quartz vein-type tungsten deposit (Tibet): Insights from textural and compositional variations of wolframite and scheelite

Abstract We evaluate the controlling factors of hydrothermal wolframite and scheelite precipitation in the quartz vein-type Jiaoxi tungsten deposit situated in the western part of the Lhasa terrane (Tibet, China) using texture, major and trace element mineral geochemistry, and sulfur stable isotope geochemistry. Pyrite and chalcopyrite that are intergrown with Fe-enriched wolframite and siderite, have distinct in situ S isotope compositions (δ34SV-CDT) of −31.38 to +1.77‰, and +2.07 to +2.30‰, respectively. Major and trace element contents and in situ S isotope compositions of pyrite and chalcopyrite indicate that the hydrothermal evolution involved fluid-fluid mixing and greisenization. We report evidence for an early magmatic fluid, which is characterized by the enrichment of W, Mn, Zr, Ti, Sc, and Sn and depletion of Fe. This magmatic fluid was diluted by meteoric water and interacted with biotite monzogranite porphyry to leach Fe, Mg, and Zn into the system to form wolframites with variable Fe/(Fe+Mn) ratios ranging between 0.06–0.84. The late Fe-enriched magmatic fluid released from the muscovite granite mixed with meteoric water that leached minor Fe and S from shale to form late shale-hosted wolframite with a Fe/(Fe+Mn) mass ratio of >0.75 and coeval siderite and sulfides. This study highlights that multiple Fe sources were present in the system, including muscovite granite-released Fe through fluid exsolution, biotite monzogranite porphyry-released Fe during greisenization, and minor Fe released from the shale as a result of meteoric water leaching.

Role of fractional crystallization, fluid-melt separation, and alteration on the Li and B isotopic composition of a highly evolved composite granite pluton: The case of the Eibenstock granite, Erzgebirge, Germany

We present a comprehensive Li and B isotope study of granites, aplites, and igneous enclaves from the multi-phase Eibenstock granite in the Western Erzgebirge-Vogtland metallogenic province of Germany. The studied samples cover the entire compositional range of the granites from moderately to highly evolved and include variably altered types as obtained by magmatic fractionation, post-magmatic high- to medium-temperature and near-surface low-temperature alteration. Fractionation and alteration processes are unequivocally documented by the chemical variability of the rocks. Despite the marked imprint of these processes on bulk-rock compositions, our granite samples show only little variation in δ 7 Li (−0.52 to 0.75‰) and δ 11 B (−17.46 to −14.78‰), with surface samples defining the lower end of the δ 7 Li range. The narrow range in δ 7 Li suggests that magmatic fractionation and high-temperature overprint have a very minor effect on δ 7 Li. The B budget of the samples is dominated by tourmaline, which makes δ 11 B values insensitive for later high- to medium-temperature overprint or surficial low-temperature alteration. Depending on whether tourmaline crystallized before or after exsolution and loss of magmatic fluids, whole-rock samples have higher or lower δ 11 B values. Granite enclaves have δ 7 Li and δ 11 B values ranging from −1.51 to −0.81‰ and − 14.55 to −13.89‰, respectively. Some samples have chemical and mineralogical evidence for wall-rock interaction during emplacement or later overprint by external fluids. These samples show broader ranges in δ 7 Li (−2.61 to 2.21‰) and δ 11 B (−21.58 to −9.85‰). These values show that wall-rock interaction via assimilation and external fluids may affect δ 7 Li and δ 11 B to a larger extent than intra-magmatic processes, such as fractional crystallization, fluid-mediated autometasomatic overprinting, or exsolution of fluids from the melt. The offset of δ 7 Li and δ 11 B values towards the compositions of the wall rocks reflects the contrasting composition of granite and country rock and the addition of country-rock material to the granite. The magnitude of the offset reflects both the relative contribution of wall-rock derived Li and B to the granite and the magnitude of the difference in the Li and B isotopic compositions between them. • Whole rock δ 7 Li and δ 11 B are essentially unaffected by magmatic fractionation. • Fluid-melt separation may reduce δ 7 Li and δ 11 B in residual melt. • Effect of intra-granitic alteration on δ 7 Li and δ 11 B of granitic rocks is small. • Effects on δ 7 Li and δ 11 B during alteration depend on fluid source.

Barium isotopes reveal the role of deep magmatic fluids in magmatic-hydrothermal evolution and tin enrichment in granites

Although deep magmatic fluids are important for the shallow magmatic-hydrothermal evolution and transport/enrichment of ore-forming elements in transcrustal magmatic system, it is still difficult to identify the influences of such fluids by using conventional geochemical indicators. Here we report Ba isotope compositions for granites from the Jurassic Qitianling batholith in the Nanling Range, South China that hosts several large tin deposits. This composite batholith consists of three stages of granites with an age range of ∼15 Ma and sharp contacts between each other, which suggest an underlying long-lived crystal mush-dominated transcrustal magmatic system. The small variation of δ138/134Ba in the first-stage less evolved granites (−0.24–0.37‰) indicates that fractional crystallization in deep crystal mush of this system does not cause significant Ba isotope fractionation. In contrary, the latter-two stage highly differentiated granites show more variable and overall lower δ138/134Ba (−1.79–0.14‰), which cannot be explained by K-feldspar-controlled fractional crystallization in shallow crystal mush. Instead, the distinctively low Ba contents (<100 μg/g) and magmatic-hydrothermal evolution features of the latter-two stage granites suggest that their low δ138/134Ba are due to modification by magmatic fluids. Further modeling demonstrates that exsolved fluids from deep crystal mush can explain the light Ba isotope compositions of the latter-two stage granites. Because these ascending deep magmatic fluids can efficiently scavenge Sn and other fluid-mobile elements then transport to shallow level, we propose that these fluids could provide critical materials for the Sn-polymetallic mineralization.

Co–Ni decoupling indicates fluid exsolution during the formation of podiform chromitites: Insights from the Luobusa ophiolite, southern Tibet

Podiform chromitites typically occur in harzburgites near the petrological Moho in ophiolite sections, and are petrologically and economically significant. Hydrous fluids may have an important role in the formation of podiform chromitites, although unambiguous evidence for the involvement of such fluids is rare. To examine whether hydrous fluids are involved in the formation of podiform chromitites, we measured the Ni, Co, Mn, and Zn contents of olivines and chromites in representative samples from the Luobusa ophiolite, southern Tibet, taking account of the different geochemical behaviors of these elements in melts and hydrous fluids. In the harzburgites, the olivine (Ol) has a limited compositional range (2605–2919 ppm Ni, 117–135 ppm Co, 736–1005 ppm Mn, and 28.9–35.7 ppm Zn), whereas the chromite (Chr) has a relatively variable composition (718–1239 ppm Ni, 364–493 ppm Co, 1314–2733 ppm Mn, and 1411–1900 ppm Zn). From the dunite lenses to the dunite envelopes, and to the chromitites, both olivine and chromite exhibit significant decreases in Co (Ol = 137 to 45 ppm; Chr = 542 to 168 ppm), Mn (Ol = 889 to 273 ppm; Chr = 2426 to 862 ppm), and Zn (Ol = 37.7 to 4.0 ppm; Chr = 1746 to 206 ppm) contents, and an increase in Ni contents (Ol = 3009 to 5946 ppm; Chr = 413 to 1462 ppm). These features cannot be fully explained by subsolidus re-equilibration, partial melting, fractional crystallization, and melt–rock reactions. Olivine relicts in chromite indicate the dissolution of pre-existing olivine in dunitic channels. The olivine dissolution may have been due to the water-rich nature of the parental melts of the chromitites. These observations and mass-balance calculations suggest that, during the formation of podiform chromitites, chromite crystallization was accompanied by olivine dissolution and exsolution of a hydrous fluid phase. This resulted in the preferential transfer of Ni into the melt and olivine relicts, and Co, Mn, and Zn into the fluid phase. As such, the chromite and olivine in the chromitites became Ni-rich and Co-, Mn-, and Zn-poor, which led to Ni–Co decoupling.

Volatile accumulation for the mineralization of Li–Be pegmatites in the northeastern Pamir, Western Kunlun, China

ABSTRACT Li–Be mineralization in the northeastern Pamir is the most westerly portion of the newly defined Western Kunlun–Songpan–Ganze rare metal belt in China. This paper discusses the geochemistry and isotopic U–Pb–Sr–Nd–Hf systems of two Triassic granites and Li–Be pegmatites, trying to identify their genetic relationship. Our study shows that although the biotite monzogranites, the muscovite granites, and the Li–Be pegmatites have similar zircon and coltan U–Pb ages of 210 Ma, continuous crystallization could not completely account for their compositional and Sr–Nd–Hf isotopic diversity. In comparison with the biotite monzogranites, the muscovite granites exhibit strongly fractionated characteristics, with higher levels of SiO2, K2O, P2O5, Li, Be, Cs, Rb, and Ta, and are sourced from a partial melt of meta-pelite rocks, showing a genetical relationship with the Li–Be pegmatites. A model of volatile accumulation is an attempt to interpret some characteristics of the strongly fractionated muscovite granites and Li-Be pegmatites, which show dramatically elevated contents of P2O5 of 0.20–0.23% to Li–Be pegmatites of 0.4–2.64%, respectively. P2O5 might be an agency of the accumulation of fluxing compositions also including B, F, and H2O. Succeeding pegmatite-forming fluid exsolution and selective removal of Ca–Na–K to precipitate feldspar minerals far away from the parent granites should be accountable for some strongly fractionated characteristics in the residual melt, e.g. the enrichment of Rb, loss of Sr and Ba, and elevated negative Eu anomaly. Furthermore, this selective extractant of alkalis metal triggered by the accumulation of flux notably enhanced the contents of Li and Be in the remaining melts by three times, which might be also an important mechanism for the rare metal concentration in the Li–Be pegmatites by thousands of times.

Formation of the Lened W-(Be) Skarn Deposit by Neutralization of a Magmatic Fluid—Evidence from H3BO3-Rich Fluids

Magmatic-hydrothermal systems, especially those causing the formation of tungsten deposits, may be enriched in boron, as is suggested by the presence of hydrothermal tourmaline. This study examines the boron and metal (including tungsten) concentrations of quartz-hosted fluid inclusions in the Lened W-(Be) deposit of the Canadian Cordillera and resolves (i) the analytical challenges involved during fluid salinity calculations of B-rich fluids and (ii) the relationship between fluid chemical composition and ore-forming processes involved at Lened. The aqueous fluid inclusions from this study have high CO2 and boron contents, indicated by the presence of a carbonic phase and sassolite crystals (H3BO3) in fluid inclusions. The boron content of the aqueous liquid phase (0.5 wt. %) was determined using microthermometric and Raman spectroscopic analyses. Boron was judged the most appropriate internal standard for quantifying the LA-ICP-MS data from these inclusions after calculation of salinity in the H2O-NaCl-H3BO3 system (3.5 to 5 wt. % NaCleq). Trace element data of the fluids show relatively high concentrations of Li (40 to 474 ppm), Al (56 to 1003 ppm), As (36 to 490 ppm) and Cs (68 to 296 ppm); and lower concentrations of Rb (3.6 to 77 ppm), Sr (0.4 to 23 ppm), Sb (1 to 32 ppm), Ba (0.6 to 163 ppm), Mg (6.9 to 7.6 ppm) and other metals, such as Be (2.4 to 10.2 ppm), W (2.4 to 27 ppm) and Cu (5.1 to 73 ppm). The high Cs and Li concentrations suggest a magmatic origin of the metals, while the moderate concentrations in Sr and Ba are indicative of fluid–rock interaction with the surrounding limestone. The presence of sassolite suggests that these fluids were highly acidic. The neutralization of this fluid through interaction with the surrounding limestone is the most probable trigger for scheelite precipitation. The presence of such high boron content in the magmatic fluid at Lened indicates the potential role in the enrichment of the source melt before fluid exsolution.

Tourmaline as a recorder of the magmatic-hydrothermal evolution in the formation of pegmatite: In-situ elemental and boron isotopic compositions of tourmaline from the Qinghe pegmatite, Chinese Altay orogen

• Tourmalines record the magma-hydrothermal evolution of pegmatite-forming magma . • Extremely low REEs of tourmaline indicate direct melting of metapelite to form pegmatite . • Boron isotopic variation reflects the scale of fluid exsolution during pegmatitic evolution. We conducted elemental and boron isotope studies on tourmalines from a single pegmatite dyke of the Altay orogen to investigate its origin and evolution. The pegmatite dyke is composed of border, intermediate and core zones. The border and intermediate zone tourmalines mostly belong to schorl, while the core zone and magmatic hydrothermal tourmalines to dravite. Most tourmalines follow the (Na + Mg) (Al + X vac ) -1 , FeMg -1 and MnMg -1 exchange vectors. The core zone tourmalines show positively correlated FeO t and MgO, and negatively correlated FeO t and Al 2 O 3 , suggesting a rise of f O2 in this zone. Moreover, they show higher B, Mg, Sr and Eu than other tourmalines due to magmatic evolution. Tourmaline from the Altay pegmatite is characterized by extremely low REE abundances (<3 ppm) when compared with those from granites, suggesting an origin from direct melting of metapelitic rocks. The border and intermediate zone tourmalines have comparable boron isotopic compositions with δ 11 B = −14.0 to −12.2‰, which are similar to the mica schist (−13.2 ‰ to −12.2‰), but slightly heavier than the core zone and magmatic hydrothermal tourmalines (−14.4‰ to −13.3‰). This suggests derivation of the pegmatite from melting of the metapelite without causing boron isotope fractionation, whereas late-stage fluid exsolution caused somewhat boron fractionation. This may be indicative of only small amounts of fluids being released from the barren pegmatite-forming magma. Tourmalines from mineralized pegmatites basically exhibit heavier boron isotope compositions than those from barren pegmatites, due probably to more involvement of evaporate in the source of the former.

The role of crystal mush in porphyry systems: A case study from the Baishiya ore field, East Kunlun orogenic belt, northern Qinghai-Tibet plateau

Since a cold storage crystal mush reservoir has been gradually used to describe magmas in shallow crust, traditional mechanisms, such as wall-rock assimilation, melt-crystal separation, as well as magma mixing and mingling, must be used with caution because recycled antecrysts can play a significant role. As a result, magmatic processes accounted for texture, geochemistry, and mineralization diversity among ore-related porphyry, overlying (sub)volcanic rocks, and underlying pluton in porphyry systems remain ambiguous. The Baishiya igneous suite (East Kunlun orogenic belt, China) with porphyry-related skarn mineralization provides an opportunity to resolve this issue. Within the crystal mush concept, we propose a comprehensive model based on the subtle changes. First and foremost, large K-feldspar grains characterized by high Sr/Y ratios (52–134) in the top area of granodiorite have a slightly enriched lead isotopic signature, indicating a previously existed crystal mush equilibrated with K-feldspar. The locked crystal mush was injected by mantle-derived mafic melts, leading to a transfer in heat and volatiles from hot melts to cold crystal mush. The buoyant volatiles and loaded metals introduced by episodic mafic replenishments concentrated on the roof, providing a fluid-rich and thermal cycling condition for K-feldspar coarse graining. Some enlarged K-feldspar grains sunk into the bottom area due to gravity, and subsequently reacted with upwelling hot melts to form rapakivi textures. As a gradually waning of injection of mafic melts, crystallization drove fluid exsolution, forming magmatic-hydrothermal events. The finally solidified granodiorite acted as a host rock for a newly produced magma reservoir, but the following development was different. Catastrophic eruptions induced by tectonic stress transition led to a strong depletion of volatiles and associated metal endowment in the underlying magma reservoir, and subsequently left a barren magma source, terminating potential mineralization events. The residual felsic melts migrated upwards along previously formed magma conduits, and thus emplaced as barren porphyry dike. Our work reveals that recycling of K-feldspar antecryst induced by mafic injection contributes to high Sr/Y ratio in ore-related porphyritic granodiorite. Within a long-lived replenishment of mantle-derived magma, a crystal mush reservoir at shallow crustal levels can act as a ‘cooling pond’ to quench these hot melts, but accept volatiles and loaded ore-forming elements, concentrating metals by volatile buoyancy with the help of a compressional stress, which is presumably essential for late fluid exsolution from the reservoir’s roof.

Genetic Relationship between Subduction of Slab Topographic Anomalies and Porphyry Deposit Formation: Insight from the Source and Evolution of Rio Blanco Magmas

AbstractThe Rio Blanco deposit, which is one of the largest porphyry Cu–Mo deposits in northern Peru, formed coevally with the subduction of the Inca Oceanic Plateau at 12–10 Ma. However, the genetic relationship between the subduction of oceanic plateaus and the porphyry deposit formation remains unclear. Igneous rocks emplaced at 23–12 Ma in northern Peru, including the Portachuela batholith (which hosts the Rio Blanco porphyry complex), are normal calc-alkaline to weakly adakitic. In comparison, the 12–8 Ma igneous rocks, including the ore-related Rio Blanco porphyry complex, have typical adakitic signatures, such as high Sr/Y ratios (up to 180) and LaN/YbN ratios (up to 32). The Rio Blanco igneous rocks (Portachuela batholith and Rio Blanco porphyry complex) have uniform zircon εHf(t) values (+0.3 ± 1.2) and δ18O values (6.5 ± 0.14‰). These geochemical characteristics indicate that the Rio Blanco igneous rocks evolved from mantle-derived parental melts in a long-lived, stable, homogeneous isotopic reservoir at the crust–mantle boundary. However, whereas both the Portachuela batholith and the Rio Blanco porphyry complex formed from hydrous parental magmas (&amp;gt;5 wt %; based on plagioclase hygrometry), the ones of the Rio Blanco porphyry complex seem to be more oxidized, hydrous, and sulfur-rich compared with the older batholitic rocks. Reverse zoning in plagioclase phenocrysts, with a systematic core–mantle–rim variation in An (anorthite) and Fe (total iron) contents, are common in the intermineralization rocks. The An content of the mantles of the plagioclase phenocrysts correlates positively with the Fe content, but in the rims, the An contents significantly decrease while Fe remains constant. The apatite inclusions in the mantles are richer in S (0.24 ± 0.06 wt %) and Cl (1.42 ± 0.32 wt %) than those in the phenocryst cores (S: 0.09 ± 0.07 wt %; Cl: 1.03 ± 0.56 wt %) and rims (S: 0.14 ± 0.09 wt %; Cl: 0.83 ± 0. 35 wt %). These systemic geochemical variations in the plagioclase phenocrysts suggest recharge by S- and Cl-rich melts followed by fluid exsolution. This magma recharge and subsequent fluid exsolution may have triggered porphyry Cu mineralization at Rio Blanco. The coincidence of timing between the geochemical transition and collision (initial subduction) of the Inca Oceanic Plateau with the South American plate may indicate a change in the tectonic regime to a compressional state of stress and a thickening of the crust during the collision. The tectonic transition would have facilitated the fractionation of mantle-derived magma in a deep crustal hot zone, resulting in oxidized, volatile-rich residual melts. Replenishment of the upper-crustal magma chamber by such volatile-rich magmas and the subsequent discharge of fluids are interpreted to be fundamental for porphyry Cu mineralization at Rio Blanco and plausibly for the formation of Late Miocene porphyry ore deposits in northern Peru in general.

Crucial control on magmatic-hydrothermal Sn deposit in the Tengchong block, SW China: Evidence from magma differentiation and zircon geochemistry

Magmatic-hydrothermal Sn deposits are commonly associated with high silica magmas, but why most global high silica granites do not bear economic Sn ore grades remains unclear. Two crucial factors controlling magmatic-hydrothermal Sn mineralization, including advanced fractionation and depressurization-induced rapid cooling, were revealed in the case study of the Guyong granitic pluton linked with the Xiaolonghe Sn deposit, in the Tengchong block, SW China. The Guyong granitic pluton comprises three petrological facies: less evolved biotite syenogranite, evolved alkali granite and leucogranite, and highly evolved facies (the protolith of greisenized granite). Similar crystallization ages (∼77 Ma) and gradual contact between different petrological facies indicate the Guyong granitic pluton records a continuous fractionation process. Monte Carlo-revised Rayleigh fractionation model suggests the fractionation degree of the Guyong pluton is markedly high (>87 wt.%) that can only be achieved by a high initial water (≥4 wt.%) content in the parent granitic magma revealed by rhyolite-MELTS calculation. Advanced degree fractionation causes the first Sn enrichment but it also significantly increases the viscosity of evolved magmas, suppressing the exsolution and transport of hydrothermal fluids. Hence, it must be compensated by the second critical factor: depressurization-induced rapid cooling, reflected by the occurrence of highly metamict zircons in the greisenized granite. The highly metamict feature, indicated by the large full width at half maximum (FWHM) values of zircon ν3(SiO4) peak (>19.5 cm−1), suggests these zircons do not experience thermal annealing but rapidly ascend into a shallow cooling environment. Depressurization-induced rapid cooling facilitates exsolution and transport of hydrothermal fluids, interacting with wall rocks and resulting in Sn mineralization.

Deep ore‑forming fluid characteristics of the Jiaodong gold province: Evidence from the Qianchen gold deposit in the Jiaojia gold belt

The Jiaodong gold province hosts world-class gold deposits with giant resources of >5000 t, where the Jiaojia-Xincheng fault belt and the Sanshandao-Cangshang fault belt occupy more than two-thirds of the gold deposits. Recent developments of exploration have advanced in tracking deep mineralization. The nature of the ore-forming fluid, source, and the extreme enrichment mechanism of the gold in this region remain debated. Here, we investigate the deep ore body of the Qianchen gold deposit at the southern part of the Jiaojia gold belt based on mineralogy, fluid inclusion, elemental and isotopic composition of pyrite, and the primary halo geochemistry. Natural gold is the important form of Au in this deposit with a relatively large (up to 200 μm) grain size, suggesting the prospect of mineralization at deeper domains. The accompanying PGE minerals (Hexaferrum) hint at contributions from mantle material. The ore-forming fluids of the Qianchen gold deposit are characterized by medium–low temperatures and salinities and have a general H2O–NaCl–CO2 ± CH4 composition, which indicates that the Jiaodong gold province has a unified ore-forming fluid system from shallow to deep. However, the trace elements and S isotopes of pyrite at different elevations show slight differences, suggesting the composition and properties of the fluid changed during the process of ascending, affected by ore-forming conditions such as temperature, pressure, and oxygen fugacity. Furthermore, the evidence of a decrease of salinity with depth and temperature and fluid immiscibility indicates that fluids from different sources continuously mixed during the upward migration of fluid, with the mineralization triggered possibly by phase separation. The primary halo model suggests promising prospecting potential. We formulate a comprehensive metallogenic model for the mineralization process in the Jiaodong gold province. The Au-bearing hydrous magma derived from the partial melting of the metasomatized SCLM, underwent further magmatic-hydrothermal differentiation and fluid exsolution, and thus the Au was enriched and partitioned into exsolved fluids that evolved to auriferous fluids. During the upward migration of ore-forming fluids (magmatic-hydrothermal), many factors affected mineralization, mainly including fluid-rock interaction and mixing of the metamorphic-hydrothermal and meteoric fluids. The fluid activities, multiple sources of Au in the mantle and ancient basement, together with the large fault structures resulted in the formation of the giant Jiaodong gold province.

Trace elements in apatite from Gejiu Sn polymetallic district: Implications for petrogenesis, metallogenesis and exploration

The world-class Gejiu tin polymetallic district is located in southeastern Yunnan Province. Yet, the largest deposits are concentrated in the Masong and Laoka granites in the eastern area of Gejiu. To investigate the factors controlling Sn mineralization among intrusions in the Gejiu area, we collected apatite from eight different intrusions (including gabbro, monzonite, and granite) and determined the fertility indices of Sn deposits through the chemical composition of apatite. Analysis of the redox-sensitive elements in apatite showed that the ore-bearing granites have high Mn and low S content, high Y/∑REE and low La/Sm ratios, extreme negative Eu anomalies, and weak Ce positive anomalies compared to the barren intrusions, indicating that the formation environment of the ore-bearing granites was more reducing. The apatites in ore-bearing intrusions have lower Sr and higher Y contents, which suggests that these intrusions have a higher degree of fractionation than barren intrusions. The apatites in the Masong and Laoka granites have a high F content, which reflects the high F content of magma that contributed to the enrichment of Sn. The decreasing (Sm/Yb)N ratios with stable Sr content imply exsolution of Cl-rich fluids or crystallization of monazite and allanite. The exsolution of Cl-rich fluids can increase Sn solubility in the melt and promote the extraction of Cu from basalt. The apatites in the ore-bearing intrusions have high Li, Mn, Fe, and F contents and extremely low (Eu/Eu*)N ratios (<0.05). These results show that apatite can be used to distinguish Sn deposits from other deposit types and guide the Sn exploration.

The roles of emplacement depth, magma volume and local geologic conditions in the formation of the giant Yulong copper deposit, Eastern Tibet

The relative importance of the oxidation state, water content, and copper and sulfur content of the causative magma in the formation of porphyry Cu deposits is hotly debated. The giant Yulong Cu deposit in Eastern Tibet, with proven reserves of 6.5 Mt Cu at 0.62%, is an example of an economically important porphyry deposit. We investigate the relative roles of factors and different processes controlling the formation of the ores. Zircon U-Pb age data and whole rock, amphibole and apatite major and trace element geochemistry are reported together with previously published data. The results indicate a focused sub-alkaline magma event at 41 Ma in the North Yulong district and contemporaneous mineralization between 41.0 and 40.9 Ma at Yulong. Among the Yulong intrusive complex, the causative K-feldspar granite porphyry with high K2O (up to 7 wt%) and SiO2 (∼70 wt%) contents and elevated Sr/Y ratios (70–90), is interpreted to be a product of fractional crystallization of the evolved sub-alkaline magma. When the Yulong deposit is compared with the global porphyry Cu deposits with similar metal endowment and coeval sub-economic intrusions and barren igneous rocks in the belt, we find that high oxidation states (ΔFMQ + 1.0∼+2.5) and water contents (∼4–6 wt%) coupled with elevated background Cu contents (∼40–120 ppm) in the causative magma are essential in the formation of porphyry Cu deposits. However, the porphyry Cu deposits have magmatic S concentrations (∼10–100 ppm; estimated from apatite SO3 contents) comparable to that of the barren igneous rocks, independent of the relative timing of apatite crystallization to fluid exsolution. Overall, these conditions are necessary but not sufficient alone to form a porphyry deposit. The economic porphyries commonly have shallower emplacement depths (∼4–5 km) which favor extensive fluid exsolution and larger magma volume (∼50–250 km3 at Yulong) which provide enough storage of ore elements than the barren igneous rocks. In the North Yulong district, extensive fluid exsolution is absent from the two sub-economic intrusions indicated by their relatively high and fixed oxygen fugacity and H2O and Cl contents in the magma evolution. In contrast, at Yulong, a local anticlinal structure provided a trap in which exsolved fluid from the magma was concentrated, and carbonate wall rocks triggered the efficient precipitation of metals during intense fluid-rock interactions and skarn alteration. Both are also critical processes in the formation of the giant Yulong Cu deposit.

Boron isotopes of tourmalines from the central Himalaya: Implications for fluid activity and anatexis in the Himalayan orogen

The relationship between fluid activity and anatexis in the Himalaya is ambiguous in regard to whether it is dominated by fluid-saturated melting triggered by Lesser Himalayan Sequence (LHS) fluids or dehydration melting of Greater Himalayan Sequence (GHS) metasedimentary rocks. Boron isotopes are a sensitive probe for tracing the source of fluids and their role during partial melting. However, a lack of constraints on LHS fluid activity has limited our understanding of anatectic processes in the Himalayan orogen. Here we report the results of in situ tourmaline and muscovite B isotope analyses of a suite of samples from the LHS and GHS, including chlorite schist from the LHS, quartz–tourmaline rock from the Main Central Thrust Zone, and leucogranites, leucosomes, mica schist, and garnet amphibolite from the GHS. Tourmaline B isotopic compositions fall in the range of δ11B = −18.3‰ to −8.2‰, with tourmalines from leucogranites showing δ11B values of −17.2‰ to −8.2‰, from leucosomes of −11.8‰ to −10.7‰, from schists of −17.8‰ to −13.9‰, from quartz–tourmaline rock of −18.3‰ to −12.8‰, and from garnet amphibolite of −16.9‰ to −13.6‰. Muscovites from leucogranite and mica schist yield relatively low B isotopes of −23.3‰ to −17.2‰ and −22.4‰ to −15.4‰, respectively. Different source compositions, prograde devolatilization, tourmaline fractionation, fluid exsolution, and external fluid metasomatism are inferred to be the main mechanisms responsible for the varied B isotopes in this study. Our data suggest that the LHS is characterized by depleted 11B, with LHS-derived dehydration fluids calculated to have δ11B values of −15.2‰ to −12.0‰ at 600 °C, whereas metamorphic fluids in the GHS have δ11B values of −11.8‰ to −4.8‰. Thus, we propose that dehydration of LHS hydrous minerals produced relatively light-B fluids. Consequently, leucogranites derived from hydration melting triggered by LHS fluids exhibit light B isotopic compositions compared with those derived from dehydration melting in the GHS. To investigate the source of heavy B in GHS leucogranites, we modeled B isotopic fractionation during muscovite prograde devolatilization, with results showing that substantial fractionation (∆11B > 7‰) would occur during muscovite dehydration. Hence, B lost from micas could be a source of heavy B in the leucogranites. On the basis of these results, we propose a model whereby LHS hydrous metasediments underwent intense dehydration, with the resultant fluids migrating upward and triggering hydration melting on the edge of melt channel in the GHS. Hence, tourmalines that crystallized from the hydrated magmas inherited the low δ11B characteristic of the LHS fluids, whereas those derived from dehydration melting of micas in the interior of melt channel acquired higher δ11B values.

Geology and factors controlling the formation of the newly discovered Beimulang porphyry Cu deposit in the western Gangdese, southern Tibet

The porphyry Cu deposits in the Gangdese metallogenic belt were mainly formed in the eastern portion. The Beimulang is a newly discovered deposit formed in post-collisional setting in the western part of the belt and adjacent to the super-large Zhunuo porphyry Cu deposit. There are three epochs of magmatism at Beimulang, including pre-mineralization quartz porphyry (PQP; 49.7 ± 0.4 Ma), inter-mineralization monzogranite porphyry (IMGP; 14.8 ± 0.2 Ma), monzogranite (IMG; 14.1 ± 0.2 Ma), and late-mineralization granite porphyry (LGP; 11.7 ± 0.1 Ma). The various degrees of alteration and sulfide occurred at the IMG(P) and LGP indicate that there are at least two episodes of fluid exsolution events associated with different intrusions spanning ∼ 3 m.y. at Beimulang, which are unlike other Gangdese porphyry Cu deposits showing single fluid exsolution around only one porphyry at Miocene (e.g., Qulong, Zhunuo, Dabu). The PQP is characterized by high SiO2 (75.6–81.4 wt%) and K2O (2.1–4.3 wt%) concentrations, high Nb/Ta (8.4–11.3) ratios, and low Mg# (22–43), Ni (1.5–2.7 ppm) contents. They have high (87Sr/86Sr)i (0.705098–0.707767) ratios and low εNd(t) (-4.9 to −1.9) values. These geochemical characteristics indicate that they were probably derived from a juvenile basaltic lower crust during the ongoing Indian-Asian collision. The IMGP and IMG show similar petrography and geochemical characteristics, such as high SiO2 (65.0–69.8 wt%), K2O (3.6–4.3 wt%), Sr (550–809 ppm) contents, high Sr/Y (79–89) ratios, and low Mg# (43–48), Th/Yb, and Nb/Yb values, combined with high (87Sr/86Sr)i (0.707254–0.708017) and low εNd(t) (-7.9 to −6.0), implying a similar source probably originated in the subduction-modified lower crust metasomatized by Neo-Tethyan oceanic slab-derived fluids. The LGP displays trends produced by fractionation of magma with a composition similar to the IMG(P), but wall rock assimilation plays a significant role during magmatic evolution. Zircons from the IMG(P) show higher Eu/Eu* (>0.3), 10000*(Eu/Eu*)/Y (>4), and (Ce/Nd)/Y (>0.04) ratios than those of the PQP, indicating the fertile magmas were more oxidized and hydrous. The high magmatic oxidation state for the IMG(P) is also supported by using the zircon (ΔFMQ = +4.1 to + 6.1), and amphibole (ΔFMQ =+2.6 to + 3.4) oxybarometers. Their high magmatic water contents are supported by evaluations using amphibole (∼3.5 wt%) and plagioclase (∼6.7 wt%) compositions, respectively. However, the LGP has higher magmatic oxygen fugacity (Eu/Eu* >0.3, ΔFMQ = +4.2 to + 7.3) caused probably by auto-oxidation of the magmas, but lower magmatic water contents (Sr/Y = 31 ± 15; V/Sc = 6 ± 4) caused presumably by previous fluid exsolution and/or fractionation of hydrous silicate minerals during magma ascent than those of the IMG(P). Therefore, the highly hydrous and oxidized magmas with juvenile material are favorable to form porphyry Cu deposits in Tibetan orogen. On the contrary, magmatic evolution can probably reduce the water content of magma chamber, which is not beneficial to porphyry Cu mineralization.

Stress changes caused by exsolution of magmatic fluids within an axisymmetric inclusion

SUMMARY In volcanic regions ascending magma is subject to depressurization and is generally accompanied by exsolution of volatiles. We assume a process in which these volatiles propagate upward across newly fractured and permeable rock layers, bringing a sharp increase of pore pressure and temperature within a thin disc-shaped region (inclusion). Thermo-poro-elastic (TPE) inclusion models provide a mechanism to explain seismicity and deformation induced by p and T changes in absence of new magma emplacement in volcanic contexts. They are also suitable to represent the mechanical effects due to fluid extraction and re-injection in geothermal fields. In the present work analytic solutions are provided for the displacement, strain and stress fields assuming a TPE unbounded medium. Significant deviatoric stress is generated by positive increments of pore pressure and temperature: the stress field is fully deviatoric outside the TPE inclusion, but a strong isotropic stress component is present within, leading to highly heterogeneous faulting mechanisms: if the disc plane is horizontal, thrust faulting mechanisms are favoured within the TPE disc over optimally oriented faults and normal mechanisms above. The model is easily generalized to a vertically thick disc with variable temperature and pore-pressure changes: then, an extensional environment can be obtained even within the TPE inclusion assuming upward decreasing of pore pressure and temperature changes. The supplied analytical solution may be used to model near-field TPE inclusion effects and to validate more complex numerical modelling.

Magmatic and Hydrothermal Evolution at Qian’echong, Central-Eastern China: Insights into Dabie-Type Porphyry Mo Mineralization

Abstract Dabie-type porphyry Mo deposits have recently been identified as a new subtype of porphyry Mo deposits, but several questions remain about the role of ore-related magmas in the formation of this type of deposit, as well as distinctions in genetic processes with the well-studied Climax-type porphyry Mo deposits. Here, mineral and melt inclusions from the giant Qian’echong deposit, Dabie orogen, central-eastern China, were studied in order to improve our understanding of the nature and the role of ore-related magmas in the genesis of Dabie-type porphyry Mo deposits. The magmatic and hydrothermal evolution of the system was reconstructed based primarily on the analysis of quartz-hosted silicate melt and mineral inclusions, in concert with field and petrographic relations as well as previously published U–Pb and Re-Os geochronology. Ore-related magmas at Qian’echong include, from early to late, quartz porphyry dikes, rhyolite porphyry dikes, granite porphyries (stock and dikes) and a newly discovered, deep-seated monzogranite porphyry stock. Based on TitaniQ thermobarometry, these lithologies were sourced from a ~16–19-km deep (500–600 MPa) magma chamber and underwent nearly isothermal decompression to ~7 km (200 MPa), at temperatures ranging from 720°C to 690°C. According to the trace element composition of melt inclusions, in combination with published whole-rock Nd isotopic compositions, the early magma evolved from quartz porphyry to rhyolite porphyry through fractionation crystallization and thus became a crystal mush. This viscous crystal mush was subsequently re-melted to produce the granite porphyries and was injected with melts from a different source to generate the monzogranite porphyry. At Qian’echong, all melt inclusions have low concentrations of Mo (2–8 ppm), &amp;gt;5 wt. % H2O, and little to no F (≤0.26 wt. %), arguing against the requirement for ore-related magmas to be enriched in Mo to form large porphyry Mo deposits. Rayleigh fractionation modeling shows that the concentration of Mo in the quartz and granite porphyries increased through fractionation of quartz, feldspars, biotite, magnetite, and ilmenite and that Mo was subsequently depleted in the melt through fluid exsolution, exclusively in the rhyolite and monzogranite porphyries, as a result of magma ascent and decompression. This suggests that, in addition to ore-related granite porphyry, both the rhyolite and the monzogranite porphyries also contributed to the hydrothermal mineralization. This study confirms that the formation of porphyry Mo deposits does not rely on abnormally high concentrations of Mo in ore-related magmas but instead requires efficient extraction of Mo from large volumes of magmas, with normal concentrations of Mo. Unlike the Climax-type deposits where multiple pulses of ore-forming fluids are delivered from convecting shallow magma chambers, Mo mineralization in Dabie-type deposits was achieved by the assembling of ore-forming fluids from successively emplaced, relatively deep intrusions. Although the Dabie- and Climax-type Mo deposits are respectively associated with I-type and A-type granitoids in the Dabie orogen, it is suggested that both types of magmas are derived from the partial melting of subducted Yangtze continental crust and that it is the tectonic transition from compressional to extensional settings that controlled the different styles of porphyry Mo mineralization in this orogen.

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 (&amp;lt;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 &amp;gt;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.