Low-salinity Vapor Research Articles

Improvements in the Quantification of End-Member Vapor Compositions in Fluid Inclusion Boiling Assemblages and Implications for Metal Transport by Low-Density Fluids

Abstract Under conditions typically found in the Earth’s crust, there is a large pressure-temperature-composition range in the H2O-NaCl system where fluids may separate into a low-salinity vapor end member and a high-salinity liquid end member. However, heterogeneous trapping is common during the formation of fluid inclusions in an immiscible fluid system, violating the fundamental assumption of homogeneous entrapment for fluid inclusion microthermometry. This has profound consequences on the composition of these fluid phases and consequently on the formation of ore deposits from hydrothermal systems. At the same time, minor mixture of the high-salinity liquid phase with the low-salinity vapor phase cannot be distinguished from the end-member vapor-rich fluid inclusions by their bubble sizes. Precise determination of the salinities of vapor-rich fluid inclusions using microthermometry is deterred by the very small proportion of the liquid phase and the limitation of the analytical precision. All this will lead to erroneous compositional data from fluid inclusion analysis. We have quantitively calculated the variations of fluid inclusion properties caused by heterogeneous entrapment during phase separation in the H2O-NaCl system and showed that the salinity, and thereby the element contents and element/Na ratios of the vapor-rich fluid inclusions, is significantly changed in heterogeneously trapped fluid inclusions. The addition of 1 vol % of the high-salinity end member to the low-salinity end member results in a salinity change from 0.99 to 4.4 wt % NaCl equiv at the phase separation condition of 800 bar and 750°C. This will have a significant impact when it comes to determining the concentration of elements transported as chlorine complexes and typically leads to an overestimation of the mass transport capability by the vapor phase. Therefore, every effort should be taken during microthermometry to find and measure the least affected vapor-rich fluid inclusions. Our numerical calculations and synthetic fluid inclusions reveal that there is a linear relationship between the salinity and element contents for fluid inclusions at different extents of heterogeneous entrapment. Therefore, linear correction to the measured lowest vapor salinity can obtain a good approximation of the element contents in the end-member vapor phase.

Porphyry copper mineralization triggered by sulfate reduction and alkali metasomatism: Constraints from an experimental investigation

The potassium silicate (K-silicate) alteration zone is the main ore contributor in porphyry copper deposits worldwide. Knowledge of element behaviors in the alteration and mineralization processes is essential for an improved understanding of porphyry copper mineralization, but they are still not well understood. In this study, we reacted synthetic Cl-rich fluids, containing K, Na, Cu, Mo, Zn, etc., with andesite in a complex experimental system to simulate the shallow porphyry copper mineralization process. We aimed to bridge the gap between simple experimental studies and complex natural systems and to evaluate the contribution of sulfate reduction to porphyry ore formation and its relationship with early alkali metasomatism. The results show that increasing temperature (from 300 to 500 °C) enhances the K-silicate alteration by promoting ion-exchange reactions, and the K-feldspar is mainly formed by the transformation of plagioclase via a dissolution-reprecipitation processes. The low-salinity vapor phase has a stronger capacity for K-silicate alteration than the liquid phase at similar temperatures. In addition, increasing temperature from 300 to 500 °C favors sulfate reduction to further enhance metal sulfide precipitation. The limited availability of reduced sulfur in the fluid causes preferential precipitation of Cu-(Mo) sulfides, while most of the Zn is soluble in the fluid, and Cu precipitation as sulfides in the vapor is much more efficient than in the coexisting liquid. The overlap between the K-silicate alteration zone and the mineralization triggered by sulfate reduction in porphyry copper deposits is controlled by several concomitant factors, e.g., relatively high temperature (e.g., at 400−500 °C), vapor formation, and decompression. Moreover, K-silicate alteration would further promote mineralization by changing fluid compositions, e.g., removing K from the fluid.

New insights from thermohaline multiphase simulations into the mechanisms controlling vent fluid salinity following a diking event at fast-spreading ridges

Hydrothermal fluids expelled at mid-ocean ridge vent sites show a large spatial and temporal variability in exit temperatures and chlorinities. At the East Pacific Rise (EPR), 9°50.3'N, time series data for a 25+ year period reveal a correlation between these variations and magmatic diking events. Heat input from dikes appears to cause phase separation within the rising fluids splitting them into low-salinity vapor and high-salinity brine phases. The intrusion of a new dike is therefore likely to result in a characteristic salinity signal, with early post-eruptive fluids showing vapor-influenced low salinity and later brine-influenced fluids showing high salinity values.We here use a 2-D multiphase hydrothermal flow model to relate these observations to processes and properties within the sub-seafloor such as permeability, porosity, background flow rates, and phase separation as well as segregation phenomena. We have grouped the time evolution of vent fluid salinity into four temporal stages and have identified how multiphase flow phenomena control vertical salt mass fluxes within each stage. Rock porosity and permeability as well as background temperature of the undisturbed hydrothermal system control the duration of the four stages and maximum venting salinity. Based on our results, we are able to reproduce the characteristics of time-series data from the EPR at 9°50.3'N and infer the likely ranges of rock properties and the hydrothermal conditions within the oceanic crust beneath.

Temporal and spatial separation mechanisms of the Cu and Mo mineralization in the Dabate porphyry deposit, Western Tianshan, Xinjiang, China

Dabate deposit is a typical porphyry Cu-Mo deposit in the Western Tianshan, Xinjiang, China. Both the Cu and Mo mineralization are mainly associated with intrusion of rhyolite porphyry. Distinguishing from typical porphyry deposits, the Cu mineralization and Mo mineralization are separated from each other on both the temporal and spatial scales. The Cu orebodies are mainly hosted in the contact zone between the rhyolite porphyry and Tuosiku’ertawu Formation and predominantly occur as fluorite-arsenopyrite-Cu-sulfides ± K-feldspar ± quartz veins (Cu-1 stage), whereas the Mo orebodies are mostly hosted in the inner part of the rhyolite porphyry and are typically presented as Mo-2 stage quartz-molybdenite veins (external Cu and internal Mo). Total homogenization temperatures (Th, L-V) for boiling fluid inclusion assemblage of the Mo-2 stage occurred at 302 to 358 °C, with corresponding salinities of 1.4 to 38.1 wt% NaCleqv. H-O isotope data indicate that the Mo mineralization fluids were mainly derived from magmatic water. The non-boiling fluids of the Cu-1 stage originated from magmatic fluids and meteoric water, which were characterized by high temperature (323 to 379 °C) and low salinity (2.6 to 5.9 wt% NaCleqv) as well as Cu, As, and volatile enrichment. These features are different from the initial magmatic fluids of this deposit, indicating that the Cu mineralization may occur before the Mo mineralization (early Cu and late Mo) and the magmatic fluid components of the Cu-1 stage should be derived from the condensation of the low-salinity vapor generated by boiling of the hydrothermal fluids (351 to 405 °C; 1.8 to 40.1 wt% NaCleqv) for the earliest Mo-1 stage barren quartz veins. The positive δ34S values (5.5 to 9.8‰) of sulfides indicate that the sulfur in the ore-forming fluids was derived from magma and country rocks. The subtle change in fluid conditions from the high temperature (peak range 340 to 360 °C), alkaline, and slightly more oxidized of the Cu-1 stage to high temperature (peak range = 320 to 340 °C), more reduced, and acidic of the Mo-2 stage gave rise to the temporal separation of Cu and Mo mineralization. The Cu, As, and volatiles (e.g., CO2, HF), preferentially partitioned into the vapor phase, were highly enriched in Cu ore-forming fluids, whereas Mo mineralization fluids contained the majority of Mo with strong preference for the brine phase. Therefore, the spatial separation of the Cu and Mo mineralization could be attributed to selectively fractionating between the vapor phase and hypersaline liquid of the Cu and Mo metals during the process of fluid boiling.

Genesis of the distal axis East Longjing-2 hydrothermal field on the ultraslow-spreading Southwest Indian Ridge: Evidences from fluid inclusions

Tectonic-related hydrothermal fields constitute about half of the hydrothermal activities on slow-ultraslow spreading ridges, and may form sulfides large in size and rich in precious metals. Ore-forming fluids are critical for element migration progressions, thus are key parameter to reveal their genesis and sulfide mineralization processes. However, previous studies of this kind of hydrothermal activities primarily focused on vent fluids which have transient effects or based on surface samples, while the ore-forming fluid evolution and related sub-seafloor mineralization process are still poorly constrained. In this study, we analyzed the characteristics of fluid inclusions in the stockwork mineralization of the East Longjing-2 hydrothermal field (ELHF-2) on the Southwest Indian Ridge. The results showed that the fluids evolved gradually from medium-high temperatures (260–315 °C, mean of 284 °C) and low salinities (0.70–3.70 wt.% NaCl eq.) in the disseminated pyrite mineralization stage (Stage I) to medium temperatures (239–261 °C, mean of 249 °C) and low salinities (1.05–3.85 wt.% NaCl eq.) in the chalcopyrite–pyrite–quartz veinlet mineralization stage (Stage II). Laser Raman specular analysis indicated the fluid inclusions were mainly composed of H2O. The low salinity (lowest 0.7 wt.% NaCl eq.) and various vapor volume (0.05–0.5) observed in these fluid inclusions suggested the ore-forming fluids were probably mixtures of the low-salinity vapor phase formed by phase separation with large amounts of seawater. The precipitation of the ore-forming elements was likely induced by temperature decreasing. We propose that hydrothermal circulation in ELHF-2 was a result of coupled high-permeability detachment fault and shallow gabbro intrusion. Our results suggest that high temperature hydrothermal activities could be developed on distal axis area on ultraslow spreading ridges.

Hydrothermal evolution and ore precipitation of the No. 2 porphyry Cu–Au deposit in the Xiongcun district, Tibet: Evidence from cathodoluminescence, fluid inclusions, and isotopes

The No. 2 porphyry Cu–Au deposit in the Xiongcun district forms part of the Gangdese porphyry copper belt of Tibet. This study clarifies the fluid evolution and ore precipitation mechanisms of the magmatic-hydrothermal system that formed the deposit by combining the results of fluid inclusion petrography, microthermometry, scanning electron microscope-cathodoluminescence imaging, and stable isotope analyses of multiple quartz-dominated veins. Four distinct sets of quartz-dominated veins and eight generations of quartz (Q1–Q8) within the veins were identified, namely from early to late, early barren quartz veins (V1) comprised of equigranular CL-bright Q1 quartz, quartz–pyrite–chalcopyrite ± magnetite veins (V2) containing early equigranular CL-bright Q1 quartz and later CL-gray or CL-dark Q2–3 (Q2 + Q3) quartz grains intergrown with Cu–Fe sulfides, quartz–molybdenite ± pyrite ± chalcopyrite veins (V3) comprised of quartz grains (Q4–Q6) characterized by subhedral comb-like textures with well-developed oscillatory growth zoning, and late quartz veins (V4) containing CL-gray Q7 quartz with euhedral growth zones and later CL-dark Q8 quartz. Fluid inclusions in these veins can be classified into six types: B40, B40H, B80, B15H, B15H+, and B15. The B40 and B40H inclusions; B80 inclusions; and B15, B15H, and B15H+ inclusions contain vapor bubbles occupying 30–60 vol%, >70 vol%, and 5–25 vol%, respectively; in addition, B15H, B15H+, and B40H inclusions contain transparent daughter minerals (halite, and in some cases, sylvite). Raman spectra showed that CO2 is only present in B40 and B80 inclusions. In the Q1 quartz in V1 and V2 veins, the dominant type of inclusions is B40, which have homogenization temperatures (Th) of 278 °C–389 °C (peaking at 310 °C–330 °C) and salinities of 2.0–12.8 wt% NaCl equiv. In Q2–3 quartz in V2 veins, all inclusion types were observed. These were trapped at temperatures of approximately 325 °C–340 °C and had salinities of 1.8–69.7 wt% NaCl equiv. In V3 veins, only B15H and B15 inclusions were observed, which had Th values of 171 °C–320 °C (peaking at 240 °C–260 °C) and 173 °C–317 °C (peaking at 210 °C–230 °C) and salinities of 28.7–33.9 wt% NaCl equiv and 5.6–23.1 wt% NaCl equiv, respectively. In V4 veins, only B15 inclusions were observed. Primary B15 inclusions in V4 veins yielded Th values of 166 °C–229 °C (peaking at 170 °C–200 °C) and salinities of 5.1–17.1 wt% NaCl equiv. The 3He/4He and 40Ar/36Ar ratios of the fluid inclusions exhibited the ranges of 0.08–0.84 Ra and 451.3–1567.8, respectively, and the δ18Ofluid and δDfluid values varied from −3.2‰ to 5.8‰ and −92.8‰ to −80.3‰, respectively. By integrating all results from the fluid inclusion, cathodoluminescence, and isotopic analyses, we conclude that the initial ore-forming fluids of the No. 2 deposit were low-salinity, CO2-rich single-phase fluids of magmatic origin. Subsequently, fluid immiscibility developed in the initial ore-forming fluids, generating hypersaline liquid and low-salinity vapor phases and leading to the separation of a CO2 phase plus chalcopyrite precipitation from the fluids. As meteoric water was injected into the hydrothermal system, the ore-forming fluids gradually evolved to become meteoric water-dominated, low temperature, low-salinity, and CO2-poor; in addition, fluid-cooling due to the meteoric water input resulted in molybdenite precipitation.

How do metals escape from magmas to form porphyry-type ore deposits?

Many precious and base metals (Cu, Au, Ag, Mo, W, Sn) in porphyry-type deposits are extracted from a granitic to granodioritic silicate melt and transported by magmatic-hydrothermal fluids to the site of ore deposition. Metals abundance increases from some ppb in the melt to % in ore bodies. Assuming a unique magmatic source for metals, it implies an exceptional combination of enrichment process acting over up to 4 orders of magnitude. Previous models of magma differentiation and hydrothermal circulations omitted to consider the role of metal segregation during magma chamber evolution. Models for the formation and evolution of intrusions have recently shifted from the so-called melting-storage-assimilation-homogenization (MASH) paradigm, basically steady state, to the mantle-melting-segregation-ascent-emplacement (m(M-SAE)) paradigm, dynamic and discontinuous in time. Successive magma injections of variable compositions progressively build the magma chamber. This is supported by field relations (e.g. cross-cutting dykes and stocks), textures (e.g. partially resorbed enclaves, or mineral fabrics), geochemistry (e.g. hybridization signatures), and age dating aswell as istopic studies. As melt crystallizes and forms a mush (>50% crystals), volatiles also exsolve forming a magmatic volatile phase (MVP) while melt motion slows down. Metals partition between the three phases: melt; crystals; and MVP. They generally prefer the MVP owing to more favorable partition coefficients. This is indicated by metal content in coeval melt and fluid inclusions with homogenization temperatures above 600 °C, or in volcanic fumaroles. Here, by simulating the physical interactions between the three phases we suggest a model of fluid sparging, during which metals segregate towards the MVP by diffusion and are further transported by advection as metal complexes. The model also provides estimates of metal enrichment. An undimensional Péclet number rules the competition between diffusion and advection, basically scaling with respect to the inverse of the product of melt viscosity (η) by metal diffusivity (D). The threshold value of the Péclet number between diffusion and advection is roughly 10−9. Fast diffusive metals (Au, Cu, Ag, W) readily diffuse from silicate melt and towards bubbles of the MVP. To escape the mush through tubular structures, the MVP must overcome a critical gas saturation level (about 20% vol.) usually reached after several magma injections. Advection then takes over and transports the metal-enriched MVP towards the top of the magma chamber. This leads to nearly coeval (1) separation between a high salinity-liquid phase and a low-salinity vapor phase, (2) fluid-rock interactions resulting in potassic, advanced argillic and phyllic alterations and (3) metal deposition. The metal enrichment scales as the ratio between partition coefficient (i.e. related to the gradient in chemical potential), diffusivity (i.e. related to the gradient of concentration), and melt viscosity (i.e. related to the gradient of momentum). The rates at which all such gradients relax determe metals enrichment, inducing chemical and physical instabilities, leading to a cyclic process. The whole cycle also encompasses the case of a partial, non-completed, full enrichment yielding to barren intrusions. A tentative model generalizes the sparging fluid model to other metal deposits linked to an intrusion. Such generalization should be interpreted as predicting metal enrichment by 3–4 orders of magnitude, rather than predicting an exact value.

An Analytical Model for Flow of Seawater Adjacent to an Igneous Wall

We develop an analytical model describing the flow of NaCl– $$\hbox {H}_2\hbox {O}$$ in a saturated porous medium adjacent to a hot vertical wall and apply the model to flow along a 450-m high dike in a generic, high-temperature seafloor hydrothermal setting. After 0.5–4 days, a steady two-phase layer forms along the wall with a maximum width of approximately 20 cm. The layer consists of rising, low salinity vapor and sinking, high salinity brine; its width increases as height to 1/2 power. The brine salinity is highest at the dike–fluid interface and decreases toward the edge of the layer. Vapor dominates the vertical heat transport, while brine dominates salt transport; the resulting phase separation leads to increasing liquid volume saturation and bulk salinity with increasing depth. The bulk salinity reaches 10% NaCl by weight (wt%) at the lowest height for which the model is valid. The two-phase zone begins to decay from the bottom upward at around 7 days. Phase separation at the top of the system persists for 21 days. The heat required to sustain the layer is derived primarily from the latent heat of magma crystallization.

Fluid and metal sources of the Wenquan porphyry molybdenum deposit, Western Qinling, NW China

The Wenquan porphyry moybdenium deposit, Western Qinling, NW China, with a resource of 247 million tonnes at 0.048% Mo, formed during Triassic collision between South China and North China blocks. Ore fluids at Wenquan vary widely in composition from single-phase, low-salinity aqueous to low-salinity vapor, and hypersaline inclusions. Fluid inclusion assemblages in a quartz-molybdenite vein contain >60vol% vapor, are equant to negative-crystal shaped, and form clusters or distinct inclusion trails. They yield homogenization temperatures of 285–295°C, with an estimated trapping temperature of 425°C, suggesting a paleodepth of about 5km calculated at a pressure correction of 100–150MPa. Hydrothermal K-feldspar from early stockwork veins related to potassic alteration have calculated δ18Ofluid values of −1.9‰ to +1.9‰. Hydrothermal sericite from an overprinting phyllic alteration associated with late quartz-pyrite veins has calculated δDfluid values between −68 and −60‰, and δ18Ofluid values from −3.7 to +1.4‰. These isotopic data suggest that both early- and late-stage fluids are dominated by magmatic fluids, with influx of meteoric water during the late stage. Exsolution of volatiles from magma in a late-stage open system, results in more variable δD values than an earlier closed-system.Molybdenite and pyrite have δ34S values ranging from 1.1‰ to 6.6‰, indicating that sulfur at Wenquan had a magmatic source. A linear relationship between δ34S values of sulfides separated from early potassic alteration and late phyllic alteration could reflect incorporation of isotopically heavy evaporate sulfate into source magmas from underlying Devonian sedimentary rocks during late alteration. The δ56Fe whole-rock values of altered porphyries range from 0.08‰ to 0.26‰, similar to δ56Fe values of 0.15‰–0.32‰ for pyrite from quartz veins related to both the potassic and phyllic alteration assemblages. The δ56Fe values of pyrite are positively correlated to, but in general slightly lighter than, those of altered porphyries, indicating similar metal sources. Moreover, both altered porphyries and the pyrite are progressively enriched in heavy Fe isotopes from biotite, through potassic to late phyllic alteration, consistent with isotopic evolution from an early lithostatic load to a late hydrostatic load.The Triassic intrusive rocks and fluids responsible for mineralization were derived from a deeply-sourced hybrid mantle-crustal magma crystallizing at a paleodepth of 5km under lithostatic load. External Late Triassic meteoric or Devonian formational fluids became a part of the hydrothermal system during the post-fracturing final stage of ore formation under a hydrostatic regime. The formation of the Wenquan magmatic-hydrothermal systems spans the ductile-brittle transition based on a normal thermal gradient as constrained by the temperature range of 550–300°C. The early, high-temperature potassic alteration developed under a ductile regime, whereas the later, low-temperature phyllic alteration correlates to a brittle environment, where hydraulic rock fracturing enhances permeability and fluid circulation. Most sulfur and metals were precipitated from vapor-rich fluids resulting from cooling and expansion of a single-phase fluid exsolved from a hybrid magma.

Fluid inclusions from the Jinchang Cu–Au deposit, Heilongjiang Province, NE China: Genetic style and magmatic-hydrothermal evolution

The Jinchang Cu–Au deposit is located in the easternmost part of the Central Asian Orogenic Belt (CAOB). The mineralization here is associated with Cretaceous porphyritic intrusions of ca. 120–110Ma and is mainly hosted within intensely altered Mesozoic granites, breccia pipes, ringed and radial faults. The gold ores are disseminated in the wall rock, and also occur as massive auriferous quartz-sulfide veins in fractures and breccias. Fluid inclusion data from breccias, chalcopyrite–quartz vein, and pyrite–quartz vein display homogenization temperatures in the range of 200 to >550°C and the fluids show variable salinity in the range of 0–58wt.% (NaCl equiv.). The halite absent fluid inclusions shows a slight increase in salinity with a sharp decrease in temperature. We interpret that the high-salinity brine and low salinity vapor- or aqueous-rich fluids from ores represent fluid unmixing from magma. The fluid inclusion data from Jinchang demonstrate that the main mineralization took place at temperatures from 480°C to 200°C, and evolved from high to low salinities. Geology and fluid inclusion data suggest that the Jinchang deposit is an oxidized intrusion-related Cu–Au deposit. The voluminous hydrothermal magnetite formed during high temperature potassic alteration of wall rocks in this ore deposit can be used as a potential prospecting tool.

An experimental study of high temperature potassic alteration

A magmatic volatile phase (MVP) may be a supercritical fluid, low salinity vapor, or high salinity brine. The MVP causes hydrothermal alteration and can also transport metals in ore forming environments. Unfortunately, there is a lack of experimental data regarding mineral alteration by high salinity liquids (brines) in porphyry systems. This study was designed to expand the experimental data set on mineral alteration by chloride-rich subcritical, vapor-undersaturated high-salinity brines. The phase stability and equilibrium brine compositions in the K-feldspar–andalusite/sillimanite–quartz–brine system (the brine was 70wt% KCl equivalent; KCl–HCl–H2O) were determined from 600 to 750°C and 50 to 80MPa for the equilibrium:KAlSi3O8+HCl=1/2Al2SiO5+KCl+5/2SiO2+1/2H2O. The stable mineral phase was ascertained by an examination of crystal morphology, optical microscopy, and by the compositions of the run products. The mineral stability fields were plotted as a function of temperature, pressure, and molar (KCl/HCl) concentration ratios, CKClbrine/CHClbrine, of the brine. K-feldspar was observed to be stable over the entire temperature range of the experiments whereas andalusite was only stable at temperatures <750°C, sillimanite was found to be the stable Al2SiO5 polymorph at 750°C. The K-feldspar–andalusite/sillimanite–quartz–brine equilibrium boundaries at 80MPa and 600, 650, 700, and 750°C are at CKClbrine/CHClbrine of 38±6, 29.5±0.5, 22±8, and 6±2, respectively (uncertainties are half-widths of the reversal brackets). Pressure effects were examined by fixing temperature at 600°C while varying pressure to 50, 65 and 80MPa. The K-feldspar–andalusite–quartz–brine phase boundaries at 600°C and 50, 65 and 80MPa were determined to be at CKClbrine/CHClbrine of 11±3, 30±10 and 38±6, respectively. These data show that with increasing pressure and/or decreasing temperature, the equilibrium boundaries shift to higher CKClbrine/CHClbrine values in the brine. These phase boundaries are not consistent with the metastable extensions of the reaction boundaries involving low salinity fluids and illustrate that the chloride content of the MVP has an impact on the equilibrium KCl/HCl during potassic alteration. A thermodynamic model was developed from the determined equilibrium boundaries that allows for the comparison of low- and high-salinity systems. The model shows that there is a more substantial deviation from ideality in high-salinity systems when compared to low-salinity systems and that these deviations can be quantified by the calculated activity, aKClbrine/aHClbrine, and activity coefficient, γKClbrine/γHClbrine, ratios determined in this study.

High-temperature magmatic fluid exsolved from magma at the Duobuza porphyry copper-gold deposit, Northern Tibet

The Duobuza porphyry copper–gold deposit (proven Cu resources of 2.7 Mt, 0.94% Cu and 13 t gold, 0.21 g t−1 Au) is located at the northern margin of the Bangong-Nujiang suture zone separating the Qiangtang and Lhasa Terranes. The major ore-bearing porphyry consists of granodiorite. The alteration zone extends from silicification and potassic alteration close to the porphyry stock to moderate argillic alteration and propylitization further out. Phyllic alteration is not well developed. Sericite-quartz veins only occur locally. High-temperature, high-salinity fluid inclusions were observed in quartz phenocrysts and various quartz veins. These fluid inclusions are characterized by sylvite dissolution between 180 and 360°C and halite dissolution between 240 and 540°C, followed by homogenization through vapor disappearance between 620 and 960°C. Daughter minerals were identified by SEM as chalcopyrite, halite, sylvite, rutile, K–feldspar, and Fe–Mn-chloride. They indicate that the fluid is rich in ore-forming elements and of high oxidation state. The fluid belongs to a complex hydrothermal system containing H2O – NaCl – KCl ± FeCl2 ± CaCl2 ± MnCl2. With decreasing homogenization temperature, the fluid salinity tends to increase from 34 to 82 wt% NaCl equiv., possibly suggesting a pressure or Cl/H2O increase in the original magma. No coexisting vapor-rich fluid inclusions with similar homogenization temperatures were found, so the brines are interpreted to have formed by direct exsolution from magma rather than trough boiling off of a low-salinity vapor. Estimated minimum pressure of 160 MPa imply approximately 7-km depth. This indicates that the deposit represents an orthomagmatic end member of the porphyry copper deposit continuum. Two key factors are proposed for the fluid evolution responsible for the large size of the gold-rich porphyry copper deposit of Duobuza: (i) ore-forming fluids separated early from the magma, and (ii) the hydrothermal fluid system was of magmatic origin and highly oxidized.

The partitioning behavior of silver in a vapor–brine–rhyolite melt assemblage

The partitioning of silver in a sulfur-free rhyolite melt–vapor–brine assemblage has been quantified at 800 °C, pressures of 100 and 140 MPa and f O 2 ≈ NNO (nickel–nickel oxide). Silver solubility (±2 σ) in rhyolite increases 5-fold from 105 ± 21 to 675 ± 98 μg/g as pressure increases from 100 to 140 MPa. Nernst-type partition coefficients ( D Ag i , j ± 2 σ ) describing the mass transfer of silver at 100 MPa between vapor and melt, brine and melt and vapor and brine are 32 ± 30, 1151 ± 238 and 0.026 ± 0.004, respectively. At 140 MPa, values for D Ag i , j ( ± 2 σ ) for vapor and melt, brine and melt, and vapor and brine are 32 ± 10, 413 ± 172 and 0.06 ± 0.03, respectively. Apparent equilibrium constant values (±2 σ) describing the exchange of silver and sodium between vapor and melt, K Ag , Na v / m , at 100 and 140 MPa are 105 ± 68 and 14 ± 6. The average values (±2 σ) for silver and sodium exchange between brine and melt, K Ag , Na b / m , at 100 and 140 MPa are 313 ± 288 and 65 ± 12. These data indicate that the mass transfer of silver from rhyolite melt to an exsolved volatile phase(s) is enhanced at 100 MPa relative to 140 MPa, suggesting that decompression increases the silver ore-generative potential of an evolving silicate magma. Model calculations using the new data suggest that the evolution of low-density, aqueous fluid (i.e., vapor) may be responsible for the the silver tonnage of many porphyry-type and perhaps epithermal-type ore deposits. For example, Halter et al. (Halter W. E., Pettke T. and Heinrich C. A. (2002) The origin of Cu/Au ratios in porphyry-type ore deposits. Science 296, 1842–1844) used detailed silicate and sulfide melt inclusion and vapor and brine fluid inclusions analyses to estimate a melt volume on the order of 15 km 3 to satisfy the copper budget at the Bajo de la Alumbrera copper-, gold-, silver-ore deposit. Using their melt volume estimate with the data presented here, model calculations for a 15-km 3 felsic melt, saturated with pyrrhotite and magnetite, suggest that a low-salinity magmatic vapor may scavenge on the order of 7 × 10 12 g of silver from the melt. This quantity of silver exceeds the discovered 2 × 10 9 g of Ag at Alumbrera. Calculated tonnages for numerous other deposits yield similar results. The excess silver in the vapor, remaining after porphyry formation, is then available to precipitate at lower PTconditions in the stratigraphically higher epithermal environment. These data suggest that silver, and perhaps other ore metals, in the porphyry-epithermal continuum may be derived solely from the time-integrated flux of dominantly low-salinity vapor exsolved from a series of sequential magma batches.

Characteristics and evolution of ore-forming fluids of the Chongjiang copper deposit in the Gangdise porphyry copper belt, Tibet

Petrography, microthermometry, and scanning electron microscope/energy dispersive spectrometer (SEM/EDS) studies were performed on the fluid inclusions in the ore-bearing quartz veins and quartz phenocrysts in the porphyry of the Chongjiang porphyry copper deposit. The analyses of the fluid inclusions indicate that the ore-forming fluids were exsolved from magma. They are near-saturated, supercritical, rich in volatile constituents, and have the capture temperature of 362–389°C and salinities of 17.7wt%-18.9wt% NaCl eq. With the decreasing of temperature and pressure, the supercritical fluids were separated into a low salinity vapor phase and a high salinity liquid phase. During quartz-sericitization, the high salinity fluid boiled and separated into a low salinity vapor phase and a high salinity liquid phase. The high salinity inclusions that formed in the boiling process had daughter mineral melting temperatures higher than the homogenization temperatures of the vapor and liquid phases. The late fluids that are responsible for argillization are of lower temperature and salinity.

Yulong Deposit, Eastern Tibet: A High-Sulfidation Cu-Au Porphyry Copper Deposit in the Eastern Indo-Asian Collision Zone

The Yulong ore body is the largest Cu deposit (6.22 million metric tons [Mt] at 0.99% Cu) in the 300 km long Himalayan porphyry copper belt, and is controlled by major Cenozoic strike-slip faults in the eastern Indo-Asian collision zone. It is associated with a steeply dipping, pipe-like multiphase (42-35 Ma) monzogranitic stock. The host rocks are potassic calc-alkaline or shoshonitic, and show geochemical affinities with adakites. They appear to have been derived from a thickened lower crustal source in East Tibet. The Yulong deposit consists of a ring-shaped, high-grade Cu-Au zone overlying and/or surrounding a porphyry-type Cu-Mo ore body. Cu-Mo mineralization produced a steeply dipping, pipe-like, veinlet-disseminated ore body within the stock. Associated hydrothermal alteration produced K-silicate and quartz-sericite assemblages within the stock, and contemporaneous propylitic alteration in the Upper Triassic sandy-slate wall rock. Fluid inclusion and δ18O-δD data indicate that the ore-forming fluid was supercritical, and exsolved from a high-level magma chamber at >620°C; it then separated into a hypersaline aqueous liquid and a coexisting low-salinity vapor at 340°-600°C. The high-grade Cu-Au zone (3 Mt at 4.74% Cu, and 4.5 g/t Au) is dominated by a supergene chalcocite-malachite blanket resting on an underlying supergene/hypogene sulfide transition unit and a hypogene pyrite-chalcopyrite sulfide unit. The Cu-Au zone was controlled by a subhorizontal or gently outward dipping breccia horizon developed along the marginal fracture zone near the roof of the stock, produced by hydrothermal brecciation due to regional uplift and/or fluid boiling. Alteration associated with the hypogene Cu-Au mineralization was texture-destructive advanced argillic alteration, characterized by associations of quartz, kaolinite, dickite, endellite, montmorillonite, hydromica, and minor alunite. It mainly developed within the breccia horizons, and partially over-printed the early-formed K-silicate zone and the quartz-sericite zone. Associated mineralization was of the high-sulfidation epithermal-type, characterized by chalcocite, tennantite, covellite, bornite, and minor pyrite, which formed the main ore body in the high-grade Cu-Au zone. Epithermal fluids also caused the dissolution of early-formed sulfides and remobilization of Cu-Mo, the latter transported into the intense advanced argillic alteration halo within the mineralized stock. This late-stage alteration and mineralization is attributed to a CO2-rich, low-temperature (<350°C), low-salinity (<12 wt% NaCl equiv.) meteoric fluid, involving input of magmatic fluid. Based on alteration, mineralization, fluid inclusion and stable isotopic data, a two-stage genetic history has been reconstructed for the Yulong deposit. It spans (1) a magmatic hydrothermal environment reflecting the emplacement of the monzogranite stock and Cu-Mo introduction through (2) hydrothermal fluid infiltration of breccia zones to epithermal overprinting.

Trace element partitioning between vapor, brine and halite under extreme phase separation conditions

Experiments were conducted to investigate the partitioning of Li, Br, Rb, Cs and B between vapor, brine and halite during subcritical and supercritical phase separation in the NaCl–H 2O system (388–550 °C, 250–350 bars). Results indicate that Li and Br partition preferentially into the low-salinity vapor fluids, while Rb and Cs become more enriched in the coexisting brines. Under more extreme conditions of pressure and temperature in the two-phase region, especially near the vapor–brine–halite boundary, strong salting-out effects imposed on neutral aqueous species enhance significantly partitioning of all trace elements into the low-salinity fluid. Dissolved boron is strongly affected by this and a particularly strong enrichment into vapors is observed, a trend that can be effectively correlated with changes in reduced density. Exclusion of Li, Br, Rb, Cs and B from halite, when precipitated, further increases the solubility of these species in the coexisting Cl-poor fluid. In general, the lack of distortion in the partitioning behavior of trace elements between vapor, brine and/or halite with the transition from subcritical to supercritical conditions in the NaCl–H 2O system precludes the need for special reference to the critical point of seawater when interpreting phase relations in submarine hydrothermal systems. The combination of experimentally determined trace element partitioning data with constraints imposed by mineral solubility provides a means to better understand the origin and evolution of hot spring vent fluids. For example, in Brandon hydrothermal system (21°S EPR) supercritical phase separation and subseafloor mixing appear to be the main heat and mass transport mechanisms fueled by a shallow magmatic intrusion, with boron systematics ruling out major contributions from magmatic degassing processes accompanying the near-seafloor volcanism.

The partitioning behavior of As and Au in S-free and S-bearing magmatic assemblages

The partitioning of As and Au between rhyolite melt and low-salinity vapor (2 wt% NaCl eq.) in a melt–vapor–Au metal ± magnetite ± pyrrhotite assemblage has been quantified at 800 °C, 120 MPa and f O 2 = NNO . The S-bearing runs have calculated values for the fugacities of H 2S, SO 2 and S 2 of log f H 2 S = 1.1 , log f SO 2 = - 1.5 , and log f S 2 = - 3.0 . The ratio of H 2S to SO 2 is on the order of 400. The experiments constrain the effect of S on the partitioning behavior of As and Au at magmatic conditions. Calculated average Nernst-type partition coefficients (±1 σ) for As between vapor and melt, D As v / m , are 1.0 ± 0.1 and 2.5 ± 0.3 in the S-free and S-bearing assemblages, respectively. These results suggest that sulfur has a small, but statistically meaningful, effect on the mass transfer of As between silicate melt and low-salinity vapor at the experimental conditions. Efficiencies of removal, calculated following Candela and Holland (1986), suggest that the S-free and S-bearing low-salinity vapor can scavenge approximately 41% and 63% As from water-saturated rhyolite melt, respectively, during devolatilization assuming that As is partitioned into magnetite and pyrrhotite during second boiling. The S-free data are consistent with the presence of arsenous acid, As(OH) 3 in the vapor phase. However, the S-bearing data suggest the presence of both arsenous acid and a As–S complex in S-bearing magmatic vapor. Apparent equilibrium constants, log K As ′ ( ± 1 σ ) , describing the partitioning of As between melt and vapor are −1.3 (0.1) and −1.1 (0.1) for the S-free and S-bearing runs, respectively. The increase in the value of K As ′ with the addition of S suggests a role for S in complexing and scavenging As from the melt during degassing. The calculated vapor/melt partition coefficients (±1 σ) for Au between vapor and melt, D Au v / m , in S-free and S-bearing assemblages are 15 ± 2.5 and 12 ± 0.3, respectively. Efficiencies of removal ( Candela and Holland, 1986) for the S-free melt, calculated assuming that magnetite is the dominant Au-sequestering solid phase during crystallization ( Simon et al., 2003), suggest that magmatic vapor may scavenge on the order of 72% Au from a water-saturated melt. Efficiencies of removal calculated for the S-bearing assemblage, assuming pyrrhotite and magnetite are the dominant Au-sequestering solid phases, indicate that vapor may scavenge on the order of 60% Au from the melt. These model calculations suggest that the loss of pyrrhotite and magnetite from a melt, owing to punctuated differentiation during ascent and emplacement, does not prohibit the ability of a rhyolite melt to generate a large-tonnage Au deposit. Apparent equilibrium constants describing the partitioning of Au between melt and vapor were calculated using the mean D Au v / m values for the S-free and S-bearing assemblages; only S-bearing data from runs longer than 400 h were used as shorter runs may not have reached equilibrium with respect only to vapor/melt partitioning of Au. The values for log K Au ′ ( ± 1 σ ) are −4.4 (0.1) and −4.2 (0.2) for the S-free and S-bearing runs, respectively. These data suggest that the presence of S does not affect the mass transfer of Au from degassing silicate melt to an exsolved, low-salinity vapor in a low- f S 2 assemblage (i.e., pyrrhotite–magnetite at NNO) at the experimental conditions reported here. Efficiencies of removal are calculated and used to model the mass transfer of Au from a crystallizing silicate melt to an exsolved, low-salinity vapor phase. The calculations suggest that the model, absolute tonnage of Au scavenged and transported by S-free and S-bearing vapors, from a crystallizing melt, would be comparable and that the time-integrated flux of low-salinity vapor could be responsible for a significant quantity of the Au in magmatic-hydrothermal ore deposits.

Multiphase Thermohaline Convection in the Earth’s Crust: II. Benchmarking and Application of a Finite Element – Finite Volume Solution Technique with a NaCl–H2O Equation of State

We present the benchmarking of a new finite element – finite volume (FEFV) solution technique capable of modeling transient multiphase thermohaline convection for geological realistic p-T-X conditions. The algorithm embeds a new and accurate equation of state for the NaCl–H2O system. Benchmarks are carried out to compare the numerical results for the various component-processes of multiphase thermohaline convection. They include simulations of (i) convection driven by temperature and/or concentration gradients in a single-phase fluid (i.e., the Elder problem, thermal convection at different Rayleigh numbers, and a free thermohaline convection example), (ii) multiphase flow (i.e., the Buckley–Leverett problem), and (iii) energy transport in a pure H2O fluid at liquid, vapor, supercritical, and two-phase conditions (i.e., comparison to the U.S. Geological Survey Code HYDROTHERM). The results produced with the new FEFV technique are in good agreement with the reference solutions. We further present the application of the FEFV technique to the simulation of thermohaline convection of a 400°C hot and 10 wt.% saline fluid rising from 4 km depth. During the buoyant rise, the fluid boils and separates into a high-density, high-salinity liquid phase and a low-density, low-salinity vapor phase.

Multiphase Thermohaline Convection in the Earth’s Crust: I. A New Finite Element – Finite Volume Solution Technique Combined With a New Equation of State for NaCl–H2O

We present a new finite element – finite volume (FEFV) method combined with a realistic equation of state for NaCl–H2O to model fluid convection driven by temperature and salinity gradients. This method can deal with the nonlinear variations in fluid properties, separation of a saline fluid into a high-density, high-salinity brine phase and low-density, low-salinity vapor phase well above the critical point of pure H2O, and geometrically complex geological structures. Similar to the well-known implicit pressure explicit saturation formulation, this approach decouples the governing equations. We formulate a fluid pressure equation that is solved using an implicit finite element method. We derive the fluid velocities from the updated pressure field and employ them in a higher-order, mass conserving finite volume formulation to solve hyperbolic parts of the conservation laws. The parabolic parts are solved by finite element methods. This FEFV method provides for geometric flexibility and numerical efficiency. The equation of state for NaCl–H2O is valid from 0 to 750°C, 0 to 4000 bar, and 0–100 wt.% NaCl. This allows the simulation of thermohaline convection in high-temperature and high-pressure environments, such as continental or oceanic hydrothermal systems where phase separation is common.

Geochemistry of low-molecular weight hydrocarbons in hydrothermal fluids from Middle Valley, northern Juan de Fuca Ridge

Hydrothermal vent fluids from Middle Valley, a sediment-covered vent field located on the northern Juan de Fuca Ridge, were sampled in July, 2000. Eight different vents with exit temperatures of 186–281 °C were sampled from two areas of venting: the Dead Dog and ODP Mound fields. Fluids from the Dead Dog field are characterized by higher concentrations of ΣNH 3 and organic compounds (C 1–C 4 alkanes, ethene, propene, benzene and toluene) compared with fluids from the ODP Mound field. The ODP Mound fluids, however, are characterized by higher C 1/(C 2 + C 3) and benzene:toluene ratios than those from the Dead Dog field. The aqueous organic compounds in these fluids have been derived from both bacterial processes (methanogenesis in low temperature regions during recharge) as well as from thermogenic processes in higher temperature portions of the subsurface reaction zone. As the sediments undergo hydrothermal alteration, carbon dioxide and hydrocarbons are released to solution as organic matter degrades via a stepwise oxidation process. Compositional and isotopic differences in the aqueous hydrocarbons indicate that maximum subsurface temperatures at the ODP Mound are greater than those at the Dead Dog field. Maximum subsurface temperatures were calculated assuming that thermodynamic equilibrium is attained between alkenes and alkanes, benzene and toluene, and carbon dioxide and methane. The calculated temperatures for alkene–alkane equilibrium are consistent with differences in the dissolved Cl concentrations in fluids from the two fields, and confirm that subsurface temperatures at the ODP Mound are hotter than those at the Dead Dog field. Temperatures calculated assuming benzene–toluene equilibrium and carbon dioxide–methane equilibrium are similar to observed exit temperatures, and do not record the hottest subsurface conditions. The difference in subsurface temperatures estimated using organic geochemical thermometers reflects subsurface cooling processes via mixing of a hot, low salinity vapor with a cooler, seawater salinity fluid. Because of the disparate temperature dependence of alkene–alkane and benzene–toluene equilibria, the mixed fluid records both the high and low temperature equilibrium conditions. These calculations indicate that vapor-rich fluids are presently being formed in the crust beneath the ODP Mound, yet do not reach the surface due to mixing with the lower temperature fluids.