Search

The partition coefficients of Cu, Au and Mo between liquid and vapor were determined at P = 130 MPa and T = 900 °C, and P = 90 MPa and T = 650 °C and redox conditions favoring the dominance of reduced S species in the fluid. The experiments at 900 °C were conducted in rapid-quench Molybdenum-Hafnium Carbide externally-heated pressure vessel assemblies, whereas those at 650 °C were run in René41 pressure vessels. The fluids were sampled at run conditions using the synthetic fluid inclusion technique. The host quartz was fractured in situ during the experiments ensuring the entrapment of equilibrium fluids. A new method was developed to quantify the composition of the vapor inclusions from LA-ICPMS analyses relying on the use of boron as an internal standard, an element that fractionates between vapor and liquid to a very small degree. The bulk starting fluid compositions closely represented those expected to exsolve from felsic silicate melts in upper crustal magma reservoirs (0.64 m NaCl, 0.32 m KCl, ±0.2 m HCl and/or 4 wt% S). The experiments were conducted in Au 97 Cu 3 alloy capsules allowing the simultaneous determination of apparent Au and Cu solubilities in the liquid and the vapor phase. Though the apparent metal solubilities were strongly affected by the addition of HCl and S in both phases, all three elements were found to preferentially partition to a liquid phase at all studied conditions with an increasing degree of preference for the liquid in the following order Au < Cu < Mo. The presence of HCl and S did not have a significant effect on the liquid/vapor partition coefficients of either Au or Cu, whereas the presence of HCl slightly shifted the partitioning of Mo in favor of the vapor. Ore metal partition coefficients normalized to that of Na ( K i - Na liq / vap = D i liq / vap / D Na liq / vap ) fall in the following ranges respectively for each studied metal: K Au - Na liq / vap = 0.20 ± 0.07–0.50 ± 0.19 (1 σ ); K Cu - Na liq / vap = 0.36 ± 0.12–0.76 ± 0.22; K Mo - Na liq / vap = 0.67 ± 0.15–2.5 ± 0.8. Decreasing T from 900 °C to 650 °C slightly shifted K Au - Na liq / vap and K Cu - Na liq / vap to the lower end of the reported ranges. A consequence of K Au - Na liq / vap and K Cu - Na liq / vap being significantly smaller than 1 is that much of the Au and a significant fraction of Cu may be carried to shallower levels of magmatic-hydrothermal systems by residual vapors despite potentially extensive brine condensation. [ABSTRACT FROM AUTHOR]

Analysis of fluid inclusions from porphyry copper deposits reveals that magmatic vapor and brine are vital for the removal of copper from arc magmas and its transport to the site of ore deposition. Experiments in melt–vapor–brine systems allow for investigation of the partitioning of copper between silicate melts and volatile phases at magmatic conditions. The presence of CO 2 affects both the pressure at which a melt saturates with respect to volatile phases. Therefore, the partitioning of copper among felsic (rhyolitic) melt, vapor and brine in CO 2 -bearing experiments was examined to provide insights into copper partitioning and the generation of porphyry copper and related deposits. We present results from experiments performed at 800 °C and 100 MPa in CO 2 -bearing melt–vapor–brine systems with X CO 2 v + b = 0.10 and 0.38. The compositions of vapor and brine inclusions, and run-product glasses, were determined during the course of this investigation. Microthermometric measurements of fluid inclusions show an increase in the salinity of the magmatic brine (∼65 to ∼70 wt% NaCl eq ) and decrease in the salinity of the vapor (∼3.5 to ∼1 wt% NaCl eq ) as X CO 2 is increased from 0.10 to 0.38. The partitioning of copper between brine and vapor ( D Cu b / v ± 2 σ ) increases from 25 (±6) at X CO 2 = 0.10, to 100 (±30) at X CO 2 = 0.38 . The partitioning of copper between vapor and melt ( D Cu v / m ± 2 σ ) decreases from 9.6 (±3.3) at X CO 2 = 0.10 , to 2 (±0.8) at X CO 2 = 0.38 . These data demonstrate that copper partitioning in sulfur-free, CO 2 -bearing systems is controlled by the changes in the salinity of the vapor and brine that, in turn, are functions of X CO 2 . No change in the apparent equilibrium constants for Cu–Na exchange was observed in Fe-bearing experiments which supports a salinity-dependent model for copper partitioning. An existing model (MVPart) for ore metal partitioning between melt and volatile phases was modified to incorporate partitioning data from CO 2 -bearing experiments. The model can be used to predict trends in efficiency of removal of copper from melts into exsolving CO 2 -bearing magmatic volatile phases at a fixed pressure and temperature. The CO 2 -MVPart model predicts that CO 2 -rich ( X CO 2 = 0.38 ) magmatic vapor will remove half of the available copper from the melt, without a contribution from the brine, compared to low CO 2 ( X CO 2 ≤ 0.10 ) systems. Thus, periods of CO 2 -rich vapor exsolution are not expected to be associated with efficient removal of copper from shallow felsic melts. [ABSTRACT FROM AUTHOR]

Experiments have been conducted at 200MPa, 800–1030°C, and fO2 0.8 log units below the Ni–NiO buffer to determine the solubility of Au, Cu and Ag in silicate melts and pyrrhotite. The metal activities were imposed by using Au965Ag020Cu015 alloy capsules. Sulfur-free and sulfur-bearing systems were studied with otherwise identical melt compositions to assess the relative effect of S on the solubility of these metals. The data show that the major element composition of the silicate melt only moderately affects the solubility of Au, Ag and Cu between basalt and dacite, yielding solubilities identical within 50% relative. In comparison, solubilities in the rhyolite melts are lower by a factor of 1.5–2.5 for Cu and higher by up to a factor of 5 for Ag, depending on the aluminum saturation index of the melt. The solubilitiy of Ag significantly increases with increasing peraluminousity above an aluminum saturation index of 1. The effect of melt composition is significant on the solubility of Au in S-bearing melts, in part due to its effect on the sulfur concentration at sulfide saturation. The effect of S is the most pronounced in peralkaline rhyolites and mafic melts, and minimal in peraluminous rhyolites. The solubilities of all three metals significantly decrease with decreasing temperature. The concentration of sulfur in the melt at sulfide saturation and its volatile/melt partition coefficient are primarily determined by the FeO activity in the melt and the activity coefficients of dissolved FeS species, which appear to correlate with the degree of melt polymerization. The volatile/melt partition coefficient of reduced S increases from 79±4 (1σ) to 635±80 as the melt composition changes from basalt to slightly peraluminous rhyolite, and from 225±13 to 776±148 as the aluminium saturation index increases from 0.7 to 1.1 in rhyolites. At 1000°C, pyrrhotite/silicate melt partition coefficients for Cu increase from 540±30 (1σ) to 1140±110 from basalt to dacite, whereas the partition coefficients for Ag are nearly constant at 50±10. The partition coefficients of Au increase from 180±20 to 900±210 from basalt to dacite. Pyrrhotite/rhyolite melt partition coefficients for Cu, Ag and Au increase by about an order of magnitude as temperature drops from 1000°C to 800°C. At typical S concentrations for arc magmas (500–2000μg/g), the budget of Ag in the magma will not be controlled by pyrrhotite, whereas primary control on the Au and Cu budget by pyrrhotite may only be relevant in intermediate to felsic magmas. [ABSTRACT FROM AUTHOR]

Abstract: The solubility of Au and Cu in hydrous andesite melts was determined at 1000°C and 200MPa. The data show that the solubility of Au is highly influenced by reduced S (S2−), and to a lesser degree, by the Cl concentration in the melt. Other melt compositional parameters such as K2O/Na2O ratio, or FeO concentration have no significant effect. The simultaneous presence of S2− and high concentrations of Cl (0.7–1.3wt.%) yield Au solubilities higher than expected from separate contributions of Au-sulfide and Au-chloride species. The strong effect of S and Cl on Au solubilities suggests dissolution mechanisms similar to those observed in magmatic volatiles. The most likely dominant dissolved species are AuSH or AuS(K/Na) in sulfur-bearing, and AuCl in Cl-bearing, S-free systems. At the S2−/S6+ transition (∼NNO+0.2), the solubility of Au sharply drops by a factor of 5. Copper shows markedly different behavior from that of Au; its solubility is influenced only slightly by the presence of S and Cl in the melt. This shows that the dominant Cu species is CuO0.5, with the exception of melts with unrealistically high Cl concentrations for natural systems, where Cu-chloride complexes become significant. Calculated volatile/melt partition coefficients of Au based on solubility data in melts and volatiles reach values up to ∼200 in a pyrrhotite saturated high-K calc-alkaline andesite melt below NNO+0.5, but sharply drop by an order of magnitude at the H2S to SO2 transition in the volatile phase. The predicted volatile/melt partition coefficients of Cu are much lower and vary in a narrow range (0.5–2). Consequently, volatiles released from mafic to intermediate magmas below NNO+1 may be important for the formation of magmatic-hydrothermal Au deposits and will favor high Au/Cu ratios in the overlying hydrothermal systems. [Copyright &y& Elsevier]

Abstract: Andesite melts were equilibrated with an H–O–S-bearing volatile phase to determine the partition coefficients for S and Cl as a function of melt composition and oxygen fugacity. The experiments were conducted in rapid-quench MHC vessel assemblies at 200MPa and 1000°C, and over a range of imposed fO2 between NNO−1.2 and NNO+1.8. High fluid/melt mass ratios (∼15) were employed, allowing precise and accurate partition coefficients to be obtained by mass balance calculations. Chlorine exhibits Henrian behavior at ClO−0.5 activities typical for arc magmas, with (1σ) below 0.2wt.% Cl in the melt; at higher ClO−0.5 activities, increases linearly to 2.11±0.02 at 1wt.% Cl in the melt. In the volatile phase: FeCl2 ∼NaCl>KCl∼HCl. The determination of cation exchange coefficients for major cations yielded: (1σ) and (1σ). Under these conditions, the concentration of HCl in the vapor is negatively correlated with the (Na+K)/(Al+Fe3+) ratio in the melt. Reduced sulfur (S2−) appears to obey Henry’s law in andesite melt–volatile system at fH2S below pyrrhotite saturation. The partition coefficient for S at fO2 =NNO−0.5 correlates negatively with the FeO concentration in the melt, changing from 254±25 at 4.0wt.% FeO to 88±6 at 7.5wt.% FeO. Pyrrhotite saturation is reached when approximately 3.2mol% S is present in the volatile phase at fO2 =NNO−0.5. At the sulfide/sulfate transition, the partition coefficient of S drops from 171±23 to 21±1 at a constant FeO content of ∼6wt.% in the melt. At fO2 =NNO+1.8, anhydrite saturation is reached at ∼3.3mol% S present in the volatile phase. Aqueous volatiles exsolving from intermediate to mafic magmas can efficiently extract S, and effect its transfer to sites of magmatic-hydrothermal ore deposit formation. [Copyright &y& Elsevier]

Many experimental studies have been performed to evaluate the composition of coexisting silicate melts and magmatic volatile phases (MVP). However, few studies have attempted to define the relationship between melt chemistry and the acidity of a chloride-bearing fluid. Here we report data on melt composition as a function of the HCl concentration of coexisting brines. We performed 35 experimental runs with a NaCl-KCl-HCl-H2O brine (70 wt% NaCl [equivalent])-silicate melt (starting composition of Qtz0.38Ab0.33Or0.29, anhydrous) assemblage at 800°C and 100 MPa. We determined an apparent equilibrium constant K′meas (K, Na)=(CNam×CKClb)/(CNaClb×CKm)for the equilibrium NaClb+ΣKm=ΣNam+KClb,(where CKClb, CNaClb, CKm, and CNam are total concentrations of potassium and sodium chloride in the brine, and potassium and sodium in the melt, respectively) as a function of the HCl concentration in the brine (CHClb). Although K′meas (K, Na) was not affected by variations in KCl/NaCl of the brine, it did vary inversely with CHClb. The relationship is given by K′meas (K, Na)=K′ex (K, Na)+SHAPE="BUILT" ALIGN="C" STYLE="S">aCHClb[where CHClb is in wt% and a = 0.03; K′ex (K, Na) = 0.40 ± 0.03 (1σ) and represents the exchange of model sodium and potassium between chloride components in the brine and the aluminate components (NaAlO2 and KAlO2) in the melt. This empirical result will be discussed in light of a structural hypothesis; however, validation of the model awaits determinations based on spectroscopy or transport properties–thermodynamic relations alone cannot be used as evidence of structure. The form of this equation is consistent with a model wherein sodium is present in the melt as both sodium aluminate and sodium hydroxide components, and HCl reacts with the NaOH component in the melt to produce NaCl and H2O.The correlation between fugacity of H2O (fH2Osys), model NaOHm/ΣNam, aluminum saturation index (ASI), and the ratio (HCl/NaCl)b of an exsolving MVP is complex. fH2Osys and the ASI are the main controls on model NaOHm/ΣNam in the system, with model NaOHm/ΣNam increasing with increasing fH2Osys. This relationship can be used to estimate the CHClb in subaluminous systems, an improvement over previous models. Data for metal partitioning between a volatile phase and melt are commonly presented in the literature as metal–sodium exchange equilibria (i.e., KCu,Na for the exchange of copper and sodium). However, the variation in K′meas (K, Na) observed in this study implies that the treatment of metal partitioning between a volatile phase and melt as metal–alkali exchange equilibria is complex because alkali partitioning is not constant and suggests that experimental partitioning studies need to carefully control the HCl/NaCl in experimental vapors and brines. This effect may explain discrepancies in metal–alkali exchange equilibria presented in the literature. Therefore, metal–alkali exchange cannot be described fully by a single metal–alkali equilibrium but must be examined by multiple equilibria. [Copyright &y& Elsevier]

A vapor-undersaturated synthetic brine was equilibrated with metallic gold and a haplogranitic melt at 800°C and 100 MPa to examine the solubility, speciation and partitioning of gold in the silicate melt-brine-metallic gold system. The starting composition of the NaCl-KCl-HCl-H2O brine was 70 wt.% NaCl (equivalent) with starting KCl/NaCl ranging from 0.5 to 1. KCl/HCl was varied from 3.2 to 104 to evaluate the solubility and partitioning of gold as a function of the concentration of HCl in the brine. Inclusions of brine were trapped in a silicate glass during quench. Inclusion-poor and inclusion-rich portions of glass were analyzed for gold and chloride by using neutron activation analysis. The inclusion-poor glass yielded an estimate of the solubility of gold and chloride in the silicate melt. The solubility of gold in the melt, at gold metal saturation, was estimated as &ap;1 ppm. The solubility of gold in the brine was estimated by mass balance, given the concentration of gold and chloride in the inclusion-poor and inclusion-rich glasses. The solubility of gold metal at low-HCl concentrations in the brine, CHClb, (3 × 103 to 1.1 × 104 ppm) is &ap;40 ppm (by weight) and is independent of the HCl concentration under those conditions. For CHClb of 1.1 × 104 to 4.0 × 104 ppm, the solubility of gold increased from 40 to 840 ppm, and the solubility is given by: log CAub = [2.2 · log CHClb] − 7.2(1) These data suggest that a significant amount of gold is not chloride complexed in brines at low-HCl concentrations (< 1.1 × 104 ppm), but that gold-chloride complexes, possibly AuCl2H, are important at elevated concentrations of HCl (> 1.1 × 104 ppm). The calculated Nernst partition coefficient (DAub/m) for gold between a brine and melt varied from 40 to 830 over a range of brine HCl concentrations of 3 × 103 to 1.1 × 104 ppm. Our results indicate a significant amount of gold can be transported by a brine in the magmatic-hydrothermal environment independent of the fugacity of sulfur in the system. Thus brines provide an effective mechanism for the scavenging of gold from a crystallizing melt and transport into an associated magmatic-hydrothermal system, regardless of their sulfur contents. [Copyright &y& Elsevier]

In order to determine whether sulfides are major reservoirs for Mo in the upper continental crust (UCC), we determined the composition and mode of occurrence of sulfides and evaluated their contribution to the molybdenum budget in twelve glacial diamictites with ages ranging from 2900 to 300 Ma. The diamictites provide a snapshot of UCC mineralogy and composition at the time of their deposition and show systematic depletion in bulk rock Mo concentrations after the Great Oxidation Event (GOE), reflecting the effects of oxidative weathering in their provenance (Gaschnig et al., 2014; Li et al., 2016). Sulfides are generally confined to Archean and Paleoproterozoic diamictites, although they also have been found in one Phanerozoic sample with an ancient provenance. We classify the sulfides based on their compositions and morphologies. Detrital sulfides are generally rounded, may be a single mineral, or an assemblage of minerals, and show a very wide range in mineralogy, including a single molybdenite grain. Sedimentary sulfides are pyrites, generally with framboidal-like textures. Pyrites also include non-framboidal textured authigenic pyrites. Epigenetic sulfides consist of irregular pyrrhotite aggregates (sometimes pyrrhotite intergrown with chalcopyrite and cobaltite), late-stage euhedral pyrites and pyrite aggregates in veins. High Mo concentrations (up to ∼230 ppm) are found in some sedimentary framboidal-like pyrites from the Mesoarchean Coronation and Paleoproterozoic Makganyene Formations, epigenetic pyrrhotite aggregates and chalcopyrite in the Ramsay Lake diamictite, and in detrital sulfides in Timeball Hill diamictites that may have originated from hydrothermal fluids in the sedimentary basins. Other detrital sulfides have widely variable Mo concentrations (0.5–36 ppm). Mass balance calculations show that sulfides can account for only <8% of the whole rock Mo contents in all glacial diamictites except for the Makganyene Formation, and thus sulfides (including molybdenite) are unlikely to be a significant host of Mo within the UCC before the GOE. In the Makganyene diamictites, ∼37% of whole rock Mo is contained within sulfides, which are predominantly framboidal-like pyrites interpreted to have grown in the matrix at the time of deposition. In these, the Mo contents correlate with the size of the microcrystals: smaller-sized crystals contain significantly higher concentrations than larger-sized crystals, suggesting that rapid nucleation and growth of framboidal pyrites leads to incorporation of large amounts of Mo available in the pore water or Mo expulsion during recrystallization. The presence of Mo in sedimentary pyrites from a pre-GOE deposit – the 2900 Ma Coronation Formation – points to the availability of Mo and S in pre-GOE seawater (and associated pore fluids), and is consistent with previously reported evidence for isolated pulses of atmospheric oxygen and oxidative continental weathering prior to the GOE. However, the lack of significant whole rock Mo depletion in pre-GOE diamictites limits the amount of Mo released from the continents prior to the GOE. [ABSTRACT FROM AUTHOR]

Abstract The oxygen fugacities of nine mantle-derived komatiitic and picritic systems ranging in age from 3.55 Ga to modern day were determined using the redox-sensitive partitioning of V between liquidus olivine and komatiitic/picritic melt. The combined set of the oxygen fugacity data for seven systems from this study and the six komatiite systems studied by Nicklas et al. (2018), all of which likely represent large regions of the mantle, defines a well-constrained trend indicating an increase in oxygen fugacity of the lavas of ∼1.3 ΔFMQ log units from 3.48 to 1.87 Ga, and a nearly constant oxygen fugacity from 1.87 Ga to the present. The oxygen fugacity data for the 3.55 Ga Schapenburg komatiite system, the mantle source region of which was previously argued to have been isolated from mantle convection within the first 30 Ma of the Solar System history, plot well above the trend and were not included in the regression. These komatiite's anomalously high oxygen fugacity data likely reflect preservation of early-formed magma ocean redox heterogeneities until at least the Paleoarchean. The observed increase in the oxygen fugacity of the studied komatiite and picrite systems of ∼1.3 ΔFMQ log units is shown to be a feature of their mantle source regions and is interpreted to indicate secular oxidation of the mantle between 3.48 and 1.87 Ga. Three mechanisms are considered to account for the observed change in the redox state of the mantle: (1) recycling of altered oceanic crust, (2) venting of oxygen from the core due to inner core crystallization, and (3) convection-driven homogenization of an initially redox-heterogeneous primordial mantle. It is demonstrated that none of the three mechanisms alone can fully explain the observed trend, although mechanism (3) is best supported by the available geochemical data. These new data provide further evidence for mantle involvement in the dramatic increase in the oxygen concentration of the atmosphere leading up to the Great Oxidation Event at ∼2.4 Ga. [ABSTRACT FROM AUTHOR]

We quantify the behavior of Cu, Ga, Ge, As, Mo, Ag, Cd, In, Sn, Sb, W, Tl, Pb, and Bi during the differentiation of a picritic magma in the Kilauea Iki lava lake, Hawaii, using whole rock and glass differentiation trends, as well as partition coefficients in Cu-rich sulfide blebs and minerals. Such data allow us to constrain the partitioning behavior of these elements between sulfide and silicate melts, as well as the chalcophile element characteristics of the mantle source of the Kilauea lavas. Nearly all of the elements are generally incompatible on a whole-rock scale, with concentrations increasing exponentially below ∼6 wt% MgO. However, in-situ laser ablation data reveal that Cu, Ag, Bi, Cd, In, Pb, and Sn are chalcophile; As, Ge, Sb, and Tl are weakly chalcophile to lithophile; and Mo, Ga, and W are lithophile. The average D sulfide/silicate melt values are: D Ag = 1252 ± 1201 (2SD), D Bi = 663 ± 576, D Cd = 380 ± 566, D In = 40 ± 34, D Pb = 34 ± 18, D Sn = 5.3 ± 3.6, D As = 2.4 ± 7.6, D Ge = 1.6 ± 1.4, D Sb = 1.3 ± 1.5, D Tl = 1.1 ± 1.7, D Mo = 0.56 ± 0.6, D Ga = 0.10 ± 0.3, and D W = 0.11 ± 0.1. These findings are consistent with experimental partitioning studies and observations of Ni-rich sulfide liquid in mid-ocean ridge basalts (MORB), despite the different compositions of the KI sulfides. The KI glasses and whole rocks are enriched in As, Ag, Sb, W, and Bi, relative to elements of similar compatibility (as established by abundances in MORB), mimicking enrichments found in basalts from the Manus back arc basin (Jenner et al., 2012) and the upper continental crust (UCC). These enrichments suggest the presence of terrigenous sediments in the Kilauea mantle source. The KI source is calculated to be a mixture of depleted MORB mantle (DMM) and 10–20% recycled crust composed of MORB and minor terrigenous sediments. [ABSTRACT FROM AUTHOR]

The Kenya Rift Valley contains many ochre sources that are currently used by indigenous peoples for adornment, rituals, and art. Ochre pigments occur in rock art and archaeological sites spanning over 250,000 years. Chemical analysis for provenience of geological sources is the first step in the process of reconstructing provenance of archaeological artifacts for cultural heritage, archaeological, and paleoanthropological research. Development of an ochre source chemical composition database can facilitate reconstruction of social interaction networks and cultural heritage conservation efforts in this region. Techniques such as Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) and Instrumental Neutron Activation Analysis (INAA) are often used for compositional analysis and sourcing of ferruginous mineral pigments. Sourcing has proven challenging due to the diverse range of rocks and minerals that are classified as red and yellow ochres, and the diverse processes that induce variation in composition, including modes of formation, sedimentary transport of parent materials, and diagenesis. Attribution of samples to specific sources is possible only when variation within sources is less than differences between sources (the Provenience Postulate). Here we present the results of a study using LA-ICPMS to determine inter- and intra-source geochemical variations for ten ochre sources associated with three large volcanic centers in the central Rift Valley of Kenya. Our results show that differences in chemical composition among sources are greater than variation within sources, both at the scale of large volcanic centers and of individual ochre outcrops within these centers. Clear differentiation of source chemical fingerprints at local and regional scales satisfies the Provenience Postulate, and suggests that provenance studies of ochre artifacts, residues, and rock art in Kenya will be feasible. [ABSTRACT FROM AUTHOR]

Abstract: The petrology, mineral compositions, whole rock major/trace element concentrations, including highly siderophile elements, and Re–Os isotopes of 99 peridotite xenoliths from the central North China Craton were determined in order to constrain the structure and evolution of the deep lithosphere. Samples from seven Early Cretaceous–Tertiary volcanic centers display distinct geochemical characteristics from north to south. Peridotites from the northern section are generally more fertile (e.g., Al2O3 =0.9–4.0%) than those from the south (e.g., Al2O3 =0.2–2.2%), and have maximum whole-rock Re-depletion Os model ages (T RD) of ∼1.8Ga suggesting their coeval formation in the latest Paleoproterozoic. By contrast, peridotites from the south have maximum T RD model ages that span the Archean–Proterozoic boundary (2.1–2.5Ga). Peridotites with model ages from both groups are found at Fansi, the southernmost locality in the northern group, which likely marks a lithospheric boundary. The Neoarchean age of the lithospheric mantle in the southern section matches that of the overlying crust and likely reflects the time of amalgamation of the North China Craton via collision between the Eastern and Western blocks. The Late Paleoproterozoic (∼1.8Ga) lithospheric mantle beneath the northern section is significantly younger than the overlying Archean crust, indicating that the original lithospheric mantle was replaced in this region, either during a major north–south continent–continent collision that occurred during assembly of the Columbia supercontinent at ∼1.8–1.9Ga, or from extrusion of ∼1.9Ga lithosphere from the Khondalite Belt beneath the northern Trans-North China Orogen, during the ∼1.85Ga continental collision between Eastern and Western blocks. Post-Cretaceous heating of the southern section is indicated by high temperatures (>1000°C) recorded in peridotites from the 4Ma Hebi suite, which are significantly higher than the temperatures recorded in peridotites from the nearby Early Cretaceous Fushan suite (<720°C), and likely reflects significant lithospheric thinning after the Early Cretaceous. Combining previous Os isotope results on mantle xenoliths from the eastern North China Craton with our new data, it appears that lithospheric thinning and replacement may have evolved from east to west with time, commencing before the Triassic on the eastern edge of the craton, occurring during the Jurassic–Cretaceous within the interior, and post-dating 125Ma on the westernmost boundary. [Copyright &y& Elsevier]

Abstract: We conducted experiments to determine the effect of various chemical components (NaCl, KCl, HCl, FeCl2, H2S, SO2) on the solubility of Cu in single phase aqueous vapors at 1000°C and 150MPa. The experiments were conducted in Au97Cu3 alloy capsules buffering Cu activities at 0.01. The volatile phase was sampled at run conditions by the entrapment of synthetic fluid inclusions in quartz. To test if the volatile phase had reached equilibrium before the isolation of the inclusions by fracture healing, we trapped two inclusion generations, one in an initially prefractured chip and another in a quartz chip that was fractured in situ during the experiments. The synthetic fluid inclusions were subsequently analyzed by laser ablation inductively coupled plasma mass spectrometry. In pure water, the apparent solubility of Cu is below the limits of detection of 6μg/g, showing the low stability of hydroxy Cu complexes at our experimental conditions. The presence of alkali chlorides supports modest Cu solubility likely in the form of NaCuCl2 and KCuCl2 complexes. In the H2O–H2S (+SiO2 and Au97Cu3) system at an fH2S of 10.4MPa the apparent solubility of Cu is lower by a factor of ∼5 than that in a S-free 0.5m NaCl solution, showing that copper hydrosulfide complexes are only moderately stable at these conditions. Addition of 4.7mol% of sulfur to the H2O–NaCl system at an fO2 of 0.4 log units below the Ni–NiO buffer, yielding dominantly H2S species, results in only a moderate increase in apparent Cu solubility, which diminishes in the presence of HCl. The addition of KCl results in a strong increase of apparent Cu solubility in the presence of H2S. The solubility of Cu increases with the fugacity of oxygen in both the H2O–NaCl and the H2O–S–NaCl system following an approximately fourth root relationship as expected based on the stoichiometry of the involved redox reactions. Replacement of NaCl by FeCl2 exerted only a minor effect on the Cu solubility. Results of our experiments, combined with thermochemical data obtained by ab initio quantum chemical calculations, suggest dissolution of Cu dominantly as Na(/K)CuCl2, Na(/K)Cu(HS)2, H2SCuHS, and Na(/K)ClCuHS, the relative abundance of which are dictated by the H2S/total chloride and HCl/alkali chloride ratios. [Copyright &y& Elsevier]

Abstract: Porphyry-type ore deposits sometimes contain fluid inclusion compositions consistent with the partitioning of copper and gold into vapor relative to coexisting brine at the depositional stage. However, this has not been reproduced experimentally at magmatic conditions. In an attempt to determine the conditions under which copper and gold may partition preferentially into vapor relative to brine at temperatures above the solidus of granitic magmas, we performed experiments at 800°C, 100MPa, oxygen fugacity () buffered by Ni–NiO, and fixed at either 3.5×10−2 by using intermediate solid solution–pyrrhotite, or 1.2×10−4 by using intermediate solid solution–pyrrhotite–bornite. The coexisting vapor (∼3wt.% NaCl eq.) and brine (∼68wt.% NaCl eq.) were composed initially of NaCl+KCl+HCl+H2O, with starting HCl set to <1000μg/g in the aqueous mixture. Synthetic vapor and brine fluid inclusions were trapped at run conditions and subsequently analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Our experiments demonstrate that copper and gold partitioned strongly into the magmatic volatile phase(s) (MVP) (i.e., vapor or brine) relative to a silicate melt over the entire imposed range of . Nernst style partition coefficients between coexisting brine (b) and melt (m), Db/m (±1σ), range from 3.6(±2.2)×101 to 4(±2)×102 for copper and from 1.2(±0.6)×102 to 2.4(±2.4)×103 for gold. Partition coefficients between coexisting vapor (v) and melt, Dv/m range from 2.1±0.7 to 18±5 and 7(±3)×101 to 1.6(±1.6)×102 for copper and gold, respectively. Partition coefficients for all experiments between coexisting brine and vapor, Db/v (±1σ), range from 7(±2) to 1.0(±0.4)×102 and 1.7(±0.2) to 15(±2) for copper and gold, respectively. Observed average Db/v at an of 1.2×10−4 were elevated, 95(±5) and 15±1 for copper and gold, respectively, relative to those at the higher of 3.5×10−2 where Db/v were 10(±5) for copper and 7(±6) for gold. Thus, there is an inverse relationship between the and the Db/v for both copper and gold with increasing resulting in a decrease in the Db/v signifying increased importance of the vapor phase for copper and gold transport. This suggests that copper and gold may complex with volatile S-species as well as Cl-species at magmatic conditions, however, none of the experiments of our study at 800°C and 100MPa had a Db/v ⩽1. We did not directly determine speciation, but infer the existence of some metal–sulfur complexes based on the reported data. We suggest that copper and gold partition preferentially into the brine in most instances at or above the wet solidus. However, in most systems, the mass of vapor is greater than the mass of brine, and vapor transport of copper and gold may become more important in the magmatic environment at higher , lower , or near the critical point in a salt-water system. A Db/v ⩽1 at subsolidus hydrothermal conditions may also occur in response to changes in temperature, , , and/or acidity. Additionally, both copper and gold were observed to partition into intermediate solid solution and bornite much more strongly than into vapor, brine or silicate melt. This suggests that, although vapor and brine are both efficient at removing copper and gold from a silicate melt, the presence of Cu–Fe sulfides can sequester a substantial portion of the copper and gold contained within a silicate melt if the Cu–Fe sulfides are abundant. [Copyright &y& Elsevier]

Abstract: Exposure of the ca. 6Ma Taitao ophiolite, Chile, located ∼50km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly siderophile elements (HSE). Taitao peridotites have compositions indicative of variable extents of partial melting and melt extraction. Low δ18O values for most whole rock samples suggest some open-system, high-temperature water–rock interaction, most likely during serpentinization, but relict olivine grains have δ18O values consistent with primary mantle values. Most of the peridotites analyzed for Nd–Sr isotopes have compositions consistent with estimates for the modern DMM, although several samples are characterized by 87Sr/86Sr and 143Nd/144Nd indicative of crustal contamination, most likely via interactions with seawater. The peridotites have initial 187Os/188Os ratios that range widely from 0.1168 to 0.1288 (γ Os =−8.0 to +1.1), averaging 0.1239 (γ Os =−2.4), which is comparable to the average for modern abyssal peridotites. A negative correlation between the Mg# of relict olivine grains and Os isotopic compositions of whole rock peridotites suggests that the Os isotopic compositions reflect primary mantle Re/Os fractionation produced by variable extents of partial melting at approximately 1.6Ga. Recent re-melting at or near the spatially associated Chile Ridge further modified these rocks, and Re, and minor Pt and Pd were subsequently added back into some rocks by late-stage melt–rock or fluid–rock interactions. In contrast to the peridotites, approximately half of the mafic rocks examined have whole rock δ18O values within the range of mantle compositions, and their Nd and Sr isotopic compositions are all generally within the range of modern DMM. These rocks have initial 187Os/188Os ratios, calculated for 6Ma, that range from 0.126 (γ Os =−1) to as high as 0.561 (γ Os =+342). The Os isotopic systematics of each of these rocks may reflect derivation from mixed lithologies that include the peridotites, but may also include pyroxenites with considerably more radiogenic Os than the peridotites. This observation supports the view that suprachondritic Os present in MORB derives from mixed mantle source lithologies, accounting for some of the worldwide dichotomy in 187Os/188Os between MORB and abyssal peridotites. The collective results of this study suggest that this >500km3 block of the mantle underwent at least two stages of melting. The first stage occurred at ∼1.6Ga, after which the block remained isolated and unmixed within the DMM. A final stage of melting recently occurred at or near the Chile Ridge, resulting in the production of at least some of the mafic rocks. Convective stirring of this mantle domain during a >1Ga period was remarkably inefficient, at least with regard to Os isotopes. [Copyright &y& Elsevier]

Abstract: The partitioning of silver in a sulfur-free rhyolite melt–vapor–brine assemblage has been quantified at 800°C, pressures of 100 and 140MPa and (nickel–nickel oxide). Silver solubility (±2σ) in rhyolite increases 5-fold from 105±21 to 675±98μg/g as pressure increases from 100 to 140MPa. Nernst-type partition coefficients describing the mass transfer of silver at 100MPa between vapor and melt, brine and melt and vapor and brine are 32±30, 1151±238 and 0.026±0.004, respectively. At 140MPa, values for 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 (±2σ) describing the exchange of silver and sodium between vapor and melt, , at 100 and 140MPa are 105±68 and 14±6. The average values (±2σ) for silver and sodium exchange between brine and melt, , at 100 and 140MPa 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 100MPa relative to 140MPa, 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 15km3 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-km3 felsic melt, saturated with pyrrhotite and magnetite, suggest that a low-salinity magmatic vapor may scavenge on the order of 7×1012 g of silver from the melt. This quantity of silver exceeds the discovered 2×109 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. [Copyright &y& Elsevier]

Abstract: 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, 120MPa and . The S-bearing runs have calculated values for the fugacities of H2S, SO2 and S2 of , , and . The ratio of H2S to SO2 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 (±1σ) for As between vapor and melt, , 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 , 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, , 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 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 (±1σ) for Au between vapor and melt, , in S-free and S-bearing assemblages are 15±2.5 and 12±0.3, respectively. Efficiencies of removal () for the S-free melt, calculated assuming that magnetite is the dominant Au-sequestering solid phase during crystallization (), 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 values for the S-free and S-bearing assemblages; only S-bearing data from runs longer than 400h were used as shorter runs may not have reached equilibrium with respect only to vapor/melt partitioning of Au. The values for 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- 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. [Copyright &y& Elsevier]

Abstract: The effect of sulfur on the partitioning of Cu in a melt–vapor–brine±magnetite±pyrrhotite assemblage has been quantified at 800°C, 140MPa, =nickel–nickel oxide (NNO), (i.e., on the magnetite–pyrrhotite curve at NNO), and . All experiments were vapor+brine saturated. Vapor and brine fluid inclusions were trapped in silicate glass and self-healed quartz fractures. Vapor and brine are dominated by NaCl, KCl and HCl in the S-free runs and NaCl, KCl and FeCl2 in S-bearing runs. Pyrrhotite served as the source of sulfur in S-bearing experiments. The composition of fluid inclusions, glass and crystals were quantified by laser-ablation inductively coupled plasma mass spectrometry. Major element, chlorine and sulfur concentrations in glass were quantified by using electron probe microanalysis. Calculated Nernst-type partition coefficients (±2σ) for Cu between melt–vapor, melt–brine and vapor–brine are , , and , respectively, in the S-free system. The partition coefficients (±2σ) for Cu between melt–vapor, melt–brine and vapor–brine are , , and , respectively, in the S-bearing system. Apparent equilibrium constants (±1σ) describing Cu and Na exchange between vapor and melt and brine and melt were also calculated. The values of are 34±21 and 128±29 in the S-free and S-bearing runs, respectively. The values of are 33±22 and60±5 in the S-free and S-bearing runs, respectively. The data presented here indicate that the presence of sulfur increases the mass transfer of Cu into vapor from silicate melt. Further, the nearly threefold increase in suggests that Cu may be transported as both a chloride and sulfide complex in magmatic vapor, in agreement with hypotheses based on data from natural systems. Most significantly, the data demonstrate that the presence of sulfur enhances the partitioning of Cu from melt into magmatic volatile phases. [Copyright &y& Elsevier]

Abstract: We used laser-ablation inductively coupled plasma mass spectrometry to measure the solubility of gold in synthetic sulfur-free vapor and brine fluid inclusions in a vapor + brine + haplogranite + magnetite + gold metal assemblage. Experiments were conducted at 800°C, oxygen fugacity buffered at Ni-NiO (NNO), and pressures ranging from 110 to 145 MPa. The wt% NaCl eq. of vapor increases from 2.3 to 19 and that of brine decreases from 57 to 35 with increasing pressure. The composition of the vapors and brines are dominated by NaCl + KCl + FeCl2 + H2O. Gold concentrations in vapor and brine decrease from 36 to 5 and 50 to 28 μg/g, respectively, and the calculated vapor:brine partition coefficients for gold decrease from 0.72 to 0.17 as pressure decreases from 145 to 110 MPa. These data are consistent with the thermodynamic boundary condition that the concentration of gold in the vapor and brine must approach a common value as the critical pressure is approached along the 800°C isotherm in the NaCl-KCl-FeCl2-HCl-H2O system. We use the equilibrium constant for gold dissolution as AuOH 0 , extrapolated from lower temperature and overlapping pressure range, to calculate expected concentrations of AuOH 0 in our experimental vapors. These calculations suggest that a significant quantity of gold in our experimental vapors is present as a non-hydroxide species. Possible chloridogold(I) species are hypothesized based on the positively correlated gold and chloride concentrations in our experimental vapors. The absolute concentration of gold in our synthetic vapor, brine, and melt and calculated mass partition coefficients for gold between these physicochemically distinct magmatic phases suggests that gold solubility in aqueous fluids is a function of aqueous phase salinity, specifically total chloride concentration, at magmatic conditions. However, though we highlight here the effect of salinity, the combination of our data with data sets from lower temperatures evinces a significant decrease in gold solubility as temperature drops from 800°C to 600°C. This decrease in solubility has implications for gold deposition from ascending magmatic fluids. [Copyright &y& Elsevier]

Abstract: We have examined the effect of pressure on the apparent equilibrium constant, K′, for magnetite solubility (Fe3O4mt + 6HClfluid + H2fluid = 3FeCl2fluid + 4H2Ofluid) and the relative iron-carrying capacities of magmatic vapor and brine by conducting experiments in a rhyolite melt-vapor-brine-magnetite system at 800°C, fO2 = NNO and pressures ranging from 100 to 145 MPa. Iron concentrations in synthetic vapor and brine fluid inclusions were quantified by using laser-ablation inductively-coupled-plasma-mass-spectrometry (LA-ICPMS). Hydrogen chloride (HCl) concentrations in magmatic vapor were inferred by potentiometric measurements of H+ in quenched run product fluids. These data yield calculated values for log K′, assuming aH2O = XH2O, of 1.7, 4.9, 6.2, 6.8 and 9.1 at 100, 110, 130, 140 and 145 MPa, respectively. The concentration of iron in magmatic vapor increases by an order of magnitude, whereas the concentration of iron in magmatic brine remains constant (within 1σ) with increasing pressure as the 800°C critical pressure is approached along the vapor-brine solvus. The concentrations of iron in vapor and brine fluid inclusions yield calculated partition coefficients (DFev/b) of 0.05, 0.14, 0.27 and 0.56 at 110, 130, 140 and 145 MPa, respectively. Our data reveal that pressure fluctuations may significantly affect the value of log K′. More importantly, the data demonstrate conclusively that a significant amount of iron can be transported by a low-density aqueous vapor in the magmatic-hydrothermal environment. [Copyright &y& Elsevier]