An experimental study of the solubility and speciation of neodymium (III) fluoride in F-bearing aqueous solutions

The solubility of neodymium (III) fluoride was investigated at temperatures of 150, 200 and 250 °C, saturated water vapor pressure, and a total fluoride concentration (HF°_aq + F⁻) ranging from 2.0 × 10⁻³ to 0.23 mol/l. The results of the experiments show that Nd³⁺ and NdF²⁺ are the dominant species in solution at the temperatures investigated and were used to derive formation constants for NdF²⁺ and a solubility product for NdF₃. The solubility product of NdF₃(logK_sp=loga_Nd³⁺+3loga_F^-) is −24.4 ± 0.2, −22.8 ± 0.1, and −21.5 ± 0.2 at 250, 200 and 150 °C, respectively, and the formation constant of NdF²⁺(logβ=loga_NdF²⁺-loga_Nd³⁺-loga_F^-) is 6.8 ± 0.1, 6.2 ± 0.1, and 5.5 ± 0.2 at 250, 200 and 150 °C, respectively. The results of this study show that published theoretical predictions significantly overestimate the stability of NdF²⁺ and the solubility of NdF₃.The potential impact of the results on natural systems was evaluated for a hypothetical fluid with a composition similar to that responsible for REE mineralization in the Capitan pluton, New Mexico. In contrast to results obtained using the theoretical predictions of Haas [Haas J. R., Shock E. L., and Sassani D. C. (1995) Rare earth elements in hydrothermal systems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures. Geochim. Cosmochim. Acta59, 4329-4350.], which indicate that NdF²⁺ is the dominant species in solution, calculations employing the data presented in this paper and previously published experimental data for chloride and sulfate species [Migdisov A. A., and Williams-Jones A. E. (2002) A spectrophotometric study of neodymium(III) complexation in chloride solutions. Geochim. Cosmochim. Acta66, 4311-4323; Migdisov A. A., Reukov V. V., and Williams-Jones A. E. (2006) A spectrophotometric study of neodymium(III) complexation in sulfate solutions at elevated temperatures. Geochim. Cosmochim. Acta70, 983-992.] show that neodymium chloride species predominate and that neodymium fluoride species are relatively unimportant. This suggests that accepted models for REE deposits that invoke fluoride complexation as the method of hydrothermal REE transport may need to be re-evaluated.     

The solubility of gold in hydrogen sulfide gas: An experimental study

The solubility of gold in H₂S gas has been investigated at temperatures of 300, 350 and 400 °C and pressures up to 230 bars. Experimentally determined values of the solubility of Au are 0.4-1.4 ppb at 300 °C, 1-8 ppb at 350 °C and 8.6-95 ppb at 400 °C. Owing to a positive dependence of the logarithm of the fugacity of gold on the logarithm of the fugacity of H₂S, it is proposed that the solubility of Au can be attributed to formation of a solvated gaseous sulfide or bisulfide complex through reactions of the type:

(A)     

四川冕宁牦牛坪稀土矿床地质特征

四川牦牛平矿床是世界上最大的稀土矿床之一,该矿床位于攀西裂谷西缘,同位素年龄表明稀土化发育于裂谷期后阶段。矿区内碳酸岩、碱性基性伟晶岩、花岗斑岩、煌斑岩以及辉绿岩岩墙（脉）构成了与矿化有关的碳酸岩杂岩。围绕碳酸岩杂岩广泛发育着霓长岩化,蚀变作用形成的霓长岩具有显显的分带特征,由内向外可分为：钠铁闪石黑云母带、霓石霓辉石带、碎裂带。该矿床具有三种矿石类型：碱性基性伟晶岩型、岩酸岩型和细网脉裂。氟碳铈     

Partitioning of boron among melt, brine and vapor in the system haplogranite–H2O–NaCl at 800 °C and 100 MPa

Fluid-saturated experiments were conducted to investigate the partitioning of boron among haplogranitic melt, aqueous vapor and brine at 800 °C and 100 MPa. Experiments were carried out in cold-seal pressure vessels for 1 to 21 days, and utilized powdered synthetic subaluminous haplogranite glass doped with 1000 ppm B (crystalline H₃BO₃) and variable amounts of NaCl and H₂O at a fluid/haplogranite mass RATIO=1:1. Run-product glasses were analyzed for boron concentration by secondary ion mass spectrometry (SIMS) and for major elements and chlorine by electron microprobe. The composition of the coexisting fluid was calculated by mass balance. Boron partition coefficients between aqueous vapor and hydrous granitic melt range from 3.1 to 6.3, and demonstrate a clear preference of boron for the vapor over the hydrous melt. Partition coefficients between brine and hydrous granitic melt vary from 0.45 to 1.1, suggesting that boron has no preference for the brine or the melt. The bulk fluid–melt partition coefficients for low-salinity and high-salinity experiments are D_B^(vapor/melt)=4.6±1.3 and D_B^(brine/melt)=0.91±0.49, respectively. The corresponding vapor–brine partition coefficient is 5.0±3.1, demonstrating that boron partitions preferentially into the vapor over the brine at the conditions of this study. The preferential incorporation of boron in the aqueous vapor is controlled by borate speciation and solution mechanism. The dominant borate species in aqueous fluids, H₃BO₃^o, is highly soluble in aqueous vapor (X_B2O3=0.187); however, B₂O₃ is immiscible in NaCl liquid. Consequently, concentrations of boron in aqueous vapor are significantly higher than in the coexisting brine. Furthermore, Na–B complexing in the melt at high chlorine fluid contents stabilizes boron in the melt thereby contributing to the non-preferential partitioning of boron between brine and melt. The commonly observed association of tourmalinization (boron metasomatism), brecciation and ore deposition in nature is consistent with the preferential partitioning of boron into aqueous vapor of magmatic-hydrothermal systems predicted by this study.     

Genesis of the Sb-W-Au deposits at Ixtahuacan,Guatemala: evidence from fluid inclusions and stable isotopes

The Ixtahuacan Sb-W deposits are hosted by upper Pennsylvanian to Permian metasedimentary rocks of the central Cordillera of Guatemala. The deposits consist of gold-bearing arsenopyrite, stibnite and scheelite. Arsenopyrite and scheelite are early in the paragenesis, occurring as disseminations in pyritiferous black shale/sandstone and in argillaceous limestone, respectively. Some stibnite is disseminated, but the bulk of the stibnite occurs as massive stratabound lenses in black shales and in quartz-ankerite veins and breccias, locally containing scheelite.Microthermometric measurements on fluid inclusions in quartz and scheelite point to a low temperature (160–190°C) and low to moderate salinity (5–15 wt% NaCl eq.) aqueous ore fluid. Abundant vapour-rich inclusions suggest that the fluid boiled. Carbon dioxide was produced locally as a result of interaction of the aqueous fluid with the argillaceous limestone. Bulk leaching experiments and SEM-EDS analyses of decrepitated fluid inclusion residues indicate that the ore-bearing solution was NaCl-dominated. The ¹⁸O values of quartz, ankerite and scheelite from mineralized veins range from 19.7 to 20.5, 18.1 to 20.0 and 7.0 to 8.4 respectively. The average temperature calculated from quartz-scheelite oxygen isotopic fractionation is 170°C. The oxygen isotopic composition of the fluid, interpreted to have been in equilibrium with these minerals, ranged from 5.7 to 7.6, and is considered to represent an evolved meteoric water. Diagenetic or syngenetic pyrite has a sulphur isotopic composition of 0.5±0.3 which is consistent with bacterial reduction of sulphate. The ³⁴S values of arsenopyrite and stibnite range from –2.8 to 2.0 and –2.7 to –2.3 respectively, and are though to reflect sulphur derived from pyrite.The Ixtahuacan deposits are interpreted to have formed at low temperature (<200°C) and a depth of a few hundred metres from a low fO₂ (10^–49–10^–57), high pH (7–8) fluid. Arsenic was probably transported as arsenious acid, antimony and gold as thio-complexes and tungsten as the complex HWO ₄ ^– .A model is proposed in which a meteoric fluid, heated by a felsic intrusion at depth, was focused to shallow levels along faults. The interaction of the fluid with pyritiferous beds caused the deposition of arsenopyrite as a result of sulphidation and/or decreasing fO₂; gold probably co-precipitated with As or was adsorbed onto the arsenopyrite. The precipitation of stibnite was caused by boiling. Scheelite deposited in response to the increase in Ca²⁺ activity which accompanied interaction of the ore fluid with the argillaceous limestones.     

The role of magmatic and hydrothermal processes in the chemical evolution of the Strange Lake plutonic complex,Québec-Labrador

The Strange Lake plutonic complex consists of three annular Mid-Proterozoic arfvedsonite-aegirine-bearing alkali granites emplaced in the Rae province of the Canadian Shield. The mineralogy, chemistry and structural setting of the complex are very similar to that of many peralkaline central salic complexes associated with the development of the Gardar rift in southern Greeland. The Strange Lake granites are highly fractionated (Rb/Sr=5 to 160 and K/Rb=27 to 120) and carry unusually high abundances of HFSE and REE-bearing exotic minerals (e.g. pyrochlore, gittinsite, elpidite, gadolinite and kainosite) which are reflected in the elevated HFSE (e.g.Zr=307 to 16800 ppm) and REE (e.g. La=84 to 1337 ppm) contents of the granites. HFSE and REE increase from the oldest intrusive unit, which is hypersolvus and unaltered, to the youngest, which is subsolvus and metasomatized. The unaltered granites display a restricted range of ¹⁸O values (+8.2 to+9.6) and low ¹⁸O signatures for fresh arfvedsonite/aegirine (+4.8 to+5.2). Anomalously high CaO (0.7 to 3.2 wt%) and MgO (0.1 to 0.6wt%) concentrations characterize the altered subsolvus granites. These rocks also have elevated whole rock ¹⁸O values (+9.6 to +11.9), negative ¹⁸O_{quartz-alk.feld.} (-0.1 to-1.6), and high ¹⁸O values of altered arfvedsonite (i.e.+6.5 to 13.75) that correlates positively with whole rock ¹⁸O values. The chemical and isotopic data are consistent with a model in which the least evolved alkali granites are formed through differentiation from trachytic (syenitic) parents. Extreme HFSE and REE-enrichment may have been accomplished by differentiation through fractional crystallization and heterogenous distribution of F-rich silicic residual melts in which the REE and HFSE are transported as fluorocomplexes. The O-isotopic values are consistent with the circulation of low temperature (lt;200°C) hydrothermal fluids in the youngest subsolvus intrusive unit which caused extensive Ca (Mg and Sr) metasomatism and fluorine leaching, widespread hematization, and remobilization of the HFSE and REE.     

A spectrophotometric study of samarium (III) speciation in chloride solutions at elevated temperatures

The speciation of samarium (III) in chloride-bearing solutions was investigated spectrophotometrically at temperatures of 100-250 °C and a pressure of 100 bars. The simple hydrated ion, Sm³⁺, is predominant at ambient temperature, but chloride complexes are the dominant species at elevated temperatures. Cumulative formation constants for samarium chloride species were calculated for the following reactions:

     

Molecular research on the systematically challenging smoothhound shark genus Mustelus: a synthesis of the past 30 years

The species-rich genus Mustelus (smoothhounds) of the shark family Triakidae is one of the most bio-economically important groups of elasmobranchs in the world’s oceans. Despite the commercial value of Mustelus, the systematics of the group remains largely unresolved and there is no global review or synthesis of knowledge about the conservation status and conservation genetics of smoothhounds across all oceanic regions. Here, we analysed published studies as well as grey literature to gain insight into the biogeographic, ecological and behavioural factors that shape genetic diversity in smoothhounds, and we identify critical knowledge gaps. From a series of molecular phylogenetic studies it can be inferred that the genus Mustelus is paraphyletic and that the aplacental species evolved secondarily from the placental species of the genus. The increasing availability of genetic data aids in disentangling systematic issues, such that more meaningful morphological characters can be chosen for use in practical field-identification keys for co-occurring smoothhounds. An integrative taxonomic approach to the genus Mustelus may offer the best chance of recording and protecting the biodiversity of these sharks. Furthermore, it is evident that different smoothhound species exhibit unique gene-flow patterns, suggesting varying rates within species and hence that species-level conservation approaches would be most appropriate. Molecular studies have advanced our understanding of smoothhound biology (including reproductive traits), ecology and evolution. While many knowledge gaps remain, a crucial lesson from this review is that, when doing assessments on a molecular level, it is important to place genetic results in a broader context, by assimilating biological and ecological data, if definitive conclusions are to be drawn.     

San Rafael, Peru: geology and structure of the worlds richest tin lode

The San Rafael mine exploits an unusually high grade, lode-type Sn–Cu deposit in the Eastern Cordillera of the Peruvian Central Andes. The lode is centered on a shallow-level, Late Oligocene granitoid stock, which was emplaced into early Paleozoic metasedimentary rocks. It has a known vertical extent exceeding 1,200 m and displays marked vertical primary metal zoning, with copper overlying tin. The tin mineralization occurs mainly as cassiterite–quartz–chlorite veins and as cassiterite in breccias. The bulk of it is hosted by a K-feldspar megacrystic, biotite- and cordierite-bearing leucomonzogranite, which is the most distinctive phase of the pluton. Copper mineralization occurs predominantly in the veins that straddle the metasedimentary rock–intrusion contact or are hosted entirely by slates. Both tin and copper mineralization are associated with strong chloritic alteration, which is superimposed on an earlier episode of sericitization and tourmaline–quartz veining. Based on the distribution of alteration and ore mineralogy, cassiterite deposition and subsequent chalcopyrite precipitation are believed to have been the result of a single, prolonged hydrothermal event. The source of the metals is inferred to be a highly evolved, peraluminous magma, related to the leucomonzogranitic phase of the San Rafael pluton. Preliminary fluid inclusion microthermometry suggests that ore deposition took place during the mixing of moderate and low salinity fluids, which were introduced in a series of pulses. Several large fault-jogs, created by sinistral-normal, strike-slip movement, are interpreted to have focused synkinematic magmatic fluids and permitted their effective mixing with meteoric waters. It is proposed that this mixing led to rapid oxidation of Sn (II) chloride species and caused supersaturation of the fluids in cassiterite, resulting in the development of localized, high-grade ore shoots. A favorable structural regime that promoted large-scale mixing of two fluids originating under very different physico-chemical conditions appears to have been the key factor responsible for the unusual richness of the deposit.     

Epithermal mineralization and ore controls of the Shasta Au-Ag deposit, Toodoggone District, British Columbia, Canada

The Shasta gold-silver deposit, British Columbia, Canada, is an adularia-sericite-type epithermal deposit in which deposition of precious metals coincided with the transition of quartz- to calcite-dominant gangue. Mineralization is associated with stockwork-breccia zones in potassically altered dacitic lapilli tuffs and flows, and consists of pyrite, sphalerite, chalcopyrite, galena, acanthite, electrum and native silver. Pre- and post-ore veins consist solely of quartz and calcite, respectively. Fluid inclusion microthermometry indicates that ore minerals were deposited between 280 ° and 225 °C, from a relatively dilute hydrothermal fluid (˜1.5 wt.% NaCl equivalent). Abundant vapor-rich inclusions in ore-stage calcite are consistent with boiling. Oxygen and hydrogen isotopic data (δ¹⁸O_fluid = −1.5 to −4.1‰; δD_fluid = −148 to −171‰) suggest that the fluid had a meteoric origin, but was ¹⁸O-enriched by interaction with volcanic wallrocks. Initial (˜280 °C) fluid pH and log f O₂ conditions are estimated at 5.3 to 6.0, and −32.5 to −33 bar, respectively; during ore deposition, the fluid became more alkaline and oxidizing. Ore deposition at Shasta is attributed to localization of meteoric hydrothermal fluids by extensional faults; mineralization was controlled by boiling in response to hydraulic brecciation. Calcite and base metal sulfides precipitated due to the increase in pH that accompanied boiling, and the associated decrease in H₂S concentration led to precipitation of gold and silver. Received: 23 February 1995 / Accepted: 16 April 1996