Geochimica et Cosmochimica Acta最新文献

{"title":"Gold in sulfide fluids revisited","authors":"Boris R. Tagirov ,&nbsp;Nikolay N. Akinfiev ,&nbsp;Mariia E. Tarnopolskaia ,&nbsp;Irina Yu. Nikolaeva ,&nbsp;Irina Yu. Zlivko ,&nbsp;Valentina A. Volchenkova ,&nbsp;Luydmila A. Koroleva ,&nbsp;Alexander V. Zotov","doi":"10.1016/j.gca.2024.08.022","DOIUrl":"10.1016/j.gca.2024.08.022","url":null,"abstract":"<div><div>Gold solubility was measured at temperatures of 350, 400, 450, and 490 °C and pressures of 500 and 1000 bar in an ’oxidized sulfide’ system, as a function of pH_T<span> (2 – 10) and sulfur concentration (</span><em>m</em>(S_total) = 0.03 – 1.2 [mol·(kg H₂O)^-1]). In this system, sulfur primarily exists as H₂S, H₂SO₃, H₂SO₄, their dissociation products, and radical species such as S₂^- and S₃^-. The complexes Au(HS)₂^-, Au₂S₂^2-, AuHS_(aq), AuHS(H₂S)_3(aq), and AuOH_(aq) were identified as the primary gold species in the experimental fluids, varying with pH and <em>m</em>(S_total). The solubility constants for Au(HS)₂^-<span>, a critical (hydro)sulfide complex, align excellently with literature for ’reduced sulfide’ fluids, where sulfur predominantly exists in the 2- oxidation state. New experimental data from the ’oxidized sulfide’ system were regressed along with reliable literature data from ’reduced sulfide’ systems to calculate standard thermodynamic properties and parameters of the Helgeson-Kirkham-Flowers (HKF) model. The solubility constants for charged complexes, Au(HS)</span>₂^- and Au₂S₂^2-, increase sharply with temperature, whereas those for neutral species, AuHS_(aq) and AuHS(H₂S)_3(aq), show a pronounced peak near 300 °C. These (hydro)sulfide complexes account for gold solubility ranging from a few tens of ppb to a few tens of ppm in natural sulfide fluids, depending on the fluid pH. Thermodynamic calculations also indicate that, in addition to (hydro)sulfide species and the hydroxide complex, AuCl₂^-<span> significantly contributes to Au mobility in high-temperature acidic chloride fluids. Based on new experimental data and prior studies using solubility and X-ray absorption spectroscopy methods, other gold complexes including mixed Au-HS-Cl, Au-HS-S</span>₃^-<span> species, and complexes with alkali metal cations, are deemed redundant. Above 250 °C, the influence of chloride salts on gold solubility can be accurately modeled using a simple extended Debye-Hückel equation with the term </span><em>b</em>_γ·<em>I</em> = 0. The Setchenov coefficient <em>b</em>_n<span> = 0 suffices for calculating the activity coefficients of neutral species. This streamlined thermodynamic model aligns closely with earlier experimental work by Terry Seward and his team, effectively describing the state of Au in all types of natural fluids where Au exists in the 1+ oxidation state, under any set of </span><em>P</em>-<em>T</em>-<em>f</em>(O₂)-<em>f</em>(S₂)-compositional parameters.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 285-304"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142245687","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Hydrothermal geochemistry: a tribute to Terry M. Seward","authors":"Jean-François Boily ,&nbsp;Kono H. Lemke ,&nbsp;Andri Stefánsson","doi":"10.1016/j.gca.2025.08.033","DOIUrl":"10.1016/j.gca.2025.08.033","url":null,"abstract":"","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 1-4"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144923896","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Silica solubility and molecular speciation in water vapor at 400–800 °C","authors":"Andri Stefánsson ,&nbsp;Samuel W. Scott ,&nbsp;Anna B. Bjarkadóttir ,&nbsp;Adolph Jr. Manadao Bravo ,&nbsp;Sigríður M. Aðalsteinsdóttir ,&nbsp;Erlend Straume ,&nbsp;Kono H. Lemke","doi":"10.1016/j.gca.2024.10.022","DOIUrl":"10.1016/j.gca.2024.10.022","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Silica solubility and molecular speciation in hydrothermal water vapor have been determined through quartz solubility experiments at 400–800 °C and 50–270 bar using a novel U-tube flow-through reactor system and theoretical calculations. The results demonstrate that silica concentrations are low in water vapor (&lt;em&gt;m&lt;/em&gt;&lt;sub&gt;Si,tot&lt;/sub&gt; = 0.11–4.56 mmol/kg or &lt;em&gt;x&lt;/em&gt;&lt;sub&gt;Si,tot&lt;/sub&gt; = 8.21 × 10&lt;sup&gt;−5&lt;/sup&gt;–1.98 × 10&lt;sup&gt;−6&lt;/sup&gt; mol/mol) increase with both temperature and pressure, which is attributed to the dissolution of quartz according to the reaction:&lt;/div&gt;&lt;div&gt;&lt;span&gt;&lt;span&gt;&lt;span&gt;SiO&lt;sub&gt;2(s)&lt;/sub&gt; + (&lt;em&gt;n&lt;/em&gt; + 2)H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;(g)&lt;/sub&gt; ⇋ Si(OH)&lt;sub&gt;4&lt;/sub&gt;·(H&lt;sub&gt;2&lt;/sub&gt;O)&lt;em&gt;&lt;sub&gt;n&lt;/sub&gt;&lt;/em&gt;&lt;sub&gt;(g)&lt;/sub&gt;&lt;/span&gt;&lt;/span&gt;&lt;/span&gt;&lt;/div&gt;&lt;div&gt;&lt;span&gt;Thermodynamic modeling and theoretical calculations employing density functional theory (B3LYP-D3), and MP2 &lt;/span&gt;&lt;em&gt;ab initio&lt;/em&gt; calculations reveal the stable structures to be Si(OH)&lt;sub&gt;4(g)&lt;/sub&gt;, Si(OH)&lt;sub&gt;4&lt;/sub&gt;·(H&lt;sub&gt;2&lt;/sub&gt;O)&lt;sub&gt;2(g)&lt;/sub&gt;, Si(OH)&lt;sub&gt;4&lt;/sub&gt;·(H&lt;sub&gt;2&lt;/sub&gt;O)&lt;sub&gt;4(g)&lt;/sub&gt; and Si(OH)&lt;sub&gt;4&lt;/sub&gt;·(H&lt;sub&gt;2&lt;/sub&gt;O)&lt;sub&gt;7(g)&lt;/sub&gt; (&lt;em&gt;n&lt;/em&gt; = 0, 2, 4, 7) under the temperature and pressure conditions of interest, with higher-order hydrated structures also present at the lowest temperatures and highest pressures. Various isomers of the gaseous silica species were identified, with the number of energetically favorable structures increasing with hydration level and the motifs shifting from silanol-water bonds to complex water-water networks. Over the temperature range of interest, the logarithm of the quartz equilibrium solubility constant (log&lt;em&gt;K&lt;sub&gt;n&lt;/sub&gt;&lt;/em&gt;) rises from −7.40 to −6.55 and −12.23 to −11.65 at 400 to 800 °C for the formation of Si(OH)&lt;sub&gt;4(g)&lt;/sub&gt; and Si(OH)&lt;sub&gt;4&lt;/sub&gt;·(H&lt;sub&gt;2&lt;/sub&gt;O)&lt;sub&gt;2(g)&lt;/sub&gt;, respectively, and decreases from −16.20 to −19.15 and –22.61 to −28.74 for Si(OH)&lt;sub&gt;4&lt;/sub&gt;·(H&lt;sub&gt;2&lt;/sub&gt;O)&lt;sub&gt;4(g)&lt;/sub&gt; and Si(OH)&lt;sub&gt;4&lt;/sub&gt;·(H&lt;sub&gt;2&lt;/sub&gt;O)&lt;sub&gt;7(g)&lt;/sub&gt; at the same temperature range, respectively. Standard thermodynamic properties were derived based on the experimental results, revealing temperature-independent enthalpy (&lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;msubsup&gt;&lt;mi&gt;H&lt;/mi&gt;&lt;mrow&gt;&lt;mi&gt;n&lt;/mi&gt;&lt;mo&gt;,&lt;/mo&gt;&lt;mi&gt;r&lt;/mi&gt;&lt;/mrow&gt;&lt;mi&gt;o&lt;/mi&gt;&lt;/msubsup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;), entropy (&lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;msubsup&gt;&lt;mi&gt;S&lt;/mi&gt;&lt;mrow&gt;&lt;mi&gt;n&lt;/mi&gt;&lt;mo&gt;,&lt;/mo&gt;&lt;mi&gt;r&lt;/mi&gt;&lt;/mrow&gt;&lt;mi&gt;o&lt;/mi&gt;&lt;/msubsup&gt;&lt;mrow&gt;&lt;mo&gt;)&lt;/mo&gt;&lt;/mrow&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; and heat capacity (&lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;msubsup&gt;&lt;mi&gt;C&lt;/mi&gt;&lt;mrow&gt;&lt;mi&gt;p&lt;/mi&gt;&lt;mo&gt;,&lt;/mo&gt;&lt;mi&gt;n&lt;/mi&gt;&lt;mo&gt;,&lt;/mo&gt;&lt;mi&gt;r&lt;/mi&gt;&lt;/mrow&gt;&lt;mi&gt;o&lt;/mi&gt;&lt;/msubsup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;) of reaction for each gaseous silica species. The enthalpy of the reaction is nearly constant, whereas the entropy and heat capacity decrease with increasing hydration, resulting in higher-level hydrated species becoming less important with increasing temperature. Our quartz solubility results are in good agreement with pr","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 44-56"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673651","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Solubility of NaCl in water vapor at 400–700 °C","authors":"Sigríður María Aðalsteinsdóttir,&nbsp;Guðmundur Sverrisson,&nbsp;Samuel Warren Scott,&nbsp;Erlend Straume,&nbsp;Adolph M. Bravo,&nbsp;Andri Stefánsson","doi":"10.1016/j.gca.2024.11.002","DOIUrl":"10.1016/j.gca.2024.11.002","url":null,"abstract":"&lt;div&gt;&lt;div&gt;&lt;span&gt;&lt;span&gt;&lt;span&gt;Water significantly impacts the chemical evolution of the Earth’s crust, affecting environments from volcanic settings to hydrothermal systems&lt;span&gt;. These fluids transport elements essential for geological processes, such as metal &lt;/span&gt;&lt;/span&gt;ore deposit formation. At high temperatures, as water transitions from liquid to vapor, its molecular structure changes, drastically reducing its capacity to dissolve solids and solvate ions. Here, we report experimental results of &lt;/span&gt;halite (NaCl&lt;/span&gt;&lt;sub&gt;(s)&lt;/sub&gt;) solubility in water vapor at 400–700 °C and 30–300 bar using a novel U-tube flow-through reactor system. The results show that halite solubility is low (&lt;em&gt;x&lt;/em&gt;&lt;sub&gt;NaCl,tot&lt;/sub&gt; = 3.2 × 10&lt;sup&gt;−9&lt;/sup&gt; to 2.9 × 10&lt;sup&gt;−4&lt;/sup&gt; mol/mol) and increases with temperature and pressure, attributed to the dissolution of NaCl followed by its hydration according to the reaction:&lt;/div&gt;&lt;div&gt;&lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mtext&gt;NaC&lt;/mtext&gt;&lt;msub&gt;&lt;mtext&gt;l&lt;/mtext&gt;&lt;mtext&gt;(s)&lt;/mtext&gt;&lt;/msub&gt;&lt;mo&gt;+&lt;/mo&gt;&lt;mi&gt;n&lt;/mi&gt;&lt;msub&gt;&lt;mtext&gt;H&lt;/mtext&gt;&lt;mtext&gt;2&lt;/mtext&gt;&lt;/msub&gt;&lt;msub&gt;&lt;mtext&gt;O&lt;/mtext&gt;&lt;mtext&gt;(g)&lt;/mtext&gt;&lt;/msub&gt;&lt;mo&gt;⇋&lt;/mo&gt;&lt;mtext&gt;NaCl&lt;/mtext&gt;&lt;mo&gt;·&lt;/mo&gt;&lt;msub&gt;&lt;mrow&gt;&lt;mfenced&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mtext&gt;H&lt;/mtext&gt;&lt;mtext&gt;2&lt;/mtext&gt;&lt;/msub&gt;&lt;mtext&gt;O&lt;/mtext&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mi&gt;n&lt;/mi&gt;&lt;mtext&gt;(g)&lt;/mtext&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;&lt;/div&gt;&lt;div&gt;Thermodynamic modeling of the experimental results indicates a preference for specific hydrated species, including &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mtext&gt;NaC&lt;/mtext&gt;&lt;msub&gt;&lt;mtext&gt;l&lt;/mtext&gt;&lt;mrow&gt;&lt;mfenced&gt;&lt;mrow&gt;&lt;mtext&gt;g&lt;/mtext&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;, &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mtext&gt;NaCl&lt;/mtext&gt;&lt;mo&gt;·&lt;/mo&gt;&lt;msub&gt;&lt;mrow&gt;&lt;mfenced&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mtext&gt;H&lt;/mtext&gt;&lt;mtext&gt;2&lt;/mtext&gt;&lt;/msub&gt;&lt;mtext&gt;O&lt;/mtext&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mn&gt;4&lt;/mn&gt;&lt;mrow&gt;&lt;mfenced&gt;&lt;mrow&gt;&lt;mtext&gt;g&lt;/mtext&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;, &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mtext&gt;NaCl&lt;/mtext&gt;&lt;mo&gt;·&lt;/mo&gt;&lt;msub&gt;&lt;mrow&gt;&lt;mfenced&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mtext&gt;H&lt;/mtext&gt;&lt;mtext&gt;2&lt;/mtext&gt;&lt;/msub&gt;&lt;mtext&gt;O&lt;/mtext&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mn&gt;6&lt;/mn&gt;&lt;mrow&gt;&lt;mfenced&gt;&lt;mrow&gt;&lt;mtext&gt;g&lt;/mtext&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; and &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mtext&gt;NaCl&lt;/mtext&gt;&lt;mo&gt;·&lt;/mo&gt;&lt;msub&gt;&lt;mrow&gt;&lt;mfenced&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mtext&gt;H&lt;/mtext&gt;&lt;mtext&gt;2&lt;/mtext&gt;&lt;/msub&gt;&lt;mtext&gt;O&lt;/mtext&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mn&gt;8&lt;/mn&gt;&lt;mrow&gt;&lt;mfenced&gt;&lt;mrow&gt;&lt;mtext&gt;g&lt;/mtext&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;. The less hydrated gaseous NaCl species become more prevalent with increasing temperature and decreasing pressure. At temperatures below 600 °C and pressures above 50 bar, halite solubility is close to stoichiometric, whereas at higher temperatures and lower pressures, the hydrolysis of NaCl&lt;sub&gt;(s)&lt;/sub&gt; to form NaOH&lt;sub&gt;(lq,s)&lt;/sub&gt; or a NaCl&lt;sub&gt;x&lt;/sub&gt;OH&lt;sub&gt;(1-x)(lq,s)&lt;/sub&gt; (x &lt; 1) solid solution is evident, resulting in the formation of HCl&lt;sub&gt;(g)&lt;/sub&gt;. The logarithm of the halite equilibrium solubility constant (logK&lt;sub&gt;&lt;em&gt;n&lt;/em&gt;&lt;/su","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 5-19"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673614","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Are scandium sulfate complexes effective in mobilizing scandium?","authors":"Jia-Xin Wang ,&nbsp;A.E. Williams-Jones ,&nbsp;Xue-Ni Zhang ,&nbsp;Shun-Da Yuan","doi":"10.1016/j.gca.2025.01.038","DOIUrl":"10.1016/j.gca.2025.01.038","url":null,"abstract":"<div><div><span>Sulfate has been increasingly acknowledged as a key ligand for the mobilization and enrichment of rare earth elements (REEs). Here, we report the results of an investigation of the solubility of Sc</span>₂O₃(s), scandium speciation, and the coordination geometry of scandium species in sulfate-bearing solutions using solubility experiments and <em>ab initio</em> molecular dynamics (AIMD) simulations. The investigation was conducted for temperatures of 175 to 250 °C at vapor-saturated water pressure. From the results of our experiments, we conclude that Sc(SO₄)₂⁻ is the dominant scandium species, and that its formation constant (log <em>β₁</em>) varies from 11.20 ± 0.08 at 175 °C to 14.21 ± 0.12 at 250 °C. The AIMD simulations show that the Sc(SO₄)₂⁻ complex is either doubly monodentate or has a mixed monodentate-bidentate configuration, and is coordinated with four water molecules. The species ScSO₄ + was also identified in our experiments, but has a relatively low formation constant (log <em>β₂</em><span>) varying from 7.24 ± 0.46 at 175 °C to 9.51 ± 3.50 at 250 °C. Modeling of the transport and deposition of scandium provides convincing evidence that sulfate scandium complexes can transport scandium efficiently in acidic fluids (pH below 4). Our simulations emphasize the critical roles played by fluid-rock interaction and fluid–fluid mixing in the genesis of scandium ores. This study presents the key thermodynamic data needed to evaluate scandium mobilization in sulfate-rich hydrothermal systems.</span></div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 90-100"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143401584","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The role of potassium in lode gold mineralization: insights from ab initio molecular dynamics and geochemical modeling","authors":"Gao-Hua Fan ,&nbsp;Jian-Wei Li ,&nbsp;Yuan Mei ,&nbsp;Si-Yu Hu ,&nbsp;Ri-Chen Zhong ,&nbsp;Chang Yu ,&nbsp;Xiao-Dong Deng ,&nbsp;Hao Cui ,&nbsp;Wen-Sheng Gao","doi":"10.1016/j.gca.2024.12.022","DOIUrl":"10.1016/j.gca.2024.12.022","url":null,"abstract":"<div><div>Potassium ions (K⁺) are abundant in ore fluids of lode gold deposits, largely illustrated by pervasive potassic alteration and commonly expressed as K-feldspar and muscovite on both sides of individual gold lodes. However, their potential roles in gold mineralization remain elusive. Here, we present results from <em>ab initio</em> molecular dynamics simulation and geochemical modeling to address this question. Molecular dynamics simulation results show that the ability of K⁺ pairing with Au(HS)₂^– complex over a wide temperature–pressure range has a negative linear correlation to fluid density. In high-density liquid-like fluids, little K⁺ is coordinated with the Au(HS)₂^– complex. In contrast, this complex can be nearly neutralized by ion association with K⁺ in low-density, vapor-like fluids, but such a neutral complexation is not very stable, even if under conditions typical of lode gold mineralization. Thus, K⁺ has a limited role in the complexing and transporting of Au in hydrothermal fluids forming lode gold deposits. We conducted geochemical modeling that integrates geological context and mineral paragenesis. The results reveal that potassic alteration in lode gold deposits, characterized by the transition from K-feldspar to muscovite, occurs alongside decreasing temperature, pH, and oxygen fugacity of the ore-forming fluids. Among these factors, the drop in temperature is the most significant mechanism driving potassic alteration, while also causing the destabilization of Au-bisulfide complex and spatially associated deposition of gold. These results suggest that gold mineralization during potassic alteration is primarily driven by the cooling of ore fluids, which also explains the transition from K-feldspar to muscovite alteration. The combination of molecular simulation and geochemical modeling indicates that the role of potassic alteration in lode gold mineralization reflects the influence of fluid evolution particularly fluid cooling on gold precipitation, rather than a direct control of K⁺ on Au transport in lode gold deposits. Therefore, potassic alteration can serve as an effective indicator for lode gold exploration and has been widely applied in practical fieldwork.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 305-316"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841589","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Assessing the validity and limits of linear density models for predicting dissociation–association equilibria in supercritical water","authors":"Maximilian Schulze ,&nbsp;Thomas Driesner ,&nbsp;Sandro Jahn","doi":"10.1016/j.gca.2024.10.008","DOIUrl":"10.1016/j.gca.2024.10.008","url":null,"abstract":"<div><div>A linear relationship between the logarithms of solute dissociation constants, <span><math><mrow><mo>log</mo><mrow><mo>(</mo><mi>K</mi><mo>)</mo></mrow></mrow></math></span>, and the density of water, <span><math><mo>log</mo></math></span> (<span><math><msub><mrow><mi>ρ</mi></mrow><mrow><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub><mi>O</mi></mrow></msub></math></span>), has empirically been demonstrated for decades and raised hope for an universal formalism to describe solute thermodynamic properties over wide ranges of temperature and density at supercritical conditions. Yet, neither a theoretical foundation nor an assessment of the ranges of validity have been presented. Here, we use classical molecular dynamics (MD) simulations as a complementary tool to assess the validity of this linear relationship for the example of NaCl and reveal its limits at water densities below ca. 0.3<!--> <!-->g<!--> <!-->cm⁻³. The derivative <span><math><mrow><mi>∂</mi><mo>log</mo><mrow><mo>(</mo><mi>K</mi><mo>)</mo></mrow><mo>/</mo><mi>∂</mi><mo>log</mo><mrow><mo>(</mo><msub><mrow><mi>ρ</mi></mrow><mrow><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub><mi>O</mi></mrow></msub><mo>)</mo></mrow></mrow></math></span> is calculated based on the volume of reaction and water compressibility derived from the simulations performed in the isothermal–isobaric ensemble at 673<!--> <!-->K. Our results corroborate the linear dependence of <span><math><mrow><mo>log</mo><mrow><mo>(</mo><mi>K</mi><mo>)</mo></mrow></mrow></math></span> vs. <span><math><mrow><mo>log</mo><mrow><mo>(</mo><msub><mrow><mi>ρ</mi></mrow><mrow><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub><mi>O</mi></mrow></msub><mo>)</mo></mrow></mrow></math></span> in the experimentally studied density range and suggest that the linear dependence also extends to higher densities. However, towards lower densities, <span><math><mrow><mo>log</mo><mrow><mo>(</mo><mi>K</mi><mo>)</mo></mrow></mrow></math></span> decreases and takes on values that are lower than would be expected by simply extrapolating the linear behavior. This decrease is consistent with earlier theoretical predictions for the behavior of <span><math><mrow><mo>log</mo><mrow><mo>(</mo><mi>K</mi><mo>)</mo></mrow></mrow></math></span> at vapor-like densities but questions the relevance of some indirect experimental evidence obtained at low temperatures. Although the function described by <span><math><mrow><mo>log</mo><mrow><mo>(</mo><mi>K</mi><mo>)</mo></mrow></mrow></math></span> vs <span><math><mrow><mo>log</mo><mrow><mo>(</mo><msub><mrow><mi>ρ</mi></mrow><mrow><msub><mrow><mi>H</mi></mrow><mrow><mn>2</mn></mrow></msub><mi>O</mi></mrow></msub><mo>)</mo></mrow></mrow></math></span> is non-linear in the low density range, it can be considered well-behaved even at near critical conditions.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 34-43"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673655","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Temperature-, pressure-, and composition-dependence of 18O/16O fractionation involving H2O-NaCl fluids","authors":"Thomas Driesner","doi":"10.1016/j.gca.2025.04.015","DOIUrl":"10.1016/j.gca.2025.04.015","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Oxygen stable isotope fractionation involving water is a function of temperature, water density (or pressure), and dissolved components, namely salts. While temperature effects are well understood, knowledge of pressure and salinity effects is still patchy. A review of available experimental &lt;sup&gt;18&lt;/sup&gt;O/&lt;sup&gt;16&lt;/sup&gt;O quartz-water and calcite-water fractionation data provides consistent evidence for a measurable pressure effect, i.e., both mineral–water fractionation factors decrease similarly with increasing pressure. While the decrease is small to negligible at temperatures above ca. 500 °C it amounts to ca. 1.5 ‰ when going from saturated water vapor pressure to 1500 MPa at 250 °C (equivalent to a water density increase from 800 to 1200 kg m&lt;sup&gt;−3&lt;/sup&gt;) and explains most of the differences between different experimental calibrations. Fits to the experimental data and a theoretical analysis based on published concepts are used to construct a novel graphic representation of the &lt;sup&gt;18&lt;/sup&gt;O/&lt;sup&gt;16&lt;/sup&gt;O reduced isotope partition function ratio (RIPFR) of water, demonstrating in a single diagram the effects of pressure and liquid–vapor fractionation. In a plot of the RIPFR vs. inverse squared temperature, isochores show much simpler trends than isobars and the change of the RIPFR can be represented as a single, empirical function of temperature and water density as&lt;/div&gt;&lt;div&gt;&lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mn&gt;1000&lt;/mn&gt;&lt;mi&gt;ln&lt;/mi&gt;&lt;msup&gt;&lt;mi&gt;β&lt;/mi&gt;&lt;mo&gt;∗&lt;/mo&gt;&lt;/msup&gt;&lt;mo&gt;=&lt;/mo&gt;&lt;mrow&gt;&lt;mfenced&gt;&lt;mrow&gt;&lt;mrow&gt;&lt;mn&gt;3.695&lt;/mn&gt;&lt;mo&gt;×&lt;/mo&gt;&lt;msup&gt;&lt;mn&gt;10&lt;/mn&gt;&lt;mrow&gt;&lt;mo&gt;-&lt;/mo&gt;&lt;mn&gt;5&lt;/mn&gt;&lt;/mrow&gt;&lt;/msup&gt;&lt;mfrac&gt;&lt;msup&gt;&lt;mn&gt;10&lt;/mn&gt;&lt;mn&gt;6&lt;/mn&gt;&lt;/msup&gt;&lt;msup&gt;&lt;mi&gt;T&lt;/mi&gt;&lt;mn&gt;2&lt;/mn&gt;&lt;/msup&gt;&lt;/mfrac&gt;&lt;mo&gt;-&lt;/mo&gt;&lt;mn&gt;3.508&lt;/mn&gt;&lt;mo&gt;×&lt;/mo&gt;&lt;msup&gt;&lt;mn&gt;10&lt;/mn&gt;&lt;mrow&gt;&lt;mo&gt;-&lt;/mo&gt;&lt;mn&gt;5&lt;/mn&gt;&lt;/mrow&gt;&lt;/msup&gt;&lt;mfrac&gt;&lt;msup&gt;&lt;mn&gt;10&lt;/mn&gt;&lt;mn&gt;3&lt;/mn&gt;&lt;/msup&gt;&lt;mi&gt;T&lt;/mi&gt;&lt;/mfrac&gt;&lt;/mrow&gt;&lt;/mrow&gt;&lt;/mfenced&gt;&lt;/mrow&gt;&lt;msup&gt;&lt;mi&gt;ρ&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;1.5&lt;/mn&gt;&lt;/mrow&gt;&lt;/msup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;&lt;/div&gt;&lt;div&gt;with &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mn&gt;1000&lt;/mn&gt;&lt;mi&gt;ln&lt;/mi&gt;&lt;msup&gt;&lt;mi&gt;β&lt;/mi&gt;&lt;mo&gt;∗&lt;/mo&gt;&lt;/msup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; being the normalized RIPFR obtained by subtracting the ideal gas contribution, &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mi&gt;T&lt;/mi&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; temperature in Kelvin, and &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mi&gt;ρ&lt;/mi&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; water density [kg m&lt;sup&gt;−3&lt;/sup&gt;]. It is then demonstrated that at high temperatures the formula can also be used for aqueous NaCl solutions, using the solution’s density rather than the pure water density. Differences to low temperature data can be eliminated with a simple deviation term such that eventually a single formula allows reproducing almost all mineral–water/-solution and liquid–vapor (water or NaCl solution) experimental results within their uncertainties. Applying this formula to published experimental mineral–water fractionation data allows deriving mineral–water fractionation curves at constant water density. These corrected mineral–water fractionation lines can then be combined to derive consistent mineral–mineral","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 20-33"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145327270","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"NdPO4 solubility and aqueous Neodymium speciation in supercritical fluids: An experimental study at 500–700 °C and 1.7 kbar","authors":"Debarati Banerjee ,&nbsp;Laura E. Waters ,&nbsp;Nicole C. Hurtig ,&nbsp;Alexander P. Gysi ,&nbsp;Daniel Harlov ,&nbsp;Chen Zhu ,&nbsp;Artaches Migdisov","doi":"10.1016/j.gca.2025.01.004","DOIUrl":"10.1016/j.gca.2025.01.004","url":null,"abstract":"<div><div><span>A key aspect in the formation of rare earth elements (REE) deposits is the role of REE transport as aqueous REE complexes in supercritical hydrothermal solutions, where the nature of the aqueous complex is controlled by solution composition, temperature and pressure. Despite chloride being considered as one of the most abundant transporting ligands in magmatic-hydrothermal fluids, experimental investigations on the stability of aqueous REE chloride complexes are scarce above 300 °C. In this study, synthetic NdPO</span>₄ crystals were reacted with non-saline and saline (0, 0.05 and 0.5 mNaCl), acidic (0.01 mHCl) aqueous solutions in a series of solubility experiments conducted at 500 − 700 °C and 1.7 kbar, where the solubilities were determined using a stable Nd isotope (¹⁴⁵Nd isotope spike) dilution technique. NdPO₄<span> solubility ranges between 28 ppm and 10,858 ppm, where solubility increases with both temperature and salinity. At 500 °C, log mNdPO</span>₄ increases from −3.93 to −1.60 and there is a strong correlation between NdPO₄<span> solubility and NaCl concentrations (slope of 1.2 ± 0.3), indicating stabilization of the Nd chloride aqueous complexes with a stoichiometry corresponding to NdCl</span>²⁺. At 600 °C, this correlation is weaker (slope of 0.4, log mNdPO₄ increases from −2.63 to −1.88) indicating the stabilization of both Nd chloride and hydroxyl species controlling solubility. At 700 °C, NdPO₄ solubility is largely independent of NaCl concentration indicating that solubility is controlled by Nd hydroxyl complexes, where stoichiometry suggests the neutral Nd(OH)₃⁰ species is dominant. The solubility product (<em>K</em>_sp) of NdPO₄ is derived from experimental data with the relation: log <em>K</em>_sp = -41.81 – 0.057<em>T</em> – 20987/<em>T</em>, with <em>T</em><span> temperature in Kelvin. Comparison of the measured Nd phosphate solubility to thermodynamic predictions using the available Helgeson-Kirkham-Flowers equation of state<span> parameters for aqueous Nd complexes indicate that predictions are up to three orders of magnitude lower compared to experimental observations. This discrepancy is most pronounced in saline solutions, suggesting that thermodynamic properties of the REE chloride species in supercritical fluids require revision. Numerical simulations of fluid-rock interaction between acidic, saline fluids and a Strange Lake felsic mineral assemblage demonstrates that NdPO</span></span>₄ solubility predictions from models are four to six orders of magnitude lower than those calculated based on empirical fits from experiments, which suggests that acidic, saline fluids may play an important role in mobilizing large amounts of light REE from 450 to 700 °C.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 211-226"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143055249","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Phosphate effects on rare earth element sorption onto kaolinite: molecular-scale insights at circumneutral pH","authors":"Hang Xu ,&nbsp;Johannes Leisen ,&nbsp;Alicia S. Robang ,&nbsp;Yinghao Wen ,&nbsp;Biao Wan ,&nbsp;Simin Zhao ,&nbsp;Anant Paravastu ,&nbsp;Brian L. Phillips ,&nbsp;Yuanzhi Tang","doi":"10.1016/j.gca.2025.09.040","DOIUrl":"10.1016/j.gca.2025.09.040","url":null,"abstract":"<div><div>Rare earth elements (REEs) are critical minerals that are indispensable for the clean energy transition. Understanding their occurrence and behavior in highly weathered environments provides valuable insights into the identification and prediction of potential REE resources. Phosphate (P) plays an important role in controlling the geochemical behaviors of REE during weathering and secondary deposition. REE-phosphate minerals, such as monazite and xenotime, are among the important natural sources of REE. However, the molecular-scale reaction mechanisms underlying phosphate-REE interactions in weathering environments remain unclear. This study investigates the interaction between phosphate and yttrium on the surface of kaolinite, a representative and abundant clay mineral in highly weathered environments. At circumneutral pH 6, phosphate inhibits yttrium mobilization by forming ternary kaolinite-yttrium-phosphate complexes and surface precipitation. The reaction mechanisms identified in this study are distinct from previously reported adsorption-dominated processes on clay minerals that govern REE immobilization during chemical weathering. Solid-state nuclear magnetic resonance (NMR) spectroscopy, including ³¹P direct polarization magic angle spinning (DP/MAS) and ¹H – ³¹P cross polarization (CP) rotational echo adiabatic passage double resonance (REAPDOR) analyses, provides direct molecular-scale evidence for the yttrium-phosphate surface complexation on kaolinite. Quantification of atomic distances by numerical simulations further substantiates the formation of ternary surface complexes. These mechanistic findings enhance our understanding of phosphate-mediated REE mobilization, transport, and redeposition in natural environments.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"408 ","pages":"Pages 132-143"},"PeriodicalIF":5.0,"publicationDate":"2025-09-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145263667","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}