摘要:

IntroductionIt is well established through field observations, experiments, and chemical models that oxidation (redox) state and pH exert a strong influence on the speciation of dissolved components and the solubility of minerals in hydrothermal fluids. These parameters are also known to act as major constraints on the stability of specific minerals and mineral assemblages in hydrothermal systems and to control sulfur and carbon isotope systematics in these systems. Of particular interest to this study is the influence of pH and redox state on the speciation of metals, ligands, and metal complexes. For example, in the model presented below, zinc monoacetate complexation, as represented by the reactionis directly influenced by pH through the control of acetate activity by protonation,and the control of Zn2+activity by sphalerite solubility,The oxygen fugacity indirectly affects reaction (1) because HS−in reaction (3) is constrained, assuming constant total dissolved sulfur, constant pH and equilibrium, by the SO42−to HS−ratio of the fluid through the reaction,and corresponding mass action condition at equilibrium:In eqn. (5), the bracketed species represent the activity of the enclosed species andfO2is the fugacity of oxygen gas in equilibrium with the fluid. It has been suggested (cf.Shock,1–3Helgesonet al.4) that dissolved acetate is coupled to dissolved carbonate in basinal fluidsviametastable redox equilibria including the oxidation of acetate by molecular oxygen,This type of metastable link between carboxylate species and carbonate is not assumed in the model described below. In this model total acetate and total malonate concentrations are independent of total carbonate.Oxygen fugacity (or activity) is the most commonly used redox variable for describing the oxidation state of hydrothermal systems. Its common use stems from the convenience of writing many hydrothermal reactions in terms of oxygen gas as a reactant or product and the generally close approach to (or actual attainment of) stable or metastable equilibrium under hydrothermal conditions. Consequently, logfO2–pH (or logaO2–pH) diagrams have become the most widely used approach to illustrate the combined effects of pH and redox state in the control of hydrothermal aqueous speciation and related phase equilibria (Barnes and Kullerud,5Barnes and Czamanske,6Ripley and Ohmoto,7Henleyet al.,8McPhail,9Wood10). These diagrams can also be employed to graphically represent aqueous carbon and sulfur isotope systematics (Ohmoto,11Ohmoto and Rye,12Zhang13) and organic matter composition (Helgesonet al.,4Gize14) under hydrothermal conditions. In addition to logfO2–pH diagrams, other graphical depictions of hydrothermal redox dependency are used, including: (a) plots in log (ΣSO4/ΣH2S)–pH and log (ΣSO4/ΣH2S)–Tspace (Ripley and Ohmoto,7,15Ahmadet al.,16Ohmoto and Goldhaber17), (b) logfS2–Tand logfS2–1/Tdiagrams (Ripley and Ohmoto,15Ohmotoet al.,18Henleyet al.8), (c) logfO2–T, logfO2− 1/T, and log(fH2O/fH2)–Tdiagrams (Ripley and Ohmoto,7Ohmoto and Rye,12Eugster,19Helgesonet al.,4Ohmoto and Goldhaber,17Giggenbach,20Cookeet al.,21Wood and Samson22), (d) plots in Eh (pe)–pH space (Rose,23,24Wintsch,25Henleyet al.,8Poundet al.,26Pourbaix and Pourbaix27), (e) logfH2–Tdiagrams (Shock3), and (f) the activity diagrams logai– logaCO2(Helgesonet al.4), logfO2− logaH2S(Kettleret al.28), logaSO4–logaH2S(Ohmotoet al.18), and logfO2– logfS2(Bartonet al.,29Casadevall and Ohmoto,30Ohmotoet al.,18Henleyet al.,8Eugster,19Sverjensky,31,32Ahmadet al.16).Over the past forty years these diagrams, particularly logfO2–pH diagrams, have been used to investigate transport and deposition mechanism of metals in hydrothermal ore fluids. In the majority of these studies, the principal focus is on inorganic complexation mechanisms that mobilize metals in solution and associated processes that cause complex destabilization and concomitant ore deposition. Although a large number of inorganic ligands are known to be complexing agents in hydrothermal ore fluids, inorganic transport of Hg, Pb, Zn, Cu, REE, and the precious metals appears to be dominated by the ligands Cl−, HS−, NH3, and OH−(Seward and Barnes33). Thus, it is not surprising, that ore transport and deposition processes have been commonly evaluated using concentration contours of metal complexes involving these ligands in logfO2–pH space. This approach has been particularly fruitful in developing or evaluating genetic models for a wide variety of hydrothermal deposit-types, including mercury deposits (Wells and Ghiorso,34Barnes and Seward,35Fein and Williams-Jones36), Mississippi Valley-type (MVT) deposits (Anderson,37,38Giordano and Barnes,39Barnes,40Sverjensky41), red-bed related base metal (RBRBM) deposits (Sverjensky31,32), McArthur-type and Selwyn-type sedex deposits (Cookeet al.21), porphyry copper deposits (Crerar and Barnes42), gold and silver deposits (Bartonet al.,29Casadevall and Ohmoto,30Ahmadet al.,16Gammons and Williams-Jones43), as well as deposits of the more exotic metals including the platinum group elements (Mountain and Wood,44Woodet al.,45Gammonset al.46).Although inorganic transport mechanisms adequately account for the genesis of many medium- and high-temperature hydrothermal deposits (e.g., porphyry Cu–Mo–W deposits; skarn deposits; and epithermal base metal, Hg, Au, and Ag deposits) it is less certain that such mechanisms can fully account for most lower-temperature hydrothermal deposits formed in the 50 to 200 °C temperature range, especially those deposits intimately associated with organic matter and formed by basinal diagenetic processes (e.g., MVT and RBRBM deposits). As a means of addressing metal transport in these lower-temperature hydrothermal ore systems, alternative complexing mechanisms have been tested including organic transport, involving transport of metalsviaorganic ligands in the form of metal-organic complexes (cf.Giordano and Barnes,39Giordano,47Drummond and Palmer,48Manning,49Hennetet al.,50Giordano and Kharaka,51Sicree and Barnes,52Giordano53) andviapressure-sensitive species such as metal-thiocarbonate complexes (Hennetet al.54). Furthermore, many epithermal precious metal and mercury deposits contain organic matter with paragenetic relationships closely related to the timing of ore-formation. This observation has sparked speculation that a significant role for organic complexing may supplement or dominate apparently viable inorganic mechanisms of metal transport in these systems (cf.Fein and Williams-Jones,36Leventhal and Giordano55).Giordano53gives an overview of those investigations conducted during the past 25 years and designed to evaluate organic transport in hydrothermal ore systems. The majority of these investigations (experimental and theoretical) have focused on Pb and Zn transport in MVT and RBRBM deposits and deep sedimentary basin brines, generally thought to be ore fluids for these deposits. In all of these studies Pb and Zn concentrations and speciation are either measured or calculated for very restricted fluid compositions in terms offO2, pH, and other solute activities. This approach is fine for evaluating the significance of complexing mechanisms under specific ore-fluid conditions. More useful, however, would be an evaluation based on metal concentration contours in logfO2–pH space. This is the method employed in those studies mentioned above in which logfO2–pH diagrams were used to develop transport and deposition models based on inorganic complexing. This approach allows a broad-based evaluation of a particular complexing mechanism over a wider range of possiblefO2–pH conditions and the simultaneous evaluation of deposition processes as a function offO2and pH. The author is not aware of any published studies that take this approach in evaluating organic-transport mechanisms of base or precious metals in low- to moderate-temperature hydrothermal ore fluids. However, organic transport of mercury has been evaluated in logfO2–pH space for hydrothermal ore-forming systems (Wells and Ghiorso,34Fein and Williams-Jones36). These authors found that methyl mercury as well as acetate and oxalate complexes of mercury do not contribute significantly to Hg transport in the formation of hydrothermal mercury deposits.The purpose of this paper is to present a preliminary evaluation of the influence of oxygen fugacity and pH on monocarboxylate- and dicarboxylate-transport of Pb and Zn in low-temperature (100 °C) hydrothermal ore fluids that are related to diagenetic processes in deep sedimentary basins. First a description of the chemical model used in this study will be presented. This is followed by a presentation of model results illustrating Pb and Zn solubility and speciation and a discussion of these results in terms of fluid parameters and the compositional characteristics of modern petroleum-field brines. Finally, some conclusions will be presented regarding (a) the similarities between modern petroleum-field brines and ore fluid models of MVT and RBRBM deposits and (b) the systematics of Zn and Pb transport by carboxylate complexation in these basinal brines.

ISSN:1467-4866    

DOI:10.1039/b204459e

出版商:RSC    

年代:2002

数据来源: RSC