Thermodynamic Model of Oxidized Solubility of Arsenopyrite and Arsenic Mobility Under
Surface Conditions

G. R. Kolonin United Institute of Geology, Geophysics and Mineralogy, Novosibirsk 630090, Russia

G. A. Palyanova United Institute of Geology, Geophysics and Mineralogy, Novosibirsk 630090, Russia

Arsenopyrite is very abundant mineral of Au-Ag-bearing, polymetallic and other hydrothermal ore deposits. There are a lot of sites in West and East Siberia with arsenopyrite-containing mine and plant tailings and dumps. It is the reason why the problem of As migration and subsequent pollution of surface water will be under attention in regions with mining activity during long time period Porter and Peterson, 1977; Probst et al., 1994). Additionally to potential toxic properties arsenopyrite as the sulphide with high anion:cation ratio is the second after pyrite reason of acid mine drainage. Thermodynamic model is developed by authors to estimate the stability of arsenopyrite and its replacing mineral assemblages in solutions, including both hydrothermal (Kolonin et al., 1988; Palyanova and Kolonin, 1991) and surface (Palyanova and Kolonin, 1989; Kolonin and Palyanova, 1991) conditions. There is the comprehensive description of arsenopyrite behaviour within natural oxidized zones of sulphide deposits (Smirnov, 1951) , but experimental and theoretical data are very restricted. Special interest is in recent spectroscopic investigations of altered arsenopyrite surfaces after short-run oxidation by air-saturated water and inorganic oxidants (Bucley and Walker, 1988; Richardson and Vaughan, 1989; Nesbitt et al., 1995). A comparison of these experimental results with our thermodynamic data is an additional goal of this report.

A variety of types of computer modeling of arsenopyrite solubility process have been carried out, including dependence of compositions of solid phase and solution on reaction coordinate as well as influence of water:mineral ratio. Three principle oxidation stages were fixed with formation of following mineral products, when oxygen fugacity increases: 1) pyrite + native As + magnetite; 2) pyrite + goethite or goethite only (intermediate stage); 3) scorodite only. It is very important that the presence of most toxic As(III) takes place only at the first and second stages, when some amounts of native arsenic, magnetite or pyrite can be formed and Fe(II) species predominate in solution. Naturally, a big excess of sulfuric acid forms if As(V) and Fe(III) are precipitated as scorodite at the third stage. The last phase is equal to goethite (or hematite) during pyrite oxidation. Moreover, the replacement of scorodite by goethite takes place as the result of increase of H2SO4 concentration, especially in presence of oxidizing pyrite. It is important, that other principle changes of relative As, Fe and S mobility in solutions describe as a function of both pH or composition of initial mineral assemblage.

The thermodynamic model is in a good agreement with natural observations (Smirnov, 1951), emphasizing the possibility of appearance of realgar, auripigment, and native As as secondary minerals in acid reduced conditions, when arsenic is inert component. Arsenopyrite demonstrates maximal stability in near neutral solutions, when all its components (As, Fe, S) are enough inert. High mobility of As and S, accompanied by replacement of arsenopyrite by magnetite and other Fe minerals, can be expected in alkaline solutions. There are some common features in thermodynamic and spectroscopic models too, in spite of a principle difference between these two approaches: spectroscopic results highlight details of solution-mineral interface, including intermediate state and metastable phases, whereas thermodynamic calculations show more bulk results of phase transitions of initial mineral. In particular surface enrichment by As, presence of iron oxides and hydroxides as well as arsenate phases, including possible scorodite, were obtained. Nevertheless, arsenic oxides (As2O3 and As2O5) and iron sulphates were fixed in Richardson and Vaughan (1989) but are absent in our model (as very soluble phases). The general and important conclusion, following from both models under discussion is a reality of intermediate oxidized state for
As(0 or III), Fe(II) and S (thiosulphate, polysulphides and elemental sulphur).


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