Experimental study of the solubilities of pyrite in NaCl-bearing aqueous solutions at 250–350°C

Abstract

A total of sixty-three silica capsule experiments were performed to determine the solubilities of pyrite in NaCl-bearing aqueous solutions (0, 0.1, 0.5, 1, 2, 3, and 4 m) at 250, 300, and 350°C at pressures of vapor/liquid coexistence. The starting materials in the capsules were H 2 O(1) + FeS 2( s) + S ° ( s) ± NaCl ( s). After reaction times up to ~ 60 days, the quenched solutions were analyzed for ΣFe, σH 2 S, ΣSO 4 2−, and pH; the ΣFe content, ranging 5–1,300 ppm, generally increased with increasing temperature and ΣCl content of solution. The calculated solution compositions at the experimental P-T conditions fall mostly in the following ranges: pH = 2.0 to 3.2, log aH 2 s = −1.9 to −1.0, log aHSO 4 − = −3.8 to −2.0, and log aH 2( aq) = −7.0 to −5.0. Evaluation of the experimental data suggests that the various redox equilibria between solution and mineral were attained in most of the experimental solutions. The pH, a H 2 S( aq) , and a H 2( aq) of the solutions were controlled by the sulfur hydrolysis reaction (48° + 4 H 2 O( l) = 3 H 2 S( aq) + HSO 4 − + H +) and the sulfide/sulfate reaction ( H 2 S( aq) + 4 H 2 O( l) = 4 H 2( aq) + H + + HSO 4 −). The pyrite solubility is controlled by a general reaction: FeS 2( s) + nCl − + 2 H + + H 2( aq) = FeCl n 2− n + 2 H 2 S( aq). The equilibrium constants for this reaction, as well as those for association of ferrous chloride complexes ( Fe 2+ + nCl − = FeCl n 2− n ), were obtained at 250, 300, and 350°C; they were used also to compute the equilibrium constants for the reactions controlling the solubilities of pyrrhotite, magnetite, and hematite: FeS( s) + 2 H + + nCl − = FeCl n 2− n + H 2 S( aq); Fe 3 O 4( s) + 6 H + + 3 nCl − + H 2( aq) = 3 FeCl n 2− n + H 2 O( aq); Fe 2 O 3( s) + 4 H + + 2 nCl − + H 2( aq) = 2 FeCl n 2− n + 3 H 2 O( aq). Our experimental data suggest that the dominant Fe-Cl complex is FeCl + in solutions of ΣCl ≤ 0.5 m at 250°C and ΣCl ≤ 0.1 m at 300 and 350°C; FeCl 2 0 is dominant in solutions of the higher ΣCl contents at each temperature. The association constants for FeCl + and FeCl 2 estimated from this study are in good agreement with those estimated recently by Heinrich and Seward (1990), Ding and Seyfried (1992), Fein et al. (1992), and Palmer and Hyde (1992). Our solubility constants for pyrite are in good agreement with those obtained by Crerar et al. (1978) and Wood et al. (1987) for 3 m ΣCl solution at 350°C, but are 0.5–2 orders of magnitude higher than those obtained by them at lower temperatures and/or at lower ΣCl values. Our data suggest that natural hydrothermal fluids that are in equilibrium with pyrite, the most abundant sulfide mineral in the upper crust, are able to transport sufficient amounts (> 10 − m) of both Fe and H 2S to produce pyrite-rich ore deposits at temperatures above 250°C, and possibly at lower temperatures. The solubility of pyrite (and of other Fe-bearing minerals) is affected very little by a change of temperature, provided the pH, a H 2( aq) , a H 2 S( aq) , and ΣCl values remain constant.