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NOTE Estimates of Selenium and Sulfur Fugacities and Formation

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NOTE Estimates of Selenium and Sulfur Fugacities and Formation Geochemical Journal, Vol. 18, pp. 263 to 268, 1984 NOTE Estimates of selenium and sulfur fugacities and formation temperature for selenium-rich gold-silver vein-type deposits NAOTATSU SHIKAZONO' and KOICHI TAKEUCHI2 Geological Institute, University of Tokyo, Tokyo 113,1 and Institute for Ceramics of Nagasaki Prefecture, Nagasaki 859-37,2 Japan (Received April 21, 1984: Accepted June 19, 1984) It was theoretically derived that the iron content of sphalerite, silver content of electrum and selenium contents of argentite and galena, which all coexist with pyrite, are related to temperature, and selenium and sulfur fugacities. Based on this relation and analytical data on these coexisting minerals, selenium and sulfur fugacities and formation temperature for the selenium-rich gold-silver vein-type deposits were de duced. The temperature estimated from this assemblage (ca. 150 300°C) is in agreement with the homo genization temperature for fluid inclusions in the Kushikino and Takatama deposits. INTRODUCTION and mineral assemblage, TAKEUCHI and SHIKA ZONO (1984) have estimated fs2 and formation A large number of gold-silver vein-type temperature for the Arakawa No. 4 vein of the deposits occur in Tertiary and Quaternary vol Kushikino Au-Ag vein-type deposits, in which canic regions of Japan. Characteristic features large amounts of Se-bearing minerals occur, but of these deposits vary widely. For instance, they have not estimated selenium fugacity gold/silver total production ratio, kinds of (fse2 ). In the argentite-poor deposits, selenium opaque minerals and metals concentrated in rich minerals such as naumannite and aguilarite the deposits are different for each deposit. are commonly observed. The stability of these Based on these features, the gold-silvervein-type minerals might be largely controlled by f$,2 deposits in Japan are classified into two; argen together with temperature and fs2. tite-poor deposits and argentite-rich deposits In the present paper, an attempt is made to (SHIKAZONO,1984a). Argentite-poor deposits estimate fse2, fs2 and formation temperature are characterized by the selenium mineralization for the argentite-poor gold-silver vein-type and poor amounts of base metal elements. In deposits on the basis of the silver content of contrast, argentite-rich deposits are not as electrum, iron content of sphalerite, selenium sociated with selenium mineralization, but base contents of argentite and of galena which all metal elements (Cu, Pb, Zn, Mn) are rich in coexist with pyrite. Temperature estimated some of this type of deposits. from this mineral assemblage will then be corn Depositional environment of several argen pared with the homogenization temperature of tite-rich deposits such as formation temperature, fluid inclusions in order to evaluate the validity sulfur fugacity (fs2) and oxygen fugacity (foe) of the mineral assemblage mentioned above as a has been estimated (e.g., SHIKAZONO, 1978, possible indicator of fse2, fs2 and formation 1984b). In contrast, studies on environmental temperature. conditions for the argentite-poor deposits are Although a large number of studies on very few, although several fluid inclusions have gaseous fugacities such as fs2 and foe for been studied for this type of deposits. For various types of hydrothermal ore deposits instance, based on the studies on fluid inclusions have been carried out, no attention has been 263 264 N. SHIKAZONO and K. TAKEUCHI paid on f S12 not only for the Se-rich Au-Ag (equation (5) in Table 1) was used. By combin vein-type deposits but also for the other hydro ing equations (1) (5) in Table 1 and the rela thermal ore deposits. The study on f see is es tion between activity coefficient of Ag in sentially important for considering the deposi electrum, Ag content of electrum and tempera tional mechanisms for the Se-rich Au-Ag vein ture obtained by WHITE etal. (1957), we derived type deposits. relationships between temperature, fs2, .fse2, silver content of electrum, FeS content of sphalerite, selenium contents of argentite (or THEORETICAL CONSIDERATION acanthite), and of galena, and depicted them in If coexisting electrum, sphalerite, pyrite, Figs. 1 and 2. From these relations and ana argentite and galena are in equilibrium, the lytical data on coexisting galena, sphalerite, relation between the Ag content of electrum, argentite and electrum, we can estimate the selenium contents of argentite and galena, iron formation temperature, fs2 and fse2. To derive content of sphalerite, temperature, fs2 and these relations shown in Figs. 1 and 2, unity of fse2 can be derived from the equilibrium rela activity coefficients of FeS2 in pyrite, Ag2S, tions for the chemical reactions given in Table 1. and Ag2Se in argentite (or acanthite) and of PbS Uncertainties of free energy changes for reac and PbSe in galena was assumed. This assump tions (1) (4) in Table 1 are within 1 kcal and tion, however, is not obvious and it is difficult that for reaction (5) is 1-2 kcal (BARTONand to estimate the activity coefficients of PbS and SKINNER,1979). SCOTT and BARNES (1971) PbSe in galena, and those of Ag2S and Ag2Se in have obtained an equation representing FeS argentite (or acanthite). Activity coefficient of content of sphalerite in equilibrium with pyrite FeS2 in pyrite should be very close to unity, as functions of fs2 and temperature. The tem because the concentrations of minor elements perature which can be estimated at constant fs2 (e.g., Ni, Co) in pyrite are very low, less than on the basis of their equation is 10-20'C lower 0.0n wt%. than that by BARTONand SKINNER'sequation BETHKE and BARTON (1971) have suggested (equation (5) in Table 1) in the temperature from their experimental study on the distribu range considered here (ca. 180-300°C). For tion of selenium between coexisting galena and drawing Figs. I and 2, BARTONand SKINNER'S sphalerite that the PbS-PbSe system behaves as equation in the temperature range concerned ideal solid solution at least above 600'C. Table 1. Chemical reactions and equations representing the equilibrium relations used for drawing Figs. 1 and 2 Temperature Chemical reactions Equilibrium relations References range ('C) 4Ag + S2 (g) logfS2 (-9790.21/T)+4.83 25 176 KUBASHEWSKI et al. (1967) 2Ag2S (acanthite) (1) + 2logaAg 2 S 4 logaAg (1) 4Ag + S2 (g) logfS2 (-9173.95/T) + 3.61 176-804 KUBASHEWSKI et al. (1967) 2Ag2 S (argentite) (2) + 2logaAB 2 S 4logapg (2) 4Ag + See (g) IogfSe2 _ (-10644 .67/T) + 3.12 133 727 MILLS (1974) 2Ag2 Se (naumannite) (3) + 2logaA82Se 4logaAg (3) BARTON and SKINNER (1979) 2PbS + See (g) logfS2 = logfSe 2 + 755.68/T 25 327 MILLS (1974) 2PbSe + S2 (g) (4) 0.24 21og(apbSe/aPbS) (4) FeS + 1/2S2 (g) logfS2 = -15460/T+14 .32 ca.400 700 BARTON and SKINNER (1979) FeS2 (pyrite) (5) 2logXFeS (5) T: absolute temperature, fS2: sulfur fugacity, fse2: selenium fugacity, aAg2S: activity of Ag2S in argentite, aAg2Se: activity of Ag2Se in argentite, aAg: activity of Ag in electrum, apbSe: activity of PbSe in galena, apbS: activity of PbS in galena, XFeS: FeS mole fraction of FeS in sphalerite. N. SHIKAZONO and K. TAKEUCHI 265 in argentite (or acanthite), FeS in sphalerite and and fs2 estimated on the basis of fs2 tempera Ag in electrum are 5 mole%, ca. 20 mole%, ture diagram (Fig. 1) is ca. 220-300°C and 10-9 1.2-2.4 mole% and 46-60 atom.%, respectively, 10-13 atm for Kushikino and ca. 150-250° C and for Kushikino deposits and 2 mole%, ca. 50 10-11 10-18 atm for Takatama. Temperature mole%, 0.4-0.7 mole%, and 50-70 atom.%, respec and fse2 estimated on the basis of fSe2 tem tively, for Takatama deposits. Detailed descrip perature diagram (Fig. 2) is ca. 200 300"C and tions on these mines can be referred to in KITAMI 10-12 10-18 atm for Kushikino and ca. 150 (1973), SUKESHITA and UEMURA (1976), YA 250°C and 10-14 10-23 atm for Takatama. MAOKA and NEDACHI(197 8), IZAWA et al. (1981) Homogenization temperatures of fluid inclu and TAKEUCHIand SHIKAZONO(1984). Based on sions in quartz for the electrum-sphalerite these analytical data on the minerals mentioned pyrite-argentite-galena stage of the Kushikino above and thermochemical consideration carried and Takatama deposits are in the range of ca. out in the previous section, formation tempera 180 250°C (TAKEUCHI, 1979; IzAwA et al., ture, fs2 and fSe2 for these deposits are esti 1981) and ca._. 160 240'C (YAMAOKA and mated as shown in Figs. 1 and 2. Temperature NEDACHI, 1978; WATANABE, 1979), respectively. -10 -10 v d ; N m d 19 C r cb L 20 01 dC 0 a a 0 C b 21 -15 -15 rn a 22 23 24 13 14 -20 -20 15 16 17 H. T. 18 H. T. -25 -25 150 200 250 300 150 200 250 300 Temperature (*C) Temperature (•C) Fig. 2a Fig. 2b Fig. 2. Selenium fugacity temperature diagram 13: Argentite (or acanthite) electrum galena Se2(g) equilibrium curve for XAg2S = 0.8, XPbSe ° 0.05 and XAg = 0.4. 14: Argentite (or acanthite) electrum S2(g) equilibrium curve for XAg2Se = 0.2 and XAg = 0.4. 15: Argentite (or acanthite) electrum galena Se2(g) equilibrium curve for XAg2S = 0.8, XPbSe = 0.05, and XAg = 0.8. 16: Argentite (or acanthite) electrum Se2(g) equilibrium curve for XA82Se = 0.2 and XAg = 0.6. 17: Sphalerite pyrite galena Se2(g) equilibrium curve for XpbSe = 0.05 and XFeS = 0.01. 18: Sphalerite pyrite galena Se2(g) equilibrium curve for XPbSe 0.05 and XFeS = 0.02. 19: Sphalerite pyrite galena Se2(g) equilibrium curve for XpbSe = 0.02 and XFeS = 0.004.
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