8, April 2003 GS 553. THERMODYNAMICS AND PHASE EQUILIBRIA Lecture 27: Au Cu Fe S Next lecture: Zn Fe S, Ni Fe S, Fe As S readings: CS 41-57, 77-90; Sharp et al. (1985) A. Fe S list of minerals in system (Craig & Scott, 1974) Table CS1 several phases near FeS (pyrrhotite superstructures) and two near FeS2 various representations of phase equilibria for Fe S Fig. S12 stable phases at high T: Fe, troilite (tr), pyrrhotite (po), pyrite (py), FexSy (l), S liquid/vapor Fig. CS1 solid solution of excess S in po is a measure of fS2, T, a(FeS) Figs. S-19, CS6-9 application to most rocks shows low T, suggesting po reset to lower S during cooling possible phase diagram constructed at < 300C consistent w/ natural assemblages Fig. CS3 B. Cu S list of minerals in system (Craig & Scott, 1974) several phases close to Cu2S: chalcocite (cc), djurleite (dj), digenite (dg) stable phases at high T: Cu (l), digenite (dg) Cu2S, CuxSy (l), S liquid/vapor variable solid solution of excess S in dg is a measure of fS2, T, a(Cu2S) Table CS6 Fig. CS22 CS23-25
C. Au Cu list of minerals in system: Au, Cu, tetraauracupride (AuCu), auricupride (AuCu3), unnamed (Au3Cu) intermetallic minerals formed by replacement of Au (Knipe & Fleet, 1997) KF97, Fig. 2 continuous solution of metal above 400C, continuous liquid solution (Okamoto et al., 1987) Fig. 1 thermodynamic data available for ordered and disordered Au-Cu solids (Hultgren et al., 1973) D. Au Cu S no additional phases known in this system phase equilibria may be calculated from available G, a/X data for Au-Cu solids at high T, continuous solid and liquid solution for Au-Cu, consider displacement of reaction: (1) 4Cu + S2 = 2Cu2S: curve shifts upward in fS2-T space with increasing Au solution in Cu at T < 400C, stable intermetallic phases which are related by fS2-T curves for reactions: (2) 4AuCu2 + S2 = 4AuCu + 2Cu2S (3) 6CuAu + S2 = 2Au3Cu + 2Cu2S (4) 2Au3Cu + S2 = 6Au + 2Cu2S (1) (4) located at successively higher fS2 (similar relations in Au-Cu-O forming Cu2O) E. Cu Fe S list of minerals in system (Craig & Scott, 1974) Table CS7 stable ternary phases at high T: bornite, intermediate solid solution (iss) Fig. CS22 variable solid solution of iss must be a measure of fS2, T, a(CuFeS2) CS23-25 close to CuFeS2: chalcopyrite (cpy), talnakhite (tal), mooihoekite (mo), iss, haycockite (hc) solid solution of iss ranges beyond tal to mo, cpy, hc and cb at 600C (Cabri, 1973) Fig. CS28 phases plotted in mol % Cu-Fe-S Fig. CS27 melting above 1000C, immiscible sulfide melts in basalts (Kullerud et al., 1969) KYM69, Fig. x such melts also take up O, best described as sulfide-oxide melts (McKinstry, 1959) low T ternary phases: idaite (id), cubanite (cb), fukuchilite (fk), ferroan digenite (cc-dg) at 400C: cp and id appear, bn ss to dg, bn stable with py, iss field still large schematic phase equilibria at 300C: iss shrinks, cc-dg and tal appear possible phase equilibria at 25C, iss disappears, complex phase relations (if stable) Fig. CS32 fS2 T plot showing iss, cpy sulfidizing to py assemblages (Barton, 1973) Fig. CS34
equivalent to moving up to S on triangular plot with Cu: Fe constant F. Au Cu Fe S little systematic work: Simon et al. (2000) undertook experiments on Cu Fe S with excess Au synthesis runs only starting with mixtures of end-member sulfides (CuS, FeS, FeS2, fine Au metal) if at equilibrium, experiments represent saturation of bn, cpy with native Au References Cited Barton, P.B. Jr. (1973) Solid solutions in the system Cu-Fe-S. Part I. The Cu-S and CuFe-S joins. Econ. Geol. 68, 461. Cabri, J.L. (1973) New data on phase relations in the Cu-Fe-S system. Econ. Geol. 68, 443-454. Craig, J.R. & Scott, S.D. (1974) Sulfide phase equilibria. In Sulfide Mineralogy, Ribbe, P.H., ed., Mineral. Soc. Am. Short Course Notes 1, CS1-110, reprinted as Reviews in Mineralogy. Hultgren R.R., Desai, P.D., Hawkins, D.T., Gleiser, M. & Kelley, K.K. (1973) Selected Values of the Thermodynamic Properties of Binary Alloys. Amer. Soc. Metals, Metals Park, OH, 1435 p. Kullerud, G., Yoder, H.S. & Moh, G.H. (1969) Phase equilibria in the systems Cu-Fe-S. Cu-Ni-S and FeNi-S system, In Wilson, H.D.B., ed., Magmatic Ore Deposits, Econ. Geol. Monogr. 4, 323-343. Okamoto, H., Chakrabarti, D.J., Laughlin, D.E. & Massalski, T.B. (1987) system. Bull. Alloy Phase Diagrams 8, 454-474. The Au-Cu (gold-copper)
Roseboom, E.H. (1966) An investigation of the system Cu-S and some natural copper sulfides between 25 and 700C. Econ. Geol. 61, 641-672. Scott, S.D. (1974) Experimental methods in sulfide synthesis. In Sulfide Mineralogy, Ribbe, P.H., ed., Mineral. Soc. Am. Short Course Notes 1, S1-S38, reprinted as Reviews in Mineralogy. Simon, G., Kesler, S.E., Essene, E.J. & Chryssoulis, S. (2000) Gold in porphyry copper deposits: experimental determination of the distribution of gold in the Cu-Fe-S system at 400-700C. Econ. Geol. 95, 259-270. Sugaki, A., Shima, H., Kitakaze, A. & Harada, H. (1975) Isothermal phase relations in the system Cu-FeS under hydrothermal conditions at 350 and 300C. Econ. Geol. 70, 806-823. References on Ordered Au-Cu Minerals Chen, K., Yu, T., Zhang, Y. & Peng, Z. (1982) Tetraauricupride, CuAu, discovered in China. Sci. Geol. Sinica 11, 111-116. Knipe, S.W. & Fleet, M.E. (1997) Gold-copper alloy minerals from the Kerr Mine, Ontario. Can. Mineral. 35, 573-586. Meisser, N. & Brugger, J. (2000) Alluvial native gold, tetraauricupride and AuSn2 from western Switzerland. Schweiz. Mineral. Petrogr. Mitt. 80, 291-298. Murzin, V.V. & Sustavov, S. (1989) Solid phase transformations in natural copper-bearing gold. Izv. Akad. Nauk SSSR, Ser. Geol. 1989, 94-104. Piestrzynski, A. & Pieczonka, J. (1998) Tetraauricupride from the Kupferschiefer type deposit, SW Poland: the first occurrence. Mineral. Polonica 29, 11-17. Tarkian, M., Economou-Eliopoulos, M. & Eliopoulos, D.G. (1992) Platinum-group minerals and tetraauricupride in ophiolitic rocks of Skyros Island, Greece. Mineral. Petrol. 47, 55-66.
Papers with Abstracts Barton, P.B. Jr. & Toulmin, P. III (1964) Experimental determination of chalcopyrite+sulfur=pyrite+bornite from 350 to 500C. Econ. Geol. 59, 747-752. the reaction
Abstract. The univariant equilibrium curve determined by the electrum-tarnish method, is expressed as a function of temperature and fugacity of sulfur, and extended upward, with gentle curvature, to the invariant point with liquid sulfur at 568C. The data provide a basis for recognizing disequilibrium in certain assemblages. Brett, R. (1964) Experimental data from the system Cu-Fe-S and their bearing on exsolution textures in ores. Econ. Geol. 59, 1241-1269. Abstract. Ss along bn-dg, bn-cc and bn-cpy joins were cooled at various rates, experiments in up to 7 mo. The stability of exsolution lamellae is controlled by several factors in addition to rate of cooling, so these lamellae need not indicate rapid cooling. Neither this nor any of the other textures formed was diagnostic of any particular T or cooling rate. The experiments suggest a number of rules on the relative stability of the various exsolution textures. The experiments support earlier work in showing that veining, mutual boundary, textures, and textures suggesting replacement may all result from exsolution. They also point to dangers in the acquisition of thermometric data by rehomogenization. Lusk, J. & Maxwell Bray, D. (2002) Phase relations and the electrochemical determination of sulfur fugacity for selected reactions in the Cu-Fe-S and Fe-S systems at 1 bar and temperatures between 185 and 460C. Chem. Geol. 192, 227-248. Abstract. Schneeberg (1973) used the Ag/AgI/Ag(2+x)S electrochemical cell for directly measuring the fugacity of sulfur vapor in equilibrium sulfide reaction mixtures in the Cu-Fe-S, Ni-S and Fe-S systems between 210-445C. The present study has re-examined three reactions investigated by Schneeberg (1973), and included four others in the low-sulfur portion of the Cu-Fe-S system. The abundant iron sulfides pyrite (py) and/or pyrrhotite (po) are commonly associated with chalcopyrite (cp) and sphalerite. Other Cu-Fe-S sulfides, including covellite (cv), idaite (id), bornite (bn) and cubanite (cb), may also occur, but are less common because they require relatively high-sulfur or low-sulfur conditions in order to form. The range of logfS2-T conditions that apply to most ore assemblages is therefore contained by reactions that involve these minerals. Accordingly, sulfur fugacities were determined for the following reactions: (1) py+Id = bn+S2, (2) py+bn = cp+S2, (3) py+cp = cb+S2, (4) po+cp = cb+S2, (5) py+cb = po+cb+S2, (6) py+cp = po+cp+S 2 and (7) py = po+S2. Our results for reaction (1) indicate lower sulfur fugacities than were obtained by Schneeberg (1973). For reactions (2) and (7), the present results show excellent agreement at higher T. However, at lower T, previously undetected inflections in logfS 2 vs. 1000/T plots occur at ca. 221 and 237C for reactions (1) and (2), respectively. An invariant point occurs at approximately 335C and a sulfur fugacity of approximately 10-9 where py, cp, cb and po coexist with S vapor. These results confirm findings that reactions (3) and (5) occur above the invariant temperature and are replaced by reactions (4) and (6) below it. For reaction (7), a previously unidentified inflection occurs at ca. 291C. The latter implies a phase transition in hexagonal pyrrhotite that appears to coincide with the lower temperature boundary of the sphalerite geobarometer. The present data provide a logfS2-T framework that will facilitate future calibration of the FeS content in sphalerite. McKinstry, H.E. (1959) Mineral assemblages in sulfide ores: the system Cu-Fe-S-O. Econ. Geol. 54, 975-1001. Abstract. A quaternary diagram is constructed making use of observed mineral associations to supplement available chemical and experimental data. Phase relations explain the rarity or absence of a number of associations such as magnetite-covellite, pyrrhotite-hematite and, in the presence of magnetite, bornite-pyrite. Although most commonly observed assemblages are apparently stable over a wide range of temperatures there is a strong suggestion of shifting of tie-lines with change of temperature within the field of composition bounded by chalcocite, covellite, chalcopyrite, and pyrite. The observed sequence in ore deposition is a set assemblages, each higher than the preceding one in ratio of S to O and usually of Cu to Fe. However, it seldom reaches the ultimate high-S assemblage, covellitepyrite. Late stage reversals of the trend (e.g., chalcocite replacing covellite) could be occasioned by
waning of S supply before cooling was far advanced. Certain minerals such as pyrrhotite and magnetite are early members of the sequence, not because they are in themselves "high temperature" minerals, but because they are stable in low-S high-Fe assemblages. Robie, R.A., Seal, R.R. II, & Hemingway, B.S. (1994) Heat capacity and entropy of bornite (Cu5FeS4) between 6 and 760 K and the thermodynamic properties of phases in the system Cu-Fe-S. Can. Mineral. 32, 945-956. Abstract Heat capacities of synthetic bornite (Cu5FeS4) have been measured by quasi-adiabatic calorimetry between 5 and 351 K and between 338-761 K by DSC. The heat capacity of bornite exhibits a lambda-type anomaly at 651 K associated with the antiferromagnetic ordering of the spins of the Fe3+ ions. Between 140 and 255 K, there is a broad hump in CP, which may be caused by charge transfer involving Fe2+-Fe3+ and Cu+-Cu2+. At 4702 and 5352 K there are sharp peaks in CP caused by ordering of the copper, iron and vacancies and consequent crystallographic changes. At 298.15 K, the CP and S of bornite are 242.90.6 and 398.51.0 J.mol -1K-1, respectively. Enthalpies and Gibbs free energies of formation of chalcopyrite and bornite are evaluated based on heat capacities (5 to 760 K) and the entropy for bornite, published calorimetric (H and CP) data for chalcopyrite, thermodynamic data for pyrite, and sulfur fugacity data for the reaction: 5CuFeS 2 + S2 = Cu5FeS4 + 4FeS2. Previous studies show that all phases in this reaction are stoichiometric within analytical uncertainty from 673-773 K, and no adjustments for solid-solution effects are needed. Revised expressions for the Gibbs free energy of formation for selected phases in the system Cu-Fe-S are presented relative to the elements and ideal S2 gas. Yund, R.A. & Kullerud, G. (1966) Thermal stability of assemblages in the Cu-Fe-S system. J. Petrol. 7, 454-488. Abstract. Phase relations in the Cu-Fe-S system were determined from 700-200C in most portions of the system and <100C in restricted areas. Approximate solid solution limits for bn, cpy, and po were determined at elevated T. At low T emphasis was placed on establishing the stable assemblages and less on determining the compositions of coexisting phases. The experimental results indicate that the mineral commonly identified as chalcopyrrhotite is in reality tetragonal or isometric cb. Experimental data could not be obtained for the existence of Cu3Fe4S7 or Cu2Fe4S7, older formulae given for val. The thermal breakdown of natural material supports that val is a low-T polymorph of cpy. The relatively uncommon occurrence of id in comparison to cv is attributed to the greater difficulty in nucleating idaite. The possibility of stable coexistence of cc and py was investigated but was found to be prohibited by tie-lines between bn and dg even as low as 100C.