An Unusual Pyrite-Sulphur-Jarosite Assemblage from Arkaroola, South Australia
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SHORT COMMUNICATIONS 139 compositions of the Broken Hill ferroan tephroites Acknowledgements. Dr F. L. Sutherland kindly provided from this study, gives a predicted fl refractive index specimens from the Australian Museum collection for this close to fl = 1.820. Refractive indices determined study. Samples from the NBHC Mine were provided by by Dr J. McAndrew on the Broken Hill type Dr Ian Plimer. I am grateful to Mr Oliver Chalmers for specimen (cited in Stillwell's report) were a = 1.791, providing a copy of Stillwell's unpublished report and for reviewing the manuscript. Dr E. R. Segnit also offered fl = 1.819, ~ = 1.829, with errors of +0.002. Thus, helpful comments. The microprobe analyses were under- the measured fl agrees with that predicted by the taken in the Geology Department, University of Mossman-Pawson grid. Melbourne. The terms knebelite, tephroite, and roepperite have all been used for what appears to be essentially REFERENCES the same mineral at Broken Hill. Their use, how- ever, has been arbitrary in the absence of sufficient Birch, W. D., Chapman, A., and Pecover, S. R. (1982) In data to establish natural compositional limits. The Minerals of Broken Hill (H. K. Worner and R. Mitchell, results of this study indicate that most (if not all) of eds.). Australian Mining and Smelting Ltd., Mel- bourne, 68-195. the Broken Hill occurrences are ferroan tephroite, Brush, G. J. (1872) Dana's Syst. Min., 5th edn., App. 1, with a compositional range defined by Fe/Mn 13. between 0.61 and 0.30. However, more data are Fleischer, M. (1980) Glossary of Mineral Species, 1980. needed to determine whether there are any com- Mineral Records, Tucson, 192 pp. positional variations related to either position in Hodgson, C. J. (1975) J. Geol. Soc. Austral. 22, 33-50. the lode horizon or the nature of the tephroite- Hurlbut, C. S. (1961) Am. Mineral. 46, 549-59. bearing assemblage. Mason, B. (1973) J. Geol. Soc. Austral. 20, 397-404. The term knebelite, now defined as a synonym Mossman, D. J., and Pawson, D. J. (1976) Can. Mineral. for manganoan fayalite (Fleischer, 1980) is not 14, 479-86. Roepper, W. T. (1870) Am. J. Sci., ser. 2, 50, 35-7. generally applicable to any Broken Hill mineral Segnit, E. R. (1977) Austral Mineral. 9, 37-9. and roepperite, on the basis of this study, is not a Smith, G. (1922) Geol. Surv. N.S.W., Mere. 8, 403-16. valid mineral at Broken Hill (Birch et al., 1982). --(1926) Geol. Surv. N.S.W. Mineral Res. 34, 145 pp. The nature of the original roepperite specimen Stillwell, F. L. (1956) Roepperitefrom Broken Hill. Report from Sterling Hill is unknown. However, there may to Australian Museum (unpubl.). be some doubt cast on its validity, since Hurlbut (1959) Proc. Australas. Inst. Mining Met. 190, 1-84. (1961) demonstrated that the solid solubility of Zn in tephroite is limited and, of the four Franklin [Manuscript received 3 May 1983; roepperite specimens available for Hurlbut's study, revised 4 July 1983] three proved to be black willemite and the fourth to be ferroan tephroite with exsolved willemite. Copyright the Mineralogical Society Department of Mineralogy and Petrology, National Museum of Victoria, W. D. BIRCH 285 Russell Street, Melbourne, Victoria 3000, Australia MINERALOGICAL MAGAZINE, MARCH 1984, VOL. 48, PP. 139 42 An unusual pyrite-sulphur-jarosite assemblage from Arkaroola, South Australia AN occurrence of co-existing pyrite, elemental The occurrence was discovered by Mr Edward sulphur, and jarosite (KFe3(SO4)2(OH)6) has Madden of Echunga, South Australia, in a meta- recently been discovered in South Australia. morphic formation known as the Woodnamoka Although jarosite is a common alteration product Phyllite, which is in the lower part of the Protero- of pyrite (e.g. Furbish, 1963; Bladh, 1982), this is zoic Adelaidean system in the Mount Painter apparently the first time that native sulphur has Province of South Australia. A description of the been reported in association with the two. regional geology is provided by Coats and Blissett 140 SHORT COMMUNICATIONS FIGS. l and 2. FIG. 1 (left). Photograph of broken surface, showing jarosite boxwork containing sulphur and pyrite. FIG. 2. (right). SEM micrograph of sulphur crystals on a bed of finer-grained jarosite crystals. (1971). The site of the discovery is about 2 km Thermodynamic calculations indicate that these north-west of Wywyana Park (Lat. 30 ~ 18' S, Long. three minerals do not form a stable assemblage. 139 ~ 19'), on the Arkaroola pastoral lease, in the This is illustrated by fig. 4, an Eh-pH diagram Flinders Range of South Australia. constructed from data published by Brown (1971), The pyrite-sulphur-jarosite assemblage occurs and adopting Brown's assumptions about the at and near the surface of the weathered rock in the activities of the various species involved, which form of fist-sized pseudomorphs of pyrite crystals approximate the activities found in naturally occur- which, when broken open, are seen to consist of ring acid waters (Hem, 1959). cellular jarosite boxworks (fig. 1) with small crystals of elemental sulphur in many of the cells, and pyrite remnants in a few of the larger ones. The pseudo- morphs under consideration contain virtually no goethite, although some small goethitic pseudo- morphs of pyrite have been recovered from the same weathered rock. The jarosite comprising the external surfaces of the pseudomorphs has an earthy, yellow-brown appearance, with the darker shades being due to traces of goethite. The cell walls of the internal boxwork consist of finely crystallinejarosite (fig. 2). The jarosite is yellow to light brown in colour. In some places the boxwork has a rectilinear pattern, but mostly it is quite irregular (fig. 1). The sulphur crystals are quite small, generally less than 0.1 mm in diameter, and are multi-faceted (fig. 2). They are transparent, pale yellow, and have a vitreous lustre; X-ray diffraction patterns show that the sulphur is in the orthorhornbic (~) modi- fication. The pyrite grains in the boxwork cells are FIG. 3. SEM micrograph of corroded pyrite remnant in corroded in appearance (fig. 3), and many of them jarosite. have clear, splendant surfaces; others are lightly coated by jarosite. It is obvious that the pyrite grains are unoxidized remnants of the original Fig. 4 shows that the jarosite predominance field pyrite crystals that resulted in the pyrite-sulphur- is widely separated from those of pyrite and sulphur jarosite assemblage. by an area where Fe z + remains in solution; the SHORT COMMUNICATIONS 141 separation approximates about 0.4 V. The pre- elemental sulphur crystallizes, and which react to dominance field of elemental sulphur is very small, form SO 2- only at a considerable over-potential, and indicates a stable existence for this phase only estimated by Hamilton and Woods (1981) to be at at a pH below 1, which is probably unrealistic for about 0.8 V, which is in the predominance field of normal weathering conditions. Therefore, to explain jarosite (fig. 4). In the meantime, the hydrogen ions the pyrite-sulphur-jarosite assemblage, it is neces- liberated by the oxidation of the sulphide com- sary to look beyond the theoretical thermodynamic ponent of pyrite attack the surrounding potash framework and to consider the possible mechan- feldspar and/or muscovite, putting K + ions into isms involved. solution. When the activity of K § gets sufficiently high, jarosite precipitates from the solution, which ..... also contains abundant iron and sulphur species; in the process, Fe 2§ is oxidized to Fe 3§ and the metastable sulphur species are converted to SO 2-. The jarosite apparently precipitated along frac- 0.8 Jarosite tures in the pyrite, giving rise to the cellular boxwork. Following the initial development of the boxwork, the pyrite continued to dissolve, leaving 0.6 Goethite most of the cells empty except for the small sulphur crystals. The few pyrite remnants have been mostly left with clean splendant surfaces probably because, during the dissolution process, both iron and 0.4~ sulphur species went into solution simultaneously. Most of the sulphur appears to have crystallized after thejarosite, which indicates that the oxidation potential must have returned to a lower level, possibly because of a rising water table. Bladh (1982), in a computerized simulation of the oxidation of pyrite and chalcopyrite in rocks H20 ~~ containing feldspar and muscovite, predicts that --0.2 unstable ~~ goethite should precipitate before jarosite. This has I I I I ~ ~"5.,. not happened in the pyrite pseudomorphs under 0 1 2 3 4 5 pH discussion, and can probably be explained by the FIG. 4. An Eh pH diagram constructed to show the pH prevailing at the time of oxidation. In Bladh's predominance fields of pyrite, sulphur, jarosite, and simulation, the pH is maintained at a relatively high goethite. Numbered lines correspond to numbered equa- level, presumably by silicate buffering reactions, tions in the text. Activities of dissolved species are as whereas in the case of the pyrite pseudomorphs, the follows: Fe = 10 -4, K ~ = 10 -3, and total S = 10 -2. pH probably remained at a locally low level throughout the oxidation process because of the large size of the original pyrite crystals and possibly Theoretically, as shown in fig. 4, the oxidation of by the lack of effective water circulation. The pyrite can produce either sulphate ions (equation l) smaller pyrite crystals, on the other hand, were or elemental sulphur (equation 2), reactions which converted to goethite because of the relatively are shown as numbered lines 1 and 2 on fig. 4. stronger influence of the buffering wall-rock FeS 2 + 8H20 = reactions. Fe2+ +2SO2- + 16H+ + 14e - (1) In conclusion, it is evident that a purely thermo- dynamic approach is inadequate to explain the FeS2 = Fe 2 + + 2S + 2e- (2) unusual pyrite-sulphur jarosite assemblage, and However, in practice, SO~- forms only with diffi- that kinetic factors must be taken into considera- culty, and elemental sulphur precipitates over a tion.