Examples of application of equilibria in the systems Fe-Zn-S, Cu-Fe-Zn-S, Fe-As-S, and Fe-Zn-As-S are drawn from modern sulphide-forming hydrothermal vents on the East Pacific Rise, from ancient vein deposits and from metamorphosed sulphide ores. The ZnS content (1.2 and 1.3 mole %) of Cu-Fe-S intermediate solid solution (iss) from the sea-floor deposits gives temperatures of equilibration of 331° and 335 °C from experimental curves, in excellent agreement with the 350 °C expected from actual measurement of the hot springs. Heterogeneities in sphalerite in these deposits do not necessarily represent disequilibrium during deposition but can be explained simply by small fluctuations in aS2. Diagrams of log aS2vs. 1000/T, K for sphalerite and arsenopyrite are useful for estimating temperature and activity of sulphur in hydrothermal or metamorphosed deposits provided that equilibrium can be demonstrated and the systems are properly buffered.

The sphalerite geobarometer has had wide applications, some successful (mostly vein deposits) and some not (particularly metamorphosed ores in which chalcopyrite is in contact with sphalerite). Compositions of sphalerites which are totally enclosed within metablastic pyrites represent preserved high P-T equilibria which have been isolated from further reaction during subsequent retrograde conditions by the inert encapsulating pyrite and may provide more reliable estimates of pressure.

Published data from pyrrhotine-vapour equilibrium experiments are used to make inferences concerning the high-temperature thermodynamic mixing properties of pyrrhotine. Darken's quadratic formalism provides the justification for plotting activity of sulphur data against (1−X)2, where X is used in Fe1−XS to express the composition of non-stoichiometric pyrrhotine. Such plots show that the thermodynamic behaviour of pyrrhotine appears to be different on either side of X = 0.125, the Fe7S8 composition.

The limits and effect on cell size of silver substitution into synthetic tetrahedrite, Cu10(Zn,Fe,Cd2) Sb4S13, are investigated for comparison with natural tetrahedrite. The limit of Ag substitution into natural zincian tetrahedrite is ∼ 4 atoms per half unit cell and into iron tetrahedrite ∼ 6.5 atoms (with rare exceptions). The cell size of natural tetrahedrite increases with increasing Ag content up to 4 atoms but decreases with further Ag substitution. The highest Ag substitution achieved in synthetic tetrahedrites was 4.7 atoms in Zn2-tetrahedrite, 7.02 atoms in Cd2-tetrahedrite and 6.80 atoms in Fe2-tetrahedrite. The cell size of synthetic tetrahedrites shows a continuous increase with increasing Ag content, the largest cell size of a = 10.927 Å being in the Cd2-tetrahedrite with 7.02 atoms Ag. The iron content of tetrahedrite systematically increased from 1.1 to 2.0 atoms per half unit cell tetrahedrite with increase from 0 to ∼ 4 atoms Ag. The different limit of Ag substitution between Zn2 and Cd2 tetrahedrite can be explained by size constraints on the expanding structure. An explanation is given for a limit of 7 atoms Ag substitution in tetrahedrite using a combined electron band/molecular orbital approach.

The kinetics and mechanisms of pentlandite [(Fe,Ni)9S8] exsolution from the monosulphide solid solution [or MSS (Fe,Ni)1−xS] have been studied by synthesis and annealing experiments in part of the Fe-Ni-S system particularly relevant to sulphide nickel orebodies. This experimental work has been combined with the examination of the compositions and textures of pyrrhotine-pentlandite intergrowths from a variety of ore deposits. Isothermal annealing of MSS compositions with varying M:S and Fe:Ni ratios at 400 °C shows the formation of a sequence of textures which depend for their full development chiefly on initial M:S ratio and annealing time. The full sequence of textures starts with heterogeneous nucleation of pentlandite at MSS grain boundaries, the growth of these blebs to form ‘rims’, nucleation of finer bladed particles arranged ‘en échelon’ along fractures, and, finally, homogeneous nucleation of fine orientated pentlandite lamellae. Using the information from the annealing experiments, the inter-growths produced by cooling in the natural ores are interpreted using a simplified Time-Temperature-Transformation Model.

Microprobe analyses of members of the erlichmanite-laurite series from Guma Water and Senduma, Sierra Leone and Tanah Laut, Borneo, indicate that complete solid solution is possible between OsS2 and RuS2 with considerable substitution of Os and Ru by Ir, Rh, and Pt. The cell size of the erlichmanite from Guma Water is a = 5.6183±0.0003 Å at a composition (Os0.61Ru0.30Ir0.06Rh0.03)Σ0.93S2 whilst the laurite from Senduma has a composition of (Ru0.88Os0.05Ir0.04 Rh0.03)Σ0.93S2 and a cell size of a = 5.6089±0.0005 Å. Substitution of Os for Ru provides the predominant cause of the variation of cell size. Substitution by other elements of the platinum group appears to produce little effect on cell size and is presumably controlled by genesis rather than considerations of crystal chemistry or structure. The recorded analyses for these elements indicate a pre-dominance of Ir over Rh for members of the series containing more than about 15% of the laurite molecule. For the remainder of the series Rh is more important than Ir. The reflectance in air and oil of the members of the series from Sierra Leone and Borneo are presented and the microhardness of the erlichmanite from Guma Water shown to be 1854 kg/mm2. This is the first report of laurite from Senduma, Sierra Leone.

Evidence obtained from morphological and extensive trace element studies, and from the examination of mineral and fluid inclusions in Witwatersrand pyrites, shows three major types of pyrite: (i) detrital pyrite (rounded pyrite crystals transported into the depositional environment); (ii) synsedimentary pyrite (round and rounded aggregates of fine-grained pyrite formed within the depositional environmen); and (iii) authigenic pyrite (newly crystallized and/or recrystallized pyrite formed after deposition). The detrital grains contain mineral inclusions such as biotite, feldspar, apatite, zircon, sphene, and various ore minerals, and fluid inclusions with daughter minerals. Most of the inclusions are incompatible with an origin by sulphidization. Recrystallized authigenic pyrite occurs in large quantities but only in horizons or localities which have been subjected to higher temperatures during the intrusion or extrusion of younger volcanic rocks. Important additional findings are the often substantial amounts of pyrite and small amounts of particles of gold found in Archaean granites (Hallbauer, 1982) as possible source rocks for the Witwatersrand detritus. Large differences in Ag and Hg content between homogeneous single gold grains within a hand specimen indicate a lack of metamorphic homogenization. The influence of metamorphism on the Witwatersrand pyrites can therefore be described as only slight and generally negligible.

Sulphur isotope ratios in sulphides and baryte from stratabound and stratiform orebodies in a metavolcanic-sedimentary sequence in Namaqualand were found, in part, to be extreme for Precambrian sulphur. Black Mountain, Aggeneys, in the west gave an average for the sulphides of δ34S = +8.9±3.7‰ (9 samples), an average for barytes of +20.6±4.3‰ (3 samples). Broken Hill, Aggeneys, in the centre gave an average for the sulphides of δ34S = +19.8±3.1‰ (19 samples). Gamsberg, in the east, gave an average for the sulphides of δ34S = +29.2±1.8‰ (24 samples), and an average for barytes of +35.4±0.2‰ (2 samples). The δ34S values increase eastward. Their range is strongly on the positive side and does not centre around zero. The Gamsberg barytes and most Gamsberg sulphides have more positive δ34S values than those reported for other Precambrian sulphides and sulphates. We interpret the above sulphur isotope range as being mainly due to the varying contributions of submarine-exhalative sulphide sulphur with δ34S close to zero and bacterially(?) reduced sulphate with strongly positive δ34S, apparently from evaporites in the east. Metamorphism of amphibolite facies grade has partly isotopically re-equilibrated the ore minerals, as indicated by galena-pyrrhotine and sulphide-baryte isotope temperatures from single specimens, but has not destroyed the primary sulphur isotope range indicating pre-existing sulphate concentrations.

Sphalerite is a common constituent of the mineralized rocks and host metasediments of the Foss baryte-base metal deposit, located near Aberfeldy in the central Scottish Highlands. Microprobe analyses of sphalerite show a wide range in minor element content (0–17 mol. % FeS, 0–3 mol. % MnS), and sphalerites of contrasting composition are often found in the same rock. This suggests that equilibrium domains in some rocks were minute (< 1 mm), during regional metamorphism. Pressures derived from the selective application of the sphalerite geobarometer are consistent with other mineralogical evidence of peak metamorphic pressures in the range 7–10 kbar, at 540–580 °C. However, there is considerable evidence of partial retrograde re-equilibration of sphalerite, by continued buffering with pyrite+ pyrrhotine and by outward diffusion of iron. Marginal depletion of Fe and Mn in sphalerite within coarse carbonate rocks is attributed to partitioning reactions following recrystallization. Sphalerite which has retained its pre-metamorphic composition shows systematic variations in composition through profiles of mineralized beds, which may be related to depositional environments. Bimodal primary compositions can be explained by precipitation of zinc sulphides under the contrasting chemical environments of hydrothermal vents and cooler, exhaled brine layers on the sea floor.

The ultramafic-hosted disseminated nickel sulphide deposit at Mount Keith, Western Australia, originally consisted of sulphide droplets and chrome spinels interstitial to olivine. During cooling, the sulphide droplets crystallized to pentlandite-pyrrhotine. Hydration and carbonation caused rimming and replacement of pentlandite-pyrrhotine by tochilinite, magnetite, and magnesite. These textures are interpreted in terms of diffusion between serpentinite matrix and the sulphide blebs.

Sulphide and sulphosalt mineralization in the Hällefors silver mining district occurs as stratabound bodies in skarn-like black carbonates and other carbonates in the eastern part of the district, and as epigenetic, tectonically controlled vein mineralization in tuffaceous metavolcanics and, to a lesser extent in slates, in the western part.

Previous work has suggested that the Ag-bearing Hällefors deposits were formed by the action of metamorphic solutions on sediments of volcano-sedimentary origin. At the stages of sulphosalt and sulphide formation, temperature and pressure conditions were probably in the ranges 500–440 K and 4.5–3.5 kbar. Examination of a log aS2-1/T diagram and of Ag-Sb phase relations combined with study of the ores suggest the paragenesis of the Ag-bearing minerals is as follows: allargentum and dyscrasite as exsolution bodies in silver bearing galena; Ag-containing tetrahedrite → freibergite → miargyrite → pyrargyrite → stephanite (from hydrothermal solutions).

Massive stratiform and stratabound sulphide bodies in the central Appalachians exhibit a wide variety of metamorphic features which are typical of similar deposits world-wide. The ores occur as lens- to plate-like bodies which are concordant with the enclosing meta-sediments and volcanics and are interpreted as products of Late Precambrian to Early Palaeozoic submarine volcanism. Sulphide mineralization ranges from pyritic to pyrrhotinic bodies with variable, but Zn- and Cu-dominant, base-metal contents. The deposits have been subjected to metamorphism ranging from greenschist to amphibolite grade. The metamorphism has been pervasive and has resulted in thorough recrystallization of most ores and intense deformation of pyrrhotinic ores, but does not appear to have significantly altered original sulphide assemblages. Recrystallization has homogenized most sulphide minerals including pyrite and sphalerite, so that any original compositional zoning is no longer seen. The presence of chalcopyrite has apparently promoted an increase in grain size and has facilitated post-metamorphic retrograde re-equilibration. Consequently, sphalerite geobarometry is not reliable in Cu-bearing assemblages. Pyrrhotines, mostly hexagonal, have re-equilibrated to low temperatures but commonly display pressure twins and kink-banding, apparently due to the late stages of deformation. Characteristic mineralogical changes in the host rocks include changes in the abundance of minerals, changes in the Fe: Mg ratios, and the presence of gahnite.

A detailed study of pyrite in a number of metamorphosed, stratiform, sediment-hosted Pb-Zn deposits has shown the importance of cataclastic deformation, pressure-solution, and grain growth in the deformation and textural development of pyrite. Primary depositional or early diagenetic microstructures are preserved in pyritic ores deformed or metamorphosed at grades up to mid-upper greenschist facies, whereas at higher temperatures only metablastic or annealed pyrite textures are found. Brittle deformation is found at all metamorphic grades and is favoured by coarse grain-sizes. Pressure-solution is a major deformation mechanism in fine-grained pyritic ores in low-grade metamorphic environments. Grain growth and annealing dominate at higher metamorphic temperatures and are likely to have obliterated any evidence of deformation by dislocation processes. Significant macroscopic ductility of fine-grained pyritic ores in low-grade environments may be accounted for by a combination of pressure-solution, grain boundary sliding, and cataclastic flow.

Sark's Hope silver-lead lode, which was mined during the 1830s and 1840s cuts a Late Precambrian granite at the southernmost point of the island of Sark. The primary ore assemblage is pyrite, galena, chalcopyrite, tennantite, tetrahedrite, sphalerite, marcasite, arsenopyrite, pyrrhotine, bravoite, enargite, and the silver minerals pyrargyrite, pearceite, polybasite, and acanthite. Gangue minerals are hematitic quartz, calcite, and illite. Alteration products include chalcosine, covelline, blaubleibender covelline, limonite, malachite, azurite, cerussite, and anglesite. The generalized paragenesis is of early Fe, Co, Ni, As, and S species and later minerals of Pb, Cu, Ag, Zn, Fe, As, Sb, and S. The earliest alteration products are copper sulphides; these are followed by lead and copper carbonates and sulphates, and hydrated iron and manganese oxides. Growth zoning is a common feature of many of the ore minerals, and electron microprobe analysis shows that this is sometimes related to compositional differences. Tetrahedrite and tennantite, particularly, exhibit a wide range of compositions.

The distribution of REE in a zoned ultramafic pod formed by incomplete re-equilibration of ultrabasic and quartzofeldspathic reactant compositions has been studied. Transport of the heavy REE (HREE) as well as the light REE (LREE) over several metres has occurred during the diffusion-controlled metasomatism of the protolith mineral assemblages. The largest resultant concentration range (Eu) exceeds two orders of magnitude. In general, REE abundances increase towards the marginal zones, and differences between the behaviour of LREE, middle REE (MREE) and HREE subgroups are observed. LREE are least mobile in the aqueous transporting medium. Complexing by carbonate ligands is probably not an important factor in this system, and the final REE distribution is thought to be governed largely by the crystal structure of the major zonal minerals.

Högbomite occurs in a 20 × 80 cm gedrite-rich lens between meta-volcanic amphibolite and anorthosite at the top of the Fiskenaesset Complex in the Angnertussoq area 13 km SE of Fiskenaesset. Between coarse prismatic gedrites there are dusters with chlorite, spinel, sapphirine, rutile, högbomite and corundum. Högbomite forms 200 µm grains often associated with rutile, surrounded by spinel and sapphirine but never in contact with gedrite. Usually sapphirine forms rims on spinel and contains relict gedrite. Corundum is intergrown with spinel and sapphirine. Reactions suggested by the textures are: 1. Gedrite + spinel + rutile → sapphirine + högbomite + chlorite. 2. Sapphirine → chlorite + corundum + spinel. 3. Högbomite → rutile + corundum + spinel. In some areas without rutile or högbomite the reaction gedrite + spinel + ? → sapphirine + chlorite took place.

Mg/(Mg + Fe + Mn) ratios are in the following order: sapphirine (0.95–0.92) > chlorite (0.94) > hornblende (∼ 0.91) > anthophyllite (∼ 0.86) > gedrite (∼ 0.86) > spinel (0.77–0.74) > högbomite (0.75–0.71). Högbomite usually has an anhydrous total of c. 98 %; on the basis of 31 oxygens and with total Fe as FeO one representative analysis has Mg 4.59, Fe 1.73, Zn 0.02, Ni 0.05, Al 14.78, Cr 0.02, Ti 1.2 atoms per formula unit.

Ganophyllite from Franklin, New Jersey, Pajsberg, Sweden, and the Benallt Mine, Wales, has been chemically reinvestigated. Twelve new analyses confirm the general structure of Kato (1980) and yield the tentative formula (K,Na,Ca)2Mn8(Si,Al)12(O,OH)32(OH)4·8H2O. There is little solid solution among octahedral cations, and the Si:Al ratio is nearly constant at 10:2. Ca and (Na + K) are apparently differentiated, but all examined ganophyllites are K-rich. Much of the water content is loosely bound and the upper limit of water content is not well defined. Ganophyllite is relatively invariant in chemical composition from locality to locality.