Sulfide evolution in high-temperature to ultrahigh-temperature metamorphic rocks from Lützow–Holm Complex, East Antarctica

The high-temperature (HT) to ultrahigh-temperature (UHT) metamorphic rocks from Lützow–Holm Complex, East Antarctica show a systematic difference between sulfide assemblages in the rock matrix and those found as inclusions in the silicates stable in high-temperatures. Matrix sulfides are commonly pyrite with or without pentlandite and chalcopyrite. On the other hand, inclusion sulfides are pyrrhotite with or without pentlandite and chalcopyrite lamellae. When recalculated into integrated single-phase sulfide compositions, inclusion sulfides from the UHT region showed a wider range of solid–solution composition than the inclusion sulfides from the HT region. The host minerals of the sulfides with extreme solid–solution compositions are those stable at the peak of metamorphism such as orthopyroxene and garnet. One of the most extreme ones is included in orthopyroxene coexisting with sillimanite ± quartz, which is the diagnostic mineral assemblage of UHT metamorphism. These observations suggest that sulfide inclusions preserve their peak metamorphic compositions. Pyrrhotite did not revert to pyrite because of the closed system behavior of sulfur in inclusion sulfides. On the other hand, in the rock matrix where the open system behavior of sulfur is permitted, original sulfides were partly to completely altered by the later fluid activity.     

Multicomponent magnetization in western Dolpo (Tethyan Himalaya, Nepal): tectonic implications

A palaeomagnetic study has been carried out in the Tethyan Himalaya (TH; the northern margin of Greater India). Twenty-six palaeomagnetic sites have been sampled in Triassic low-grade metasediments of western Dolpo. Two remanent components have been identified. A pyrrhotite component, characterized by unblocking temperatures of 270–335 °C, yields an in situ mean direction of D=191.7°, I=−30.9° (k=29.5, α₉₅=5.7°, N=23 sites). The component fails the fold test at the 99% confidence level (k_{in situ}/k_bed=6.9) and is therefore of postfolding origin. For reason of the low metamorphic grade, this pyrrhotite magnetization is believed to be of thermo-chemical origin. Geochronological data and inclination matching indicate an acquisition age around 35 Ma. The second remanence component has higher unblocking temperatures (>400 °C and up to 500–580 °C range) and resides in magnetite. A positive fold test and comparison with expected Triassic palaeomagnetic directions suggest a primary origin.The postfolding character of the pyrrhotite component, and its interpreted age of remanence acquisition, implies that the main Himalayan folding is older than 35 Ma in the western Dolpo area. This study also suggests that the second metamorphic event (Neo-Himalayan) was more significant in the Dolpo area than the first (Eo-Himalayan) one.A clockwise rotation of 10–15° is inferred from the pyrrhotite component, which is compatible with oroclinal bending and/or rotational underthrusting models. This rotation is also supported by the magnetite component, indicating that no rotation of the Tethyan Himalaya relative to India took place before 35 Ma.     

Feasibility and cost of creating an iron-phosphate coating on pyrrhotite to prevent oxidation

 Acid mine drainage (AMD) occurs when sulfide minerals are exposed to an oxidizing environment. Most of the methods for preventing AMD are either short-term or high cost solutions. Coating with iron phosphate is a new technology for the abatement of AMD. It involves treating the sulfide with a coating solution composed of H₂O₂, KH₂PO₄, and sodium acetate as a buffer agent. The H₂O₂ oxidizes the sulfide surface and produces Fe³⁺ so that iron phosphate precipitates as a coating on the sulfide surface. Experiments performed under laboratory conditions prove that an iron phosphate coating can be established on pyrrhotite surfaces with optimal concentrations of the coating solution in the range of: 0.2M/0.01M H₂O₂, 0.2M KH₂PO₄, and 0.2M sodium acetate NaAc, depending on the experimental scale. Iron phosphate coating may be a long-term solution to the problem of AMD. The method would be easy to implement; the reagent cost, however, is not low enough, although it is lower than the conventional treatment with lime. Received: 30 March 1995 · Accepted: 6 September 1995     

辽宁红透山铜锌块状硫化物矿床的变质变形和成矿组分再活化

辽宁红透山铜-锌块状硫化物产在太古宙绿岩带中，矿床形成后经历了强烈的变形和变质，变质程度达高级角闪岩相。野外和显微镜研究表明，矿石在进变质过程中发生过强烈的机械再活化和重结晶，但各种进变质结构大部分已被变质峰期的全面重结晶所清除，目前保存着的结构主要是变质峰期和退变质过程的产物。退变质过程以黄铁矿变斑晶生长、矿石糜棱岩的形成、二次退火和化学再活化为特征。矿床中高度富集铜和金的矿石是韧性剪切形成的矿石糜棱岩受退变质流体叠加而成。磁黄铁矿主要是同生沉积后重结晶的产物，另有一部分由退变质热液形成，而黄铁矿变斑晶则有沉积一重结晶、磁黄铁矿退变质脱硫和热液叠加多种成因。世界各地块状硫化物矿床中的磁黄铁矿和黄铁矿各有三种成因类型。磁黄铁矿的类型有：同生沉积．变质重结晶、同生沉积黄铁矿变质和退变质热液充填或交代；黄铁矿的类型有：同生沉积-变质重结晶、磁黄铁矿退变质脱硫和退变质热液充填或交代。红透山矿区的退变质流体具有从早到晚氧逸度升高的趋势。     

Pristine and reacted surfaces of pyrrhotite and arsenopyrite as studied by X-ray absorption near-edge structure spectroscopy

Fe L-, S L-, and O K-edge X-ray absorption spectra of natural monoclinic and hexagonal pyrrhotites, Fe_1-xS, and arsenopyrite, FeAsS, have been measured and compared with the spectra of minerals oxidized in air and treated in aqueous acidic solutions, as well as with the previous XPS studies. The Fe L-edge X-ray absorption near-edge structure (XANES) of vacuum-cleaved pyrrhotites showed the presence of, aside from high-spin Fe²⁺, small quantity of Fe³⁺, which was higher for a monoclinic mineral. The spectra of the essentially metal-depleted surfaces produced by the non-oxidative and oxidative acidic leaching of pyrrhotites exhibit substantially enhanced contributions of Fe³⁺ and a form of high-spin Fe²⁺ with the energy of the 3d orbitals increased by 0.3–0.8 eV; low-spin Fe²⁺ was not confidently distinguished, owing probably to its rapid oxidation. The changes in the S L-edge spectra reflect the emergence of Fe³⁺ and reduced density of S s–Fe 4s antibonding states. The Fe L-edge XANES of arsenopyrite shows almost unsplit e_g band of singlet Fe²⁺ along with minor contributions attributable to high-spin Fe²⁺ and Fe³⁺. Iron retains the low-spin state in the sulphur-excessive layer formed by the oxidative leaching in 0.4 M ferric chloride and ferric sulphate acidic solutions. The S L-edge XANES of arsenopyrite leached in the ferric chloride, but not ferric sulphate, solution has considerably decreased pre-edge maxima, indicating the lesser admixture of S s states to Fe 3d orbitals in the reacted surface layer. The ferric nitrate treatment produces Fe³⁺ species and sulphur in oxidation state between +2 and +4.     

XMCD at Fe L<Subscript>2,3</Subscript> edges,Fe and S K edges on Fe<Subscript>7</Subscript>S<Subscript>8</Subscript>

Pyrrhotite (Fe₇S₈) is a natural iron sulphide that can participate in rock magnetisation. Its electronic structure is not yet surely described. X-ray magnetic circular dichroism (XMCD) at Fe L_2,3 edges on Fe₇S₈, coupled with multiplet calculations, shows that iron is present only as Fe²⁺ in this magnetic iron sulphide. It reveals a strong magnetic orbital moment. XMCD at Fe and S K edges shows the quite strong polarization of both Fe and S in Fe₇S₈.     

安徽铜陵冬瓜山铜矿床的叠加改造成矿机制：来自矿石结构的证据

长江中、下游地区块状硫化物矿床普遍受到燕山期岩浆及其热液的改造与叠加.本文以铜陵冬瓜山矿床为例,探讨这类矿床的成矿机制.该矿床主要由层状硫化物矿体组成,伴有矽卡岩型和斑岩型矿体.野外地质观察及室内矿相学的研究表明,冬瓜山层状矿体中矿石遭受了强烈的热变质作用及热液交代作用.进变质过程中形成的结构主要为黄铁矿受燕山期岩浆侵...     

Relations entre déformation et métamorphisme alpin dans les schistes noirs helvétiques : l’apport de la fabrique magnétique

Résumé

La fabrique magnétique des schistes noirs du Jurassique helvétique au pourtour des massifs de l’Aar et du Gothard (Alpes suisses) reflète, en fonction du métamorphisme croissant, l’orientation préférentielle de minéraux magnétiques d’origines diverses : la magnétite primaire, les phyllosilicates et la pyrrhotite métamorphiques. La foliation magnétique est parallèle à la première schistosité pénétrative même si celle-ci est effacée par une seconde schistosité. La linéation magnétique indique retirement lié à la direction de transport des nappes. Dans la partie la plus métamorphique où apparaît la pyrrhotite au cours de la déformation, la fabrique magnétique permet de situer l’apparition de ce minéral dans la tectonique polyphasée. Les propriétés magnétiques anisotropes de la pyrrhotite s’avèrent plus complexes que le modèle usuel de marqueurs planaires et l’interprétation des linéations observées doit en tenir compte.     

中甸普朗还原性斑岩型铜矿床：矿物组合与流体组成约束

成矿流体高氧逸度是斑岩铜矿床模式的一个基本原则。虽然亚洲单个矿体储量最大的普朗铜矿床的成矿母岩——普朗复式岩体具氧化性岩浆特点,但其矿物组合及流体成分却与还原性斑岩型铜金矿床一致:矿石中以发育大量磁黄铁矿为特征,构成黄铜矿-磁黄铁矿-黄铁矿为主的矿物组合,不发育表征高氧逸度的原生磁铁矿和硫酸盐(硬石膏等)矿物;成矿流体中含较多CO₂、CO和CH₄等还原性组分,氧逸度低于铁橄榄石-磁铁矿-石英缓冲剂。成矿流体中还原性组分可能来源于普朗复式岩体周围的含碳质千枚岩或深部铁镁质岩浆。还原性流体中铜元素的溶解度比氧化性流体中的低,但金元素的溶解度不受氧化还原条件的影响;而CH₄可使SO₂还原形成S^2-,为辉钼矿的形成提供物质基础;可能是导致普朗铜矿床Cu品位偏低而伴生大量Au、Mo矿化的主要原因之一。普朗铜矿床还原性特征的厘定有益于深入研究其矿床成因、乃至区域斑岩型铜矿床成矿机制。