西藏甲玛铜多金属矿床磁黄铁矿标型矿物学特征及其地质意义

甲玛矿床位于冈底斯成矿带东段,是西藏地区最大的铜多金属矿床之一。磁黄铁矿是甲玛矿床最常见的金属矿物之一,其标型特征不仅反映其自身形成环境,对其形成机制和矿床成因也具有指示意义。文章选取产于不同岩性中的磁黄铁矿矿石样品,利用矿相学、X射线衍射和电子探针分析等手段对磁黄铁矿的形态、成分和结构进行了分析研究。研究表明,甲玛矿床的磁黄铁矿主要分布在距离岩体中心较远的矿区远端矽卡岩和角岩中。磁黄铁矿的晶胞参数和粉晶X射线衍射曲线显示矽卡岩中的磁黄铁矿主要为高温六方磁黄铁矿,角岩中的磁黄铁矿为高温六方磁黄铁矿和低温单斜磁黄铁矿的交生体,但主要以低温单斜磁黄铁矿为主。通过对矽卡岩和角岩中的磁黄铁矿进行电子探针测试,结果显示:矽卡岩中的磁黄铁矿中w(Fe)为60.09%～60.71%,平均为60.38%,w(S)为38.18%～38.69%,平均38.35%,化学分子式为Fe_8S_9～Fe_(10)S_(11);角岩中的磁黄铁矿中w(Fe)为59.05%～59.57%,平均为59.10%,w(S)为39.28%～39.95%,平均39.59%,化学分子式为Fe_5S_6～Fe_7S_8。根据以上矿物学特征,笔者进一步探讨了该矿床磁黄铁矿的沉淀机制:炽热的岩浆热液上涌,与碳酸盐岩地层和碎屑岩地层接触发生相互作用,并有大气水的加入,使得成矿流体在角岩中先快速降温,形成高温六方磁黄铁矿和低温单斜磁黄铁矿的交生体。同时,大量的含矿热液形成,并充填于有利的成矿空间(主要为层间破碎带)沉淀成矿,形成矽卡岩矿体,然后流体在矽卡岩矿段中经历缓慢降温,形成高温六方磁黄铁矿。结合矿床地质特征和相关元素地球化学特征,认为甲玛矿床类型为斑岩-矽卡岩型。     

A continuous phase gradient in a “monocrystalline” pyrrhotite

X-ray diffraction and microprobe analyses of pseudomonocrystalline fragments of pyrrhotite from Bodenmais, Bavaria, revealed continuous gradients in composition and phase distribution. The gradients extend from the well-developed (0001) cleavage surfaces 15–30 μm into the bulk of the crystals. The phase gradient is made up two low-temperature pyrrhotites with monoclinic (4C) and hexagonal (5C) symmetry. The fraction of monoclinic pyrrhotite, expressed on the basis of recorded X-ray intensities, I, decreases exponentially according to I _(mon)/[I _(hex)+I _(mon)] = EXP (aX+b) where a is a constant ranging from ?0.04 to ?0.25, X is the depth from the (0001) cleavage surface in μm, and b is a constant determined by the intensity ratios obtained from the untreated cleavage surfaces. The phase gradient developed during retrograde reactions from a continuous composition gradient. This primary gradient was caused by the extraction of iron from a disordered, high-temperature hexagonal pyrrhotite during oxidation of the cleavage surfaces at temperatures above 254° C (upper stability limit of 4C pyrrhotite), probably above 308° C. The length of the c axis of the monoclinic superstructure slightly increases with the increase in iron and decrease in vacancy content of the bulk. This expansion is probably due to a minor compositional variation of the monoclinic phase controlled by the availability of vacancies during the transition to low-temperature phases.     

Evidence for aqueous activity on comet 81P/Wild 2 from sulfide mineral assemblages in Stardust samples and CI chondrites

The discovery of nickel-, copper-, and zinc-bearing iron sulfides from comet 81P/Wild 2 (Wild 2) represents the strongest evidence, in the Stardust collection, of grains that formed in an aqueous environment. We investigated three microtomed TEM sections which contain crystalline sulfide assemblages from Wild 2 and twelve thin sections of the hydrothermally altered CI chondrite Orgueil. Detailed structural and compositional characterizations of the sulfide grains from both collections reveal striking similarities. The Stardust samples include a cubanite (CuFe₂S₃) grain, a pyrrhotite [(Fe,Ni)_1−xS]/pentlandite [(Fe,Ni)₉S₈] assemblage, and a pyrrhotite/sphalerite [(Fe,Zn)S] assemblage. Similarly, the CI-chondrite sulfides include individual cubanite and pyrrhotite grains, cubanite/pyrrhotite assemblages, pyrrhotite/pentlandite assemblages, as well as possible sphalerite inclusions within pyrrhotite grains. The cubanite is the low temperature orthorhombic form, which constrains temperature to a maximum of 210 °C. The Stardust and Orgueil pyrrhotites are the 4C monoclinic polytype, which is not stable above ∼250 °C. The combinations of cubanite and pyrrhotite, as well as pyrrhotite and pentlandite signify even lower temperatures. The crystal structures, compositions, and petrographic relationships of these sulfides constrain formation and alteration conditions. Taken together, these constraints attest to low-temperature hydrothermal processing.Our analyses of these minerals provide constraints on large scale issues such as: heat sources in the comet-forming region; aqueous activity on cometary bodies; and the extent and mechanisms of radial mixing of material in the early nebula. The sulfides in the Wild 2 collection are most likely the products of low-temperature aqueous alteration. They provide evidence of radial mixing of material (e.g. cubanite, troilite) from the inner solar system to the comet-forming region and possible secondary aqueous processing on the cometary body.     

Formation of gold–silver sulfides and native gold in Fe–Ag–Au–S system

We carried out experiments on crystallization of Fe-containing melts FeS₂Ag_0.1–0.1xAu_0.1x (x = 0.05, 0.2, 0.4, and 0.8) with Ag/Au weight ratios from 10 to 0.1. Mixtures prepared from elements in corresponding proportions were heated in evacuated quartz ampoules to 1050 ºC and kept at this temperature for 12 h; then they were cooled to 150 ºC, annealed for 30 days, and cooled to room temperature. The solid-phase products were studied by optical and electron microscopy and X-ray spectroscopy. The crystallization products were mainly from iron sulfides: monoclinic pyrrhotite (Fe_0.47S_0.53 or Fe₇S₈) and pyrite (Fe_0.99S_2.01). Gold–silver sulfides (low-temperature modifications) are present in all synthesized samples. Depending on Ag/Au, the following sulfides are produced: acanthite (Ag/Au = 10), solid solutions Ag_2–xAu_xS (Ag/Au = 10, 2), uytenbogaardtite (Ag/Au = 2, 0.75), and petrovskaite (Ag/Au = 0.75, 0.12). They contain iron impurities (up to 3.3 wt.%). Xenomorphic micro- (<1–5 μm) and macrograins (5–50 μm) of Au–Ag sulfides are localized in pyrite or between the grains of pyrite and pyrrhotite. High-fineness gold was detected in the samples with initial ratio Ag/Au ≤ 2. It is present as fine and large rounded microinclusions or as intergrowths with Au–Ag sulfides in pyrite or, more seldom, at the boundary of pyrite and pyrrhotite grains. This gold contains up to 5.7 wt.% Fe. Based on the sample textures and phase relations, a sequence of their crystallization was determined. At ~1050 ºC, there are probably iron sulfide melt L₁ (Fe,S ? Ag,Au), gold–silver sulfide melt L₂ (Au,Ag,S ? Fe), and liquid sulfur L_S. On cooling, melt L₁ produces pyrrhotite; further cooling leads to the crystallization of high-fineness gold (macrograins from L₁ and micrograins from L₂) and Au–Ag sulfides (micrograins from L₁ and macrograins from L₂). Pyrite crystallizes after gold–silver sulfides by the peritectic reaction FeS + L_S = FeS₂ at ~743 ºC. Elemental sulfur is the last to crystallize. Gold–silver sulfides are stable and dominate over native gold and silver, especially in pyrite-containing ores with high Ag/Au ratios.     

Supergene alteration of the Mt Windarra nickel sulphide ore deposit,Western Australia

Three main zones of progressive oxidation, termed the transition, violaritepyrite and oxide zones, can be delineated in the supergene profile of the Mt Windarra massive/matrix ore deposit. In the broad transition zone from pure primary ore, pentlandite is progressively oxidised to an iron rich violarite of composition Co_0.02Fe_1.38Ni_1.60 S₄, releasing Fe²⁺ and Ni²⁺ ions into solution. Up to 43% of this Ni²⁺ moves to nearby pyrrhotite margins which are replaced firstly by nickeliferous smythite and then by a second lamellar-textured violarite with an even higher iron content but lacking in cobalt (approximately Fe_1.6Ni_1.4S₄). On completion of violaritisation of the pentlandite, violaritisation of the pyrrhotite also ceases and the remainder of the pyrrhotite is rapidly replaced by secondary pyrite/marcasite, siderite and void space, this reaction defining the top of transition zone. Both sulphur and nickel are extracted from solution and further Fe²⁺ ions are released into solution. The violarite-pyrite zone is characterised by the absence of pentlandite and pyrrhotite and continued stability of violarite and secondary iron disulphides. Most, if not all, of the iron generated by these oxidation reactions precipitates as magnesian siderite at the expense of magnesite, giving rise to solutions containing mainly Mg²⁺ and Ni²⁺ ions. At and just above the water table atmospheric oxygen is reduced while the sulphides are oxidised to sulphate and hydroxides. Much of the iron remains in situ as characteristic goethite relicts while nickel and copper are leached, producing the enrichment below the water table. The overall genetic model proposed is electrochemical and is analogous to the corrosion of a piece of metallic iron partially immersed in differentially aerated water.     

Transmission electron microscopy on iron monosulfide varieties from the Suizhou meteorite

Transmission electron microscopy on the iron monosulfide (FeS) varieties from the Suizhou meteorite (Hubei, China) reveals the intergrowth of primary hexagonal 2C troilite and minor monoclinic 4C pyrrhotite (SG: F2/d) phases as nanometer-scale domain microstructure. In addition, anti-phase domain boundaries are found to present in the 2C troilite superstructure with the displacement vector 1/4[001]_2C, which is expected to form during the translational symmetry breaking during cooling from higher symmetry, high-temperature modification of the NiAs-type (SG: P6₃/mmc) structure. Furthermore, 60° rotation twinning about the pseudo-hexagonal c-axis is observed in the 4C pyrrhotite superstructure, which may result from rotation symmetry reduction induced by the ordered arrangements of metal vacancies through solid-state transformation during further cooling. All the above microstructural characteristics are discussed with consideration to the thermal metamorphism history experienced by the Suizhou meteorite.     

The thermodynamic properties of pyrrhotite and pyrite: A re-evaluation

On a plot of log sulfur activity versus inverse absolute temperature, the variation in published pyrite/pyrrhotite curves below 500°C is larger than expected from the precision of the measurements. The precise data by Rau (1976) fall between interpretations by Scott and Barnes (1971) and by Toulmin and Barton (1964) and are recommended.Scott and Barnes calibrated sulfur fugacities in the system Fe-Zn-S, against the data of Toulmin and Barton, but this involved a double extrapolation of empirical relationships, to and from a region where fugacities in pyrrhotite are unmeasured. Regular-solution models offer no improvement. An apparent interruption in the properties of the high-temperature pyrrhotite solid solution, at the composition Fe₇S₈ (Powell, 1983) is probably due to the inclusion of metastable microdomains of monoclinic pyrrhotite in some of Rau's experimental runs, rather than to an equilibrium change of structure. Hence, the uncertainties of extrapolation are unlikely to account for the displacement of the pyrite/pyrrhotite curve of Scott and Barnes. There may be a systematic error in the composition of pyrrhotite inferred by Scott and Barnes from X-ray lattice spacings, due to the effects of preparation-dependent ordering.Other influences on pyrrhotite thermodynamics are discussed. There is a maximum in the pyrrhotite fundamental unit-cell parameter, “a,” as composition is changed. This maximum shifts towards the Fe-rich boundary of pyrrhotite as temperature is increased, so it suggests a contribution from intrinsic defects, even at low temperatures. The thermodynamic effects of pressure need recalculating to suit these unit-cell data.     

氮气气氛下黄铁矿热分解的矿物相变研究

通过差热-热重分析、X射线粉末衍射（XRD）及磁化率分析等手段,对天然黄铁矿样品在氮气中受热发生的矿物相 变过程进行了综合研究。不同温度下黄铁矿煅烧产物的XRD物相分析结果显示,低于500℃时,黄铁矿无显著变化;随着 温度的升高（500~600℃）,黄铁矿开始转变为单斜磁黄铁矿,进而生成六方磁黄铁矿,磁化率显著升高;700℃~800℃的 煅烧产物主要为六方磁黄铁矿,磁化率明显下降,直至900℃进一步形成更稳定的陨硫铁（FeS）,磁化率接近于零。在黄 铁矿物相开始转变的温度（500~600℃）区间,黄铁矿生成单斜磁黄铁矿的速率大于单斜磁黄铁矿转化为六方磁黄铁矿的速 率;高温（700~900℃）时,黄铁矿转化为单斜磁黄铁矿的速率低于单斜磁黄铁矿转化为六方磁黄铁矿的速率,表现为黄铁 矿直接生成六方磁黄铁矿。     

镍铜硫化物矿石中磁黄铁矿固溶体的退火及其选矿意义

磁黄铁矿固治体从硫化物熔体结晶后，在缓慢冷却过程中经历了显著的退火。出治和出治体的租化是固治体退火的两种方式。叶片状的单斜磁黄铁矿和“火焰状”的镍黄铁矿原始出治相在降温过程中均可发生退火和租化。分布于磁黄铁矿等矿物粒间或包于磁黄铁矿粒内的粒状镍黄铁矿，不只是高温出治的直接产物，有一部分可能是由火焰状出治体租化而成的。磁黄铁矿中单斜变体的出治和租化可使矿石的磁性发生改变，镍黄铁矿出治体的租化使含镍矿物的粒度加大。因而，退火作用对矿石的选矿工艺性能有着显著影响。     

Towards pyrrhotite/magnetite geothermometry in low-grade metamorphic carbonates of the Tethyan Himalayas (Shiar Khola,Central Nepal)

In Mesozoic metacarbonates of the Tethyan Himalayas (Shiar Khola area, Central Nepal) two characteristic remanent magnetisations (ChRM₁ and ChRM₂) were identified by their unblocking temperature spectra. The ChRM₁ is carried by pyrrhotite (unblocking temperature: 270–360°C) and the ChRM₂ by magnetite (unblocking temperature spectra: 430–580°C). The temperature-related formation of pyrrhotite at the expense of primary magnetite during low-grade metamorphism in marly carbonates allows the determination of thermal gradients by the pyrrhotite/magnetite ratio. This new method can be used as a geothermometer for T≤300°C in low-grade metamorphic carbonates, where other methods are not available. This method is applied for the first time in the Tethyan Himalayas of Central Nepal.In the Shiar Khola valley, systematic variations in the ferrimagnetic content of the metacarbonates along an E–W profile were detected by the ratio of remanence intensity of pyrrhotite to magnetite, derived from natural remanent magnetisation (R_PYR/MAG) and saturation magnetisation (S_PYR/MAG). Over a stretch of 10 km the R_PYR/MAG and S_PYR/MAG increase from W to E from ~0.42 to ~0.91 and ~0.48 to ~1.0, respectively. Based on temperature estimates, the eastern part experienced upper anchizone–epizone (~250–300°C) conditions, while the western part underwent only diagenesis (~200°C). The temperature gradient and the temperature ranges suggested are consistent with the findings of the calcite twin lamellae geothermometry which is a non-magnetic method.     

Carbon–sulfur–iron relationships in sedimentary rocks from southwestern Taiwan: influence of geochemical environment on greigite and pyrrhotite formation

The importance of the magnetic iron sulfide minerals, greigite (Fe₃S₄) and pyrrhotite (Fe₇S₈), is often underappreciated in geochemical studies because they are metastable with respect to pyrite (FeS₂). Based on magnetic properties and X-ray diffraction analysis, previous studies have reported widespread occurrences of these magnetic minerals along with magnetite (Fe₃O₄) in two thick Plio-Pleistocene marine sedimentary sequences from southwestern Taiwan. Different stratigraphic zones were classified according to the dominant magnetic mineral assemblages (greigite-, pyrrhotite-, and magnetite-dominated zones). Greigite and pyrrhotite are intimately associated with fine-grained sediments, whereas magnetite is more abundant in coarse-grained sediments. We measured total organic carbon (TOC), total sulfur (TS), total iron (Fe_T), 1N HCl extractable iron (Fe_A), and bulk sediment grain size for different stratigraphic zones in order to understand the factors governing the formation and preservation of the two magnetic iron sulfide minerals. The studied sediments have low TS/Fe_A weight ratios (0.03–0.2), far below that of pyrite (1.15), which indicates that an excess of reactive iron was available for pyritization. Observed low TS (0.05–0.27%) is attributed to the low organic carbon contents (TOC=0.25–0.55%), which resulted from dilution by rapid terrigenous sedimentation. The fine-grained sediments also have the highest Fe_T and Fe_A values. We suggest that under conditions of low organic carbon provision, the high iron activity in the fine-grained sediments may have removed reduced sulfur so effectively that pyritization was arrested or retarded, which, in turn, favored preservation of the intermediate magnetic iron sulfides. The relative abundances of reactive iron and labile organic carbon appear to have controlled the transformation pathway of amorphous FeS into greigite or into pyrrhotite. Compared to pyrrhotite-dominated sediments, greigite-dominated sediments are finer-grained and have higher Fe_A but lower TS. We suggest that diagenetic environments with higher supply of reactive iron, lower supply of labile organic matter, and, consequently, lower sulfide concentration result in relatively high Eh conditions, which favor formation of greigite relative to pyrrhotite.     

Tectonic control of pyrrhotite phase distribution studied on a fold structure

A fold structure within a pyrite ore specimen has been analysed with respect to the amount and distribution of pyrrhotite modifications present. The relative distribution of the two types of pyrrhotite, hexagonal 5C and monoclinic 4C, was found to be strongly dependent upon the stress distribution. Within very short distances the hexagonal fraction of the total amount of pyrrhotite varied from nearly zero up to 0.65 giving steep and structurally well defined gradients. The monoclinic phase was preferably located to regions deformed by intense shearing. It is suggested that the influence of anisotropic stresses accelerated the conversion of hexagonal to monoclinic pyrrhotite. Variations in the distribution of the total amount of pyrrhotite were also observed and partly ascribed to migration of pyrrhotite.     

Chalcopyrite formation through the metathesis of pyrrhotite with aqueous copper

The heterogeneous chemical environment which develops in the heap leaching of some pyrrhotite-containing copper ores can promote covellite and chalcopyrite formation particularly in acid-depleted regions of a heap. In such circumstances, copper recovery will be delayed until the acid and oxidation fronts move through the bed of ore and these secondary copper sulfides are re-leached. The transition from pyrrhotite to chalcopyrite most probably follows the sequence, pyrrhotite to copper-pyrrhotite to unnamed mineral CuFe₃S₄ to isocubanite to chalcopyrite, with a major structural expansion occurring prior to CuFe₃S₄. The mechanism is one in which copper is incorporated into pyrrhotite, which maintains its NiAs-type structure up to a stability limit, above which the structure rearranges to a chalcopyrite-like structure followed by isomorphic substitution of copper for iron. The structural rearrangement proceeds with significant expansion in one of the hexagonal axis directions and contractions in the other directions. Depending on the orientation, this expansion induces different levels of strain in the product chalcopyrite. The level of strain subsequently impacts on the rate of chalcopyrite metathesis to covellite. The depth of chalcopyrite formation into the pyrrhotite varies with pyrrhotite orientation.     

Composition features of isocubanite and polymorphous modifications of CuFe2S3 compound

The results of studying isocubanite from sulfide ores of recent oceanic black smokers and sulfide mud of the Red Sea are compared with those of isocubanite from ores of the Noril’sk ore field and isocubanite synthesized in the course of experimental study of the Cu–Fe–S system. Isocubanite associated with chalcopyrite is enriched in Cu, whereas that associated with pyrrhotite or with pyrrhotite and haycockite is enriched in Fe. According to data in the literature, the CuFe₂S₃ compound has four polymorphous modifications: orthorhombic cubanite, tetragonal, hexagonal, and cubic isocubanite. Cubanite, and tetragonal and hexagonal modifications of the CuFe₂S₃ compound are high-pressure minerals. Therefore, they may be used as barometers.     

Edgarite, FeNb3S6, first natural niobium-rich sulfide from the Khibina alkaline complex, Russian Far North: evidence for chalcophile behavior of Nb in a fenite

The new mineral species edgarite, FeNb₃S₆, was discovered in a feldspar-rich fenite, in a fenitized xenolith enclosed by nepheline syenite of the Khibina alkaline complex, Kola Peninsula, northwestern Russia. It occurs as platy inclusions (up to 0.15?mm) in Ti-(V)-rich pyrrhotite and ferroan alabandite, and as dark gray aggregates of platy grains located on the surface of the pyrrhotite. The associated minerals include Ti-(V)-rich marcasite, Mn-Fe-rich wurtzite-2H, corundum, nearly end member phlogopite, rutile, monazite-(Ce), and a graphite-like material. Edgarite is soft (VHN_5;10= 135–205?kg/mm²), distinctly bireflectant, and has a strong anisotropy. Its reflectance in air (and in oil) (R₁ and R₂ in percent, respectively) is: 470?nm: 28.1, 40.2 (13.0, 24.2), 546?nm: 27.4, 39.3 (12.3, 22.7), 589?nm: 27.0, 38.5 (12.2, 21.7), and 650?nm: 27.0, 36.9 (12.4, 20.3). The composition is Nb 52.87, Fe 10.12, V 0.36, Mn 0.10, Ti 0.04, S 35.86, sum 99.35?wt%, which corresponds to (Fe_0.96V_0.04Mn_0.01)_Σ1.01Nb_3.03S_5.95 (basis: Σ atoms=10). By analogy with synthetic FeNb₃S₆, the X-ray powder pattern of edgarite was indexed on a hexagonal cell, a=5.771(1), c=12.190(6)?Å, and V=351.6(3)?ų, D _calc is 4.99?g/cm³. The space group is most probably P6₃22, with Z=2. The strongest lines of the pattern [d in Å (I, hkl)] are: 6.11 (8, 002), 3.04 (6, 004), 2.88 (5, 110), 2.606 (8, 112), 2.096 (10, 114), 1.665 (8, 300), 1.524 (6, 008), 1.126 (7, 322), and 1.027 (6, 414). Edgarite appears to have formed at a very late or final stage of metasomatism, after the main event of fenitization, from a highly reduced, subalkaline S-C-H-rich fluid, which may have remobilized Nb as a result of destabilization of oxide minerals. These reducing conditions promoted the chalcophile behavior of lithophile elements (Nb, Ti, V and Mn) on a local scale in the fenite.     

Calcite twinning strain variations across the Proterozoic Grenville orogen and Keweenaw-Kapuskasing inverted foreland,USA and Canada

We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Ordovician Taconic allochthon. Fifty four carbonates (marbles, calcite veins, Ordovician limestone) were collected resulting in 68 strain analyses on mechanically twinned calcite (n = 2337 grains) across the Central Gneiss Belt (CGB; 3 samples), the Central Metasedimentary Belt (CMB; 27 samples), the Central Granulite Terrane (CGT; Adirondack's; 13 samples) and the Ottawan Orogenic Lid (OOL; 11 samples). Twinning strains in the greenschist-grade OOL marbles preserve N–S shortening and U-Pb titanite ages (～1150 Ma; n = 4) document these marbles formed during the Shawinigan (1190–1140 Ma) part of the Grenville orogen. From northwest to southeast, the Ottawan (1095–1020 Ma) twinning strain is dominantly a layer-parallel shortening fabric oriented N–S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW–SE) across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW–NE). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone (n = 11) and a roadcut on the southeast side of the Adirondack Mountains (Ft. Ann, NY; n = 8). Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae (e₁ and e₂). The better-developed e₁ sets (n = 406) record a horizontal fabric oriented NW–SE whereas the younger e₂ lamellae (n = 146) preserve a margin-parallel (SW–NE) horizontal fabric. Both the e₁ and e₂ strains record an overprint vertical shortening strain (NEV), perhaps related to extensional orogenic collapse. We also report an Ottawan orogen-aged granoblastic mylonite (1093 Ma, U-Pb zircon; 1102 Ma Ar-Ar biotite) in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics.     

Observations on coexisting pyrrhotite phases by transmission electron microscopy

Electron optical observations of coexisting pyrrhotite phases in a specimen of bulk composition near Fe₇S₈ are described. The implications of the coexistence of FeS and Fe₇S₈ are discussed in terms of the thermodynamic stability of the intermediate pyrrhotites and it is concluded that the intermediate pyrrhotites represent metastable phases at room temperatures.     

The experimental calibration of the iron isotope fractionation factor between pyrrhotite and peralkaline rhyolitic melt

A first experimental study was conducted to determine the equilibrium iron isotope fractionation between pyrrhotite and silicate melt at magmatic conditions. Experiments were performed in an internally heated gas pressure vessel at 500 MPa and temperatures between 840 and 1000 °C for 120-168 h. Three different types of experiments were conducted and after phase separation the iron isotope composition of the run products was measured by MC-ICP-MS. (i) Kinetic experiments using ⁵⁷Fe-enriched glass and natural pyrrhotite revealed that a close approach to equilibrium is attained already after 48 h. (ii) Isotope exchange experiments—using mixtures of hydrous peralkaline rhyolitic glass powder (∼4 wt% H₂O) and natural pyrrhotites (Fe_1 − xS) as starting materials— and (iii) crystallisation experiments, in which pyrrhotite was formed by reaction between elemental sulphur and rhyolitic melt, consistently showed that pyrrhotite preferentially incorporates light iron. No temperature dependence of the fractionation factor was found between 840 and 1000 °C, within experimental and analytical precision. An average fractionation factor of Δ ⁵⁶Fe/⁵⁴Fe_{pyrrhotite-melt} = −0. 35 ± 0.04‰ (2SE, n = 13) was determined for this temperature range. Predictions of Fe isotope fractionation between FeS and ferric iron-dominated silicate minerals are consistent with our experimental results, indicating that the marked contrast in both ligand and redox state of iron control the isotope fractionation between pyrrhotite and silicate melt. Consequently, the fractionation factor determined in this study is representative for the specific Fe²⁺/ΣFe ratio of our peralkaline rhyolitic melt of 0.38 ± 0.02. At higher Fe²⁺/ΣFe ratios a smaller fractionation factor is expected. Further investigation on Fe isotope fractionation between other mineral phases and silicate melts is needed, but the presented experimental results already suggest that even at high temperatures resolvable variations in the Fe isotope composition can be generated by equilibrium isotope fractionation in natural magmatic systems.     

Pseudosinhalite: discovery of the hydrous MgAl-borate as a new mineral in the Tayozhnoye, Siberia, skarn deposit

After its initial synthesis as the new compound Mg₂Al₃B₂O₉(OH) (Daniels et al. 1997) pseudosinhalite has now been discovered as a new mineral. It occurs, together with hydrotalcite, as a replacement product of sinhalite, MgAlBO₄, in an impure marble of the contact metasomatic iron boron deposit of Tayozhnoye in the Aldan Shield of Siberia. Its chemical composition determined by electron microprobe is (wt%): Al₂O₃ 46.88; MgO 25.12; FeO 1.99; B₂O₃ (calculated) 21.75; H₂O (calculated) 2.81 giving a total of 98.55 and leading to the empirical formula (Mg_2.00 Fe²⁺ _0.09)_Σ=2.09 Al_2.94 B₂O₉(OH). The small deviation from the ideal stoichiometry with (Mg?+?Fe²⁺):Al?≠?2:3 may be caused by either solid solution towards, or submicroscopic interlayering with lamellae of, the structurally similar mineral sinhalite. The underlying substitution involving also B and H would be (Mg?+?Fe)+?B=Al+2H. Pseudosinhalite is monoclinic, space group P2₁/c, with a=7.49(1), b=4.33(1), c=9.85(2) Å; β=110.7(1)°; V?=?299(1) ų; Z?=?2. Calculated density is 3.508?g/cm³. Pseudosinhalite is colourless with white streak and has a vitreous lustre. It is transparent; no fluorescence was detected. There is no cleavage and parting; fractures are concoidal. Optical constants could not be measured properly due to polysynthetic microtwinning, but α<1.72<γ. For synthetic pseudosinhalite α=1.691(1); β=1.713(1); γ=1.730(1); Δ=0.039; 2?V=80°. The temperature of pseudosinhalite formation was below about 400?°C at low pressures and with a hydrous, CO₂-bearing fluid participating in the reaction.     

Phase equilibria in the KFeS<Subscript>2</Subscript>–Fe–S system at 300–600 °C and bartonite stability

The article deals with phase relations in the KFeS₂–Fe–S system studied by the dry synthesis method in the range of 300–600 °C and at a pressure of 1 bar. At the temperature below 513?±?3 °C, pyrite coexists with rasvumite and there are pyrite–rasvumite–KFeS₂ and pyrite–rasvumite–pyrrhotite equilibria established. Above 513?±?3 °C pyrite and rasvumite react to form KFeS₂ and pyrrhotite, limiting the pyrite–rasvumite association to temperatures below this in nature. The experiments also outline the compositional stability range of the copper-free analog of murunskite (K _x Fe_2?yS₂) and suggest that mineral called bartonite is not stable in the Cl-free system, at least at atmospheric pressure and the temperature in the experiments. Chlorbartonite could be easily produced after adding KCl in the experiment. Possible parageneses in the quaternary K–Fe–S–Cl system were described based on the data obtained in this research and found in the previous studies. The factors affecting the formation of potassium–iron sulfides in nature were discussed.