Pyrrhotite Crystals Research Articles

Low-temperature magnetic properties of monoclinic pyrrhotite with particular relevance to the Besnus transition

Monoclinic pyrrhotite (Fe7S8) owes its ferrimagnetism to an ordered array of Fe vacancies. Its magnetic properties change markedly around 30 K, in what is known as the Besnus transition. Plausible explanations for the Besnus transition are either due to changes in crystalline anisotropy from a transformation in crystal symmetry or from the establishment of a two-phase system with magnetic interaction between the two phases. To help resolve this discrepancy, we measured hysteresis loops every 5° and backfield curves every 10° in the basal plane of an oriented single crystal of monoclinic pyrrhotite at 300 K and every 2 K from 50 K through the Besnus transition until 20 K. Between 50 and 30 K, hysteresis loops possess double inflections between crystallographic a-axes and only a single inflection parallel to the a-axes. Magnetization energy calculations and relative differences of the loops show a sixfold symmetry in this temperature range. We propose that the inflections stem from magnetic axis switching, which is both field and temperature dependent, in a manner somewhat analogous to an isotropic point where magnetocrystalline constants change their sign. The Besnus transition is best characterized by changes in magnetic remanence and coercivity over a 6° temperature span (28–34 K) with a maximum rate of change at 30 K. A surprising yet puzzling finding is that the coercivity ratio becomes less than unity below the transition when fourfold symmetry arises. Because the changes in magnetic parameters are linked to the crystal structure, we conclude the Besnus transition owes its origin to a distortion of the crystallographic axes below 30 K rather than an apparition of a two-phase system. An isothermal magnetization of natural pyrrhotite cycled from room temperature to successively lower temperatures through the Besnus transition decreases 2–4 times less than equivalent grain sizes of magnetite, with less than a 10 per cent loss in remanence between 300 and 150 K in pseudo-single-domain (PSD) pyrrhotite. As PSD monoclinic pyrrhotite carries the magnetic remanence in some meteorites, it is likely that low-temperature cycling in space then warming to ambient conditions at the Earth's surface will have only a minor influence on palaeointensity values derived from those meteorites.

Pyrite-pyrrhotite intergrowths in calcite marble from Bistriški Vintgar, Slovenia

Roman marble quarry in Bistrica gorge in southern Pohorje Mt. (north-eastern Slovenia) is situated in a 20 m thick lens of layered marble, at the contact zone between granodiorite and metamorphites. Grey and yellowish non-homogenous calcite marble is heavily included by mica, quartz, feldspars, zoisite, pyrite and amphiboles. In the present research, we have studied numerous pyrite (FeS2) crystals associated with yellowish-bronze non-stoichiometric pyrrhotite (Fe1−xS), not previously reported from this locality. SEM investigation revealed unusual sequence of crystallisation: primary skeletal pyrrhotite matrix is sparsely overgrown by well-crystalline pyrite, both being overgrown by smaller, well-developed hexagonal pyrrhotite crystals of the second generation. With TEM we identify the pyrrhotite as 5T-Fe1-xS phase, where x is about 0.1 and is equivalent to Fe9S10. The pyrite-pyrrhotite coexistence allows us a construction of fO2-pH diagram of stability fields, which reflects geochemical conditions at the time of marble re-crystallisation.

Variable defect structures cause the magnetic low-temperature transition in natural monoclinic pyrrhotite

Non-stoichiometric monoclinic 4C pyrrhotite (Fe7S8) is a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials. Because of its low-temperature magnetic transition around 30 K also known as Besnus transition, which is considered to be an intrinsic property, this mineral phase is easily detectable in natural samples. Although the physical properties of pyrrhotite have intensively been studied, the mechanism behind the pronounced change in magnetization at the low-temperature transition is still debated. Here we report magnetization experiments on a pyrrhotite crystal (Fe6.6S8) that consists of a 4C and an incommensurate 5C* superstructure that are different in their defect structure. The occurrence of two superstructures is magnetically confirmed by symmetric inflection points in hysteresis measurements above the transition at about 30 K. The disappearance of the inflection points and the associated change of the hysteresis parameters indicate that the two superstructures become strongly coupled to form a unitary magnetic anisotropy system at the transition. From this it follows that the Besnus transition in monoclinic pyrrhotite is an extrinsic magnetic phenomenon with respect to the 4C superstructure and therefore the physics behind it is in fact different from that of the well-known Verwey transition.

Full vector low‐temperature magnetic measurements of geologic materials

AbstractThe magnetic properties of geologic materials offer insights into an enormous range of important geophysical phenomena ranging from inner core dynamics to paleoclimate. Often it is the low‐temperature behavior (<300 K) of magnetic minerals that provides the most useful and highest sensitivity information for a given problem. Conventional measurements of low‐temperature remanence are typically conducted on instruments that are limited to measuring one single‐axis component of the magnetization vector and are optimized for measurements in strong fields. These instrumental limitations have prevented fully optimized applications and have motivated the development of a low‐temperature probe that can be used for low‐temperature remanence measurements between 17 and 300 K along three orthogonal axes using a standard 2G Enterprises SQuID rock magnetometer. In this contribution, we describe the design and implementation of this instrument and present data from five case studies that demonstrate the probe's considerable potential for future research: a polycrystalline hematite sample, a polycrystalline hematite and magnetite mixture, a single crystal of magnetite, a single crystal of pyrrhotite, and samples of Umkondo Large Igneous Province diabase sills.

An early solar system magnetic field recorded in CM chondrites

Abstract We present a paleomagnetic study of seven CM carbonaceous chondrites. CM chondrites are believed to be some of the most chemically primitive materials available in our solar system and may sample the continuum of transitional objects between asteroids and comets formed in the outer solar system. As such, CM chondrites can help us to understand primordial aspects of the history of the early solar system including protoplanetary disk and planetesimal magnetism. The ferromagnetic assemblage of CM chondrites is composed of a mixture of primary metallic iron, pyrrhotite, and magnetite. The remanent properties are usually dominated by secondary pyrrhotite. Paleomagnetic analyses using thermal and alternating field demagnetization identified a stable origin-trending component of magnetization in the seven studied CM chondrites. In each meteorite, this component is homogeneous in direction at least at the cm scale and is therefore post-accretional. We interpret this stable component as a pre-terrestrial chemical remanent magnetization acquired during crystallization of magnetite and pyrrhotite during parent body aqueous alteration in a field of at least a few μ T ( 2 ± 1.5 μ T ) . Considering the timescale and intensities of primordial magnetic fields, both internally generated fields from a putative dynamo and external fields, generated in the protoplanetary disk, may have been recorded by CM chondrites. It is presently difficult to discriminate between the two hypotheses. Regardless, CM chondrites likely contain the oldest paleomagnetic record yet identified.

Magnetoacoustic emission of natural pyrrhotite

We consider data of study of the parameters of magnetoacoustic emission performed on pyrrhotite from magnetite and pyrite ores from the deposits of the Urals. It is shown that the difference in signals is mainly due to different types of domain structure which forms during the crystallization of pyrrhotite as pinacoids or prisms. Five types of pyrrhotite are recognized depending on the parameters of magnetoacoustic emission. This information can be used as typomorphic features of pyrrhotite of different geneses.

Clasts in theCM2 carbonaceous chondrite Lonewolf Nunataks 94101: Evidence for aqueous alteration prior to complex mixing

AbstractClasts in theCM2 carbonaceous chondrite Lonewolf Nunataks (LON) 94101 have been characterized using scanning and transmission electron microscopy and electron microprobe analysis to determine their degrees of aqueous alteration, and the timing of alteration relative to incorporation of clasts into the host. The provenance of the clasts, and the mechanism by which they were incorporated and mixed with their host material are also considered. Results show that at least five distinct types of clasts occur inLON94101, of which four have been aqueously altered to various degrees and one is largely anhydrous. The fact that they have had different alteration histories implies that the main part of aqueous activity occurred prior to the mixing and assimilation of the clasts with their host. Further, the presence of such a variety of clasts suggests complex mixing in a dynamic environment involving material from various sources. Two of the clasts, one containing approximately 46 vol% carbonate and the other featuring crystals of pyrrhotite up to approximately 1 mm in size, are examples of unusual lithologies and indicate concentration of chemical elements in discrete areas of the parent body(ies), possibly by flow of aqueous solutions.

The nanoscale mineralogy of Fe,Ni sulfides in pristine and metamorphosed CM and CM/CI‐like chondrites: Tapping a petrogenetic record

AbstractWe have sampled sulfide grains from one pristine CM2 chondrite (Yamato [Y‐] 791198), one thermally metamorphosed CM2 chondrite (Y‐793321), and two anomalous, metamorphosed CM/CI‐like chondrites (Y‐86720 and Belgica [B‐] 7904) by the focused ion beam (FIB) technique and studied them by analytical transmission electron microscopy (TEM). Our study aims at exploring the potential of sulfide assemblages and microstructures to decipher processes and conditions of chondrite petrogenesis. Complex exsolution textures of pyrrhotite (crystallographic NC‐type with N ≈ 6), troilite, and pentlandite occur in grains of Y‐791198 and Y‐793321. Additionally, polycrystalline 4C‐pyrrhotite‐pentlandite‐magnetite aggregates occur in Y‐791198, pointing to diverse conditions of gas–solid interactions in the solar nebula. Coarser exsolution textures of Y‐793321 grains indicate higher long‐term average temperatures in the <100 °C range compared to Y‐791198 and other CM chondrites. Sulfide mineralogy of Y‐86720 and B‐7904 is dominated by aggregates of pure troilite and metal, indicating metamorphic equilibration at sulfur fugacities (fS2) of the iron‐troilite buffer. Absence of magnetite in equilibrium with sulfide and metal in Y‐86720 indicates higher peak temperatures compared with B‐7904, in which coexistence of troilite, metal, and magnetite constrains metamorphic temperature to less than 570 °C. NC‐pyrrhotite occurs in both meteorites as nm‐wide rims on troilite grains and, together with frequent anhydrite, indicates a retrograde metamorphic stage at higher fS2 slightly above the fayalite‐magnetite‐quartz‐pyrrhotite buffer. Fine‐grained troilite‐olivine intergrowths in both meteorites suggest the pre‐metamorphic presence of tochilinite‐serpentine interlayer phases, pointing to mineralogical CM affinity. Pseudomorphs after euhedral pyrrhotite crystals in Y‐86720 in turn suggest CI affinity as do previously published O isotopic data of both meteorites.

Redetermination of the structure of 5C pyrrhotite at low temperature and at room temperature

The crystal structure of a 5C pyrrhotite from Silberberg Mine, Bodenmais, Germany, has been determined at 120 and 293 K in space group P21. The low-temperature structure refined to R = 0.0261 for 5727 data with Io > 2σ(Io) and R = 0.0354 for all 7121 data. The room-temperature structure of the same crystal refined to R = 0.0383 for 2471 data with Io > 2σ(Io) and R = 0.0550 for all 3419 data. In addition, the diffraction data of a 5C pyrrhotite crystal from Copper Cliff Mine, Sudbury, Canada, previously refined in space group Cmce, has been transformed and also refined in space group P21. This structure refined to R = 0.0441 for 2701 data with Io > 2σ(Io) and R = 0.0672 for all 3843 data, which is a substantial improvement over the previous refinement. The structure is characterized by iron vacancy avoidance within a layer, and with partially occupied sites projecting on top of each other in adjacent layers. Two half-occupied sites and two sites each with occupancy of 0.88 are present in the Bodenmais structure, together with one site with occupancy of 0.28. The distribution in the crystal from Sudbury is slightly different and cannot be described with the same number of partially occupied sites. Broadly, however, the two structures are similar.

S saturation history of Nain Plutonic Suite mafic intrusions: origin of the Voisey’s Bay Ni–Cu–Co sulfide deposit, Labrador, Canada

The Voisey’s Bay deposit is hosted in a 1.34-Ga intrusion composed of troctolite, olivine gabbro, and ferrogabbro. The sulfide mineralization is associated with magmatic breccias that are enveloped by weakly mineralized olivine gabbros and troctolites, and also occurs as veins along structures in adjacent paragneiss. A dyke is connected to the base of the north wall of the Eastern Deeps Intrusion, and the entry point of this dyke into the chamber is the locus of the Eastern Deeps nickel sulfide deposit. A detailed exploration in the area between the Eastern Deeps and the Ovoid has shown that these intrusions and ore deposits are connected by a splayed dyke. The Eastern Deeps Deposit is surrounded by a halo of moderately to weakly mineralized variable-textured troctolite (VTT) that reaches a maximum thickness above the axis of the Eastern Deeps Deposit along the northern wall of the Eastern Deeps Intrusion. The massive sulfides and breccia sulfides are petrologically and chemically different when compared to the disseminated sulfides in the VTT, and there is a marked break in Ni tenor of sulfide between the two. Sulfides hosted in the dyke tend to have low metal tenors ([Ni]100 = 2.5–3.5%), sulfides in Eastern Deeps massive and breccia ores have intermediate Ni tenors ([Ni]100 = 3.5–4%), and disseminated sulfides in overlying rocks have high Ni tenors ([Ni]100 = 4–8%). Four principal processes control the compositions of the Voisey’s Bay sulfides. Coarse-grained loop-textured ores consisting of pyrrhotite crystals separated by chalcopyrite and pentlandite exhibit a two orders of magnitude variation in the Pd/Ir ratio which is due to mineralogical variations where pentlandite is enriched in Pd and Ir is dispersed throughout the mineral assemblage. A decrease in Ir and Rh from the margin of the Ovoid toward cubanite-rich parts at the central part of the Ovoid is consistent with fractionation of the sulfide from the margins toward the center of the Ovoid. The Ovoid ores have higher Ni and Pd tenor than the Eastern Deeps massive sulfides; this is consistent with both a higher R factor and greater degree of silicate parental magma evolution in the Ovoid than the Eastern Deeps. The disseminated sulfides surrounding the Eastern Deeps deposit have some of the highest Ni and Pd tenors at Voisey’s Bay, which are indicative of not only more primitive magmas but also higher R factors than the Ovoid or the Eastern Deeps. VTT and normal-textured troctolite of the Eastern Deeps that contain trace sulfide have 0.1–3 ppb Pt and 0.1–3 ppb Pd, whereas weakly to heavily mineralized variable troctolites in the same unit have one to two orders of magnitude higher abundances of Pt and Pd. Troctolites and olivine gabbros from other parts of the Voisey’s Bay Intrusion and other Nain Plutonic Suite Intrusions, including the Kiglapait, Newark Bay, Barth Island, Mushua, and Nain Bay South Intrusion, also have low platinum group element abundances. Although it is possible that this is a signature of a widespread sulfide saturation event that pre-dated ore formation at Voisey’s Bay, it is more likely that platinum group element (PGE) depletion is a product of the source melting process where low degrees of melting resulted in the retention of PGE in the mantle source. If so, this indicates that PGE depletion should be used with caution as an exploration tool in the Nain Plutonic Suite.

S-rich apatite-hosted glass inclusions in xenoliths from La Palma: constraints on the volatile partitioning in evolved alkaline magmas

The composition of S-rich apatite, of volatile-rich glass inclusions in apatite, and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in La Palma has been investigated to constrain the partitioning of volatiles between apatite and alkali-rich melts. The xenoliths are interpreted as cumulates from alkaline La Palma magmas. Apatite contains up to 0.89 wt% SO3 (3560 ppm S), 0.31 wt% Cl, and 0.66 wt% Ce2O3. Sulfur is incorporated in apatite via several independent exchange reactions involving (P5+, Ca2+) vs. (S6+, Si4+, Na+, and Ce3+). The concentration of halogens in phonolitic to trachytic glasses ranges from 0.15 to 0.44 wt% for Cl and from 0.2 wt% SO3). DSapatite/glass is only slightly dependent on the melt composition and can be expressed as: SO3 apatite (wt%) = 0.157 * ln SO3 glass (wt%) + 0.9834. The phonolitic compositions of glass inclusions in amphibole and hauyne are very similar to evolved melts erupted on La Palma. The lower sulfur content and the higher Cl content in the phonolitic melt compared to basaltic magmas erupted in La Palma suggest that during magma evolution the crystallization of hauyne and pyrrhotite probably buffered the sulfur content of the melt, whereas the evolution of Cl concentration reflects an incompatible behavior. Trachytic compositions similar to those of the (water-rich) glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products. These compositions are interpreted to be formed by the reaction between water-rich phonolitic melt and peridotite wall-rock.

Sphalerite Associated with Pyrrhotite-Chalcopyrite Ore Occurring in the Kotana Fe-Skarn Deposit (Giresun, NE Turkey): Exsolution or Replacement

The Kotana prospect is located about 30 km south of Giresun (NE Turkey). The ore mineralization is a Fe-skarn occurring within the low-grade pre-Lower Jurassic Pınarlar metamorphics, consisting of marble-phyllite intruded by the Upper Cretaceous-Eocene Aksu biotite monzogranite. The principal primary ore minerals include pyrrhotite and magnetite along with minor pyrite (I) and chalcopyrite, accompanied by trace sphalerite. Sphalerite is closely associated with chalcopyrite and to a lesser extent with hexagonal pyrrhotite. Secondary ore minerals include pyrite (II), marcasite, martite, hematite, goethite, lepidocrocite, and intermediate Fe-oxides-hydroxides. Gangue minerals are mainly calcite and quartz. Oxidation of the primary sulphides resulted in formation of diverse secondary ore textures containing bird's eye, martitic, spheroidal, colloform, rim, and veinlets. Distinct crystal shapes of sphalerite are of particular interest to this investigation due mainly to their proposed formation mechanisms. Three alternative mechanisms for their formation were considered: (i) quasi-exsolved bodies developed by hexagonal pyrrhotite replacement of chalcopyrite, (ii) interstitial formation between coalescing pyrrhotite crystals during crystal growth, and (iii) as genuinely exsolved bodies, and as such conflict with previous experimental results. Although the general absence of solute mineral outside the solvent mineral suggests solid solution at high temperatures (favouring the third mechanism), textures, modal, microprobe and sulfur-isotope data suggest that these are more likely to be pseudoexsolved bodies formed as a result of replacement of chalcopyrite by hexagonal pyrrhotite. The \delta^3^4S values of pyrrhotite and chalcopyrite are between 5.23 and 6.73 per mil (n= 12) and 2.29 and 3.26 per mil (n= 8), respectively, indicating continuous enrichment in heavy sulphur isotopes from prograde stage to retrograde stage within the typical range for skarn-type mineralization. Fluid inclusion analyses of calcite and quartz gangues indicate that the minimum homogenization temperature (Th) averaged 400±20 ºC with salinities < 15 wt% NaCl equivalent.

Magnetic anisotropy behaviour of pyrrhotite as determined by low- and high-field experiments

SUMMARY Here we report on the sources of magnetic anisotropy in pyrrhotite, an iron sulphide present in many rocks as an important carrier of the Natural Remanent Magnetization. While the magnetic hysteresis parameters of pyrrhotite are well known, the existing database concerning its anisotropy behaviour is patchy and ambiguous. Therefore, a collection of 11 seemingly single crystals of natural pyrrhotite was scrutinized. Before embarking on the anisotropy determinations the set of single crystals was extensively characterized rock magnetically by measuring Curie temperatures, hysteresis loops, IRM acquisition curves, and FORC diagrams (the latter three all at room temperature). First the variation of the low-field susceptibility as function of applied field and grain size was evaluated for fields ranging from 1 to 450 A m −1 . Existing grain size dependent data and the present larger crystals show a logarithmic grain size dependence. This enables estimating the grain size for unimodal pyrrhotite distributions in rocks. Measured trends are better fitted with an exponential function than with a Rayleigh Law style function. Based on the rock magnetic characterization and the behaviour of the anisotropy of magnetic susceptibility six samples (of the original 11) were selected for the high-field anisotropy determinations within the basal plane. Those data were acquired with a torque cantilever-type magnetometer. As expected, most single crystals showed a pure 6‐ θ curve within their basal plane because of the easy axis configuration. In some crystals, however, lower harmonic terms overlapped the 6‐θ term. This may be the dominant source of the observed variation in magnetic anisotropy properties. Torque data of three of the six samples were of sufficient quality to allow evaluation of K 1. Re-evaluation of existing torque data and including the present newly derived determinations, yields for the anisotropy constant of pyrrhotite within the basal plane K 1: (2.7 ± 0.2) 10 4 Jm −3 . This is over an order of magnitude more precise than the sparse existing K 1 data; only the value reported by Mikami and co-authors in 1959 agrees with the new determination. With this firmly established K 1 value meaningful anisotropy models are now possible for pyrrhotite-bearing rocks.

Sulfur concentration at sulfide saturation (SCSS) in magmatic silicate melts

The sulfur concentration in silicate melts at sulfide saturation (SCSS) was experimentally investigated in a temperature range from 1150 to 1450 °C and a pressure range from 500 MPa to 1 GPa in a piston–cylinder apparatus. The investigated melt compositions varied from rhyolitic to basaltic and water concentrations varied from 0 to ∼9 wt%. All experiments were saturated with FeS melt or pyrrhotite crystals. Temperature was confirmed to have a positive effect on the SCSS. Experimental oxygen fugacities were either near the carbon–carbon monoxide buffer or one log unit above the nickel–nickel oxide buffer, and found to positively affect the SCSS. Combining our results with data from the literature we constructed a model to predict the SCSS in melts ranging in composition from komatiitic to rhyolitic, with water concentrations from 0 to 9 wt%, at pressures from 1 bar to 9 GPa and oxygen fugacities between ∼2 log units below the fayalite–magnetite–quartz buffer to ∼2 log units above it. The coefficients were obtained by multiple linear regression of experimental data and the best model found for the prediction of the SCSS is: ln ( S in ppm ) SCSS = 11.35251 - 4454.6 T - 0.03190 P T + 0.71006 ln ( MFM ) - 1.98063 [ ( MFM ) ( X H 2 O melt ) ] + 0.21867 ln ( X H 2 O melt ) + 0.36192 ln X FeO melt where P is in bar, T is in K, MFM is a compositional parameter describing the melt based upon cation mole fractions: MFM = Na + K + 2 ( Ca + Mg + Fe 2 + ) Si × ( Al + Fe 3 + ) , X H 2 O melt is the mole fraction of water in the melt, and X FeO melt is the mole fraction of FeO in the melt. This model was independently tested against experiments performed on anhydrous and hydrous melts in the temperature range from 800 to 1800 °C and 1–9 GPa. The model typically predicts the measured values of the natural log of the SCSS (in ppm) for komatiitic to rhyolitic (∼42 to ∼74 wt% SiO 2) melts to within 5% relative, but is less accurate for high-silica (>76 wt% SiO 2) rhyolites, especially those with molar ratios of iron to sulfur below 2. We demonstrate how this model can be used with the MELTS software to calculate the saturation of natural magmas with a sulfide phase and evolution of the SCSS during magmatic differentiation.

The role of microorganisms on the mineralogical and geochemical characteristics of the Parnassos-Ghiona bauxite deposits, Greece

The Parnassos-Ghiona bauxite deposits of Greece are hosted within carbonate rocks and have been formed during different geological ages. The most economically important deposits occur in the B3 bauxite horizon, which is developed over long distance as a continuous layer of 1–10 m in thickness, within Cretaceous limestones. Due to intense tectonics, a significant (approximately 30 vol.%) bauxite ores along and near their contact with faults show a brittle deformation and change in the color from red to black-gray, in a distance of tens of meter. Commonly gray to whitish bauxites are aluminum-enriched (> 65 wt.% Al 2O 3) and iron depleted. The most salient feature of bauxite ores of black-gray, grayish-brown or yellowish color along faults is the abundance of secondary pyrite of varying size (from less than 1 μm to a few hundreds μm) and form, which is closely associated with organic matter. Frambroidal pyrite and pyrite veins crosscutting sulfide-rich zones are occasionally present. Dismembered pyrrhotite and pentlandite crystals are only occasionally present. Pyrite is Co, Ni free, while Se varies from 0.7 to 0.9 wt.% Se. Pyrrhotite is Ni-bearing (up to 0.5 wt.% Ni) and pentlandite contains cobalt (average 1.5 wt.% Co). Textural relationships between Fe-sulfides/Fe-oxides, and the total organic carbon (TOC) content, ranging from 0.06 wt.% in red-color bauxite ores to 2.78 wt.% in gray-color ores, is consistent with a biological activity. Bacteria may play a significant role in the mineralogical and chemical composition of laterite ores, and in leaching and beneficiation of low-grade ores. Due to the behaviour of ferric iron as an electron acceptor for energy metabolism in a wide spectrum of bacteria the mobility of iron in the bauxite ores may be the result of an effective leaching by percolating pore waters, depending mostly on the pH–Eh conditions.

Emplacement of the Whistle Dike, the Whistle Embayment and Hosted Sulfides, Sudbury Impact Structure, Based on Anisotropies of Magnetic Susceptibility and Magnetic Remanence

The Sudbury impact structure is one of the few multi-ring meteorite impact structures recognized on Earth. The meteorite impact occurred ca. 1850 Ma, resulting in the production of impact melts derived from the crust, possibly with a mantle component. The impact melts formed the Sudbury Igneous Complex and also were emplaced as radial and concentric dikes within the target rocks. The Whistle dike, one of the radial “offset” dikes in the Sudbury impact structure, is connected to the Sudbury Igneous Complex by the Whistle embayment structure, which is composed of noritic cumulates. We present a study of the emplacement flow patterns in the Whistle dike and the embayment, using magnetic anisotropy methods to measure the petrofabrics that record emplacement. Petrographic and scanning electron microscope observations, combined with thermal demagnetization measurements, and tests of the acquisition and demagnetization of anhysteretic magnetic remanence indicate that the primary sources of the magnetic signals in the sample suite are coarse-grained, soft coercivity magnetite and pyrrhotite that are interpreted to be primary igneous phases. Some samples also contain hard coercivity phases of possible metamorphic origin. The anisotropy of magnetic susceptibility (AMS), the anisotropy of an-hysteretic remanent magnetization (AARM) and the anisotropy of partial anhysteretic remanent magnetization (ApARM) were measured for the samples collected at 20 sites within the Whistle dike and the embayment structure, including an exposed chalcopyrite-rich massive sulfide body. The AMS method provided well-defined, interpretable fabrics for many of the sample sites. However, the AMS and AARM methods were unsuccessful in providing interpretable fabric measurements at some sample sites due to contributions from the hard coercivity ferromagnetic minerals. For those samples, the ApARM method was successful in isolating the sub-fabric defined by the primary magnetite ± pyrrhotite crystals. The fabric maps are consistent with lateral (horizontal) injection of the Whistle offset dike and subsequent sinking of molten massive sulfides within the still ductile dike. Sinking of the massive sulfides is recorded by steeply plunging magnetic lineations. The magnetic fabrics in the cumulate norites of the embayment structure show that the norites were deposited in a dynamic environment that resulted in a well-oriented lineation, which may record flow of gravity or thermal currents, channelled along the embayment axis. In the embayment, the lineations plunge toward a massive sulfide body, suggesting the flow that led to the formation of the lineation may also have led to emplacement of the massive sulfide ore. It is suggested that steeply plunging lineations elsewhere in the Whistle offset dike, and possibly in other Sudbury offset dikes, might indicate the trails of sunken massive sulfides in the subsurface. It is also suggested that magnetic fabrics might be useful to locate massive sulfide bodies hosted in embayment structures.

The Solubility of Sulphur in Hydrous Rhyolitic Melts

Experiments performed at 2 kbar, in the temperature range 800–1000°C, with fO2 between NNO–2·3 and NNO+2·9 (where NNO is the nickel–nickel oxide buffer), and varying amounts of sulphur added to hydrous metaluminous rhyolite bulk compositions, were used to constrain the solubility of sulphur in rhyolite melts. The results show that fS2 exerts a dominant control on the sulphur solubility in hydrous silicate melts and that, depending on fO2, a rhyolitic melt can reach sulphur contents close to 1000 ppm at high fS2. At fO2 below NNO+1, the addition of iron to a sulphur-bearing rhyolite magma produces massive crystallization of pyrrhotite and does not enhance the sulphur solubility of the melt. For a given fO2, the melt-sulphur-content increases with fS2. For fixed fO2 and fS2, temperature exerts a positive control on sulphur solubilities, at least for fO2 below NNO+1. The mole fraction of dissolved sulphur exhibits essentially linear dependence on fH2S at low fO2 and, although the experimental evidence is less clear, on fSO2 at high fO2. The minimum in sulphur solubility corresponds to the redox range where both fH2S and fSO2 are approximately equal. A thermodynamic model of sulphur solubility in hydrous rhyolite melts is derived assuming that total dissolved sulphur results from the additive effects of H2S and SO2 dissolution reactions. The model reproduces well the minimum of sulphur solubility at around NNO+1, in addition to the variation of the sulphide to sulphate ratio with fO2. A simple empirical model of sulphur solubility in rhyolitic melts is derived, and shows good correspondence between model and observations for high-silica rhyolites.

Gold distribution in pyrrhotite crystals grown under hydrothermal conditions

The distribution of gold between pyrrhotite and greenockite was studied by the method of hydrothermal thermal-gradient cocrystallization of the components at a temperature of 450°C, a pressure of 1 kbar, and different activities of sulfur in the presence of As and Se impurities. The structural component of Au impurity in these minerals was selected by studying statistical samplings of the analytical data for single crystals. Ferrous greenockite incorporates a maximum of 10 ± 2 ppm Au in solid solution. The incorporation limit of Au in stoichiometric pyrrhotite was estimated to be 25 ± 9 ppm; for nonstoichiometric pyrrhotites, it is lower. The As and Se impurities barely affect the concentration of structural gold in the minerals. It was shown that constitutional vacancies in pyrrhotite do not capture Au impurity. The observed phenomenon may be the growth effect related to the surface properties of nonstoichiometric pyrrhotite crystals grown under hydrothermal conditions. Differences in the surface structure of pyrrhotites of different composition and the products of their sorption of gold were revealed by X-ray photoelectron spectroscopy.

Characteristics of mineralization and gold occurrence in Shaxi porphyry copper-gold deposit, central Anhui, China

Through field geological surveys, mineralogical observation by microscopy, compositional analyses on minerals using EPMA, geochemical analyses on Cu-Au-bearing intrusive rocks, fluid inclusion and isotope studies, and element distributions detected by PIXE system, the characteristics of mineralization and gold occurrence in the Shaxi porphyry copper-gold deposit (Central Anhui Province, East China) have been fully investigated. The results of the study are summarized as the follows: (1) Altogether 5 types of copper (gold) ore can be detected, namely chalcopyrite ore, Cu-bearing pyrite ore, magnetite-chalcopyrite ore, pyrite-bornite-chalcopyrite ore and chalcopyrite-molybdenite ore; (2) The assemblage of ores mainly consists of chalcopyrite, pyrite, psedomorph hematite, bornite, magnetite, chalcocite, covellite and arsenopyrite; the non-metal minerals are quartz, K-feldspar, calcite, gypsum, anhydrite; (3) Liquid-vapour conditions have been documented and show that fluid inclusions frequently occur in quartz veins accompanied by pyritization-chalcopyritization-gold mineralization, mostly consisting of a coexisting liquid-gas phase and some are only with pure liquid phase. The ore-forming temperature is between 230-350°C; (4) Sulfur isotope studies show that the δ 3 4 S values are between 0.28-0.80 per mills for most of sulfides, δ 3 4 S values of chalcopyrite are somewhat homogenous those of pyrites; (5) The gold occurred in micro-inclusion sizes (<1 μm), heterogeneously distributed in chalcopyrite and pyrite in the ore deposit. The Au mineralization is positively correlated with As, S, Cu, Fe, Zn and Ti in chalcopyrite and pyrite. (6) The major controlling deposition of Au in the region is probably the oxidation potential of the red sedimentary rocks of shale and fine grain sandstone may be crucial as a geochemical barrier to the reducing fluid. Some fine-grained crystals of pyrite and pyrrhotite formed under conditions of very rapid crystallization from the hydrothermal solutions, in which the content of H 2 S decrease rapidly accompanied by destabilization of Au disulfides complexes. This study has provided the first evidences for the theoretical study of gold mineralization as well as its possible utilization during the mining process of the porphyry copper-gold deposit.

Diffusion of osmium in pyrrhotite and pyrite: implications for closure of the Re–Os isotopic system

In order to better constrain the extent to which common sulfide minerals will retain their osmium isotopic composition subsequent to crystallization, we have conducted experiments to quantify the diffusion behavior of osmium in pyrite and pyrrhotite. Experiments consisted of either (1) isothermal soaking of diffusion couples consisting of natural pyrite or pyrrhotite crystals packed against powdered Os-bearing Fe-sulfide or (2) ‘relaxation’ of initially high near-surface osmium concentrations produced in the latter experiments (pyrite only). Osmium penetration into samples was characterized by depth profiling using Rutherford backscattering spectroscopy (RBS) (pyrite) or electron microprobe analyses across sectioned run products (pyrrhotite). Results of the first type of diffusion experiment involving pyrite show only limited osmium penetration into sample surfaces, with the extent of penetration uncorrelated with run duration. Images of pyrite samples using atomic force microscopy show roughening of initially smooth surfaces as a consequence of step formation and suggest that osmium incorporation into the near-surface occurred by solute uptake during step growth and not by volume diffusion. Prolonged (1000 + h) ‘relaxation’ experiments revealed no additional osmium penetration into pyrite surfaces and based on the depth resolution for RBS, a maximum diffusion coefficient of 2.5×10 −23 m 2/s at 500°C was calculated. Experiments involving pyrrhotite over the temperature range of 950–1100°C showed extensive osmium uptake and osmium concentration gradients that conform with Fickian diffusion behavior. We found that pyrrhotite Fe/S could be varied by changes in the composition of the starting material and osmium source and over the range of Fe/S produced in experiments (molar Fe/S=0.83–0.90), we observed no systematic variation in the osmium diffusion coefficient. Diffusion coefficients measured parallel to the a crystallographic axis were on average ∼1.4× higher than values measured parallel to c and regression of the c-axis data yielded the Arrhenius relation: D(T)=1.31±0.16×10 −5 m 2 / s exp −211.5±14.7 kJ/ mol RT The application of these diffusion data to simple models of diffusive exchange during static or polythermal time–temperature histories is used to assess the conditions under which radiogenic osmium will be retained. During isothermal annealing, calculations indicate that the cores of millimeter-sized spherical pyrrhotite crystals undergoing diffusive exchange with an external osmium reservoir will have their initial compositions perturbed in ≤0.5 Ma at temperatures exceeding 400°C. Pyrite undergoing the same process at 500°C requires in excess of 10 Ma before crystal cores are affected. The relatively short ‘core retention’ time-scales for pyrrhotite indicates that this mineral may be prone to isotopic resetting following relatively brief crustal thermal events, thus possibly accounting for the scatter that commonly occurs in Re–Os isochrons generated from massive sulfide samples. Calculated closure temperatures ( T c) for osmium exchange in pyrrhotite yielded values of 300–400°C for grain sizes ranging from 10 to 1000 μm. These values of T c are similar to those calculated for Ar retention in biotite, and considerably lower than for Sr in apatite and plagioclase, for example. Such low closure temperatures for pyrrhotite suggest this mineral will date the final stage in the cooling of a magmatic system and possibly be susceptible to open system osmium exchange in the presence of late-stage hydrothermal fluids. This latter result infers that caution be applied when interpreting elevated initial osmium isotopic ratios as a product of crustal assimilation at the magmatic stage.