Accessory Pyrite Research Articles

Crystal chemistry and origin of REE-bearing mukhinite from carbonate veins of the Svetlinsky gold deposit, South Urals, Russia

AbstractA rare earth element (REE)-, Cr- and Mg-bearing variety of the vanadium epidote-group mineral mukhinite occurs in a calcite–dolomite carbonatite dyke cutting metamorphosed volcano-sedimentary rocks exposed in the walls of the quarry of the Svetlinsky gold deposit, South Urals. This mineral was found in a paragene assemblage including native sulphur, phlogopite and fluorophlogopite, together with accessory pyrite, other sulfides and sulfosalts, gold, Cr- and V-bearing muscovite, margarite, Cr- and V-bearing dravite, fluoro-tremolite, actinolite, fluoro-pargasite, anhydrite, apatite, uranium hydroxides, V-rich titanite, V- and Nb-rich rutile, spinel and corundum. The contents of ΣREE2O3 and V2O3 in mukhinite vary in the ranges of 4.01–9.69 and 5.34–7.46 wt.%, respectively. A Raman spectrum of REE-rich mukhinite is provided. The main schemes of isomorphic substitutions in mukhinite are ΣREE + Mg ↔ Ca + Al and V+Cr ↔ Al. The crystal structure of REE-rich mukhinite has been studied by single-crystal X-ray diffraction analysis. The mineral is monoclinic, with the space group P21/m, and unit-cell parameters are: a = 8.8972(11) Å, b = 5.6221(6) Å, c = 10.1519(12) Å, β = 115.169° and V = 459.60(11) Å3. The crystal structure of REE-rich mukhinite is similar to that of its synthetic analogue; the refined crystal-chemical formula of the sample studied is (Z = 2): {A1CaA2(Ca0.8REE0.2)}{M1(Al0.95Cr0.05)M2AlM3[(V,Cr)3+0.40Al0.35Mg0.25]}(Si2O7)(SiO4)O(OH).

Specific green zonal silica nodules of serpentinite weathering: unusual products of silicification in laterite-like residuum (Moldanubian Zone, Bohemian Massif)

Massive quartz-chakedony +/- opal nodules (plasma in gemology) represent a specific silica variety, which occurs in the laterite-like residues of pre-Miocene paleo-weathering of ultramafites in western Moravia (Moldanubian Zone, Bohemian Massit). These zonal silica nodules (ZSN) tend to have concentric texture with a dark green to green-brown core, pale green margin and a narrow white rim (outer surface zone). The most typical microscopic feature of ZSN is vermiform microstructure particularly in the two outer zones. Individual mines consist of micro- to non-crystalline SiO2 polymorphs with variable contents of H2O (quartz, chalcedony, moganite, opal-C/CT and opal-A). The predominant green colour is due to submicroscopic smectite pigment, while the brownish colour originated from decomposition of smectite to iron oxohydroxides. ZSN formed in subaerial, partially reducing conditions in the lower part of weathering crusts covering serpentinites. The whole process was preceded by component exchange (chloritization) along serpentinite -felsic rocks (granulite. migmatite, pegmatite veins) boundaries. The gradual silica migration and subsequent redistribution associated with the removal of aluminium, magnesium and iron led up to the formation of a zonal nodular texture dominated by SiO2 polymorphs. Newly formed minerals in micro-cavities and cracks of ZSN are represented by accessory pyrite and sporadic barite. Zonal silica nodules-bearing residues on serpentinites occur only in a narrow area which was originally covered by clay-sandy Miocene sediments of the Carpathian Foredeep in western Moravia. Probably late low-temperature fluid interaction between silicified serpentinite residuum (chlorite montmorillonite saprolite) and marine sediments may be the main factor controlling formation of ZSN.

The “intraorogenic” Svecofennian Herräng mafic dyke swarm in east-central Sweden: age, geochemistry and tectonic significance

ABSTRACTThe Herräng mafic dykes form an E-W-trending dyke swarm within the Bergslagen lithotectonic unit of the Svecofennian orogen in east-central Sweden. They intrude the Svecofennian supracrustal rocks and early-orogenic granitoids, but are themselves cut by late Svecofennian pegmatites, and have undergone Svecofennian amphibolite-facies metamorphism, leading to their classification as “intraorogenic” Svecofennian dykes. They can be assigned an age between 1870 and 1850 Ma, with metamorphism of the dykes dated at 1848 ± 13 Ma by U-Pb in titanite. Their current mineralogy is dominated by metamorphic plagioclase and amphibole, with variable amounts of quartz and biotite, and minor to accessory titanite, apatite, epidote, pyrite, magnetite, ilmenite and zircon. Textures range from massive to strongly foliated. Twenty samples of dyke rocks from three subareas in the Roslagen region, including the Herräng type area, range in composition from basaltic to andesitic with 47 to 60 wt% SiO2, broadly similar to the Dannemora dykes and the Avesta-Östhammar gabbros and diorites. Initial 87Sr/86Sr ratios (at 1870 Ma) varies between 0.7026 and 0.7038, corresponding to initial εSr between +5 and +21, and initial εNd between −0.4 and +1.3, suggesting a slightly enriched to mildly depleted mantle source, similar to other Svecofennian mafic rocks. The dykes dominantly show a calc-alkaline volcanic arc signature related to subduction. They formed during an extensional episode, possibly related to incipient back-arc spreading or subduction roll-back following the main early-orogenic subduction-related Svecofennian magmatism, but penecontemporaneous with amphibolite-facies metamorphism in the area.

Mineralogická charakteristika fosforitové konkrece s rodochrozitem z lokality Tabarky, severní Chřiby

The second occurrence of phosphorite in the Chřiby Mts. was found in a secondary position (pebble from stream gravel) in the northern part of the mountain massif at the Tabarky site. Its original host rock environment were Cretaceous-to-Palaeocene flysch sediments of the Soláň Formation, belonging to the Rača Unit of the flysch belt of the Outer Western Carpathians. Based on bulk chemical composition, the studied phosphorite concretion is formed by ca. 47 wt. % of carbonate-fluorapatite, 31 wt. % of carbonate (rhodochrosite to Fe-rich rhodochrosite), and 21 wt. % of detritic admixture. The grains of carbonate are zoned with increasing Fe/Mn ratio from core to rim. Accessory pyrite with elevated contents (0.X wt. %) of Mn, Ni, Co, Cu, As and Pb as well as very rare sphalerite were also found. Phosphorite is product of early diagenetic processes operating in unconsolidated host deep-sea sediments. The material source of this mineralization was in unstable components of host sediments, which were remobilized by pore fluids under reducing conditions associated with shallow burial. The geochemical signature suggests that material resembling oceanic manganese nodules could have participate during formation of the studied authigenic mineralization.

Hydrothermal alteration products in the vicinity of the Ahırözü kaolin deposits, Mihalıççık-Eskişehir,Turkey

ABSTRACTThe mineralogy, texture and composition of rocks associated with the kaolin deposits in the Ahırözü-Hamidiye-Üçbaşlı area, SE of Mihalıççık, Eskişehir, Turkey, were investigated. In the study area, Triassic, blue-green schists and serpentinized ultramafic rocks are exposed. Kaolinization occurs at the contact between a Triassic granitic intrusion and metamorphic rocks. Textural and mineralogical characteristics were identified by X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) analyses. The effects of hydrothermal alteration on the whole-rock chemical composition were determined by inductively coupled plasma mass spectrometry (ICP-MS) analysis. Homogenization temperature (Th, °C), salinity, (wt.% NaCl) and melting temperature (Tm, °C) were detected by fluid-inclusion analyses.The argillic alteration zone (Zone A) includes kaolinite, smectite, natroalunite and accessory pyrite, hematite and goethite. This paragenesis corresponds to a low–intermediate argillic alteration caused by fluids having neutral to acidic pH, at a temperature of <200°C. The propylitic alteration zone (Zone B) is characterized by epidote, chlorite, illite and pyrite and was caused by fluids with a neutral pH and a temperature of >250°C. The rocks in Zone C are mainly silicified and represent sinter formation in the study area. The rocks in the kaolin deposit are depleted in Rb, Pb and Y and enriched in Cs, U and Sr, and represent the products of epithermal hydrothermal alteration of granitic intrusions emplaced in a volcanic arc.

Pseudobrookite From the Nine Mile Pluton, Marathon County, Wisconsin: An Occurrence From A Plutonic Environment

Abstract Pseudobrookite has been found in vuggy episyenite surrounding a thin vein in granite of the Nine Mile pluton, the youngest and most silicic of four plutons comprising the Wausau Complex. The pseudobrookite occurs as black to dark grey prismatic crystals up to approximately 1.5 mm in length, clustered in radiating sheaves or as single crystals. Electron microprobe analyses show no significant components other than Ti, Fe, and minor amounts of Mn, Nb, and Ta. Major associated minerals are K-feldspar, biotite, and quartz, and accessory ilmenite, rutile, anatase, sphalerite, chalcopyrite, pyrite, arsenopyrite or marcasite replaced by goethite, and molybdenite, cassiterite, fluorapatite, fluorite, monazite, zircon, rhabdophane-(Ce), and bastnasite-(Ce). Small sprays of secondary gypsum crystals are also present. The cassiterite crystals commonly contain inclusions of fluorapatite and monazite-(Ce). This association suggests that the pseudobrookite formed in a high-temperature volatile- and Ti-rich environment (Parodi et al . 1989). Conditions were likely close to a vapor phase environment, consistent with the typical pseudobrookite environments. The ambient pressure was likely somewhat higher, although it is believed that the Nine Mile granite was intruded at a shallow level in the crust (Myers et al . 1984). Hence the conditions of formation of the Nine Mile pseudobrookite probably bore at least some similarities to the more typical volcanic volatile-rich, high-temperature paragenesis in lithophysae of felsic volcanic rocks. The key to the preservation of pseudobrookite from this plutonic environment was the rapid evacuation of all fluid phases during a pressure quench episode.

TRACE ELEMENT SIGNATURE OF PYRITE FROM THE LOS COLORADOS IRON OXIDE-APATITE (IOA) DEPOSIT, CHILE: A MISSING LINK BETWEEN ANDEAN IOA AND IRON OXIDE COPPER-GOLD SYSTEMS?

Although studies have proposed that iron oxide-apatite (IOA) deposits may represent the deeper roots of some Andean iron oxide copper-gold (IOCG) systems, their genetic links remain obscure and controversial. A key question when considering an integrated genetic model is whether a magmatic-hydrothermal fluid that precipitates massive magnetite will continue transporting significant amounts of dissolved Fe, Cu, and Au after IOA precipitation. Here we provide new geochemical data for accessory pyrite from the Los Colorados IOA deposit in the Chilean iron belt that confirm the role of this sulfide as a relevant repository for economic metals in IOA deposits. Pyrite occurs at Los Colorados as disseminated grains and as veinlets associated with magnetite and actinolite that postdate the main igneous magnetite stage. Electron probe microanalysis (EPMA) data for pyrite show anomalously high Co and Ni concentrations (up ~3.9 and ~1.5 wt %, respectively) and relatively high As contents (100s of ppm to a maximum of ~2,000 ppm). When combined with results from secondary ion mass spectrometry (SIMS) spot analyses, pyrite data show significant amounts of Cu that range from sub-ppm values (~100 ppb) up to 1,000s of ppm, plus nonnegligible concentrations of Zn, Pb, Cd, Sb, Se, and Te (up to ~100 ppm). The highest contents of Cu measured (wt % level) most likely record the presence of Cu-bearing submicron-sized mineral inclusions. Contents of Au and Ag are up to ~1 and 10 ppm, respectively, with maximum concentrations that can rise up to ~800 ppm Au and ~300 ppm Ag due to the presence of submicron-sized inclusions. The high Co/Ni ratios of pyrite from Los Colorados are consistent with a magmatic-hydrothermal origin associated with a greater mafic affinity, compared to pyrite from porphyry Cu deposits. Furthermore, the geochemical signature of Los Colorados pyrite shares important similarities of composition and microtexture with the few published data for pyrite from IOCG deposits (e.g., Ernest Henry, Australia, and Manto Verde, Chile). These findings, combined with recent geochemical and isotopic studies that support an igneous origin for the dike-shaped magnetite orebodies at Los Colorados, point to a magmatic source of mafic to intermediate composition for the contained metals, and support the hypothesis that IOA systems can source Fe-Cu-Au-rich fluids. Based on experimental studies, these IOA-derived fluids may continue transporting significant amounts of metals to form IOCG mineralization at shallower levels in the crust.

Nickeliferous pyrite tracks pervasive hydrothermal alteration in Martian regolith breccia: A study in NWA 7533

AbstractMartian regolith breccia NWA 7533 (and the seven paired samples) is unique among Martian meteorites in showing accessory pyrite (up to 1% by weight). Pyrite is a late mineral, crystallized after the final assembly of the breccia. It is present in all of the lithologies, i.e., the fine‐grained matrix (ICM), clast‐laden impact melt rocks (CLIMR), melt spherules, microbasalts, lithic clasts, and mineral clasts, all lacking magmatic sulfides due to degassing. Pyrite crystals show combinations of cubes, truncated cubes, and octahedra. Polycrystalline clusters can reach 200 μm in maximum dimensions. Regardless of their shape, pyrite crystals display evidence of very weak shock metamorphism such as planar features, fracture networks, and disruption into subgrains. The late fracture systems acted as preferential pathways for partial replacement of pyrite by iron oxyhydroxides interpreted as resulting from hot desert terrestrial alteration. The distribution and shape of pyrite crystals argue for growth at moderate to low growth rate from just‐saturated near neutral (6 < pH<10), H2S‐HS‐rich fluids at minimum log fO2 of >FMQ + 2 log units. It is inferred from the maximum Ni contents (4.5 wt%) that pyrite started crystallizing at 400–500 °C, during or shortly after a short‐duration, relatively low temperature, thermal event that lithified and sintered the regolith breccias, 1.4 Ga ago as deduced from disturbance in several isotope systematics.

High tenor Ni–PGE sulfide mineralization in the South Manasan ultramafic intrusion, Paleoproterozoic Thompson Nickel Belt, Manitoba, Canada

The South Manasan ultramafic intrusion, located in the Early Proterozoic Thompson Nickel Belt (TNB), contains Ni and platinum group element (PGE) mineralization in the form of disseminated sulfide. Whole-rock Ni values range from 0.3 to 1.7wt.% and total precious metals range from 0 to 1.3ppm Pt+Pd+Au. In 100% sulfide the concentration of Ni ranges from 11 to 39wt.%, and the concentration of Pt+Pd+Au ranges from 8 to 27ppm. The intrusion is a steeply dipping, boudinaged, sill-like body with a strike length of approximately 1200m, an average width of 125m, and a depth extent of >1000m. The intrusion is composed of ~25% fresh dunite, ~50% serpentine-altered dunite and ~25% tectonized and carbonate altered dunite. The most intense alteration is found near the margin of the intrusion where it is in contact with Pipe Formation metasedimentary rocks of the Ospwagan Group. The sulfide assemblage in fresh dunite is interstitial between cumulus olivine and is composed predominantly of pentlandite with accessory pyrite with symplectic intergrowths of the two same sulfides. The interstitial sulfide assemblage displays a primary magmatic position between olivine grains, but the sulfides are penetrated by platy intergrowths of chlorite and serpentine. In contrast, the most intensely carbonate altered dunite contains irregular patches of pyrrhotite and pentlandite.Representative drill core samples through the South Manasan Intrusion have been used to establish variations in whole rock geochemistry and mineralogy; these variations coupled with the stratigraphy of the host sedimentary rocks are consistent with magmatic differentiation in a sill-like body which was serpentinized and then tilted upright such that tops now face west. Intrusion of the original komatiitic magma and assimilation of sulfur from Ospwagan Group sedimentary rocks triggered sulfide saturation and formation of an immiscible sulfide melt. The early crystallization of olivine inhibited the sulfide melt from settling to the bottom of the magma column and, as a consequence, the sulfides now have a primary interstitial magmatic texture. Calculated Ni and PGE tenor values indicate an R factor of 500–2500. The current sulfide association dominated by pentlandite≫pyrite>chalcopyrite has a mineral paragenesis that is consistent with crystallization from a monosulfide solid solution which cooled down to pentlandite and pyrite. The subsequent processes of serpentinization, deformation and carbonate alteration modified the primary sulfide assemblages to pentlandite and pyrrhotite. The alteration of the dunite and serpentine to talc-carbonate occurred by open-system hydrothermal processes that produced a reduction in metal tenor through a dilution process accompanying the addition of carbonate and water.It is proposed that the enrichment of the sulfide assemblage at South Manasan in Ni and PGEs is a consequence of a primary magmatic process involving a high-R factor and that the effects of later overprinting processes (alteration and deformation) are not responsible for the high-tenor sulfide association. The study has important exploration implications, as it helps to establish the characteristics of a new type of Ni–PGE mineralization in the TNB, and thus provides a basis on which to better understand the formation of other low-grade ultramafic-hosted nickel deposits.

Evolution of porosity and geochemistry in Marcellus Formation black shale during weathering

Soils developed on the Oatka Creek member of the Marcellus Formation in Huntingdon, Pennsylvania were analyzed to understand the evolution of black shale matrix porosity and the associated changes in elemental and mineralogical composition during infiltration of water into organic-rich shale. Making the reasonable assumption that soil erosion rates are the same as those measured in a nearby location on a less organic-rich shale, we suggest that soil production rates have on average been faster for this black shale compared to the gray shale in similar climate settings. This difference is attributed to differences in composition: both shales are dominantly quartz, illite, and chlorite, but the Oatka Creek member at this location has more organic matter (1.25wt.% organic carbon in rock fragments recovered from the bottom of the auger cores and nearby outcrops) and accessory pyrite. During weathering, the extremely low-porosity bedrock slowly disaggregates into shale chips with intergranular pores and fractures. Some of these pores are either filled with organic matter or air-filled but remain unconnected, and thus inaccessible to water. Based on weathering bedrock/soil profiles, disintegration is initiated with oxidation of pyrite and organic matter, which increases the overall porosity and most importantly allows water penetration. Water infiltration exposes fresh surface area and thus promotes dissolution of plagioclase and clays. As these dissolution reactions proceed, the porosity in the deepest shale chips recovered from the soil decrease from 9 to 7% while kaolinite and Fe oxyhydroxides precipitate. Eventually, near the land surface, mineral precipitation is outcompeted by dissolution or particle loss of illite and chlorite and porosity in shale chips increases to 20%. As imaged by computed tomographic analysis, weathering causes i) greater porosity, ii) greater average length of connected pores, and iii) a more branched pore network compared to the unweathered sample.This work highlights the impact of shale–water–O2 interactions in near-surface environments: (1) black shale weathering is important for global carbon cycles as previously buried organic matter is quickly oxidized; and (2) black shales weather more quickly than less organic- and sulfide-rich shales, leading to high porosity and mineral surface areas exposed for clay weathering. The fast rates of shale gas exploitation that are ongoing in Pennsylvania, Texas and other regions in the United States may furthermore lead to release of metals to the environment if reactions between water and black shale are accelerated by gas development activities in the subsurface just as they are by low-temperature processes in our field study.

Natural CO2 accumulations in the western Williston Basin: A mineralogical analog for CO2 injection at the Weyburn site

The Devonian carbonates of the Duperow Formation on the western flank of the Williston Basin in southwest Saskatchewan contain natural accumulations of CO2, and may have done so for as long as 50 million years. These carbonate sediments are characterized by a succession of carbonate cycles capped by anhydrite-rich evaporites that are thought to act as seals to fluid migration. The Weyburn CO2 injection site lies 400km to the east in a series of Mississippian carbonates that were deposited in a similar depositional environment. That long-term isolation of natural CO2 can be accomplished within carbonate strata has motivated the investigation of the Duperow rocks as a potential natural analog for storage of anthropogenic CO2 in carbonate lithologies. For the Duperow strata to represent a legitimate analog for Midale injection and storage, the similarity in lithofacies, whole rock compositions, mineral compositions and porosity with the Midale Beds must be established. Here we compare lithofacies, whole rock compositions, mineralogy and mineral compositions from both locales. The major mineral phases at both locales are calcite, dolomite and anhydrite. In addition, accessory pyrite, fluorite, quartz and celestine (strontium sulfate) are also observed. Dawsonite, a potential CO2-trapping mineral, is not observed within the CO2-bearing horizons of the Duperow Formation, however. The distribution of porosity in the Midale Vuggy units is similar to that of the Duperow Formation, but the Marly units of the Midale have significantly higher porosity. The Duperow Formation is topped by the Dinesmore evaporite that is rich in anhydrite, and often contains authigenic K-feldspar. The chemistry of dolomite and calcite from the two localities also overlaps. Silicate minerals are in low abundance (<3%) within the analyzed Duperow samples, with quartz and K-feldspar the only silicates observed petrographically or in X-ray diffraction patterns. The Midale Beds contain significantly higher silica/silicate concentrations (Durocher et al., 2003), but the paucity of mono- and divalent cations that can be derived from dissolution of these silicate minerals likely precludes significant carbonate mineral formation. Hence, physical and solution trapping are likely to be the primary CO2 trapping mechanisms at both sites.

Factors influencing the release rate of uranium, thorium, yttrium and rare earth elements from a low grade ore

This paper presents data from laboratory leaching of a mineralogically complex low grade ore containing uranium (U), thorium (Th), yttrium (Y), rare earth elements (REEs) and accessory pyrite. The study examines the influence of varying the leaching protocol on the rate of release of U, Th, Y and REEs. Leaching protocols were designed to simulate a range of heap/in situ stope leaching scenarios. Protocol variants included flushing frequency, leachate recycle, nutrient (9K salts) addition and ferric sulphate addition to cells. Maximum extractions over 52weeks were: 58% U, 50% Th, 36% Y and 45% of the REEs; observed in cells flushed fortnightly with a ferric sulphate lixiviant (0.5g/l, pH 3.5). Flushing with tap water once monthly resulted in second highest observed extractions (57% U, 6% Th, 27% Y, 37% REEs) with the exception of Th, where the lower release was due to insufficiently low pH for Th mobilisation. Pourbaix plots indicate that redox potentials within the leaching system were buffered by coffinite dissolution. The high redox potentials, acidic pH and elevated sulphate concentrations required for metals dissolution were found to be a consequence of (a) direct addition of ferric sulphate as a lixiviant or (b) in situ generation of acidity and ferric sulphate via microbially-mediated oxidation of accessory pyrite. Generation and maintenance of acidic sulphate-rich interstitial water appear to control the dissolution of Th, Y and REEs in addition to U. The presented data highlight that significant U, Th, Y and REEs can be mobilised from rock materials by the addition of a ferric sulphate lixiviant, furthermore periodic flushing with water alone is sufficient to achieve similar release of U, Y and REEs where accessory pyrite is available in the reacting system. These findings have significant implications for recovery of valuable elements from low grade and marginal ores or materials previously considered as wastes.

A Devonian to Carboniferous intra-oceanic subduction system in Western Junggar, NW China

The Devonian and Carboniferous rock assemblages in Western Junggar, southern Altaids, include ophiolitic mélanges and coherent units. At Baijiantan, Yeyagou and Dagun Devonian gabbros and one group of volcanic rocks (type I lava) generally crop out as blocks in a matrix of ultramafic rocks and/or mudstone. They have a depleted light rare earth element (LREE) and mid-oceanic ridge basalt (MORB)-like signature with a small negative Nb anomaly, suggesting formation in a back-arc basin. Another group of volcanic rocks (type II lava) from Baijiantan, Yeyagou, Western Karamay and Maliya includes alkaline basalt, basanite, andesite, dacite and trachyte that contain microphenocrysts of clinopyroxene–plagioclase, amygdales filled with chalcedony and calcite, and accessory pyrite and ilmenite. The type II lavas are further subdivided into five sub-types: (1) Type II1 lava is strongly enriched in LREE with incompatible trace element ratios similar to oceanic island basalt (OIB), suggesting derivation from enriched mantle. (2) Type II2 lava and tuff are slightly LREE-enriched, with a marked negative Nb anomaly and Th/Yb-enrichment indicating that they were generated in a supra-subduction zone (SSZ). (3) Type II3 lava is also moderately enriched in LREE suggesting derivation from a more-enriched MORB mantle. (4) Type II4 lava is enriched in LREE and has high Nb/Yb values close to those of OIB. It is extensively depleted in Nb suggesting a source containing a mixture of enriched and SSZ mantle. (5) Type II5 lava has MORB-like REE patterns, and Nb/Yb and Th/Yb ratios. Based on field structural data, and the contemporaneity of the Baogutu adakite, a high-Mg diorite dike and the Miaoergou charnockite, these enriched and depleted magmatic features suggest generation in an intra-oceanic subduction zone influenced by ridge-trench interaction. The Altaids in Western Junggar are characterized by multiple intra-oceanic subduction systems, which may have continued to operate to the late Carboniferous.

Meteoric lithification of catastrophic rockslide deposits: Diagenesis and significance

Deposits of catastrophic rockslides composed of lithologies rich in carbonate minerals may undergo precipitation of cements that can be used to proxy-date the rockslide event and/or subsequent geomorphic changes of the rockslide mass. In the Alps, localized to widespread lithification of post-Glacial rockslide deposits is observed in lithologies ranging from limestones and dolostones to metacarbonates to calcphyllites. Lithification of rockslide deposits to breccias may be localized to meteoric ‘runoff-shadows’ below larger boulders, or may comprise a layer of breccia or may affect a rockslide mass down its base. In addition, precipitation of cements and small stalactites may take place in megapores on boulder undersides. Cements found in rockslide deposits comprise skalenohedral calcite, prismatic calcite, blocky calcite, calcitic micrite and micropeloidal calcitic cement and, rarely, botryoidal aragonite. Initial cement formation probably is driven by meteoric dissolution–re-precipitation of (mini-) micritic abrasive rock powder generated by dynamic disintegration during the rockslide event. Preliminary 234 U/ 230 Th ages of rockslide cements support a concept that cementation starts immediately or early after a rockslide event. In rockslide deposits of calcphyllite with accessory pyrite, oxidation of pyrite probably also propels the process of carbonate dissolution–re-precipitation. Limestone-precipitating springs emerging from rockslide masses, and well-cemented talus slopes and fluvial conglomerates percolated by rockslide-derived groundwaters, indicate that rockslide deposits remain diagenetically active long after emplacement.

10.1007/s11720-008-1003-y

More than 200 analyses of the sulfur isotopic composition of sulfides from various terrigenous and intrusive host rocks, metasomatically altered wall rocks, and gold lodes of the Upper Kolyma region are presented. In accessory pyrite of the metaterrigenous rocks, δ34S varies from −23.1 to +5.7‰ δ34S of pyrite and arsenopyrite from gold-quartz mineralization is within the range −10.6 to −0.4‰ and is close to the average δ34S of pyrite from the metaterrigenous rocks (−4.4‰). In the intrusive rocks, δ34S of pyrite varies from −3.8 to +2.6‰ (+0.7‰, on average) and drastically differs from δ34S of arsenopyrite from postmagmatic gold-rare-metal mineralization (−7.9 to −2.7‰; −5.2‰, on average). The comparison of the δ34S of accessory sulfides from the host rocks with δ34S of sulfides from the gold deposits suggests that sulfur mobilized from the terrigenous sequences participated in the hydrothermal process. The results obtained are consistent with the metamorphic model of the formation of gold-quartz deposits in the Upper Kolyma region.

Abundance and distribution of platinum-group elements in orogenic lherzolites; a case study in a Fontete Rouge lherzolite (French Pyrénées)

Orogenic lherzolites exhibit a specific (although reproducible) platinum-group element signature characterized by slight positive deviations of Pd/Ir, Rh/Ir and Ru/Ir ratios from the canonical chondritic model. Such a signature was alternatively considered to be a compositional feature of the primitive upper mantle or resulting from sulfide melt addition by refertilisation reactions in the continental lithosphere. To shed additional light on this conundrum, the distribution of PGE in FON B 93, an unserpentinized, fertile (3.24% Al 2O 3) orogenic lherzolite (French Pyrenees) used as in-house standard by our group, has been studied to different scales, from the bulk rock to trace minerals. In addition to new ICPMS analyses after separation into NiS and Te coprecipitation, its whole-rock PGE concentrations were redetermined by three different laboratories using very precise ID-ICP-MS methods after digestion of the sample in Carius tube at T = 250 °C and 320 °C or in high-pressure asher (HPA-S). The four methods produce reproducible Ru, Rh and Pd contents (7.1 ± 0.18 ppb; 1.43 ± 0.05 ppb; 7.1 ± 0.30 ppb) whereas the non-ID NiS-fire assay method underestimates the Os and Ir concentrations by c.a. 10 and 15% compared to ID-ICP-MS analyses (4.40 ± 0.07 and 4.00 ± 0.17 ppb, respectively). Platinum was the most difficult to analyse. If performed on powder aliquots smaller than 3 g, the ID-ICP-MS analyses generate strong nugget effects while the non-ID NiS-fire assay method yields statistically lower (but highly reproducible) Pt concentrations (6.92 ± 0.26 ppb). These features reflect the strong partitioning of Pt into trace phases that SEM and laser ablation-ICP-MS analyses on thin sections identify as both high-temperature Pt–Ir–Os alloys and Pt–(Pd) tellurides of likely subsolidus origin; ten to fifteen grains ranging in maximum dimension from a few micrometres to a few hundreds of nanometers, were identified by standard polished thin section. LA-ICP-MS data on base metal sulfides (25 grains analysed), coupled with the whole-rock S concentration (277 ± 10 ppm) and modal composition of the BMS (90% pentlandite + accessory pyrite and secondary pyrrhotite + 10% chalcopyrite) allowed the contribution of the BMS phase to the PGE budget of FON B 93 to be estimated. Except Pt that exhibits a 95% deficit in the BMS phase, the PGE concentrations measured by ID-ICP-MS can be balanced by BMS while Al-spinel is a negligible contributor, accounting for less than 0.5% of the Ru budget. The occurrence of Pt in trace phases may bias the whole-rock Pt concentrations because 1) mechanical collection of Pt-rich trace phases remains problematic with the NiS button, and 2) Pt–Ir–Os alloys may be prone to digestion problems in conventional Carius tube procedures. Since they could be stable at mantle depth, such refractory alloys that contain Os and Ir may also have enhanced the heavy PGEs/light PGEs fractionation. Our observations likely pertain to orogenic lherzolites as a whole because BMS assemblages in these mantle rocks record evolution at low sulfur fugacity, which prevents Pt–Ir–Os alloy from entering the Mss in the mantle; moreover, at subsolidus temperature, pentlandite, the major BMS, cannot accommodate 0 valence state Pt.

Oxidation of the arsenic-rich concentrate at the Přebuz abandoned mine (Erzgebirge Mts., CZ): mineralogical evolution

Ore concentrate with up to 65 wt.% of arsenic (by-product of cassiterite extraction) exposed to climatic conditions was studied from the mineralogical point of view. Detailed sampling, X-ray diffraction analyses, energy-dispersive microanalysis (EDAX) and especially scanning electron microscopy (SEM) were applied to study the arsenopyrite–löllingite-concentrate weathering. The studied concentrate contains very small proportion (<5 vol.%) of gangue minerals such as quartz and feldspars; the oxidation of arsenopyrite and löllingite (and accessory pyrite) is thus practically not complicated by interference with additional minerals and elements. Arsenolite, scorodite, kaatialaite and native sulphur were found to be the main secondary phases originating by dissolution of arsenopyrite and löllingite. New secondary phases precipitate on the surface of the ore-concentrate body but also form cement among the grains of finely milled material. The following succession of secondary minerals was determined: arsenolite, scorodite+native sulphur and kaatialaite. Significant arsenic migration into the proximal environment was revealed: 2580 and 13 622 mg kg −1 were the highest arsenic concentrations in two sections excavated at distances of 0.5 and 1.5 m from the concentrate body.

Geology and genesis of Variscan porphyry-style gold mineralization, Petr�?kova hora deposit, Bohemian Massif, Czech Republic

A large number of Variscan mesothermal gold deposits are located in the central part of the Bohemian Massif, close to the Central Bohemian Plutonic Complex. The Petrackova hora deposit has many features that distinguish it from other deposits in the region and suggest its mineralization is closely related to the late magmatic processes associated with the Petrackova hora granodiorite. The gold ores occur as sheeted arrays of quartz veins and veinlets hosted by the small Petrackova hora granodiorite stock. Gold is found mainly as free grains of >900 fineness, and is accompanied by abundant pyrrhotite and chalcopyrite, and accessory pyrite, arsenopyrite, loellingite, and molybdenite. Molybdenite from the Petrackova hora deposit has been dated by the Re–Os method at 344.4±2.8 Ma. Hydrothermal alteration in the Petrackova hora deposit exhibits a distinct temporal paragenesis. Selectively pervasive, early K-alteration and silicification are the oldest hydrothermal phases. These were followed by early quartz veins (Q1 to Q4) that contain most of the gold mineralization. Late quartz veins (Q5) and fracture-controlled silicification are gold-poor or barren. Barren calcite veins are the youngest hydrothermal product. Extensive low-temperature, meteoric-water dominated alteration, as is typical of classic porphyry deposits, is absent. However, the lower δ18O whole rock values for Petrackova hora granodiorite and aplite (+2.4 to +5.1‰ SMOW) compared to other intrusions in the region reflect either interaction with isotopically light external fluids or magma assimilation of small volumes of hydrothermally altered country rock. The δ18O isotopic compositions for quartz, scheelite and hornblende (7.7 to 13.4‰ SMOW) and the δ34S compositions for sulfide minerals (–1 to +3.5‰ CDT) from early, gold-rich quartz veins indicate formation at high temperatures (590 to 400 °C) from fluids with a magmatic isotopic signature (δ18OFLUID of 5.7 to 7.2‰). Fluids related to late quartz veins (Q5) suggest the presence of a significant component of non-magmatic water (δ18OFLUID: +2.5 to +4.0‰). The δ34S values of post-Q5 sulfide minerals (–4.5 to –3.5‰) reflect at least partial derivation of late-stage sulfur from a source external to the intrusions. Aqueous, aqueous–carbonic and nitrogen-bearing fluid inclusions were identified in hydrothermal and igneous quartz, with the aqueous inclusions being the most common. In hydrothermal vein quartz, the salinity of primary aqueous inclusions falls into ranges 6 to 23 and 33 to 41 equiv. wt% NaCl; in igneous quartz, populations in salinity were observed between 5 to 16, 35 to 40 and 62 to 70 equiv. wt% NaCl. The salt component of these fluids is best, and minimally, approximated by the NaCl–KCl–CaCl2 system. Low- and high-salinity aqueous–carbonic inclusions are accessory in many of the analyzed samples. Three large successive pulses of fluids are recognized. Each pulse begins with a high-salinity (>30 equiv. wt% NaCl) magmatic fluid and evolves toward a lower salinity (~5 equiv. wt% NaCl) fluid. Data suggest that external (meteoric?) water(s) were significant for only the third fluid pulse, which formed the late Q5 quartz veins and the calcite veins. Polyphase fluid inclusions hosted by igneous quartz of the Petrackova hora granodiorite indicate minimum trapping conditions of about 3 kbar and 550 °C. The gold-rich Q1 to Q4 veins may have formed along a quasi-isobaric cooling path at 2.5 to 1.5 kbar and 590 to 400 °C. This was followed by uplift, and formation of late Q5 quartz veins (0.5 to 1.5 kbar; ~300 °C) and post-ore calcite veins (<0.5 kbar; 100 to 140 °C). The characteristics of the Petrackova hora deposit suggest that it may represent a position intermediate between intrusion-related gold systems (e.g., Fort Knox deposit, Alaska) and gold-rich, copper-poor porphyry deposits (e.g., Maricunga Belt in Chile). As such, the Petrackova hora deposit might be an example of the reduced gold sub-type of porphyry deposit.

Solute generation and transfer from a chemically reactive alpine glacial-proglacial system

The environs of the Glacier de Tsanfleuron, Switzerland, was used as a study site to investigate the controls on the relative efficiency of solute generation and removal from glacial and proglacial environments. Here, a 1500 m wide glacier forefield consists of a karstic limestone plateau flanked to the north by a till-floored valley. Bedrocks and glacial debris are composed of chemically reactive pure and impure Mesozoic and Tertiary limestones with accessory pyrite. Spot sampling of ice, snow and meltwaters in the late melt season was supplemented by systematic measurements of the main meltstream, including during periods of rainfall, and simple laboratory leaching and weathering experiments. Isotopic parameters were used to investigate water sources. Most meltwater and glacier ice samples lay close to a meteoric water line (δ D= 8·3 δ18O + 14) defined by waters from small tributary streams. Heavy isotopic excursions of bulk meltwater chemistry were caused by rainfall events, recovering within days to a δ18O baseline around −12 permil. No regular diurnal variations in δ18 O were apparent. The atmosphere is the source of Cl− and most Na+, but the bulk of other solutes are generated in the environment. Ion loads of up to 1 meq l−1 are rapidly attained by calcite dissolution. Over periods of weeks to months pyrite oxidation generates sulphate and acidity that drives further calcite dissolution. Low water–rock ratio weathering environments have characteristically high SO42−, Mg2+, Sr2+, and ratios of these species to calcium. The characteristic cation ratios are influenced by non-congruent calcite dissolution. The ratio of sulphate to other species is highest where water–rock contact times are highest, although this relationship is complicated by spatial variations in pyrite abundance. Meltstream time series illustrate that 90 per cent of daily ion yields in fine weather are concentrated in the 12 h time period of higher discharge. Ion yields increase downstream mainly by a combination of dissolution of calcareous suspended sediment and input from tributaries and seepage from the till banks. Rainstorms lead to increased solute concentrations and resulting hourly fluxes can match daily fine weather ion fluxes. Excess nitrate appears to be largely sourced from the proglacial surface. The capacity of the proglacial environment for yielding significant subsurface water as a result of the storm seems low, unlike non-glacial environments. This implies that most of the excess solutes mobilized by storms comes from subglacial sources. Increased efficiency of yield of solutes from low water/rock ratio subglacial weathering environments persists after the isotopic signature of the rainfall event has died away. A simple conceptual model of the sources of water and solutes agrees with conclusions from attempts at hydrograph separation, that mixing of water reservoirs of fixed solute composition cannot be used for quantitative descriptions of the system. Estimated annual solute yields (17 ton per km2 per m precipitation) are high, but cannot be readily expressed in purely areal terms because of likely significant losses to the underlying karstic system. A tentative conclusion is that the proglacial environment is overall less efficient at producing solutes than the glacial environment, but more information is required on processes in the early melt season to substantiate this statement. Copyright © 1999 John Wiley & Sons, Ltd.

The Nevoria gold skarn deposit in Archean iron-formation, Southern Cross greenstone belt, Western Australia; I, Tectonic setting, petrography, and classification

The Nevoria gold skarn deposit is located in the Southern Cross belt of the Archean Yilgarn craton, Western Australia, and occurs in amphibolite facies greenstones between two dome-shaped granitoid batholiths. Regional pressure estimates suggest that the terrane is deeply eroded and that the skarn formed at a 10- to 15-km depth. The orebodies are confined to the folded contacts of three beds of silicate facies iron-formation and extend over a strike length of more than 1 km. Iron-formation-hosted skarn contains 6 to 7 g/t gold, 2 to 3 g/t silver, and less than 0.1 wt percent base metals. The gangue is calcic, highly reduced, and composed of two distinct mineralogic types: hedenbergite-actinolite and almandine-hornblende skarn. The calc-silicates are intergrown with abundant pyrrhotite (10 vol %) and with accessory (< or =0.5%) pyrite, arsenopyrite-loellingite, and chalcopyrite. Native gold is enclosed in hedenbergite, actinolite, almandine, and quartz and occurs together with the alloy maldonite (Au 2 Bi) and a suite of bismuth tellurides. Skarns replacing amphibolite and metakomatiite adjacent to calcic skarn in banded iron-formation form broad (up to 17 m) contact zones of outer biotite-rich alteration. These zones are composed of numerous alternating bands of reduced diopside-hornblende, almandine-hornblende, and almandine-cummingtonite-plagioclase skarn. Veins of oxidized grossular-diopside skarn occur in the amphibolites between and adjacent to the banded iron-formations. The skarns are cut by barren pegmatite dikes and are underlain by a massive pegmatite-granite pluton. These postskarn intrusions, in turn, are displaced by brittle-ductile faults, which control zones of mesothermal muscovite-chlorite-calcite alteration replacing amphibolite. These zones lack pyrite and gold mineralization. The Nevoria gold skarn is part of a larger group of reduced skarn deposits, which formed in a deep midcrustal environment (10-15 km) during late Archean magmatism, possibly in broad continental arcs at convergent plate margins. The Archean skarns are distinct from the copper-gold skarns in Cenozoic continental margins and from gold-rich iron-copper skarns in Cenozoic to Mesozoic island-arc terranes, which all formed at much shallower depths of 2 to 5 km.