Showing papers in "Canadian Mineralogist in 2005"

The classification of granitic pegmatites revisited

Abstract: The classification of granitic pegmatites was frequently attempted during the past century, with variable degrees of success and applicability. Internal structure, paragenetic relationships, bulk chemical composition, petrogenetic aspects, nature of parent medium, and geochemical features were applied. However, all schemes were marked by contemporary degrees of understanding of these parameters, and most attempts were hindered by ignoring differences in geological environment. Substantial progress was achieved only since the late 1970s. The classification is approached here from two directions, based on but broadened and refined from earlier works by Ginsburg and Cerný. The first concept deals with geological location, leading to division of granitic pegmatites into five classes (abyssal, muscovite, muscovite – rare-element, rare-element, and miarolitic), most of which are subdivided into subclasses with fundamentally different geochemical (and in part geological) characteristics. Further subdivision of most subclasses into types and subtypes follows more subtle differences in geochemical signatures or P–T conditions of solidification, expressed in variable assemblages of accessory minerals. The second approach is petrogenetic, developed for pegmatites derived by igneous differentiation from plutonic parents. Three families are distinguished: an NYF family with progressive accumulation of Nb, Y and F (besides Be, REE, Sc, Ti, Zr, Th and U), fractionated from subaluminous to metaluminous A- and I-type granites that can be generated by a variety of processes involving depleted crust or mantle contributions; a peraluminous LCT family marked by prominent accumulation of Li, Cs and Ta (besides Rb, Be, Sn, B, P and F), derived mainly from S-type granites, less commonly from I-type granites, and a mixed NYF + LCT family of diverse origins, such as contamination of NYF plutons by digestion of undepleted supracrustal rocks.

U6+ minerals and inorganic compounds: insights into an expanded structural hierarchy of crystal structures

Abstract: The crystal structures of uranyl minerals and inorganic uranyl compounds are important for understanding the genesis of U deposits, the interaction of U mine and mill tailings with the environment, transport of actinides in soils and the vadose zone, the performance of geological repositories for nuclear waste, and for the development of advanced materials with novel applications. Over the past decade, the number of inorganic uranyl compounds (including minerals) with known structures has more than doubled, and reconsideration of the structural hierarchy of uranyl compounds is warranted. Here, 368 inorganic crystal structures that contain essential U6+ are considered (of which 89 are minerals). They are arranged on the basis of the topological details of their structural units, which are formed by the polymerization of polyhedra containing higher-valence cations. Overarching structural categories correspond to those based upon isolated polyhedra (8), finite clusters (43), chains (57), sheets (204), and frameworks (56) of polyhedra. Within these categories, structures are organized and compared upon the basis of either their graphical representations, or in the case of sheets involving sharing of edges of polyhedra, upon the topological arrangement of anions within the sheets.

Carbonatites and carbonatites and carbonatites

Abstract: Le clan des carbonatites est ici redefini dans le contexte d'une classification mineralogique et genetique, et subdivise en deux groupes: carbonatites primaires et residus carbothermaux. On souligne le fait que "carbonatite" est a la fois un terme petrographique applicable a un type de roche particulier, et un terme faisant reference a un groupe de roches, et applique a des roches carbonatees and silicatees genetiquement liees dans un complexe magmatique ou extrusif. On peut diviser les carbonatites primaires, en termes de classifications mineralogiques et genetiques plutot que de simples classifications modales, en un groupe de carbonatites veritablement magmatiques formees a partir de divers magmas mantelliques, i.e., carbonatites associees aux clans de melilitite, nephelinite, aillikite et kimberlite, avec ces dernieres manifestations preferablement appelees kimberlites a calcite. Chaque type de magma et les carbonatites associees seraient genetiquement distincts, et formees a des profondeurs differentes du manteau superieur selon des taux distincts de fusion partielle. Les carbonatites associees aux suites de melilitite et de nephelinite peuvent avoir une multiplicite d'origines, impliquant possiblement fusion fractionnee, cristallisation fractionnee ou immiscibilite liquide. Les kimberlites a calcite seraient des differencies tardifs a volume limite qui ne sont pas apparentees aux autres carbonatites ou a leur magmas parents. L'origine et les relations genetiques de la natrocarbonatite de Oldoinyo Lengai ne peuvent pas etre precisees sans ambiguite, quoique ces roches sont considerees comme variete distincte de carbonatite primaire. Les roches riches en carbonates associees aux divers complexes hyperalcalins potassiques ou sodiques satures ou sous-satures et derives surtout a partir du manteau lithospherique ayant subi une metasomatose, de meme que les roches enrichies en carbonates de terres rares, d'origine indeterminee, seraient plutot des residus carbothermaux que des carbonatites. Il pourrait y avoir convergence mineralogique (ou modale) entre de telles roches et les produits veritables a faible pression de carbonatites primaires enrichis en terres rares. On ne devrait pas inclure les roches riches en carbonates formees par reactions pneumatolytiques ou par anatexie de roches de la croute dans le clan des carbonatites.

Crystallographic controls on trace-element incorporation in auriferous pyrite from the pascua epithermal high-sulfidation deposit, chile–argentina

Abstract: The distribution of gold and other trace elements in the ore-stage pyrite from the high-sulfi dation Au–Ag–Cu Pascua deposit in the El Indio belt in north-central Chile was studied using electron-microprobe and secondary-ion mass spectrometry (SIMS) imaging and analysis. The images show that concentrations of Au and Cu are concentrically zoned, and those of Ag, As, Se and Te are zoned both concentrically and sectorally. The principal trace-element associations are As–Ag, Au–Cu and Se–Te. We propose that the fi rst two associations, As–Ag and Au–Cu, refl ect coupled substitutions for Fe, whereas Te and Se replace S by direct-ion exchange. The near-identical patterns of sector zoning of Ag and As suggest that silver enters the structure of pyrite via a coupled substitution with As, in which one atom of Ag and one of As substitute for two Fe atoms, yielding (Ag + 0.5As 3+ 0.5)S2. Gold and copper were excluded during an interval in which growth conditions promoted the development of | hk0 | sectors at the expense of | 111 | sectors; their enrichment patterns show only slight evidence of sectoral preferences. With the analytical methods employed, we could not distinguish unequivocally between incorporation of gold as minute inclusions of the native metal or a coupled substitution of gold as an Au 3+ ion and copper as a Cu + ion for two Fe 2+ ions, yielding (Au 3+ 0.5Cu + 0.5)S2. However, we favor the latter interpretation. Incorporation of gold as Au 3+ is also consistent with evidence of unusually oxidizing conditions during the formation of the Pascua deposit. The zonation of invisible gold and associated trace elements in the ore-stage pyrite at Pascua is the fi rst documented example of crystallographic structural control of the surface on the incorporation of multiple trace elements in pyrite. Moreover, the nature of this zonation provides clear evidence that incorporation of gold in pyrite does not necessarily involve coupled substitution with arsenic, as proposed for many other deposits. Not only can arsenic behave as a metal in the structure of pyrite, but its sectoral pattern of incorporation is a potentially sensitive indicator of redox conditions during mineralization.

Carbonated serpentinite (listwanite) at atlin, british columbia: a geological analogue to carbon dioxide sequestration

Abstract: Listwanite, a carbonate-altered serpentinite commonly associated with gold and mercury mineralization, also represents a natural analogue to CO2 sequestration via in situ carbonation of minerals. The reaction pathways and permeability structure controlling listwanite formation are preserved and exposed at Atlin, British Columbia, where listwanite extends tens of meters into surrounding wallrock. The overall mineralogical transformation is the same as that being considered for industrial sequestration of CO2. In nature, this reaction proceeds via subreactions that are fossilized as spatially distinct zones. Serpentine + olivine + brucite reacted with CO2 to form serpentine + magnesite, then magnesite + talc, and finally magnesite + quartz. These mineralogical transformations are achieved isochemically, except for the volatile species H2O and CO2. Although the first stage of the reaction only accounts for about 5‐15% of the carbonation potential of serpentinite, it is very widespread, and therefore may have sequestered a significant portion of the total bound CO2. Moreover, within intact bedrock, the progress of the magnesite + talc reaction generates fracture permeability, which appears to have locally enhanced reaction. The first two reactions combined account for about half of the carbonation potential for serpentinite and have a small associated increase in the volume of solids, which limits porosity loss. They thus hold the greatest promise for in situ mineral carbonation. The carbonation reactions are controlled by the activity of CO2 in the fluid phase. Proponents of industrial mineral-carbonation processes therefore may seek to control the composition of the input gas to preferentially drive carbonation reactions that minimize porosity loss and maximize permeability generation. Magnetite was progressively destroyed during carbonation, which allowed magnetic susceptibility to be used as a proxy for carbonation-reaction progress. It facilitated the mapping of the permeability structure of these systems and delineated subtle variations in reaction progress that might otherwise have gone unnoticed in the field.

The patterns of enrichment in felsic pegmatites ultimately depend on tectonic setting

Abstract: Le systeme de classification des pegmatites granitiques actuellement en vogue est largement derive de la classification des granites selon la profondeur de leur mise en place. Nous proposons comme alternative une classification fondee sur les processus petrogenetiques responsables, et nous incluons ici les pegmatites des systemes satures et sous-satures en silice. Des venues pegmatitiques a petite echelle sont souvent formees en milieu anatectique, au cours d'une culmination metamorphique. Nous incluons une categorie recemment decrite de pegmatites anatectiques a corindon, formees dans les massifs de Iherzolite ayant subi une metasomatose mantellique prealable en presence d'une phase fluide a H 2 O + CO 2 . En revanche, la plupart des venues de pegmatite sont le produit du fractionnement d'un volume important de magma. Nous considerons les pegmatites LCT (enrichies en Li-Cs-Ta) comme membres de suites orogeniques (calco-alcalines), signalant un contexte de subduction ou de collision; par contre, nous attribuons aux pegmatites NYF (enrichies en Nb-Y-F) une affiliation aux suites anorogeniques, formees dans un contexte d'extension, et sans difference fondamentale du point de vue petrogenetique avec les essaims pegmatitiques des complexes sous-satures classiques, y inclus les complexes carbonatitiques. Les pegmatites de types LCT et NYF contiennent typiquement des composantes de croute et de manteau, mais dans le cas d'un magmatisme anorogenique, il ya eu une etape de preparation metasomatique du socle en presence d'une phase fluide a H 2 O + CO 2 avant de passer a l'etape de fusion, afin d'assurer un produit a tendance alcaline. Un signal NYF + LCT inhabituel peut se developper dans des suites shoshonitiques. Nous decrivons ensuite les pegmatites hybrides dans lesquelles un ensemble NYF change a la fin de l'intervalle de cristallisation a un assemblage de type LCT, temoignant d'une contamination locale du systeme. La possibilite existe qu'une pegmatite LCT puisse aussi etre affectee par une telle surimposition tardive, pour donner une pegmatite qualifiee de "super-LCT". Une documentation des processus petrogenetiques serait essentielle a une evaluation du potentiel economique des venues pegmatitiques.

Xanes spectroscopy of sulfur in earth materials

Abstract: X-ray absorption near-edge structure (XANES) spectroscopy is the ideal non-destructive technique for characterizing and quantifying S species in compositionally complex natural materials such as silicate glasses and minerals, coals, asphalts and asphaltenes, kerogens and humic substances. Sulfur absorption edges represent the transition of S 1 s and 2 p core electrons to unoccupied antibonding orbitals at the bottom of the conduction band. Shifts in the position of the absorption-edge feature of S K - and L -edge XANES spectra constitute a chemical ruler for oxidation state of both inorganic and organic species of S; the S K edge shifts from 2469.5 eV for chalcopyrite (2− oxidation state) to 2482 eV for gypsum (6+). However, chemical state of S in Earth materials is most readily assigned by comparing the overall XANES profile with spectra for reference compounds. Sulfur XANES spectra are reviewed for pyrite, troilite, pyrrhotite and NiAs-type Co 0.923 S and Ni 0.923 S, niningerite (MgS), oldhamite (CaS), alabandite (MnS) and cubic FeS, and sphalerite and related phases, as well as for selected solid-solutions of the monosulfides. Sulfur XANES spectra for FeS, CoS, NiS, MgS, CaS, MnS and ZnS have been simulated by multiple scattering calculations. The S K -edge XANES of transition-metal-bearing monosulfides generally show anomalous absorption consistent with hybridization of the final S 3 p σ* antibonding states with empty 3 d orbitals on the metal atoms. Various applications of S K - and L -edge XANES fingerprinting are discussed, including speciation of inorganic S in basaltic glasses, lazurite and hauyne, identification of organic functional groups of S in coals, kerogens and humic substances extracted from subtropical soils and marine sediments, and the association of sulfated sugars with calcification of coral aragonite skeletons.

TECTONOMETAMORPHISM AT ca. 2.35 AND 1.85 Ga IN THE RAE DOMAIN, WESTERN CHURCHILL PROVINCE, NUNAVUT, CANADA: INSIGHTS FROM STRUCTURAL, METAMORPHIC AND IN SITU GEOCHRONOLOGICAL ANALYSIS OF THE SOUTHWESTERN COMMITTEE BAY BELT

Abstract: De nouvelles contraintes sont maintenant disponibles pour elucider l'evolution paleoproterozoique du domaine Rae, dans le secteur ouest de la Province de Churchill, au Nunavut; on se sert d'etudes integrees des aspects structuraux, metamorphiques et geochronologiques, ces derniers etablis in situ, du secteur sud-ouest de la ceinture archeenne de Committee Bay. Au sein des zones protegees de la deformation D 2 voisines d'un pluton synvolcanique d'environ 2.72 Ga, nous avons etabli que la foliation Si est orientee vers le nord, a pendage vers l'ouest, en association avec des plis D 1 et possiblement des chevauchements diriges vers l'ouest. Les structures D 1 sont partiellement obliterees par une foliation S 2 orientee vers le nord-est, a pendage vers le sud-est, en association avec des plis F 2 a plongement axial legerement vers le nord-est et indiquant une contraction vers le nord-ouest. Quoique certaines observations texturales font penser a un evenement thermique cryptique pre-D 1 qui pourrait resulter d'un plutonisme granitique repandu a environ 2.61-2.58 Ga, les relations entre porphyroblastes et leurs matrices dans les roches metapelitiques demontrent bien que deux evenements principaux ont eu lieu, d'abord avec et suivant D 1 (M 1 ), et ensuite avec et suivant D 2 (M 2 ). Les donnees thermobarometriques et l'utilisation de diagrammes de phase quantitatifs indiquent des traces d'evolution metamorphiques P-T-t a faible pression, dans le sens de l'horloge, pour atteindre une croissance post-tectonique de l'andalousite pendant M 1 et pendant M 2 . Les inclusions de monazite dans le grenat et la staurolite tardifs ou posterieurs par rapport a D 1 menent a une population d'âges de 2344 ′ 6 Ma; a la lumiere des microtextures, nous attribuons cet intervalle d'âge a l'evenement M 1 , developpe a un stade tardif de la deformation D 1 . La croissance de la monazite a eu lieu entre 520 et 560°C. L'âge de l'evenement M 2 est fixe par l'âge de la monazite dans la matrice, qui donne 1838 ′ 5 Ma. D'apres l'absence de roches magmatiques d'âge approprie pouvant fournir la chaleur necessaire, les traces P-T-t dans le sens de l'horloge, et la croissance tardive de porphyroblastes associes aux deux evenements de deformation contractionnelle, D 1 et D 2 , le metamorphisme etait une reponse a la contraction et l'epaississement de la croute. Les forces de compression qui ont cause ces evenements d'epaississement de part et d'autre de la ceinture de Committee Bay a environ 2.35 et 1.85 Ga, grosso modo, seraient des effets a distance dans la plaque superieure de deux evenements orogeniques. Le premier (l'orogenese "Arrowsmith") pourrait temoigner d'une collision a environ 2.35 Ga posterieure a une periode de magmatisme dans un arc continental le long de la bordure ouest de la ceinture de Rae. Le second serait lie a une accretion precoce au cours de l'orogenese trans-hudsonnienne impliquant une collision a environ 1.88-1.86 Ga de microcontinents dans la region de la baie d'Hudson.

Estimating p-t conditions of garnet growth with isochemical phase-diagram sections and the problem of effective bulk-composition

Abstract: Nous utilisons des sections de pression-temperature isochimiques d'un diagramme de phases approprie pour obtenir la distribution a l'equilibre des assemblages de mineraux et leur composition pour une compositionglobale donnee. L'utilisation de telles sections isochimiques pour deriver les conditions P-T de croissance du grenat requiert une composition globale effective, que nous definissons comme la composition de la roche disponible a l'assemblage de phases lors de la reaction. Nous avons calcule de telles sections isochimiques pour une pelite contenant l'assemblage grenat - muscovite - biotite - kyanite - plagioclase - quartz - rutile - ilmenite - graphite provenant de la zone a kyanite a Mica Dam, en Colombie-Britannique, Cordilleres canadiennes, en termes du systeme MnO-Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 en utilisant des compositions globales derivees par fluorescence X (XRF) et par repartition quantitative des elements par cartographie des rayons X avec une microsonde electronique (EPMA). Les isoplethes de compositions mesurees du coeur du grenat sont posees sur les sections isochimiques pour evaluer les conditions P-T de croissance du coeur. Une comparaison des compositions mesurees du coeur des cristaux avec celles qui sont predites a partir des sections construites avec la composition globale XRF indique un debut de croissance a environ 7.7 kilobars et 555°C, et une approche par thermobarometrie mufti-equilibre utilisant la bordure des grains de grenat mene a 7.2 kilobars, 680°C. Il n'y a donc aucune difference discernable en pression entre le coeur et la bordure des grains de grenat. D'apres la litterature recente, les compositions derivees par fluorescence X ne seraient pas appropriees pour estimer la composition globale effective, mais nous croyons qu'elles peuvent l'etre dans certaines situations. L'alternative pour des echantillons a grains grossiers serait de cartographier avec rayons X des superficies beaucoup trop grandes. Dans ce travail, les sections isochimiques sont utilisees pour delimiter les conditions P-T de croissance du coeur de grenat dans une pelite a grains moyens a grossiers. Les compositions derivees d'une analyse XRF sont plus aptes a etre appropriees dans ce cas. Parmi les benefices des compositions globales calculees a partie d'analyses ponctuelles des mineraux, il y a l'abilite d'exclure certaines phases de la composition globale calculee, ou bien des portions zonees de phases, afin d'evaluer l'evolution de la composition globale effective pendant la croissance des phases zonees. Seul le grenat montre une zonation importante dans cet echantillon. Des cartes quantitatives de repartition des rayons X ont ete utilisees pour calculer la composition integree du grenat, laquelle a ensuite ete soustraite de la composition XRF afin d'evaluer le fractionnement de la roche globale au cours de la croissance du grenat. Une section isochimique calculee avec cette composition fractionnee predit que le grenat paraitra dans cet assemblage a des conditions P-T semblables a celles preconisees par thermobarometrie effectuee avec la bordure du grenat, ce qui fait penser que la methode utilisee pour simuler le fractionnement est appropriee.

The speciation of arsenic in iron oxides in mine wastes from the giant gold mine, n.w.t.: application of synchrotron micro-xrd and micro-xanes at the grain scale

Abstract: Understanding the solid-phase speciation of arsenic in soils and sediments is important in evaluations of the potential mobility of arsenic and of its bio-availability in the environment. This is especially true in mine-infl uenced environments, where arsenic commonly is present at concentrations two and three orders of magnitude above quality criteria for soils and sediments. Arsenicbearing particulates dispersed through hydraulic transport or aerosol emissions can represent a persistent source of contamination in sediments and soils adjacent to past mining and metallurgical operations. The stability and mobility of arsenic associated with these phases depend on the chemical form and oxidation state of the arsenic and the interaction with post-depositional geochemical conditions. The Giant mine in Yellowknife, Northwest Territories, roasted arsenic-bearing gold ore from 1949 to 1999. The roasting process decomposed arsenic-bearing sulfi des (pyrite and arsenopyrite) to produce a calcine containing fi ne (generally <50 � m) arsenic-bearing iron oxides. We have applied synchrotron As K-edge micro X-ray Absorption Near-Edge Structure

Truth and beauty in metamorphic phase-equilibria: conjugate variables and phase diagrams

Abstract: Dans l'utilisation des equilibres calcules impliquant les mineraux necessaires pour comprendre les processus metamorphiques, il y a des choix naturels des axes a utiliser pour construire les diagrammes de phases, dependant des suppositions faites a propos des processus importants dans les roches. Les variables thermodynamiques se presentent en paires conjuguees, l'un etant intensif, et l'autre, extensif, et le choix naturel a faire depend de quelle paire mene a un trace evolutif des processus metamorphiques qui soit le plus facile a visualiser. Un choix non naturel mene a un diagramme pour lequel les traces ne sont pas evidents de facon intuitive, et un tel diagramme doit en general etre calcule. Nous illustrons les concepts liant les paires conjuguees en utilisant les equilibres portant sur les mineraux typiques du facies amphibolite a granulite, calcules pour une composition representative d'une pelite alumineuse en termes du systeme Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O (NCKFMASH). Nous examinons tout particulierement les paires conjuguees pression-volume et teneur en H 2 O-aH 2 O (i.e., la paire H 2 O-μ H 2 O ). Les diagrammes P-T versus V-T et T-M H 2 O versus T-a H 2 O servent a evaluer les processus metamorphiques. Nous montrons que la variable la moins evidente, par exemple V plutot que P, peut etre plus utile dans certaines situations, generalement transitoires.

Effects of cationic substitutions and alteration in uraninite, and implications for the dating of uranium deposits

Abstract: Des grains d'uraninite provenant de deux zones de mineralisation en uranium de type discordance, le gisement a teneur elevee de McArthur River, situe dans le socle, et la mineralisation de Virgin River, situee dans les gres, dans le bassin Paleoproterozoique d'Athabasca, Saskatchewan, Canada, ont ete etudies par microscope electronique a balayage et par microsonde electronique. L'imagerie en electrons retrodiffuses, la composition chimique, et les ages chimiques U-Pb calcules pour l'uraninite indiquent un remplacement du plomb radiogenique par Ca, Si et Fe, au cours d'evenements de circulation de fluide apres la mineralisation initiale, survenue a environ 1600 Ma. La conversion de l'uraninite a la coffinite, avec pres de 10% poids de SiO 2 , est survenue vers 700 Ma. L'intensite et le caractere des substitutions dependent de la quantite et la composition chimique de la phase fluide, qui pourrait avoir ete influencee par les caracteristiques des roches encaissantes.

The structure of silicate melts: a glass perspective

Abstract: On se sert couramment des verres pour etudier les bains fondus silicates. Quoiqu'ils sont des materiaux solides, leur structure est naturellement complexe et difficile a etudier. Toutefois, des progres ont ete faits dans l'etude de cette structure, sa relation a la composition, et son comportement a temperatures et pressions elevees. Les projets de recherche recents font ressortir les implications importantes aux bains fondus naturels. A titre d'exemple, la structure dependrait de la sorte et la taille d'ion alcalin ou alcalino-terreux qui se trouve incorpore dans le reseau du verre. Le Li se comporte de facon tres differente des autres alcalins. De plus, la presence de plus d'une sorte d'alcalin peut causer des comportements physiques non-lineaires. Les elements tels Al, Ti et Fe, dont l'importance petrologique est evidente, peuvent adopter une coordinence inhabituelle, cinq par exemple, et un arrangement polyedrique, des groupements par trois, par exemple, qui ne sont pas communs dans les phases cristallines. De plus, des transitions de phases polyamorphiques pourraient avoir lieu a temperatures et a pressions elevees. L'application de nouveaux modeles theoriques, par exemple le modele de contrainte moyenne du champ, fait penser que plusieurs phenomenes impliquant les verres, et tout probablement les bains fondus, pourraient resulter de transitions de materiau rigide face aux contraintes a materiau deformable. Quoique les connaissances de la structure du verre sont rudimentaires en comparaison de celles des materiaux cristallins, les connaissances ont augmente exponentiellement au cours des dernieres decennies, souvent grâce aux decouvertes de chercheurs oeuvrant dans le domaine des sciences de la Terre.

Variation in mineralogy, temperature, and oxygen fugacity in a suite of strongly peralkaline lavas and tuffs, pantelleria, italy

Abstract: Nous avons analyse huit echantillons de lave et de tuff pantelleritiques et une inclusion lithique de trachyte provenant de Pantelleria, en Italie, au moyen d'une microsonde electronique. Ces echantillons revelent cinq assemblages differents de mineraux selon la presence de fayalite, aenigmatite, ilmenite. et magnetite: (1) augite + fayalite + ilmenite + magnetite, (2) augite + fayalite + ilmenite, (3) hedenbergite plus ou moins sodique + fayalite + ilmenite + aenigmatite + quartz, (4) hedenbergite sodique ou aegirine-augite + ilmenite + aenigmatite + quartz ′ ferrorichterite, et (5) aegirine-augite + aenigmatite + quartz. Un feldspath alcalin (Or 3 5 - 3 7 ) constitue la phase phenocristique dominante avec chaque assemblage. La teneur de la roche en silice et le degre d'hyperalcalinite montrent une forte correlation avec l'assemblage de ces mineraux. L'assemblage 1 caracterise l'echantillon le moins fortement hyperalcalin [indice d'agpaicite, I.A. = (Na + K)/Al, proportion molaire] et la plus faible teneur en silice (I.A. < 1..31, SiO 2 < 64.8%, poids), et s'est equilibre a 991-888°C a une fugacite d'oxygene entre 0.7 et 1.1 unites logarithmiques en dessous du tampon FMQ (FMQ - 0.7 to FMQ - 1.1). L'assemblage 2 est associe a un indice d'agpaicite et une teneur en silice plus eleves (I.A. = 1.42, SiO 2 = 67.1%) et aurait equilibre a environ 794°C et a FMQ-0.5. L'assemblage 3 est associe a indice d'agpaicite et une teneur en silice encore plus eleves (I.A. dans l'intervalle 1.55 - 1.63, 66.8 < SiO 2 < 67.8%) et aurait equilibre a 764-756°C a entre FMQ - 0.5 et FMQ - 0.2. L'assemblage 4 est associe a un indice d'agpaicite legerement plus eleve et une teneur en silice encore plus elevee (1.61 < 1.A. < 1.75, 67.6 < SiO 2 < 72.0%) et marquerait un equilibre entre 740 et 700°C a une fugacite d'oxygene soit a ou legerement sous le tampon FMQ. L'assemblage 5 est associe a l'indice d'agpaicite et la teneur en silice les plus eleves (I.A. = 1.97, SiO 2 = 69.7%) et marquerait un equilibre a <700°C a une fugacite d'oxygene depassant legerement le tampon FMQ, dans un champ sans oxyde stable. Malgre la rarete d'assemblages a deux oxydes, deux pyroxenes ou deux feldspaths, il semble donc possible de delimiter les conditions de temperature et de fugacite d'oxgene dans les roches hyperalcalines au moyen des equilibres QUI1F impliquant un assemblage de fayalite, ilmenite, et clinopyroxene a l'equilibre.

IDENTIFICATION AND ALTERATION TRENDS OF GRANITIC-PEGMATITE-HOSTED (Y,REE,U,Th)–(Nb,Ta,Ti) OXIDE MINERALS: A STATISTICAL APPROACH

Abstract: The (Y,REE,U,Th)–(Nb,Ta,Ti) oxide minerals in REE-enriched granitic pegmatite and A- and I-type granites are primarily comprised of the fergusonite, samarskite, euxenite, aeschynite and pyrochlore groups. They are typically metamict and altered, and can be difficult to identify on a structural basis (XRD); consequently, examination of sample composition may be the most reliable approach to their identification. Canonical discriminant analysis can be used to discriminate among the various groups, and to assess the style and degree of alteration affecting samples. Identification of individual species should be made on the basis of previously published guidelines or, in their absence, on the basis of the “50% rule” of the IMA. Vector analysis shows that alteration typically pushes the compositions of samarskite-, euxenite- and aeschynite-group minerals in a direction that causes them to resemble pyrochlore-group minerals. For this reason, it has been historically difficult to determine the exact nature of “viethofingite”, “hjelmite” and “ampangabeite”; all are shown to be altered varieties of samarskite-group minerals. Yttrotantalite-(Y) is shown to be a member of the samarskite group, not the fergusonite group.

Natural radiation-induced damage in quartz. i. correlations between cathodoluminence colors and paramagnetic defects

Abstract: Quartz grains from the McArthur River uranium deposit of the Athabasca Basin, Saskatchewan, are characterized by three distinct types of cathodoluminescence (CL): 1) halos surrounding U- and Th-bearing mineral inclusions, and 2) patches and 3) continuous rims along grain boundaries and fractures. These three types of CL have a constant width of ~35 to 45 m, consistent with the maximum depth of penetration of alpha particles, and therefore they record alpha-particle-induced radiation damage. Relative to the host grains, the radiation-damaged areas are characterized by pronounced but broad CL bands in the ultraviolet (~350 nm) and red (~620–650 nm) regions. Isochronal annealing experiments reveal that the ultraviolet CL persists to 500°C but is annealed out at 600°C, whereas the red CL persists to at least 800°C. Electron paramagnetic resonance (EPR) spectroscopy, including detailed measurements on saturation behavior and thermal properties, revealed six paramagnetic defects: one oxygen vacancy center (E1’), three silicon vacancy hole centers [O2 3– /H + (I), O2 3– /H + (II) and O2 3– /M + ] and two O2 – peroxy centers. Spectral simulations confi rm the presence of these centers. Moreover, EPR spectra of HF-treated samples show that the silicon vacancy-hole centers and the peroxy centers are concentrated in the radiation-damaged rims and fractures. Center E1’ appears to occur throughout the quartz grains and is annealed out at ~500°C; it thus cannot be responsible for the ultraviolet or red CL. The silicon vacancy-hole centers are all annealed out between 550° and 600°C, similar to the annealing temperature of the ultraviolet CL. The peroxy centers are the only paramagnetic defects stable above 600°C, corresponding to the preservation of red CL in radiation-damaged areas at high temperatures. Therefore, the silicon vacancy-hole centers and the peroxy centers are probably responsible for the characteristic ultraviolet and red CL, respectively, associated with radiation-damaged halos, patches and rims in quartz.

Zincian spinel associated with metamorphosed proterozoic base-metal sulfide occurrences, colorado: a re-evaluation of gahnite composition as a guide in exploration

Abstract: Zincian spinel [(Zn,Fe,Mg)Al2O4] is an accessory mineral in a variety of rock types, in particular, metamorphosed massivesulfide deposits, pegmatites, aluminous metasedimentary rocks, skarns, marbles, and sulfide-bearing granulites. Textural and compositional studies of zincian spinel in thirteen Proterozoic sulfide deposits metamorphosed to the amphibolite and granulite facies in Colorado, many of which are spatially associated with Mg-rich alteration zones, shed new light on the use of zincian spinel composition as a guide to sulfide ores. Zincian spinel from the Independence, Bon Ton, and Sedalia deposits and some cases from the Green Mountain, Betty, Caprock, Cinderella, and Cotopaxi deposits show compositions that correspond to zincian spinel associated with metamorphosed massive-sulfide deposits hosted by hydrothermally altered Fe‐Al-rich metasedimentary and metavolcanic rock units elsewhere in the world (Gahnite44‐90Hercynite0‐45Spinel0‐25). However, other compositions of zincian spinel from these last five deposits and from three locations in the Wet Mountains (Marion and Amethyst prospects and an unnamed prospect) are depleted in Zn and enriched in Mg and Fe. These examples of Mg-rich zincian spinel occur in rocks in or adjacent to metamorphosed massive sulfides dominated by Mg-rich minerals such as phlogopite, anthophyllite, gedrite, tremolite, forsterite, cummingtonite, sapphirine, cordierite, enstatite, and clinohumite. The composition of zincian spinel in and surrounding metamorphosed massive-sulfide deposits is influenced by reactions involving Zn and Fe sulfides, as well as by bulk-rock composition, which in the case of many metamorphosed massive-sulfide deposits in Mg-rich alteration zones in Colorado, causes a marked enrichment in the spinel component of zincian spinel, to Gahnite0‐65Hercynite0‐50Spinel25‐90. Knowledge of the geological setting must be considered when using zincian spinel as a guide to metamorphosed massive-sulfide deposits.

Aluminum phosphate-sulfate minerals associated with proterozoic unconformity-type uranium deposits in the east alligator river uranium field, northern territories, australia

Abstract: Aluminum phosphate–sulfate (APS) minerals occur as disseminated crystals in a wide range of geological environments near the Earth’s surface, including weathering, sedimentary, diagenetic, hydrothermal, metamorphic and also postmagmatic systems. Their general formula is AB3(XO4)2(OH)6, in which A, B and X represent three different crystallographic sites. These minerals are known to incorporate a great number of chemical elements in their lattice and to form complex solid-solution series controlled by the physicochemical conditions of their formation (Eh, pH, activities of constituent cations, P and T). These minerals are particularly widespread and spatially related to hydrothermal clay-mineral parageneses in the East Alligator River Uranium Field (EARUF) environment associated with uranium orebodies in the Proterozoic Kombolgie basin of the Northern Territories, Australia. This field contains several high-grade unconformity-related uranium deposits, including Jabiluka and Ranger. Both petrography and chemical data are used to discuss the significance of APS minerals in the alteration processes from the EARUF. The wide range of chemical compositions recorded in APS is essentially due to coupled substitutions of Sr for the LREE and of S for P at the A and X sites, respectively. The major variations of the APS solid-solution series mainly consist of the relative proportions of svanbergite, goyazite and florencite end-members. The APS minerals result from the interaction of oxidizing and relatively acidic fluids with aluminous host-rocks enriched in monazite. The spatial distribution of these minerals and their compositional variation around the uranium orebodies allow us to consider them as good tracers of redox and pH paleo-conditions responsible for the development of fronts during the alteration processes, and hence as potential tools for mineral exploration.

Sulfur k-edge xanes spectroscopy: chemical state and content of sulfur in silicate glasses

Abstract: The chemical state and local environment of sulfur in basaltic glasses from the eastern Galapagos rift and quenched laboratory experiments have been investigated by synchrotron-radiation S K-edge X-ray absorption near-edge structure (XANES) spectroscopy, using an energy-selective fl uorescence detector. We have also investigated sulfur in melts in the system 6NaAlSiO4–CaS– CaSO4 quenched from 1500°C and various reference compounds, using conventional fl uorescence yield and total-electron yield (TEY) recording modes. The S K-edge XANES spectrum of CaS dissolved in Na-aluminosilicate glass is dominated by a broad singlet edge-feature near 2475 eV, consistent with isolated SCan coordination polyhedra in the melt, whereas that of quenched end-member CaSO4-bearing melt is similar to XANES spectra of crystalline sulfates. The linear correlation between relative XANES area and wt% content of sulfur has been used to estimate the content of sulfur in samples of lazurite (5.6–6.3 wt% by EPMA), scapolite-group minerals (0.4 wt%) and the basaltic glasses (0.04–0.15 wt%). The S K-edge XANES spectra of basaltic glasses synthesized under reducing (WM and IQF solid oxygen-buffer) conditions are dominated by a single broad absorption peak at 2470–2482 eV in combination with a weak peak or shoulder at 2470–2471 eV, and are reproduced by mixing XANES spectra of FeS and alkaline-earth monosulfi des. The S K-edge XANES spectra for glasses from the eastern Galapagos rift are also dominated by S 2– , but one TEY spectrum shows possible thiosulfate and another, trace sulfate.

NEW INSIGHT INTO STRUCTURAL AND COMPOSITIONAL VARIABILITY IN SOME ANCIENT EXCESS-Ca DOLOMITE

Abstract: Samples of non-stoichiometric dolomite of different age and origin have been studied by powder X-ray diffraction, scanning electron microscopy and electron-microprobe analysis. Most samples containing a high excess of Ca consist of two or even three phases, differing in the degree of excess Ca uptake and degree of structural order. On the basis of Rietveld refi nements of samples with two 3D periodic phases, only the B sites have a mixed occupancy of Ca and Mg, and the phases differ from each other only in the content of Ca at the B site. A simulation of the experimental XRD patterns of samples showing a high and a low excess of Ca with a defective layer-structure was carried out in terms of mixed-layer structures in which different proportions of nonstoichiometric dolomite layers are interstratifi ed with a small amount of calcite-like as well as stoichiometric dolomite layers. Electron-microprobe analysis and BSE images show that large crystals in the two-phase excess-Ca dolomite samples have sharply bounded zones with different levels of Ca concentrations. Each BSE contrast zone has the same composition (within error), and in each sample there are three or four non-overlapping groups of such zones differing in composition. Remarkably, fi ne-grained micritic portions of these samples contain the same discrete sets of Ca concentration zones as the large zoned euhedral crystals. This result supports the hypothesis that crystals of excess-Ca dolomite adopt only certain non-continuous compositions, and that growth and composition are structurally controlled.

Contrasting sequences of metapelitic mineral-assemblages in the aureole of the tilted nelson batholith, british columbia: implications for phase equilibria and pressure determination in andalusite–sillimanite-type settings

Abstract: The Nelson Batholith is a ca. 1,800 km 2 Jurassic intrusive body in southeastern British Columbia surrounded by a contact aureole, 0.7–1.8 km wide, developed in graphitic argillaceous rocks that show only minor variations in bulk composition. Contrasting prograde sequences of mineral assemblages are developed around the aureole in a regular pattern, reflecting different pressures of contact metamorphism. The following assemblages are seen going from lower to higher pressure (all assemblages contain muscovite + biotite + quartz ± Mn-rich garnet): (1) cordierite-only assemblages, (2) mix of cordierite-only and cordierite + andalusite assemblages, locally with cordierite + K-feldspar and andalusite + K-feldspar assemblages at higher grade, (3) andalusite-only assemblages, with sillimanite + andalusite assemblages and locally sillimanite + K-feldspar assemblages at higher grade, (4) staurolite-only assemblages, (5) staurolite ± andalusite assemblages, with sillimanite-bearing and locally sillimanite + K-feldspar assemblages at higher grade. The higher-pressure sequences with staurolite ± andalusite are restricted to the aureole surrounding the east half of the batholith, whereas the lower-pressure cordierite ± andalusite are restricted to the aureole surrounding the west half of the batholith and its northern and southern tips. The sequences of mineral assemblages correspond closely to the facies series of Pattison & Tracy (1991) and are interpreted to represent a series of approximately isobaric metamorphic field-gradients below the Al2SiO5 triple point, providing an excellent opportunity to evaluate thermodynamically calculated low-pressure phase equilibria in the metapelitic system. The total difference in pressure represented by the contrasting assemblages is about 1.0 kbar, showing that they are a sensitive measure of small differences in pressure within the stability field of andalusite. Thermobarometry results from the aureole are moderately consistent with the mineral assemblage constraints, but carry pressure uncertainties larger than the total range of pressure represented by the aureole's assemblages. Pressures of the intrusive rocks derived from hornblende barometry are scattered, and many do not agree with the pressure constraints from the aureole. The mineral-assemblage constraints indicate down-to-the-west post-metamorphic tilting of the batholith and aureole of about 10°, interpreted to be due to a combination of eastward thrusting of the Nelson Batholith over crustal-scale ramps during Cretaceous–Paleocene shortening and Eocene east-side-down normal motion on the Slocan Lake – Champion Lakes fault system that forms the western boundary of the batholith.

The composition of coexisting jarosite-group minerals and water from the richmond mine, iron mountain, california

Abstract: Jarosite-group minerals accumulate in the form of stalactites and fi ne-grained mud on massive pyrite in the D drift of the Richmond mine, Iron Mountain, California. Water samples were collected by placing beakers under the dripping stalactites and by extracting pore water from the mud using a centrifuge. The water is rich in Fe 3+ and SO4 2– , with a pH of approximately 2.1, which is signifi cantly higher than the extremely acidic waters found elsewhere in the mine. Electron-microprobe analysis and X-ray mapping indicate that the small crystals (<10 � m in diameter) are compositionally zoned with respect to Na and K, and include hydronium jarosite corresponding to the formula (H3O)0.6K0.3Na0.1Fe 3+ 3(SO4)2(OH)6. The proton-microprobe analyses indicate that the jarosite-group minerals contain signifi cant amounts of As, Pb and Zn, and minor levels of Bi, Rb, Sb, Se, Sn and Sr. Speciation modeling indicates that the drip waters are supersaturated with respect to jarosite-group minerals. The expected range in composition of jarosite-group solid-solution in equilibrium with the pore water extracted from the mud was found to be consistent with the observed range in composition.

A methodology to determine the acid-neutralization capacity of rock samples

Abstract: Des calculs fondes sur la quantite molaire des cations neutralisateurs (Na, K, Mg, Ca, Mn) relâches dans une solution associee a la dissolution de mineraux en milieu acide fournissent une methode de quantifier chimiquement la capacite a neutraliser les acides de carbonates, silicates, de simples assemblages de mineraux mixtes, et des echantillons de roches constituant des dechets d'operations minieres internationales. L'acidite neutralisee par chaque cation serait equivalente a sa valence dans le mineral hote. Le Fe et l'Al sont hydrolyses lors de titration inversee de l'evaluation, et donc ne joueraient aucun role dans la neutralisation. Le soufre derive de l'oxydation de la pyrite serait l'equivalent a la production de deux moles de H + , quoique le soufre non apte a former un acide, par exemple le soufre du gypse, devrait aussi etre evalue. Pour les echantillons etudies, les calculs fondes sur ces principes montrent une bonne correlation avec la valeur du pourvoir neutralisateur determinee dans des tests de titration.

Foliation development and reaction softening by dissolution and precipitation in the transformation of granodiorite to orthogneiss, glastonbury complex, connecticut, u.s.a.

Abstract: Les textures, microstructures, et les schemas de zonation chimique des mineraux d'un orthogneiss granodioritique provenant du complexe de Glastonbury, au Connecticut, menent a une hypothese que le developpement de la foliation a ete facilite par des reactions retrograde d'hydratation en presence d'une phase fluide aqueuse. Un remplacement partiel des mineraux magmatiques metastables feldspath potassique + hastingsite + magnetite a produit l'assemblage biotite + epidote + quartz, qui definissent la foliation. Ces produits de reaction n'ont pas remplace les interfaces feldspath potassique - hastingsite; plutot, c'est soit la biotite ou l'epidote qui a remplace l'amphibole, et le plagioclase qui a remplace le feldspath potassique. La biotite et l'epidote se sont formees en couches distinctes qui definissent la foliation de l'orthogneiss au cours de la deformation, tandis que le quartz s'est forme surtout en rubans, contribuant ainsi au developpement de la texture. La titanite primaire, riche en terres rares et metastable, est aussi passee en solution, et a ete remplacee partiellement par une titanite metamorphique alumineuse et a faible teneur en terres rares. Les teneurs semblables en terres rares de l'epidote et de la titanite montrent que les terres rares issues de la dissolution de la titanite ont contribue localement a la formation de la composante allanite des grains adjacents d'epidote. Le processus au complet etait syntectonique, et la plupart des grains font preuve de surcroissances multiples dans la direction de l'extension, telle que definie par les xenolithes etires. Il y avait une quantite suffisante d'uranium dans les surcroissances de titanite pour permettre une datation par la methode SHRIMP des coeurs, des zones intermediaires et des bordures des cristaux. Ces resultats semblent indiquer au moins trois evenements de croissance retrograde d'âge alleghanien dans un intervalle d'environ 30 million d'annees. Le processus de dissolution - transfert - precipitation decrit non seulement le mecanisme de reaction, mais il a mene aussi a la redistribution des produits de reaction pour former des couches presque monomineraliques, contribuant ainsi a la differenciation metamorphique et au developpement de la foliation. L'orthogneiss qui en a resulte est beaucoup plus deformable que le protolithe granodioritique a cause de cette reaction et du ramollissement textural.

ON THE ORIGIN OF THE PGE MINERALIZATION IN THE ELATSITE PORPHYRY Cu–Au DEPOSIT, BULGARIA: COMPARISON WITH THE BAULA–NUASAHI COMPLEX, INDIA, AND OTHER ALKALINE PGE-RICH PORPHYRIES

Abstract: The Elatsite porphyry-copper deposit contains platinum-group minerals associated with base-metal sulfi des, i.e., merenskyite, moncheite, palladoarsenide and an undetermined Pd–Ag–Te–Bi mineral, all in a magnetite – bornite – chalcopyrite assemblage. Other minerals such as linnaeite, carrollite, siegenite, rammelsbergite, which are uncommon in typical porphyry-copper deposits, indicate a mantle derivation for the specifi c PGE–Co–Ni episode of mineralization. The Pt–Pd-rich facies is characterized by extremely low Os, Ir and Ru contents (respectively <3.0, <0.6 and <1.0 ppb), with up to 349 ppb Pt and 3440 ppb Pd. Mineralization results either from (1) multi-stage events involving a preconcentration of the PGE in magmatic disseminated (Ni–Cu–Co) sulfi des related to a mantle-derived magmatic event, with subsequent selective hydrothermal remobilization, or (more probably) from (2) particular conditions in the partial melting of the mantle and magmatic evolution that rendered the precious metals available in the magma, along with a specifi c evolution that prevented the early formation of the sulfi des whilst the PGE (mainly Pd) were concentrated in the hydrothermal fl uids. This PGE–Co–Ni episode is a very specifi c event in the main mineralizing process giving rise to the massive deposition of the Cu-porphyry system. The characteristics of the PGE mineralization deviate from those of a purely hydrothermal PGE mineralization in a mafi c environment, as exemplifi ed by the Baula–Nuasahi mineralization (India), in that they tend to converge toward (and present striking similarities with) the PGE mineralization in alkaline porphyry deposits, such as are found in British Columbia.

Modification of detrital platinum-group minerals from the eastern bushveld complex, south africa

Abstract: Les particules de mineraux du groupe du platine (MGP) sont presentes en grand nombre dans les alluvions des rivieres drainant la partie orientale du complexe de Bushveld, en Afrique du Sud. Environ 32% d'un total de790 pepites de MGP, mesurant entre 50 et 1600 μm, font preuve de modifications chimique ou texturale. Les alliages Pt-Fe sont les plus susceptibles, et revelent une grande variete de types texturaux, parmi lesquels les assemblages a Ni et a Cu sont les plus repandus. Dans le premier groupe, les grains de l'alliage Pt-Fe [Pt 2 . 3 - 2 . 7 Fe] deviennent aureoles par des couches de ferronickelplatinum [Pt 2 NiFe] et recouverts d'un alliage de composition voisine de [Ni 2 PtFe]. Dans l'assemblage riche en Cu, l'alliage Pt-Fe [Pt 2 . 2 - 2 . 6 Fe] est recouvert de facon concentrique par [Pt(Fe,Cu)], i.e., une solution solide entre tetraferroplatinum [PtFe] et tulameenite [Pt 2 CuFe]. Une variete de phases Pd-Pt-Te-Sb-Sn, d'alliages a Au-Ag, et des sulfures de metaux de base sont presents en inclusions, et de petits grains de zvyagintsevite [Pd 3 Pb] sont rattaches a la bordure externe. L'alliage Pt-Fe peut aussi se voir remplacer par des "oxydes" heterogenes de Pt-Fe(Cu,Pd) qui peuvent contenir des concentrations importantes de SiO 2 et MgO. D'autres grains de l'alliage Pt-Fe sont recouverts d'un mince lisere de PtS. Cooperite et braggite, et a un degre moindre, sperrylite, sont recouverts d'une couche de platine natif poreux, qui penetre les grains aussi en canaux ressemblant un reseau de racines. Des grains monomineraliques d'un alliage poreux riche en Pt representeraient le terme de ce type d'alteration. Dans de rares cas, la laurite est remplacee par deux differents types d'oxydes de Ru-Rh-Fe-(Ca-V-Mn), de stoechiometrie RuO 2 et RuO, respectivement. Par la suite, les oxydes sont transformes en ruthenium et rhodium natifs. Des oxydes riches en Pd sont presents dans la bordure de certains grains ayant subi une alteration, et sont associes aux oxydes-hydroxydes palladiferes de Fe et aux phases silicatees palladiferes riches en Mn et en Fe (e.g., smectite, chlorite). Les grains modifies sont genetiquement attribuables a trois processus: (1) reactions de sulfuration au cours de processus magmatiques et hydrothermaux affectant l'alliage Pt-Fe, la braggite et les sulfures de Rh, (2) alteration de l'alliage Pt-Fe pour donner des alliages Pt(Fe,Cu,Ni) a temperature moyenne dans un milieu reducteur, et (3) oxydation des sulfures des elements du groupe du platine dans un milieu oxydant a faible temperature. Ces modifications et les phenomenes d'alteration se seraient deroules sur un grand laps de temps, possiblement sous conditions de climat et d'oxydation variables au cours de stades postmagmatiques, y inclus la serpentinisation, de stades de lessivage des roches du socle porteuses des elements du groupe du platine, et suite a la deposition des placers.

Mineralogy of permeable reactive barriers for the attenuation of subsurface contaminants

Abstract: L'installation de barrieres reactives permeables est un developpement assez recent pour depolluer les eaux souterraines contaminees par des produits organiques ou inorganiques. L'ecoulement souterrain suit un gradient naturel qui passe au travers d'un rideau permeable contenant un medium traitant qui soit precipite les contaminants sous forme relativement insoluble, ou bien les transforme en especes acceptables ou benignes pour l'environnement. Le traitement le plus repandu utilise le fer metallique submillimetrique, substance tres reactive, acceptable pour l'environnement, et facilement disponible comme produit du recyclage de ferraille et d'acier. On se sert aussi des dechets de composts organiques afin d'eliminer les contaminants inorganiques. Nous presentons a titre d'exemples deux cas d'utilisation de composts pour reduire la teneur en sulfate et pour precipiter les metaux presents, surtout d'une perspective mineralogique. Dans les carottes du medium de traitement ayant servi, le produit secondaire le plus abondant forme in situ serait un oxyhydroxyde de fer, mais divers precipites ont ete identifies. Par exemple, les phases secondaires pyrite, greigite, et nickel natif sont presentes au site ou il y a remplacement de materiaux organiques par des sulfures. A un site industriel, la pyrite, covellite, chalcopyrite, et bomite secondaires se sont formes dans le medium de traitement. Tandis que le remplacement de materiaux organiques par des oxyhydroxydes de fer est repandu, un remplacement par des sulfures semble rare. Les sulfures secondaires n'occupent qu'un petit volume, et sont peu propices a reduire la permeabilite du medium de traitement, mais la formation des oxyhydroxydes de fer et des carbonates secondaires en presence de fer metallique requiert une surveillance continue afin de determiner si les precipites secondaires et la consommation du Fe° diminuent de facon importante l'efficacite de telles barrieres a long terme. Il semble clair, d'apres les indications actuelles, que ces barrieres sont environnementalement efficaces et rentables.

A history of our understanding of magmatic Ni-Cu sulfide deposits

Abstract: Progress in the understanding of the origins of magmatic Ni–Cu sulfide deposits underwent a major acceleration with the advent of the 1960s. Prior to this decade, thinking had largely been influenced by observations on the Sudbury area, in Ontario, which was by far the dominant Ni producer. Discussion focused on the nature of the Sudbury Igneous Complex, and whether the ores were gravitational segregates from the complex, or whether they had been introduced by hot aqueous fluids. During the 1960s, the concept that Sudbury is an astrobleme was first proposed, the discovery of the Talnakh ore junction (Russia) elevated Noril’sk from minor to major status, and a new class of deposit related to komatiitic volcanism was recognized at Kambalda (Australia). The 1960s were also a turning point with respect to research funding, which led to an explosion both in the amount of research conducted, and in its global nature. Thereafter, progress was stimulated by the new thinking about Sudbury, and the very different environments of ore deposition observed at Noril’sk and Kambalda. It has turned out that a number of broad themes have evolved under which much of the progress over the past 40 years can be grouped. (i) Magmas rising directly from the mantle are unlikely to reach crustal depths saturated in sulfide, and contamination with crustal rocks is required for sulfide immiscibility to occur early in the crystallization process. (ii) With appropriate experimentally derived partition-coefficients, the relationship between the compositions of magma and sulfide can be modeled, and this modeling provides important constraints on geologically based hypotheses. (iii) The development of sulfide immiscibility commonly leaves a mark on the composition of the source magma and the rocks crystallizing from it that can act as a signpost for exploration. (iv) Sudbury is unique, probably because of the high degree of superheat that it experienced; sulfides have settled and accumulated over much of the base of the complex; in most other deposits, it has been the flow of magma carrying immiscible sulfides that has caused the sulfides to concentrate in economically exploitable proportions. The physical environment represented by a given part of an igneous body is therefore important when considering its potential. (v) Once concentrated, sulfide magmas cool and fractionate, and the fractionated residual liquid may migrate away from the initial site of crystallization to form rich concentrations of Cu, Pt, Pd and Au elsewhere. Observations at Voisey’s Bay (Canada) over the past 10 years have confirmed the importance of these five themes.

IDENTIFICATION OF SCORODITE IN FINE-GRAINED, HIGH-SULFIDE, ARSENOPYRITE MINE-WASTE USING MICRO X-RAY DIFFRACTION (μXRD)

Abstract: Secondary phases precipitated during oxidation of sulfides in mine wastes can be very fine-grained and poorly crystalline, making accurate identification difficult. As part of a study to examine arsenic mobility within the arsenopyrite residue stockpile, Snow Lake, Manitoba, arsenic-rich secondary phases were examined by a combination of micro X-ray diffraction (μXRD) and electron-probe micro-analysis (EPMA). With EPMA, we found one of the secondary As-containing phases to have an Fe:As ratio of 1:1. Examination of μXRD data allowed positive identification of this phase as scorodite (FeAsO4· 2H2O). Two texturally distinct occurrences of scorodite were identified in specific areas of the polished thin sections. Type-1 scorodite occurs around grains of primary arsenopyrite, and Type-2 scorodite is disseminated throughout an amorphous iron sulfo-arsenate (AISA) matrix in highly altered material. Our observations suggest that μXRD can be used to routinely identify phases comprising <1% of the bulk sample, provided that these phases are positioned under the X-ray beam. All of the scorodite examined is very fine-grained polycrystalline material, displaying homogeneous Debye powder rings in the two-dimensional (2D) General Area Diffraction Detector System (GADDS) image, using either a 500 or 50 μm X-ray beam diameter. The homogeneous texture of fine-grained secondary scorodite makes it easily discernable from relatively coarse-grained primary minerals, which give large discrete diffraction-spots or discontinuous “grainy” Debye rings in the GADDS image.

COMPOSITION AND PARAGENESIS OF Na-, Nb- AND Zr-BEARING TITANITE FROM KHIBINA, RUSSIA, AND CRYSTAL-STRUCTURE DATA FOR SYNTHETIC ANALOGUES

Abstract: We describe titanite with unusually high contents of Na, Nb, and Zr from a hydrothermal natrolite-rich vein cutting kalsilite‐ nepheline syenite at Mount Rasvumchorr, Khibina peralkaline complex, in the Kola Alkaline Province, Russia. This titanite is associated with astrophyllite, ceriobetafite, yttrobetafite, henrymeyerite, and banalsite. We recognize four generations of titanite: nearly stoichiometric titanite-I, titanite-II, containing up to 16.4 wt.% Nb2O5 (0.25 apfu Nb) and 3.2% Na2O (0.21 apfu Na), titanite-III, with up to 9% ZrO2 (0.15 apfu Zr) and zoned from 12.4 to 2.2% Nb2O5, and (Nb‐Zr)-poor titanite-IV, with up to 3.6% Al2O3 and 2.2% Fe2O3. In titanite-II, substitution of Nb at the octahedral site is accompanied by Na at the seven-fold site and, probably, by (F,OH) ‐ and vacancies. A compositional analogue of titanite-III, and synthetic titanite containing 0.25 apfu Zr, similar to the most Zr-rich titanite known, have been prepared by standard ceramic techniques, and their crystal structure determined by Rietveld refinement of powder X-ray-diffraction patterns. The synthetic variants doped with Zr, or with Zr, Na, and Nb, adopt space group A2/a, and consist of distorted YO7 polyhedra, XO6 octahedra, and SiO4 tetrahedra. The (Ca0.85Na0.15) (Ti0.70Zr0.15Nb0.15)OSiO4 analogue of natural titanite from the Rasvumchorr zeolite vein contains the least-distorted coordination polyhedra. The polyhedra in Ca(Ti0.75Zr0.25)OSiO4 are moderately distorted, and the most strongly distorted polyhedra are those in the CaTiOSiO4 end-member.