Showing papers in "Canadian Mineralogist in 1997"

The crystal chemistry of hexavalent uranium; polyhedron geometries, bond-valence parameters, and polymerization of polyhedra

Abstract: The geomeffy, bond valences, and polymerization ofhexavalent uranium polyhedra from 105 well-refined structures are analyzed. The Utu cation is almost always present in crystal stnrctures as part of a nearly linear (UOr)z* uranyl ion that is coordinated by four, five or six equatorial anions in an approximately planar arangement perpendicular to the uranyl ion, giving square, pentagonal and hexagonal bipyramids, respectively. The Utu-O7\" bond length (Oy,: uranyl-ion O atom) is independent of the equatorial anions of the polyhedra;-averages of all polyhedra tlat contain uranyl ions ffs; I6lIJ6f-Or. = 1.79(3), mg0.-.9 a,= 1.79(4), and t8lu6+-Our = 1.78(3) A. Not a[ r6lu6+ polyhedra contain uranyl ions; there is a continuous series of coordiaation polyhedr4 from square bipyramidal polyhedra with uranyl ions to holosymmehic octahedral geometry. The mUo* and t8lu6+ polyhedra invariably contaitl a uranyl ion. The equatorial U6.-0 (0: O,-, OH-) bond-lengths of uranyl polyhedra depend upon coordhation number; averages for all polyhedra are t6lu6+-dq = 2,28(5), rlUot-$* = 2.37(9), afi t8tlJ6+-$q2.47 (12) A. Cunently available bond-valence parameters for U& are unsatisfactory for determining bond-valence sums. Coordination-specific bond-valence paxameters have been derived for U6|, together with parameters applicable to all coordination geometries. The parameters give bond-valence sums for Ue of -6 vlr and reasonable bond-valences for Uc,-Ou, bonds. The bond-valence paraneters facilitate the recognition of Ua, U5+ and U6| catiotrs in refined crystal structures. The crystal-chemical consfraints ofpolyhedral polymerization in uranyl phases are discussed.

Characterization of multiple fluid-flow events and rare-earth-element mobility associated with formation of unconformity-type uranium deposits in the Athabasca Basin, Saskatchewan

Abstract: The hoterozoic Athabasca Basin in northem Saskatchewan is host to the world's largest and richest uranium deposits. Tbree main stages of uranium mineralization have been identified on the basis of cross-cutting relationships, textures observed in thin section and by SEM, oxygen and U-Pb isotopic mmpositions, and chemical compositions. All thee stages of uranium mineralization have been variably altered to becquerelite, calcio-uranoite, and coffinite. U-Pb chemical ages and 207PbP06Pb ages of uranium mineralization indicate that tle three main uranium-deposition events occurred at -1500 Ma during peak diagenesis, at 950 Ma' and as late as 300 Ma. Stage-l and -2 uraninite and pirchblende have the lowest 618O values, which range from -32to -1.9.5Voo, whereas stage-3 uraninite has 6180 values near -l07oo. The alteration minerals becquerelite, calcio-uranoite, and coffinite have 618O values near 0%o. 875r/865r ratios of uranium minerals strongly reflect the proportion of basin and basement fluid involved in the formation of the deposits. Uranium minerals entirely hosted by basement rocks have relatively low 8751165r mtios, which indicate that the najority of the fluid involved in the formation of fhese deposits was basinal brine, whereas tlte relatively high rSrF6Sr ratios of uranium minerals partially hosted by the overlying sandstones indicate that a high proportion of the Sr and by inference, the fluid involved in the formation of these deposits was derived from the basement rocks. High concentrations of REE-rich hydrothermal crandallite-group minerals, and xenotime are associated witl uranium nineralization in the Athabasca Basir In addition, the uranium minerals have up to 12,000 ppm total REE contents atd are H&EE-eniched. The relations among REE-rich minerals, diagenetic clays and uranium mineralization in the basin and basement rocks indicate extensive REE mobility during diagenesis of the Athabasca Basin. The REE and U most likd were derived from detrital fluorapatite and zircon in the sandstone, gamet in the basement and to a lesser extent, diagenetic clay minerals and zircon in basement roctrs. The ftEE, and possibly the uranium, were likely transported as F-complexes. The high concentration of uranium (427 million kg) and RZE in deposits of tle Athabasca Basin render unconformity-type deposits viable sources of.REE as well as uranium, and demonstrate that REE mobility can be extreme in certain geological environments.

Primitive magmas at five Cascade volcanic fields; melts from hot, heterogeneous sub-arc mantle

Abstract: Major and trace element concenftations, including REE by isotope dilution, and Sr, Nd, Pb, and O isotope ratios have been determined for 38 mafic lavas from the Mount Adams, Crater Lake, Mount Shasta Medicine Lake, and Lassen volcanic flelds, in the Cascade arc, northwestem part of the United States. Many of the samples have a high Mg# tl@Mg/(Mg + FeD > 601 and Ni content (>140 ppm) such that we consider them to be primitive. We rccognlze rhree end-member p:,jmillrve magma groups in the Cascades, characterized mainly by their trace-element and alkali-metal abundances: (1) High-alumina olivine tholeiite (HAOT) has trace element abundaaces imilarto N-MORB, except for slightly elevatedLllE, and has Eu/Eu* > 1. (2) Arc basalt and basaltic andesite have notably higher L/lE contents, generally have higher SiO2 contents, are more oxidized and have higher Cr for a given Ni abundance than HAOT. These lavas show relative depletioninl/F.i4 have lowerl/ftEE andhigherl,ftEEthan HAOT, and have smaller EulEu* (0.94-1.06). (3) Alkali basalt from the Simcoe volcanic field east of Mount Adams reprcsents the third end-membr, which contributes an intraplate geochemical sigpature to magna compositions. Notable geochemical features among the volcanic flelds are: (1) Mount Adams rocks are richest in Fe and most incompatible elements including I/FSE; (2) the most inconpatible-element depleted lavas occur at Medicine Lake; (3) all centers have relatively primitive lavas with high ULEIHFSE nttos but only the Mount Adams, Lassen, and Medicine Lake volcanic fields also have relatively primitive rocks with an intraplate geochemical signature; (4) there is a tendency for increasing 87SrF6Sr, 2r7Pbp4Pb, and 6180 and decreasing 236Pbp@fb and 143Nd,f144Nd from north to south. The tlree end-member Cascade magma types reflect contributions from three mantle components: depleted sub-arc mantle modestly enriched in ZIZE during ancient subduction; a moder& hydrous subduction componenl and OlB-source-like domains. Lavas with arc and intraplate (OIB) geochemical signatures were erupted close to FIAOT, and many lavas are blends of two or more magma t)?es. pre-eruptive HzO contents of FIAOT, coupted with phase-equilibrium studies, suggest that these magmas were relatively dry and last equiJibrated in tle mantle wedge at temperatures of -1300\"C and depths of^40 km, virtually at the base ofthe crust. Arc basalt and basaltic andesite reprcsent grcater extents of melting than HAOT, presumably in the same general thermal regime but at somewhat lower mantle separation temperatures, ofdomains ofsub-arc mantle that have been enriched by a hydrous suMuction component derived from the young, relatively hot Juan de Fuca plate. The primitive magmas originated by partial melting in response to adiabatic upwelling within the mantle wedge. Tectonic extension in this part ofthe Cascade arc, one characterized by slow oblique convergence, contributes to mantle upwelling and facilitates eruption of primitive magmas. Keyvords: arc magmatism, primitive basalt, geochemistry, isotopes, tace elements, Cascades, northwestern United States.

The variable role of slab-derived fluids in the generation of a suite of primitive calc-alkaline lavas from the Southernmost Cascades, California

Abstract: The compositional continuum observed in primitive calc-alkaline lavas erupted from small volcanoes across t}te southemmost Cascade arc is lnoduced by the introduction of a variable proportion of slab-derived fluid itrto the superjacent peridotite layer of the mantle wedge. Magnas derived from fluid-rich sources are erupted primarily in the foremc and are characterized by Sr and Pb enrichment (prinitive mantle-normalized Sr/P > 5.5), depletions of Ta and Nb, low incompatible-element abundances, and MORB-like Sr and Pb isotopic ratios. Magnas derived from fluid-poor sources are erupted primarily in the arc axis and behind the arc, and are characterized by weak enrichment in Sr [l.0 < (Sr/P),v < 1.3], weak depletions in Ta aad M, highsr incompatible-element abundances, and OIB-like Sr, Nd, and Pb isotopic ratios. Fluxing the mantle wedge above the subducting slab with Hzo-rich fluid stabilizes amphibole and emiches the wedge peridotites in incompatible elements, particularly unradiogenic Sr and Pb. The hydrated amphibole-bearing portion ofthe mantle wedge is downdragged beneath the forearc, where its solidus is exceeded yielding melts that are enriched in Sr and Pb, and depleted in Ta and Nb (reflecting both high Sr and Pb relative to Ta and Nb in the fluid, and the greater compatibility ofTa and Nb in amphibole compared to other silicate phases in the wedge). A steady decrease ofthe fluid-contributed geochemical signature away from the trench is produced by the progressive dehydration of tle downdragged portion of the mantle wedge witl depth, resulting from melt extraction and increased temperature at tle slab-wedge interface. lnverse correlation between incompatible-element abundances and the size of the fluid-confiibuted geochemical signature is generated by melting of more depleted peridotites, rather than by significant differences in the degree of melting. High-(Sr/P)ri' lavas of the forearc are generated by melting of a MORB-source-like peridotite that has been fluxed with a greater proportion of slab-derived flui{ and low (Sr/P)rv lavas of the arc axis are produced by melting of an OlB-source-like peridotite in the presence of a smaller proportion of slab-derived fluid. This study documents the contol that a slab-derived fluid can have on incompatible element and isotopic systematics of arc magmas by 1) the addition of incompatible elements to the wedge, 2) the stabilization of hydrous phases in the wedge, and 3) the lowering of peridotite solidi. Kewords: Cascade arc, calc-alkaline, primitive lav4 slab-derived fluid, Sr enrichmen! Califomia.

The incorporation of actinides in monazite and xenotime from placer deposits in western australia

Abstract: The composition of several samples of monazite-(Ce) and xenotime from placer deposits in Western Australia has been determined from the electron-microprobe analysis ofmany individual grains ofmineral sand. A regression analysis ofthe resulls was used to identiff tlhe mechanisms of incorporation of the actinides Th and U in each mineral. Both of the substitution mechani sms (Th,U)A + C*+ = 2REfu and (Ih,I)a+ + S i4 = RER+ + Ps+ @cur together in each of the monazite and xenotime samples. The incorporation ofthe actinides in monazite can, in general, be described by an approximately equal contribution of both tle above.nechanisms. The incorporation of the actinides in xenotime, albeit less extensive than in monazite, can be largely accounted for by the second mechanism. The relative role of each mechanism of substitution in the two minerals is explained in terms of the size restrictions inherent in their structures. Mauy mineral grains exhibit combined levels of Ca and Si in excess of those expected as a result ofthe incorporation ofTh and U. Additional mechanisms ofsubstitution seem to be involved.

Diversity of precious-metal mineralization in footwall Cu-Ni-PGE deposits, Sudbury, Ontario; implications for hydrothermal models of formation

Abstract: The magmatic origin of contact orebodies of the Sudbury structure, in Ontario, is widely accepted. The remobilization of some of the metals from the magmatic envhonment to the footwall by hydrothermal processes is not as widely accepted as a possible mode of formation of Cu-Ni-PGE orebodies in the footwall. However, precious-metal minerals (PMM) occur most commonly in chalcopyritenear, or at, thecontactwith alteration minerals such as actinolite, quaftand epidotein footwall Cu-Ni-PGEmineralization of the Sudbury Struchre. Study loca.tions include the Barnet Property, Deep Copper and Copper zones of the Strathcona mine, McCreedyWastmine(Cu stringen), andtheFrasermine@pidoteZone).The mostcommonPMM areft-" PdandAg-richtelluride{ and bismuthides. Each deposit has a characteristic assemblage of PMM, with distinct compositions of these minerals. Among Cu-Ni-PGE mineralized zones, distinct Pt, P4 Bi and Sb variations are most common in sobolevskite, michenerite, merenskyite, moncheite and melonite. Comparison of whole-rock concentrations of Cu, Ni, Ir, Rh, Pt, Pd and Au for the Bamet Property with that of other Cu-rich footwall deposits further illusrates the individuality of each depsit The close spatial association of PMM with hydrous, locally Cl-bearing alteration and other rare minerals (including halides) is compatible with the i.nfluence of a H-€l-S-Fe{u-Ni-Te-Se-Bi-As-bearing fluid in deposition of the PMM. The assemblages of the PMM and geochemical distinctions of each Cu-rich footwall deposit are afunction of the interaction of contactmagmatic s"lfide asseoblages with fluids whose compositions were buffered by rock compositions in the footwall. The rretals scavenged from these primary base-metal sulfides were transported into the footwall and subsequently deposited in hydrothermal cells.

Olivine and chromian spinel in primitive calc-alkaline and tholeiitic lavas from the southernmost Cascade Range, California; a reflection of relative fertility of the source

Abstract: Chromian spinel and coexis '.9 olivine phenocrysts from a geochemically diverse suite of primitive tholeiitic and calc-alkaline basalts and maguesian andesites from the Lassen region, in the southernmost Cascade Range, in California, show that the sub-arc mantle is zoned. Depleted calc-alkaline basalts aad magnesian andesites erupt in the forearc region, and calc-alkaline basalts contain increasing abundances of incompatible elements toward the backmc. High-alumina olivine tholeiites erupt from the arc and backarc areas. Olivine from all these lavas displays a limited compositional range, from Fo86 to Fo9l, and crystallized at high temperature, generally 1225-Ln5"C. Chromian spinel trapped in the olivine phenocrysts displays a large raage of composition: Cr# values span the range 9J6. Excess Al in the spinel relative to that in l-atm spinel suggests that it crystallized at elevated pressure. The phenocrysts in these lavas are in equilibrium with their host liquids. The fulI range of Cr# of the spinel compositions cannot be explained by differentiation or variable pressure, variations in.t(Oz), subsolidus equilibration or variations in degree of partial melting of a single peridotitic source. Rather, the systematic compositional differences among phenocrysts in these primitive lavas result from bulk chemical variability in tlefu mantle sources. Correlations between spinel and host-rock compositions support the assertion that the geochemical diversity of Lassen basalts reflects tle relative fertility of their mantle sources.

Primitive ankaramitic magmas in volcanic arcs; a melt-inclusion approach

Abstract: Alkaramite is more mafic than basal! commonly porphyritic, and generally interpreted as a variety of picrite or olivine basalt that has been emiched in clinqryroxene crystals rather than a product of crystallization of an ankaramitic magma. Samples of ar:karamite from the lllakan Formation, Bali, and Rinjani volcano, Lombok, botl in the Sunda arc, and from Merelava and Epi, in the Vanuatu arc, contain olivine and clinopyroxene phenocrysts with vitreous and crystalline inclusions. Heated and homogenized silicate melt inclusions hosted by olivine with Fo,sohave CaOl/tlzOt@t%o) values > 1, and are rich in Mg (>14 wtvo MgO) and Ca (>13 wt%o CaO), and therefore have ankmamitic affinities and are quite unlike picrites. Although the composition of the inclusions provides evidence that primitive ankaramitic melts exis! they are consistently more silica-undersaturated than the ankaramitic bulk-rock compositions. Primitive ankararnitic melts are substantially higher in their normative diopside contents than the compositions of experimental melts of dry lherzolite at all pressures up to 4 GPa\" which typically are picritic. Olivine fractionation of primary komatiite-like melts with CaO/AlzO: (wt7o) values > 1, derived by partial melting of lherzolite at high pressures (>5 GPa), could produce compositions sinilar to primitive ankaramitic bulk-rock compositions. Altematively\" melting of lherzolite in the presence of mantle fluids bearing HzO and COz could produce compositions similar to those of the silica-undersaturated melt inclusions trapped in the olivine (Fqqo) of ankaramitic samples. Such melts would be likely to react with mantle during ascent o become less silica-undersaturated and more like ankammitic bulk-compositions.

Aspects of magma sources and processes in the honshu arc

Abstract: The volcanoes of the northern lzu - south-cental Honshu and northeastem Honshu arcs, Japan, occur in a region where vmiables such as age, conposition, and nature ofthe sediment cover oftle subducting plate are relatively uniform. In this study, v/e attempt to isolate some ofthe variables associated with suprasubduction zone (SSZ) magma genesis, in order to provide insight into ttre nature of sources and petrogenetic processes. The chemical compositions of selected parental and evolved magmas in fifteeu Plio-Pleistocene centers along the strike ofthe volcanic front allow us to assess tle significance of over-riding lithospheric input for SSZ magmatism. These volcanoes are distributed across a number of major terrane boundaries, including the lithosphere-penetrating Tanakura Tectonic Line. Conpositions range from basalt to rhyolite, with a preponderance of basaltic andesite and andesite; an exception is basalt-dominated Fuji. Most basalts are signiffcmfly fractionate4 with low MgO ( 3) and Dy/Yb, indicative of gamet involvement in their gercsis. The Nd and Sr isotope ratios also indicate regional variations in chemical composition of the source(s) (ong-term depleted to enriched). Pb isotopic data are distinctive in northern Honshu, with elevated 48Pb/2I/.Pb afi,NTPbPePb at specific 2n6Pb2oaPb (south versas north of the Tanakura Tectonic Line), are collectively elevated above the Northem Hemisphere Reference Line (and lzu-Mariana arcs), appear similax to portions of the Philippines and Tertiary basalts of eastern Asia and probably reflect an Asian lithospheric heritage. The combined trace-element and isotope geochemistry of parental basalts distinguish different domains within the Izu-Bonin, central and northeastern Honshu arcs. The ultimate cause of these is most likely multicomponent, and some crustal assimilation is higbly probable. However, tlese domains may in some paxt represent nagma interaction with a subcontinental lithosphere dominated by Indian-Ocean-type or alt€matively, derivation from an Indian-Ocean-type asthenosphere. The correlation of some magma compositional characteristics with nature of the basement terrane shows that the lithosphere plays a subtle but significant role in determining parental island-arc basalt chaxacteristics. The longevity and character ofthe continental lithosphere in Asian margin arc systems, and its interaction with asthenosphere-derived melts, is a1 imFcntant process in island-arc basalt genesis.

A new method of synthesis of boltwoodite and of formation of sodium boltwoodite, uranophane, sklodowskite and kasolite from boltwoodite

Abstract: Boltwoodite has been syntlesized at 185"C (1.3 MPa) from quartz, KCI and uranyl nitrate by a new method avoiding local supersatmation of silicat€ species in solution. Sodium boltwoodite, uranophane, sklodowskite and kasolite were synthesized from boltwoodite. Unit-cell paramet€n and space groups were determined from the X-ray powder-diffraction data. Infrared spectra have intense absorptionbands at 3458 and 3377 cm-l, which conespondto OH-stretching vibrations. Bands at 1384 cm-l indicate the presence of SiO:OH groups in the structures. Luminescence spoctra and solubility data are reported.

Primitive K-rich magmas from Clark Volcano, southern Kermadec Arc; a paradox in the K-depth relationship

Abstract: Clark volcano is the last significant volcanic edifice in the oceanic segment of the nearly 3000-km-long Tonga Kermadec New Zealand volcanic arc system, which becomes continental in the Taupo Volcanic Zone ofNew Zealand. Clark volcano is a submarine basaltic andesite dacite edi8ce with broadly similar petrochemical affinity to the rest of the Kermadec isla:rd arc (i.e., lavas are mostly of basalt-basaltic andesite composition, v/ith A,2VoKzO at507o SiOz, MgO in therange 5 ta7Vo, (Ce/Yb), between 100 ppm) and Cr (>200 ppm), and 5 < (Ce/Yb)r < 7; these basalts are unique within the Kermadec New Zealand subduction regime. Phenocryst assemblages in the K-rich basalts at Clark are olivine (t chromian qpinel) + clino,pyroxene, which contast with tle plagioclase + olivine t clinopgoxene assemblages in \"typical' Kermadec Arc basalts. In addition fe high lgv6ls 6f K and the iignt rare-earth elements, the K-rich suite shows higb concentrations of Ba (^6@ ppm), Rb (40 60 ppm) and Cs (-1 ppm). Radiogenic isotqres of Sr, Nd and Pb in the K-rich basalts are enriched relative to other maguas of the oceanic Kermadec Arc, including those of the more 'typical\" Clark basalts, and overlap with those of basalts from the continental Taupo Volcanic Zone of New Zealand. We interpret these pdmitive K-rich lavas as a rare example of near-slab, small-volume melts of a mantle wedge source enriched by sediment and fluid transfer from the descending slab of lithosphere. The nature of this subducting slab is significan! because it conprises crust ofthe Hikuangr Plateau, an anomalously thick sequence ofcretaceous basalt basement and sedimentary basin fill, which is subducting beneath tle present-day continentalTaupo Volcanic Tnne and the oceanic so$hern Kermadec Arc.

Primitive magmas in arc.type volcanic associations: examples from the southwest pacific

Abstract: In widely accepted models for tle origin of volcanic axcs at convergent plate margins, magmas originate in subductionmodified peridotitic mantle. Primary arc magrnas are predictably unique in ffierent arcs because of the variety of potential components and processes involved in their generation, but they should sharc the common characteristics of high concentrations of Mg, Ni and Cr, reflecting their mantle origin. An examination of suites of samples taken from tbree southwest Pacific volcanic arcs (the Kermadec, New Zealand" and Papuan arcs) shows contrasting geochemical pat[erns that correlate with different tectonic settings. Magmas with primitive chemical characteristics are comparatively rare, and appear to occur where an extensional tectonic se6ing has allowed patls ofrelatively rapid ascenl In typical arc settings, magma ponds at one or more positions above its source and is modified by a combination of fractionation, eruption, assimilalion and recharge prccesses, so that the most primitive magma erupted is i chemically evolved high-Al basalt. The factors that dictate the geochemical nature of magnas in volcanic arcs-are tectonic sefling togefher with the environment (depth) at which primary nantle-derived magmas are modifiedKewords: arcs, primary magma, high-Mg basalt, Eu anomaly, southwest Pacific.