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Compositional and Crystallographic Data on REE-Bearing Coffinite from the Grants Utani'm Regio& Nolthwestern New Mexico

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Compositional and Crystallographic Data on REE-Bearing Coffinite from the Grants Utani'm Regio& Nolthwestern New Mexico ' Amerkan Mineralogist,Volume 74, pages 263-270, 1989 Compositional and crystallographic data on REE-bearing coffinite from the Grants utani'm regio& nolthwestern New Mexico P,c.ur.AL. H,l,Nsrxv,JoaN J. Frtzparrrcr U.S. Geological Survey, Box 25046, M.S. 939, Denver Federal Center, Denver, Colorado 80225, U.S.A. Arsrnagr Paired substitution of Y and P in coarse-grained coffinite from the Upper Jurassic Mor- rison Formation in the Grants uranium region, northwestern New Mexico, indicates that coffinite forms an isomorphous serieswith xenotime, which is isostructural with thorite and zircon. Quantitative electron-microprobeanalyses ofcoftnite crystals(> l0 g.m)dem- onstrate that significant amounts of Ca, Y, and rare-earth elements EEEs) proxy for U as P substitutes for Si, effectively maintaining the charge balance. The abundant rhyolitic volcanic ash present in the Morrison Formation at the time of deposition is believed to be the source for U, Y, and REEs. Petrographicand stable-isotopedata suggestthat the coarse-grained,pore-filling coffinite anahyzedin this study was formed at elevated temperatures (at least 100 "C) during max- imum burial of the Morrison Formation in the mid-Tertiary. Supporting this relatively young age is the anisotropy and transparency of the coffinite crystals, indicating that they are not highly metamict. The coffinite owes its brownish color primarily to microscopic inclusions ofcarbonaceous material. The ubiquitous presenceofcarbonaceous inclusions in sedimentary coffinites suggeststhat organic C is critical to the formation of cofrnite at low temperatures(<200 "C). of Crownpoint, New Mexico. This drillhole intersects a Irrnonuc-rroN limf sf the nearby Nose Rock uranium deposit, which Coffinite [u(SiO4)r-"(O]I)J was frnt describedby Stieff also contains coarsely crystalline coffinite (Rhett, 1979, et al. (1955) in an investigation of sedimentaryuranirrm- 1980),Cstrnite-fearing samplesfrom the Nose Rock de- vanadium deposits in the Upper Jurassic Morrison For- posit (provided by D, W. Rhett) and a polished thin sec- mation from the Uravan mineral belt in southwestern tion containing coffinile (provided by S. S. Adams, Col- Colorado and southeasternUrah. Coffinite was subse- orado School of Mines) from a Homestake Mining quently found to be the major ore mineral in unoxidized Company drillhole Gl26-22) located about 16 km south- sandstone-hosteduranium-vanadium deposits in the eastof Crownpoint were also studied. The large size(> l0 Morrison Formation throughout the Colorado Plateau. It pm) of crystals from these localities enabled determina- has also been identified in many other stratiform sedi- tionoftheunit-cellparametersandcompositionsofthese mentary uranium deposits and in hydrothermal vein de- gqffinirc5. posits around the world (Ramdohr, 1980). The compo- Sedimentaryooffinite crystals of large enough size and sition and crystal chemistry sf seffinite in sedimentary purity from which petrographic and crystallographic data deposits are poorly known, because it commonly forms can be gathered are rare, alillsrgh hydrothermal veil de- submicrometer-sized crystals that are intimately mixed posits may contain massesof radiating coffinite crystals withamorphousorganicmatterand/orclayminerals.Cof- up to 5 mm long @amdohr, 1980). Most noted occur- finite is usually detected only from the presenceof broad rences of coarsely crystalline coffinite from sedimentary peaks on X-ray diftaction patterns. p"u1 6lsadening is deposits are from the Colorado Plateau and nearby areas. due to the small size of coherent X-ray domains and to Colloform coffinite crystals(>20 pmin lengft) were found the tendency of older coffinites to be metamict @am- in the Woodrow pipe, a mineralized collapse feature in dohr, 196l). the Morrison Formation il the southeasternpart of the This report describes extraordinarly large and well- Grants uranium region (Moench, 1962). Ludwig and formed coffinite crystals in the Morrison Formation from Grauch (1980) described and analyzed coarse-grained three localities in the Grants uranium region of north- colloform cofinite from Wyoming roll-front uranium de- western New Mexico, formerly the most productive ura- posits. Doubly terminated tetragonal prismatic crystals nium mining region in the United States (Chenoweth and up to 5 pm in length are intergrown with vanadiferous Holen, 1980).Mostcoffinilssampleswere obtainedfrom chlorite in uranium-vanadium ore from the Morrison a U.S. Geological Survey drillhole (no.7) located north FormationinsoutheasternUtah(Goldhaberetal., 1987), 0003{04v89/0r024263502.00 263 264 HANSLEY AND FITZPATRICK: REE-BEARING COFFINITE Fig. l. Scanningelectron micrograph of submicroscopiccof- finite crystals in remnant primary ore. Note interpenetrating cof- aggregatesof cofrnite finite crystals (C). Matrix is a mixture of clay minerals and amor- Fig. 2. Photomicrograph of colloform (gray) framework grains. Black material is pri- phous organic matter (OM). Sample 4-1933. Scalebar : 1 pm. crystals rimming mary carbonaceousore and pyrite. Plane-polarizedlight. Sample H26-22. TABLE1. X-ray powder-diffraction data for coffinite from the Psrnocrupnv Grantsuranium region, New Mexico Coffinite occurs primarily in fine- to medium-grained rcah arkosic litharenites of the Morrison Formation. In hand d*. I-i (anhydrous) specimens,coffinite-rich areas are black with a dull to 101 4.654 4.661 a 5b adamantine luster. Crystals are less than 2 pm in length 200 3.473 3.472 S 100 and are intimately mixed with organic matter and clay 211 2.783 2.779 MS 26 112 2.642 2.641 S 90 minerals so that definitive petrographic analysis is diffi- 220 2.456 2.458 M 25 cult (Fig. l). In a few samples,however, coffinite crystals 301 2.172 2.173 M 14 large enough to be studied optically. These crystals 103 2.001 2.000 M 10 are 321 1.841 1.842 MW 15 commonly form colloform rinds on detrital grains, and 312 1.799 1.800 J the long axes of coffinite crystals are perpendicular to 400 1.737 1.735 M 2'l grains no evidence 213 1.734 I grain margins (Fig. 2). Detrital show 411 1.627 1.626 MW 7 of replacementwhere in contact with coffinite. In direct 004 1.567 5 reflectedlight in oil, coffinite is brownish gray with a re- 420 1.553 1.551 M 21 332 1.451 1.451 M 22 204 1.428 1.428 W 18 323 1.416 4 431 1.356 1.354 W 6 TABLE2, Comparisonof unit-celldimensions (A) of selectedcof- 501 1.356 2 finites 244 1.321 1.316 VW 1a 413 1.311 J Reference O@urrence 521 1.263 3 512 1.249 1.250 W 22 This report 6.946(1) 6.268(2) Jm* sandstone 105 1.234 1 Dubinchuket al. (1981) 6.s8(3) 6.30(3) nypergene 440 1.228 1.228 W 6 6.93(3) 6.30(3) hydrothermal 404 1.163 I Dubinchuket al. (1982) 6.98(1) 6.24(1) sandstone 215 1.163 4 Belovaet al. (1980) 6.90 6.21 sedimentary 600 1.158 1.158 vw(d) b Nord(1977) 6.s380) 6.2s1(2)Jm sandstone 433 1.157 3 Bayushkinand ll'menev(1969) 6.92 6.2s Mo-U ore 503 1.'157 2 Tayloret al. (1966) 6.e7(2) 6.22(31 phyllite 611 1.123 3 Darnleyet al. (1965) 6.95(2) 6.26(3) hydrothermal 532 1.114 1.113 W 17 6.s8(2) 6.19(3) hydrothermal 424 1.103 1.103 W 18 Dymkov and Nazarenko(1962) 7.O1 6.26 hydrothermal (1961) 6.93(5) 6.21(21 hydrothermal ' Simmset al. Intensitiesby visual estimate: S : strongi MS : medium strong; M 6.97(1) 6.28(6) hydrothermal : : : : medium; MW : medium weak; W weak; VW very weak; (d) Fuchs and Gebert (1958) 6.981(4)6.2s0(5) synthetic diffuse. (1955) 6.94 6.31 Jm sandstone '" stieff et al. Intensitiescalculated using anhydrous formula with refinedzircon oxy- Grunerand Smith(1955) 6.92 6.22 Jm sandstone gen positionsand uraniumscattering factors. Allowedreflections with zero ' intensity not shown. Jm : Upper Jurassic Morrison Formationon the Colorado Plateau' HANSLEY AND FITZPATRICK: REE-BEARING COFFINITE 265 Fig. 3. Photomicrograph of cofrnite (C) texturesin Nose Rock ore. Q, quartz; PY, pyrite. Organic matter (OM) contains traces of U and other elements.Direct reflectedlight, oil immersion. flectivity only slightly higher than associatedquartz grains cell data (Table 2). The unit-cell parameterswere refined and much lower than nearby pyrite (Fig. 3). Coffinite is with a least-squarescomputer program (Appleman and light to medium brown in transmitted light and light to Evans, 1973). The refined unit-cell parametersdo not medium green under crossedpolarizers. Elongatecofrn- differ significantly from those reported for other coffin- ite crystals are length-slow with parallel extinction, and rtes. they exhibit a distinct anisotropy suggestingthat they are not highly metamict (Ramdohr, 1980; Speer, 1982a). Crystal aggregatesexhibit undulatory extinction, and the narrow basesofradiating crystal groupsare opaqueowing to numerous inclusions of titanium dioxides, pyrite, and carbonaceousmaterial. Under the scanningelectron mi- croscope,the morphology of coffinite aggregatesis clearly visible (Fig. a). Cnysr.lLLocRAPHY Coffinite is tetragonal (SG 14,/amd) and is isostructur- al with zircon and thorite (Speer, 1982a).Cofrnite crys- tals were handpicked from cofrnite-organic matter mix- tures for X-ray powder-diffraction analyses. An 8-h exposurein a Gandolfl camerausing Cu radiation yielded a pattern of 2l well-defined X-ray diffraction lines, in- cluding severallines in the back-reflectionregion. Quartz 2un was used as an internal standard. Unit-cell parameters calculated from measured 2-d angles and corresponding Fig. 4. Scanningelectron micrograph of clustersof elongate d values (Table l) were compared with published unit- cofrnite crystalswith squarecross sections.Sample 7-3260. 266 HANSLEY AND FITZPATRICK: REE-BEARING COFFINITE Fig. 5. Expandedwavelength-dispersive spectrum showing the Ia peaksofrare-earth elementsin cofrnite. Spectrumreads from upper left to lower right: Yb (7.41 kev) to Ce (4.84 keV). LiF spectrometer; 15 kV; sample current, l0 nA; time constant, 1.5 s. Sample7-3185. Er,ncrnoN-vrrcRoPRoBE ANALYSES the energy-dispersivesystem was usedduring the analyses Analytical rnethods to check for the presenceof other elements.
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