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 maximum 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 coffinite 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 isostructural with zircon and thorite (Speer, 1982a).Cofrnite crystals 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. Data were reduced using the MAGICIv computer program (Colby, Becausemost comnite crystalsare submicroscopicand I 97l). are commonly intergrown with other phases,electron- microprobe analysis is the most reliable method for de- termining the composition of coffinite. Electron-micro- Results probe analyseswere performed on polished thin sections Quantitative electron-microprobeanalyses revealed the of coffinite-bearing sampleswith the wavelength-disper- presenceof major Ca, Y, P, and minor Pb besidesU and sive system of an ARL-sEMqelectron microprobe. Both Si in the analyzedcoffinites (Table 3). In addition, REEs synthetic and natural standardswere used: diopside for were detected in all coffinites by energy-dispersiveanal- Ca and Si, apatite for P, and YAG garnet for Y (all pre- ysis. Due to their low individual abundances,however, paredcommercially by C. M. Taylor Corp.); natural urani- the REEs were not included in the quantitative analysis nite (Smellie et al., 1978, Table l) was used for U and scheme.Estimates of REE abundancesbased on relative Pb. Backgroundswere determined by offset on the high peak heightsin energy-dispersivespectra ranged from less and low side ofeach peak.All analyzedspots were verified than loloto 4010.Because the Z X-ray spectrallines of the as being homogeneousand flat by viewing (at 5000 x ) the REEs overlap those of the common elementsAl and Mg, secondaryelectron image ofthe areato be analyzed.Each the presenceof REEs was confirmed by plotting expanded reported analysisis an averageof4-9 spot analyseswithin wavelength-dispersivespectra of the REE region, which a small (< 100 pm) area.IJ, Si, Ca, Y, P, and Pb (assumed revealed REEs and traces of V, Fe, and Th(?). Sample to be radiogenic)were included in the analytical program; 7-3185 containedthe most REEs and Y with Gd > Dy

TABLE3. Semiquantitativeelectron-microprobe analyses of coffinitesfrom the Grantsuranium region

l- I- 7- 7- 7- 7- 3260- 3260- 3260- 3260- 3260- H26- H26- H26- H26- H26- H26- H26- H26- H26- 3185 1 2a b 3 4 22-1 22-2 22-3 22-4 22-5 22-6 22-7 22-8 22-9 UO, 63.8(5) 71.2(5)70.3(s) 70.4(s) 68.7(5) 6s.e(5) 68.9(5) 68.7(5) 69.0(5) 68.3(4) 68.2(5) 68.7(5) 6s.0(5) 69.0(5) 68.3(5) cao 1.s(7) 1.s(8) 1.8(8) 1.8(8) 2.2(8) 1.8(7) 2.3(s) 2.0(1) 2.0(5) 2.0(8) 2.1(112.s(s) 2.5(5) 2.4(91 2.4(9) Y"O" 6.0(3) 1.0(1) 1.0(1) 1.0(1) 0.4(1) 1.0(1) 0.4(2) 0.2(7) 0.2(1) 0.30) 0.2(1) 0.4(1)0.4(1) 0.4(1) 0.4(1) sio, 15.7(3) 15.8(3)15.8(3) 15.2(3) 19.3(3) 16.6(3)18.3(3) 18.s(3) 17.6(3)18.1(3)18.5(3) 18.6(3) 18-s(3) 18.6(3) 18.q3) P"Ou 3.3(1) 1.4(1) 1.4(1) 1.4(1) 1.0(1) 1.5(1) 0.s(1) 0.7(1) 0.70) 0.8(1) 0.8(1) 1 1(1) 1.1(1) 1.0(1) 1.1(1) Pbo 0.4(1) 0.q1) 0.3(1) 0.3(1) 0.3(1) 0.q1) 0.4(1)n.d. n.d. n.d. n.d. 0.q1) _9E1)_9E1) _9E1) Total 90.8 91.6 90.s 90.2 92.0 91.1 91.2 90.1 89.4 90.2 89.8 91.6 91.8 91.7 91.2 Note.'The numbers in parenthesesrepresent 2 standard deviationsin tenths of one percent; thus 63.8(5)indicates a standard deviation of 0.5; see text for discussionof other sources of error. Analyses (weight percent) are the averages of 4-9 spot analyseswithin an area of <100-pm diameter. Operatingconditions were 1s-kv acceleratingpotential and 1o-nA current on brass. Spot size was 1 pm; counting time was 40 s on Y and Pb and 80 s on the other elements.n.d. : not detected. HANSLEY AND FITZPATRICK: REE-BEARING COFFINITE 267

: Ho > Er : Nd > Ce; traces of Yb, Tm, and Eu were Accuracy of analyses also found (Fig. 5). The light rare-earthelements (LREEs) Our microprobe data are consideredto be semiquan- predominated in coffinites from sample 7-3260, which titative, becausethe oxide analysesdo not total l00o/0.In containedCe > Nd > Gd, and in sampleH26-22,which general, the low analytical totals typical of microprobe : contained Sm > Ce Nd > La. Apparently, no parlic- analysesof coffinite may be attributed to a combination ular REE or groups of REEs are favored by the coffinite of the following factors: (l) substitution of hydroxyl for structure. silica or absorption of nonstructural hydroxyl during metamictization; (2) the presenceof IJ6*; (3) angstrom- sized domains of C, titanium dioxides, and amorphous Discussionof results silica; (4) the additive effectsof trace amounts of elements Ca, which has commonly been detected in previous not in the analytical program, especially the REEs; (5) analysesof coffinite, and Y may proxy for U and P for Si microcracks in coffinite crystals; (6) absorption and flu- becauseof similar ionic radii. Pb is considered to be a orescencenot accountedfor in the correction procedure; product of radioactive decayof U. Coffinite that contains and finally (7) instrument and operator errors. Thesepo- no detectablePb may have lost radiogenic Pb becauseof tential sourcesoferrors are discussedbelow. migration of Pb from the coffinite structure (Ramdohr, Hydroxyl was originally thought to be an essential 1980) or the coffinite may be too young to contain de- component of coffinite, but anhydrous coffinite has been tectable amounts of Pb. Qualitative electron-microprobe synthesized(Fuchs and Gebert, 1958; Fuchs and Hoek- analysesof areasnot containing coffinite in ore samples stra, I 9 59), and infrared and ora studieshave shown that commonly detectedsmall areasof clausthalite (PbSe). hydroxyl is not an essentialconstituent of coffinite (Ab- Microprobe analysesof coffinite from roll-type urani- del-Gawadand Kerr, 1961; Belova et al., 1969).Smith um depositsin Wyoming detectedU, Si, and Ca (Ludwig (1984) suggestedthat the presence of organometallic and Grauch, I 980). On the other hand, Kim ( I 978) found complexes may account for deviation of the U:Si ratio significant P (averageP2Or, 2.18o/o) and trace amounts of from unity rather than the presenceof (OH)-. Nonethe- Al, V, Ca, and Fe in coffinite from the Woodrow mine in less,(OH)X- may substitute for (SiOo)'?-,or nonstructural the Grants uranium region. The inferred AlrO, (0.790lo) hydroxyl may be absorbed during metamictization due may actually be a composite of REE peaksbecause of the to radioactive decay. Partial metamictization causesdis- overlap betweenX-ray peaksof Al and REEs. Supporting ordering of crystal domains and, finally, may lead to a this hypothesis, photomicrographs of the cofrnite that complete breakdown of coffinite into amorphous silica Kim analyzed(Moench, 1962) show apparently pure cof- and uraninite (Brodin et al., 1980).However, the analyses finite crystals devoid of clay-mineral contamination. To of this study show excessP and Si, suggestingthat (OH) our knowledge,the only other Y- and P-rich coftnite that has not substituted for silica. In addition, the anisotropy has been reported in the literature is coffinite in organic- and transparencyof the coarselycrystalline coffinite sug- rich nodules from the Triassic Mercia Mudstone Group gest that it is not highly metamict and, thus, probably in England, which contained 2l wto/oYrO, (analysesnor- does not contain significant hydroxyl. malizedto l00o/o;Harrison et al., 1983). The presenceof U6* has been confirmed in uraninite Most bulk chemical analysesof coffinite list elements (Frondel, 1958) and in coffinite by X-ray photoelectron that are unlikely to fit into the coffinite structure owing spectroscopy(Goldhaber, 1977). The uranyl ion, there- to incompatible ionic radii and,/orionic charge.Chemical fore, may partly account for low analytical totals and for analysesof the first described coffinite included As and apparent underpopulation of the uranium site, because Al (srieffer al.,1956); Arribas (1966)listed Mg, Na, Al, the data reduction programs did not allow for the pres- S, and Fe as elementsin coffinite;Belova et al. (1969) enceofIJ5*. Even ifall U were IJ6*,however, only 34 found Mg, Fe3*,Al, and S in coffinite;Belova et al. (1980) wto/owould be added to the analytical totals. noted Fe and P in cofrnite pseudomorphsafter ningyoite Visible inclusions of titanium dioxide, pyrite, and C in [(U,Ca,Ce),(PO),.l-2H,O]; Khalezov and Avalonin coffinite crystals suggestthat submicrometer-sizedinclu- (1974) reported coffinite with Ti, Y, Fe3*,Th, Ce, V, and sions may also be present.These phases, therefore, may Cr; and Bylinskaya (1980) found metamict cofrnite that constitute a significant portion of the missing percentage contained tracesof Y and Ce. Coffinite that had replaced in analysistotals. In addition, secondaryelectron images pitchblende in ore from Cornwall contained traces of La of some ore samplesat >40000x revealedthat botryoi- (Taylor and Harrison, 1958).Because of similar ionic ra- dal material containing varying proportions of U, Si, and dii and charges(see Shannon and Prewitt, 1969),As and C(?) was closely associatedwith the coarsely crystalline P could proxy for Si whereasY, Ce, La, and Th could coffinite (Fig. 6). Although spots known to contain this substitute for U, but Al, Fe, Mg, S, Ti, Cr, and V were amorphous material were avoided, small amounts may probably contained in admixed iron oxides, titanium lie underneath analyzedareas. dioxides, and clay minerals. Theseanalyses undoubtedly As discussedunder Results,the REEs may account for reflect the impurity of the samplesanalyzed, because of a significant portion of the missing percentagein the an- the known difficulty in obtaining pure coffinite separates. alytical totals. The isostructural minerals thorite and zir- 268 HANSLEY AND FITZPATRICK: REE.BEARING COFFINITE

1982a):a lrace of Th was detected in only one coffinite analyzed in the present study. In synthesisexperiments (1350'C), Mumpton and Roy (1961)found that zircon can accommodate up to 6 molo/oUSiOo, but only traces of ZrSiOo have been found in natural coffinite (Speer, 1982a;this study). Isomorphism between coffinite and the isostructural phosphatexenotime is predicted by (l) isomorphism between zircon and xenotime evidenced by the intermediate varietal species"oyamalite," and (2) isomorphism between thorite and xenotime evidenced by the intermediate varietal species "auerlite" (Dymkov and Nazarenko, 1962). Zircon may exhibit coupled substitution of REEs plus Y and P for Zr and Si, respectively (Speer, 1982b). Similarly, substitution of Y and P for U and Si, respec- Fig. 6. Scanningelectron micrograph of cofrnite and asso- tively, strongly indicates that isomorphism betweencof- ciatedsiliceous microspheres containing major U, Ti, Si, andlor tracesof otherelements. Sample 7-3260. Scale bar : 5 rrm. finite and xenotime exists. Y3* and the REEs (2+ and 3+) can proxy for IJ4*,Tho*, andZra*; and P5*can substitutefor Sio*because of similar con may also contain significant amounts of REEs. Up ionic radii. Coupled substitution occurs becausethe charge to 20 wto/oY and REEs have been detected in natural imbalance causedby the substitution of 3+ (e.g.,Y) for thorite (Staatz etal., 1976),and zircon can accommodate 4+ elements(e.g., U) is compensatedby substitution of up to 25 wto/oREEs and Y (Speer, 1982b). In a compre- a 5+ (e.9., P) element for Si. Whether a new mineral hensivegeochemical study of the Morrison Formation in phaseintermediate betweencoffinite and xenotime exists the Grants uranium region, Della Valle (1981) also found owing to paired substitution is unknown. The diffraction REE enrichment in ore zones, but he assumedthat the pattern of coffinite in sample 7-3185, which contains up REEs were adsorbedonto clay minerals. to 6 wto/oYrO, and 3.3 wto/oPrOr, was distinctly different Visible shrinkage(?)cracks in large crystalswere avoid- from that of the other coffinites of this study, but insuf- ed by checking the secondaryelectron image before each ficient sample was available to establish whether a new analysis;thus, error due to an uneven surfaceis thought mineral phasewas present. to be minimal. Absorption and fluorescencenot correctedfor by rvrac- GnNosrs oF coARSE-cRATNEDsEDIMENTARy rc rv may be significant owing presence to the of elements COFFINITE that were not included in the analytical program and to The coarselycrystalline coffinite analyzedin the pres- matrix effectsarising from compositional differencesbe- probably tween the unknown and standards. Elemental interfer- ent study formed from recrystallization of ear- lier U-bearing phasesduring burial diagenesis(Hansley, enceswere minimized by selectingbackground values us- 1988). remained locally high ing the expanded wavelength-dispersive spectra as guides. Silica activity throughout Additional sourcesof error due to microprobe hard- diagenesisbecause of the dissolution of unstable silicate framework grains, such as rock fragments and plagio- ware (e.g.,instrument drift), software (e.g.,errors not ac- Thermodynamic have high counted for by the data-reduction program), inhomoge- clase. studies shown that a silica activity (10-,,; favors the formation of coffinite neous standards(particularly uraninite), and operator error were kept to a minimum. (Hemingway,1982,and Goldhaberet al., 1987).Stable- isotope minerals in In summary, it is difficult to quantify the potential an- data on authigenic clay coffinite-bearing warm (130'C), alytical sourcesof error discussedin this section; how- sandstonessuggest that deep-basin fluids moved updip through ore-bearing units in the ever, they may add as much as 5oloerror to the counting region maximum statistic errors listed on Table 3. Grants uranium during burial of the Morrison Formation in the mid-Tertiary (Northrop and Whitney, 1985). The presenceof regularly interstratified Isovronprrrslr illite-smectite (Whitney, 1986),ankerite, and albite in cof- On the basis of experimental studies and the compo- finite-bearing ore zones (Hansley, 1986a) also indicate sitions of natural coffinites, substitution betweencoffinite that temperaturesof at least 100 oC were attained during and the isostructural minerals thorite and zircon is ap- diagenesis(Perry and Hower, 1970;Boles, 1978,1982). parently limited. Mumpton and Roy (1961) were able to Warm fluids (>80 .C) would have acceleratedthe mat- synthesizethorite hydrothermally (1350 "C) with up to uration oforganic matter in ore zones,causing the release 20 to 30 molo/oUSiOo substitution.Foord et al. (1985) of organic acids, which promoted the dissolution of de- described a uranoan thorite occurring in rhyolite with trital grainsand authigeniccements (Surdam et al., I 984). about 30 molo/oUSiOo. On the other hand, only small The precipitation and growth of large coffinite crystalsin amounts of ThSiOo have been found in cofiinite (Speer, secondarypores were facilitated by warm fluids, for de- HANSLEY AND FITZPATRICK: REE-BEARING COFFINITE 269 spite its common occurrence in low-temperature sedi- tron microprobe. Carol Gerlitz gave invaluable assistancein refining the mentary deposits, coffinite has not yet been synthesized analytical data by computer. Gene Foord, David Frishman, Bruce Hem- reviewer provided thorough technical re- (Hemingway, ingway, and an anonymous at temperaturesless than 200 "C 1982). views, which greatly improved the manuscnpt. The ubiquitous association of coffinite with organic matter and the occurrenceof inclusions of organic matter in many sedimentary coffinites (Fuchs and Hoekstra, Rrnnnr,Ncrs crrno 1959;Smith, 1984)imply that the presenceof amorphous Abdel-Gawad, A.M., and Kerr, P.F. (1961) Urano-organic mineral as- organic matter as a strong sorbant and reductant (or as a sociation.American Mineralogist, 46, 402419. substrate for biogenic HrS) may be critical to coffinite Appleman, D.E., and Evans, H.T., Jr. (1973) Job 9214: Indexing and formation at temperatures less than 200 'C. The black least-squaresrefinement of powder diffraction data. U.S. National Technical Information Service,Document PB-216188. color, characteristicof coffinite, may be due to the pres- Arribas, P.A. (1966)Nuevos datos sobrela cofrnita. EstudiosGeologicos, ence of C and/or U6* (Smith, 1984). X-ray elemental 22, nos. l-2, 47-59. mapping (U, Si, and C) of ore samplesperformed on the Bayushkin, I.M., and Il'menev, E.S.(1969) Coffinite crystalsin a molyb- electron microprobe indicated that not all U is in cofrn- denum-uranium deposit. Ministerstvo VysshegoObrazovaniya SSSR, Zavadeniy, Geologiya i Razvedka, ite. Some U may be adsorbedonto organic matter (Lev- Izvestiya Vysshikh Uchebnykh Moscow, 12, 3942 (in Russian). enthal, 1980),whereas some may be presentas uraninite. Belova, L.N., Tananaeva,G.A., and Frolova, K.E. (1969) Coffinite (a Associatedrhyolitic volcanic ash (Watersand Granger, uranium mineral). Atomnaia Eneryiia, 27, 6143 (in Russian). 1953) may have been the sourcefor REEs in coffinite as Belova,L.N, Gorshkov,A.J., Ivanova,O.A., and Sivtsov,A.V. (1980) acidic volcanic rocks are enriched in REEs (Humphris, Nature of the so-called phosphorus-containing coffinite. Akademiya Nauk SSSRDoklady, Moscow, 255, 428430 (in Russian). 1984). Phosphoruscontained in the coffinite and associ- Boles,J.R. (1978) Active ankerite cementation in the subsurfaceEocene ated authigenic euhedral apatite may have been derived of southwest Texas. Contributions to Mineralogy and Petrology, 68, from organic material during diagenesis,as fossil bones 13-22. and plants are abundant locally in the Morrison Forma- -(1982) Active albitization of plagioclase,Gulf Coast Tertiary. 282, 165-180. tion. Detrital apatite grains in ore zoneswere not a source AmericanJournal ofScience, Brodin, 8.V., Sidorenko,G.A., and Dubinchuk, V.T. (1980)Structure and of phosphorus for they are generallyunaltered (Hansley, composition of metamict coffinite (concerning the nature of metamict 1986b). minerafs). International GeologyReview, 22, l7 +182 [translatedfrom Stroyeniye i sostav metamiktnogo koffnita (o prirode metamiktnykh CoNcr,usroNs mineralov), Akademiya Nauk SSSR, Izvestiya, Seriya Geologiche- skaya,Moscow, 7, 96-107, 19781. Semiquantitative electron-microprobe analysesof un- Bylinskaya,L.V. (1980) Characteristicsofthe composition and properties usually large crystals of coffinite from sandstone-hosted of coffinites.Zapiski VsesoluznogoMineralogicheskoe Obshchestva, 5, uranium deposits in the Grants uranium region demon- 589-592 (in Russian). stratethat coffinite can accommodatesignificant amounts Chenoweth, W.L., and Holen, H.K. (1980) Exploration in the Grants Mines Mineral of Ca, Y, P, and REEs. The apparent paired uranium region since 1963.New Mexico Bureauof and substitution ResourcesMemoir 38, 75-85. of Y and REEs for U and of P for Si suggeststhat cofrnite, Colby, J W. (1971) MAGIC IV-A computer program for quantitative like zircon and thorite, is isomorphous with xenotime. electronmicroprobe analysis.Bell TelephoneLaboratories, Inc , Allen- The degreeto which REEs and other elementssubstitute town, Pennsylvania. in the coffinite structureis probably related to the relative Darnley, A.G., English, T.H., Sprake, O., Preece,E.R., and Avery, D. (1965) and coffinite from southwestEngland. Min- proportions Ages ofuraninite of eachelement in interstitial solutions at the eralogicalMagazine, 34, 159-17 6. time of coffinite formation. In the case of the Morrison Della Valle, R.S. (1981) Geochemicalstudies of the Grants mineral belt, Formation, the REEs, Y, and U were readily available New Mexico. Ph.D. thesis, 350 p. University of New Mexico, Albu- becauseofabundant rhyolitic volcanic ash. querque,New Mexico. The 8-100/olacking in the microprobe analysesis due Dubinchuk, V.T., Naumova, I.S., Kravtsova, L Yu., and Sidorenko,G.A. (1981) Determination of the crystal structure of naturally occurring mainly to the presenceof REEs, organic C, and U6* that coffinite. MineralogicheskiiZhurnal, 3, 81-85 (in Russian). were not included in the analytical scheme.Optical prop- Dubinchuk, V.T., Rasulova, S.D., and Trostyanskii, G.D. (1982) Lito- erties of the coarsely crystalline coffinite suggestthat it logiya i PoleznyeIskopaemye, 6,27-36 (in Russian). contains an insignificant amount of hydroxyl. The in- Dymkov, Yu. M., and Nazarenko,N.G. (1962) Cofrnite and the nature of pitchblende pseudocrystals.Geochemistry, 4, 348-358 (translated ferred low hydroxyl content ofthe coffinite, reported sta- from Geokhimiya, 4, 304-312, 1962). ble-isotopevalues, and types ofauthigenic phasesin cof- Foord, E.E., Cobban, R.R., and Brownfield, I.K. (1985) Uranoan thorite flnite-bearing ore zones suggestthat the large coffinite in lithophysal rhyolite-Topaz Mountain, Utah, USA. Mineralogical crystals were formed near 100 "C and are no older than Magazine, 49, 729-731. mid-Tertiary. Ubiquitous inclusions of carbonaceous Frondel, C. (1958) Systematicmineralogy of uranium and thorium. U.S. GeologicalSurvey Bulletin 1064,400 p. material (probably remnants of primary ore) in the cof- Fuchs, L.H., and Gebert, E. (1958) X-ray studies of synthetic cofrnite, finites of this study and in other sedimentary coffinites thorite, and uranothorites.American Mineralogist, 43, 243-248. suggestthat organic C plays an important role in the for- Fuchs, L.H., and Hoekstra, H.R. (1959) The preparation and properties mation of coffinite at low temperature. of uranium (IV) silicate. American Mineralogist, 44, 1057-1063. Goldhaber, M.B. (l 977) Evaluation of X-ray photoelectronspectroscopy AcrNowr,r'ncMENTS as a technique for analysis of uranium ore samples.U.S. Geological Survey Open-File Report 77-798,25 p. We thank Richard Grauch for aid in settingup the quantitative analysis Goldhaber, M.B., Hemingway, B.S., Mohagheehi,Ali, Reynolds, R.L., program for cofrnite and Ralph Christian for help in operatingthe elec- and Northrop, H.R. (1987) Origin of coffinite in sedimentaryrocks by 270 HANSLEY AND FITZPATRICK: REE-BEARING COFFINITE

a sequentialadsorption-reduction mechanism Bulletin de Min6ralogie, Ramdohr, P. (1961) Das vorkommen von Coffinit in hydro-thermalen ll0,131-144. Uranerzgangen,besonders von Co-Ni-Bi-typ. NeuesJahrbuch fiir Mi- Gruner, J.W, and Smith, D.K., Jr. (1955) The problem of coffinite. In neralogieAbhandlungen, 9 5, 313-324. U.S. Atomic EnergyCommission Report (April l, 1954 to March 31, -(1980) The ore minerals and their intergowths (2nd edition, vol. 1955), p. 16-24. National Technical Information Service,Documenr 2), 1207 p. PergamonPress, New York. RME-3020. Rhett, D.W (1979) Mechanism ofuranium retention in intractable ura- Hansley, P.L. (1986a) Regional diageneticpatterns and uranium miner- nium ores from northwesternNew Mexico. Journal of Metals, 31,45- alization acrossthe Grants uranium region,northwestern New Mexico. 50. American Association of Petroleum Geologists Studies in Geology 22, - (1980) Heavy-mineral criteria for subsurfaceuranium exploration, 3r5-329. 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