The Reduced Uraniferous Mineralizations Associated with the Volcanic Rocks of the Sierra Pena Blanca (Chihuahua, Mexico)

American Mineralogist, Volume 70, pages 1290-1297, 1985

The reduced uraniferous mineralizations associated with the volcanic rocks of the Sierra Pena Blanca (Chihuahua, Mexico)

BRtcIrrn ANru, Centre de Recherchessur la Giologie de I'Urqnium BP 23, 54 5 0 1, V andoeuure-les-Nancy C6dex, F rance

,lwo hcquns LERoY

Centre de Recherchessur la Giologie de l'Uranium BP 23,54501, Vaniloeuure-les-Nancy Cidex, France and Ecole Nationale Supirieure ile Gdologie Appliquie et de Prospection Miniire ile Nancy, BP 452,54001, Nancy Cidex, France

Abstrect The uraniferousmineralizations of the Nopal I deposit (Sierra Pena Blanca, Chihuahua, Mexico) are related to a breccia pipe in a tertiary vitroclastic tuff (Nopal Formation). De- tailed mineralogicaland fluid inclusion studies led to the discovery of a primary mineralization stageof tetravalenturanium as part of an ilmenite-hematitephase. This stageoccurs soon after the depositionof the tuff and is relatedto H2O-CO, N, fluids, similar to those of the vapor phase,under temperaturesranging from 300 to 350'C. The well developedkaolini- zation of the Nopal tuff is associatedwith a secondtetravalent uranium stage(pitchblende- pyrite association).Fluids are aqueousand their temperatureranges from 250 to 200'C. The precipitation of pitchblende with pyrite within the pipe is due to a HrS activity increase strictlylimited to this structure.The remainderof mineralizingevents, either hydrothermal or supergene,led to the hexavalenturanium minerals that prevailtoday.

Introduction of Chihuahua city. This Sierra belongs to the north-south trending Basin and Range Province of Mexico (Fig. 1). It The uraniferous minerals mined in the Pena Blanca dis- roughly consists of a tertiary volcanic pile (rhyolitic vit- trict, north of Chihuahua city, Mexico, are commonly oxi- roclastic tuffs) overlying a calcareous basement of cre- dized minerals (uranophane, weeksite, haweeite, margar- taceous age (Alba and Chavez, 1974; Cardenas-Flores, itasite, metatyuyamunite, soddyite) (Calas, 1977; Uramex, The Nopal I deposit 19E0; Goodell, 1981; Wenrich et al., 1982).However tet- 1984; Magonthier, 1984a, 1984b). ravalent U minerals (pitchblende) have been described, but looks like a breccia pipe and the mineralization is strictly presently are only known in the Nopal I deposit (Rodri- located within this structure. A barren ring of altered tuffs guez, 1976; Calas, 1977; (Jramex, 1980; Goodell, 1981; (kaolinization and locally development of montmorillonite) Aniel, 1983). The relations between pitchblende, oxidized is observed up to 100 meters horizontally away from the minerals, and wall-rocks alterations in this Nopal I deposit "pipe" (Aniel, 1983; George-Aniel et al., 1984). have not been clearly defined. According to Calas (1977), The unaltered Nopal formation is a partially melted vit- pitchblende may have the same age as the oxidized min- roclastic tuff. Phenocrysts of Na-K feldspars (anortho- erals and may have resulted from a local reaction between clase), quartz and oxide minerals (Ti-magnetite and ilmen- uraniferous solutions and already present pyrite. ite) with zircon inclusions and hematite exsolutions (Fig. Detailed mineralogical and chemical studies of Nopal I 24, and 2B), are disseminated in a crystalline matrix, mineralizations (Aniel, 1983) led to the discovery of a pri- characterized by cristobalite and feldspar shards with ax- mary depositional stage of tetravalent uranium minerals iolitic texture. In the Nopal Formation, vapor phase min- (stage 1) before the development of the previously men- erals (quartz and K feldspar) are locally observed (Aniel, tioned pitchblende (stage2). The purpose ofthis paper is to 1983).In the altered ring, feldspar phenocrysts are progres- describe these two tetravalent U stagesand their relation- sively replaced by kaolinite; Fe-Ti and Fe minerals are pipe, phenocrysts feld- ship with alteration products such as kaolinite and mont- stable. Within the both and matrix kaolinized. The ilmenite and mag- morillonite in the Nopal I deposit. spars are completely netite are no longer stable. The magnetites are replaced by Geologic setting a pitchblende-pyrite phase (Fig. 2C). Ilmenites appear to be The mining district is located in the eastern border of the stable (Fig. 2D), but detailed mineralogical and chemical central part of Sierra Pena Blanca, 50 km to the northeast studies of these minerals within the pipe led to the dis- 0003{04x/85/1 I I 2-1290$02.00 1290 ANIEL AND LEROY: REDUCED URANIFEROUSMINERALIZATIONS t291

Fig. 1. (a) Distribution of tertiary volcanic rocks (from Bagby et al, 1976) (b) location map of Sierra Pena Blanca in the basin and range system (from Goodell, 1981)

covery of a primary stage of uranium deposition, before the rutile. Such variations in Ti or Fe would, in fact, produce development of the present pitchblende-pyrite phase. the opposite effect, an increase in the reflectance. After a magnetic separation of minerals, powder diffrac- Primary hydrothermal uranium oxides-ilmenite phase tion patterns were made on ilmenites from the non-altered (phase l) rocks and from the altered rocks located within the breccia pipe (Debye-Scherrer and Guinier de Wolf Cameras). Within the pipe, ilmenite phenocrysts seem to be similar The positions of the X-ray powder lines obtained for the to those of the fresh Nopal tuff, with the same elongation two types of ilmenite are identical and are close to those of ratio (2.3) and the same zircon inclusions. However they ilmenite. For the same time (48 hours) the only difference can be distinguished by their reflectance, their chemical between the two diffraction patterns is the width and the composition, and by small pitchblende veinlets within crys- clearness of the lines. For ilmenites from the barren rock, tals. lines are well defined, indicating well-crystallized material. Polished sections were used to make measurements of In contrast, lines from ilmenites located within the pipe are reflectance(Standard SiC with R: 2O.4%for i : 540 nm). diffuse. No other mineral phase is present in the diffraction Outside the zones of pitchblende veinlets, these minerals pattern. If one exists, it is amorphous (metamict). Thus, have reflectance values ranging from 12.5 to 17.5oh, X-ray diffraction studies confirm the existence, in both whereas the ilmenites of the non-mineralized rocks have mineralized and barren rocks, of a phase with an ilmenite- values between 19 and 21,'/.. ln hematite-enriched zones type structure. (exsolution lamellae and the crystal rim) the reflectance Analyses of these ilmenite-type structure minerals were values are from 2l to 26,'/o(Fig. 3). The decrease of reflec- obtained with an electron microprobe (automated Came- tance in the ilmenites of mineralized samples does not bax electron microprobe). The chemical composition of this result from increase in Ti or decrease in Fe content, or mineral is very homogeneous (Table 1) away from the from exsolution or inclusions of hematite, magnetite or pitchblende veinlets. The three most abundant elements are ANIEL AND LEROY: REDUCED URANIFEROUS MINERALIZATIONS

Fig. 2. Photomicrographs of phases occuring in the unaltered Nopal formation (A and B) and in the Nopal I deposit (C, D, E and F). (A) Stable microphenocryst of magnetite (Mt) (reflected light photomicrograph). (B) Ilmenite (Ilm) microphenocryst with zircon inclusions (Z) (reflected light photomicrograph). (C) Pitchblende (Pe) and pyrite (Py) pseudomorphs after magnetite (reflected light photomicrographs). (D) Ilmenite uranium oxide phase (Ilm) with 3 zircon inclusions (Z), surrounded by pitchblende (Pe) and pyrite (Py). The pitchblende apparently formed prior to the pyrite (backscatter electron photomicrograph).(E) Backscatter electron photo of ilmenite-uranium oxide phase: (Z) zircon; (Pe) pitchblende; (1) boxworks after uranium was leached.(F) Backscatter electron photo of pitchblende (Pe) in fracture;(1) ilmenite-uranium oxide associationwhere UOrcontent is about 62%;(2) uranium poor zone. uranium(locally up to 67 wt.%),iron, and titanium(Fig. a). from 0.30 to 0.47 insteadof 0.90 to 1.25as in ilmenites Minor elementsare Ca. Pb and Zr. Th and the rare from the freshtuff. This changein Fe/Ti ratio is interpreted earth elementsCe and Y are less than the limit of detec- asan iron lossduring the uraniumenrichment. tion. The Fe/Ti ratio in theseuranium-rich mineralsranges Traverses were performed on single crystals with a ANIEL AND LEROY: REDUCED URANIFEROUSMINERALIZATIONS 1293

Nl llmenite

Im E Ulvoxide-ilmenrtephase Y22 pi1sh61an6s

E Magnetite -l I Mineralsof reference (Picot and Johan,1977) 3 * | (\,1 ol 9 F

Pit Pit Mr Mt It -^^------lf------>Hm %------r-o * IIfo 1s 20 2:s 30 I REFLECTANCE I Fig. 3. Reflectancemeasurements vs. TiO, content in wtok in different oxides. Ilm hematite. Ia, Ib: magmatic phase; II: association 1; IIIa, IIIb and IIIc: association 2.

cAMEcA MS 46 electron microprobe to study the distri- neouszones, (as seenwith the electron microprobe analy- bution of U, Ti, and Fe (Fig. 5A). It was found that signifi- ses),light and heavy elementsare distributed in a uniform cant concentrations ofuranium (as pitchblende) and oftita- pattern.Uranium rich zoneshave a constantorientation nium (as titanium oxides) clearly appear in the veinlets which is independentof the pitchblendeveinlets. They re- occurring in the ilmenite-type structure mineral (Fig. 5B). flect the direction of the exsolution lamellae(hematite la- Outside these veinlets, the distribution of U, Ti, and Fe is mellae)in the ilmenitesfrom the unalteredtuff. rather homogeneous. In backscatter electron microscope photomicrographs FeO (Figs. 2E and F), heavy elements, such as uranium, are ;,\ -t \ lighter in color. Areas enriched in titanium and iron, which ,^\z-\ are lighter elements, are greyish. In the apparently homoge- /\ zor" /\ \8o /\ Table l. Representativeanalyses by automatedCamebax electron /\ microprobe(wt.%): (1) ilmenite from the fresh Nopal tutr; (2) /'t 401 \60\ pitchblendefrom the Nopal I deposit;(3) ilmenite-uraniumoxide t\ t- association;(4) U-depletedzones in association1 from both sides li of pitchblendeveinlets (stage 2). r'\ \40

L oxide-ilnenlEe (^ssociarion I ) (l) (2)

\20 sio2 0.45 o '16 0,15 0.06 A o.l0 t^ \ ll,oi - 0.09 A\ Feil 1a.72 to.ql 024 6,18 tJ.66 tla Fe20l - I 5.45 rlno 0.56 4 ,44 Tiol 17,33 4n.00 O.15 16.01 2q,82 Cao' L65 3,81 2.92 l.8l I 5l I 05 I'Oz 91.90 89,30 9i.96 7t.45 49.73 Pbo I .i5 o 75 0.:4 0 16 z.o2 0.58 0 16 n 03 Pitchblende P^O. 0.10 0.3l n.l7 a 11

IOT^L a7 16 Z 9r 05 ): 98 35 7 95.OJZ 9i,5A 7 97.32 7" 95-2f 7,. 96.56 Z pnase linnbers of lons on tlre basls of pnase 6(o) 2(0)

0,025 0 009 - o or4 Titanium oxide r .844 1.418 - 0Oct *' Magnetire 0.024 0 201 ^ l- T 2,026 t.672 o ol4 0 nol El 0.075 0.164 ll llmenite 0 834 0 802 h o.:70 0 008 Fig. 4. Microprobe analyses of different oxides in a UO2-TiO2 FeO diagram (wt.%,)(Aniel, 1983).(A) sampleslocated within the Nopal I deposit; (B) oxides coming from the unaltered \.c37 1.975 I OOO Nopal Formation. r294 ANIEL AND LEROY: REDUCED U RANIF EROUS M I N ERALIZ AT IONS

Pitchblende-pyrite phase (phase 2) @ The pitchblende-pyrite phase (phase 2) occurs as dis- seminations in the kaolinized matrix, pseudomorphs of magnetites, or rings around the uranium oxides-ilmenite phase (phase 1). Pitchblende, without pyrite but with local TiO, concentrations, is also observed as small veinlets within phase 1. As for phase 1, the pitchblende-pyrite phase is strictly located within the pipe. This pitchblende has a reflectance ranging from 12 to 14% (Standard SIC, R :20.4% for ).:540 nm) (Fig. 3) and the value of the unit cell, a, determined by X-ray diffraction is 5.42A. The pitchblende composition shows no systematic variation with its distribution in the mineralized tuff Ca is the main trace element in this pitchblende (from 1.50 to 3.80 wt.% CaO) (Table 1). Pb content is also vari- Ti able (0.06 to 2.35 wl% PbO). Microscope observations of dltered and mineralized tuffs from the Nopal I deposit (Aniel, 1983) indicate that this pitchblende-pyrite phase is related to the kaolinization oI the Nopal tuff. Hexavalent uranium minerals precipitated Fe after this stage under oxidizing conditions; iron sulfldes were transformed to oxides and hydroxides at the same @ time. The relations between the two reduced phases, I and 2, T--'l UlVcx idc ilr'lenrte t_____Jphasc can be seen on pseudomorphs after ilmenite where they both coexist. The pitchblende veinlets (phase 2) intersect l---l pitchbtenoc phase I and are connected to the pitchblende-pyrite rings Zl ri oridc around the ilmenites (Fig. 5B). On both sides of these veinlets, there is a U-depleted zone (Fig. 6). Ti is also leached z,r.o,, f] but to a lesser degree. This zone shows a regular lattice of Fl Pyrrtc lens-shaped holes as seen by scanning electron microscopy (Fig. 2E). Their orientation, shape, and size are constant Fig. 5. Traverses for U, Ti and Fe obtained with a cmmcn us and similar to the U-enriched zongs in the ilmenite (phase 46 electron microprobe (A) on the ilmenite-uranium oxide associ- 1). These holes (boxworks) are considered to have been the ation (B). sites of the primary uranium deposition in the magmatic ilmenites (phase 1, Figs. 2E and 2F). These uranium-rich minerals are interpreted as a uranium oxide-ilmenite phase.It correspondsto a first con- c€ntration of uranium within the Nopal I pipe. Uranium precipitated within volcanic ilmenites, perhaps in micro- fractures controlled by the hematite exsolution lamellae lattice. Such a uranium-iron andfor titanium phasehas already beendescribed in the literature.Zielinski (1978)mentions this phasein rhyolitesof the westernUnited States,but he did not determinethe exactrelations between uranium and the Fe-Ti-Mn oxides.ln the mineralizedsystems of Duo- blon (Sweden)(Lindros and Smellie,1979; Smellie, L982), the uranium releasedduring the devitrification of the ignimbrites precipitates pitchblende as flne-grained in con- phase tact with ilmenite and ilmeno-magnetite.The destabiliza- 1, UlVoxide-ilmenite tion of this phaseleads to the formation of urano-titanate 2, UlVoxide-ilmenitephase depleated complexes.Elsewhere, Caruso et al. (1982)describe pitch- in U and to a lesser degree in Ti blendedeposition in micro-fracturesoccurring in ilmenites and in hematizedfeldspars. This depositionwould result 3, Veinlet (pitchblendeand Ti-oxide) from an adsorptionof uraniumspecies on iron oxidessuch Fig- 6. Schematic relations between the phases 1,2 and the ashematite. pitchblende veinlets. ANIEL AND LEROY: REDUCED URANIFEROUS MINERALIZATIONS B

A U' o c o)

a (u c) E o o -o E z

SCALE FOR CRUSHINGSTAGE

o12345 CO release Fig. 7. (A) Scale used to estimate the CO, release with a crushing stage. Sketches show abundance and size of CO, bubbles as seen in anhydrous glycerin under the microscope (Leroy, 1978). 0. no release, l. very weak, 2. weak, 3. medium,4. abundant, 5. very abundant. (B) CO, release during the crushing. (a) vapor phase quartz grains; (b) rhyolitic quartz from tuff sampled in the faulted bordcr of the breccia pipe; (c) rhyolitic quartz located within the breccia pipe; (d) rhyolitic quartz ofsamples located in the alteration ring; (e) late opal associated with uranophane.

Fluid inclusion studies and genesisof the reduced within the altered tufl and the late opal associatedwith uranium phases ufanophane. The upper level of the Nopal tuff is characterizedby Fluid inclusionsare relativelyrare in the Nopal I deposit vapor phasecrystallization. The quartz is associatedwith and their study could only be performedon three types of euhedral potassic feldsparsin small vugs. The fluid in- minerals: the vapor phase quartz, the rhyolitic quartz clusionsobserved in the quartz are polygonal-shapedtwo- I r296 ANIEL AND LEROY: REDUCED URANIFEROUS MINERALIZATIONS phaseinclusions with a high vapor/iquid volume ratio Combining these results with accurate petrological and ( = 0.90). mineralogical studies, Aniel (1983) shows that phase I de- In the rhyolitic quartz, fluid inclusionsonly appearwhen veloped in an environment rich in complex H2O-CO2-N2 the tuff is altered. The one or two-phase inclusions are fluids, as indicated by gas chromatography analyses, and either disseminated with variable vapor/liquid volume under temperatures ranging from 300 to 350"C. Uranium ratios or distributed in healedfractures. For this latter type was leached by these complex fluids, probably within the of occurrence,the vapor/liquid volume ratio remainscon- breccia pipe, and transported as uranyl carbonate com- stant(=0.20). plexes. The precipitation of uranium oxides is due to a Lastly, fluid inclusionswere observedin the opal which change in oxygen fugacity of the fluids which were in con- coats euhedraluranophane crystals. This opal precipitated tact with the more reduced ilmenite-hematite phase (ilmen- during the latest hydrothermal event recognizedby Aniel ite with hematite exsolution). Such complex fluids are only (1983)in the Nopal I deposit.The fluids are trapped in found in relation to the vapor phase and the phase 1. The protoplasmicshaped one or two phaseinclusions located chemical compositions and temperatures are similar in betweenthe opal spherules. both cases. This means that the primary concentration Two analytical techniqueswere used to define the com- stage of uranium was early and occurred soon after the position of the trapped fluids. Small quattz or opal grains deposition of the tuff, during its cooling. were crushed under the microscopewith a crushing stage The kaolinization of the tufl and the precipitation of the (Roedder,1970) in anhydrousglycerin to detectthe pres- pitchblende-pyrite phase within the pipe were related to enceof insolublegases such as CO2 and CHn. If gasbub- aqueous salt-poor fluids (2.5 wt.o/o eq. NaCl). The temper- bles appear in the glycerin another grain of the same atures ranged from 300 to 150 with the most common samplewas then crushedin Ba(OH), to checkfor the pres- temperature about 250"C. The crystallization of pyrite with enceof CO2. The fluid inclusionswere then studied with a pitchblende during kaolinization indicates the presence of Chaixmecamicrothermometry apparatus as describedby HrS in the fluids circulating within the pipe' Outside the Poty et al. (1976). pipe, similar kaolinization occurred without any transfor- Figure 7 illustratesthe variations in the CO, content of mation of the volcanic ilmenite-magnetite association. This fluids obtained by crushing of the different samples.The means that outside the pipe the HrS activity was low. main resultsare: (l) CO, is very abundantin the vapor These variations of HrS activity explain the diflerent be- phasequartz (Fig. 7Ba); (2) CO2 is also presentin samples havior of Fe minerals. U was leached with the kaoliniza- collected in the shear zones which limit the breccia pipe tion (4 ppm U in kaolinized rock instead of 8 ppm U in and its content decreaseswith increasingdistance to the fresh rock) and precipitated as pitchblende within the main pipe; (3) no CO, is observedin opal (Fig. 7Be).All the structure (the pipe) in relation to the local H2S activity resultsregarding salt contentsand homogenizationtemper- increase which led to a pH decrease. The quantity of aturesare given in Table 2. leached U in the kaolinized tuff is suflicient to explain the Fluid inclusionsfrom the vapor phasequartz have ho- origin of the Nopal I deposit. mogenizationtemperatures of about zt00"C.The salt content of the liquid phaseof thesewas not determinedbe- Conclusion causeof the largevolume of the vapor phase(::0.90). The Detailed mineralogicaland fluid inclusion studiesof the fluids in the rhyolitic quartz are salt-poor (0 to 5 equivalent Nopal I deposit (Sierra Pena Blanca,Chihuahua, Mexico) wt.% NaCl) and the homogenization temperaturesvary suggestthat two stagesof tetravalent uranium mineral- from 350 to 150"C.the salt contentin the opal is lower (0 ization occurred prior to the developmentof hexavalent to 1.7equiv. wt.% NaCl) and the homogenizationtemper- uraniumminerals that prevailtoday. aturesdo not exceed150"C. PhaseI (uranium oxides-ilmenite):the primary uranium mineralizationoccurred in contact with the volcanicilmen- ites of the breccia pipe and seemto have been controlled Table 2. Main results of the crushing and of the by their hematiteexsolution lamellae. The leachingof U is microthermometric studies. related to Hro-cor-N, fluids at a temperatureof 300- 350'C. U may have precipitateddue to an oxygenfugacity Tl,pr of e ront T"c when the fluids encounteredthe ilmenite-hematite (wt Z aq:laCl) change phase. r2c \:aPor Di)asc co2 + + ::2 n.d,* 420-150"C Phase2 (pitchblende-pyrite):this secondstage is related Breccia fipe it21 + i:^2 +'2 c-5 /, 350-3CN'a to the kaolinization of the tuff. Uranium from the kaolinized tuff and from phase 1 was leached.Uranium then {aolinizlrjon r,2. + [2s 4-\'/ 150-180'c + pit.jhhlex.re precipitated as pitchblende within the pipe, probably in

'1 to a local HrS'activity increase(pH variation). l,2o o-5 30O-t 50"c response Fluids were aqueouswith a low salt content (2.5 wt.%), Opal l':o L + 50"C and the temperatureranged from 200 to 250'C. All the following stages,either hydrothermal or super- * n,d. not dctcreinoC gene,occurred under oxidizing conditions and led to the ANIEL AND LEROY: REDUCED URANIFEROUS MINERALIZATIONS 1297

hexavalent uranium mineralization, primarily uranophane, Goodell, P. C. (1981)Geology of the Pena Blanca Uranium De- but also soddyite, weeksite (Aniel, 1983), haweeite, bolt- posits,Chihuahua, Mexico. In P. Goodell and A. Waters,Eds., woodite, and becquerelite. Uranium in Volcanic and VolcanoclasticRocks. AAPG studies in Geology,13. Acknowledgments Leroy, Jacques(1978) The Margnac and Fanay Uranium Deposits of the La Crouzille district (Western Massif Central, France): This study is part of the research programs of the Centre de geologic and fluid inclusions studies. Economic Geology, 73, Recherches sur la G6ologie de I'Uranium of Nancy-Vandoeuvre 16tt-1634. (France). It was undertaken in cooperation with Uranio Mexicano and Smellie,John (1979) A stratabound Ura- Company and the Universit6 Pierre et Marie Curie (Paris VI) Lindroos, Hardy within Middle Precambrian Ignimbrites at (Prof. D. Velde et M. Treuil). nium occurrence EconomicGeology,74, 1118-1130. The authors would like to thank the researchers of the different Duobblon,North Sweden. Magonthier, M. C. (198aa)La Sierrade Pena Blanca(Chihuahua, organizations, specifically C. Nguyen Trung and also Professor J. petrograficasy geoquimicasde las uni- Protas (Universitd de Nancy I, France) for the constructive dis- Mexico): caracteristicas portadoras de mineralisacion de uranio. cussions. and the reviewers for their comments and for all the dades ignimbriticas El Paso, TC 490-1(in English language corrections. Proc. Symp. IAEA, U and volcanism, press). References Magonthier,M. C. (1984b)Contribution d la p'Etrographieet d la g6ochimiedes ignimbritesde la Sierra Madre Occidentaleet de Alba, L. A. and Chavez, R. (1974) K-Ar ages of volcanic rocks la Province Uranifire de la Sierra de Pena Blanca, Mexique. from the Central Sierra de Pena Blanca, Chihuahua, Mexico. M6m. Thdsed'Etat, Paris 6, 183p, 2 vol. Isochron west.. 10. 21-23. Picot, Paul and Johan, Zdenek (19771 Atlas des min6raux (1983) gisements Aniel, Brigitte Les d'uranium associ6s au vol- m6talliques.M6m. du BRGM, n"90. canisme acide tertiaire de la Sierra de Pena Blanca (Chihuahua, RodriguezTorres, R. (1976)Rocas volcanicas acidas y su potential Mexico). G6ologie et G6ochimie de I'Uranium, M6moire Nancy, como objectivos.IAEA SM 208/59.Proceeding ofa syrnposium, 2. Vienna. (1976) Bagby,W. C.,Cameron, K. L., Cameron,M. andGill, J. B. Roedder,Edwin (1970)Application of an improved crushing mi- Petrology of tertiary ash flow tuffs and related volcanics from croscopestage to studiesof the gasesin fluid inclusions.Schwei- Barranca de Batopilas and Gell region of the Sierra Madre zerischeMineralogische und PetrographischeMitterlungen, 50, Occidentale, Chihuahua, Mexico. (abstr.) Geological Society of 41-58. America Abstracts, 8, 5, 566. Smellie,John (1982)The mineralogy and genesisof uranium in (1977) phenomdnes Calas, Georges Les d'alt6ration hydrothermale rhyolitic ignimbrites of Precambrian age from Duobblon, et leur relation avec des min6ralisations uranifdres en milieu Sweden.Mineralogical Magazine, 46, 187-199. volcanique: le cas des ignirnbrites tertiaires de la Sierra de Pena Uramex (1980)Nopal I deposit,Las Margaritas deposit,Puerto I Blanca, Chihuahua. Sciences G6ologiques Bulletin Strasbourg, deposit,Internal Reports. 30,3-18. Wenrich, K. J., Modreski, P. J., Zielinski, R. A. and Seeley,J. L. Cardenas-Flores, David (1984) Volcanic stratigraphy and uranium (1982) Margaritasite: a new mineral of hydrothermal origin molibdenum mineralization of the Sierra de Pena Blanca dis- from the Pena Blanca uranium district. Mexico. American Min- trict, Chihuahu4 Mexico. Proc. Symp. I.A.E.A. Vol- U and eralogist,6'7, 127 3-1289. canism, El Paso, TC 49G31 (in press). Zielinski, R. A. (1978)Uranium abundancesand distribution in Caruso, L., Swinden, T. and Simmons, G. (,1982)Uranium migra- associatedglassy and crystallinerhyolites of the westernUnited tion through microcracks, Geevor Tin Mine, Cornwall, En- States.Geological Society of AmericaBulletin, 82,4W414. gland. EOS, 63, 45, 1128. George-Aniel, Brigitte, Uramex, Leroy, Jacques, and Poty, Ber- nard (1984) Uranium deposits of Sierra de Pena Blanca. Three examples of mechanisms of ore deposit formation in a volcanic environment. Proc. Symp. IAEA U and volcanism, El Paso, TC Manuscriptreceiued, June 20, 1983; 49G.8 (in press). acceptedforpublication, April 30, 1985.