Margaritasite: a New Mineral of Hydrothermal Origin from the Pefrablanca Uranium District, Mexico
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American Mineralogist, Volume 67, pages 1273-12E9,l9E2 Margaritasite: a new mineral of hydrothermal origin from the PefraBlanca Uranium District, Mexico KnnBN J. weNnrcn, PETERJ. MoonBsxr, Rosenr A. Zrer-rNsrrAND Jeues L. Seerey U.S. GeologicalSurvey Box 25046,MS 916, Denver Federal Center Denver. Colorado 80225 Abstract Margaritasite, a Cs-rich analogueof carnotite, is a newly discovered uranium mineral that is part of the ore at the Margaritas deposit in the Peffa Blanca uranium district near Chihuahua, Mexico. The margaritasite occurs as disseminated pore fillings and relict phenocryst linings within a rhyodacitic tuff breccia of the lower Escuadra Formation (Oligocene) and provides significant reserves of both uranium and cesium. It is a fine- grained yellow mineral and, with the possible exception of the index of refraction, is optically indistinguishablefrom carnotite. Margaritasiteis most easily recognizedby X-ray diffraction through a shift in the (001) reflection representing an increase, relative to carnotite, in the c dimension. This increaseis due to the large Cs atom in sites normally occupied by K in the carnotite structure. This cs-K uranyl vanadate, with cs:K about 5, is the natural equivalent of the compound Cs2(UO2)2V2O6synthesized by fusion (Barton, 1958).Chemical analysesof the mineral give the formula (Cs1 3s(H3O)s.rsKo.zq)>z o2(UOt r.ee(V2Odr.o0. l.07H2O correspondingto the generalizedformula (Cs,K,HrO)z(UO2)2V2O6.nH2Owhere Cs > K,H3O and n : I Unitcellparametersarea:10.514,b:8.425,andc=7.254,p:106.01"(F4/a,Z:2). Microprobe analyses of margaritasite and Cs-enriched carnotites synthesized by ion exchangeof carnotite in aqueousCsCl solution at 2fi)"C suggesta solid solution between carnotite and margaritasite, but X-ray powder patterns reveal that two discrete c dimensions exist with no intermediate value between them. The margaritasitehas a (001) reflection at 12.7"(20) while that of carnotite lies at 13.8"(20); thesepeaks do not shift. It is likely that there are two distinct phases, perhaps as interlayered lamellae which are intergrown on a smaller scale than can be resolved by the electron microprobe beam. The discovery of Cs-rich carnotite in the Pefra Blanca uranium district provides important evidencefor local hydrothermal or pneumatolitic activity during or after uranium mineralization. Data from the geochemicalliterature indicate that the high Csltotal alkali elementratios required to produce Cs-rich minerals can be generatedand sustainedonly in high temperature environments. Synthesis experiments show that margaritasitecan form by reaction of Cs-rich solutions with natural carnotite at 2fi)'C, but the samereaction does not occur or is too slow to be observedin 61 days at 80"C. Nevertheless,it appearsunlikely that margaritasite is part of the ore in any Colorado Plateau-type uranium deposit. Reported "carnotite" occurrencesfrom uranium depositsofprobable hydrothermal origin are likely sites for new discoveries of margaritasite. Introduction uranium district near Chihuahua. Mexico. The rec- ognition of margaritasite within these rocks is im- Margaritasite, a newly-recognized uranium min- portant for the characteization of this deposit and eral and a Cs-rich analogue of carnotite, is an ore is of interest in the exploration for other depositsof mineral at the Margaritas deposit in the PefraBlanca similar origin. The physical and chemical con- 0003-fixxE2l | | rLr27 3$02.00 1273 1274 WENRICH ET AL,: MARGARITASITE straints determined to be necessaryfor the forma- ably on Cretaceouslimestones. The volcanicshave tion of margaritasitecan contribute to knowledge of been datedby Alba and Chavez (1974)at 43.810.9 the genesisof ore depositshosting this mineral.The m.y. for the Nopal Formation,38.310.8 m.y. for mineral and name margaritasite were approved by the EscuadraFormation, and 37.3-r0.7 m.y.for the the Commission on New Minerals and mineral Mesa Formation (Fig. 1). The PenaBlanca volcani- names of the International Mineralogical Associa- clastic unit lies between the later two formations. tion. Margaritasite is named after the Margaritas Uranium mineralization occurs in stratiform and deposit where it was discovered. A specimenof fracture-controlled deposits within the Escuadra margaritasitehas been provided to the U.S. Nation- and Nopal Formations. al Museum of Natural History, Washington,D.C. Uranium depositsof PefraBlanca Cesium-carnotitewas first synthesizedby a fu- sion technique,using metavanadatefluxes, by Bar- The Pefla Blanca uranium district is unusual for ton (1958). This synthesis provided sufficiently North America becauseof its geologic setting in largecrystals that Applemanand Evans (1965)were silicic volcanic rocks. Over the past 20 years, able to determine the crystal structure of Cs-carno- numerous uranium occurrences at Pena Blanca tite. The X-ray diffraction pattern of the synthetic have been systematicallydrilled and developedby Cs-carnotitewas publishedby Barton (1958)and is underground mining (Goodell, 1981). In recent very similar to that of margaritasite(which contains years, severalof these have proven to be of com- about I wt.% KzO) except for a slightly larger c- mercial interest and are in active stagesof develop- dimension. ment. Presently the Nopal I deposit is partially exploited, an incline is being driven into the Puerto Geologicsetting III deposit and the Margaritas deposit is being The Pefla Blanca district is located within the prepared for exploitation by open-pit mining (Goo- Basinand RangeProvince of Mexico approximately dell, 1981).Based upon projected estimatesof U 70 km north of Chihuahua City in the north-central resources,the Pefla Blanca district could prove to portion of Sierra PefraBlanca (Fig. l). SierraPena be the most significant volcanic uranium district in Blancalies amongother fault-boundedranges con- North America. Reservesat the Margaritasdeposit tainingsimilar sequencesof Tertiary volcanicunits. aloneare believedto be 4.6 million poundsof U3Og A 300-m-thickTertiary sequenceof rhyolitic ash- with an averageore grade of 0.l03Vo(G6lvez and flow tuffs and volcaniclastic sediments caps the Y6lez,1974)along with molybdenumreserves at an bulk of this range. Three formations within this averagegrade of 0.3% (Goodell et al., 1978).In sequencecontain ignimbrites constituting outflow addition, the margaritasite provides a significant facies from an as yet unidentified caldera source reserveof cesium. (Goodell, 1981).The uppermost(Mesa) and lower- The Margaritas deposit lies within a northwest- most (Nopal) formations are particularly volumi- trending syncline (Goodell, 1981)in the lower Es- nousand of regionalextent, being most widespread cuadra and underlying Nopal Formations (Fig. 2). in the Sierradel Nido volcanicprovince to the west This deposit lies at the intersection of two fracture and northwest (Fig. 1). This range is the major zonesalong which most uraniumoccurrences of the volcanic source area in the region, and contains PefraBlanca district occur: one trends northwest- rocks which are compositionally and temporally southeastand the other north-south(Calas,1977). more variable than rocks of the Pena Blanca range A geologiccross sectionof this depositwith inter- (Goodell,1981). Although the older rhyolitic tuffs of pretations by the Instituto Nacional de Energia Sierra Pena Blanca, Sierra Pastorias,and Majalca Nuclear shows the location of the analyzedsample Canyon are not peralkaline (Table 1), the younger of margaritasite(Fig.2). Occurrencesof margarita- Campanaand Cryptic Tuffs of the northern Sierra site are by no means minor, being scattered on the del Nido are (Dayvault 1980,Table 1; seealso Table surface toward the axis of the syncline. The other 1, this report). More information on the volcanic uranium ore minerals found in the Margaritas de- geology of the region can be found in Goodell and posit are uranophane, betauranophane, autunite Waters,1981. and weeksite (Goodell, 1981);all contain hexava- At the Pefla Blanca uranium district, the Tertiary lent uranium and are associatedwith anomalously volcanics overlie a regional continental molasse, high concentrations of Mo probably in powellite the Pozos conglomerate,which rests unconform- (Goodell, 1982)and V in metatyuyamunite(Goo- WENRICH ET AL,: MARGARITASITE 1275 106"30' 106000' 1050j0' J0000' Sierra de la Tinaja Sierra Lisa de las Damas rre€rmpana la Mesa 37.0m.v. Pefia Blanca Chontes Escuadra 38.0 Nopal 44,0 Po2os Sierra Pefia Sierra Blanca de la Campana 29"00' CER.RODE RANCHERIA Capella Amarillo Tuff Quintas Tuff 44.7m.y. Acantilado Tuff Al dama Sierra de 1a Sierra Gloria 1a rLana i huahua \ NORIH Eu[.a..LLa oro Rhyolite I L2 ) D\l; 28-33m.y. x CHIHUAHUA''!a.i-'.r'r x13 Sierra Carreras 0 10 Miles Pastorias F--r -Y 0 10 Kilometers Sot Fig. l. Location map for the Peia Blanca Uranium District, Mexico, (located in the center of the diagram). Sample locations are shown for analyseslisted in Table l. The Margaritas deposit is located at site 19. The stratigraphiccolumn for Sierra pefia Blanca is shown to the upper right of the area. The sampleof margaritasitecame from the Escuadraformation. (Modified from Goodell. l98l). dell, 1982)as well as in margaritasite.No tetrava- and a breccia of reworked tuff clasts. Margaritasite lent U minerals have been observed in the Margari- occurs as pore fillings and phenocryst casts within tas deposit, although uraninite occurs in the Nopal I the rhyodacitic breccia of reworked tuffclasts in the deposit (Table l). At the Margaritas deposit miner-