larly altered; epidote and associated secondary minerals are developed. In addition, some fractures are filled with coarse-grained low-temperatrue gypsum that ...
Canadian Mineralogist Vol. 25, pp. 539-543(1987)
RARE-EARTHABUNDANCESIN HOST GRANITICROCKSAND FRACTURE-FILLING GYPSUM ASSOCIATEDWITH SALINEGROUNDWATERS FROM A DEEPBOREHOLE, ATIKOKAN, ONTARIO JAMES E. MUNGALL, SHAUN K. FRAPE ANDIAN L. GIGSON Departmentof Earth Sciences,Universityol Woterloo,Waterloo,OntartoN2L 3GI D. CHOUDARI KAMINENI GeologicalSumeyof Canada,601Booth Street,Ottawa,OntarioKIA 0E8
Ansrnacr
INTRODUCTIoN
Data axepresentedon the concentrationof the rare-earth elements(REE) in the granitic host-rock and associated fracture-filling gypsum from a borehole located 30 km northwest of Atikokan, Ontario, within the Archean EyeDashwalakespluton. The granite is relativelyhomogeneous and has a steeplight-RgE-enrichedpattern. It is irregularly altered; epidoteand associatedsecondaryminerals are developed.In addition, some fractures are filled with coarse-grainedlow-temperatruegypsum that appearsto be spatially associatedwith highly saline groundwater. The gypsum samplesare also characterizedby steeplight-REE enrichmentand, in somecases,REE concentrationsgreatly exceedingthoseof the adjacentgranite. The data suggest that the R.EEcontent of the g;'psum is a product of effective partitioning from the parental brines, which in turn reflectsthe R^EEdistributionsof the host graniteacquired during long-term rock-water interaction.
Salinegroundwaterswith very high loads of dissolvedsaltsare now known to be widely distributed in the ancientshieldareasof the world (Pinneker& Lomonosovl964,Fritz et al. l979,Korotkov et al. 1980,Frapeet al.1984). Thesewatersoccurin faults and shearzonesat depthsin excessof 650 metres, and havea rangeof isotopic and chemicalcompositions distinct from seawater,meteoric water and brinesin sedimentarybasins.The origin of thesewaters is unknown. We report heresomepreliminary resultsof a study of the rare-earth-element(REE) chemistry of eight samplesof granitic hcist-rockand nine samplesof as$ociatedfracture-filling gypsum from a borehole on the CanadianShieldthat is also a sourceof very salinegroundwater. The REEs in the rock and mineral samples were determined by instrumental neutron-activationprocdures in order to throw light on any relationshipbetweenthe compositionof the host rocks and that of the secondaryminerals and the saline groundwater. The geochemicalrelationship between groundwater and fracture-filling minerals may have important implications to the transport, adsorption and desorption of important ions within such fiacture systems.This aspect,in turn, is important in the program of storageof nuclear fuel waste.
Keywords: rare-earth elements,gypsum, fracture fillings, salinegroundwaters,plutonic rocks, Atikokan, Ontario, water-rock interaction.
Souvernr On pr6sentedesdonn6essur les concentrationsdesterres rares dans un granite h6te et du glpse qui en tapisse les fissures;les dchantillonsproviennentd'un trou de forage situ6 d 30 km au nord-ouestd'Atikokan, dans le pluton archdendes lacs Eye et Dashwa(Ontario). C'est un granite relativementhomogbnequi montre un enrichissement marqu€ en terres rares l6gdres.Il est altdrd de fagon non homogBne,i une associationd'dpidoteet autresmindraux secondaires.Certainesfissuresont 6t6rempliesd bassetemp6raturede cristauxgrossiersdegypsequi montreraient une association spatiale ir une saumure souterraine. Ce gypse montre aussil'enrichissementmarqu6en lerresraresldgeres;danscertainscas,le facteurd'enrichissementsurpasse m€mecelui du granite h6te. Les concentrationsdesterres rares dans le gypser6sulteraient d'une r6partition efficace de ces€l6mentsd partir des saumures,dont la composition en terres rares reflbte la r6partition de ces6l6mentsau coursd'une interactionprolongdeimpliquant granite h6te et saumure. (Traduit par la R6daction) Mots-clesi teres rare$,g1pse,remplissagede fissures,saumures, rochesplutoniques, Atikokan, Ontario, interaction roche - eau.
Gsolocy oF THEEvr-Dessw.r Lerss PLuroN. ONrnnto The zonedEye-Dashwa granodiorite-granite pluton, which is of Archean.age, occurs within the Superior Province of the Canadian Shield, and is located about 30 km northwest of Atikokan, Ontario. The l4x9 km pluton is being studied in detail as part of an Atomic Energy of Canada Limited program on the suitability of granitic plutons for the disposalof nuclearfuel waste.Geological studieshaveincludedwork on the petrology, fractures, and fracture fillings (Kamineni & Brown 1981, Stone& Kamineni 1982),rock alteration (Kamineni & Dugal 1982) and geochronology (Kamineni & Stone 1983).
539
540
THE CANADIAN MINERALOGIST
The granite is coarsegrained and affected by alterThe resultsfor the rock and mineral samplesare ation, which is concentratednear fracture zones shown in Tables I and 2. In Table 2, resultsfor La (Kamineni & Dugal 1982).Alteration of the feldspars 'were not included as there were analytical difficulis indicatedby irregular discolorationto shadesof ties in counting. The precisionfor the REE data for pink and greenfrom their original grey and by the both the rock and mineral samplesis better than developmentof epidote,carbonate,sericite, +l0o/o Qo) for all elements. and, occasionally,kaolinite. Fracturesand mi DrscusstoN shear-zonesare very common throughout the roc mass,and many are considered(Stone& Kami Host-rock geochemistry 1982)to have beensealedduring the closing of pluton emplaeementand cooling. The REE data for the unaltered material from the Much of the alteration is consideredto be of the ATKI boreholeindicate that the granite was origisameage.However, somefracturescutting the plunally quite homogeneousover the 600 m sampled. ton are predominantly filled with coarse-grainedgyponly anomaloussamplewastaken at 1024.6m. sum, which is assumedto be of low-temperatureorihieh REE concentrationsof this samplemay gin and much youngerin age(Kamineni 1983).Our an unusually high modal content of a REEproject focusedon thesegypsum-bearingfractures. phase.The meanR-EEcontentof the ing minor Five boreholes(ATKI - ATK5) were drilled in a (excluding 1024.6)is plotted in Figure l. The small study-areain the southwesternpart of the plute has the steeplight-rare-earth-enrichedpatton. A corefrom one of the deepestof these,ATKI, typical of granitic rocks of the Precambrian was chosenfor study, and from this a representa(Ronov et ol,1967, Hanson 1980). tive suite of fresh granite samples and gypsum fracture-fillings wascollected.In addition, three samgypsum ples of salinegroundwaterfrom different depthsin REE concentrotions in the the hole weresampledby Atomic Energyof Canada The REE contents of nine samplesof fractureLimited for analysis. filling gypsum from drill hole ATK1 are shown in Figure 2. The gypsum is associatedwith other MErHoos ANDREsuLTs secondary minerals, including chlorite and fluorapophyllite IKCaaSisOt6(F,OH).8H2O]. The whole-rock sampleswere analyzedby Dr. However, no REE-rich secondary phases were John Ludden, Universit6 de Montr6al, using observedas fracture fillings, and no correlation is instrumental neutron-activation procedures(Gordon apparent between the amount of chlorite and et ol.1968, Gibson& Jagam1980).The sampleswere apophylliteand the REE contentof the gypsum.Xactivated for four hours in a flux of I x 10lz neu- ray and microscopicexaminationssuggestthat the tron cm-2s-1.Counting was performed using a gypsumis free of inclusionsof suchR,EE-richphases Li(Ge) detector,with a resolutionof l.6keY at 122 as apatite and zircon. We contendthat the rare-earth keV. elementsare presentin the structureof the gypsum, Specimensof the fracture-filling minerals were substituting for calcium. hand-separatedunder a binocular microscopeand Three featuresof the gypsumresultsare imporidentified optically and by X-ray diffractometry. tant: (a) the dissimilarity betweenthe R-EEpatterns They weresubsequentlycrushedin an agatemortar of hydrothermal anhydrite of Morgan & Wandless until no particleswere distinguishablewith the naked (1980)and thoseofthe gypsumfrom Atikokan; (b) eye.The mineral separateswereanalyzedby the same the extremelight-rare-earthenrichmentin some of proceduresand countedusing a low-energyphoton the gypsum samples,with valuesof the chondritedetector,with a resolution of 0.65 keV at 122keV. normalized Ce/Yb ratio of over 1000;and (c) the
TIAEE
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IT
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1.86 1.34 L.2A 1.28 1.38 1.40 5.27 1.37 1.d1 14.3
0.66 0.39 0.33 0.26 0.46 0.60 1. 01 o,3S 0.a1 24.4
0.46 0.6? o .? s 0,19 o.2a o.27 !.27 0.34 o.40 60.0
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REE IN GRANITES AND FRACTURE.FILLING GYPSUM 3em2.
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patternof v in the CelYb ratio (chondrire- o normalizedval of the gypsumwith depth in the zo borehole. There is little publishedinformation on the = R.EE contents of either gypsum or anhydrite; l U' hotrrrever, & Wandless(1980)presenteddata (! on two of hydrothermalanhydrite(Fig. 2). (9 The di crystal-structuresof CaSOo.2HrO and CaSOui alidate a direct comparison of the REE ns of the two minerals.However. if the gypsum found in the fracturesis the hydratedequi of hydrothermalanhydrite,the REE contents remain unchanged.The disA GEOTHERMALANHYDRITE similarities of he plots for anhydrite and gypsum o ATIKOKANGYPSUM indicatethat th hasnot occurred,or elsethe anhydrite re-equi with groundwater during the Sm Eu Gd Tb Yb Lu hydration. It more likely that the observedpatFIG. 2. Chondrite-normalized abundances of the rare-earth terns refer to m formed in equilibrium with elements in gypsumfrom boreholeATKI, togetherwith low-temperatu groundwater. datafor someexamples of hydrothermal anhydritefrom The of someof the gypsumpatternsis (1980).Note:(a) the similarities Morgan& Wandless of interest it indicateseither R,EEfractionof theREEabundances in thegypsumandgranitehost (Frg.t), (b)thestrongdissimilarities in REEabundances between hydrothermal anhydriteandlow-temperature gypsum,and(c)theextreme IRE'Eenrichment in both samples of gypsumfrom927.76, whichsuggests that theseg;,psumsamples differ in theirorigin from others RANGE OF OTHER GRANITOID ROCKS FROM LAKE DESPAIR collectedfrom the core. AREA, PC SHIELD (LONGSTAFFEot ot. t982)
ation during precipitation of gypsum or precipitation from LREE-enrichedparental solutions. It is noted abovethat the host rock of the fracture-filling o z material is granitic, and that it has a steep,LREE7/.r,,u o ''/,2enrichedpattern. Waters in equilibrium with such o rocks would inherit REE characteristicscontrolled by the mineralogy of the host rock, the prevailing F pressure-temperature conditions,and the appropri z ate distribution-coefficients.Fractionation of the E, o R,EE would certainly be possible (Nesbitt 1979, Duddy 1980)but is not required,and we assumethat watersin equilibrium with the graniteswereLREEenriched.Unfortunately, it is difficult to demonstrate this by analyzingthe samplesof water from the borelm Eu Tm Yb Lu hole, as the rare-earthelementsappelu to partition FIG. 1. Mean ite-normalizedabundancesof the very effectivelyinto the secondarygypsumprecipiof theR.EEin the saline rare-earth in fresh granite samplesfrom bore- tate; observedconcentrations holeATK1, Dashwalakes pluton. Bars indicate watersare very low (seebelow). We are not aware coefficient of tion. of any partition-coefficient data that would allow lrJ
tr tE f
ttz..
'o!21r,,, 1",
,/.(a
542
THE CANADIAN MINERALOGIST
:-t J
LEGEND . SAI'PLE ll3o DEPTH(m) }-
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)
E
RANGE OF TDS
; o = o
z()
vul o
lt
lrj J
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(){,
J F
o F
700 800 900 DEPTH lN BOREHOLEATKI (m) FIG. 3. Chondrite-normalizedCelYb ratio in fracture-filling gmsum and groundwater salinity rersasdepth in boreholeATKI. Note the gradual increasein both CelYb ratio and salinity with depth and the anomalousnature of the gypsum samplesfrom 927.7m, close to the narrow zone of salinewaters encountered between950 and 1000m.
theoretical calculation of the R.EE abundancesof Ca-Na-Cl brines in equilibrium with gypsum. Other considerdtionsalso suggestthat fluids in equilibrium with the granite of the Eye-Dashwapluton are likely to be enrichedin the light r€re-earths. Kamineni (1985)has demonstratedthat increasing intensity of alteration of the Eye-Dashwagranite is associatedwith decreasingabundancesof the R.EE, particularlyof the light rare-earths.If thesedata are interpretedas an indication of an absoluteloss of the rare earthsfrom the gtranite,the fluids instrumental in that loss must also have beenenrichedin the light rare-earths. A further point concerningthe gypsum data is illustrated in Figure 3. Although there are exceptions, the salinity of the groundwatersgradually increases with depth in the sampledsection,with little change in the value of the CelYb ratio for the associated gypsum.However,a sampleof stronglysalinewater was collectedfrom 950 to 1000m. This may have resultedfrom the injection of a brine of deeporigin along a fracture into a regime of brackish water. Solubility data for CaCl2 (Runnels 1969) suggest that the injection of brackish water saturatedwith respectto sulfate (Frapeet o1.1984)would result in the rapid precipitation of large volumesof gypsum in the zoneof mixing. Whereasgypsumfrom within this zone (at 960 m) appearsnormal, samplesfrom abovethis zorreat927 mhave an anomalouslyhigh value of Ce,/Yband high absoluteconcentrationsof
REE. This coincidenceof the depth of anomalous featuresin both R.EE chemistry and groundwater salinity in this particular caseis taken as evidence of a geneticrelationship betweenthe gypsumand the highly salinegroundwater. Rare-earth geochemistry of the CaCl, brines Attempts were made at the University of Waterloo to determinethe R.EEconcentrationsof the three Atikokan water samples supplied by AECL. However,it was only possibleto determinethat the concentrationsof the REE elementsare below the detection limit for the instrumental neutronactivation procedure employed. These detection limits are approximately:0.2mE/L Ce,0.02 mg/L Eu, 0.04 mg/L Tb and 0.2 mg/L Yb. Theselow abundancesmay be the result of partitioning of the REE into eypsum. CoNctustoNs 1. The Cypsumin the Atikokan borelole is characterized by steep LREE-enriched chondritenormalized patterns. 2. The observed R.EE concentrations in the Atikokan brinesare low (lessthan 2 mglLrotal REE) and may be the result of effectivepartitioning of the REE into gypsum precipitated from the brines. 3. Two samplesof gypsumfrom a singlefracture
REE IN GRANITES AND FRACTURE.FILLING GYPSUM
show high valuesof the CelYb ratio. This fracture is associatedwith a zone of high-salinity groundwater. The gypsummay have been precipitated as a result of mixing of highly salinebrine with brackish groundwater. 4. Long-term rock-water interaction at the Atikokan site would produce a water with a steep pattern inherited from the light-rare-earth-enriched host granite.This pattern may be subjectto modification, but is consideredthe primary control of the steepLREE enrichmentobservedin the gypsum. AcKNowLEDGEMENTS
543
(1985):Distribution of rare earth elementsin core samples from the Eye-Dashwa pluton, Atikokan: significance to the migration of radionuclides. Proc, ITth Information Meet., Atontic Energy oJ Canada Ltd., Tech. Record 299*.
& Bnowx,P.A. (1981):A preliminaryreport on the petrology and fracture fillings of the EyeDashwa lakes pluton , Atikokan, northwestern . Ontario. Atomic Energy of Canada Ltd., Tech. Record 123*
& Ducru-,J.J.B.(1982):A studyof rock alteration in the Eye-Dashwb lakes pluton, Atikokan, northwestern Ontario, Canada. Chem. Geol. 36, 35-37.
The authors thank Atomic Energy of Canada & Sroun,D. (1983):The agesof fracturesin Limited for financial and logisticalsupport. In parthe Eye-Dashwa pluton, Atikokan, Carnda. Contr. ticular, Drs. M. Gascoyneand J. Kramerswerevery Mineral. Petrology t3, 237-246. helpful in the preparation of the manuscript. Continued NSERC support-toS.K. Frapeand I.L. GibKonotrov, A.I., Gnrrsrn, Yr.L., SevamN, V.S., son is greatly appreciated.As well, the readingand Suoarurov, S.M. & SuLrvove, L.N. (1980): Salt editing of Drs. P.G. Manning, R.F. Martin and two waters and brines of the Baltic Shield. Dokl. Acad. anonymousreaderswere very beneficial. Scl. USS& Earth Sci. Sect.238,182-187. REFERENCES
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