EconomicGeology

Vol. 77, 1982,pp. 430-440

The Sourceof Lead in the OsenSandstoneLead Depositon the

Baltic Shield, Norway
ARNE BJI•RLYKKE
GeologicalSurveyof Norway, Postboks
3006, 7001 Trondheim,Norway

AND RALPH I. THORPE

GeologicalSurveyof Canada,601 BoothStreet, Ottawa, Ontario K1A OE8,Canada

Abstract

The sandstone-hosted lead depositson the Baltic Shield occurin marine late Precambrian
to Early Cambriansedimentaryrocks.A ground-watermodel in which the basementis pos-
tulated as the metal sourcehasbeen proposedfor thesedeposits.To test this model the Osen
depositwasselectedbecauseit liesdirectlyon a homogeneous granitebasement.
On the basisof U-Th-Pb whole-rockisotopicanalysesfor three samplesof unweathered
basementgranite (Trysil granite), a basementage of 1,598 m.y. was established.The lead
isotopiccompositions of four out of five specimensof galenafrom the Osendepositplot
betweenisochrons calculatedfor the graniteat 500 m.y. and 540 m.y. This is strongsupport
for the hypothesisthat the lead sourcewasbasementgraniteand suggests that mineralization
took placesoonafter sedimentation.A basementsourcefor the lead is further substantiated
by the regionalpatternof lead isotopiccompositions of sandstoneleaddeposits, whichreflects
the characterof adjacentbasementterranes.

Introduction andby Wickmanet al. (1963).In spiteof large2ø4pb

errors,the datashowthat the leadisradiogenic, 1with
SEVERAL lead depositsand occurrencesare present compositions similar to many of the Mississippi
Valley
in Cambrian to late Precambrian sandstones on the
deposits(Heyl et al., 1966).The objectiveof the pres-
Baltic Shield.Laisvallis the largestdepositwith 80 ent studywasto testthe relationship,in a much more
millionmetrictonsof orewith an averageleadgrade restricted area, between the basement rocks and the
of 4 percent(Rickardet al., 1979).Galenais the main lead occurrences in the overlyingsandstones by ob-
basemetal sulfidein the Baltic Shielddeposits,and taining lead isotopeanalysesfor both.
Pb/Zn variesfrom 5 to 15, but locally within each In one of the geneticmodelssuggested for sand-
depositZn may be dominant. Galena and sphalerite stonelead depositson the Baltic Shield (Bjorlykke,
are disseminatedin sandstones that were deposited
1977;Bjorlykkeand Sangster,1981),it is postulated
undershallowmarineconditionsalongthe marginal that the lead was releasedduring weatheringfrom
areasof the former Lower Cambrian epicontinental the underlyingbasementeither directly or via for-
sea(Fig. 1) (Bje•rlykkeand Sangster,1981). mation of continentalarkoses.Ground water may
Two factorscritical in explainingore formationin have then carried the lead (and zinc) into marine
intracratonicsulfidedeposits,the sourceof basemet- sandstone, wherea reducingenvironmentcausedpre-
als and the timing of ore deposition,have been de-
cipitationof mainlygalena.Geologicevidence(Bj•r-
batedfor a longtime. Lead isotopeanalysisisa useful lykke and Sangster,1981) indicatesthat ore deposi-
aid in testingdifferent geneticmodelsfor suchde- tion tookplaceshortlyafter sedimentationof the host
positsand has been usedby, for example, Doe and rock and that the isotopiccompositionof the base-
Delevaux(1972) for carbonate-hosted lead-zinc(Mis- ment should be reflected in the sulfides. To test this
sissippiValley-type)depositsin the southeasternMis- model, the mineralization at Osen, southeasternNor-
souridistrict.They analyzedthe lead isotopiccom- way, wasselectedbecausethe basementthereconsists
positionsof basement,the different sedimentaryunits
of granitesof uniformcompositionand becausemin-
overlyingbasement,and the ore itself. One of the eralizationoccursin sandstonedepositeddirectly on
main problemswith testingdifferent modelsin such the basement.This is a simplerpalcogeographic re-
a large area as southeastern Missouriis to get a re-
presentativesamplingof basementand host rocks • Relative to volcanic-hosted and shale-hosted massive sulfide
with a reasonablenumber of samples. depositsand othersingle-stage
deposits
on whichterrestriallead
Lead isotopedata for depositsof the Baltic Shield evolutionmodelsare based(Cummingand Richards,1975;Stacey
have been publishedby Moorbathand Vokes(1963• and Kramers, 1975).

0561-0128/82/12/450-1052.50 430

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by Society of Economic Geologists, Cesar Ricardo Vargas
 SANDSTONE
PbDEPOSITS,
BALTICSHIELD 4131

lationshipbetweenpostulatedsourcerock (basement)
and hostrock (sandstone)than in many other Baltic
Shielddepositsof this type. This alsosuggested the
possibilityof a simpleisotopicrelationshipbetween
postulatedsourcerock (basement)and ore. The study
of sandstone-hosted depositson the Baltic Shield is
continuingwith the useof samplesfrom severalad-
ditionaldeposits,and it will further testthe regional
correlationbetween basementcompositionand the
lead isotopiccompositionof the galenain the over-
lying sandstone.
Geologic Setting
In theOsenarea(Fig.2) thePrecambrianbasement
is overlainby Cambrian sandstoneand shale;this
autochthonoussequenceis overlain by the Osen
nappe which consists mainly of late Precambrian
feldspathicsandstoneand quartzite (VangsasFor-
mation).
Basement

The sub-Jotnianbasementcomplex in the Osen

0 10 20 km
area consists
mainly of Trysil granite,althoughelse- 12•-•'---• I ' • I • I
LEGEND
OSEN NAPPE BASEMENT
"?•....'• LATE
PREEAMBRIAN-ORDOVIOAN
SEDIMENTS
E•:.• TRYSILORANITES
AUTOCHTHONOUS F.':i:i:::• "SANDSTONES
GURROGAISSA CA•BR,A.
SA,DSTONESA,DS.A,.ES
• "
b--E• G.E,SSES
Fi(;. 2. Geologyof the Osenarea,Norway(modifiedfrom Nys-
tuen, 1969b).

where it includesfelsic plutonic, volcanic,and sedi-

mentaryrocks(accordingto Piper and Smith, 1980,
the Jotniansuiteof rocksspansthe approximatetime
range1,600to 1,250m.y.). The plutonicrocksof the
Trysil Complexare coarsegrained, usuallyreddish
or greenish,and range in composition from granite
;NERTINGDAL • to alkali graniteand alkali syenite(Priemet al., 1970).
The main componentsare quartz, microcline-per-
thite, and albite.Whole-rockRb-Srdata for plutonic
and volcanicrocksresultin an ageof 1,595___ 69 m.y.
usinga decay constantof 1.42X 10-11 yr-1 (Priem
et al., 1970).2The Trysilbasement complexcontinues
OGOTLAN
•' • Lanareas into Sweden(Dala porphyriesand granite),and Rb-
Sr whole-rockdata have been publishedby Welin

I • Shallow
• Continental marine and Lundqvist(1970). Priem et al. (1970) combined
margin their own data for Trysil plutonicand volcanicrocks
with thoseof Welin and Lundqvistand obtainedan
• Major galena deposit. . ß isochron indicatingan age of 1,624 ___42 m.y.
Minor disseminated
o Kilometres
500 galena deposit ....... 0 The Trysil area was also affected by a tectono-
Minor galena vein thermal event about925 m.y. ago, as evidencedby
occurrence .......... •
the Rb-Srand K-Ar agesof separatedbiotites(Priem
FIG. 1. Distributionof sandstonelead depositson the Baltic 2In accordance with presentconvention,Rb-Sragesreferredto
Shield(modifiedslightlyfrom Bjorlykke,1974). in the text are with referenceto the 1.42decayconstant.

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by Society of Economic Geologists, Cesar Ricardo Vargas
 452 A. BJ•RLYKKE AND R. I. THORPE

SAMPLE NO
et al., 1970).This corresponds
to the widespreadSve-
conorwegian orogeny.
The unweathered graniteshavean averagecontent
BLA[K SHALE
of 20 ppm Pb, 49 ppm Zn, and 5 ppm Cu (H•y,
1977).Usingan increasein the ratio (A12Os+ K20)/
(MgO + Na•O) as an index of the degreeof weath-
ering,lead and zinccontentswere foundto decrease CONGLOMERATE

with an increasein the intensityof weatheringof '18A.--,-.- GREEN SILTSTONE

rocksin the paleoweatheredzone under the Cam- AND SHALE

brian sediments. This indicates that the metals were

4629--,,-
releasedduringthe late Precambrianto earliestEarly DARK SANDSTONE

Cambrian peneplanation. 6615 -----

Lower Cambrian sandstones and shale CiUARTZ SANDSTONE

6/,..'18
---'-'-
Between the basement and the lowermost Cale- 6620_,.
6636 ARKOSE
donian nappe (Osennappe) is an autochthonous se- ZZ---'-

quenceof Cambrian sedimentaryrocks20 to 40 m WEATHERED GRANITE

thick (Fig. $). Thesesedimentaryrocks,representing METRES

5
an increasinglyupward-finingsequence,can be sub-
dividedintoa lowermostsandstone that issuccessively 3
overlainby dark, fine-grainedsandstone, greensilt- 2
stone, and black shale. GRANITE

Basal sandstone:The transitionfrom granite to 0

overlyingsandstone
isgradualin mostplaces.Passing
upwardsfrom the unweatheredgranitethe orienta- lO-----
tion of the mica becomes more horizontal and the
feldsparand mica contentsdecrease.The thickness
of this basalarkoseis <1 m. The arkosegradesinto
a unit i to 4 m thick of coarseblue quartziticto feld- FIG. 3. Lithostratigraphic
sectionof the Osenarea showingthe
spathic sandstone.The latter unit is cemented by relativestratigraphicpositionsof galenaand granitesamples.
quartz,but pressuresolutionbetweenprimary quartz
grainsis uncommon.Illite and someamorphouscar-
bonoccurin the matrix.The oreassemblage includes Depositionalenvironments:The Lower Cambrian
galena, sphalerite,barite, and calcite occurringas sedimentaryrocksare interpretedto have been de-
disseminations in the quartz cement or as poikilo- positedas a marine transgressive succession. Depo-
blasts. sitionof basalsandstone wasprobablyin a beachen-
Dark fine-grainedsandstone: The basalsandstone vironment.The dark, fine-grainedsandstone unit was
becomesfiner upwardand passes into a unit of dark probablydeposited,with severalsmallbreaksin sed-
fine-grainedsandstones 2 to 4 m thickwhichcontains imentation,under tidal conditions(Nystuen,1969a).
thin conglomerateand green shaleinterbeds.Both Overlying green siltstoneand shalemay have been
verticaland horizontalburrowsare present,and rip- depositedin a subtidalenvironment.
ple marks,crosslaminations, and ball and pillow struc- Middle Cambrian black shales
tures have been observed(Nystuen, 1969a). Frag-
mentsof fossils(1-10 mm) from the conglomerate After a break in sedimentation,the Middle Cam-
have been identified as Hyolithes sp. and Torellella brian sedimentarysequencestartswith a conglom-
laevigata(Nystuen,1969a). The conglomeratealso erate20 to $0 cm thickcontainingfragmentsof sand-
containsroundeddiscoidalfragmentsof fossiliferousstoneand shalethat are in somecasesphosphatic.
dolomite. Immediately above the conglomeratefollow dark-
A weak dissemination of fine-grainedpyrite and gray to black shales(Alum shales)with nodulesof
galenaoccursin the sandstone, and someclustersof carbonateand pyrite depositedin a relativelyshallow
the samemineralsoccur in the conglomerate. marine environment.The autochthonous sequenceis
Green siltstone and shale: The dark sandstone interrupted 10 to $0 m from the base of the black
gradesinto a thin unit, i to 4 m thick, of greensilt- shalesby the Osennappe,which here consists of late
stoneand shale.The primary beddingin this unit is Precambriansandstones (VangsasFormation).
disruptedby a heavy bioturbation,but thin, coarse, Becausethis study attemptsto place an absolute
often more calcitic beds 5 to 10 mm thick can still age on lead mineralizationhostedby Lower Cam-
be seen.Large nodulesof pyrite are common. brian sandstone,it is relevant to considerthe ap-

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by Society of Economic Geologists, Cesar Ricardo Vargas
 SANDSTONEPb DEPOSITS,BALTIC SHIELD 455

TABI.E1. AnalyticalData for Trysil Granite

Sample
number "ø•Pb/2ø4Pb 2ø7pb/2ø4Pb "ø•Pb/2ø4pb U(ppm) Th(ppm) Pb(ppm) '2•2Th/"ø4Pb v•U/•ø4Pb

I 25.262 16.281 45.979 4.950 14.99 11.$6 101.95 $2.575

4 24.424 16.201 45.065 $.874 15.25 10.25 112.$4 27.657
10 20.158 15.776 $9.204 $.615 11.45 16.66 46.65 14.275
22 20.599 15.808 40.256 $.$94 15.25 15.29 79.45 17.155
24 19.455 15.666 $9.157 $.076 16.49 17.50 65.$1 11.429

proximateupper age boundaryfor Early Cambrian resultingsolutionwasaliquotedfor Pb isotopiccom-

sedimentation. Accordingto CowieandCribb (1978), position(unspiked)and for lead, uranium, and tho-
this Early to Middle Cambrian boundaryis at ap- rium concentrations(spiked).
proximately580 m.y. On the basisof the ground- The Pb wasseparatedusingHBr-HCI ion exchange
water model, ore formation would be expectedat columns.The eluant from the spikedaliquot was re-
roughlythis time, beforecappingMiddle Cambrian coveredfor U/Th separation.U and Th were sepa-
shaleswere depositedon a widespreadshallowmarine ratedtogetherusingthe samecolumns,but using7N
shelfthat existedthroughoutthe Cambrianand until HNOa, and the U and Th were loadedtogetheron
at leastArenig time (Early to Middle Ordovician;K. a doublefilament. U was analyzedfirst, the filament
Bjorlykke, 1974). Following depositionof Middle temperaturewas increased,and Th data then taken.
Cambrian shalesit is thereforehighly unlikely that The 2a error limits for U/Pb and Th/Pb ratiosare
LowerCambriansandstones wereaccessible
to ground lessthan 1 percent, and the correlation coefficients
water for at least60 million years. are greaterthan 0.9 for the variousU/Pb and Th/Pb
ratios.
Analytical Methods and Precision of Data The 2a errorlimitsfor the •ø•Pb/•ø4pb,
•øTPb/2ø4pb,
and "SPb/"4pbratiosfor the Klukengalenaspeci-
Analyses were performedby GeospecConsultants, men, analyzedby the GeologicalSurveyof Canada
Edmonton, Alberta, Canada, under contract to the
in 1976,are 0.10, 0.14, and 0.18 percent,respectively.
GeologicalSurvey of Norway. Theseincluded lead For the analysessuppliedby B. R. Doe of the U. S.
isotopeanalyses of one pyrite and five galenaspeci- GeologicalSurvey,the aboveseriesof lead isotope
mens from Osen and two galena specimensfrom ratiosis consideredto be within 0.1 percent of ab-
Gurrogaissa,and lead isotopeanalysesand quanti- solute.
tative U, Th, and Pb determinations of five granite
specimensfrom Osen. U-Th-Pb Relationshipsof Trysil Granite
Sulfides were dissolved in about 8 ml 2N HC1 and
slowlyevaporatedto drynessovernight.The pyrite For U-Th-Pb analysisof the Trysil granite,three
samplesolutionwasfurther processed usinga HBr- sampleswere selectedfrom unalteredgranitein dif-
HC1 ion exchangecolumn,and abouti ttg Pb was ferent drill holesas far below the Cambrian palco-
loadedon an Re filamentusingthe silicagel-phos- surfaceas possible(Fig. $). Two additionalsamples
phoricacid technique.For Pb analysesthe 2a error were takenfrom the weatheredzonenear the palco-
limits for the ratios •3Pb/2ø4pb,2ø7Pb/•ø4pb,
and surface. Earlier chemical work on the weathered zone
"SPb/"4pbare 0.069,0.079,and 0.111 percent,re- had showna leachingof lead (H•y, 1977), and the
spectively,as determinedfrom 21 analysesof the main purposein analyzingtheseweatheredsamples
U.S. National Bureau of Standards SRM981 common wasto determinewhether or not there was isotopic
lead standard. Correlation coefficients for individual fractionationof lead duringweathering.
analyses,owing to analyticprocedures,are 0.71 be- Table 1 givesthe analysesfor the five samplesof
tween•ø6Pb/2ø4pb and •ø7Pb/•ø4pb
and 0.64 between Trysil granite. The three lead isotopeanalysesfor
"6Pb/"4pband2øSPb/"4pb, respectively. unalteredgranite are almostexactlycollinear(Fig.
For whole-rocksamples,approximately 1 g of care- 4), and the slopeof this line is 0.0988 _+0.0016 (la),
fully homogenized rockpowderwasdissolved in 10 with a mean squareof weighted deviatesvalue of
ml of concentratedHF and i ml HNO3 by refiuxing 0.12, and yieldsan isochronage of 1,602 _+60 m.y.
overnightin a coveredbeaker.After drying,the res- The two alteredgranitecompositions plot belowthe
idue wasrepeatedlyheatedwith a few dropsof con- line for freshgranite,which suggests that there was
centratedHNO3 and then taken up in 4N HNO•, the an enrichmentin uranium relative to lead during
evaporationbeaker being stripped twice with 1:1 palcoweathering or possiblylater. This indicatesthat
HNO• to ensurecompletetransferof the sample.The the palcoweatheredzone has been subjectto several

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by Society of Economic Geologists, Cesar Ricardo Vargas
 4134 A. BJO•RLYKKE
AND R. I. THORPE

of either sample1 or sample10 to causethem to plot

exactly on concordia,and to compare thesediffer-
16,2
- 4.J ences to the error limits for these ratios. An increase

161-
in •ø•Pb/•ø4pb for sampleI from 25.262 to 25.272 is
sufficientto give agreementin calculatedagesat
1,594m.y. This increasein •ø•Pb/•ø4pb of about0.04
16.0- percentis a little greater than the averageanalytic
15.9- error for the laboratory but is still within the ex-
pectablerange of error. This adjustmentcould be
15.8 - distributedbetweensamplesI and 10 and wouldthen
be well within the averagedeviationexpected.If the
15.7- Weathered granite....... o
adjustment ismadeto the•7Pb/•4pb ratio,a decrease
024 Unweathered granite ..... ß
of a little lessthan0.01 percentin thisratiofor sample
21.5 22.5 23.5 1, from 16.281 to 16.2794,will give agreementat a
206 Pb/204 Pb
calculatedage of 1,591 m.y. This adjustmentis well
FIG. 4. Lead isochronplot for the Trysil granite. The slope within the limit of accuracyof measurement.How-
shownis for the unweatheredgraniteand corresponds to an age ever, in spiteof the fact that agreementis achieved
of 1,602 q- 60 m.y. betweencalculatedagesfor the z•sU-•ø•Pb and z•sU-
•7Pb systems with suchsmalladjustments to the mea-
stagesof ground-wateractivity,and the relativeen- suredlead isotoperatiosof either sampleI or 10 (or
richmentof uranium can possiblybe related to de- both),the measuredratiosmay still be correct.Small
positionof the overlyinguranium-richblack Alum errorsin the measured • values(•sU/•ø4pbratios)for
shale. the samples,for which the error limits are greater
In orderto obtainthe bestagefor the granitefrom than for the lead isotoperatios,couldalsoaccountfor
a plot of the data on a concordiadiagram,it is first the slight discordanceobserved.Thus it is obvious
necessaryto selecta reasonablelead isotopiccom- that in leadisotopestudiesof rocks,assuming that the
positionfor the commonleadin the granitesamples. samplesrepresentperfectlyclosedsystemsinsofaras
The commonlead compositionis subtractedfrom the U, Th, and Pb are concerned,very accuratemea-
measuredratiosto obtainthe radiogeniccomponent. surementsof (1) the lead isotoperatios•and in par-
This processis particularly critical in the case of ticularof the •?Pb/•4pb ratios•and (2) the amounts
whole-rockspecimens becausethe proportionof com- of U, Th, and Pb presentare necessaryin order for
monleadismuchgreaterthanit is in a uranium-rich them to fall exactlyon concordia.
systemsuchasthat providedby zircons.The common Concordiaplotscombiningthe U-Pb and Th-Pb
lead compositionwill lie on or very near the rock systems wereusedin orderto assess the 2øSPb/•ø4pb
leadisochron shownin Figure4. Parent-daughter is- ratio of the common(initial) lead in the graniteand
ochronplotsin the U-Pb system(Figs.5 and 6) for the concordancy of the Th-Pb systemin the samples
the unweatheredgranitesuggest a commonleadcom- studied.In a plot of •øSPb/•ø4pb against•Th/•ø4Pb
position of 2ø•Pb/•ø4pb= 16.142 and •?Pb/2ø4pb (Fig. 8), the three unweatheredgranitesamplesare
= 15.1382, when relianceis placedon samplesI and not collinear.A dashedline joiningsamplesI and 10
10. This relianceappearsto be justifiedby (1) the in theplotof Figure8 givesanintercept(initial•øSPb/
closeagreementbetweenparent-daughterisochron
agesof 1,591m.y. (Fig. 5) and 1,594m.y. (Fig. 6)
basedon thesesamples, and the Pb-Pbisochronage 26 - 1'
of 1,602 _+60 m.y., (2) the fact that testcalculations 25-

with other commonlead compositions yieldedages

lesscloselyin agreement,and (13)the fact that sample
4 (Figs.5, 6, and 7) appearsto showa slightdistur-
banceof its U-Pb system,either lossof uraniumor
23 -
24
-
22-

• 21--
• 20-
\•0

gain of lead, a resultin accordwith the fact that it

wascollectednearerthe paleosurface (Fig. :3)than
samplesI and 10. Usingthe aboveratiosfor common 18-

17-
lead givesa concordiaplot (Fig. 7) in which a dis-
16- . 2
cordialine, drawn throughthe point for sample4,
15
intersects concordiaat 1,598 m.y. This is in excellent
agreementwith the Pb-Pb isochronage. 238U/204 Pb

It is interestingto calculatethe small adjustments FIC. 5. This parent-daughterplot for granitesamplesi and l0
that are necessary to the measuredlead isotoperatios suggests an initial (commonlead)e•Pb/•4Pb ratio of 16.142.

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by Society of Economic Geologists, Cesar Ricardo Vargas
 SANDSTONE
PbDEPOSITS,BALTICSHIELD 455

16.3 -
cent, and the reasonfor the discordanceremains ob-
16.2 -
scure.
16.1 -
If it is assumed
that at 540 m.y. weatheredgranite
16.0 -
samples9•9•and 9•4had lead isotopiccompositions that
•. 15.9-
fell on the 540 m.y. isoehronfor the unweathered
o4158-
•. 157-
Trysilgranite(seeFig. 9), thenthe calculatedtzvalues
o4 15•6-
for thesesamplesfor the past540 m.y. are 18.47and
155-
19.63, respectively.In comparison,measuredvalues
15,4 -
are 17.13and 11.43.This suggests that the U-Pb sys-
15.3 -
tem of sample9•4,in particular,hasbeen disturbed
152
between540 m.y. andthe present,andthat itspresent
lead isotopiccompositionis thus the result of more
238U/204 Pb than two stagesof growthsinceintrusionof the Trysil
F]c. 6. Similar parent-daughterplot to Figure 5. An initial granite 1,598 m.y. ago. If the calculatedtz valuesof
•ø?Pb/•ø•Pb
ratio of 15.382is indicated. 18.47and 19.6:3represent the averagevaluesfor sam-
ples22 and 24 duringthe last540 m.y., then the first
stagetzvalues(•asu/•ø4pb ratios),from 1,598m.y. to
2ø4pbratio)of :35.178,andtheslopeof thelinesuggests540 m.y. ago,calculatedfor thesesamplesare 14.67
an unacceptablegranite age of 1,674 m.y. Conse- and8.14, respectively. Thesevaluesindicatethat sub-
quently,two approximations of the initial ratio were sequent to the first stageof evolution,and perhaps
obtainedby drawinglineswith a slopecorresponding during palcoweathering about 540 m.y. ago,the sam-
to an age of 1,597 m.y. throughthe data pointsfor pleswereenrichedin uraniumrelativeto leador were
samplesI and 10 in Figure 8. The valuesobtained subjectedto lesslossof uranium than of lead.
for the initial 2øspb/•ø4pb ratio were :35.599and
Lead IsotopeData for Galena and Pyrite
:35.$70,respectively.When theseratiosare usedto
make the common lead correction to the measured The isotopicanalysesfor a singlepyrite and five
•øSPb/•ø4pbratio,samples I and 10, respectively, plot galenaspecimensfrom drill holesthroughthe Osen
nearlyon a U-Th concordiain the combinedsystems. sandstone lead depositare presentedin Table 2. The
With an initial ratio of :35.:370,
an increasein •a2Th/ generalstratigraphicpositions of thesespecimens are
•ø4Pbfor sampleI of about2.8 percentfrom 101.9:3 shownin Figure :3,and their lead isotopiccomposi-
to 104.8wouldbe necessary in order for it to plot on tionsare plottedin Figure 9.
the U-Th concordiawith sample10. In view of the The pyriteisnotconsidered at the momentbecause
estimated i percent error in these ratios, it seems it is from a shale-siltstone unit above the mineralized
unlikelythat the analyticerror is asgreatas2.8 per- horizon and for an additional reason noted below.

4
44- 11

43-

42-

41-
o 24

. 40-
1598 Ma

280- mtersechOn/•"
1580
Me 1600
o//1.10Ma . 39-
o4 38-
Intercept J•/
37-

36-

1500
/ //'• Weathered
granite
o •'""'"•
Int
er•;•i
•'• 35-

Ma
ø 022 Unweathered
gramte
.ß I I I i

i
120
33 314 315 316 317 318 39 41o 411 412 232 Th/204 Pb
207 Pb/235U
FIG. 8. Th-Pb parent-daughterplot for unweatheredTrysil
FIc. 7. Concordiaplot for the Trysil granite usingthe lead granitesamplesshowingthe range of commonlead •ø•pb/2ø•pb
isotoperatiosfor commonleadasderivedin Figures5 and 6. ratios (interceptvalues)discussed
in the text.

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by Society of Economic Geologists, Cesar Ricardo Vargas
 456 ,4. BJ•RLYKKEAND R. I. THORPE

validity, this interceptsuggestsa sourceage of about

1,581 m.y. for a mineralizationage of 540 m.y., or
a sourceageof about1,594 m.y. for a mineralization
age of 520 m.y. The pyrite analysisobviouslyplots
just beyond its error limits from this line, and this
justifiesits exclusionfrom slope (intercept) calcula-
15.8
tions. The pyrite is from Lower Cambrian shales
(sample18A,Fig. $). It ispossible thata smallamount
of uraniumis presentin or intimatelyassociated with
the pyrite and accounts for the positionof the pyrite
pointbelowthe galenaline. A possible alternativeis
that the pyrite hassuffereda smallepisodicaddition
of radiogenicleadat sometime betweenitsformation
f
190 I 9•.5 i
200 i
205
and the present.
2O6 Pb/204 Pb

FIG. 9. Lead isotopedata for Osengalenaand pyrite in relation

If the Trysil granitewere the sourceof lead for the
to calculated500- and 540-m.y isochronsfor the Trysil basement Osengalena,the galenaanalysesshouldfall on a cal-
granite. culatedisochronfor the graniteat approximately580
m.y. This date is selectedbecauseof the previously
discussed stratigraphicconstraintson the ageof min-
The five galenasdefine a secondaryisochronfor eralization.Figure 9 showsthe galenadata in relation
which a calculatedslopeof 0.1287 ___
0.056 was ob- to calculated 500- and 540-m.y. isochronsfor the
tained usingthe methodof York (1969). For an as- Trysil granite.Four of the five galenaanalysesplot
sumeddepositionage of 540 m.y., roughlythe age between the 500- and 540-m.y. isochrons,and the
of the hostingCambriansedimentaryrocks,two-stage other plotsa little belowthe 500-m.y. isochron.Al-
calculations yield an approximatesourceageof 1,718 thoughall the galenaanalyseslie within their error
m.y. (max 2,415 m.y.; min 540 m.y.). Becauseof the limits of a 5$0-m.y. isochron,correspondingto their
broaderror limitsthiscalculatedsourceage haslittle presumedage,they bestfit a graniteisochronat ap-
meaning,but it doesnot precludethe Trysil granite proximately520 m.y.
as a source.Alternatively,at a sourceage of 1,598
m.y. a mineralizationage of 710 m.y. can be calcu- Summaryand Discussionof Isotopic Results
lated, but the broad error limits permit any age for
mineralizationin the range1,598m.y. to the present. Considerable attention has been devoted to inter-
A plot of 2ø4pb/2ø6pb-2ø7pb/2ø6pb (Fig. 10) shows pretationof the U-Th-Pb data for the Trysil granite
that the galenadata fall near a line that has been becausethey indicatethat parent-daughtersystems
drawn to passthroughthe calculatedinitial lead iso- are undisturbed in the case of two of the unweathered
topiccomposition of the Trysil granite.The intercept samplesand becausethisconclusion suggests that the
for thisline isabout0.1158;if the graniteand galenas isotopicand quantitativeanalyticdataare of excellent
are geneticallyrelated and the line thus has some quality. The corollaryisthat suchdata for whole-rock

TABLE2. Lead IsotopeData for Sulfidesfrom the OsenDrill Holes,Norway, and for Other Sandstone-Hosted
Deposits

Sample
Deposit number 2•'Pb/2"4pb 2ø7pb/2ø4Pb a"sPb/aø4Pb Laboratory(reference)
Osen Galena 4429 20.487 15.885 39.462 GeospecConsultants,1980
Osen Galena 4436 20.328 15.868 39.454 GeospecConsultants,1980
Osen Galena 4418 20.238 15.850 39.353 GeospecConsultants 1980
Osen Galena 4420 20.296 15.866 39.394 GeospecConsultants 1980
Osen Galena 4415 20.273 15.862 39.410 GeospecConsultants 1980
Osen Pyrite 18A 20.026 15.819 39.337 GeospecConsultants1980
Vassbo V7 20.849 15.901 39.823 GeospecConsultants 1980
Vassbo V3 20.852 15.927 39.947 GeospecConsultants 1980
Vassbo V2 20.878 15.923 39.948 GeospecConsultants 1980
V assbo 20.790 15.901 39.882 B. R. Doe (pets. commun.)
La isvail 21.406 16.003 40.039 B. R. Doe (pers. commun.)
Kluken TQ75-3 19.737 15.793 38.407 Geol. Surveyof Canada, 1976
Gurrogaissa 79 18.546 15.712 38.533 GeospecConsultants,1981
G urrogaissa 83 18.152 15.642 38.088 GeospecConsultants,1981

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by Society of Economic Geologists, Cesar Ricardo Vargas
 SANDSTONE Pb DEPOSITS, BALTIC SHIELD 437

to showany strictrelationship to the isotopicchar-

acteristics
of the Trysilgranite.Also,it is anticipated
that thesedata wouldthen plot in the vicinityof a
youngerisochron.
The isotopiccomposition to be expectedfor lead
derivedby weatheringof the Trysil graniteis de-
t'xl .0495 -
pendenton the character anddegreeof weathering.
Doe and Delevaux(1972)showedthat leachingof
.0497
sediments andbasement rocksresultedin an isotopic
.0499
fractionation, with lessradiogenic leadin the residue.
With onlythreeunweathered samples of thegranite,
it isdifficultto establish
itsaverageleadisotopic com-
.7 785 780 .775 7 0
< 207 Pb/206 Pb
positionat the time of weathering,but Figure4 and
FIG.10. Leadisotopedatafor Osengalenaandpyritein relation
Table2 indicatethat the leadin the galenaliesbe-
to a line drawnto passthroughthe commonlead composition tweenthe granitesamples in composition. Thisdem-
and onstrates
derivedin thisstudyfor the Trysilgranite.The 2ø4pb/2ø•Pb that a significantpart of the total lead went
axesare invertedsothatthe dataplotin the same into the ore solution.
2ø?Pb?ø•Pb
relativepositions
ason the morefamiliar•ø•Pb/•ø4pb:øTPb/•ø4pb The reason
for the rangein
lead isotopiccompo-
plot.
sitionof Osengalena(Figs.9 and 10) is not known.
Possible explanations are that theremay havebeen
needto be of very goodquality for the derived from different proportions
specimens changes in the relative of groundwater
basement areas within the
most precise determination of common lead com-
positionand rockage. drainage basin,or in the natureof the weathering
process.
An age of 1,598m.y. is indicatedfor the Trysil
granite.While a strictanalysis
of possible
errorswould As a preliminaryattempt to determinethe lead
placebroader errorlimitsonthisage,thegoodagree- isotopic composition of the interstitialwaterduring
mentbetweenPb-Pbisochron, parent-daughter, and diagenesis in theLowerCambriangreensiltstone and
concordia plotsleadsusto conclude thatit isunlikely shale (Fig. 3), one sample of pyrite from the shale
thatthe trueageliesoutsidethe 1,591-to 1,610-m.y. wasanalyzed.The pyriteis considered as probably
range. Initial (common)lead isotoperatiosfor the diagenetic because cleavage in the shale deflects
granite were approximately2ø•Pb/2ø4pb= 16.241, aroundsubhedral pyritegrains.The pyriteis a little
2ø?Pb/•ø4pb
= 15.382,and•-•sPb/•ø4Pb
= 35.37to35.60. lessradiogenicthanthe galenas, indicatinga slight
The 1,598-m.y.agefor Trysilgraniteisin excellent differencein composition betweenthe ore-forming
agreement with the whole-rock Pb-Srisochron age fluidand the interstitial
waterin the shaleduring
of 1,595__+
69 m.y. reportedby Priemet al. (1970) diagenesis.An alternativehypothesis
is that part of
forplutonic
andvolcanic rocksbelonging totheTrysil the lead in the pyrite wascontributedby ground
basementcomplex. water, similar to the groundwater that introduced
The slopeof the line definedby the lead isotope the leadfor the deposit,and that thislead wasmixed
compositionsof the five Osen galenassuggests a with lessradiogenic leadin seawater.Moresamples
of the pyrite must be
sourcewith an age a little greaterthan that of the clusions can be drawn. analyzedbeforedefinitecon-
Trysil granite, but the broad error limits do not ex-
cludethisgranite.The galenacompositions lie within Regional Implications and General Conclusions
theirerrorlimitsof a 520-m.y.isochronfor theTrysil
granite,an age that is in excellentagreementwith Vassbo (Christofferson
et al., 1979),Laisvall(Rick-
the presumedage of mineralizationwhich, in turn, ard et al., 1979),andKluken(Wolff, 1973)are other
is in agreementwith an early diageneticgenesis as sandstone lead depositshostedby Cambriansedi-
proposed by Samama(1976)for a similardepositin mentary rocksthat lie on Proterozoicbasementrocks
France(seealsoBj•rlykkeand Sangster, 1981).Thus of the BalticShield(Fig. 1). Recentleadisotope data
theisotopedatacanbeconsidered tosupport strongly
for thesedepositsare alsopresentedin Table 2 and
the ground-watermodel. plottedin Figure11. Thesedatalie reasonably near
The alternativebasinalbrinemodel(Rickardet al., a 520-m.y.isochron (Fig. 9) for the Trysil granite.
1979),in whicha muchyoungerPaleozoic agefor The basementin theseareasis fairly similarin
themineralization isproposed,iseffectively
negated compositionand age to that at Osen (Wilson and
by the isotope
data.If theoreswereformedby this Nicholson,1973), but differencesin initial lead iso-
mechanism, theirisotopedatawouldnotbe expected toperatiosand U/Pb ratios(/• values)probablyac-

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by Society of Economic Geologists, Cesar Ricardo Vargas
 4138 A. BJOCRLYKKE
AND R. 1. THORPE

Lalsvall+ north and northeast of the St. Francois Mountains

(Doeand Delevaux,1972).The lead,however,rather
than havingbeenderivedby leachingfrom the La-
motte Sandstone,could have been contributed more
Osen directly from the samebasementarea that servedas
the source for Lamotte Sandstone.
207p
b •$.e
204pb Kluken
{Osen
Pyrde Both a basinalbrine model (Rickard et al., 1979)
and a ground-watermodel (Bj•rlykke, 1977; Bj•r-
lykke and Sangster,1981) have recentlybeen pro-
posedfor the genesisof the sandstonelead deposits
15.6 .... J
• 8.5
.... •
19.0
'
19.5
•
zo.o
.... of the Baltic Shield.Accordingto the basinalbrine
,
20.5
....
z i .0
, , , I
z 1.5

206pb model,the mineralizationwasformed by interaction

of metal-bearingand sulfide-bearing brines.Rickard
FIG. 11. Lead isotopedata for othersandstone lead depositsof et al. (1979)suggestedthat the metalliferoussolution
the BalticShieldin relationto the Osengalenaandpyriteanalyses. wasa deepbasinalbrine, possibly developedduring
the late stagesof the Caledoniangeosyncline to the
count for the observedscatterof lead isotopedata. westand movedin front of the nappeseastwarddur-
This scatteris thusnot evidenceagainsta basement ing the Late Silurianor Early Devonian.
sourcemodelfor thesedeposits.It is, however,more In the ground-watermodel(Fig. 12) the mostcrit-
difficult to test the model for Vassbo and Laisvall than
ical factoris considered
to be the presence
of a large
hasbeen the casefor Osenbecausetheselarge de- drainagearea,possiblywith a high averageleadcon-
positshave a lesshomogeneous potentialbasement tent. Stabletectonicconditionsare necessary to allow
source.However,the data definitelyindicatea higher prolongedweatheringof bothbasementand overly-
uranium to lead ratio in the basement at Laisvall than ing pediment.During weathering,lead is released
at Osen and Vassbo. from feldsparand carriedin groundwaterto the site
The analyses for the Gurogaissadeposit(Moorbath of ore formation.Within the oxidizingenvironment
and Vokes, 19613),in which the lead occursboth in of meteoricand shallowgroundwater, lead will re-
veins and as disseminations, indicate that the lead main mobile.Uponreachinga reducingenvironment,
thereis lessradiogenicthan that in the otherdeposits producedeither by an accumulationof organicma-
discussed (Fig. 11).Thisisin accordwith thegranulite terial in the sediments(as found in post-Ordovician
terrane that forms the adjacentbasement(Sk•tlvoll, sandstone leaddeposits)or by a marginalmarineen-
1972),becauseotherlead isotopestudieshaveshown vironment(asin olderdeposits),lead wouldcombine
that suchterranesbecomevery depletedin uranium with biogenicallyreducedsulfur and precipitateas
during high-grade metamorphism(Lambert and galena.Becausethe solubilityof galenais approxi-
Heier, 1968; Heier and Thoresen, 1971; Black et al., matelysixordersof magnitudelessthanthatof sphal-
1971). erite under these conditions, low concentrationsof
A similar correlation has been described between biogenicallyreducedsulfurwouldpreferentiallypre-
the lead isotopiccompositions
of recent sediments
from the Baltic and Canadian Shields and their base-
MOUNTAIN AREA PEDIMENT SHALLOW MARINE
ment areas.The generallyhigher uranium content Max/mum ramfat/

in the CanadianShieldgivesriseto a moreradiogenic

composition of leadin the sediment(Doe, 1970).The
Kupferschieferis anothercasein which a regional
relationshiphasbeen documentedbetweenthe lead
• •
Increasmg salmtty
SEA-L
isotopiccomposition and the U-Pb-Th ratiosin the ofgroundwater
> Precipitation
Mechantca/
sourceareas(Wedepohlet al., 1978). weathering
Chemtca/weathering of roetatsand sthca

A basementsourcefor the lead in many of the Granttic

---> Arkose -•> Quartz+ clay -• Quartz/tic sandstone
Mississippi Valley depositswasproposedby Heyl et basement

al. (1966) basedon its isotopiccomposition.Doe and Ht-h P• Enrichment

of
Delevaux (1972) found that the Lamotte Sandstone -oa
•kgrouno
u ? Pbre/attve
toCuandZn
__•so/ubon
PbandSt• PbS+sd•cacement
comp/exes tn

wasthe mostprobablesourceof the lead after they

Kilometres
had investigatedthe whole-rocklead isotopiccom- i
lO
i
positionof the basementin the St.FrancoisMountains
aswellasthe overlyingsediments. The LamotteSand- FIG. 12. Schematicillustrationof the preferredgeneticmodel
stonewas mostlyderived from a sourcearea to the for sandstone
lead deposits(after Bjerlykkeand Sangster,1981).

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by Society of Economic Geologists, Cesar Ricardo Vargas
 SANDSTONE Pb DEPOSITS, BALTIC SHIELD 489

cipitategalenaand allow any zinc presentto remain We are gratefulto J. M. Franklin,D. F. Sangster,
and
in solutionand passonward in the system. G. L. Cummingfor their commentson an early draft
Conclusions of the manuscript,and to D. F. Sangsterfor helpful
suggestionson a later draft.
1. Whole-rockU-Th-Pb analyses for threesamples This paper is a contributionto the IGCP Proiect
of the basementTrysil granite from beneaththe No. 60, Correlation of Caledonian Strata-bound Sul-
Osensandstone leaddepositestablishan ageof about phides.
1,598 m.y.
April 28, July 6, 1981
2. Five lead isotopiccompositions for galenafrom
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by Society of Economic Geologists, Cesar Ricardo Vargas