0361-0128/09/3852/1065-15 1065

IntroductionThe Anti-Atlas of Morocco hosts a large number and vari-

ety (Au, Ag, Cu, Co, Ni) of ore deposits. The Bou Azzer min-ing district produced about 1,600 t of cobalt in 2008 (U.S. Ge-ological Survey, 2009), with major by-products includingnickel, gold, and arsenic. A number of underground minesare in operation. More than 60 variably sized orebodies areknown from the Bou Azzer district, all being spatially associ-ated with a Neoproterozoic ophiolite sequence (Fig. 1). Dur-ing the past years, rising commodity prices have led to re-newed interest in the area, and both exploration and researchactivities have been increasing considerably (e.g., El Ghorfi,2006; El Ghorfi et al., 2006, 2008; Ahmed et al., 2009).

Studies on the geologic setting, mineral content, and fluidcharacteristics of the ores in the Bou Azzer mining districthave been presented by a number of workers (e.g., Leblanc,1975, 1981; En-Naciri, 1995; En-Naciri et al., 1997; Esserrajet al., 2005; El Ghorfi, 2006; El Ghorfi et al., 2006, 2008;Ahmed et al., 2009). However, the timing of the mineraliza-tion is still controversial and mineralization ages ranging be-tween ca. 685 and 215 Ma have been proposed by various au-thors. In general, dating of ore formation processes isproblematic, as ores of base and noble metals are commonlybarren of minerals that offer chances for direct age determi-nation. Fortunately, however, inspection of the Co-As miner-alization at Bou Azzer revealed different opportunities fordating by using several less conventional methods applied todifferent mineralization-related minerals—namely, Re-Os on

molybdenite, Sm-Nd on carbonates, and U-Pb on brannerite.This paper reports new age data that constrain the timing ofCo-As-(Au) mineralization at Bou Azzer within the widercontext of crustal evolution of the Anti-Atlas.

Geology and MiningThe Bou Azzer-El Graara inlier (Fig. 1) of the Anti-Atlas in

Morocco represents a geological window (“inlier” or “bouton-nière”) into the Proterozoic basement surrounded by a dis-cordantly overlying infra-Cambrian to Paleozoic cover se-quence. The inlier is a segment of a Pan-African (685–580Ma) orogenic belt, located along the main Anti-Atlas thrust,an E-W trending fault zone which constitutes the northernboundary of the West African craton (ca. 2.0 Ga; Leblanc,1981). The Bou Azzer-El Graara inlier is well known for itsvein-type cobalt-arsenide deposits with gold as a by-product,podiform chromitites, the mined-out SEDEX-type Bleïdacopper deposit, and the Bleïda Far West gold-palladium de-posit (Leblanc and Billaud, 1978; Leblanc, 1981; En-Naciri,1995; Mouttaqi and Sagon, 1999; El Ghorfi et al., 2006,2008).

The Co-Ni-As-(Au) ores of the Bou Azzer district arehosted in variably steep shear zones which occur mainly at orclose to the contact of serpentinites with other rock types(e.g., diorites at Filon 7, Bou Azzer mine). Mineralization hasbeen traced locally for ~1 km within structures that can befollowed up to 5 km. Veins and lodes that make up payableore zones are usually between 0.2 and 2 m wide. The pristineore mineral assemblage mainly consists of Co-Ni-Fe ar-senides and sulfarsenides. Currently, about 150,000 t of ore

HERCYNIAN AGE OF THE COBALT-NICKEL-ARSENIDE-(GOLD) ORES, BOU AZZER, ANTI-ATLAS, MOROCCO: Re-Os, Sm-Nd, AND U-Pb AGE DETERMINATIONS

THOMAS OBERTHÜR,1,† FRANK MELCHER,1 FRIEDHELM HENJES-KUNST,1 AXEL GERDES,2 HOLLY STEIN,3AARON ZIMMERMAN,3 AND MUSTAPHA EL GHORFI4

1 Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany2 Goethe Universität Frankfurt, Institut für Geowissenschaften, Petrologie & Geochemie, Altenhöferallee 1, D-60438 Frankfurt, Germany

3 AIRIE Program, Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482 USA and Geological Survey of Norway, Leiv Eirikssons vei 39, 7491 Trondheim, Norway

4 REMINEX Exploration, Route de Casablanca, Lot. Elhamra Nr. 235, Marrakech - 40 000, Morocco

AbstractThe age of the hydrothermal Co-Ni-As-(Au) mineralization in the Bou Azzer-El Graara inlier was deter-

mined using different isotopic systems on ore and mineralization-related gangue minerals, namely Re-Os onmolybdenite, Sm-Nd on carbonates, and U-Pb on brannerite.

Re-Os dating of molybdenite was difficult owing to the intimate intergrowth of molybdenite with Co arsenides and the fact that sampling by drilling complex sulfide mixtures resulted in poor concentration ofmolybdenite. Accordingly, Re-Os dating of the impure molybdenite-sulfide mixtures yielded inconsistent agesin the range 400 to 350 Ma. This scatter is attributed to (1) inadequate concentration of pure molybdenite, (2)complex Re-Os systematics, possibly involving excess radiogenic Os derived from older generation(s) of sulfide,and/or (3) mixture of at least two Re-Os systems consisting of a component with pre-Hercynian history andlater disturbance by Hercynian metamorphism. Carbonates and brannerite that coexist with molybdenite gavemore consistent ages of 308 ± 31 Ma (Sm-Nd) and 310 ± 5 Ma (U-Pb), respectively. Because both ages agreewithin their errors, we regard the brannerite U-Pb age of 310 ± 5 (2σ) Ma as the best and most precise esti-mate for the age of the Co-Ni-As-(Au) mineralization. Although an earlier onset of mineralization cannot beignored totally, the new age data underline that the principal Co-Ni-As-(Au) mineralization at Bou Azzer occupying the main ore-bearing structures was driven by and formed during the end of the Hercynian orogeny.

† Corresponding author: e-mail: thomas.oberthuer@bgr.de

©2009 Society of Economic Geologists, Inc.Economic Geology, v. 104, pp. 1065–1079

Submitted: September 11, 2009Accepted: October 17, 2009

with grades of ca. 1 percent Co, 1 percent Ni and 3-4 g/t Auis mined annually from several mines in the Bou Azzer dis-trict. The ores are treated in a central processing plant at BouAzzer where they are upgraded by jigging machines and flota-tion. The concentrates are sent to Marrakech for further met-allurgical treatment. The resulting 1,600 t of cobalt producedin 2008 (U.S. Geologial Society, 2009) represent about 2.2percent of the world production of cobalt. Most of the pro-duction stems from pristine underground ore, however, oxideores were primary targets in the past and are mined occa-sionally today. These ores and oxidized material from olderdumps are treated at Bou Azzer in a separate plant that pro-duces CoCO3 as a saleable end-product.

Samples and MineralizationIn view of the fact that little geochemical information, es-

pecially on trace elements, is available in the literature, somewhole-rock analyses of selected elements of ores and concen-trates from the whole Bou Azzer mining district are pre-sented in Table 1 (El Ghorfi, 2006). Note that all ore samplescommonly carry appreciable contents of gold, but insignifi-cant amounts of Pt and Pd. Noteworthy also are distinct Mocontents, reaching up to ca. 1 wt percent (bound to molyb-denite), and U concentrations, up to 2,000 ppm, which aremineralogically hosted by brannerite and lesser uraninite(pitchblende). El Ghorfi (2006) noted that high molybdenumcontents appear to be present in most ores of the Bou Azzermining district; however, elevated contents of uranium weremainly found in the western part of the mining district

(Tarouni, Bou Azzer mine, Bou Azzer East). Further, sulfurisotope compositions (δ34S) show a wide range, from –8.4 to+10.3 per mil CDT throughout the mining district.

Samples of primary Co-Ni arsenide ores were collected inactive underground workings of the Bou Azzer mining district(Fig. 1), stretching for about 40 km from Tarouni in the westto Aït Ahmane in the east. Major ore minerals are skutteru-dite [CoAs3], safflorite [CoAs2], niccolite [NiAs], rammels-bergite [NiAs2], löllingite [FeAs2], various sulfarsenides(cobaltite, gersdorffite, arsenopyrite), subordinate base metalsulfides, and rare free gold (Leblanc, 1981; En-Naciri, 1995;El Ghorfi, 2006; Ahmed et al., 2009).

In the context of the present study of the ores, the ubiqui-tous, granular and webby-looking mats and rosettes of mi-cron-scale anhedral molybdenite [MoS2] and subidiomorphicto idiomorphic grains of brannerite [UTi2O6], often in closeassociation with each other, are of particular interest. Bothminerals occur either in quartz-carbonate (mainly calcite)gangue (Figs. 2, 3), or in the outer zones of skutterudite, sug-gesting a close temporal relationship to the main Co-As min-eralization.

Methods

(1) Re-Os analyses of ore mixtures with molybdenite

Re-Os dating of molybdenite and other sulfides has foundincreasing application (e.g., Stein et al., 2000, 2001). However,in the Bou Azzer samples, molybdenite was not visible to thenaked eye for routine extraction of a pure or silica-diluted

1066 SCIENTIFIC COMMUNICATIONS

0361-0128/98/000/000-00 $6.00 1066

FIG. 1. The Bou Azzer inlier and location of mines (redrawn after Leblanc, 1981).

SCIENTIFIC COMMUNICATIONS 1067

0361-0128/98/000/000-00 $6.00 1067

TAB

LE

1. S

elec

ted

Maj

or a

nd T

race

Ele

men

t Dat

a of

Ore

Sam

ples

and

Con

cent

rate

s (c

c) fr

om th

e B

ou A

zzer

Min

ing

Dis

tric

t

Sam

ple

no.

Loc

ality

Fe 2

O3

Co

Ni

Cu

PbZn

As

SSe

Ag

Bi

TeM

oU

Au

PdPt

(wt %

)(p

pm)

(ppm

)(p

pm)

(ppm

)(p

pm)

(wt %

)(w

t %)

(ppm

)(p

pm)

(ppm

)(p

pm)

(ppm

)(p

pm)

(ppb

)(p

pb)

(ppb

)

BA

04-0

1B

A m

ine

Filo

n 7

12.5

360

377

6635

3310

428

.50

0.64

113

0.5

831.

0511

923

2630

7.9

9.9

BA

04-0

2B

A m

ine

Filo

n 7

0.25

5508

351

179

0.83

0.44

2445

1420

1.3

0.3

BA

04-0

3B

A m

ine

Filo

n 7

13.5

648

513

4703

6663

358

22.5

04.

5637

618

.933

111

.16

9554

126

2630

6.4

9.9

BA

04-0

4B

A m

ine

Filo

n 7

3.58

1172

8231

8512

0690

4134

.60

1.62

224

19.4

200

5.49

6192

2007

2850

BA

04-0

5B

A m

ine

Filo

n 7

1.99

9618

134

9414

48

1329

.10

1.39

212

33.2

165

7.38

5021

489

2270

BA

04-0

6B

A m

ine

Filo

n 7

5.94

5666

216

0213

0915

873

16.4

02.

0230

342

.539

213

.57

1362

517

9234

20B

A04

-07

BA

min

e F

ilon

73.

2388

3127

2565

0.04

0.29

4833

.265

3.14

1226

125

347

BA

04-0

9B

A E

ast

11.5

562

057

722

7335

772

16.3

08.

1513

63.

217

53.

3043

6869

2100

6.6

20.8

BA

04-1

0A

ghba

r1.

7013

3193

4393

831

465

47.0

00.

3963

0.8

100

1.00

4522

2730

BA

04-1

1A

ghba

r2.

1711

3861

8207

2482

182

36.5

01.

0040

4.1

130

0.24

3271

415

001.

8B

A04

-12

Taro

uni

13.9

975

9113

246

3190

8.40

4.11

2016

0.18

152

815

80B

A04

-13

Taro

uni

7.09

9476

127

0155

040

9625

.50

2.41

168

3.1

452.

2924

710

055

50B

A04

-14

BA

Eas

t-Pu

it 3

5.02

9847

530

4711

444

121

33.0

01.

9191

1.1

472.

7520

915

629

40B

A04

-15

BA

Eas

t-Pu

it 5

3.42

4353

122

3211

790

177

11.5

01.

7210

62.

473

4.54

1438

1384

703.

717

.6B

A04

-16

Bui

smas

-Axe

33.4

827

263

403

876

485

46.2

00.

4628

0.8

172

0.86

9621

111

0.2

BA

04-1

7B

uism

as-A

xe31

.92

1839

716

4185

212

1343

.90

0.39

160.

923

20.

6913

726

8760

5.1

16.4

BA

04-1

8B

uism

as-A

xe34

.45

2044

015

5733

410

547

.10

0.43

160.

613

21.

0947

1532

6B

A04

-19

Oum

lil2.

6314

7000

1159

268

1820

43.8

00.

9785

14.3

157

0.55

2227

3130

70B

A04

-20

Oum

lil5.

6596

822

1474

138

925

715

835

.80

1.35

229

30.0

147

1.15

2661

4686

70B

A04

-21

Ago

udal

1.52

9358

512

5643

545

206

19.2

03.

8227

55.

912

60.

9492

922

2690

BA

04-2

2A

goud

al2.

0612

3464

2632

6764

3821

.40

6.05

715

15.5

253

2.70

2249

1555

800.

1B

A04

-23

Ago

udal

2.45

7392

215

4420

4751

14.6

03.

3932

914

.722

90.

6422

1914

2370

BA

04-2

5A

mbe

d0.

7811

110

1797

452

1863

0.12

0.02

244

2075

648.

711

.3B

A04

-26

Am

bed

12.3

781

750

2186

619

345

1475

18.3

00.

0511

2.5

134

0.35

691

6912

7011

.141

.2B

A04

-27

Am

bed

22.5

014

049

9637

2073

4221

50.

880.

8215

16.3

137

0.11

6924

1.8

8.3

BA

04-2

8A

ghba

r4.

5013

9197

2350

224

317

4260

.40

0.67

136

27.0

182

0.58

5319

3747

601.

57.

0B

A04

-33

Ait

Ahm

ane_

JO5.

6146

061

4377

122

728

80.

010.

952

45.5

950.

1819

376

545.

516

.7B

A04

-34

Ait

Ahm

ane_

JO4.

9435

528

370

134

131

0.01

0.94

140

.243

0.16

1320

7655

12.9

12.7

BA

04-3

5A

it A

hman

e F

ilon

5111

.37

244

1490

1595

126

153

0.02

1.66

20.9

243.

7910

1653

17.0

28.0

BA

04-3

6A

it A

hman

e F

ilon

5114

.67

142

2398

1756

175

219

0.02

0.22

0.9

74.

3812

1362

14.6

20.7

BA

04-3

7A

it A

hman

e F

ilon

514.

9757

2389

9932

934

1112

63.

592.

5424

2.3

390.

8948

131

603

10.7

43.0

BA

04-2

9B

A E

ast j

ig_c

c4.

5113

1188

2875

941

5878

957

147

.00

2.18

335

13.9

179

9.73

2791

8643

300

3.4

23.9

BA

04-3

0B

A E

ast s

pira

le_c

c6.

5016

0370

1896

867

6731

860

50.3

03.

2421

413

.020

88.

6719

3678

2870

0.6

8.3

BA

04-3

1B

A m

ine

final

_cc

9.51

6876

110

565

1046

962

1546

32.0

02.

2713

121

.626

22.

5219

4818

846

003.

16.

8B

A04

-32

Bui

smas

fina

l_cc

22.2

954

588

1570

716

8068

1167

51.6

01.

0861

47.8

1617

1.13

1524

7948

700

3.9

20.3

Not

es: O

btai

ned

by c

ombi

natio

n of

XR

F (B

GR

) and

aci

d di

gest

ion

(AC

TL

AB

S, C

anad

a) a

naly

sis;

no

num

bers

(ope

n fie

lds)

are

pro

vide

d fo

r an

alys

es b

elow

the

resp

ectiv

e de

tect

ion

limits

of t

he v

ar-

ious

ele

men

ts; d

etec

tion

limits

: Cu

= 10

ppm

, Pb

= 10

ppm

, Se

= 0.

1 pp

m, A

g =

0.05

ppm

, Bi =

0.0

2 pp

m, T

e =

0.1

ppm

, Mo

= 10

ppm

, Pt =

0.1

ppb

, Pd

= 0.

1 pp

b

separate. Instead, in a nonconventional hit-or-miss fashion, apowder was obtained from Mo-rich ore samples (cf. Table 1)in the hope that molybdenite would be present in the mix-ture, common Os would be minimal, and thus direct dating ofthe mixture using the model age approach would be possible(Stein et al., 2001). Re and Os concentrations were deter-mined at AIRIE, Colorado State University. A Carius-tube di-gestion using HNO3-HCl equilibrated with a mixed-doubleOs spike was used (Markey et al., 2003). Os is recovered bydistillation into HBr and purified by microdistillation. Re isrecovered by anion exchange. Isotopic compositions were de-termined by NTIMS on NBS 12-inch radius, 90° sector massspectrometer at AIRIE-CSU, using both Faraday cup andelectron multiplier, as warranted.

(2) REE analysis and Sm-Nd dating of carbonates

For REE analysis and Sm-Nd dating, optical homogeneouscarbonate fractions were handpicked from coarsely crushedgangue. Carbonate fractions were leached from the pow-dered samples with diluted acetic acid and separated fromimpurities (insoluble phases) by repeated centrifuging andwashing. For REE determination, the sample solutions wereevaporated to dryness, dissolved in hot HNO3 and evaporatedto dryness again. REE concentrations were determined usingan Agilent 7500 ICP-MS. Procedural blanks are negligibleand have not been subtracted. The respective analyticaldata are available on request. Sm-Nd element fractions were

obtained from the sample solutions after addition of a mixed148Nd-147Sm tracer by conventional cation exchange chro-matography in a first step and using columns with HDEHP-coated Teflon powder in a second step. Sm and Nd were mea-sured on a ThermoFinnigan Triton multicollector massspectrometer in static mode, using a double Re filament as-sembly. Nd isotope ratios were normalized to 146Nd/144Nd =0.7219. An Nd element standard (Merck™) run routinely inthe course of the sample measurements yielded 143Nd/144Nd= 0.512401 (1SD = 0.000002; n = 4), which corresponds to143Nd/144Nd = 0.511851 for the LaJolla Nd standard (cross-calibrated at the BGR). Uncertainties at the 95 percent con-fidence level were 0.005 percent for 143Nd/144Nd and 1 per-cent for 147Sm/144Nd. Procedural blanks were negligible at<0.1 percent of the relevant sample concentration. Sm-Ndisochron diagram and age calculation were done using Iso-plot/Ex 3.41b (Ludwig, 2003).

(3) U-Pb on brannerite

Uranium-lead isotope analyses were performed on bran-nerite grains in situ in polished sections by LA-SF-ICPMS atthe University of Frankfurt using a Thermo-Scientific Ele-ment II sector field ICP-MS coupled to a New Wave UP-213ultraviolet laser system. Brannerites were ablated with a laserintensity of <1 Jcm–2, repetition rate of 5 Hz and spot sizesof 5 to 16 µm. Data were acquired in peak jumping modeover 800 mass scans during 20 s background measurement

1068 SCIENTIFIC COMMUNICATIONS

0361-0128/98/000/000-00 $6.00 1068

serpentinite

shearedserpentinite

mineralization

diorite

diorite

FIG. 2. Cobalt-arsenide mineralization in shear zone at the contact between serpentinite and diorite. Bou Azzer mine,Filon 7, level –380 m.

followed by 30 s sample ablation. Depth penetration wasabout 10 µm or less. A teardrop-shaped, low volume (<2.5cm3) laser cell was used (cf. Janoušek et al., 2006). With a re-sponse time of <1 s (time until maximum signal strength wasachieved) and wash-out time of ~4 s (signal decreased to

below 1%), the cell enables sequential sampling of heteroge-neous grains (e.g., growth zones) during time-resolved dataacquisition. Signal was tuned for maximum sensitivity for Pband U while keeping oxide production, monitored as254UO/238U, well below 1 percent. Raw data were corrected

SCIENTIFIC COMMUNICATIONS 1069

0361-0128/98/000/000-00 $6.00 1069

skut

carb

mo

brn

zr

mo

skut

carb

mo + brn

100 µm

250 µm

100 µm

100 µm

100 µm

C D

E

B

F

A

FIG. 3. Reflected light photomicrographs in air (A-E), and backscatter electron image (F) of ore mineral assemblagesfrom the Bou Azzer mine; all samples from Filon 7. A. Association of skutterudite (skut), molybdenite (mo), brannerite (brn)and carbonate (carb). B. Skutterudite (skut) crystals growing into carbonate (carb; top) and parallel vein below consisting offine-grained molybdenite and brannerite (mo + brn). C. Brannerite crystals enclosed in skutterudite. Rosettes of molybden-ite (mo) within and adjacent to the skutterudite aggregate. D. Brannerite crystals at periphery of skutterudite. Note rosettesof molybdenite within and adjacent to the skutterudite aggregate. E. As in Figure 3C, after laser ablation (holes in branner-ite crystals). F. Brannerite crystal (center) showing internal inhomogeneity (lighter areas). Groups of dark gray rosettes aremolybdenite.

for background signal, common Pb, laser-induced elementalfractionation, instrumental mass discrimination, and time-de-pendent elemental fractionation using an in-house Excelspreadsheet program (Gerdes and Zeh, 2006). Analytical re-producibility (Manangotry monazite and GJ-1 zircon refer-ence standards) for 206Pb/238U was ~1.0 percent and for 207Pb/206Pb, ~0.5 percent. The total offset of the measured drift-corrected 206Pb/238U ratio from the “true” ID-TIMS variedbetween 8 and 20 percent for the different analytical sessions.

Previous studies have shown the ability to use non-matrixmatched standardization for LA-ICP-MS U-Pb dating (e.g.,Horstwood et al., 2003; Meier et al., 2006; Melcher et al.,2007). Meier et al. (2006), for instance, dated Late Protero-zoic monazite and xenotime using zircon as an external refer-ence standard and yielded concordant results where the207Pb/206Pb and the 206Pb/238U ages agreed to better than 1percent. This implies, in accordance with the results of thisstudy, a negligible difference in the U-Pb fractionation be-tween phosphates, silicates, and oxides (this study) after cor-rection of the time-dependent element fraction.

A common Pb correction based on the interference- andbackground-corrected 204Pb signal and a model Pb composi-tion (Stacey and Kramers, 1975) was carried out in caseswhere the corrected 207Pb/206Pb exceeded the internal errorsof the measured ratios. The interference of 204Hg (mean =294 ± 32 counts per second) on the mass 204 was correctedusing a 204Hg/202Hg of 0.228 and the measured 202Hg. Re-ported uncertainties (95% confidence level) were propagatedby quadratic addition of the external reproducibility (2 SD,standard deviation) obtained from the reference standardsduring the individual analytical session and the within-runprecision of each analysis (2 SE, standard error). Multipleanalyses of the Elk Mountains monazite and of the Plesovicezircon yielded weighted mean ages of 1395 ± 13 Ma(207Pb/206Pb) and of 339.0 ± 2.4 Ma (206Pb/238U), respectively.Both ages are within uncertainty indentical to conventionalTIMS analyses (Sláma et al., 2008). Concordia diagrams (2σerror ellipses) and concordia ages at 95 percent confidencelevel were produced using Isoplot/Ex 2.49 (Ludwig, 2001).

Results

(1) Re-Os analyses of ore mixtures with molybdenite

Dating very fine grained and intricately intergrown oreminerals with microscopic granular mats of poorly formedmolybdenite presents a challenge, as evident from the vari-ous photomicrographs of Figure 3. The hope was to capture

adequate “invisible” molybdenite on drilling sulfide masses,to provide good model ages. This approach requires that ar-senide-sulfarsenide-sulfide mineralogies have minimal com-mon Os, as is nearly always the case for ores derived fromgranitoids or fluids of typical upper crustal compositions.Such ores and associated pyrites are classified LLHR (low-level, highly radiogenic) with regard to their Re-Os isotopecharacter (Stein et al., 2000). Despite Re levels in the low ppbto ppt range, these LLHR sulfides can be treated like molyb-denite, as nearly all of their Os is radiogenic 187Os daughter.With little to no common Os to account for, determination ofreliable model ages, regardless of selection of the initial Osratio, is straightforward for LLHR samples.

The investigated ores from Bou Azzer, however, have er-ratic and sometimes significant levels of common Os (Table2). In some cases, the Os isotope composition is even domi-nated by common Os. Six analytical runs were made on foursamples from four different mine localities; however, onlythree runs provide Re-Os isotope data amenable to a modelage calculation, as their common Os is minimal and/or wellconstrained. Also, their Re concentrations suggest that somemolybdenite was captured in the sulfide powder. Selection oftwo very different initial 187Os/188Os ratios (0.2 and 0.9, eachwith an assigned uncertainty of ±0.1) has little effect on thecalculated age (Table 2). The other three runs yielded signif-icant common Os that cannot be well determined with thedouble 188Os-190Os spike (Markey et al., 2003). Yet using a sin-gle 190Os spike ran the risk of not correctly determining theisotopic composition of a drilled sample powder should itcontain micro-molybdenite. The high levels of common Os inthree of the samples preclude a fractionation correction andcause the radiogenic 187Os to be poorly known.

Unfortunately, the analysis yielding the highest Re (10.84ppm) also yielded the highest common Os (Table 2). Thesamples with the higher radiogenic to common Os ratios(MDID-542, MDID-560) provide the stronger age results(370–350 Ma), even though their Re concentrations are sub-ppm level. The three model ages obtained, however, are notin good agreement, spanning the time range of 400 to 350Ma. If pure crystalline molybdenite separates were possible,it would be highly unlikely to find a range of ages, given theconfined occurrence of molybdenite in the Bou Azzer ores. Ifa molybdenite separate is diluted by silicate minerals (e.g.,quartz or feldspar), model ages are not disturbed as silicatesdo not carry Re and Os. The Re-Os compositions and rangein model ages indicate variable mixtures of molybdenite-ar-senides-sulfarsenides-sulfides that must have a component of

1070 SCIENTIFIC COMMUNICATIONS

0361-0128/98/000/000-00 $6.00 1070

TABLE 2. Re-Os Data for Molybdenite-rich Sulfides from the Bou Azzer Co-Ni-As-Au Deposits, Anti-Atlas, Morocco

AIRIE run no. Sample no. Sample number, locality Re (ppm) 187Os (ppb) common Os (ppb) Age (Ma)

MDID-542 BA04-06 Bou Azzer, Puit-III, Filon 7 Central (-380m) 0.1553 (1) 0.571 (1) 0.010 (19) 350.4 ± 1.3MDID-543 BA04-11 Aghbar, Principal Trenches South (-105m) 10.84 (5) 45.6 (1) 7.38 (19) 400.2 ± 2.5MDID-560 BA04-06(b) Bou Azzer, Puit-III, Filon 7 Central (-380m) 0.345 (1) 1.346 (8) 0.004 (13) 371.0 ± 2.7

Notes: Run MDID-560 is a second separate from complex massive sulfide; absolute uncertainties shown, all at 2-sigma level, for last digit(s) indicated;decay constant used for 187Re is 1.666 × 10-11yr-1 (Smoliar et al. 1996); for MDID-560, ages corrected for Re blank = 1.85 ± 0.06 pg, total Os = 1.14 ± 0.02pg, 187Os/188Os = 0.307 ± 0.006, for MDID-542 and MDID-543, ages corrected for Re blank = 4.5 ± 0.1 pg, total Os = 0.344 ± 0.008 pg, 187Os/188Os = 0.381± 0.023, ages calculated using 187Os = 187Re (eλt - 1), assume initial 187Os/188Os = 0.2, and include all analytical and 187Re decay constant uncertainties; Cariustube dissolution with double Os spike method (Markey et al. 2003) on 60-100 mg targeted separates drilled as a powder from massive sulfide samples

pre-Hercynian history. The Re-Os systematics were mostlikely disturbed during Hercynian metamorphism.

(2) REE abundances and Sm-Nd dating of carbonates

Sm-Nd isochron dating of hydrothermal carbonates is a rel-atively young tool to determine the formation age of associ-ated metal deposits. So far, only a few applications of this dat-ing method are known. It has been used to date amesothermal gold-copper gangue mineralization (Nie et al.,1999), a stratiform antimony deposit (Peng et al., 2003), anda strata-bound Carlin-type gold deposit (Su et al., 2009). Sm-Nd isochron dating has also been applied to determine theformation age of vein-type calcite in non-metal deposits(Uysal et al., 2007) and of hydrothermal Mg carbonate inevaporates and in sparry magnesite deposits (Henjes-Kunst etal., 2008). In addition, Nd isotopes can help to constrain thesource of the metals or of the mineralizing fluid path (Nie etal., 1999; Uysal et al., 2007).

A basic assumption in the application of the Sm-Ndisochron method is that the investigated minerals provide sig-nificant variations in Sm/Nd. Carbonates from the Bou Azzermining district show a wide range in REE concentrationswith, in part, very high amounts of the total REE (rangingfrom 23 to 596 ppm ΣREE). No positive correlation exists be-tween the concentrations of the REE and those of As, Co,and Ni in the carbonate leachates, indicating that the REEare not contributed by an easily leachable ore mineral oflikely secondary origin. Furthermore, the REE do not corre-late with U abundance, indicating that brannerite inclusionsin the carbonates are not controlling the REE budget of theleachates. Therefore, we conclude that the REE form part ofthe carbonate lattice (cf. Hein, 1993) and that there is no sig-nificant contribution to the REE budget of the leachates fromnoncarbonate minerals.

In the sample set studied, carbonates from the eastern partof the ore district (Buismas, Oumlil, Agoudal, Ait Ahmane)show the largest variations in REE abundances and, further-more, elevated degrees of light and heavy REEcn (REEcn:chondrite-normalized concentrations) enrichment and/or de-pletion (Fig. 4A), indicative of complex mineralizing processes.Most carbonate samples from the Bou Azzer mining districtdisplay a pronounced negative Eucn anomaly. Samples withweak or missing negative Eucn anomalies are all from filon 51at Aït Ahmane in the eastern part of the Bou Azzer district(BA 04-35, -36, -37). Eu anomalies are controlled by chem-istry, temperature, pH, and redox conditions of the hy-drothermal fluids from which the carbonates precipitated(Sverjensky, 1984; Bau and Möller, 1992; Brugger et al.,2008). Strongly negative Eu anomalies as displayed by mostcarbonates in this study are likely a primary chemical charac-teristic of the mineralizing fluids. However, at temperaturesin excess of ~250°C, Eu2+ dominates over Eu3+ and may sub-stitute for Ca2+ preferentially over trivalent REEs, leading topositive Eu anomalies in mineral precipitates (Sverjensky,1984; Bau and Möller, 1992). The lack of any positive Eucn

anomaly for the calcites from ore samples indicates that thetemperature during fluid precipitation probably did not reachsuch high values. In addition, the Eu2+/ Eu3+ is also very sen-sitive to small variations in pH (Brugger et al., 2008). It canthus be assumed that the differences in the Eu anomalies

probably represent slightly different geochemical milieus ofthe mineralization processes in the different areas. A detaileddiscussion of the processes leading to the differences in theREE budgets of the samples, however, is beyond the scope ofthis study.

Five samples from the eastern part of the ore district (indi-cated by solid lines in Fig. 4A) were selected for Sm-Nd dat-ing according to their wide spread in Sm/Nd between 0.19(BA 04-35) and 0.47 (BA 04-20). Carbonates from the BouAzzer mine itself (Filon 7) exhibit relatively uniform REEcn

patterns (Fig. 4B). Significant variations are only found in theextent of negative Eucn anomalies and fractionation betweenmiddle and heavy REEcn. Because of the low spread in

SCIENTIFIC COMMUNICATIONS 1071

0361-0128/98/000/000-00 $6.00 1071

0.1

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sam

ple

/Cho

ndri

te

BA 04-16

BA 04-17

BA 04-20

BA 04-35

BA 04-37

BA 04-11

BA 04-18

BA 04-36

BA 04-23

East of Bou Azzer

A

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sam

ple

/Cho

ndri

te

BA 04-01

BA 04-04

MAR 06-09

BA 04-02

BA 04-03

BA 04-07

MAR 06-10

B

Bou Azzer Est, Tarouni

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sam

ple

/Cho

ndri

te

BA 04-09

BA 04-15

BA 04-12

C

FIG. 4. Chondrite (C1) normalized plot of REE concentrations in car-bonates from Co-Ni-As mineralization from (A) the eastern sector of themining district (Buismas, Oumlil, Aït Ahmane), (B) Bou Azzer mine, Filon 7,and (C) Bou Azzer East and Tarouni.

Sm/Nd (0.32–0.40), only three samples were selected for Sm-Nd dating from this part of the district (indicated by solidlines in Fig. 4B). Two samples from Bou Azzer East showconsistent patterns with maximum chondrite-normalizedabundances for the middle REE and strongly negative Eucn

anomalies (Fig. 4C). Because of the difference in Sm/Nd(0.38; 0.52) both samples were chosen for Sm-Nd dating.

Among the nine carbonate samples investigated by the Sm-Nd method (Table 3), five samples from the eastern part ofthe ore district (Buismas, Oumlil, Aït Ahmane: BA 04-16, -17,-20, -35, -37) are approximately colinear (MSWD of 2.1) inthe isochron diagram (Fig. 5) corresponding to an age of 308± 31 (2σ) Ma. An initial Nd isotope ratio corresponding to εNd

+1 suggests a significant component of Nd from a primitivesource. The low spread in 147Sm/144Nd of the three samplesfrom Bou Azzer mine Filon 7 (BA 04-01, -04; Mar 06-09)does not allow isochron calculation. However, the data pointsfit to a similar isochron age, although projecting to a lower,more crustal-like initial εNd of –5 (dashed line in Fig. 5). BA04-09 (analyzed in replicate) and BA 04-15 from Bou Azzer

East also fit to the isochron age (dashed-dotted line in Fig. 5),however, projecting to εNd –1.5 just in between the lines fromthe eastern part of the Bou Azzer district and Bou Azzer mine(Filon 7).

(3) U-Pb dating of brannerite

Brannerite [UTi2O6] is a rare mineral best known from an-cient, uranium-bearing conglomerate ores (Elliott Lake,Canada; Witwatersrand, South Africa), where it formedfrom uraninite and TiO2 phases following the so-called“Pronto-reaction” of Ramdohr (1957, 1979): UO2 + 2TiO2 →UTi2O6. As this reaction involves U4+ only, the presence ofbrannerite points to a reducing hydrothermal environment.In the Bou Azzer ores, brannerite forms idiomorphic tosubidiomorphic grains and aggregates, up to some hundredmicrons in diameter (Fig. 3). In the electron backscattermode of the SEM, the individual brannerite crystals areoften internally inhomogeneous (Fig. 3F). This is also re-flected by microprobe analyses (Table 4) which demonstratethat the Bou Azzer brannerite contains appreciable amounts

1072 SCIENTIFIC COMMUNICATIONS

0361-0128/98/000/000-00 $6.00 1072

TABLE 3. Sm-Nd Element Concentrations and Isotope Ratios of Carbonates from the Bou Azzer District

εNd εNd

Location Sample no. Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd (present day) (t = 308 Ma)

East of Bou Azzer BA 04-16 40.98 150.39 0.1641 0.512621 –0.3 1.0BA 04-17 13.75 60.18 0.1376 0.512553 –1.7 0.7BA 04-20 4.54 9.91 0.2759 0.512832 3.8 0.7BA 04-35 0.91 4.76 0.1153 0.512516 –2.4 0.8BA 04-37 11.89 37.20 0.1924 0.512703 1.3 1.4

Bou Azzer (East) BA 04-15 38.89 101.62 0.2314 0.512621 –0.3 –1.7BA 04-09 33.89 63.69 0.3204 0.512823 3.6 –1.3BA 04-09 (repl.) 33.81 63.91 0.3199 0.512815 3.5 –1.4

Bou Azzer (Central), Filon 7 BA 04-01 9.90 27.70 0.2153 0.512466 –3.4 –4.1BA 04-04 32.03 77.90 0.2476 0.512560 –1.5 –3.5Mar 06-09 14.06 45.06 0.1879 0.512401 –4.6 –4.3

0.5120

0.5122

0.5124

0.5126

0.5128

0.5130

0.10 0.14 0.18 0.22 0.26 0.30 0.34

147Sm/144Nd

Buismas, Oumlil, Ait Ahmane (in black):Age = 308 ± 31 Ma

Initial 143Nd/144Nd = 0.512288 ± 0.000038

data-point error ellipses are 2s

BA 04-20

BA 04-37

BA 04-16

BA 04-17

BA 04-35

143Nd144Nd

Mar 06-09

BA 04-01

BA 04-04

BA 04-09 (2x)

BA 04-15

FIG. 5. Sm-Nd isochron plot for carbonates grouped according to different mining sectors. Samples from (A) the easternsector of the mining district (Buismas, Oumlil, Aït Ahmane) in gray, (B) Bou Azzer East in blue, and (C) Bou Azzer mine,Filon 7, in red.

(in the percent range) of Ca and REE (only Y, Ce, and Ndwere analyzed quantitatively).

The results of the U-Pb LA-SF-ICPMS analyses of bran-nerite are shown in Figures 6 to 9 and are listed as Table A1in the Appendix. Eight to 17 spot analyses were performed on6 different brannerite clusters or groups in three polishedsections. Most spots yield discordant results with U-Pb agesscattering mainly between 70 and 270 Ma (Figs. 6A-B, 9).However, 13 of 64 analyses yielded concordant and equiva-lent results with a concordia age of 310 ± 5 Ma (Fig. 7). Con-sidering the large scatter in the U-Pb system, this age has tobe interpreted with some care. Individual analyses are gener-ally characterized by very variable Pb/U with typical within-run precision (SE, standard error) of 2 to 10 percent, which isabout 3 to 20 times that of the reference material, e.g., thePlesovice zircon and Elk Mountains monazite. This indicatesheterogeneity of the Pb/U on the µm-scale most likely due tomobility of the Pb and U. The 207Pb206Pb* (* denotes radi-ogenic Pb) is much less variable. The within-run precisionvaries between 0.4 to 2.1 percent (mean 0.9%), which is about2 to 4 times higher than typical monazite and zircon analysesat similar signal strength. This can be to some degree attrib-uted to elevated common Pb contents. About 70 percent ofthe brannerite analyses have 206Pb/204Pb <10,000, indicatingthe presence of common Pb.

The analyses of the six brannerite clusters yielded well-de-fined discordias (MSWD = 0.28–1.08) with upper interceptsof 307 +14/–13, 303 +14/–13, 302 ± 28, 295 +20/–19, 299+34/–29, and 323 +29/–27 Ma, repectively (Fig. 8). The lowerintercepts vary from 51 to 74 Ma. The weighted mean age ofthe 6 upper intercepts is 303 ± 9 Ma. The upper intercept age

of the discordia defined by all 64 analyses is 302 ± 8 Ma (Fig.9), which is within uncertainty indentical to the concordia ageof 310 ± 5 Ma defined by 13 analyses (Fig. 7). The concordiaage of 310 ± 5 Ma is interpreted as best estimate of the timeof brannerite formation. The lower intercept at 63 ± 6 Mapoints most likely to a later remobilization event.

Discussion and ConclusionsThe lithostratigraphic-geodynamic framework and devel-

opment of the Anti-Atlas of Morocco is a theme of ongoingdifference of opinion, and work on modern, comprehensivemodels of crustal evolution and metallogenesis is in continu-ous progress (Choubert, 1963; Leblanc and Lancelot, 1980;Saquaque et al., 1989, 1992; Hefferan et al., 1992, 2000, 2002;Leblanc and Moussine-Pouchkine, 1994; Piqué, 2001; Gas-quet et al., 2004, 2005; Levresse et al., 2004; Thomas et al.,

SCIENTIFIC COMMUNICATIONS 1073

0361-0128/98/000/000-00 $6.00 1073

TABLE 4. Representative Analyses of Brannerite, Bou Azzer (3# - filon 7)

Section AS7080 AS7080 AS7080 AS7081 AS7081Analysis 17 35 45 44 40

SiO2 0.40 0.23 0.74 0.74CaO 3.19 1.96 2.02 2.94 2.76TiO2 35.81 38.88 39.63 36.24 35.61MnO 0.63 0.44 0.60 0.71 0.62FeO 1.69 0.93 1.07 1.56 1.22Y2O3 0.87 2.50 5.25 0.77 0.63Ce2O3 1.60 0.39 0.62Nd2O3 0.43 1.94 0.87 0.54 0.33PbO 0.72 0.89 0.68 0.49 0.40ThO2 0.31 0.46 0.25 0.18UO2 53.06 43.59 42.35 52.82 53.61Total 97.12 93.41 92.86 97.68 96.12

Cations normalized to 6 oxygen

Si 0.028 0.016 0.052 0.053Ca 0.241 0.146 0.149 0.220 0.211Ti 1.903 2.042 2.054 1.900 1.907Mn 0.038 0.026 0.035 0.042 0.037Fe 0.100 0.054 0.062 0.091 0.072Y 0.032 0.092 0.191 0.028 0.024Ce 0.041 0.010 0.016Nd 0.011 0.048 0.021 0.013 0.008Pb 0.014 0.017 0.013 0.009 0.008Th 0.005 0.007 0.004 0.003U 0.835 0.677 0.650 0.820 0.849Total 3.207 3.167 3.185 3.195 3.172

Al, P, S, As, La and empty fields = below detection limits

400

360

320

280

240

200

160

120

0.047

0.049

0.051

0.053

0.055

10 20 30 40 50 60

238U/ 206Pb

Intercepts at74 +12/-12 & 303 +14/-13 Ma

MSWD = 0.95, probability = 0.50

data-point error ellipses are 2σ

branneritecluster-2

207 P

b/20

6 Pb a)

207 P

b/20

6 Pb

400

360

320

280

240

200

160

120

80

0.046

0.048

0.050

0.052

0.054

0 20 40 60 80 100

238U/ 206Pb

branneriteclusters 1+3

Intercepts at60.6 +9.1/-10 & 306 +14/-13 MaMSWD = 0.61, probability = 0.88

b)

FIG. 6. A-B.Two examples of LA-ICPMS spot analyses of different bran-nerite clusters or groups. For further details, see text.

2004; D’Lemos et al., 2006; Burkhard et al., 2006; Ennih andLiégeois, 2008). In general, the more recent reconstruction ofthe Anti-Atlas by Thomas et al. (2004) and Gasquet et al.(2005) is followed here, which views the Anti-Atlas as repre-sentative of a complex orogenic front that developed at thenorthern edge of the Eburnian (ca. 2.1–2.0 Ga) West Africancraton during Pan-African times. In the Anti-Atlas, the Pre-cambrian basement comprises several Paleoproterozoic toNeoproterozoic units, which are unconformably overlain bylate Ediacaran to Paleozoic rocks. The Precambrian base-ment crops out in several inliers, whereby the Bou Azzer-ElGraara inlier is regarded to be the structurally most complexpart of the whole Anti-Atlas (Gasquet et al., 2005).

The Bou Azzer-El Graara inlier was divided by Leblanc(1981) into a western oceanic domain and an eastern conti-nental margin domain. The oceanic domain of Bou Azzer ischaracterized by an ophiolite complex consisting of a basal se-quence of serpentinized peridotites, followed by ultrabasicand basic cumulates (layered gabbros), large stocks of quartzdiorite, basic lavas, and a mixed volcanic and sedimentary se-quence (Leblanc, 1981). The continental margin domain ofBleïd-Tachdamt is characterized by a thick, intimately imbri-cated sequence of sedimentary and volcanic rocks (Leblanc,1981).

Leblanc (1981) and Gasquet et al. (2005) identified twoPan-African tectonic events at 690–660 Ma and 605 Ma, re-spectively, as well as for larger parts of the Anti-Atlas. The in-trusive Bleïda granodiorite has been dated at 579.4 ± 1.2 Ma(Inglis et al., 2004).

Metamorphism in the area is generally of greenschist facies(actinolite, albite, epidote-clinozoisite, chlorite). From theMiddle Cambrian through the Middle Carboniferous, thewestern Anti-Atlas basin is characterized by a strong and es-sentially linear subsidence trend, leading to the accummula-tion of more than 10 km of mostly fine grained sediments,shed into an epicontinental sea from the African craton(Burkhard et al., 2006). In Late Carboniferous (to Permian?)time, compression led to an event of strong inversion andfolding; the basement was uplifted and folded into huge an-tiformal culminations (“buttonnières”) which punctate thesouthwestern Anti-Atlas fold belt (Burkhard et al., 2006).Gasquet et al. (2005) elaborate further that hydrothermal ac-tivity is recorded at 330 and 300 Ma, and that at least part ofthe gold mineralization in the Iourirn deposit has been at-tributed to a late Variscan (Hercynian) event at 301 ± 7 Ma.

The timing of the various types of mineralization of theAnti-Atlas of Morocco is a matter of dispute. Apparently, var-ious temporally different episodes of mineralization must beenvisaged. Previous work on Anti-Atlas ore deposits revealeda variety of possible mineralization ages. For example, the sil-ver ores of the world-class Imiter silver deposit were em-placed at ca. 550 Ma (Cheilletz et al., 2002; Levresse et al.,2004), whereas the gold mineralization of the Iourirne minewas dated at 301 ± 7 (Gasquet et al., 2005).

In previous studies of the Bou Azzer ores, Leblanc (1981)proposed a multistage model of ore formation commencingwith a late Pan-African (~685–580 Ma) event followed bylater remobilization during the Hercynian. According toLeblanc (1981), at least part of the host structures of the min-eralization represent earlier structures that were rejuvenated

1074 SCIENTIFIC COMMUNICATIONS

0361-0128/98/000/000-00 $6.00 1074

0.050

0.051

0.052

0.053

0.054

15 17 19 21 23 25

238U/ 206Pb

207 P

b/20

6 Pb

data-point error ellipses are 2σ

260

300

340

380

Concordia Age = (2 )310.4 ±5.1 Ma σMSWD = 0.93, probability = 0.57,C+E 13 spots

FIG. 7. Plot of concordant U-Pb results (13 out of 64 analyses) ofbrannerite.

br-2 br-3 br-4 br-5 br-6br-1270

290

310

330

350

370

Mean = [ ]302.7 ± 8.7 Ma 95% conf.

MSWD = 0.68, probability = 0.69

box heights are 2σ

207

206

Pb

/P

b a

ge

FIG. 8. Analyses of six groups or cluster of brannerite. Shown are theupper intercept ages of each group with 2 sigma uncertainty. The lower in-tercepts of the corresponding discordias (see Fig. 6) range from 51 to 74 Ma.

350

250

150

50

0.00

0.02

0.04

0.06

0.0 0.1 0.2 0.3 0.4

206Pb238U

Intercepts at

62.7 ± 6.4 & 301.6 ± 8.4 Ma

MSWD = 1.13

207Pb/235U

64 analyses

FIG. 9. Discordia defined by all 64 analyses performed on six groups orclusters of brannerite.

during the Hercynian. Levresse (2001) arrived at a maximummineralization age of 533 ± 2 Ma, represented by the age ofthe Aghbar trachyte that is cut by the mineralization. Ledent(1960) determined an age of 240 ± 10 Ma (Pb-Pb) for syn-mineralization brannerite; we recalculated the age by usingthe 207Pb/206Pb ratio to ca. 325 Ma. En-Naciri et al. (1997) re-ported a SIMS U-Pb age of ca. 550 Ma for brannerite. Lev-resse (2001) obtained an age of 215 ± 8 Ma (i.e., end of theTriassic) using 40Ar/39Ar dating of adularia from Filon 7, BouAzzer mine, and Esseraj et al. (2005) even propose mineral-ization ages later than the Triassic (<200 Ma).

The focus of the present study is on the application of dif-ferent isotopic systems to diverse ore- and mineralization-re-lated gangue minerals—i.e., Re-Os on molybdenite, Sm-Ndon carbonates, and U-Pb on brannerite—of the Co-Ni-As-(Au) mineralization of the Bou Azzer district, with the aim ofdating the mineralization. Re-Os dating of very fine grainedand intricately intergrown ore minerals with microscopicgranular mats of molybdenite presented a particular chal-lenge. The aim of capturing adequately pure and homoge-neous molybdenite when drilling sulfide masses was notachieved at acceptable levels. The analyzed Bou Azzer molyb-denite-arsenide ore mixtures have erratic and sometimes sig-nificant levels of common Os, as also found by Brauns (pers.commun., 2009) in a whole-rock Re-Os study of ore samplesfrom Bou Azzer mine. Brauns (pers. commun., 2009) con-cluded that the data he obtained were inadequate to producean isochrone and did not allow meaningful Re-Os dating. Hisinterpretation points to the fact that the ore samples appearto contain inhomogeneous mixtures of two different Re-Ossystems, one with radiogenic Os and one dominated by com-mon Os.

From six analytical runs in the present study, only threeprovided Re-Os isotope data amenable to a model age calcu-lation, as their common Os is minimal and/or well con-strained, and their Re concentrations suggest the presence ofsome molybdenite in the sulfide powder. The three modelages obtained, however, are not in good agreement, spanningthe time range from 400 to 350 Ma. The Re-Os compositionsand range in model ages indicate variable mixtures of molyb-denite-arsenides-sulfarsenides-sulfides that must have a com-ponent of pre-Hercynian history. The Re-Os systematics weremost likely disturbed during Hercynian metamorphism.

In conclusion, the Re-Os data for drilled ore mixtures con-taining micron-scale molybdenite yielded rather inconsistentages, in the range 400 to 350 Ma, which are attributed tocomplex Re-Os systematics, possibly involving excess radi-ogenic Os derived from older generation(s) of sulfide. Appar-ently, the Re-Os isotope clock in the analyzed ore mixtures isdisturbed, possibly due to recrystallization and formation ofnew sulfide intergrown with older generations, events whichtook place during the Variscan-Hercynian orogeny.

Regarding Sm-Nd dating of ore-related carbonates, thepreceding study of REE concentrations reveals some remark-able relationships. In most samples, the REE concentrationsof the carbonates are characterized by pronounced negativeEu anomalies, indicating high-temperature (>200°C) and re-ducing mineralizing fluids. Carbonates from the eastern partof the Bou Azzer district show the largest variations in REEabundances as well as elevated degrees of light to heavy REE

enrichment and depletion, indicative of complex mineralizingprocesses. Furthermore, samples from one location (Aït Ah-mane) display weak or missing negative Eu anomalies and,therefore, probably indicate ore mineralization in a differentgeochemical milieu.

Carbonate samples investigated by the Sm-Nd method fallinto three groups:

1. Five samples from the eastern part of the ore district areapproximately colinear (MSWD of 2.1) on an isochron dia-gram corresponding to an age of 308 ± 31 (2σ) Ma. An initialNd isotope ratio corresponding to εNd +1 suggests a signifi-cant component of Nd from a primitive source.

2. Samples from Bou Azzer East also fit to the isochron age,projecting, however, to εNd –1.5, just below the line from theeastern part of the Bou Azzer district.

3. Three samples from Bou Azzer mine Filon 7 revealed alow spread in 147Sm/144Nd and do not qualify for isochron cal-culation. However, the data points fit to a similar isochron ageas sample set (1), although projecting to an even lower, morecrustal-like initial εNd of –5. This trend of decreasing Nd fromeast to west is notable. However, a geologic interpretation ofthis trend would appear speculative at present.

In general, the REE characteristics corroborate fluid inclu-sion and stable isotope studies of the Bou Azzer mineraliza-tion (En Naciri et al., 1995; El Ghorfi, 2006; El Ghorfi et al.,2006), which points to an origin of the mineralization frompercolating basinal brines, under reducing conditions, and inthe temperature range between ca. 300° to 200°C. The threeisotopically different groups of carbonates within the BouAzzer mining district, as outlined above, probably representsomewhat different subsystems of the larger, dominating fluidsystem. Carbonates formed within the subsystems obtainedtheir specific isotopic signatures from fluids that experiencedfluid-rock interaction with different host rocks. In conclusion,the use of the Sm-Nd method for the dating of carbonatesamples has proven highly successful in the case of the BouAzzer district. The isochron diagram corresponding to an ageof 308 ± 31 (2σ) Ma is regarded as reflecting the age of oremineralization.

The new approach of U-Pb dating of brannerite by LA-SF-ICPMS proved to be a great achievement. The analysesof six different brannerite clusters or groups generallyyielded discordant results with U-Pb ages scattering, mainlybetween 70 and 270 Ma. However, 13 of 64 analyses yieldedconcordant and equivalent results with a concordia age of310 ± 5 Ma. The analyses of six different brannerite clustersyielded well-defined discordias (MSWD = 0.28 to 1.08) withthe weighted mean age of the upper intercepts being 303 ±9 Ma. The upper intercept age of the discordia defined byall 64 analyses is 302 ± 8 Ma, within uncertainty indenticalto the concordia age of 310 ± 5 Ma. Hence, the concordiaage of 310 ± 5 Ma is interpreted as the best estimate of thetiming of the mineralization. The lower intercept at 62 ± 6Ma points most likely to a later remobilization event(Alpine?).

The complexities of both the U-Pb and Sm-Nd data sets,however, would also permit a somewhat older age for the oresassociated with primitive source material. Reworking of older

SCIENTIFIC COMMUNICATIONS 1075

0361-0128/98/000/000-00 $6.00 1075

sulfide assemblages, particularly at smaller deposits that areless chemically isolated and more susceptible to communica-tion with later metamorphic fluids, could explain the scatterobserved in the isotopic data sets. However, at this time, andwith these diverse data, it is not clear which component mighthave been older and reworked, recrystallized, or remobilizedduring the Hercynian-Variscan orogeny.

In conclusion, although a certain spectrum of ages due torepeated pulses of ore formation appears possible, we preferto regard the carbonate and brannerite ages to represent thebest estimate for the age of the Co-Ni-As-(Au) mineralizationof the Bou Azzer district. Carbonates and brannerite that co-exist with molybdenite yielded consistent ages of 308 ± 31 Ma(Sm-Nd) and 310 ± 5 Ma (U-Pb), respectively. Because bothages agree within their errors, we regard the brannerite U-Pbage of 310 ± 5 Ma (2σ) as the best and most precise estimatefor the age of the Co-Ni-As-(Au) mineralization. Although anearlier onset of mineralization cannot be totally ignored, thenew age data underline that the principal Co-Ni-As-(Au) min-eralization at Bou Azzer occupying the main ore-bearingstructures was driven by and formed during the end of theHercynian orogeny.

AcknowledgmentsSincere thanks to the geology department at the Bou Azzer

mine, especially Mr. Mhaili and Mr. Mustapha Souhassou, forlogistic assistance and valuable discussions on site. ReminexExploration provided support during field work. Thanks alsoto Siegrid Gerlach, Kirsten Fromme, Hans Lorenz, JerzyLodziak, and Peter Macaj for analytical assistance at theBGR. M. El-G. acknowledges support from the German Aca-demic Exchange Service (DAAD) and the SEG Foundation.The AIRIE Program thanks Edward M. Warner for his sup-port. Michael Brauns, Darmstadt, and Bruce Eglington, asso-ciate editor, are thanked for their insightful comments on theisotopic data presented in this study. Finally, our thanks goto the editor, Larry Meinert, for fast-track handling of themanuscript.

REFERENCESAhmed, A.H., Arai, S., and Ikenne, M., 2009, Mineralogy and paragenesis of

the Co-Ni arsenide ores of Bou Azzer, Anti-Atlas, Morocco: ECONOMIC GE-OLOGY, v. 104, p. 249–266.

Bau, M., and Möller, P., 1992, Rare-earth element fractionation in metamor-phogenic hydrothermal calcite, magnesite and siderite: Mineralogy andPetrology, v. 45, p. 231–246.

Brugger, J., Etschmann, B., Pownceby, M., Liu, W., Grundler, P., and Brewe,D., 2008, Oxidation state of europium in scheelite: Tracking fluid-rock in-teraction in gold deposits: Chemical Geology, v. 257, p. 26–33.

Burkhard, M., Caritg, S., Helg, U., Robert-Charrue, C., and Soulaimani, A.,2006, Tectonics of the Anti-Atlas in Morocco: Comptes Rendus Geo-science, v. 338, p. 11–24.

Cheilletz, A., Levresse, G., Gasquet, D., Azizi Samir, M.R., Zyadi, R., and Ar-chibald, D.A., 2002, The Imiter epithermal deposit (Morocco): New petro-graphic, microtectonic and geochronological data. Importance of the Pre-cambrian-Cambrian transition for major precious metal deposits in theAnti-Atlas: Mineralium Deposita, v. 37, p. 772-782.

Choubert, G., 1963, Histoire Géologique du Précambrien de l’Anti-Atlas:Notes and Mémoir, Service Geologie de Maroc, v. 162, p. 7-352.

D’Lemos, R.S., Inglis, J.D., and Samson, S.D., 2006, A newly discovered oro-genic event in Morocco: Neoproterozoic ages for supposed Eburnean ba-sement of the Bou Azzer inlier, Anti-Atlas Mountains: Precambrian Re-search, v. 147, p. 65–78.

El Ghorfi, M., 2006, Etude géochimique et métallogénique des méteaux pré-cieux (or, argent et platinoides) associés aux minéralisations à Co, Ni, Cr de

Bou Azzer-El Graara, et dans la série de Bleida Far West, Anti Atlas,Maroc: Unpublished PhD thesis, Marrakech, Morocco, Université CadyAyyad, 256 p.

El Ghorfi, M., Oberthür, T., Melcher, F., Lüders, V., El Boukhari, A., Maa-cha, L., Ziadi, R., and Baoutoul, H., 2006, Gold-Palladium mineralizationat Bleïda Far West, Bou Azzer-El Graara inlier, Anti-Atlas, Morocco: Mi-neralium Deposita, v. 41, p. 549–564.

El Ghorfi, M., Melcher, F., Oberthür, T., El Boukhari, A.E., Maacha, L.,Maddi, A., and Mhaili, M., 2008, Platinum group minerals in podiformchromitites of the Bou Azzer ophiolite, Anti Atlas, Central Morocco: Mi-neralogy and Petrology, v. 92, p. 59–80.

En-Naciri, A., 1995, Contribution a l’étude du district a Co, As, (Ni, Au, Ag)de Bou Azzer, Anti Atlas (Maroc), données mineralogiques et geochimi-ques; étude des inclusions fluides: Unpublished PhD thesis, Orleans,France, Université Orleans, 238 p.

En-Naciri, A., Barbanson, L., and Touray, J.C., 1997, Brine inclusions fromthe Co-As(Au) Bou Azzer district, Anti-Atlas Mountains, Morocco. ECO-NOMIC GEOLOGY, v. 92, p. 360–367.

Ennih, N., and Liégeois, J.-P., 2008, The Boundaries of the West African cra-ton: London, The Geological Society Special Publication, no. 297, 533 p.

Esseraj, S., Boiron, M.-C., Cathelineau, M., Banks, D.A., and Benharref, M.,2005, Penetration of surface-evaporated brines into the Proterozoic base-ment and deposition of Co and Ag at Bou Azzer (Morocco): Evidence fromfluid inclusions: Journal of African Earth Sciences, v. 41, p. 25–39.

Gasquet, D., Chèvremont, P., Baudin, T., Chalot-Prat, F., Guerrot, C.,Cocherie, A., Roger, J., Hassenforder, B., and Cheilletz, A., 2004, Poly-cyclic magmatism in the Tagragra and Kerdous-Tafeltast inliers (westernAnti-Atlas, Morocco): Journal of African Earth Sciences, v. 39, p. 267–275.

Gasquet, D., Levresse, G., Cheilletz, A., Azizi-Samir, M.R., and Mouttaqi, A.,2005, Contribution to a geodynamic reconstruction of the Anti-Atlas (Mo-rocco) during Pan-African times, with emphasis on inversion tectonics andmetallogenic activity at the Precambrian-Cambrian transition: PrecambrianResearch, v. 140, p. 157–182.

Gerdes, A., and Zeh, A., 2006, Combined U-Pb and Hf isotope LA-(MC-)ICP-MS analyses of detrital zircons: Comparison with SHRIMP and newconstraints for the provenance and age of an Armorican metasediment inCentral Germany: Earth and Planetary Science Letters, v. 249, p. 47–62.

Hefferan, K., Karson, J., Saquaque, A., 1992, Proterozoic collisional basins ina Pan-African suture zone, Anti-Atlas Mountains, Morocoo: PrecambrianResearch, v. 54, p. 295–319.

Hefferan, K., Admou, H., Karson, J., Saquaque, A., 2000, Anti-Atlas (Mo-rocco) role in Neoproterozoic Western Gondwana reconstruction: Precam-brian Research, v. 103, p. 89–96.

Hefferan, K., Admou, H., Hilal, R., Karson, J., Saquaque, A., Juteau, T.,Bohn, M., Samson, S., Kornprobst, J., 2002, Proterozoic blueschist-bearingmélange in the Anti-Atlas Mountains, Morocco: Precambrian Research, v.118, p. 179–194.

Hein, U.F., 1993, Synmetamorphic Variscan siderite mineralisation of theRhenish Massif, central Europe: Mineralogical Magazine, 57, p. 431–467.

Henjes-Kunst, F., Prochaska, W., and Schramm, M., 2008, Application of theSm-Nd isochron method to dating of evaporitic and hydrothermal carbon-ates: Goldschmidt Conference Abstracts, p. 30.

Horstwood, G.L., Foster, R.R., Parrish, S.R., Noble, P., and Nowell, G.M., 2003,Common-Pb corrected in situ U-Pb accessory mineral geochronology by LA-MC-ICP-MS: Journal of Analytical Atomic Spectrometry, v. 18, p. 837–846.

Inglis, J.D., MacLean, J.S., Samson, S.D., D´Lemos, R.S., Admou, H., Hef-feran, K., 2004, A precise U-Pb zircon age for the Bleida granodiorite, Anti-Atlas, Morocco: Implications for the timing of deformation and terrane as-sembly in the eastern Anti-Atlas: Journal of African Earth Sciences, v. 39,p. 277–283.

Janoušek, V., Gerdes, A., Vrána, S., Finger, F., Erban, V., Friedl, G., andBraithwaite, C.J.R., 2006, Low-pressure granulites of the Lišov Massif,Southern Bohemia: Viséan metamorphism of Late Devonian plutonic arcrocks: Journal of Petrology, v. 47, p. 705–744.

Leblanc, M., 1975, Ophiolites précambiennes et gîtes arséniés de cobalt(Bou Azzer-Maroc): Unpublished Ph.D. thesis, France, Université de ParisVI, 367 p.

——1981, Ophiolites précambiennes et gîtes arséniés de Cobalt (Bou Azzer-Maroc): Notes et mémoires du service géologique du Maroc, no. 280, 306p.

Leblanc, M., and Billaud, P., 1978, A volcano-sedimentary copper deposit ona continental margin of upper Proterozoic age: Bleïda (Anti-Atlas, Mo-rocco): ECONOMIC GEOLOGY, v. 73, p. 1101–1111.

1076 SCIENTIFIC COMMUNICATIONS

0361-0128/98/000/000-00 $6.00 1076

Leblanc, M., and Lancelot, J., 1980, Interprétation géodynamique du do-maine panafricain de l’Anti-Atlas (Maroc) à partir de données géologiqueset géochronologiques: Canadian Journal of Earth Sciences, v. 17, p.142–155.

Leblanc, M., and Moussine-Pouchkine, A., 1994, Sedimentary and volcanicevolution of a Neoproterozoic continental margin (Bleida, Anti-Atlas, Mo-rocco): Precambriam Research, v. 70, p. 25–44.

Ledent, D., 1960, Age absolu d’une brannérite de Bou-Azzer (Sud-Maro-cain): Brussels, Belgium, Académie des Sciences Belgique, p. 1309–1311.

Levresse, G., 2001, Contribution à l’établissement d’un modèle génétiquedes gisements d’Imiter (Ag-Hg), Bou Madine (Pb-Zn-Cu-Ag-Au) et BouAzzer (Co-Ni-As-Ag-Au) dans l’Anti-Atlas marocain: Unpublished PhDthesis, Nancy, France, CRPG-CNRS, 191 p.

Levresse, G., Cheilletz, A., Gasquet, D., Reisberg, L., Deloule, E., Marty, B.,and Kyser, K., 2004, Osmium, sulphur, and helium isotopic results from thegiant Neoproterozoic epithermal Imiter silver deposit, Morocco: Evidencefor a mantle source. Chemical Geology, v. 207, p. 59–79.

Ludwig, K.R., 2001, Users manual for Isoplot/ex rev. 2.49: A Geochronologi-cal Toolkit for Microsoft Excel: Berkeley Geochronology Center, SpecialPublication 1a, 56 p.

Ludwig, K.R., 2003, User’s manual for Isoplot 3: A Geochronological Toolkitfor Microsoft Excel: Berkeley Geochronology Center, Special Publicationno. 4, 70 p.

Markey, R.J., Hannah, J.L., Morgan, J.W., and Stein, H.J., 2003, A doublespike for osmium analysis of highly radiogenic samples: Chemical Geology,v. 200, p. 395–406.

Meier, F.M., Kolb, J., Skallaris, G.A., and Gerdes, A., 2006, New ages fromthe Mauritanides: Recognition of Archean IOCG mineralization at GuelbMoghrein, Mauritania: Terra Nova, v. 18, p. 345–352.

Melcher, F., Sitnikova, M., Oberthür, T., Henjes-Kunst, F., Gerdes, A., Brätz,H., and Davis, D.W., 2007, “Coltan“ (columbite-tantalite ores): Finerprin-ting the source by combined mineralogical and geochemical methods, inAndrews, C.J., et al., eds., Digging Deeper: Proceedings of the Ninth Bien-nial SGA Meeting, Irish Association for Economic Geology, Dublin, v. 1, p.1485–1488.

Mouttaqi, A., Sagon, J.P., 1999, Le gisement de cuivre de Bleida (Anti-Atlascentral): une interférence entre les processus de remplacement et d’exha-laison dans un contexte de rift: Chronique de la Recherche Minière, v.536–537, p. 5–21.

Nie, F.-J., Bjørlykke, A., and Nilsen, K.S., 1999, The origin of the ProterozoicBidjovagge gold–copper deposit, Finnmark, northern Norway, as deducedfrom rare earth element and Nd isotope evidences on calcites: ResourceGeology, v. 49, p. 13–25.

Peng, J.-T., Hu, R.-Z., and Burnard, P.G., 2003, Samarium-neodymium iso-tope systematics of hydrothermal calcites from the Xikuangshan antimonydeposit (Hunan, China): The potential of calcite as a geochronometer:Chemical Geology 200, p. 129–136.

Piqué, A., 2001, Geology of northwest Africa: Berlin, Gebrüder Bornträger,310 p.

Ramdohr, P., 1957, Die “Pronto-Reaktion”: Neues Jahrbuch MineralogieMonatshefte, v. 57, p. 217–222.

——1979, Daten und Bilder von Mineralien des U4+ verschiedenen Altersund verschiedener Herkunft: Zeitschrift der Deutschen Geologischen Ge-sellschaft, v. 130, p. 439–458.

Saquaque, A., Admou, H., Karson, J., Hefferan, K., and Reuber, I., 1989,Precambrian accretionary tectonics in the Bou Azzer-El Graara region,Anti-Atlas, Morocco: Geology, v. l7, p. 1107–1110.

Saquaque, A., Benharref, M., Abia, H., Mrini, Z., Reuber, I., and Karson, J.,1992, Evidence for a Pan-African volcanic arc and wrench fault tectonics inthe Jbel Saghro, Anti-Atlas, Morocco: Geologische Rundschau, v. 81, p.1–13.

Su, W., Hu, R., Xia, B., and Liu, Y., 2009, Calcite Sm-Nd isochron age of theShuiyindong Carlin-type gold deposit, Guizhou, China: Chemical Geology,v. 258, p. 269–274.

Sláma, J., Košler, J., Crowley, J.L., Gerdes, A., Horstwood, M.S.A., Morris,G.A., Nasdala, L., Norberg, N., Schaltegger, U., Tubrett, M.N., and Whi-tehouse, M.J., 2008, Plesovice zircon—a new natural standard for U-Pband Hf isotopic microanalysis: Chemical Geology, v. 249, p. 1–35.

Smoliar, M.I., Walker, R.J., and Morgan, J.W., 1996, Re-Os ages of group IIA,IIIA, IVA, and IVB iron meteorites: Science, v. 271, p. 1099–1102.

Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead iso-tope evolution by a two-stage model: Earth and Planetary Science Letters,v. 26, p. 207–221.

Stein, H.J., Morgan, J.W., and Scherstén, A., 2000, Re-Os dating of low-levelhighly radiogenic (LLHR) sulfides: The Harnäs gold deposit, southwestSweden records continental scale tectonic events: ECONOMIC GEOLOGY, v.95, p. 1657–1671.

Stein, H.J., Markey, R.J., Morgan, J.W., Hannah, J.L., and Scherstén, A.,2001, The remarkable Re-Os chronometer in molybdenite: How and whyit should work: Terra Nova, v. 13, p. 479–486.

Sverjensky, D.A., 1984, Europium redox equilibria in aqueoussolution: Earthand Planetary Science Letters, v. 67, p. 70–78.

Thomas, R.J., Fekkak, A., Ennih, N., Errami, E., Loughlin, E.S., Gresse,P.G., Chevallier, L.P., and Liégois. J.P., 2004, A new lithostratigraphic fra-mework for the Anti-Atlas orogen, Morocco: Journal of African EarthSciences, v. 39, p. 217–226.

U.S. Geological Survey, 2009, U.S. Geological Survey Mineral CommoditySummaries: Cobalt, p. 48–49.

Uysal, I.T., Zhao, J.-X., Golding, S.D., Lawrence, M.G., Glikson, M., andCollerson, K.D., 2007, Sm-Nd dating and rare-earth element tracing of cal-cite: Implications for fluid-flow events in the Bowen basin, Australia: Che-mical Geology, v. 238, p. 63–71.

SCIENTIFIC COMMUNICATIONS 1077

0361-0128/98/000/000-00 $6.00 1077

1078 SCIENTIFIC COMMUNICATIONS

0361-0128/98/000/000-00 $6.00 1078

TAB

LE

A1.

Sup

plem

enta

ry D

ata

Spot

207 P

b1U

2Pb

3T

h420

6 Pb

206 P

b32σ

207 P

b32σ

207 P

b32σ

206 P

b2σ

207 P

b2σ

207 P

b(µ

m)

(cps

)(w

t.%)

(ppm

)U

204 P

b23

8 U(%

)23

5 U%

206 P

b(%

)rh

o423

8 U(M

a)23

5 U(M

a)20

6 Pb

Bra

nner

ite c

lust

er 1

(B

ouA

_005

4)1-

116

4210

7047

1426

70.

002

3607

0.03

255

90.

2344

90.

0522

41.

70.

9820

619

214

1829

61-

216

1676

3947

6175

0.00

489

720.

0141

43.

70.

0969

3.9

0.04

970

1.3

0.95

913

944

181

1-3

1612

8956

4193

360.

001

1833

50.

0246

06.

50.

1719

6.6

0.05

069

1.2

0.98

157

1016

110

227

1-4

1640

1362

4822

793

0.00

621

985

0.05

171

6.5

0.37

376.

60.

0524

11.

20.

9832

521

322

1830

31-

516

2300

8146

1211

20.

001

1472

90.

0287

44.

00.

2053

4.3

0.05

182

1.7

0.92

183

719

08

277

1-6

1653

8300

4823

604

0.00

115

700.

0522

710

0.37

8910

0.05

258

1.5

0.99

328

3332

629

311

1-7

1641

8274

4319

893

0.00

121

723

0.05

067

5.2

0.36

825.

30.

0527

01.

10.

9831

916

318

1531

61-

816

3079

2539

1522

20.

003

1838

00.

0426

05.

30.

3085

5.4

0.05

252

1.0

0.98

269

1427

313

308

1-9

1626

9767

4113

208

0.00

215

413

0.03

483

4.7

0.24

934.

80.

0519

20.

90.

9822

110

226

1028

2

Bra

nner

ite c

lust

er 2

(B

ouA

_005

5)2-

18

1181

9743

7572

0.00

324

880.

0186

84.

30.

1298

5.4

0.05

039

3.4

0.78

119

512

46

213

2-2

812

2194

4494

160.

004

3913

0.02

271

9.6

0.15

9810

0.05

104

1.7

0.99

145

1415

114

243

2-4

422

423

3710

067

0.00

247

750.

0293

624

0.21

0224

0.05

193

2.7

0.99

187

4419

443

282

2-5

1219

7649

5021

706

0.00

240

990.

0472

15.

80.

3388

6.0

0.05

204

1.4

0.97

297

1729

616

287

2-6

1648

7998

4713

438

0.00

447

380.

0302

314

0.21

6014

0.05

183

0.9

1.00

192

2619

925

278

2-7

1665

0622

4521

516

0.00

121

870.

0508

58.

30.

3671

8.4

0.05

236

1.2

0.99

320

2631

723

301

2-8

1640

4621

3917

536

0.00

287

80.

0479

26.

60.

3453

6.7

0.05

227

1.3

0.98

302

2030

118

297

2-9

1656

6286

4817

570

0.00

255

550.

0391

18.

50.

2827

8.6

0.05

243

0.9

0.99

247

2125

319

304

2-10

1268

2652

4514

158

0.00

214

60.

0218

95.

40.

1548

6.0

0.05

128

2.5

0.91

140

714

68

254

2-11

1634

5534

4913

245

0.00

261

460.

0291

710

0.20

6110

0.05

125

1.6

0.99

185

1819

018

252

2-12

1651

1243

4822

846

0.00

313

420.

0504

55.

00.

3649

5.1

0.05

246

1.4

0.96

317

1531

614

306

2-13

1616

2274

4580

570.

001

7357

0.01

926

5.0

0.13

275.

10.

0499

80.

80.

9912

36

127

619

42-

1416

2872

7038

8308

0.00

242

00.

0217

03.

60.

1492

4.1

0.04

987

1.9

0.88

138

514

15

189

2-15

1634

1646

4112

191

0.00

477

60.

0310

511

0.22

1411

0.05

172

1.3

0.99

197

2220

321

273

2-16

1635

0023

4091

590.

003

705

0.02

242

7.9

0.15

488.

00.

0500

81.

60.

9814

311

146

1119

82-

1716

4917

1046

2182

90.

003

925

0.04

962

4.1

0.35

434.

30.

0517

81.

40.

9531

213

308

1227

6

Bra

nner

ite c

lust

er 3

(B

ouA

_005

6)3-

116

2417

9635

1355

10.

004

4362

0.04

139

110.

2977

110.

0521

72.

20.

9826

127

265

2629

33-

216

1620

8839

8439

0.00

983

450.

0236

016

0.16

5217

0.05

076

2.2

0.99

150

2415

524

230

3-3

1656

3024

4923

370

0.00

384

800.

0507

28.

60.

3668

8.7

0.05

246

1.6

0.98

319

2731

724

306

3-4

1615

5454

4450

860.

018

5092

0.01

253

120.

0850

130.

0491

82.

40.

9880

1083

1015

73-

516

1418

7042

6294

0.03

427

655

0.01

632

130.

1110

130.

0493

42.

20.

9910

413

107

1316

43-

616

1540

1539

8201

0.00

411

494

0.02

289

100.

1595

110.

0505

52.

20.

9814

615

150

1522

03-

716

2456

5945

1314

50.

004

8310

0.03

144

7.8

0.22

588.

20.

0520

92.

50.

9520

015

207

1528

93-

816

2745

4847

1458

00.

005

848

0.03

295

120.

2358

120.

0518

91.

70.

9920

925

215

2428

0

Bra

nner

ite c

lust

er 4

(B

ouA

_005

9)4-

216

7054

3044

1952

20.

020

7913

0.04

776

9.3

0.34

439.

40.

0522

81.

70.

9830

127

300

2529

84-

316

3112

6445

9219

0.01

128

754

0.02

223

130.

1547

130.

0504

91.

50.

9914

218

146

1821

74-

416

1460

9243

1011

50.

004

2688

80.

0256

04.

90.

1808

5.3

0.05

121

1.8

0.94

163

816

98

250

4-5

1614

4082

4578

960.

006

1280

50.

0188

95.

10.

1282

5.5

0.04

920

2.0

0.93

121

612

26

158

4-6

1613

0297

4756

490.

019

4536

0.01

294

6.2

0.08

786.

60.

0492

02.

30.

9483

585

515

74-

716

4735

7944

1468

50.

002

3751

0.03

601

110.

2553

110.

0514

10.

91.

0022

826

231

2425

94-

816

7784

5446

2268

60.

003

688

0.05

014

7.4

0.36

327.

50.

0525

41.

50.

9831

523

315

2130

94-

916

2423

7339

9256

0.00

723

030.

0247

36.

10.

1744

6.4

0.05

115

2.1

0.94

157

916

310

248

4-10

1621

4024

4784

630.

005

1797

20.

0195

26.

40.

1331

6.7

0.04

946

2.1

0.95

125

812

78

170

4-11

1687

4102

4723

714

0.00

744

380.

0540

411

.40.

3947

11.4

0.05

297

1.5

0.99

339

3833

833

328

4-12

1625

8350

4013

563

0.00

824

550.

0360

56.

50.

2556

6.9

0.05

143

2.3

0.94

228

1523

114

260

SCIENTIFIC COMMUNICATIONS 1079

0361-0128/98/000/000-00 $6.00 1079

Bra

nner

ite c

lust

er 5

(B

ouA

_004

0)5-

15

6693

643

1579

70.

010

6214

0.03

924

40.

2828

40.

0522

61.

70.

9124

810

253

1029

75-

25

1338

1549

1699

50.

004

8263

0.03

720

220.

2660

220.

0518

72.

21.

0023

550

240

4728

05-

35

7379

3748

650.

004

2572

0.01

429

110.

0989

110.

0501

63.

00.

9691

1096

1020

25-

412

2277

5340

8998

0.01

287

990.

0244

217

0.17

3318

0.05

147

3.0

0.99

156

2716

227

262

5-6

513

413

3610

950

0.00

368

750.

0329

88

0.23

548

0.05

178

2.3

0.96

209

1621

516

276

5-7

516

832

3888

250.

006

1155

30.

0255

919

0.18

1619

0.05

147

1.3

1.00

163

3116

931

262

5-8

592

3539

5782

0.01

929

170

0.01

627

150.

1117

150.

0498

01.

60.

9910

416

108

1618

65-

95

8378

4277

840.

002

1637

50.

0205

15

0.14

495

0.05

125

2.3

0.89

131

613

77

252

5-10

510

003

3383

450.

001

5066

0.02

744

100.

1933

100.

0511

01.

70.

9917

417

179

1724

55-

1112

1745

4541

1105

20.

001

7031

0.02

930

20.

2083

30.

0515

62.

60.

5818

63

192

626

6

Bra

nner

ite c

lust

er 6

(B

ouA

_002

8)6-

15

4886

3937

910.

013

1905

0.01

166

120.

0792

130.

0492

74.

40.

9475

977

1016

06-

25

3679

3741

180.

011

4544

0.01

208

80.

0852

90.

0511

64.

30.

8777

683

724

86-

35

1235

537

8083

0.01

331

210.

0284

714

0.20

2415

0.05

155

3.8

0.97

181

2618

726

266

6-4

829

489

5049

270.

013

2220

90.

0105

517

0.07

1517

0.04

918

2.1

0.99

6811

7012

157

6-5

836

137

4855

130.

006

7012

0.01

195

100.

0810

100.

0491

83.

00.

9677

779

815

66-

68

6643

642

1131

30.

013

8949

0.02

948

150.

2131

150.

0524

41.

51.

0018

727

196

2730

56-

78

1324

7138

1859

80.

007

687

0.04

954

100.

3559

100.

0521

02.

10.

9831

230

309

2729

06-

88

6140

648

1384

80.

011

2221

70.

0283

812

0.20

3512

0.05

200

1.2

1.00

180

2218

821

286

6-9

837

227

5265

530.

008

6985

0.01

377

70.

0930

80.

0489

92.

30.

9588

690

714

76-

108

1381

7149

1576

30.

013

1913

40.

0320

112

0.23

3212

0.05

284

2.4

0.98

203

2421

324

322

Elk

,e40

1171

590.

4111

628

3959

150.

2495

2.9

3.04

83.

60.

0886

01.

310.

7214

3112

1417

1513

96Pl

esoe

6015

709

680

360.

1170

320.

0539

91.

90.

3958

2.3

0.05

317

0.69

0.79

339.

02.

433

93.

133

7

1 M

easu

red

207 P

b si

gnal

in c

ount

s pe

r se

cond

2 U

and

Pb

cont

ents

and

Th/

U w

ere

calc

ulat

ed r

elat

ive

to G

J-1

refe

renc

e an

d ar

e ac

cura

te to

abo

ut 2

0% d

ue to

diff

eren

t abl

atio

n ra

tes

betw

een

zirc

on a

nd b

rann

erite

and

the

hete

roge

neity

of t

heG

J-1;

Not

e th

at th

is d

oes

not a

ffec

t the

acc

urac

y on

the

isot

ope

ratio

s or

, the

refo

re, t

he a

ges

3 C

orre

cted

for

back

grou

nd, m

ass

bias

, las

er-in

duce

d U

-Pb

frac

tiona

tion

and

com

mon

Pb

usin

g St

acey

and

Kra

mer

s (1

975)

mod

el P

b co

mpo

sitio

n;20

7 Pb/

235 U

cal

cula

ted

usin

g 20

7 Pb/

206 P

b/(2

38U

/206 P

b×

1/13

7.88

); er

rors

are

pro

paga

ted

by q

uadr

atic

add

ition

of w

ithin

-run

err

ors

(2SE

) an

d G

J-1

repr

oduc

ibili

ty (

2SD

)4

Rho

is th

e 20

6 Pb/

238 U

/err

207 P

b/23

5 U e

rror

cor

rela

tion

coef

ficie

nt5

Mea

n an

d 2σ

stan

dard

dev

iatio

n of

Ple

sovi

ce z

irco

n (n

= 7

) an

d E

lk M

ount

ains

mon

azite

(n

= 11

), re

spec

tivel

y; a

bsol

ute

ages

and

unc

erta

inty

giv

en a

s w

eigh

ted

mea

ns

TAB

LE

A1.

Sup

plem

enta

ry D

ata

Spot

207 P

b1U

2Pb

3T

h420

6 Pb

206 P

b32σ

207 P

b32σ

207 P

b32σ

206 P

b2σ

207 P

b2σ

207 P

b(µ

m)

(cps

)(w

t.%)

(ppm

)U

204 P

b23

8 U(%

)23

5 U%

206 P

b(%

)rh

o423

8 U(M

a)23

5 U(M

a)20

6 Pb