elements from the mantle to superficial crustal levels. The origin of metals and ligands (sulphides) was deduced from S, fluid

inclusions, He, and Re/Os data obtained on sulphide phases and gangue minerals. Sulphur isotope analyses show the existence

34 34the other in pyrites associated with a synchronous rhyolitic dome (d SCDT = 7 to 2x). The d SCDT values of the silvermineralization event range from 28 to 2xand are interpreted as resulting from preferential degassing of SO2 in ascendingfluids, as well as mixing between the magmatic isotopic reservoir and a country rock reservoir. Helium isotope analyses of

sulphides and gangue minerals yield similar results, with 3He/4He ratios ranging from 0.76 to 2.64Ra. These data and the

absence of 20Ne in the analyzed fluid inclusions suggest a mantle origin for the fluids associated with the epithermal silver

event. Os isotopic ratios have been measured for the first time in Agj and Agsulphosalts. These data and those obtained onother sulphide phases directly associated with the Ag mineralization show measured 187Os/188Os ratios of 0.1420.197

indicating a dominantly mantle source for the associated Os. Assuming that Os and Ag are derived from the same source, the

black shale country rock provided less than 10% of the silver metal stock.

This study highlights the conditions required to transfer 8000 metric tons of Ag (and other elements such as Hg, Co, Ni, Pb,

Zn) from the mantle, i.e. (i) an efficient vector such as the sulphide ligand, (ii) a major and sudden break in the geodynamic

environment similar to the one which occurred near the PrecambrianCambrian transition to provide pathways from the mantle

wedge and the lithospheric base to the surface and, (iii) a strongly extensional tectonic setting to provide traps for the

concentration of metals in ore deposits.of two distinct isotopic reservoirs, one found in pyrite from the surrounding black shale country rocks (d34SCDT = 38x) andOsmium, sulphur, and helium isotopic results from the giant

Neoproterozoic epithermal Imiter silver deposit, Morocco:

evidence for a mantle source

G. Levressea,b,*, A. Cheilletza, D. Gasqueta, L. Reisberga, E. Deloulea,B. Martya, K. Kyserc

aCRPGCNRS UPR A2300 and ENSGINPL, BP 20, 54501 Vandoeuvre-le`s-Nancy, FrancebCentro de Geociencias, UNAM, Campus Juriquilla, 76230 Santiago de Queretaro, Mexico

cDepartment of Geological Sciences and Geological Engineering, Miller Hall,

Queens University, Kingston, Ontario, K7L 3N6

Received 3 June 2003; accepted 13 February 2004

Abstract

The giant epithermal AgHg deposit of Imiter (Anti-Atlas, Morocco) was investigated to study the transfer of chalcophilewww.elsevier.com/locate/chemgeo

Chemical Geology 207 (2004) 5979D 2004 Elsevier B.V. All rights reserved.

Keywords: Osmium; Sulphur; Helium; Fluid inclusions; Epithermal deposit; Imiter; Morocco

0009-2541/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemgeo.2004.02.004

* Corresponding author. CRPGCNRS UPR A2300 and ENSGINPL, BP 20, 54501 Vandoeuvre-le`s-Nancy, France.

E-mail address: glevresse@geociencias.unam.mx (G. Levresse).

events with homogenization temperatures (Th) rang-

ing from 100 to 450 jC, Baroudi et al. (1999) have

G. Levresse et al. / Chemical Geology 207 (2004) 597960mation on only one of the geological constraints of proposed magmatic and basinal (as brine) origins,1. Introduction

The potential utility of isotopic studies for deci-

phering the sources of ore-forming elements in ore

deposits has long been recognized. However, both

the choice of the analytical targets (native minerals

or sulphides, gangue minerals or wall-rocks) and the

analytical uncertainties due to the low concentra-

tions of certain isotopic pairs (for instance, Os and

Re in some sulphides; Freydier et al., 1997) may

pose difficulties. Moreover, additional complications

arise when considering the multiple transfers of

elements involved in ore-genesis related to hydro-

thermalmagmatic processes. These types of depos-

its involve various sources located throughout the

section from the mantle to the superficial crust, thus

requiring the use of complementary geochemical

tracers. Epithermal precious-metal deposits consti-

tute an excellent subject of investigation, as they

present geological characteristics related to both

near-surface environments (geothermal systems are

recognized in numerous AuAg epithermal depos-

its; Henley, 1985) and magmatic processes (deposits

are generally hosted by volcanics with calcalkaline

affinity). In this study, we investigated osmium,

sulphur and helium isotopes in ore and gangue

minerals of the Imiter AgHg giant deposit (stock

metal estimated at ca. 8000 metric tons Agj)located in the Anti-Atlas range of Morocco. This

deposit has recently been recognized to be of

epithermal-type, genetically associated with a rhyo-

litic event dated at 550F 3 Ma (ion-probe UPbdating on zircon; Levresse, 2001; Cheilletz et al.,

2002) and synchronous with extensional tectonics

(Ouguir et al., 1996). Prior to this study, it was not

known whether the metals, particularly silver, and

fluids are derived from a later supergene secondary

enrichment event (Guillou et al., 1988), from black-

shale country-rocks (Leistel and Qadrouci, 1991;

Baroudi et al., 1999) or have a clear magmatic

signature (either mantle, crustal or both). This

knowledge is required to establish a genetic model

firmly based on the source and mode of deposition

of metals and fluids. As noted by Barnes (1997),

answering these questions is not easy, as the use of

isolated geochemical techniques may provide infor-the system (Hedenquist and Richard, 1998). There-identified two generations of epithermal fluids. The

earlier fluids were hot (homogenization tempera-

tures (Th) = 100290 jC) and display ice-meltingtemperatures (Tmi) ranging from 19 to 0.1 jC.The second generation fluids were cooler

(Th = 115192 jC) and display lower Tmi rangingfrom 27.2 to 18 jC. Baroudi et al. (1999)fore, we have performed a multi-element geochem-

ical study, including the first Re/Os analysis of Agjand Ag sulphosalts, direct ion-probe analysis of S

isotopes in sulphide minerals and helium isotope

determinations in sulphide sulphosalts and gangue

minerals. The original data presented here indicate a

likely mantle origin for most of the silver and fluids

in the Imiter deposit.

2. Geological setting

2.1. Previous studies

The Imiter deposit is located on the northern

side of the Saghro massif, which constitutes, with

the other Proterozoic inliers (Ifni, Kerdous, Akka,

Bou Azzer, Sirwa and Ougnat) the Anti-Atlas

orogenic belt of Morocco (Fig. 1) bordering the

north side of the western African craton (Clauer,

1974; Ennih and Liegeois, 2001; Fekkak et al.,

2001). The Imiter AgHg deposit is epithermal

(Levresse, 2001; Cheilletz et al., 2002); it is hosted

by black shales and volcanics of Middle and Late

Neoproterozoic age, respectively, and unconform-

ably overlain by a Paleozoic sedimentary succession

(Fig. 2). The silver mineralization is genetically

related to felsic volcanics (domes and dikes) dated

at ca. 550 Ma (ion-probe UPb dating on zircon;

Levresse, 2001), coeval with regional extensional

tectonics. This epithermal event postdates a discrete

base metal episode associated with granodiorite

intrusives dated at 572F 5 Ma (ion-probe UPbdating on zircon; Levresse, 2001). Wall-rock alter-

ation associated with the silver epithermal event

was minimized by the neutrality of the epithermal

fluids. In a succession of four silver-mineralizingrespectively, for these two fluids. They suggested

that the brines were the vector of the silver metal

(in chloride complex form) extracted from the

enclosing black shales and that the deposition of

the silver mineralization is associated with the

mixing of the brine with the hot fluid.

2.2. Mineralization events and mineral paragenesis

Re-examination of the spatial and temporal rela-

tionships between the silver orebody and the coun-

try rock indicates three distinct successive events:

Fig. 1. Major geological units of the Anti-Atlas belt in southern Morocco and position of the Imiter deposit. Modified from Levresse (2001).

G. Levresse et al. / Chemical Geology 207 (2004) 5979 61Fig. 2. Geological map of the northern part of the Imiter inlier, Anti-Atlas, Morocco. Modified from Levresse (2001).

(i) recrystallization of syngenetic sulphide from the

Neoproterozoic black shales (NBS) during the Pan-

African (ca. 650600 Ma; UPb isotopic dilution

dating on zircon; Leblanc and Lancelot, 1980)

orogenic lower greenschist metamorphism; (ii) a

discrete base metal mineralization event (BME)

Fig. 3. BSE photographs of selected mineralization associations of the four distinct stages (NBS, BME-Qtz1, ESE-Qtz2, ESE-dolo). (a and b)

(Enlargement of (a)) Neoproterozoic black shales (NBS): Brecciated metamorphic pyrite cemented by argentite from ESE association (IMI99-

57). (c and d) (Enlargement of (c)): base metal mineralization (BME-Qtz1): Pyrite with inclusions of galena, sphalerite and chalcopyrite

n (Qt

entite

of m

omitic

G. Levresse et al. / Chemical Geology 207 (2004) 597962(IMIII99-03), associated with chlorite and muscovite in quartz vei

(ESE-Qtz2): Polymetallic area composed of AgHg, polybasite, arg

of AgHg area is related to the inhomogeneous concentrations

(IMIII99-60). (g and h) (Enlargement of (g)): silver epithermal dolrims and pyrite and AgHg cores. AgHg replaces the pyrite, but notz1). (e and f) (Enlargement of (e)): silver epithermal quartz stage

and pyrite in a quartz vein (Qtz2). Variation of grey color intensity

ercury. Pyrite and argentite display myrmekytic texture (cf. (f )).

stage (ESE-dolo): Cupola texture composed of CoNi arseniurethe CoNi sulphoarseniures (cf. (h)) (IMIII99-54).

Table 1

Sample locations and types of analyses performed

Mineralized sample Elevation (m) Event Stage Analysis

Sulphur Osmium Helium

IMI IMIS33-63 m 1463 ESE Qtz2 Pyrite

IMI IMI00-18 1454 ESE Qtz2 Chalcopyrite

ESE Qtz2 Pyrite

ESE Qtz2 Galena

IMI IMIHe2 1500 ESE Dolo Arsenopyrite

IMI IMIHe4 1500 ESE Dolo Sphalerite

IMI IMIHe12 1512 ESE Dolo Arsenopyrite

ESE Dolo Galena

IMI IMI00-13 1300 ESE Dolo Cinabar

IMI IMI99-39 1350 ESE Dolo Pyrite

IMIII IMIII99-03 1450 BME Qtz1 Pyrite

IMIII IMIII00-49 1370 ESE Qtz2 AgHgjIMIII IMIII00-50 1370 ESE Qtz2 AgHgjIMIII IMIII00-51 1454 ESE Qtz2 Galena

IMIII IMIII00-52 1454 ESE Qtz2 Galena Galena Galena

IMIII IMIII99-51B 1320 ESE Qtz2 Sphalerite Sphalerite

IMIII IMIII99-60 1374 ESE Qtz2 Quartz

IMIII IMIII99-65 1325 ESE Qtz2 Sphalerite

IMIII IMIII00-22 1454 ESE Dolo Arsenopyrite

ESE Dolo Chalcopyrite

ESE Dolo Galena

IMIII IMIII00-23 1320 ESE Dolo Arsenopyrite

ESE Dolo Galena

IMIII IMIII00-29 1374 ESE Dolo Dolomite

ESE Dolo Pyrargyrite

IMIII IMIII00-37 1325 ESE Dolo Pyrargyrite Pyrargyrite

IMIV IMIV99-58 1325 BME Qtz1 Pyrite Quartz

IMIV IMIV99-37 1325 BME Qtz1 Pyrite

IMIV IMIV99-20 1325 ESE Qtz2 Pyrite Pyrite

IMV IMV00-36 1275 ESE Qtz2 Sphalerite

IMIV IMIV00-16 1325 ESE Dolo Pyrite

IMIV IMIV99-24 1325 ESE Dolo Pyrite

IMV IMV00-24 1275 ESE Dolo Galena

Country rocks samples Elevation (m) Event Stage Analysis

Sulphur Osmium Helium

Takhatert dome IMRD99-72 1700 RD Pyrite

Tachkakacht dike IMRD99-74 1550 RD Pyrite Pyrite

IMI IMIS33-33 m 1467 NBS Pyrite Pyrite

IMIII IMIII99-18 1350 NBS Whole rock

IMIII IMIII00-41 1320 NBS Pyrite

IMIV IMIV99-13 1325 NBS Pyrite

Abbreviation: IMI,II,III,IV,V: mining shaft; IMRD: rhyolitic dome/dike; ESE: epithermal silver event; BME: base metal event; NBS:

Neoproterozoic black shales; RD: rhyolitic dome/dike.

Elevation: above sea level. Example of main type of sample code name: IMIII99-60: shaft III, year 1999, sample number 60; example of other

sample code names: IMIS33-63 m (or He): shaft I, drill hole number 33, depth 63 m.

G. Levresse et al. / Chemical Geology 207 (2004) 5979 63

G. Levresse et al. / Chemical Geology 207 (2004) 597964near the metamorphic halos induced by intrusion of

granodiorite (572F 5 Ma) and (iii) the epithermalsilver mineralization event (ESE) at 550F 3 Ma.The silver mineralization is structurally controlled

by the Imiter fault zone (Fig. 2), which experienced

two successive tectonic regimes (Levresse, 2001).

The earlier and predominant regime led to the

development of normal faults trending N80jE, thesecond sinistral strike-slip regime led to reactivation

of the normal faults inducing an anastomosed

geometry.

2.2.1. NBS sulphides

Sulphides hosted by black shales are mainly pyrites,

exhibitingxenomorphicporousstructure(IMI99-13-18-

57; IMIII00-41; IMIS33-33).These pyrites are deformed

and brecciated and often serve as nucleation sites for the

ESE silver minerals (IMI99-57, Fig. 3(a),(b)). They are

considered as contemporaneous of the black shale lower

Neoproterozoic greenschist metamorphic event.

2.2.2. BME

The base metal assemblage is enclosed in the

cordierite/andalousite metamorphic halo surrounding

the granodiorite stock and is crosscut by the rhyo-

litic dikes and silver epithermal stockwork. The

BME assemblage is supported by quartz veins

emplaced in the black shale schistosity planes.

The veins do not exceed 50 cm (length) 10 cm(width), and are characterized by phyllic alteration

close to the vein walls. Quartz grains locally

display ondulose extinction, providing evidence for

dynamic crystallization (IMIII99-03). Pyrite is close-

ly associated with disseminated chlorite (BME-

Chl1) and muscovite (BME-Ms1, IMIII99-58), or

is found in veins in the quartz matrix (IMIII99-03,

IMIV99-37). Chlorite (BME-Chl1) and muscovite

(BME-Ms1) indicate crystallization temperatures of

325F 15 to 400F 15 jC and 450650 jC, respec-tively (Levresse, 2001; Cheilletz et al., 2002).

Minor sulphides include sphalerite, arsenopyrite,

galena and chalcopyrite (IMIII99-03-58, IMIV99-

37). Local zoning is observed in veins, starting

with quartz, followed by pyrite, then muscovite,

and then chlorite. Pyrite is corroded by the other

sulphides (Fig. 3(c),(d)). Close to the contact with

the granodiorite stock, quartz presents graphitictextures (IMIII00-53).2.2.3. ESE

The epithermal silver event is divided into two

successive stages characterized by two different

gangue minerals, quartz (ESE-Qtz2) and dolomite

(ESE-dolo) associated in extensional veins, hy-

draulic breccias and quartzdolomite laminations.

Hydraulic breccias are mainly developed in black

shales. In addition to the AgHgj present withinthe mineralized structures, AgHgj is also dis-seminated in the basal conglomerate and black

shales. The most impressive AgHgj and Agjoccurrences are the decakilogram plates in exten-

sive fault zones and brecciated areas. Mercury

contents in Ag amalgam increase from the

quartz- to the dolomite stage, and range from

10% to 30% and from 20% to 40%, respectively.

Mineralogical textures in both stages are complex

(association, replacement). Monosulphides are rare

(Fig. 3(e)(h)). Each mineralized stage is charac-

terized by a specific texture, i.e. myrmekitic in

the ESE-Qtz2 (IMIII99-60; Fig. 3(e),(f)), and by

cupolas in the ESE-dolo stage (IMIII99-54; Fig.

3(g),(h)). Sulphides are the earliest phases. These

are, in decreasing abundance, pyritearsenopyrite,

sphalerite, galena and chalcopyrite. Sulphides oc-

cur mostly as aggregates or as xenomorphic areas.

They are strongly corroded by native silver,

sulphides and sulphosalts (IMI99-57; IMIII99-03-

54-60; Fig. 3(e)(h)). Silver sulphosalts always

precipitated in association with or as replacement

of sulphides (Fig. 3(f)). Imiterite is most com-

monly found as needles in dolomitic geodes,

associated with cinnabar and argentite (Guillou

et al., 1985).

Oxidation is scarce and locally limited to the very

upper levels of the deposit. The oxidized assemblage

includes milky-quartz, limonite, rhodocrosite, kaolin-

ite, erythrite, malachite, azurite, marcassite, lavendu-

lane and cerusite.

The Imiter paragenetic succession is complex

and polyphased, rendering difficult the separation

of pure phases for isotopic analyses. To limit the

effect of impurities on the analytical results, we

separated and analyzed only the most representa-

tive phases of the paragenesis and used only the

purest separates, within the limits of optical deter-

mination. The exact location of analyzed samples

is given in Table 1.

G. Levresse et al. / Chemical Geology 207 (2004) 5979 653. Analytical procedure

3.1. Sulphur isotopes

In-situ analyses of d34SCDT in sulphide mineralswere carried out at the CRPGCNRS (France) using

the IMS 3F CAMECA ion-probe and purified sul-

phide at Queens University (Canada) using the

VG602 continuous flow-isotope ratio mass spec-

trometer (CF-IRMS). CF-IRMS analytical proce-

dures are given in detail in Grassineau et al.

(2001). For ion probe analyses, sulphides were

analyzed on gold-coated thin sections. Sulphur iso-

tope ratio measurements are complicated by isobaric

interferences on the 32S peak (e.g., 16O2) and by

large differences in matrix effects between the

various sulphide minerals (e.g., Eldridge et al.,

1987; Chaussidon and Demange, 1988; Deloule et

al., 1992). For this study, a 40 nA O primary beamaccelerated to 10 kV was used. The beam was

focused to about 10 Am in diameter and rasteredover a sample area with a width of 50 Am. Thesecondary positive beam was measured without

energy filtering. Intensities were measured in pulse

counting mode, 3 s on mass 32 and 10 s on mass

34. Background noise was analyzed at mass 31.5 for

50 s before each analysis and never exceeded 0.1

counts per second. Analyses consisted of 10 blocks of

10 cycles. This analytical procedure usually yielded an

in-run precision of 0.5x. The calibration was per-formed with regular analyses of internal standards

( d3 4 S C D T s t a n d a r d v a l u e s : d3 4 S C D T

pyrite = 1.41F1.2x; d34SCDT galena =0.00F0.8x,d34SCDT sphalerite = 3.00F0.6x, d34SCDT are-snopyrite = 2.24F1.5x). The sample IMRD99-74was analyzed by both instruments (IMS3F and CF-

IRMS). The lack of analytical bias is confirmed by the

similar average values obtained by the different meth-

ods. d34SCDT values in this sample (IMRD99-74) varyover a restricted range, from 4.1F1.2xto 2.0F 0.8x.

3.2. Fluid inclusion study

A Linkam stage was used to record ice-melting

temperature (Tmi) and homogenization temperature

(Th) to confirm the homogeneity of the fluids in theESE-Qtz2 inclusions. The calibration was performedwith the appropriate chemicals from Merck. Measure-

ment accuracy was about 0.2 jC for freezing runs and1 jC for heating runs. Most representative inclusionswere analyzed with a Labram-type (DilorR) Ramanmicroprobe with a NotchR filter at the G2R (Nancy,France). Technical details are provided in Debussy et

al. (2001).

3.3. Helium isotopes

Analyses were carried out at the CRPGCNRS

(France) following the extraction, purification and

analytical procedures described in Richard et al.

(1996). Samples were first crushed to large grain

size and handpicked under a binocular microscope.

Separated minerals were cleaned in distilled water

and high-grade acetone and loaded into online sole-

noid-driven crushers. Samples were loaded in crush-

ing tubes and gently baked at 80 jC overnight.Gases were extracted by sequential crushing (10

and 100 strokes for quartz grains, 100 strokes for

sulphide and dolomite grains. Helium and neon

blanks were lower than 110 14 mol 4He and5 10 15 mol 20Ne.

3.4. Re/Os isotopes

Analyses were carried out at the CRPGCNRS

(France). About 0.5 g of purified samples of Ag

amalgam, pyrite, galena, pyrargyrite, sphalerite and

black shale were spiked with 190Os and 185Re and

digested in Carius tubes at 230 jC overnight in 9ml (f 12 g) of 1:2 solutions of HCl (f 11 mol/l):HNO3 (f 15 mol/l). This procedure is based onthe one developed by Shirey and Walker (1995)

for silicate samples, but uses a more oxidizing

solution (enriched in HNO3 relative to aqua regia)

and somewhat more reagent (9 ml instead of 6 ml)

to promote a more complete oxidation. For Os

extraction, the procedure of Birk et al. (1997)

was used. Osmium analyses were performed by

negative thermal ionization mass spectrometry

(NTIMS) on a Finnigan MAT 262 instrument at

CRPG equipped with an electron multiplier and ion

counting electronics and an oxygen leak valve. The

reproducibility of the 187Os/188Os of our in-house

liquid standard (187Os/188Os = 0.1739) was F 0.3%

(2r) over the course of these measurements. Total

Table 2

d34SCDT values

Sample d34SCDTx 2r Analytical technique

Black shales (NBS)

IMIS33-33 m

Py 38.5 2.0 IMS3fPy 37.4 2.0 IMS3f

IMIII00-41

Py 38.4 2.8 IMS3fPy 38 3 IMS3f

Rhyolitic domes (RD)

IMRD99-74

Py 3.8 1.2 IMS3fPy 4.1 1.2 IMS3fPy 2.0 0.8 IMS3fPy 3.5 0.6 IMS3fPy 3.4 0.4 IMS3fPy 3.2 0.4 IMS3fPy 3 0.4 IRMSPy 2.8 0.4 IRMS

IMRD99-72

Py 7.4 0.4 IMS3fPy 7 0.4 IMS3fPy 6.5 0.8 IMS3f

Base metal event (BME-Qtz1)

IMIII99-03

Py 7.4 1.0 IMS3fPy 6.1 2.2 IMS3f

IMIV99-58

Py 4.2 0.8 IMS3fPy 2.0 0.8 IMS3f

IMIV99-37

Py 3.4 1.0 IMS3fPy 6.1 2.0 IMS3fPy 4.6 0.6 IMS3f

Epithermal silver quartz stage (ESE-Qtz2)

IMI00-18

Py 7.4 0.6 IMS3fPy 11.7 1.0 IMS3f

IMIS33-63 m

Py 9.5 1.2 IMS3fPy 4.5 0.6 IMS3fPy 11.9 0.8 IMS3f

IMIV99-20

Py 11.5 0.8 IMS3fPy 10.6 0.8 IMS3fPy 10.5 0.8 IMS3fPy 9.9 0.8 IMS3f

IMIII00-52

Gal 8.4 0.4 IRMSGal 8.4 0.4 IRMSGal 9.4 1.6 IMS3fGal 8.3 1.2 IMS3f

Table 2 (continued)

Epithermal silver quartz stage (ESE-Qtz2)

IMI00-18

Gal 5.0 1.2 IMS3fGal 6.4 1.0 IMS3fGal 4.8 1.0 IMS3f

IMIII00-51

Gal 10.0 1.0 IMS3fIMIII99-65

Sph 8.2 0.4 IMS3fSph 10.9 0.8 IMS3fSph 9.4 0.2 IMS3f

IMV00-36

Sph 9.0 0.4 IMS3fSph 8.0 0.4 IMS3f

IMI00-18

Cpy 2.4 0.8 IMS3f

Epithermal silver dolomite stage (ESE-dolo)

IMIV00-16

Py 4.1 0.8 IMS3fPy 5.3 0.4 IMS3fPy 5.8 0.6 IMS3fPy 6.2 0.6 IMS3f

IMI99-39

Py 4,6 0.4 IMS3fIMIV99-24

Py 8 0.4 IMS3fIMIII00-22

Gal 18.8 0.8 IMS3fGal 19.6 0.8 IMS3f

IMIHe12

Gal 14.1 1.0 IMS3fGal 13.4 0.8 IMS3f

IMIII00-23

Gal 10.8 1.0 IMS3fGal 13.6 1.2 IMS3fGal 7.1 1.2 IMS3fGal 9.1 0.8 IMS3f

IMV00-24

Gal 15.8 0.8 IMS3fGal 16.6 0.8 IMS3f

IMIII00-37

Pyrar 5 0.4 IRMSPyrar 5 0.4 IRMS

IMIII00-22

Cpy 3.2 0.8 IMS3fIMIII99-65

Sph 7.2 0.6 IMS3fSph 5.0 0.4 IMS3f

IMIHe4

Sph 9.3 0.8 IMS3fSph 5.2 0.6 IMS3fSph 6.3 0.8 IMS3f

IMIII99-65

Apy 1.9 2.0 IMS3f

G. Levresse et al. / Chemical Geology 207 (2004) 597966

G. Levresse et al. / Chemical Geology 207 (2004) 5979 67procedural osmium blanks were 1.3F 0.7 pg, withan 187Os/188Os ratio of 0.26F 0.17. However, thetotal quantity of osmium measured in some sul-

phide samples (not shown in table), as well as in

several silicate samples analyzed during the same

Table 2 (continued)

Sample d34SCDTx 2r Analytical technique

Epithermal silver dolomite stage (ESE-dolo)

IMI00-18

Apy 1.8 1.2 IMS3fIMIII00-22

Apy 24.2 0.8 IMS3fApy 24.3 0.4 IMS3f

IMIHe 2

Apy 23.0 0.6 IMS3fIMIII00-23

Apy 26.2 1.2 IMS3fApy 24.9 0.8 IMS3f

IMIHe12

Apy 18.1 0.8 IMS3fApy 18.5 0.8 IMS3fApy 22.6 1.2 IMS3f

IMI00-13

Cin 27.7 0.4 IRMSAbbreviations: Py, pyrite; Gal, galena; Sph, sphalerite; Cpy,

chalcopyrite; Pyrar, pyrargyrite; Apy, arsenopyrite; Cin, cinnabar;

IMS, ion probe CAMECA IMS3F; CF-IRMS, continuous flux-

isotope ratio mass spectrometry.period, was lower than the measured osmium

blank. This suggests that the osmium blank could

be overestimated. This might reflect enhanced

leaching of the Carius tube or the Teflon beaker

in the absence of sample powder. Because of this

uncertainty concerning the appropriate blank cor-

rection, in Table 5 we include both the measured187Os/188Os ratios and two extreme estimates of the

corrected 187Os/188Os ratios, one based on the

largest Os blank (2.44 pg, 187Os/188Os = 0.132) and

the other based on the most radiogenic Os blank

(0.72 pg, 187Os/188Os = 0.515). This calculation

shows that the blank correction could have a large

effect on the two sulphides associated with the

black shale and the rhyolite (IMIV99-20 and

IMRD99-74), which have highly radiogenic compo-

sitions and very low 188Os concentrations. Howev-

er, the blank correction has little effect on the Os

isotopic compositions of the minerals directly as-

sociated with the silver mineralization. Even the

two Os-poor samples of Ag Hgj amalgam(IMIII00-49 and IMIII00-50) retain

187Os/188Os ra-tios of less than 0.2 after correction for the largest

measured blank. Thus, the most important point

emerging from this study is that the non-radiogenic

nature of the silver mineralization is a robust

conclusion regardless of the uncertainty concerning

the appropriate blank correction.

After these analyses were completed, it was

suggested to us that we may not have used suffi-

cient quantities of reagent to allow complete oxida-

tion of the Os, as sulphur will be oxidized

preferentially to osmium during the Carius tube

dissolution step. We think that this is unlikely, as

the reagent/sample proportions we used (f 12 g ofsolution for 0.5 g of sample) are similar to those

(f 40 g of solution for 12 g of sample) suggestedby R. Walker (personal communication) for the

dissolution of sulphide samples and because the

HNO3-rich solution we used is significantly more

oxidizing than pure aqua regia. Nevertheless, to test

this possibility, we attempted to analyze a fresh

aliquot of sulphide from the sole sample (IMIII00-

52) for which material remained, using less sample

powder (0.09 g) and thus a higher solution/sample

ratio. The Os concentration determined for this new

aliquot (f 0.24 ppb) is much lower than thatdetermined for the first aliquot. While this could

potentially reflect a spikesample equilibration

problem with the first aliquot, it could also reflect

sample heterogeneity, given that the various sul-

phide phases are intimately intermixed and that the

two aliquots were segregated from different portions

of the rock. The small total quantity of Os in this

sample (f 2 pg) and a highly radiogenic massspectrometer blank at the time of its analysis

unfortunately prevented us from obtaining a well-

constrained isotopic ratio for this sample. However,

a maximum 187Os/188Os ratio of 0.25 could be

established, confirming the non-radiogenic nature

of the sulphides associated with the silver mineral-

ization.

The Re analyses were performed by MCICP

MS (Micromass Isoprobe). Total procedural blanks

were variable, but lower than 12 pg for Re. The

variation of the Re blank is taken into account in the

error determination of [Re], 187Re/188Os and187Os/188Osinitial. Additional details of the Re ana-

lytical procedures are available in Pierson-Wick-mann et al. (2000).

Fig. 4. Comparison of the distribution of d34SCDT (in per mil) values of pyrite, galena, sphalerite, arsenopyrite, chalcopyrite and pyrargyrite fromthe NBS, BME-Qtz1, ESE-Qtz2 and ESE-dolo. Mineralization events in the Imiter silver deposit. n=Number of analyzed samples.

G. Levresse et al. / Chemical Geology 207 (2004) 597968

7.4F0.4xto 2.0F 0.8x. Twenty-two analyseswere performed on eight samples from the silver

epithermal quartz stage mineralization (ESE-Qtz2).

The d34SCDT values of all of the analyzed sulphidesfrom this stage range from 11.9F0.8xto 2.4F0.8x. The d34SCDT ranges of the pyrites (IMIII00-18and IMIS33-63) are comparable to those of the galena

(IMIII00-18, IMIII00-51 and IMIII00-52), respectively,

from 11.7F 1.0xto 4.5F 0.6xand from 10.0F 1.0xto 4.8F 1.0x. The d34SCDT valuesdetermined in sphalerite (IM 99-65 and IM 00-36)

G. Levresse et al. / Chemical Geology 207 (2004) 5979 694. Analytical results

4.1. Sulphur isotopic study

Sulphur isotopic data are presented in Table 2 and

Fig. 4 in per mil deviation relative to the CanyonDiablo

troilite standard. Data are grouped according to miner-

alized stage from older to recent. We analyzed four

distinct pyrites separated from two samples (IMIS33-

33 and IMIII00-41) of the Neoproterozoic black shales

(NBS). d34SCDT values vary from 38.5F 2.0xto 37.4F 2.0x. The analytical statistical errors, whichrange from 1 to 2x, are higher than for the otheranalyses, and suggest a heterogeneous material. Seven

analyses were performed of pyrite from three samples

(IMIII99-03, IMIII99-58 and IMIV99-37) from the base

metal hydrothermal mineralization (BME-Qtz1 quartz

veins in the black shales). The d34SCDT values obtainedvary from 7.4F 1.0xto 2.0F 0.8x. Elevenanalyses were performed on euhedral pyrites (IMRD99-

74 and IMRD99-72) in the siliceous matrix of the

rhyolitic intrusion, with d34SCDT values ranging from

Fig. 5. (a) Silver droplets and fluid inclusions in quartz grain ESE-

Qtz2 (IMIII99-60). (b) Muscovite ESE-Ms2 in ESE-Qtz2 fluid

inclusion (BSE picture, IMIII99-60).III V

are the lightest, ranging from 10.9F 0.8xto 8.0F 0.8x. The heaviest d34SCDT value, 2.4F0.8x, was determined in the chalcopyrite area(IMIII00-18) associated with galena. We found no

evidence for d34SCDT variation within the silver epi-thermal quartz stage related to lithology and/or depth.

Thirty-three analyses were performed on eleven sam-

ples from the silver epithermal dolomite stage (ESE-

dolomite). d34SCDT values obtained on all analyzedsulphides vary from 27.7F 0.4xto 1.8F1.2x. d34SCDT values measured for pyrites (IMIV00-16, IMI99-39 et IMIV99-24), ranging from 8F0.4xto 4.1F 0.8x, are similar to the values measuredfor chalcopyrite (IMIII00-22; 3.2F 0.8x) and pyr-argyrite (IMIII00-37; 5F0.4x). d34SCDT valuesdetermined in galena (IMIII00-22, IMIHe12, IMIII00-

23 and IMV00-24) reveal a large range of variation,

from 19.6F 0.8xto 7.1F1.2x. The d34SCDTvalue measured in cinnabar (IMI00-13), 27.7F0.4x, is the most negative value determined in thismineralization stage. Cinnabar is the last sulphide to

crystallize in this paragenetic succession. The mean

value determined for the ESE-dolomite stage ( 13F8.1x) is lighter than the mean ESE-Qtz2 value( 8.5F 2.5x).

Table 3

Vapor phase composition of the primary ESE-Qtz2 fluid inclusions

(sample IMIII99-60)

CH4 (mol%) CO2 (mol%) N2 (mol%)

100

64 36

77 23

76 24

56 4452 48

4.2. Fluid inclusion study

Fluid inclusions in ESE-Qtz2 quartz grains

graphic observations, Raman and microthermomet-

ric measurements defined three kinds of inclusions

(L +V; L +V+S1; L +V+S12). All three types are

commonly found within a trail or cluster. In all

inclusion types, the visual estimate of the liquid to

vapor ratio is less than 0.8. Raman analysis of the

vapor phase reveals mixtures of CH4UCO2 orCH4UN2 (see Table 3). CO2 and N2 are neverfound together in the same inclusion. Raman anal-

ysis and BSE semi-quantitative analyses of the

solids indicate that they are muscovite (S1) and

sulphide (S2) (see Fig. 5(b)). Th and wt.% eq. NaCl

variations determined in the ESE-Qtz2 are pre-

sented in Fig. 6. Th spans from 148 to 203 jC,while NaCl concentration varies from 10 to 0.0 eq.

wt.% NaCl. The eq. wt.% NaCl concentration is

determined using the equation relating Tmi to eq.

wt.% NaCl proposed by Bodnar (1993). These

results reveal the homogeneity of the temperature

of the fluid associated with the epithermal silver

ide an

100

150

200

250

0 2 4 6 8 10

wt% eq. NaCl

Th (in C)

Fig. 6. Homogenization temperature (Th in jC) vs. wt.% NaClequivalent salinity diagram for ESE-Qtz2 fluid inclusions of the

Imiter Ag (HgPbZn) deposit. Weight percent NaCl is deter-

mined using the equation proposed by Bodnar (1993).

G. Levresse et al. / Chemical Geology 207 (2004) 597970(IMIII99-60) occur as isolated individuals or as

clusters that show a three-dimensional random

distribution throughout a single grain. They are

spatially related to the silver droplets in the quartz

grain growth zones (Fig. 5(a)). Typically, the fluid

inclusions are irregular to ovoid in shape and are

less than 30 Am in maximum dimension. Petro-

Table 4

Helium isotopic compositions and 4He and 20Ne abundances of sulphthe silver epithermal event (ESE), as well as the black shale host rock (N

Sample Mass (g) He (10-12) (molBlack shales (NBS)

IMIV99-13 (100 shots) pyrite 0.123 1.44

IMIS33-33 (100 shots) pyrite 0.132 49.73

IMIII00-41 (100 shots) sphalerite 0.083 41.51

Base metal event (BME-Qtz1)

IMIV99-58 (10 shots) quartz 0.135 22.11

IMIV99-58 (100 shots) quartz 0.135 34.57

Epithermal siver quartz stage (ESE-Qtz2)

IMIII99-60 (10 shots) quartz 0.1249 1.45

IMIII99-60 (100 shots) quartz 0.1249 4.54

IMIII00-52 (100 shots) galena 0.1071 3.24

Epithermal silver dolomite stage (ESE-dolo)

IMIII00-29 (100 shots) dolomite 0.1316 76.41

IMIII00-29 (100 shots) Pyrargyrite 0.1838 5.07mineralization event. These values are in agreement

with the supposed magmatic fluid (Th = 100290

jC; Tmi: 19 jC to 0.1 jC) described inBaroudi et al. (1999). The variation of the NaCl

concentration illustrates the superficial position,

above the boiling zone, of the silver precipitation

(Gonzalez-Partida et al., 2000).

d matrix phases from the base metal hydrothermal event (BME) and

BS)

/g) r values 3He/ 4He, (R/Ra) r values20Ne

7.5110 14 0.237 6.05 10 2 n.d.2.60 10 12 0.178 7.28 10 3 n.d.2.17 10 12 0.126 8.80 10 3 n.d.

1.15 10 12 1.207 4.26 10 2 n.a.1.8110 12 2.642 1.00 10 1 n.a.

7.55 10 14 1.848 3.33 10 1 n.a.2.37 10 13 1.050 6.80 10 2 n.a.1.69 10 13 0.768 5.43 10 2 n.d.

3.99 10 12 0.891 1.85 10 2 n.d. 13 22.65 10 1.204 6.29 10 n.d.

Table 5

Osmium isotopic compositions and Os and Re concentrations from the rhyolitic dome (RD), the black shales (NBS), and the silver epithermal mineralization (ESE)

Sample Mass

(g)

187Os/188Os

187Os/188Os

assuming

largest

blank1

187Os/188Os

assuming

most

radiogenic

blank2

Total

Os

(ppb)

188Os M/g Re (ppb)3 187Re/188Os Assumed

age

187Os/188Osinitial

4

187Os/188Osinitialpossible range5

Black shales

IMIV99-20 pyrite 0.5345 43.6F 0.6 63.7 47.8 0.097 1.0110 14 150F 30 49721F 5048 850 Ma 665F 144 651 to 868IMIV99-20 (2)

6 pyrite 0.4436 116.4F 1.4 undefined 174.8 0.075 3.23 10 15 182F 2 189261F1186 850 Ma 2582F 36 2582 to 3908IMIII99-18_ whole

rock

0.5001 2.7F 0.2 2.89 2.79 0.133 6.9110 14 7.355F 0.026 356.9F 0.6 850 Ma 2.3F 0.2 2.3 to 2.5

Rhyolitic intrusions

IMRD99-74 pyrite 0.7812 2.686F 0.01 3.63 2.86 0.016 8.14 10 15 0.544F 0.044 224F 8 550 Ma 0.619F 0.17 0.60 to 0.99IMRD99-74 (2)

6 pyrite 0.5070 3.755F 0.028 11.95 4.54 0.010 4.86 10 15 0.563F 0.050 388F 16 550 Ma 0.278F 0.34 0.03 to 1.12

Silver epithermal stage

IMIII00-52 galena 0.4813 0.181F 0.002 0.185 0.174 0.070 4.86 10 14 0.060F 0.044 4.2F 1.1 550 Ma 0.142F 0.022 0.091 to 0.143IMIII00-52 (2)

6 galena 0.0880 < 0.25 0.024 1.64 10 14IMIII99-51B

7 sphalerite 0.4964 0.197F 0.002 0.199 0.194 0.162 1.12 10 13 0.000F 0.008 0 550 Ma 0.197 0.192 to 0.197IMIII00-37 Pyrargyrite 0.5141 0.144F 0.004 0.147 0.116 0.020 1.36 10 14 0.057F 0.050 14.2F 4.3 550 Ma 0.013F 0.084 negativeIMIII00-49

7 AgHgj 0.4854 0.189F 0.004 0.195 0.182 0.006 4.46 10 14 0.002F 0.032 0 550 Ma 0.188F 0.004 0.162 to 0.198IMIII00-50 AgHgj 2.0000 0.164F 0.002 0.183 0.123 0.003 2.29 10 15 n.d. n.d. 550 Ma < 0.164n.d.: non-determined values.

1 Blank-corrected Os ratios assuming largest blank: 2.44 pg total Os, 187Os/188Os = 0.132.2 Blank-corrected Os ratios assuming most radiogenic blank: 0.72 pg total Os, 187Os/188Os = 0.515.3 Re concentration corrected for blanks (212 pg) associated with each measurement series. Re error includes uncertainty on the Re blank.4 Uncertainty listed in this column is based solely on total uncertainty of Re concentration.5 Possible range of initial 187Os/188Os ratios based on most extreme assumptions concerning blanks and errors.6 Duplicates based on separate powder aliquots.7 Measured 185Re/187Re within error of spike value thus Re concentration is appreciatively 0.

G.Levresse

etal./Chem

icalGeology207(2004)5979

71

(f 0.12) at 600 Ma.

G. Levresse et al. / Chemical Geology 207 (2004) 5979724.3. Helium isotopic study

The following samples were selected for rare gas

isotope analysis: country rocks NBS (pyrite-IMIV99-

13, pyrite-IMIS33-33, sphalerite-IMIII00-41), mineral-

ized BME (quartz-IMIII99-58) and ESE (quartz-

IMIII99-60, dolomite-IMIII00-29, galena-IMIII99-52,

pyrargyrite-IMIII00-29).20Ne concentrations (Table

4) were indistinguishable from the blank, demonstrat-

ing the extremely low Ne content and lack of signifi-

cant contribution from atmospheric gases. The black

shale (IM00I-13, IMIS33-33, IMIII00-41)3He/4He iso-

topic ratios range from 0.126F 0.008Ra to 0.237F0.006Ra, where Ra is the atmospheric

3He/4He value

(1.38 10 6). These values are higher than thoseobserved in crustal fluids (0.02Ra) that incorporated

radiogenic helium (e.g., Mamyrin and Tolstikyn, 1984;

Ozima and Podosek, 2002) and require addition of a3He-rich component. Quartz grains associated with the

base metal event (IM99III58) have similar4He con-

centrations, on the order of 10 12 mol/g, with higher3He/4He ratios of 1.21F 0.04Ra and 2.64F 0.10Ra.Samples representative of the epithermal silver stages

(quartz grains, IMIII99-60, and sulphides, IM99III-52

and IMIII00-29, from both stages) also have compara-

ble 4He contents and 3He/4Hemeasured ratios between

0.77F 0.06Ra and 1.85F 0.20Ra. Finally, the4He

content of the dolomite (IMIII00-29) is one order

of magnitude greater than those of the other epither-

mal silver event samples, but has a similar 3He/4He

ratio of 0.89F 0.02Ra. Clearly, the He contents ofthese samples, which are governed to a large degree

by the distribution of fluid inclusions, do not allow a

genetic distinction to be made. In contrast, the He

isotopic ratios are variable and aid the characteriza-

tion of the several fluid components, as discussed

below.

4.4. Re/Os isotopic study

Os isotopic compositions and Re and Os concen-

trations are given in Table 5. Re and Os concen-

trations vary over several orders of magnitude,

depending on the kind of sulphide analyzed. Black

shale pyrite IMIV99-20 was analyzed twice, yielding

different results in concentration and isotopic ratio,

though both aliquots are strikingly radiogenic. The

difference between these results is probably due tothe uncertainty on the blank correction, discussed

above, as well as to sample heterogeneity, as it was

difficult to produce identical large, pure mineral

separates from these opaque and often intimately

intergrown phases (Fig. 3). Regardless of the blank

correction applied, the initial ratios of these two

separates are strongly negative, demonstrating that

the ReOs system was not closed in the black

shale. This is confirmed by the analysis of the black

shale whole rock, which also yields a markedly

negative initial 187Os/188Os ratio. Repeat analyses

of rhyolite-hosted pyrite IMRD99-74 yielded mea-

sured 187Os/188Os ratios of 3.75 and 2.68. This

difference probably results mainly from radiogenic

ingrowth acting on fractions with distinct187Re/188Os ratios, though analytical uncertainties

(blank correction and perhaps incomplete dissolu-

tion) may have had some effect. The calculated

initial 187Os/188Os ratios, 0.28 and 0.62, though

geologically plausible, must therefore be interpreted

with caution.

While it is difficult to draw many conclusions

from the Os data of the highly radiogenic black shale

and rhyolite sulphides, the Os isotopic results from

the silver mineralization and the directly associated

sulphides are unambiguous. The measured187Os/188Os ratios of all analyzed phases formed

during the silver epithermal event are quite low and

span only a narrow range, from 0.144 to 0.197. Even

the most extreme estimates of the appropriate blank

correction do not raise these ratios above 0.2. The 50-

fold variation in Os concentration observed among

the various ESE phases is not linked to variations in

Os isotopic ratio, providing further evidence that the

low 187Os/188Os ratios of these samples do not result

from contamination during the analytical procedure.

The analyzed phases included galena (IMIII00-52)

and sphalerite (IMIII99-51B), as well as the first

analyses of pyrargyrite (IMIII00-37) and AgHgj(IMIII00-49 and IMIII00-50) that we are aware of.

The Re concentrations of these samples are quite low

and induced only small corrections for radiogenic

ingrowth, yielding initial 187Os/188Os ratios ranging

from 0.142 to 0.197 (with the exception of sample

IMIII00-37, which yielded an unrealistic initial187Os/188Os ratio of 0.013F 0.064). These valuesare comparable with the mantle 187Os/188Os ratio

G. Levresse et al. / Chemical Geology 207 (2004) 5979 735. Discussion

Barnes (1997) suggested that the source of

metals in a hydrothermal deposit could be deep

and/or shallow, but it is often quite difficult to

determine the specific source for a given deposit.

The Imiter epithermal silver deposit is a perfect

place to address this question. Two non-exclusive

possibilities, both located in the vicinity of the

mining area, can be envisaged for the metal and

fluid sources: (i) shallow sources, represented in

this case by a basinal fluid that carries metals

derived from the black shale formation; (ii) deep

sources, represented by the volcanic fluid associated

with the rhyolitic dome. To resolve this problem,

we use a wide variety of geochemical methods to

obtain an overview of the metal, ligand, and fluid

sources and to understand the evolution of the

mineralized system.

5.1. The sources of metal

The ReOs isotopic system is particularly useful

for distinguishing between mantle and crustal sour-

ces (Alle`gre and Luck, 1980). This is particularly

true in metallogenic studies where the chalcophile

and siderophile characters of Re and Os allow direct

analyses of sulphide minerals. The 187Os/188Os ratio

of the mantle at the time of formation of the Imiter

deposits was about 0.12. In contrast, crustal Os

isotopic ratios are much more radiogenic and highly

variable and would have ranged mostly between 1

and 3 at the time of mineralization. The measured

and initial 187Os/188Os ratios of the silver minerali-

zation are all less than 0.2, indicating without ambi-

guity the predominance of a mantle source for the

osmium in this mineralization. Nevertheless, these187Os/188Os values are slightly higher than the aver-

age mantle value of about 0.12 at 550 Ma. This

small difference probably reflects a minor contribu-

tion from a crustal component, notably the nearby

black shales. In order to assess the possible contri-

bution of this reservoir, we suggest that the187Os/188Os ratios of the silver mineralization result

from the mixing of two pure poles, representing the

mantle (187Os/188Os = 0.12) and the black shales

(187Os/188Os550Ma = unknown). As discussed above,the black shale ReOs systematics have been per-turbed, so we cannot obtain the ratio at 550 Ma

simply by back calculation using the measured187Re/188Os ratio. Instead, we assume that the187Re/188Os ratios of the black shales were in the

range (601500) of those measured in other black

shales with demonstrably closed ReOs systematics

(i.e., those displaying isochronal relationships;

Cohen et al., 1999; Ravizza and Turekian, 1989;

Singh et al., 1999). Not knowing the Os isotopic

composition of the late Proterozoic oceans, we use

the mantle value (0.12) as a minimum estimate for

the marine Os isotopic ratio at the time of black

shale deposition (f 850 Ma). Taking the 187Re/188Osratio to be at the low end (60) of the black shale

range given above, this suggests that the 187Os/188Os

ratio of the black shale at 550 Ma must have been at

least 1.0. Using this very conservative estimate for

the black shale Os isotopic ratio, a value of 0.12 for

the mantle Os ratio and a maximum value of 0.2 for

the initial 187Os/188Os ratio of the silver mineraliza-

tion, mass balance requires that the black shales

provided less than 10% of the osmium present in

the silver bearing and closely associated phases. The

true black shale contribution was probably consider-

ably lower than 10%, as most rocks of this type have187Re/188Os much higher than 60, and thus would

have had 187Os/188Os ratios much higher than 1.0

after 300 Ma of radiogenic ingrowth.

This calculation shows that the contribution of the

black shales to the Os budget of the silver mineral-

ization was quite limited. Instead, nearly all of the

Os in the mineralized zone was derived from the

mantle. This does not, of course, automatically

assure that the silver was also derived from the

mantle, though the intimate association of the osmi-

um and the silver argues in this sense. The mantle-

like Os isotopic signature of the silver mineralization

also provides an important argument for explaining

the giant character of the Imiter deposit. By com-

parison with the Freydier et al. (1997) study of the

Chilean porphyries, it is possible to speculate that the

volume of epithermal deposits increases with de-

creasing 187Os/188Os ratio. The conservation of a

mantle-like ratio, in a subduction context, suggests

a lack of contamination of the mantle wedge from

the slab and a rapid migration from an uncontami-

nated source to the metal trap, as described byMcInnes et al. (1999).

zoning, reflecting changes in temperature, fO2 and/or

G. Levresse et al. / Chemical Geology 207 (2004) 5979745.2. The sources of fluids

The conservative behavior of helium and the differ-

ence in isotopic composition between the crust

(3He/4He = 0.02Ra on average), the mantle (3He/

4He = 630Ra) and the atmosphere (3He/4He = 1Ra)

make helium an excellent proxy for the determination

of the sources of the fluids trapped during mineraliza-

tion and the possible interactions between these fluids

and the country rocks (e.g., Mamyrin and Tolstikyn,

1984). The low 20Ne concentrations, under the detec-

tion limit, in the sample matrices indicate a negligible

contribution of atmospheric helium to fluids trapped in

matrix inclusions. Thus, 3He/4He ratios showing

excesses of 3He relative to typical crustal fluids strong-

ly suggest the contribution of mantle volatiles to fluids

trapped in the analyzed minerals. The present He

isotope results together with ReOs systematics are

not consistent with the supergene silver enrichment

models proposed by Guillou et al. (1988), Leistel and

Qadrouci (1991) and Baroudi et al. (1999), which call

for a large crustal contribution.

The observed 3He/4He values provide clear evi-

dence for two different isotopic reservoirs. The first,

identified in sulphides from the black shale forma-

tions, has the lowest 3He/4He ratios and therefore has

incorporated crustal helium to the largest extent. The

second, richer in mantle 3He, is found in the fluid

inclusions and the matrices of samples from the base

metal and silver epithermal events. The origin of the

radiogenic helium component could be (i) linked with

the regional crust, (ii) the result of radiogenic in-

growth in the matrix, or (iii) the result of contributions

from the subducted slab in this collision context.

Possibility (ii) is not consistent with the rather con-

stant 3He/4He ratios for the different groups of sam-

ples, which contain minerals with variable U and Th

contents. The possibility that subducted material is

responsible for introducing radiogenic 4He into the

source region has been proposed in Hickley et al.

(1986) and Worner et al. (1992) based on Pb and Sr

isotopic studies. Os isotopes could, in theory, be used

to identify contributions from the subducting slab, as

the Os of the slab should be radiogenic. However,

slab-derived fluids are likely to have low Os concen-

trations and thus will have only a small effect on the

Os isotopic compositions of peridotites from the

mantle wedge (Brandon et al., 1996). So the possibil-ity of input from slab-derived fluids remains an open

question. The synchronism of the rhyolitic dome

emplacement and the silver mineralization supports

the possibility of contamination from the lower crust.

In this case, however, one would not expect similar

He isotopic ratios in the base metal mineralization,

which predates the rhyolite. The middle crustal as-

similation hypothesis is not in agreement with the

evidence for fast emplacement through the crust of the

mineralizing fluids proposed by Levresse (2001), in

accordance with the observations of Hilton et al.

(1993). Finally, possible country rock contamination

might be suggested by the fluid analyses. The pres-

ence of CH4 in the fluid inclusions provides evidence

for interaction between the mineralizing fluids and the

black shale formation. The base metal and silver

epithermal helium isotopic values could be reasonably

interpreted as mixtures of the mantle (MORB-like; R/

Ra = 8) and black shale (R/Ra = 0.240.12) isotopic

reservoirs. In addition, the evolution of the helium

isotopic composition following the paragenetic se-

quence could be explained by the variation of the

physico-chemical conditions affecting the fluids (Sim-

mons et al., 1987).

To sum up, the helium and neon results strongly

suggest a mantle source for the base metal and silver

epithermal mineralizing events. Nevertheless, a small

amount of contamination from a more radiogenic

component, most probably derived from the black

shale country rocks, is also likely.

5.3. The sources of sulphur

The temperatures of mineralization previously de-

termined for the Imiter silver deposit range from 100 to

450 jC (Baroudi et al., 1999). Based on the establish-ment of a new paragenetic sequence and fluid inclu-

sion analyses, this range of temperatures has been

refined in the present work and is now established at

100290 jC for the epithermal silver episode. Thesetemperatures are far below those required for complete

isotope equilibration (Field and Fifarek, 1985). Due to

the relatively complex geological history of the Imiter

deposit, variations of sulphur isotope ratios are diffi-

cult to interpret. Additionally, the processes responsi-

ble for the mineralogical zoning observed along the

epithermal veins could also produce some S-isotope

G. Levresse et al. / Chemical Geology 207 (2004) 5979 75pH. In the following discussion, variations in S-isotope

composition will be interpreted mainly by comparison

with those of the largest S-reservoirs available in the

country rocks, i.e., the middle Neoproterozoic black

shale formation and the rhyolitic intrusives synchro-

nous with the epithermal sulphide mineralization.

None of the cogenetic sulphide pairs yielded isoto-

pic data permitting determination of a realistic temper-

ature. This implies that the minerals were not in

isotopic equilibrium. Field and Fifarek (1985) show

that in an epithermal system (Tj < 300 jC), isotopicfractionation is predominantly controlled by kinetic

effects. Under these conditions, isotopic equilibrium is

never achieved. The main analytical method used (ion-

probe) is not favorable for this kind of determination.

The errors obtained on the measurements (higher for

the ion-probe, ranging from 0.3xto 1.5x) have asignificant influence on the precision of the calculated

temperature, yielding uncertainties from F 20 to F 60jC, for the Py/Gal pair for example. However, theseuncertainties do not explain the negative temperatures

obtained, which illustrate perturbation of the system.

Despite the uncertainty introduced by the lack of

intermineral isotopic equilibrium, and the probable

perturbation of the system, the sulphur isotopes display

some systematic trends (Fig. 4). The d34SCDT valuesbecome more variable, and on average, more negative,

with each successive mineralizing event (BME-Qtz 1,

then ESE-Qtz2, then ESE-dolo). This may reflect, in

part, increasing interaction with the black shales with

time. These systematics may be interpreted as follows:

5.3.1. The middle Neoproterozoic black shales (NBS)

Pyrite extracted from the metamorphosed black

shales in the vicinity of the Imiter fault zone exhibits

d34SCDT values ranging from 38.5F 2.0xto 37.4F 2.0x(Fig. 4), which are the lightest valuesobtained in this study. These values are in agreement

with those obtained in euxinic depositional environ-

ments (Ohmoto and Rye, 1979; Kribek, 1991). In this

case, as is often true in highly reduced rocks (Rye and

Ohmoto, 1974; Cook and Hoefs, 1997), the S-isotopic

composition was apparently preserved during the lower

greenschist Pan-African metamorphism.

5.3.2. The rhyolitic intrusives

Pyrite from the two rhyolitic intrusives (the Takha-tert dome and the Tachkakacht dike) yield relativelyrestricted d34SCDT variations ranging from 7.4F0.4xto 2.0F 0.8x. These values are similar totypical magmatic melt compositions (Rye and

Ohmoto, 1974). The slight drift towards negative

values can be explained by the degassing of SO2(and probably some SO4

2) during the eruption ofrhyolitic magma and the resulting d34SCDT fraction-ation. As a consequence, H2S and H2S-derived pyrite

will present some 32S enrichment leading to slightly

negative d34SCDT. The amount of variation from thetheoretical magmatic value observed in pyrite from

the Imiter rhyolitic intrusives is similar to S-isotope

variations described in natural examples such as the

Krakatoa volcanic suite (Mandeville et al., 1998).

5.3.3. Sulphides from the mineralized associations

Pyrites from BME have negative d34SCDT valuesvery close to the values determined in the pyrites from

the rhyolitic intrusions and similar to those obtained

for pyrite equilibrated with magmatic fluids (Field and

Gustafson, 1976). This supports the inference that the

base metal mineralization was deposited by a hydro-

thermal system driven by the cooling of the granitic

and granodioritic plutons at 572F 5 Ma.Sulphides from the two distinct stages of ESE were

analyzed, including pyrite, galena, sphalerite and chal-

copyrite from the quartz epithermal stage (ESE-Qtz2)

and pyrite, polybasite, cinnabar, chalcopyrite and ar-

senopyrite from the dolomitic stage (ESE-dolo). They

generally display a shift toward negative d34SCDT,relative to magmatic values. This shift was on average

greater, but also more variable, for the dolomitic stage

( 27.7F 0.4xto 1.8F 1.2x). As suggested inFig. 4, this shift is probably driven in part by increas-

ing contamination with the black shales, although the

magnitude of this effect is difficult to estimate due to

the presence of both syngeneticmetamorphic and

epigeneticepithermal sulphides in the late Neoproter-

ozoic black shales. Other factors are also likely to drive

the sulphur isotopic compositions to lighter values.

These include the low temperature of the epithermal

silver event (ca. < 290 jC) deduced from fluid inclu-sions, the isotopic equilibrium fractionation of sulphur

compounds relative to H2S (Ohmoto and Rye, 1979)

and the rather low values inferred from the rhyolitic

intrusion d34SCDT values.The dolomitic stage includes not only sulphideswith very low d34SCDT values, but also sulphides with

stage (1030% and 2040%, respectively).

For two samples of the ESE mineralization

G. Levresse et al. / Chemical Geology 207 (2004) 597976(IMIII00-52 and IMIII00-37), both S and Os isotopic

data are available. The d34SCDT values ( 8.4xand 5x, respectively) and the 187Os/188Os initialratios (0.142 and < 0.144, respectively) of these

samples yield coherent results that argue for only a

very limited contribution from the black shale

reservoir.

6. Conclusion

The Imiter silver deposit is an example of a Neo-

proterozoic telescoped deposit that comprises two

mineralizing events: (a) a minor episode of hydrother-

mal base metal deposition (PbZnCu), associated

with the emplacement of calc-alkaline intrusives dated

at 572F 5 Ma; (b) a major episode, characterized bythe development of a giant epithermal silver deposit,

linked mainly with rhyolitic volcanism dated at

550F 3 Ma.The mineralization of the base metal event appears

to be clearly related to a hydrothermal system driven

by a magmatic episode (Henley, 1985). This inference

is sustained by the spatial association between this

mineralization and the granodioritic intrusives (Taouz-

zakt, B3 Hill and Igoudrane) and its paragenetic

characteristics. It is reasonable to suppose that onethe highest d34SCDT values observed in this study (upto 1.8F 1.2x). These high values are observed inspite of probable changes in temperature (and pH)

accompanying the changes in gangue- and sulphide-

mineral deposition that would be expected to lower

d34SCDT. As in other precious metal vein deposits,these changes are difficult to estimate using sulphur

isotope ratios (Ohmoto and Rye, 1979). Nevertheless,

in the Imiter Ag deposit, near-zero d34SCDT valuesobserved in the dolomitic epithermal stage could

correspond to the involvement of a continuous input

of magma-equilibrated fluids into the hydrothermal

system, thus providing the large quantity of metallic

compounds needed to build a giant silver deposit (ca.

10,000 metric tons of Agj). Continuous magmaticinput during the dolomitic epithermal stage is also

supported by the increase of Hg concentration in the

Agamalgam phase from the quartz- to the dolomiteor several granodioritic intrusions provided the heat, apart of the fluids (S-isotopes in equilibrium with the

magmatic values) and probably the metals for this

mineralization.

For the silver epithermal mineralization, use of the

isotopic proxies helium, sulphur and osmium, in

conjunction with a detailed tectonic study, permits a

coherent genetic model to be drafted. This model is

summarized in Fig. 7. The source of the fluids in this

model, unlike in classic epithermal models (Henley,

1985; Sillitoe, 1993; Hedenquist et al., 2000), is

hypogene. The low salinity of the fluids (Tmi = 5.9to 0.0 jC) pleads in favor of silver transport as abisulphur complex (AgHS0) (Zotov et al., 1995;

Barnes, 1997). The low salinity of the fluids, as well

as their gas content (CO2 = 50100%, H2S = 23

36%, N2 = 2448%, of the gas phase), is characteristic

of precious metal deposits of the epithermal type

(Henley, 1985). Concerning the sources of the ele-

ments, the S and Re/Os isotopic studies (the first data

of this kind for pyrargyrite and AgHg), indicate only

contamination (much less than 10% for the Os) by the

black shale hosting the mineralization. Osmium and

helium isotopes indicate that the main source of the

elements (ligands and metals) is probably from the

mantle.

To sum up, mantle-derived metals and fluids have

been identified in an epithermal silver deposit by

using different geochemical tracers in a variety of

minerals. This does not imply that superficial hydro-

logical factors and wall-rock contamination does not

exist in Imiter (a probable input of basinal brines at

the time of mineralization may be deduced from the

data of Baroudi et al., 1999). Nevertheless, the S, He

and Os isotopic studies demonstrate that most of the

sulphur, metals and fluids were ultimately provided by

a deep source, independent of the superficial local

geothermal system. This confirms the results obtained

by Simmons et al. (1987) on arc-related hydrothermal

ore deposits. Imiter provides an example of an epi-

thermal precious metal deposit in which two critical

factors of the ore-forming system can be identified.

One is a magmatic, ultimately mantle-derived, source

of elements and ligands. The second is a superficial

geothermal system that provided metal traps in an

adequate tectonic and wall-rock environment. Al-

though in many AuAg epithermal deposits the

geochemical characteristics of the shallow geologicalprocesses can be identified (Henley, 1985; Campbell

G. Levresse et al. / Chemical Geology 207 (2004) 5979 77and Larson, 1998), the original feature of Imiter is the

preservation of the juvenile character of metals and

fluids.

In consequence, it seems likely that the develop-

ment of the Imiter silver deposit and probably of many

other epithermal precious metal deposits is associated

with the activation of an extensional fault connected

with a major crustal discontinuity rooted in a reacti-

vation zone of the lower crust and mantle. The

mobilization of felsic and mafic magmas is accompa-

nied by the ascent of fluids rich in gas and sulphur,

agents of the silver transport. The activation of the

Imiter fault at 550F 3 Ma was an important event inthe evolution of the Anti-Atlas orogenic belt during

the PrecambrianCambrian transition from a conti-

nental active margin to a passive margin context

(Villeneuve and Cornee, 1994; Cheilletz and Gasquet,

2001). Under these conditions, ascent of the astheno-

Fig. 7. Genetic model for the Imiter deposit. Boxes summarize the isoto

isotopic results are given in ratios, 187Os/88Os and 3He/4He, respectively,

synthesized geological cross-section is drawn from the AAV transect (msphere from the mantle wedge accompanied by remo-

bilization of the lower lithosphere (Davies and Von

Blanckengurg, 1995) could explain the temporal co-

incidence between the ascent of the deep mineralizing

fluids and the calc-alkaline rhyolitic magmatism, and

their spatial relationship in a superficial extensional

structure.

The genetic model proposed here highlights the

conditions required for the formation of a giant

epithermal silver deposit, namely: (i) a mantle source

for metals located in the mantle wedge or the base of

the lithosphere (McInnes et al., 1999; Freydier et al.,

1997; Patou et al., 1996); (ii) a short duration of the

mineralizing episode synchronous with a major tec-

tonic change in the geodynamic evolution (Golfarb et

al., 1998); (iii) an efficient vector for the metals

involved in the mineralization; (iv) a near-surface

extensional setting providing traps for the ascending

pic characteristics of the different reservoirs. Osmium and helium

and sulphur in per mil deviation relative to the CDT standard. The

odified from Levresse, 2001). For legend, see Fig. 2.

analyses, L. Zimmermann for the Ne/He analyses,

C. Zimmermann for the Re/Os analyses, and Th.

Min. 536537, 91111.

G. Levresse et al. / Chemical Geology 207 (2004) 597978Birk, J.L., Roy-Barman, M., Capmas, F., 1997. ReOs isotopic

measurements at the femtomole level in natural samples. Geo-

stand. Newsl. 20, 1927.

Bodnar, R.J., 1993. Revised equation and table for determination

freezing point depression of H2ONaCl solution. Geochim.

Cosmochim. Acta 57, 683684.

Brandon, A.D., Creaser, R.Z., Shirey, S.B., Carlson, R.W., 1996.

Osmium recycling in subduction zones. Science 272, 861864.

Campbell, A.R., Larson, P.B., 1998. Introduction to stable isotope

applications in hydrothermal systems. Rev. Econ. Geol. 10,

173193.Lhomme (UMR G2R) for Raman analyses, is

gratefully acknowledged. We greatly appreciate the

comments and suggestions made by Rich Walker and

an anonymous reviewer that considerably helped to

improve an earlier version. [CA]

References

Alle`gre, C.J., Luck, J.M., 1980. Osmium isotopes as petrogenetic

and geological tracers. Earth Planet. Sci. Lett. 48, 148154.

Barnes, H.L., 1997. Geochemistry of Hydrothermal Ore Deposits,

Third edition. John Wiley & Sons, Incorporated, New York,

USA. 972 pp.

Baroudi, Z., Berraouz, H., Rahimi, A., Saquaque, A., Chouhaidi,

M., 1999. Mineralisations polymetalliques argentife`res dImiter

Jbel Saghro, Anti-Atlas, Maroc): mineralogie, evolution des

fluides mineralisateurs et mecanismes de depots. Chron. Rech.magmas and fluids. Recognition of these different

factors may be very useful for identifying targets for

precious metal exploration.

This study also highlights the need to combine

geological observations and independent geochemical

techniques in an integrated approach to better under-

stand the hydrothermal magmatic processes involved

in the formation of many ore deposits.

Acknowledgements

This study was supported by several scientific

cooperation grants to A.C. and D.G. from the

REMINEX-SMI-ONA group (Morocco) and the

Ministry of Industry (France; # 98 2 24 00 30 and #

00 224 0002), and NSERC support to K.K. Technical

assistance from M. Champenois, D. Mangin and C.

Rollion-Bard for the sulphur isotope ion probeChaussidon, M., Demange, J.-C., 1988. Instrumental mass fraction-

ation in ion microprobe studies of sulphur isotopic ratios. In:

Benninghoven, A., Huber, A.M., Werner, H.M. (Eds.), SIMS VI

Proceedings. John Wiley & Sons, Incorporated, New York,

USA, pp. 937940.

Cheilletz, A., Gasquet, D., 2001. Specific plate tectonic environ-

ments for the Anti-Atlas ore deposits. Colloque Magmatisme,

Metamorphisme et Mineralisations associees, Marrakech,

Maroc, University Cady Ayyad, Marrakech, Morocco, p. 96.

Abstract.

Cheilletz, A., Levresse, G., Gasquet, D., Azizi Samir, M.R., Zyadi,

R., Archibald, D., 2002. The Imiter epithermal silver deposit: a

giant neoproterozoic epithermal mineralization in the Anti-At-

las, Morocco. Miner. Depos. 37, 772781.

Clauer, N., 1974. Utilisation de la methode RbSr pour la datation

dune schistosite de sediments peu metamorphises: application

au Precambrien II de la boutonnie`re de BouAzzerEl Graara

(Anti-Atlas, Maroc). Earth Planet. Sci. Lett. 22, 404412.

Cohen, A.S., Coe, A.L., Bartlett, J.M., Hawkesworth, C.J., 1999.

Precise ReOs ages of organic-rich mudrocks and the Os iso-

tope composition of the Jurassic seawater. Earth Planet. Sci.

Lett. 167, 159173.

Cook, N.J., Hoefs, J., 1997. Sulphur isotope characteristics of meta-

morphosed Cu (Zn) volcanogenic massive sulphide deposits in

the Norwegian Caledonides. Chem. Geol. 135, 307324.

Davies, J.W., Von Blanckengurg, F., 1995. Slab breakoff: a model

of lithosphere detachment and its test in the magmatism and

deformation of collisional orogens. Earth Planet. Sci. Lett.

129, 85102.

Debussy, J., Buchaert, S., Lamb, W., Pironon, J., Thiery, R., 2001.

Methane-bearing aqueous fluid inclusions: Raman analysis,

thermodynamic modelling and application to petroleum basins.

Chem. Geol. 173, 193205.

Deloule, E., Alle, P., Chaussidon, M., 1992. Instrumental limitations

for isotope ratios measurements with a Cameca IMS 3f ion mi-

croprobe: the example of H, B, S, Sr. Chem. Geol. 101, 187192.

Eldridge, C.S., Compston, W., Williams, J.L., Walshe, J.L., Both,

R.A., 1987. In situ microanalysis for 34S/32S ratios using the

ion microprobe SHRIMP. Int. J. Mass Spectrom. Ion Process.

76, 6583.

Ennih, N., Liegeois, J.P., 2001. The Moroccan Anti-Atlas: the West

African craton passive margin with limited Pan-African activity.

Implications for the northern limit of the craton. Precambrian

Res. 112, 289302.

Fekkak, A., Pouclet, A., Ouguir, H., Badra, L., Gasquet, D., 2001.

Geochimie et signification geotectonique des volcanites du Cry-

ogenien inferieur du Saghro (Anti-Atlas oriental, Maroc). Geo-

din. Acta 14, 373385.

Field, C.W., Fifarek, R., 1985. Systematique des isotopes stables

legers dans les environnements epithermaux. In: Berger, B.R.,

Bethke, P.M. (Eds.), Geology and Geochemistry of Epithermal

Systems. Rev. Econ. Geol., vol. 2, p. 6. Chap. 6.

Field, C.W., Gustafson, L.B., 1976. Sulfur isotopes in the por-

phyry copper deposit at El Salvador, Chile. Econ. Geol. 71,

15331548.

Freydier, C., Chesley, J., McCandless, T., Munizaga, F., 1997. Re

Os isotope systematics on sulfides from felsic igneous rocks:

application to base metal porphyry mineralization in Chile. Geo-

logy 25, 779782.

Mandeville, C.W., Sasaki, A., Saito, G., Faure, K., King, R., Hauri,

E., 1998. Open system degassing of sulfur from Krakatau 1883

magma. Earth Planet. Sci. Lett. 160, 709722.

McInnes, B., McBride, J., Evans, N., Lambert, D., Andrew, A.,

1999. Osmium isotope. Constraints on ore metal recycling in

G. Levresse et al. / Chemical Geology 207 (2004) 5979 79Golfarb, R.J., Phillips, G.N., Nokleberg, W.J., 1998. Tectonic set-

ting of synorogenic gold deposits of the Pacific Rim. Ore Geol.

Rev. 13, 185218.

Gonzalez-Partida, E., Birkle, P., Torrese-Alvarado, I., 2000. Evolu-

tion of the hydrothermal system at the geothermal field of Los

Azufres Mexico, based on fluid inclusion, isotopic and petro-

logic data. J. Volcanol. Geotherm. Res. 104, 277296.

Grassineau, N.V., Mattey, D.P., Lowry, D., 2001. Sulfur isotope

analysis of sulphide and sulfate minerals by continous flow-

isotope ratio mass spectrometry. Anal. Chem. 73, 220225.

Guillou, J.J., Monthel, J., Picot, P., Pillard, F., Protas, J.,

Samama, J.C., 1985. LImiterite, Ag2HgS2, nouvelle espe`ce

minerale; proprietes et structure cristalline. Bull. Mineral.

108, 457464.

Guillou, J.J., Monthel, J., Samama, J.C., Tijani, A., 1988. Morpho-

logie et chronologie relative des associations minerales du gise-

ment mercuro-argentife`res dImiter (Anti-Atlas, Maroc). Notes

Mem. Serv. Geol. Maroc, Rabat 334, 215228.

Hedenquist, J.W., Richard, J.P., 1998. The influence of geochemical

techniques on the development of genetic models for porphyry

copper deposits. Rev. Econ. Geol. 10, 235256.

Hedenquist, J.W., Arribas, R.A., Gonzalez-Urien, E., 2000. Ex-

ploration for epithermal gold deposits. Rev. Econ. Geol. 13,

245277.

Henley, R.W., 1985. The geothermal framework of epithermal

deposits. Geology and Geochemistry of Epithermal Deposits.

Reviews in Economic Geology, vol. 2, pp. 121.

Hickley, R.L., Frey, F.A., Gerlach, D.C., Lopez-Escobar, L., 1986.

Multiple source of basaltic arc rocks from central south Chile:

trace element and isotopic evidence for contributions from sub-

ducted oceanic crust, mantle, and continantal crust. J. Geophys.

Res. 91, 59635983.

Hilton, D.R., Hammerschmidt, K., Teufel, S., Friedrichsen, H.,

1993. Helium isotope characteristics of the Andean geothermal

fluids and lavas. Earth Planet. Sci. Lett. 120, 265282.

Kribek, B., 1991. Metallogeny, structural, lithological and time

controls of ore deposition in anoxic environments. Miner.

Depos. 26, 122131.

Leblanc, M., Lancelot, J.R., 1980. Interpretation geodynamique du

domaine pan-africain (Precambrien terminal) de lAnti-Atlas

(Maroc) a partir de donnees geologiques et geochronologiques.

Can. J. Earth Sci. 17, 142155.

Leistel, J.M., Qadrouci, A., 1991. Le gisement argentife`re dImiter

(Proterozoque superieur de lAnti-Atlas, Maroc). Controles des

mineralisations, hypothe`se genetique et perspectives pour lex-

ploration. Chron. Rech. Min. 502, 522.

Levresse, G., 2001. Contribution a` letablissement dun mode`le

genetique des gisements dImiter (Ag Hg), Bou Madine

(PbZnCuAgAu) et Bou Azzer (CoNiAsAuAg)

dans lAnti-Atlas marocain. PhD thesis, INPL, Nancy, France.

Mamyrin, B.A., Tolstikyn, I.N., 1984. Helium isotope in nature.

Dev. Geochem. 3, 273.subduction zones. Science 286, 512516.

Ohmoto, H., Rye, R.O., 1979. Isotopes of sulfur and carbon. Geo-

chemistry of Hydrothermal Ore Deposits, Second ed. Wiley,

New York, pp. 509567.

Ouguir, H., Macaudiere, J., Dagallier, G., 1996. Le Proterozoque

superieur dImiter, Saghro oriental, Maroc: un contexte geody-

namic darrie`re-arc. J. Afr. Earth Sci. 22 (2), 173189.

Ozima, M., Podosek, F.A., 2002. Noble Gas Geochemistry, 2nd

edition. Cambridge Univ. Press, Cambridge, UK. 286 pp.

Patou, L., Lorand, J.P., Gros, M., 1996. Non-chondritic plati-

num-group element ratios in the Earths mantle. Nature

379, 712715.

Pierson-Wickmann, A.C., Reisberg, L., France-Lanord, C., 2000.

The Os isotopic composition of the Himalaya river dedloads and

bedrocks: importance of black shales. Earth Planet. Sci. Lett.

176, 203218.

Ravizza, G., Turekian, K.K., 1989. Application of 187Re 187Os

system to black shale geochemistry. Geochim. Cosmochim.

Acta 53, 32573262.

Richard, D., Marty, B., Chaussidon, M., Arnd, N., 1996. Helium

isotopic evidence for a lower mantle component in depleted

Archean komatite. Nature 273, 9395.

Rye, R.O., Ohmoto, H., 1974. Sulfur and carbon isotopes and ore

genesis. Econ. Geol. 69, 826842.

Shirey, S.B., Walker, R.J., 1995. Carius tube digestion for low-

blank rheniumosmium analysis. Anal. Chem. 67, 21362141.

Sillitoe, R.H., 1993. Epithermal models: genetic types, geometrical

controls and shallow features. Spec. Pap.-Geol. Assoc. Can. 40,

403417.

Simmons, S.F., Sawkins, F.J., Schlutter, D.J., 1987. Mantle-derived

in two hydrothermal ore deposits, Peru. Nature 329, 423429.

Singh, S., Trivedi, J.R., Krishnaswami, S., 1999. Re/Os isotope

systematics in black shales from the Lesser Himalaya: their

chronology and role in the 187Os/188Os evolution seawater. Geo-

chim. Cosmochim. Acta 63 (16), 23812392.

Villeneuve, M., Cornee, J.J., 1994. Structure, evolution and palae-

geography of the West African craton and bordering belts during

the Neoproterozoic. Precambrian Res. 69, 307326.

Worner, G., Moorbath, S., Harmon, R.S., 1992. Andean Cenozoic

volcanic centers reflect basement isotopic domains. Geology 20,

11031106.

Zotov, A.V., Kudrin, A.V., Levin, K.A., Shikina, N.D., Varyash,

L.N., 1995. Experimental studies of the solubility and com-

plexing of the selected ore elements (Au, Ag, Cu, Mo, As,

Sb, Hg) in aqueous solutions. In: Shmulovitch, K.I., Yardley,

B.W.D. (Eds.), Fluids in the Crust, 95137.

Osmium, sulphur, and helium isotopic results from the giant Neoproterozoic epithermal Imiter silver deposit, Morocco: evidence for a mantle sourceIntroductionGeological settingPrevious studiesMineralization events and mineral paragenesisNBS sulphidesBMEESE

Analytical procedureSulphur isotopesFluid inclusion studyHelium isotopesRe/Os isotopes

Analytical resultsSulphur isotopic studyFluid inclusion studyHelium isotopic studyRe/Os isotopic study

DiscussionThe sources of metalThe sources of fluidsThe sources of sulphurThe middle Neoproterozoic black shales (NBS)

The rhyolitic intrusivesSulphides from the mineralized associations

ConclusionAcknowledgementsReferences