0361-0128/01/3281/1203-23 $6.00 1203

IntroductionON A GREENSTONE-BELT scale, Archean gold camps are com-monly spatially related to large-scale (>100 km long), tran-scrustal fault zones. However, on a camp scale, most of theworld-class (>100 t) and smaller gold deposits are hosted insecond- and third-order fault zones, whereas the first-ordertranscrustal faults are largely barren. In many examples, tran-scrustal faults are interpreted to penetrate into the lowercrust or even into the mantle (e.g., the San Andreas fault:Teyssier and Tikoff, 1998). Both the close spatial relationship

of world-class gold deposits and transcrustal fault zones, andthe deep penetration of the latter, stimulated the model thattranscrustal fault zones represent the main conduits for gold-bearing hydrothermal fluids from mantle and lower-crustallevels into dilatant second- and third-order shear zones in theupper crust that host the gold deposits (Eisenlohr et al., 1989;Kerrich, 1989; Groves et al., 1990).

This model requires that the transcrustal fault zones andthe gold-hosting second- and third-order shear zones werestructurally and hydraulically connected during the time ofgold mineralization. However, because most Archean tran-scrustal fault zones worldwide are poorly exposed, and their

Hydrothermal Fluid Evolution within the Cadillac Tectonic Zone, Abitibi Greenstone Belt, Canada: Relationship to Auriferous Fluids in

Adjacent Second- and Third-Order Shear Zones

PETER NEUMAYR AND STEFFEN G. HAGEMANNCentre for Global Metallogeny, Department of Geology and Geophysics, University of Western Australia,

Nedlands, Western Australia 6907, Australia

AbstractThe Cadillac tectonic zone in the Abitibi greenstone belt, Canada, is spatially associated with world-class

gold deposits that are hosted in second- and third-order faults. Five types of hydrothermal quartz veins withdistinct structural timing are distinguished in the Cadillac tectonic zone in the Val dOr camp and are relatedto D2 steep-reverse faulting and D3 oblique-transcurrent movement.

Fluid inclusions in gold-bearing and barren hydrothermal quartz veins in the transcrustal Cadillac tectoniczone record the progressive evolution of the hydrothermal fluids within the Cadillac tectonic zone. OldestH2O-CO2-CH4-NaCl fluid inclusions, which predate gold mineralization, are superseded by gold-associated,dominantly carbonic CO2-CH4 H2O NaCl fluid inclusions. On the basis of variable phase ratios and simi-lar total homogenization temperatures in contemporaneous fluid inclusions trails, the oldest H2O-CO2-CH4-NaCl fluids are interpreted to have undergone phase separation. The CH4 content increases from the older(XCH4 = 0.030.24) to the younger (XCH4 = 0.150.71) fluid inclusions, and the salinity is low to moderate in theolder fluid inclusions (mean 1 = 7.2 6.2 wt % NaCl equiv) and increases in the younger fluid inclusions(mean 1 = 16.2 6.0 wt % NaCl equiv). Trails of late-stage CO2-CH4-N2 H2O and CH4 C2H6 C3H8fluid inclusions crosscut earlier carbonic-aqueous fluid inclusion trails. In all veins, the texturally latest fluid in-clusions occur in trails that crosscut earlier fluid inclusion trails and quartz deformation bands and are com-posed of H2O-NaCl-CaCl2. Postentrapment modifications are considered unlikely for most fluid inclusions inthe Cadillac tectonic zone.

The hydrothermal fluids in the Cadillac tectonic zone were compared with those in second- and third-orderfault zones in the world-class Sigma deposit to evaluate whether the two fluid systems are of similar derivation.Carbonic aqueous fluids that deposited quartz in veins and some gold in the Cadillac tectonic zone are distinctfrom those that are hosted in structures of similar timing at Sigma. Cadillac fluids have higher CO2 content andmarginally higher salinity (0.615.3 wt % NaCl equiv and 110 wt % NaCl equiv in the Cadillac tectonic zoneand at Sigma, respectively) but contain similar amounts of CH4. The high-salinity aqueous inclusions at Sigma,which are interpreted to represent the unmixed H2O-rich end member of a combined CO2-CH4-H2O-NaClfluid, are absent in the Cadillac tectonic zone. Veins of D3 timing, which contain a high CH4 component in theCadillac tectonic zone (to 70 mole %), have not been documented at Sigma.

The magnitude of the hydrothermal quartz vein field and the homogeneity of quartz vein mineralogy andtextures in the Val dOr camp strongly suggest that the fluids that precipitated auriferous quartz veins in sec-ond- and third-order shear zones were not locally derived. It is suggested that ore-bearing fluids ascendedalong the Cadillac tectonic zone and infiltrated the second- and third-order fault network. An unknown amountof metamorphic H2O from the greenstone belt probably mixed with these fluids. Differences in the fluid com-position in the Cadillac tectonic zone and those in second- and third-order shear zones are explained by phase-separation with preferential vapor loss into the upper portions of the Cadillac tectonic zone and by in situ phaseseparation without vapor loss during ore deposition at Sigma. Methane enrichment in the late fluids in theCadillac tectonic zone was most likely controlled by an increased fluid interaction with reduced carbon in sed-imentary rocks, but increasing mantle outgassing and cooling of an fO2-buffered ascending fluid may also havecontributed to the CH4 budget. The youngest aqueous brines in the Cadillac tectonic zone are similar to flu-ids in late fracture fillings and to Canadian Shield basement brines.

Economic GeologyVol. 97, 2002, pp. 12031225

Corresponding author: e-mail: pneumayr@geol.uwa.edu.au

location, strike, and orientation are typically interpreted fromaeromagnetic data, there is a general lack of structural andfluid chemistry data about most of these structures. In theAbitibi greenstone belt in Canada, however, significant struc-tural information does exist for the transcrustal Cadillac tec-tonic zone (Robert, 1989; Hubert, 1990 and referencestherein; Wilkinson et al., 1999). The structural setting andtiming of mineralized quartz veins in transcrustal faults and ingold-bearing, second- and third-order fault zones within theVal dOr gold camp has been demonstrated in studies onsome better-exposed portions of the Cadillac tectonic zone(Robert, 1990; Neumayr et al., 2000). In these studies, it wasshown that the structural timing of quartz veins in tran-scrustal and second- and third-order structures was similar.However, this similarity indicates only that the differentstructures were activated at the same time. In order to un-equivocally demonstrate a hydraulic connection between thetranscrustal fault zones and the second- and third-order shearzones that host gold mineralization, it is necessary to demon-strate that the different structures also contained the samehydrothermal fluids.

In this study, hydrothermal fluids in quartz veins that wereformed during different deformation events in the Cadillactectonic zone in the Abitibi greenstone belt were investigatedusing microthermometry and laser-Raman spectroscopy toconstrain, for the first time, the compositions of hydrothermalfluids that infiltrated into an Archaean transcrustal fault zone.

It is the aim of this paper to compare the fluids in the tran-scrustal Cadillac tectonic zone with published data on hy-drothermal fluids in second- and third-order shear zones thathost significant gold deposits. In particular, the data are com-pared with those from the Sigma-Lamaque gold deposit in aneffort to define processes that led to world-class gold miner-alization at Sigma and not in the Cadillac tectonic zone.

Geologic and Structural Setting

Setting of the Val dOr camp

The Val dOr camp is located in the Archean Malartic com-posite block (Desrochers et al., 1993) in the eastern part ofthe Abitibi greenstone belt (Fig. 1; Superior province, Cana-dian Shield). The Malartic composite block contains north-west-southeast to east-westtrending belts of circa 2.7 Gakomatiitic to calc-alkaline dacitic volcanic rocks that are in-truded by synvolcanic and late-tectonic diorite plutons andgranitic dikes (Desrochers and Hubert, 1996). The Malarticcomposite block is bounded by narrow bands of younger sedi-mentary rocks that, in most places, are in fault contact withthe volcanic rocks of the Malartic composite block (maximumage 2691 to 2682 Ma: Davies, 1991; Mortensen and Card,1993). It is bordered to the south by the graywacke-mud-stonedominated Pontiac metasedimentary subprovince. TheMalartic composite block is separated from the Pontiac sub-province by the transcrustal Cadillac tectonic zone (Fig. 1)

1204 NEUMAYR AND HAGEMANN

CTZ

Val-dOr SigmaKiena

CadillacTectonic Zone

7800

4815 N5 kmGold deposit >30 t Au

Lake

First order deformationzoneSecond, third

Felsic syn- to late-tectonic intrusionFelsic synvolcanicintrusionVolcanic rock (Malartic Composite Block)Greywacke, mudstone,conglomerate (Cadillac, Kewagama, Lac Caste Groups)Greywacke, mudstone (Pontiac Group)

7745Orenada #2, #4

Timmins

80

BRG

MCB

100 km

Val dOr48

DPTZ

CTZ

Canadian Malartic,Barnat-Sladen,East Malartic

Sullivan

Lamaque

Malartic goldfield

Camflo

Selected Gold deposit

and is subdivided into seven fault-bounded domains(Desrochers et al., 1993). All rocks of the Malartic compositeblock have been metamorphosed to greenschist facies, butstructures and textures are generally well preserved exceptnear major faults.

Three main stages of deformation have been recognized inthe Malartic composite block (e.g., Robert, 1990; Desrocherset al., 1993; Desrochers and Hubert, 1996). Each domaincontains distinct D1 structures, such as tight to isoclinal folds,which are truncated at the domain-bounding structures. Re-gional north-south compression during D2 affected all sevendomains and juxtaposed the Pontiac subprovince and theMalartic composite block along the Cadillac tectonic zone.Both the Cadillac tectonic zone and the gold-hosting, second-and third-order shear zones, which form a 15-km-wide corri-dor to the north of the Cadillac tectonic zone, were steep-reverse to oblique-reverse faults during D2. During D3 tran-scurrent deformation, the S2 foliation in the western part ofthe Val dOr camp and in the eastern parts of the Cadillac tec-tonic zone was deformed about Z-shaped asymmetric folds.The transition from D2 to D3 deformation was progressive(Neumayr et al., 2000). Peak greenschist-facies metamorphicconditions were attained during D2 deformation in the Malar-tic composite block.

Most of the gold deposits in the Val dOr camp are relatedto networks of steeply dipping quartz shear veins and subhor-izontal extensional veins hosted by, or associated with, D2 sec-ond- and third-order (brittle-ductile) shear zones (e.g.,Robert and Brown, 1986; Robert, 1990). The total productionfrom deposits in these shear zones reached 654 t of gold by1989 (Couture, 1991). However, some gold mineralization isalso related to D3 transcurrent shearing (Robert, 1989; Neu-mayr et al., 2000). Only minor gold deposits (

1206 NEUMAYR AND HAGEMANN

N2 m

Orenada #2

70

85

85

55

70

55

V2A

V2B

V2A

V2A

V2B

V2/3V3A

0.5 m

Trace of S 2

Trace of S 33525

80

85

85

7025

85

80

85

V2A

V3A

V2A

N

B

85 Strike and dip ofquartz vein Conglomerate

Mostly Siltstone

Trace of S2Quartz+/-tourmaline vein

F3 Synform,Antiform85

70Strike anddip

S2S3

55 Plunge of F3fold hinge

A

A

B

Cadillac

Malartic

Pontiac Piche

Orenada #4Orenada #2

MonzoniteN

2km

CTZ

CO1CO2

CO9

CO10

CO17

CO18

CO28

CO13

CO10 Samplelocation

V2A V2B

V3B

V3CV3A S2

S3

1 (D2)1 (D2)

FIG. 2. Selected outcrop maps of the western part of the stripped outcrop at Orenada 2 to show the geologic setting ofsamples selected for this study (maps are from Neumayr et al., 2000). Inset in A is a schematic block diagram showing rela-tionship of hydrothermal quartz veins and their timing relative to S2 and S3 foliations. Insets in B show the location of Ore-nada 2 with respect to the Cadillac tectonic zone and the location of maps depicted in A and B with respect to the strippedoutcrop at Orenada 2.

description of the structural and textural characteristics of thequartz vein systems has been presented by Neumayr et al.(2000) and is summarized briefly below.

Vein textures

The V2A and V3A quartz veins range in thickness from afew millimeters to 15 cm, whereas all other veins are a few

centimeters thick (Table 1). All except V3B and V3C veinsshow complex quartz textures. The V2A, V2B, and V3Aquartz veins all contain coarse-grained quartz (Fig. 4A) that,in the case of V2B and V3A, is intergrown with tourmaline(Fig. 4B). Coarse-grained quartz is elongated parallel to thevein wall, which is interpreted to represent stretching of thequartz as a result of ongoing deformation during or shortly

HYDROTHERMAL FLUID EVOLUTION, CADILLAC TECTONIC ZONE, ABITIBI 1207

F

S2

qtz vein

S3

DC

S2qtzvein

A

qtz vein

S2

B

apy

E

qtz vein

S2

qtz vein

S2

0.5 cm

FIG. 3. Photographs of hydrothermal quartz veins and foliations at the Cadillac tectonic zone at Orenada 2. A. V2A quartzvein boudinaged with extension parallel to the S2 foliation. B. V2B quartz vein subparallel to the S2 foliation; both S2 folia-tion and V2B quartz vein folded about F3 folds; location of sample CO18. C. V2B quartz vein parallel to S2 foliation. D. V3Aquartz vein boudinaged with extension parallel to the S3 foliation; location of sample CO10. E. Subhorizontal V3B quartzvein crosscutting arsenopyrite, gold, and S2 foliation; arrows indicate location of arsenopyrite; location of sample CO2. F.Subvertical V3C quartz vein crosscutting the S2 foliation; note the open space filling of the hydrothermal quartz (arrow) in-dicating shallow crustal emplacement of the vein; location of sample CO44. apy = arsenopyrite, qtz = quartz.

after vein growth. Internally, coarse-grained quartz showsvariable degrees of deformation that is indicated by deforma-tion lamellae, undulose extinction, and local subgrains.Coarse-grained quartz is rimmed and crosscut by narrowbands that consist of fine-grained recrystallized quartz inV2A, V2B, and V3A quartz veins (Fig. 4A). These bands indi-cate deformation and annealing that occurred after the for-mation of individual quartz veins.

Clasts in brecciated segments of V2B and V3A quartz veinscontain bands of recrystallized quartz alternating with bands

of tourmaline. The clasts are hosted in a matrix of coarse-grained quartz that is elongated parallel to the vein wall. Thecoarse quartz grains are also brecciated and the matrix infilledby chlorite (see Neumayr et al., 2000).

V3B and V3C quartz veins are virtually undeformed (Fig.4B). Bands of tourmaline needles in V3B veins are parallel toeach quartz vein wall, but the individual needles are perpendic-ular to the vein walls, indicating that they are extension veins.

Hydrothermal alteration and vein paragenesis

Carbonate alteration is the dominant hydrothermal alterationassociated with V2A, V2B, and V3A quartz veins. Tourmalineis restricted to V2B, V3A shear, and V3B extension veins. V2Aveins locally contain arsenopyrite, and gold and arsenopyriteoccur in the wall rock adjacent to and within V2B and V3Ashear veins. These veins were either brecciated during veinformation or during subsequent deformation, and the brecciamatrix was filled with hydrothermal chlorite. Late-stage V3Cquartz veins contain chalcopyrite and pyrrhotite. On the basisof whole-rock channel samples, gold is mainly found in V2Band V3A quartz veins (Aur Resources, unpublished data;Robert, 1989).

Arsenopyrite associated with a V3A shear vein and chloritein the matrix of hydrothermal breccias are used to determineformation temperatures of the ore minerals and alterationproducts. Arsenopyrite in equilibrium with pyrrhotite containsbetween 30.3 and 32.4 at. percent As, which gives tempera-tures of 300 to 450C using the arsenopyrite geothermome-ters of Kretschmar and Scott (1976) and Sharp et al. (1985).The AlIV content of 2.67 to 2.75 in the chlorites indicates tem-peratures of 320 to 330C on the basis of the calibration ofCathelineau and Nieva (1985), which is consistent with tem-peratures derived from the arsenopyrite geothermometers.These data provide additional temperature constraints onvein formation and alteration, and they assist with estimatesof trapping pressure and temperature for fluids that have notundergone phase immiscibility.

Fluid Inclusion Analytical ProceduresFluid inclusions in 17 double-polished plates from hy-

drothermal quartz veins from the Orenada 2 outcrop werecharacterized petrographically. Microthermometric datawere collected from about 350 fluid inclusions in nine sam-ples, representing the various vein types and their respectivestructural timing. For each sample, microthermometric mea-surements were carried out on four to 10 chips that werecarefully broken off the double-polished section.

Microthermometric analyses were performed using aLinkham THMSG 600 heating and freezing stage at the Uni-versity of Western Australia. The stage was initially calibratedusing commercially available, synthetic fluid-inclusion stan-dards. The calibration was then checked before each analyti-cal session using the pure CO2 melting point and the pureH2O melting and total homogenization points. Reproducibil-ity was better then 0.1C at a heating rate of 0.1C/min. Allmeasurements were corrected using the determined calibra-tion curves (cf. MacDonald and Spooner, 1981).

Reduction of microthermometric data was carried out withMacFlinCor computer software (Brown and Hagemann,1995), using the equations of state of Kerrick and Jacobs

1208 NEUMAYR AND HAGEMANN

C

20 m

0.5 mm

B

0.1 mm

A

FIG. 4. Photomicrographs of quartz vein textures at Orenada 2. A. Coarse,elongated quartz grains are rimmed and crosscut by bands of fine-grained,recrystallized quartz (sample CO10, V3A vein; cross-polarized light). B. Sub-horizontal, unmineralized V3B quartz tourmaline extensional vein, tour-maline fibers are perpendicular to the vein wall, quartz is largely unstrained(arrow points toward top of vein; sample CO2, cross-polarized light). C.Necking down of fluid inclusions along subgrain boundaries in quartz (sam-ple CO10, V3A vein, plane-polarized light).

(1981) and Bodnar and Vityk (1994). The molar volume andXCH4 of the carbonic phase was calculated from the tempera-ture of homogenization of the carbonic phase (ThCAR) and thetemperature of melting of the carbonic phase (TmCAR) usingthe method of Thiry et al. (1994). Disequilibrium behaviorof methane-bearing (aqueous)-carbonic inclusions, in whichThCAR < TmCLA (temperature of melting of the clathrate), isnot considered to strongly affect the estimates of fluid com-position and molar volume.

Laser-Raman spectroscopic analysis was carried out at theAustralian Geological Survey Organization in Canberra to es-tablish the nature and composition of the gas phase in 40 rep-resentative fluid inclusions from all vein types. Laser-Ramanspectra of fluid inclusions were recorded on a Dilor SuperLabram spectrometer equipped with a holographic notch fil-ter, 600 and 1,800 g/mm gratings, and a liquid-N2cooled,2,000 450 pixel Charge Coupled Device detector. The in-clusions were illuminated by a 514.5 nm laser from a SpectraPhysics model 2017 argon ion laser, using 5 mW power at thesample, and a single 30 s accumulation. A 100 Olympus mi-croscope objective was used to focus the laser beam and col-lect the scattered light. The focused laser spot on the sampleswas approximately 1 m in diameter. Wave numbers are ac-curate to 1 cm1 as determined by plasma and neon emis-sion lines. For the analysis of CO2, O2, N2, H2S, and CH4 inthe vapor phase, spectra were recorded from 1,000 to 3,800cm1 using a single 20 s integration time per spectrum. Thedetection limits depend on the instrument sensitivity, the par-tial pressure of the gas, and the depth below the surface ofeach fluid inclusion. Raman detection limits (Wopenka andPasteris, 1987) are estimated to be around 0.1 mole percentfor CO2, O2, and N2 and 0.03 mole percent for H2S and CH4,and errors in the calculated gas compositions are generallyless than 1 mole percent.

Petrographic Descriptions of Fluid Inclusions

Petrographic description

All hydrothermal quartz veins within the Cadillac tectoniczone contain four types of fluid inclusions in varying abun-dance, distinguished by petrography, microthermometry, andlaser-Raman spectroscopy. These types are (1) carbonic in-clusions, with or without a variably saline aqueous compo-nent, CO2-CH4 H2O NaCl; (2) carbonic inclusions with anitrogen component, CO2-CH4-N2 H2O; (3) methane inclu-sions with a component of higher hydrocarbons, CH4 C2H6 C3H8; and (4) saline aqueous inclusions with or withoutdaughter minerals, H2O-NaCl-CaCl2 (4a) and H2O-NaCl-CaCl2-daughter mineral or minerals (4b). For the fluid inclu-sion studies, care was taken to select only weakly deformedquartz grains (i.e., weak undulose extinction), which are evenlydistributed among strongly deformed grains, in order to mini-mize chances that fluid inclusions have been modified aftertrapping by subsequent deformation and subgrain formation.

Type 1: CO2-CH4 H2O NaCl inclusions are the mostabundant fluid inclusion type in V2A, V2B, V3A, and V3Bquartz veins. The fluid inclusions have a negative crystal or ir-regular shape and are 2 to 16 m long. Most of the type 1fluid inclusions are water-absent CO2-CH4 inclusions. However,because up to 20 percent of invisible H2O may be present

along inclusion walls (Roedder, 1972), many apparently H2O-absent CO2-CH4 inclusions may contain a small, optically un-detectable quantity of H2O.

Both H2O-bearing and H2O-absent type 1 fluid inclusionsoccur in pseudosecondary internal trails parallel to the elon-gation of the quartz grains (Fig. 5A, C) and in V2A veins, insmall clusters. The trails are typically truncated by bands ofrecrystallized quartz (Fig. 5A). Quartz subgrain boundariesare decorated by necked-down type 1 fluid inclusions (Fig.4C).

Most H2O-absent type 1 fluid inclusions are monophaseand contain only supercritical CO2-CH4 at room temperature(Fig. 5C). Most fluid inclusions of this type also have a darkappearance owing to the presence of graphite along the fluidinclusion walls, as confirmed by laser-Raman analyses (seebelow). Phase ratios in H2O-absent type 1 inclusions are con-stant within individual trails and vary little within individualquartz samples. In V3B veins, which contain mostly unde-formed quartz grains, H2O-absent type 1 fluid inclusionsoccur in broad trails and clusters.

Type 1 CO2-CH4-H2O-NaCl fluid inclusions are two-phase(liquid water and carbonic vapor) at room temperature andare rare (85%) andoccur within trails of H2O-absent type 1 fluid inclusions.

Type 2: CO2-CH4-N2 H2O fluid inclusions are mono-phase, or two phase if H2O is present, at room temperature.They have an irregular shape and, less commonly, a negativecrystal shape (Fig. 5D). Type 2 fluid inclusions occur in theV3A quartz veins as conjugate sets of trails that crosscut and,therefore, postdate type 1 fluid inclusion trails. Petrographi-cally they are similar to type 1 fluid inclusions. Trails of type2 fluid inclusions are subparallel to bands of recrystallizedquartz. They range in length from 6 to 11 m. Where they areH2O bearing, the inclusions have constant phase ratios ofabout 0.6 in a single trail.

Type 3: CH4 C2H6 C3H8 fluid inclusions are monophaseat room temperature (Fig. 5E) and are also petrographicallysimilar to H2O-absent type 1 fluid inclusions. However, theyoccur in late crosscutting trails within coarse quartz grains inV3A veins. They are 4 to10 m long and have a negative crys-tal shape. Phase ratios could not be determined with micro-thermometry, since the fluid inclusions remained monophaseupon freezing to 190C.

Types 4a and 4b: Both type 4a (H2O-NaCl-CaCl2) and type4b (H2O-NaCl-CaCl2-daughter minerals) inclusions occur inclusters and trails (Fig. 5A, F) that crosscut both type 1 fluidinclusion trails and bands of fine-grained recrystallizedquartz. They are either two phase (liquid H2O-vapor H2O),three phase, or polyphase (liquid H2O-vapor H2O-daughterminerals) at room temperature and are irregular in shape.The daughter mineral in type 4b fluid inclusions is halite onthe basis of its properties in transmitted light. Some trails inV3B quartz veins contain up to three daughter minerals, one

HYDROTHERMAL FLUID EVOLUTION, CADILLAC TECTONIC ZONE, ABITIBI 1209

of which was optically identified as halite. Fluid inclusions inindividual trails display relatively constant phase ratios andrange in size from 4 to 10 m.

Microthermometry and Laser-Raman Results

Type 1CO2-CH4 H2O NaCl

Type 1 fluid inclusions show a distinct evolution from olderto younger veins in terms of microthermometric data. The

carbonic phase in H2O-absent type 1 fluid inclusions in V2Aveins melts between 59.9 and 56.6C (Fig. 6A, B) and ho-mogenizes between 24.6 and 11.6C; the mean is 0.2 6.1C (Fig. 6F, Table 2). Fluid inclusions in V2B veins arecharacterized by TmCAR between 62.2 and 57.5C. Twosamples of V2B veins have modes at 59.5C and 61.5C, re-spectively (Fig. 6C). Fluid inclusions in V3A veins haveTmCAR between 63.0 and 59.0C; the mean is 61.1 1.0C (Fig. 6D). The V3B veins contain type 1 inclusions with

1210 NEUMAYR AND HAGEMANN

C

10 m

D

10 m

A BType 4a

Type 1recryst.

F

20 m

10 m

10 m

10 m

E

FIG. 5. A. Trails of type 1 CO2-CH4 and type 4a H2O-NaCl-CaCl2 fluid inclusions (sample CO10, crossed-polarizedlight). Note that the trail of type 1 fluid inclusions is truncated at the band of recrystallized quartz, whereas the trail oftype 4a fluid inclusions crosscuts both type 1 trails and the recrystallized band. B. Trail of type 1 CO2-CH4-H2O-NaClfluid inclusions and some dark type 1 CO2-CH4 fluid inclusions (white lines indicate orientation of trail; sample CO28;plane-polarized light). Note the variable phase ratios. C. Trail of type 1 CO2-CH4 fluid inclusion (white line indicates ori-entation of trail; sample CO28; plane-polarized light). D. Trail of type 2 CO2-CH4-N2 H2O fluid inclusions (sampleCO10; plane-polarized light). E. Trail of type 3 CH4 C2H6 C3H8 fluid inclusions (sample CO10; plane-polarized light).F. Cluster of type 4b H2O-NaCl-CaCl2-daughter mineral fluid inclusions (sample CO44; plane-polarized light; arrowspoint to some of the inclusions containing daughter minerals).

the lowest TmCAR between 67.7 and 60.7C (Fig. 6E).These melting temperatures indicate that the carbonic phasecontains CH4 in addition to CO2, as confirmed by laser-Raman analyses (Fig. 7). The progressively lower TmCAR fromolder to younger veins indicates that the lowest XCH4(0.030.24) is in the V2B veins, intermediate XCH4 (0.090.31)is in V3A veins, and the highest XCH4 (0.150.71) is in V3Bveins. The ThCAR (all fluid inclusions of this type homogenizeto the liquid) decreases progressively from the older V2Aveins (25.8 to 11.6C) to V2B veins (24.0 to 13.6C) andV3A veins (49.3 to 10.1C) to the younger, V3B veins(50.8 to 7.1C: Fig. 6F, G, H, I; Table 2). Despite the largevariation of ThCAR between samples and even within a single

sample, individual fluid inclusion trails show a limited varia-tion in ThCAR. Laser-Raman analyses identified a dark coatingin most H2O-absent type 1 fluid inclusions as graphite (Fig.8A); its G peak (first order) wave number is 1,577 cm1, thesecond order peak is at 2,702 cm1, and the disorder peak isat 1,352 cm1 (e.g., Wopenka and Pasteris, 1993).

Although H2O-bearing type 1 fluid inclusions are commononly in V2A, V2B, and V3A veins and are largely absent inV3B veins, the trends in ThCAR and TmCAR are similar to thoseof H2O-absent type 1 fluid inclusions (Fig. 9A). H2O-bearingtype 1 fluid inclusions within oldest V2A veins have the high-est TmCAR (62.5 to 56.6C), whereas those in V2B andV3A veins have progressively lower TmCAR (60.4 to 58.9C

HYDROTHERMAL FLUID EVOLUTION, CADILLAC TECTONIC ZONE, ABITIBI 1211

-60

-50

-40

-30

-20

-10

0

10

20

30

ThCA

R (C

)

-69 -67 -65 -63 -61 -59 -57TmCAR (C)

LSG

X CH4

=0.

2

X CH4

=0.

3

X CH4

=0.

1

X CH4

=0.

5

critic

al cu

rve

VCAR=45cm

3

V CAR=50c

m3VCAR=60c

m3VCAR=70c

m3

XCH4=0.7

V2B qtz-tm veinV3A qtz-tm veinV3B qtz-tm vein

V2A qtz vein old

young

CO2-CH4

A

C

-68 -64 -60 -56TmCAR (C)

0

10

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t

CO18

CO9

0

10

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-70 -50 -30 -10 10ThCAR (C)

Coun

t

0

4

8

12

16

-70 -50 -30 -10 10ThCAR (C)

Coun

t

0

4

8

12

-70 -50 -30 -10 10ThCAR (C)

Coun

t0

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-68 -64 -60TmCAR (C)

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CO28

CO17V2A

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CO10

CO13V3A

-68 -64 -60 -56TmCAR (C)

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12

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E

CO2

CO1 V3B

Coun

t

V2B

-70 -50 -30 -10 10ThCAR (C)

0

10

16

Coun

t

CO28

CO17V2AF G

CO18

CO9V2BH

CO10CO13V3A

I

CO2

CO1V3B

V3A vein, N2-bearing

FIG. 6. A. TmCAR vs. ThCAR of type 1 CO2-CH4 fluid inclusions at Orenada 2 according to sample. Contours of XCH4 andVCAR (molar volume of the carbonic phase), critical curve, and liquid-solid-gas (LSG) univariant curve constructed from fig-ures in Thiry et al. (1994), assuming homogenization of the liquid and that CO2 and CH4 are the only components. Notethe clustering by sample with respect to composition and the increase in XCH4 from the oldest to the youngest quartz veins.Different symbols for the same vein type represent data derived from different veins of the same structural timing. B. His-togram of TmCAR in V2A veins. C. Histogram of TmCAR in V2B veins. D. Histogram of TmCAR in V3A veins. E. Histogram ofTmCAR in V3B veins. F. Histogram of ThCAR in V2A veins. G. Histogram of ThCAR in V2B veins. H. Histogram of ThCAR inV3A veins. I. Histogram of ThCAR in V3B veins. Tm, tourmaline. B to I labeled with vein type (e.g., V2A) and sample num-bers (e.g., CO17, CO28).

and 62.2 to 59.0C , respectively: Fig. 9B, C, D). The de-pression of the CO2 freezing temperature indicates the pres-ence of CH4 as confirmed by laser-Raman spectroscopy (Fig.7). The XCH4 increases in H2O-bearing type 1 fluid inclusionsfrom V2A veins (0.00.21; the majority are 0.1; Fig. 9). H2O-bearing type 1fluid inclusions in early V2A veins are also distinct from thosein V3A veins; TmCLA is 9.2 to 13.1C (6.4 5.4C) in V2Aveins versus 8.7 and 9C (1.2 5.7C; Fig. 10) in V3Aveins. This results in higher average salinities in the inclusionsin the V3A veins (16.2 6.0 wt % NaCl equiv) compared withthose in V2A veins (7.2 6.2 wt % NaCl equiv).

Type 2CO2-CH4-N2 H2O and type 3CH4 C2H6 C3H8

Type 2 inclusions are characterized by low TmCAR (62.3 to60.1C) and also low ThCAR (33.3 to 26.2C). Clathrate,

where present, melts between 4.9 and 0.9C. Laser-Raman spectra have a distinct peak at wave numbers around2,328 cm1, which is typical of N2 (Andersen et al., 1989,1993). The maximum N2 content of this fluid inclusion type is24 mole percent (Fig. 7). Type 3 fluid inclusions did not showany phase transitions upon cooling to 90C and subsequentheating. Laser-Raman spectra for these inclusions have peaksat wave numbers around 2,899 cm1 (C3H8), 2,917 cm1(CH4), and 2,954 cm1 (C2H6) (Fig. 8B; for Raman peak ref-erence see summary in Burke, 1994, and references therein).Methane accounts for 98 mol percent, C2H6 for 1 mol per-cent, and C3H8 for 1 mol percent in these fluid inclusions.

Type 4aH2O-NaCl-CaCl2 and type 4bH2O-NaCl-CaCl2 daughter mineral or minerals

Type 4 fluid inclusions are characterized by low eutecticmelts around 56C, which indicate the presence of CaCl2

1212 NEUMAYR AND HAGEMANN

TABLE 2. Summary of Microthermometric Data of Carbonic Inclusions in Four Vein Types in the Cadillac Tectonic Zone at Orenada 2

Vein type/ Fluid ThCAR(C) Th (C) sample inclusion VOLVAP VOLCAR TmCAR (to liquid/ TmCLA (L + V = number type Abundance Setting Shape n (vol %) (vol %) (C) vapor) (C) L [V])

V2A/CO28 CO2-CH4 Relatively Small Negative 30 1535 59.4 to 24.6 to common clusters, crystal, 56.8 11.6 (L)

trails irregular

CO2-CH4- Common Small Negative 37 1560 1090 62.5 to 22.9 to 9.2 to 230297H2O-NaCl clusters, crystal 56.6 9.7 (L) 13.1 (L, V)

trails

V2A/CO17 CO2-CH4 Common Small Negative 22 3040 56.9 to 25.8 to clusters, crystal, 62.1 10.1 (L)

trails irregular

V2B/CO18 CO2-CH4 Common Trails Irregular, 49 2050 62.2 to 24.0 to negative 57.5 13.6 (L)crystal

CO2-CH4 Rare Trails Irregular 7 3050 5090 60.4 to 2.8 to 0.5H2O-NaCl 58.9 7.0 (L)

V2B/CO9 CO2-CH4 Common Small Negative 20 2040 60.8 to 15.1 toclusters, crystal, 59.8 5.1 (L)

trails irregular

V3A/CO10 CO2-CH4 Common Trails Irregular 51 1555 63 to 59 40.6 to 10.1 (L)

CO2-CH4 Rare Trails Irregular 20 1550 3090 62.2 to 29.9 to 8.7 to 9.0 250280 H2O-NaCl 59.0 5.9 (L) (L, V)

V3A/CO13 CO2-CH4 Common Trails Irregular 20 2040 65.5 49.3 to 59.9 0.7

V3B/CO1 CO2-CH4 Common Small Negative 17 2050 63.2 to 21.1 to clusters, crystal, 66.9 50.8 (L)

trails irregular

V3B/CO2 CO2-CH4 Common Broad Negative 53 2045 67.7 to 49.4 to trails, crystal, 60.7 7.1 (L)

cluster irregular

Abbreviations: n = number of measurements, ThCAR = homogenization temperature of the carbonic phase, ThTOT = total homogenization temperature,TmCAR = melting temperature of the carbonic phase, TmCLA = melting temperature of the clathrate, VOLCAR = optical volume percent estimate of the car-bonic phase (SupercriticalCAR/SupercriticalCAR + LiquidH2O), VOLVAP = optical volume percent estimate of the carbonic vapor phase (VaporCAR/VaporCAR +LiquidCAR)

and NaCl (e.g., Borisenko, 1977; Crawford, 1981). Final icemelting is bimodal, and ice in type 4 fluid inclusions in theearlier veins (V2A, V2B, and V3A) melts at around 22C,and that in the young V3C veins melts at around 31C (Table3 and Fig. 11). Homogenization temperatures (liquid + vapor= liquid [L + V = L]) in all veins are low and scatter between40 and 160C in most inclusions. Halite, which is typicallypresent in aqueous inclusions in V3C veins but is absent inany earlier veins, dissolves between 165 and 300C. Ramananalyses on daughter minerals in some type 4b fluid inclu-sions reveal the presence of ferropyrosmallite next to halite(Fig. 12; Dong and Pollard, 1997).

Discussion

Relative chronology and variation in fluid composition with time

The successive development of multiple generations offluid inclusions is characteristic of minerals in most metamor-phic and deformed terranes (Roedder, 1984; Crawford andHollister, 1986; Mullis, 1987). In particular, the reportedstructural reactivations of the Cadillac tectonic zone (Thomp-son, 1948; Robert, 1989; Neumayr et al., 2000) indicate thepotential for multiple fluid generations and processes. There-fore, the determination of the trapping sequences of inclu-sions is critical to an understanding of fluid evolution. The

HYDROTHERMAL FLUID EVOLUTION, CADILLAC TECTONIC ZONE, ABITIBI 1213

CO2

N2 CH4

V2B qtz-tm veinV3A qtz-tm veinV3B qtz-tm vein

FIG. 7. Composition of the carbonic phase in type 1 inclusions from dif-ferent samples at Orenada 2 as determined by laser-Raman analyses. qtz =quartz, tm = tourmaline.

0

10000

20000

30000

40000

50000

60000

Inte

nsity

(a.u.

)

1000 1500 2000 2500 3000 3500Wavenumber (cm-1)

9000

12500

2800 3000

CH4C2H6C3H8

0

2000

4000

6000

8000

10000

12000

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16000

1000 1500 2000 2500 3000 3500Wavenumber (cm-1)

Inte

nsity

(a.u.

)

1352

0

6000

1400

CO2

CO2

1250

1577

Gra

phite

D-p

eak

GraphiteG-Peak

Graphitesecond order

2702

A

B

FIG. 8. Raman spectra. A. Type 1 inclusion showing CO2 and graphitepeaks (sample CO18, V2B vein), B. Type 3 inclusion showing CH4, C3H8, andC2H6 peaks (sample CO10, V3A vein).

-60

-50

-40

-30

-20

-10

0

10

20

30

ThCA

R (C

)

-69 -67 -65 -63 -61 -59 -57TmCAR (C)

LSG

X CH4

=0.

2

X CH4

=0.

3

X CH4

=0.

1

X CH4

=0.

5

critic

al cu

rve

VCAR=45cm

3

V CAR=50c

m3VCAR

=60cm3VCAR

=70cm3

XCH4=0.7

V2B qtz-tm veinV3A qtz-tm vein

V2A qtz vein old

young

A

0

5

9

Coun

t-63 -60 -57

TmCAR (C)0

3

Coun

t

-63 -60 -57TmCAR (C)

0

6

Coun

t

-63 -60 -57TmCAR (C)

B C DV2A V2B V3A

V3A vein, N 2-bearing

CO2-CH4-H2O-NaCl

FIG. 9. A. TmCAR vs. ThCAR of H2O-bearing type 1 fluid inclusions at Ore-nada 2 divided by vein type. Contours of XCH4 and VCAR, critical curve andliquid-solid-gas (LSG) univariant curve constructed assuming homogeniza-tion to the liquid from figures in Thiry et al. (1994). Note the clustering byvein type with respect to XCH4 and the increase in XCH4 from the oldest to theyoungest quartz veins. B. Histogram of TmCAR of H2O-bearing type 1 fluidinclusions in the V2A quartz vein. C. Histogram of TmCAR of H2O-bearingtype 1 fluid inclusions in the V2B quartz vein. C. Histogram of TmCAR ofH2O-bearing type 1 fluid inclusions in the V3A quartz vein. qtz = quartz, tm= tourmaline.

1214 NEUMAYR AND HAGEMANN

0

1

2

3

4

5

6

7

8

9

Coun

t

0 10 20 30Wt% NaCl

B

0

2

4

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8

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12

Coun

t

-10 -5 0 5 10 15TmCLA

A V2AMean=6.45.4

0

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6

Coun

t

-10 -5 0 5 10 15TmCLA

0

1

2

3

4

5

6

7

Coun

t

0 10 20 30Wt% NaCl

C DV3AMean=-1.25.7

V2A

Mean=7.26.2

V3A

Mean=16.26.0

FIG. 10. Histograms of TmCLA C and wt % NaCl for H2O-bearing type 1 fluid inclusions. A. TmCLA in V2A quartz vein.B. Wt % NaCl in V2A vein. C. TmCLA in V3A vein. D. Wt % NaCl in V3A vein. Note that the salinity increases statisticallyfrom fluid inclusions in V2A (older) to those in V3A (younger) veins. Salinity data have been determined using the methodsdescribed in Diamond (1992). Statistics are mean 1 calculated with the computer program Statview 4.02.

TABLE 3. Summary of Microthermometric Data of Aqueous Fluid Inclusions in Four Vein Types in the Cadillac Tectonic Zone at Orenada 2

Vein type/ Fluidsample inclusion VOLVAP Th (C) Salinitynumber type Abundance Setting Shape n (Vol %) Tmice (C) TmS (L + V = L) (wt %)

V2A/CO28 (4a) H2O- Rare Trails Irregular 20 1015 28.8 to 3 7090 4.822.6 NaCl-CaCl2 (NaCl equiv)

V2B/CO18 (4a) H2O- Rare Trails Irregular 12 1020 32 to 17 65195 20.222.1 NaCl-CaCl2 (NaCl equiv)

22.8 26.3 (CaCl2 equiv)

V3A/CO10 (4a) H2O- Rare Trails Irregular 8 1045 22.5 to 20.8 51168 22.223.0 NaCl-CaCl2 (CaCl2 equiv)

V3C/CO44 (4a) H2O- Common Trails, Irregular 14 1020 37.5 to 21.2 52155 22.427.5 NaCl-CaCl2 cluster (CaCl2 equiv)

(4b) H2O- Common Trails, Irregular 19 1020 33 to 27.6 165320 75149 25.026.5 NaCl-CaCl2- cluster (CaCl2 equiv)

dm

Salinity calculated as wt % NaCl equiv for Tmice > 20.5C and as wt % CaCl2 equiv for 50C < Tmice < 20.5CAbbreviations: n = number of measurements, Tmice = melting temperature of the water ice, TmS = dissolution temperature of daughter mineral (halite),

VOLVAP = optical volume percent estimate of the vapor phase (Vapor/liquid + vapor)

relative chronology of fluid inclusions has been determinedon two scales by use of the relative structural timing of thequartz veins and by using the crosscutting relationshipsamong different trails of fluid inclusions and of trails andquartz vein textures (e.g., bands of recrystallized quartz).Type 1 fluid inclusions occur in pseudosecondary trails inter-nal to quartz grains (i.e., the trails do not crosscut quartz grainboundaries) and in clusters, and they are interpreted to have

formed synchronously with the vein. Fluid inclusion types 1,2, and 3 in V2A, V2B and V3A veins are crosscut by bands ofrecrystallized quartz and, therefore, predate the formation ofthese bands (Fig. 13A). Type 4a and 4b fluid inclusion trailscrosscut trails of types 1, 2, and 3 fluid inclusions and thebands of recrystallized quartz. They are the only fluid inclu-sion type in the structurally latest V3C quartz veins, andtherefore they are the texturally youngest fluid inclusion gen-eration (Fig. 13A, B, C). On the basis of these textural crite-ria, two stages of fluid evolution can be distinguished.

Stage 1: The first fluid stage consists of type 1 CO2-CH4 H2O NaCl inclusions in V2A, V2B, V3A, and V3B quartzveins, and type 2 CO2-CH4-N2 H2O inclusions and type 3 CH4 C2H6 C3H8 inclusions in V3A quartz veins. The hydrother-mal fluid evolved progressively from that trapped in V2A, V2B,and V3A to the fluid trapped in V3B quartz veins. The oldestfluid trapped during stage 1 is developed in V2A veins (oldestveins recorded), represented by H2O-bearing and H2O-absenttype 1 CO2-CH4 H2O NaCl fluid inclusions. The abundanceof H2O in the carbonic inclusions, as well as the extremely vari-able volume proportions of the carbonic phase, are unique totype 1 fluids trapped in the V2A vein system.

The next younger fluids are represented by H2O-absent,type 1, CO2-CH4 inclusions and rare type 1, CO2-CH4-H2O-NaCl inclusions in the V2B and even younger V3A quartzveins. The hydrothermal fluid in these veins evolved to thattrapped in CO2-CH4 inclusion trails in undeformed V3Bquartz veins (Fig. 13B). These veins postdate the deformationthat affected V2A, V2B, and V3A veins, and fluids trapped inV3B quartz veins postdate fluids trapped in V2A, V2B, and

HYDROTHERMAL FLUID EVOLUTION, CADILLAC TECTONIC ZONE, ABITIBI 1215

-40 0-30 -20 -10

n=65

H2O-NaCl-CaCl2

Tmice (C)

V2A (-19.4/6.7/20)V2B (-22.4/3.9/11)V3A (-21.9/0.6/8)V3C (-31.3/2.7/26)

Mean/1/n

5

10

A

n

50 200

n=49

100Th(L+V=L) (C)

4

8

n

B Mean/1/nV2A: 115/41/11V2B: 106/40/10V3A: 116/52/3V3C: 114/29/25

150

n=18

200 250 300Tms (C)

24

n

C Mean/1/n243/41/18

320

FIG. 11. Histograms of Tmice, ThTOT, and Tms (halite) for type 4 fluid inclusions divided by the different vein types.

0

1000

3000

5000

7000

Inte

nsity

(a.u.

)

100 300 500 700 900 1100

615 1025326

Wavenumber (cm-1)FIG. 12. Raman spectrum of a daughter mineral in a type 4b fluid inclu-

sion (sample CO2, V3B vein). The characteristic peaks at wave numbers 326,615, and 1,025 cm1 indicate ferropyrosmalite (i.e., iron-rich end member ofthe pyrosmalite series (Fe,Mn)8Si6O15(OH,Cl)10; e.g., Dong and Pollard,1997). Note that the other peaks arise from the quartz host.

V3A quartz veins. The timing of type 2 and 3 fluids withinstage 1 is not quite clear, but on the basis of crosscutting re-lationships fluids probably formed toward the end of stage 1.

Stage 2: H2O-NaCl-CaCl2 type 4a and 4b fluids, which arethe only fluid inclusion type in the youngest vein set (V3Cveins), are chemically and physically distinct from the car-bonic-dominated fluids and, therefore, represent a differenthydrothermal fluid system (Fig. 13).

Microthermometric data indicate a progressive change inthe fluid composition (i.e., increasing CH4 content: Figs. 6, 9,13) from oldest CO2-H2O-NaCl CH4 and CO2-CH4 H2Ofluids (V2A veins), which predate or are synchronous with de-formation (V2B and V3A veins) to younger CO2-CH4 H2Odominated fluids that postdate the main deformation (V3Bveins) during stage 1. The progressive change in the carbonic

fluid chemistry may be explained in three ways: postentrap-ment modifications of the fluid inclusions; processes duringfluid entrapment, such as phase separation of a CO2-CH4 H2O NaCl fluid and subsequent heterogeneous trapping ofthe unmixed fluids; or compositional variations in the fluidsowing to differences in the fluid source or different degreesof reaction with the wall rocks.

Potential postentrapment modifications

Any model that explains the variation in fluid compositionwithin the Cadillac tectonic zone must take into considerationthe following points: (1) only the oldest veins (V2A) containabundant mixed carbonic-aqueous fluids, whereas H2O-richfluids within V2B, V3A, and V3B veins are rare; (2) carbonicfluids show a distinct increase in XCH4 from the oldest (V2A)

1216 NEUMAYR AND HAGEMANN

Pseudosecondary trails ofType 1 inclusions

Bands of recrystallizedquartz

Trail ofType 4a, binclusions

V2A

V2B

V3A

Cluster of Type 1 inclu-sions

V3B

Cluster and pseudo-secondary trails of Type4a,b inclusions

V3C

Quartz veinXCH4

0

0.7

0

0.7

0

0.7

0

0.7Trail ofType 4a, binclusions

A

B

C

FIG. 13. Sketches illustrating the relative timing relationships of fluid inclusion types and quartz textures. A. Pseudosec-ondary trails of type 1 fluid inclusions are parallel to the stretching of quartz grains and are truncated by bands of recrystal-lized quartz. This relationship implies that the deformation that caused the recrystallized quartz grains postdated type 1 flu-ids. Type 4 fluids crosscut and postdate both type 1 fluid inclusion trails and bands of recrystallized quartz. The XCH4 increasesfrom V2A to V2B to V3A veins in type 1 fluid inclusions. B. Largely undeformed quartz grains in V3B veins host clusters oftype 1 CO2-CH4 fluid inclusions. These clusters are crosscut by trails of type 4 fluid inclusions. Type 1 fluid inclusions in V2Bveins contain the highest XCH4. C. V3C veins host trails and clusters of type 4a and b fluid inclusions only.

to the younger veins (V3B); (3) the molar volume of the car-bonic phase increases (density decreases) typically from theoldest veins (V2A) to the younger veins (V3B); and (4) molarvolume variations in individual veins are limited, except in theV3A veins. Below we try to demonstrate that fluid inclusionproperties relate largely to primary features during trappingand were not caused by postentrapment modifications.

Variations in the CO2/H2O ratio in fluid inclusions may beexplained by a number of postentrapment modificationprocesses, which include H2O and H2 diffusion and/or pref-erential leakage of H2O, owing either to wicking of H2O alongmicrofractures or grain boundaries or to recrystallization(e.g., Crawford and Hollister, 1986; Bodnar et al., 1989; Hol-lister, 1990; Hall et al., 1991; Johnson and Hollister, 1995;Klemd et al., 1995).

Diffusion of H2O is typically directed out of the fluid inclu-sion during retrograde metamorphic processes. The H2O fu-gacity at grain boundaries in a rock, where retrograde hydra-tion reactions take place without the influx of externallyderived fluids, is buffered to low values (Yardley, 1981) andgenerally results in higher CO2/H2O ratios and higher salini-ties in the modified fluid inclusions. The diffusion of H2O isundoubtedly greater over longer time intervals and is mostlikely more important at higher temperatures (e.g., Ridley andHagemann, 1999). Fluid inclusions that formed in quartzveins synchronous with D2 deformation are, therefore, morelikely to be modified than those that formed in veins synchro-nous with D3 deformation. However, V2A veins at Orenada 2contain abundant mixed aqueous-carbonic inclusions, whereasstructurally younger veins (e.g., V3B veins) contain few inclu-sions of this type. Salinities are, in general, lower in fluid in-clusions trapped in D2- than in D3-controlled veins (Fig. 10).Both trends are at odds with that expected from H2O diffusionprocesses of an initially homogeneous suite of inclusions.

Hydrogen diffusion into the fluid inclusion most likelytakes place at elevated temperatures (>400450C: Ridleyand Hagemann, 1999). On the basis of arsenopyrite and chlo-rite thermometry, quartz veins at Orenada 2 formed belowthese temperatures. In addition, H2 diffusion would triggerreactions between the most common fluid species (H2O,CO2, CH4) such as these:

CO2 + 4 H2 = CH4 + 2 H2O, or (1)

CO2 + 2 H2 = C + 2 H2O. (2)

Although both reactions can explain the presence of H2O inthe oldest veins, they fail to explain increasing CH4/CO2 ratiosin progressively younger veins. Therefore, H2 diffusion is con-sidered unimportant in the samples studied.

Deformational microfracturing and subsequent H2O lossfrom the fluid inclusion could have modified the fluid inclu-sions after trapping. Carbonic inclusions are also likely to bedragged along with migrating quartz grain boundaries owingto their different wetting characteristics (e.g., Johnson andHollister, 1995), as has also been interpreted in some de-formed quartz grains in this study (Fig. 4C). However, theseprocesses are most likely to modify fluid inclusions in theveins that have been trapped during the D2 and overprintedby the D3 deformation event. Yet, these veins contain themost abundant mixed aqueous-carbonic inclusions.

The size of carbonic inclusions within the younger veins(V2B, V3A, V3B) does not show any correlation with ThCAR(Fig. 14). Because larger inclusions are more likely to bemodified than smaller inclusions (Bodnar et al., 1989), thelack of any correlation in these data suggests that modificationof most carbonic inclusions was unlikely.

Large variations in the molar volume of the carbonic phase(e.g., to 40 cm3 for V3A vein; see Fig. 6) of similar fluid inclu-sions in one sample may indicate modification of the density ofat least some fluid inclusions. However, the majority of car-bonic fluid inclusions shows a range in molar volume of

modification of these inclusions or, more likely, by fluid pres-sure fluctuations during entrapment (e.g., seismic deforma-tion: Robert et al., 1995).

Pressure-temperature conditions of fluid entrapment

Arsenopyrite associated with a V3A shear vein and chloritein the matrix of hydrothermal breccias are used to determineformation temperatures for the ore minerals and alterationproducts. These data provide additional temperature con-straints on vein formation and assist with estimates of trappingpressure and temperature for fluids that have not undergonephase immiscibility. The isochores show a distinct variation in

slope from the relatively oldest to the youngest fluid inclu-sions; this variation is used to estimate the shape of the pres-sure-temperature path.

The composition of arsenopyrite that is related to gold inV2B and V3A veins indicates mineralization temperatures of300 to 450C. The compositions of late chlorite in brecciatedV3A veins indicate temperatures of 320 to 330C. The iso-chores of type 1 carbonic-aqueous fluids in V2A veins aresteepest, whereas isochores of type 1 fluid inclusions in V2B,V3A, and V3B veins are flatter (Fig. 15). In combination, theisochores indicate that V2A vein deposits formed at higherpressures and temperatures than V2B, V3A, and V3B vein

1218 NEUMAYR AND HAGEMANN

0.5

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ar)

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A

C

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ar)

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B

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F

V2A

V2BV3A

V3B

H2O-NaCl-CaCl2

ApyChl

Halite dissolution

E

H2O-NaCl-CaCl2-dm

Type 1 CO2-CH4-H2O-NaClV2A vein

Type 1 CO2-CH4V2B vein

Type 1 CO2-CH4-H2O-NaClV3A vein

Type 1 CO2-CH4V3B vein

Type 4 H2O-NaClV3C vein

FIG. 15. Isochores for type 1 and type 4 fluid inclusions at Orenada 2 for five vein types using the MacFlinCor computersoftware of Brown and Hagemann (1995) and the equations of state of Kerrick and Jacobs (1981) and Bodnar and Vityk(1994). Notched box plots show medians, 2 confidence of median, quartile spread and upper and lower fences. A. Type 1CO2-CH4-H2O-NaCl and CO2-CH4 H2O fluid inclusions in V2A quartz vein. B. Type 1 CO2-CH4 fluid inclusions in V2Bquartz veins. C. Type 1 CO2-CH4-H2O-NaCl and CO2-CH4 H2O fluid inclusions in V3A quartz vein. D. Type 1 CO2-CH4fluid inclusions in V3B quartz veins. E. Type 4 H2O-NaCl-CaCl2 fluid inclusions in V3C quartz veins. Note that the isochoresin E have been calculated assuming H2O-NaCl compositions. H2O-CaCl2 compositions would result in marginally steeperisochores. F. Summary plot of average isochores depicted in A to E. Apy (arsenopyrite) and Chl (chlorite) indicate the rangeof temperatures derived from arsenopyrite and chlorite thermometry, respectively.

deposits. However, a pressure of >4 kbar at the upper tem-perature limit of 450C recorded by arsenopyrite (Fig. 15) isfar too high for the crustal level presently exposed in theAbitibi greenstone belt (cf. Carmichael, 1991). Higher pres-sures may be at least partly explained by pressure fluctuationsfrom hydrostatic to supralithostatic during trapping and pro-gressive healing of fractures (cf. Robert et al., 1995).

Isochores for saline aqueous fluid-inclusions (type 4a, 4b)are steeper than isochores of any of the carbonic-aqueous flu-ids. The homogenization temperatures (L + V = L) of thesefluids are low, between 40 and 200C, and all inclusionsoccur either in late trails crosscutting all other fluid trails andbands of recrystallized quartz or in the late V3C quartz veinsets. Locally, halite daughter minerals in V3C veins type 4binclusions, which are structurally unrelated to the evolution oftype 1 carbonic-aqueous fluid inclusions, dissolve at relativelyhigh temperature between 165 and 300C. The chemicalcomposition and ThTOT of these fluid inclusions are similar tothose of fluid inclusions that are interpreted to represent latebasement brine fluids unrelated to gold mineralization, as de-scribed in several gold deposits in the Abitibi (e.g., Boullier etal., 1998). Therefore, type 4a and 4b inclusions in the Cadil-lac tectonic zone are interpreted to have formed during thelate stages of fault evolution (i.e., post-D3) or during a late-Archean or Proterozoic fault reactivation. Pressures and tem-peratures of trapping of these inclusions are poorly con-strained owing to the lack of external (i.e., unrelated to fluidinclusions) temperature estimates, and only the high dissolu-tion temperatures (165300C) of halite may indicate ele-vated trapping temperatures (Fig. 15)

Fluid evolution

As determined from structural and crosscutting relation-ships, hydrothermal quartz veins formed progressively fromV2A (oldest) to V3B (youngest) quartz veins. The oldest veinscontain mixed aqueous-carbonic inclusions (type 1) with vari-able CO2/H2O ratios (volume of carbonic vapor ranges from1090%) within individual samples but also within individualfluid inclusion trails. Total homogenization temperatures (tothe liquid and vapor) within individual trails range from 260to 300C. Together these observations are taken as evidenceof phase separation. However, the most abundant fluid inclu-sions are type 1 CO2-CH4. Because the vapor-rich phase risesmore quickly than the H2O-rich phase within the fluid con-duit, phase separation probably took place structurally belowthe level of trapping, and a higher proportion of CO2-CH4rich inclusions were trapped in the veins.

The next younger vein sets (V2B, V3A, V3B) contain largelycarbonic inclusions, and CH4 content increases from theolder V2B to the younger V3B inclusions. Some fluid inclu-sions contain a small quantity of microscopically visible H2Oand most of the carbonic inclusions may also contain someH2O. Therefore, the carbonic-rich fluids in the V2B, V3A,and V3C veins might represent the unmixed carbonic-richpart of an immiscible CO2-H2O fluid. The variable, but on av-erage high, salinity in the carbonic-aqueous inclusions in theV3A vein is consistent with extensive phase separation andpreferential vapor loss (e.g., Trommsdorff and Skippen,1986). Type 1 fluid inclusions in V2B and V3A veins, whichcontain gold and are spatially related to gold in the adjacent

wall rock (Robert, 1989), are considered to have trapped themain ore fluids at Orenada 2.

The increase in CH4 from the older to the younger veinsmay be explained by increasing interaction of the fluids withsedimentary rocks, by fO2-buffered cooling of a mixed car-bonic-aqueous fluid, or by a change in the fluid source (e.g.,increasing CH4 input from mantle outgassing). Within thePontiac sedimentary province mainly turbiditic wacke are lo-cally intercalated with graphitic schists and mudstones(Desrochers and Hubert, 1996; Mortensen and Card, 1993),which could be suitable sources for CH4 if the hydrothermalfluids that ascended along the Cadillac tectonic zone reactedwith these rocks.

On the other hand, in an ascending mixed carbonic-aque-ous fluid, the CH4 content may be controlled by redox reac-tions involving C, CO2, and CH4 (Ohmoto and Kerrick, 1977).Overall, these reactions tend to favor the formation of CH4 atlower temperatures, if a constant XCAR (0.1 < XCAR < 0.9) of afreely circulating fluid is assumed (cf. fig. 10 of Ridley andHagemann, 1999). Simple cooling over a temperature inter-val of about 150C along a fO2 buffer will increase theCH4/CO2 ratio by at least a factor of 100. Likewise, a decreasein fO2 during cooling would increase the abundance of CH4relative to CO2 (e.g., Ohmoto and Kerrick, 1977; Hollaway,1981; Ridley and Hagemann, 1999).

Methane can also be derived from outgassing of juvenileCH4 from the mantle or by synthesis in inorganic reactions athigh temperatures (>300 to 400C) involving CO2 and H2 orother C-H molecules that may be derived from varioussources (Welhan, 1988). Both mantle CH4 and inorganic syn-thesis are reasonable sources of CH4 in a transcrustal shearzone. However, the extent of their contribution cannot be as-sessed with the present data set.

The increase in fluid interaction with reduced carbon inmetasedimentary rocks is the preferred interpretation here.In fact, the presence of N2 in type 2 carbonic inclusions witha nitrogen component and higher hydrocarbons in type 3 in-clusions may indicate increasing interaction of the fluids withreduced carbon during the later stages of the fault evolution.This notion is consistent with recent 15N data from severalorogenic lode gold deposits of the Superior province that in-dicate metamorphic dewatering of volcanic and siliciclasticsedimentary rocks as the N2 source (Jia and Kerrich, 1999).

The youngest fluids in the Cadillac tectonic zone are aque-ous brines with NaCl and CaCl2 as the dominant salt species.Because these fluid inclusions are characterized by low ThTOT(

(e.g., Robert and Brown, 1986; Burrows and Spooner, 1987;Robert and Kelly, 1987; Spooner et al., 1989; Robert et al.,1995; Matthai and Garofalo, 1999). Gold is hosted in second-and third-order, steep-reverse shear zones and subhorizontalextensional veins (Robert and Brown, 1986), similar to theveins in the Cadillac tectonic zone at Orenada 2. Because theSigma-Lamaque hydrothermal system is located within 5 kmof the Cadillac tectonic zone at Orenada 2, it is the ideal sys-tem to use to compare fluids in second- and third-order shearzones with those in the first-order fault.

For this comparison it is imperative to identify quartz veinsand hydrothermal fluids of similar timing. At Sigma, steeplydipping fault veins are crosscut by subhorizontal extensionalveins; both contain quartz-tourmaline and are interpreted tobe of D2 timing. Structurally, these fault veins are analogousto unmineralized V2A veins of D2 timing and mineralizedV2B veins of late D2 to early D3 timing in the Cadillac tec-tonic zone (Table 4). As D2 to D3 deformation is progressivein the Cadillac tectonic zone (Neumayr et al., 2000), it is rea-sonable to suggest that V2A and V2B quartz veins at Orenada2 are comparable to steeply dipping and subhorizontal veinsat Sigma. Mineralized V3A quartz-tourmaline veins in theCadillac tectonic zone are absent at Sigma. These veins are ofD3 timing in the Cadillac tectonic zone and probably post-date the formation of mineralized veins at Sigma. The V3Bveins in the Cadillac tectonic zone postdate both arsenopyritemineralization and D3 deformation.

Robert and Kelly (1987) and Robert et al. (1995) recog-nized three main fluid inclusion types with variable abun-dances in different vein types at Sigma: CO2 inclusions, H2O-CO2 inclusions, and H2O-NaCl-CaCl2 inclusions (Table 4).All three types occur in secondary trails and clusters relatedto late brittle fracturing of the quartz. These three fluid typesalso occur in the Cadillac tectonic zone (Table 4). Where H2Ois present, however, the Sigma fluids have a lower salinity(14 wt % NaCl equiv for the starting fluid and 010 wt %NaCl equiv after unmixing: Robert and Kelly, 1987) as com-pared with the Cadillac tectonic zone fluids (to 15.3 wt %NaCl equiv for V2A veins: Table 4). The CH4 content of type1 carbonic aqueous fluids in V2A and V2B veins in theCadillac tectonic zone is similar to that of carbonic-aqueousfluids at Sigma (Table 4).

Fluids in V3A and V3B veins in the Cadillac tectonic zone,which have no documented structural equivalent at Sigma,are clearly different: they have a higher salinity (3.823.5 wt% NaCl equiv) and higher CH4 content (XCH4 to 0.75). In ad-dition, N2, C2H4, and C3H6 have not been identified in Sigmafluids (e.g., Robert and Kelly, 1987; Robert et al., 1995; Boul-lier et al., 1998).

Within the aqueous fluids at Sigma, two genetic types aredistinguished: Th 180C and Th < 180C (Robert et al., 1995;Boullier et al., 1998). The higher-temperature fluids are inter-preted to represent the unmixed end member of a carbonic-aqueous fluid related to gold mineralization, whereas thelower-temperature fluids are interpreted to postdate gold min-eralization. In the Cadillac tectonic zone, similar aqueous flu-ids have Th

HYDROTHERMAL FLUID EVOLUTION, CADILLAC TECTONIC ZONE, ABITIBI 1221Ta

ble

4. S

ynth

esis

of C

ritic

al M

icro

ther

mom

etri

c F

luid

Inc

lusi

on D

ata

from

Hyd

roth

erm

al V

eins

in th

e C

adill

ac T

ecto

nic

Zone

at O

rena

da 2

and

the

Sigm

a M

ine

Cad

illac

tect

onic

zon

e at

Ore

nada

21

Sigm

a m

ine2

CO

2-C

H4,

CO

2-C

H4-

H2O

-NaC

l,H

2O-N

aCl-C

aCl 2

dm

3

Vein

type

type

1a

type

1b

type

4a,

4b

Vein

type

CO

2H

2O-C

O2

H2O

-NaC

l-CaC

l 2

V2A

A

bund

ance

: 28%

Abu

ndan

ce: 5

5%A

bund

ance

: 17%

shea

r ve

inT

mC

AR

= 5

9.4

to

56.8

CVo

l % C

AR

= 1

0 to

90

Tm

ice

= 2

8.8

to

3C

Th C

AR

= 2

4.6

to 1

1.6

CT

mC

AR

= 6

2.5

to

56.6

CT

h =

70

to 1

90C

Th C

AR

= 2

2.9

to 9

.7C

Salin

ity =

4.8

to 2

2.6

wt %

T

mC

LA

= 9

.2

to 1

3.1

NaC

l equ

ivSa

linity

= 0

.6 to

15.

3 w

t %

23 to

5.4

wt %

CaC

l 2N

aCl e

quiv

equi

v

V2B

A

bund

ance

: 80%

Abu

ndan

ce: 1

0%A

bund

ance

: 10%

Fau

lt ve

inA

bund

ance

: 75%

Abu

ndan

ce: 1

0%A

bund

ance

: 15%

shea

r ve

inT

mC

AR

= 6

2.2

to

57.5

C

Vol C

AR

(vol

%)

= 50

90

Tm

ice

= 3

2to

17

CT

mC

AR

= 5

8.1

to

57C

Vol C

AR

(vol

%)

= 20

to 8

5T

mic

e=

46.

4to

3.

2C

Th C

AR

= 2

4to

13.

6C

Tm

CA

R=

60.

4to

58

.9C

Th

= 65

to

195

CT

h CA

R=

27.

5to

+28

.2C

Tm

CA

R =

58

.3

to

56.8

CT

h

20.

5C

and

as

wt %

CaC

l 2eq

uiv

for

50

C