Multiple-stage diagenetic alteration and fluid history of Ordovician carbonate-hosted barite mineralization, Southern Quebec Appalachians

Sedimentary Geology 107 (1996) 121-139

Multiple-stage diagenetic alteration and fluid history of Ordovician carbonate-hosted bar&e mineralization, Southern Quebec Appalachians Suzanne Paradis *, Denis Lavoie Centre Gkoscientijque de Qukbec, Geological Survey of Canada,

2535 Boulevard Laurie< Ste-Fey, Que. GIV 4C7, Canada

Received 23 February 1995; accepted 16 April 1996

Abstract Lower Ordovician bioclastic limestone of the Upton Group, southern Quebec Appalachians, hosts stratabound Ba-ZnPb mineralization. The Upton Group, a mixed platform carbonate-siliciclastic-volcanic succession, is exposed as windows within the tectonically overlying Cambrian siliciclastics of the Granby Nappe. Mineralization consists mostly of barite and minor amounts of sulfides (sphalerite, pyrite, galena, and chalcopyrite), in addition to calcite, quartz and bitumen cements. It is hosted by a bioclastic limestone which is interbedded with and capped by a black calcareous shale, and underlain by a mudstone-siltstone-volcanic succession and a lower poorly fossiliferous limestone. The lower limestone recorded early extensive dolomitization followed by meteoric alteration (dedolomitization, sulphate dissolution, vadose cements, soil pisoids, etc.), and burial diagenesis (recrystallization, fracturation, and cementation). The vadose gravitational calcite cements yield 8r80poa values of -8.4 to -11.0%0 and Si3Cpea values of +2.4 to +2.8%0. The thin soil profiles with pisoids have a Si’Opna value of -8.2%0 and a Sr3C~na value of +2.0%0. These data suggest an evaporitive “O-enrichment of near-surface trapped soil moisture (vadose water) in a rock-dominated diagenetic system. The recrystallized limestone has S’sOpna values of -11.4 to -15.5%0 and near Early Ordovician marine 6i3Croa values of -0.2 to +2.5%0. These data suggest a final stabilization of the limestone from high temperature fluids in a rock-dominated diagenetic system. The mineralized bioclastic limestone shows rare evidence of early submarine cementation which is overprinted by significant post-depositional recrystallization and hydrothermal alteration. The latter resulted in the generation of secondary porosity and precipitation of a subhedral barite cement, a bladed barite cement, and fracture-filling barite. Fractureand void-filling calcite, sulfides, quartz and bitumen cementation followed barite mineralization. Pre-barite syntaxial calcite overgrowths on crinoids yield 613Croa values of -3.9 to -15.0%0 and Si*Opoa values of -13.7 to -14.8%0. Post-barite sparry calcite cement and fracture-filling calcite have 6r3Cpas values of -2.6 to -13.0%0 and -2.4 to -17.%0, respectively, and Sr80poa values of -13.6 to -14.2%0 and - 14.0 to -15.8%0, respectively. The 6’*0 values suggest relatively high-temperature re-equilibration in a deep-burial environment. The variable and depleted i3C values appear to reflect fluid-rock interaction and addition of significant S13C-depleted CO2 from thermochemical sulphate reduction of organic matter. Mixing of reduced, hot basinal brines with oxidizing sulphate-rich fluids resulted in barite precipitation.

* Corresponding author. Present address: Geological Survey of Canada, P.O. Box 6000, Sidney, B.C. V8L 4B2, Canada. 0037-0738/%/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PI1 SOO37-0738(96)0,0025-S

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1. Introduction Early Ordovician carbonates of the Upton Group, located 160 km SW of Quebec City, eastern Canada (Fig. 1) host vein- and breccia-type copper deposits and stratabound Ba-Zn-Pb mineralization (Fig. 2; Paradis et al., 1990). Several studies have focused on the mineralogy and genesis of the mineralization (Sassano and Procyshyn, 1988; Sassano and Scbrijver, 1989; Gauthier et al., 1989, 1994; Kumarapeli et al., 1990; Paradis et al., 1990), and the sedimentol-

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107 (1996) 121-139

ogy and diagenesis of the carbonates have been the subject of preliminary reports (Lavoie, 1990, 1992; Lavoie and Paradis, 1993). This paper documents the paragenetic relationship between the Ba-Zn-Pb mineralization and the host carbonate diagenesis, and outlines the geochemical characteristics (oxygen, carbon, and sulphur isotopes) of the carbonates and sulphides. The complex interplay between tectonic fracturing, fluid evolution, and carbonate diagenesis is outlined, indicating that minera1 and organic diagenesis and geochemistry of min-

WMMONDVILL

fl

LIMIT OF THE APPALACHIAN

4

THRUST

OROOVlCtAN DOMAIN

FAULT

PRECAMBRIAN

BASEMENT

Fig. 1. Location of the Upton Ba-Zn-Pb deposit within the Taconian orogen of the southern Quebec Appalachians (modified from St-Julien and Hubert, 1975).

S. Paradis, D. L.avoie/Sedimentary

0

Geology 107 (19%)

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123

Citadelle Formation

CAMBRO-ORDOVICIAN Upton Group

m

Basaltic lava, gabbro

F3,,Oo Graywacke, Limestonesiltstone, mudstone m Mudstone, graywacke, siltstone Granby Nappe t--l a

-

Graywacke Shale

Geological contact

Fig. 2. Geological map of the Upton-Acton Vale area, and location of the Upton deposit (after Paradis and Faure, 1994).

eralizing fluids were responsible for transformation of the cemented bioclastic limestone into a porous rock receptive to barite and sulphide deposition. 2. Methods Samples of the upper bioclastic limestone are from drill cores of the Upton deposit. Samples of the lower limestone and the black shale are from scattered outcrops in the Upton-Acton Vale area (Fig. 2), and from drill cores of the Upton deposit. Conventional and cathodoluminescence (CL) petrography was performed on more than 30 thin sections. Carbon and oxygen isotope analyses (Table 1) were carried out on calcite and dolomite crystals of various carbonate cements and fracture-filling calcite of the limestone, and on carbonate concretions of the black shale. Analyses were done at the OttawaCarleton Geoscience Centre. Samples, ranging from 0.2 to 0.5 mg, were drilled from polished rock slabs using drill bits 100 pm in diameter. The carbon and oxygen isotopes in carbonates were extracted as CO;!

by acid digestion of powdered samples at 25°C for calcite, and at 50°C for dolomite, and analysed using a VG MM602E double collector mass spectrometer. All the analyses were converted to PDB (Peedee belemnite) and corrected for 170 as described by Craig (1957). The precision of the 6L3C~B and 6l*Opoa values is 0.1 per mil(2a). Sulphur isotope analyses (Table 2) were done on barite, galena, pyrite, sphalerite, and chalcopyrite. The sulphur was converted to SO:! by direct combustion and analysed with the same mass spectrometer. The precision of the 6%co~ values is 0.2 per mil (20). 3. Geologic setting The Lower Ordovician Upton Group is located in the southern Quebec Appalachians (Fig. 1) and forms part of several thrust-bounded Taconian nappes stacked in the inverse order of their ages (St-Julien and Hubert, 1975). Uppermost is the Cambrian Granby Nappe which overlies Late Cambrian

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Geology 107 (1996) 121-139

Table 1 Range of oxygen and carbon isotope values of carbonate minerals 61*opoB (%0)

G’3Cpos (%0)

~“CSMOW

Upper limestone Syntaxial calcite cement Sparry calcite cement Fracture-filling cement

-13.7 to -14.8 -13.6 to -14.2 -14.0 to -15.8

-3.9 to -15.0 -2.6 to - 13.0 -2.4 to - 17.9

15.6 to 16.7 15.5 to 16.9 14.6 to 16.5

4 6 4

Lower limestone Meteoric vadose cement Soil pisoids Recrystallized limestone Fracture-filling calcite

- 8.4 to -11.0 - 8.2 -11.4 to -15.5 - 6.3 to -15.4

2.4 to 2.8 2.0 -0.2 to 2.5 0.5 to 3.2

19.5 to 22.2 1 14.9 to 19.1 15.0 to 24.3

3 5 17

Calcareous black shale Carbonate concretion

-11.6 to -15.3

-4.2 to -6.6

15.2 to 19.0

6

(%d

Number of analyses

Table 2 Range of sulphur isotope values of barite and sulphides of the Upton Ba-Zn-Pb deposit Mineral

Description

SYSCDT(%o)

Number of analyses

Barite Barite Barite Galena Pyrite Chalcopyrite Sphalerite

Bladed barite cement Fracture-filling Subhedral barite cement Fracture-filling Disseminated Fracture-filling Trains of grains along stylolites

28.3 28.6 29.8 1.7 4.2 5.1 2.9

11 3 4 4 2 4 1

to Middle Ordovician rocks such as the Upton Group (Globensky, 1978; Slivitsky and St-Julien, 1987). The Upton Group and the Granby Nappe are both folded (Fig. 2), and the tectonostratigraphic relationship between the two is problematic. The Upton Group has been interpreted as allochthonous blocks or olistoliths derived from the Ordovician platform of Laurentia (St-Julien and Hubert, 197.5; Beauprk, 1975; Globensky, 1978; Kumarapeli et al., 1988), or as exposures of an underlying tectonostratigraphic package, through erosional windows in the Granby Nappe (Baldwin, 1973; Sassano and Procyshyn, 1988; Lavoie, 1992). The presence of thrust faults between the Granby Nappe and the Upton Group (see Fig. 2), and similar strikes and dips for both units, support the second scenario. 4. Stratigraphy of the Upton group The 260 m-thick Upton Group consists of two limestone units, a lower mineralized (Cu) and poorly

to 34.0 to 32.2 to 31.7 to 9.7 and 7.8 to 9.6

fossiliferous limestone and an upper mineralized (Ba-Zn-Pb) bioclastic limestone, and a succession of siliciclastic, volcanic and intrusive rocks (Figs. 2 and 3). Several thrust faults occur within the Upton Group, thus some volcanic and siliciclastic successions could be tectonically repeated slices. An Early Ordovician age has been assigned to the Upton Group based on correlation with Lower Ordovician carbonate successions in southern Quebec (Beaupri, 1975; Lavoie, 1992), and on carbonate 87Sr/86Srinitial values (Kumarapeli et al., 1990). The lowermost unit of the Upton Group is a poorly fossiliferous sparry limestone that forms a 120 m-thick succession (Fig. 3). This limestone locally hosts vein- and breccia-type copper deposits (Paradis and Faure, 1994). The mudstone-siltstone succession consists of mm-thick laminae and cm-thick beds of massive and brecciated mudstone interlayered with cm-thick beds of cross-laminated dolomitic siltstone. The black calcareous shale is laminated and locally brecciated.

S. Paradis, D. Lavoie/Sedimentary Geology 107 (1996) 121-139

125

z < GRANBY NAPPE

- -

<

Green to red mudstone & shale

o

Greywacke

I

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I,-2o~)

:"i':'::":":':'::::

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Calcareous black shale

-

Baritio limestone (~<5% sulfides) --_--

(60-120m)

o

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._--_-_

~_-__-_

ol

_--_

_ _-_

.

Black to green mudstone & siltstone

_-_.

_

Volcanic rocks D ~"

0

,v v ,vvvvvvv', i t v K v A v ~ "*Jkv,i

Z

~\V

0

~.

I LOWER

LIJ

LIMESTONE

I

=~

t4. I

alcarenite

I

Calcilufite

'11 ' I I I , * I ?

I

UNIT

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o

\ ~

Copper veins and breccias

I

I !

-J p

i

•

•

Thrustfault

i

. , ,/ '. ,. ,.,

*

Carbonate concretions

Fig. 3. Schematic stratigraphic column of the Upton Group with location of the Ba-rich upper limestone unit. Note the location of the Granby Nappe.

Both the black shale and the mudstone-siltstone contain disseminated pyrite and abundant septaria-rich carbonate concretions. Volcanic rocks underlie and are interbedded with the mudstone-siltstone succession (Fig. 3). They consist mostly of massive basaltic flows and rare felsic and mafic tufts. Co-genetic mafic dikes and plutons intruded sediments of the Upton Group and the Granby Nappe. From drill hole information, the upper bioclastic limestone has a known minimum lateral extension of 600 m and a variable thickness of 0.5 m to 20 m. The black calcareous shale is interbedded with, and caps the limestone, and the mudstone-siltstone, t h e volcanic rocks, and the poorly fossiliferous limestone underlie it (Fig. 3). The bioclastic limestone consists of interspersed cm- to m-thick layers of massive to diffusely banded limestone. It has a secondary grainstone texture which is the result of recrystallization. Locally, the limestone contains up to 60 volume % barite and 1-5 volume % sulphides.

At the group-scale, the abundance of thrust planes, the lack of stratigraphic markers, and the absence of fine biostratigraphic control preclude clear definition of the spatial and temporal relationships between the upper and the lower limestones. 5. The lower limestone unit

A brief description of this unit is given in order to compare it with the upper bioclastic limestone. Details can be found in Lavoie (1990, 1992) and Lavoie and Paradis (1993). 5.1. Sedimentary facies and depositional model The lower limestone unit consists of a monotonous succession of massive sparry limestone (Fig. 4A). Microscopically, the limestone has a grainstone texture with poorly preserved small rhombs composed of various mixture of calcite and dolomite (Fig. 4B). Textural relationships indicate partial to near complete

126

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12/-139

Fig. 4. Lower limestone unit. (A) Massive sparry calcarenite with a secondary crystalline texture resulting of recrystallization. (B) Phot.omicrograph of recrystallized limestone (Rl) with a fenestral-like vug (Fe). Scale bar = 1 mm.

talc:itization of former dolomite (Lesperance, 1979; Lavde, 1992). The lower limestone is almost devoid ofa .llochems, with the rare exceptions of pelmatozoan plal:es, peloids, and micrite intraclasts.

Crinkly, laminar, peloidal (dolo)micrite showring mudcracks, laterally-linked stromatolite/thrombc Jite mounds, (dolo)micrite with fenestrae (Fig. 4LB), and dolomicrite with silica and/or sulphate ps;eu-

127

s. Paradis, D. Lavoie/Sedimentary Geology 107 (1996) 121-139

domorphs now calcite nodules having chicken wire texture, characterize the lower limestone unit. The lower limestone unit has been interpreted as a carbonate platform system with associated evaporites, a wide intertidal mud fiat, and shallow subtidal carbonates (Lavoie, 1992). This facies model agrees well with the one proposed for coeval units deposited farther north (Pratt and James, 1986; James et al., 1989) and south (Mussman and Read, 1986; Read, 1989) along the eastern seabord of Laurentia passive margin.

and intertidal muds. Interestingly, Lower Ordovician carbonates deposited on Laurentia passive margin are also typified by a similar pattern of early sabkharelated dolomitization (James et al., 1989; Montanez and Read, 1992). Pedogenic and meteoric diagenetic features of the carbonates indicate early subaerial influence on the supratidal and uppermost intertidal dolomite-rich facies (Lavoie, 1992; Lavoie and Paradis, 1993). These include early silicification and/or calcitization of evaporites, early dedolomitization, solution collapse breccias, vadose dissolution and gravitational calcite cements (6180 = - 8 . 4 to -11.0%o PDB, ~ 13C = +2.4 to +2.8%~ PDB; Table 1 and Fig. 5), and thin soil profiles with pisoids (&lSo = -8.2%~ PDB, ~13C = +2.0%o PDB; Table 1 and Fig. 5). Recrystallization of the lower limestone resulted in a mesh of anhedral calcite spar (_<10 /xm) with some ghosts of earlier grains. This recrystallized limestone has ~lSO-depleted values of -11.4 to -15.5%~ PDB) and near Early Ordovician marine ~3C values of -0.2 to +2.5%0 PDB (Table 1 and Fig. 5). Although mineralogical stabilization and recrystallization of carbonates start shortly after the initiation of burial, the stable isotopic ratios of the di-

5.2. Mineral paragenesis and stable isotopes

The diagenetic evolution of the lower limestone limit is as follows: early extensive dolomitization of peritidal muds followed by early meteoric alteration (dedolomitization, sulphate dissolution), solution (dissolution voids) and precipitation (gravitational calcite cements, soil pisoids), and burial diagenesis (limestone recrystallization, stylolites, fracturation, fracture-filling cements). The restriction of former dolomite rhombs to the peritidal facies is seen as a strong argument in favour of an early dolomitization of the supratidal

180pDB %o

0

/

t

-16-14

~

~o

CM

~

"Jr

--.-:~-o_ Zoo _ .

k5£_ _ ~ m _

-12

~

4o

- ~ -

-

5

~

m~..~

-;

I

I

-4

-2 O

[--1

Lower limestone unit

-5

o~ m

c) 13..

Upper limestone unit Meteoric vadose cement

Ni:N

-10

Q~

O9

oO

® • o

Soil pisoids Recrystallized limestone Fracture-filling calcite

-15

A

Crinoids and cement

-20

•

Sparry calcite cement

•

Carbonate concretion

Fig. 5. ~180 and ~13C cross-plot of carbonate elements for the lower and upper limestoneunits and the early diagenetic concretions in black shales. The 813Cdata of limestoneunits occur in distinctgroups. Oblique-hatchuredbox is assumedEarly Ordovicianmarine carbonate fieldfromWadleighand Veizer(1992).

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agenetic products can be easily reset through burial. The data from the matrix-recrystallized carbonates are suggestive of final stabilization from either ‘*Odepleted fluids or, more likely, high-temperature fluids in a rock-dominated diagenetic system. The late burial diagenesis of the succession is also expressed by cross-cutting stylolites, fracturefilling calcite (S’*O = -6.3 to -15.4%0 PDB, S13C = 3-0.5 to +3.2%0 PDB; Table 1 and Fig. 5) and minor quartz, sulphide and oxide cements. Minor migration of hydrocarbons and precipitation of small volume of solid bitumen occurred late, and bitumen is present in fractures, along stylolites and in some late dissolution cavities. 6. The upper limestone unit The upper limestone is a grainstone mostly composed of barite and calcite, it is crinoid-rich, and hosts stratabound Ba-Zn-Pb mineralization. 6.1. Sedimentary facies and depositional model In contrast to the hypersaline peritidal-dominated facies of the lower limestone unit, the upper limestone shows evidence for a dominant shallow subtida1 setting. The dominant lithofacies is a well-sorted, cross-laminated crinoidal biosparite (Fig. 6A). Minor intrasparite with rounded micrite clasts is locally observed. Pelsparites, well-sorted oosparites, and poorly-preserved thrombolitic textures (Fig. 6B) are less common. The dominant bioclastic facies of the limestone indicates that it was vuggy and contained a significant amount of interparticle porosity. The presence of well-sorted micrite intraclasts and oolite deposits indicates predominantly highenergy, shallow subtidal to lower intertidal shoals. The association of bioclastic, peloidal, oolitic, and thrombolite lithofacies is typical of the subtidal segment of the Early Ordovician passive margin of Laurentia, from Newfoundland (Pratt and James, 1986; James et al., 1989) to the southeast U.S.A. (Mussman and Read, 1986; Read, 1989). 6.2. Mineral paragenesis and stable isotopes The upper limestone underwent significant postdepositional alteration as indicated by abundant early

Geology

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to late diagenetic features, fracture-filling cements, and barite and sulphide mineralization. The paragenetic succession of these events is summarized in Fig. 7. The ‘main-mineralization’ events coincide with the two phases of barite mineralization. 62.1. Pre-mineralization events Evidence for early (submarine or meteoric) cementation is sparse, probably a result of pervasive post-depositional alteration. Thin broken crusts of non-luminescent palisade calcite (Fig. 8A) predating syntaxial calcite overgrowths on crinoids have been observed. The small available volume and very restricted occurrence of this cement precluded isotopic analysis. The isopachous calcite crusts share petrographic attributes with early subsea high-magnesium calcite cements, such as rhombic terminations of crystals and radiating sweeping extinction of subcrystals parallel to elongation (James and Choquette, 1990a). However, based only on these petrographic attributes, they could also be interpreted as meteoric cements (James and Choquette, 1990b). The most obvious early calcite cement is a syntaxial overgrowth on crinoids (Fig. 6A). Under CL, it is typified by a dull brown-orange luminescence with some internal orange pyramidal bands (Fig. 8B). The mixed crinoid and syntaxial cement yields Si8O values of -13.7 to -14.8%0 PDB and S’“C values of -3.9 to -15.0%0 PDB (Table 1 and Fig. 5). The syntaxial calcite overgrowths predate most stylolites and significant physical compaction of the sediments. Therefore, this cement is considered to be relatively early in the post-depositional history of the succession. Interestingly, the luminescence of this cement is different from most published descriptions of syntaxial overgrowths on crinoids (Meyers and Lohmann, 1985; Dorobek, 1987; Lavoie and Bourque, 1993). The latter are commonly nonluminescent with bright yellow luminescent laminae. The peculiar luminescence of our syntaxial cement is associated with anomalously light isotopic ratios for both oxygen (6’80pba = - 13.7 to - 14.8%0)and carbon (6’3Cpon = -3.9 to - 15.0%0);in the absence of clear evidence for dissolution-reprecipitation of this cement, this is most likely related to early precipitation in the burial realm from hot and 6i3C-depleted (biogenic?) COz-rich fluids. Anhedral calcite crystals (( 80 pm) having brown

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Fig. 6. Upper limestone unit. (A) Photomicrograph of crinoidal biosparite (Cr) with syntaxial spar overgrowths. Pyrite corona (anrows) mimic the shape of calcite overgrowths. (B) Photomicrograph of densely packed, pelletoidal material from a sample interpreted tohea thrombolite. Scale bar = 1 mm.

brown-orange luminescence under CL, coat allochems in the intrasparite, pelsparite, and oosparite lithofacies. No isotopic analysis was performed on this cement. Petrographically, the cement is likely to

coeval with the syntaxial crystal growth becau: se it predates stylolitization and physical compaction. The calcite cements described above fill prinnary pore spaces in the bioclastic limestone. Bedd ing-

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Pre-mineralization Marine cement Syntaxial calcite cement Anhedral calcite cement Stylolites Dissolution Subhedral barite cement Minor calcite fracture-filling Bladed barite cement Fracture-filling barite Fracture-filling calcite Spany calcite cement Bitumen and organic matter Sulfides Quartz Fig. 7. Paragenesis refers to precipitation

of diagenetic

elements

b

Main-mineralization

Late-mineralization

-

f

-

----I

I_ -

1

within the upper limestone

of the Upton Ba-Zn-Pb

deposit.

The main-mineralization

event

of barite.

parallel to sub-parallel stylolites which are due to compaction prior to tectonic folding, occurred prior to barite mineralization. Some pyrite mineralization also occurred early as testified by the presence of pyrite corona overlying and mimicking syntaxial overgrowths (Fig. 6A). Sassano and Procyshyn (1988) and Sassano and Schrijver (1989) have documented similar pyrite textures in the lower limestone unit. 6.2.2. Main-mineralization events Two generations of barite are observed. The first one, the subhedral barite cement (Fig. 9A), occurred prior to a minor phase of fracture formation and calcite cementation, and the second one, the bladed barite cement (Figs. 9B and C) and the fracturefilling barite, post-dated this event. The small size and restricted occurrence of this calcite cement precluded isotopic analysis. This calcite cementation is followed by precipitation of clusters of bladed and rosette-like barite in the void spaces of the limestone, and a phase of fracture formation with precipitation of bladed and blocky barite in the fractures. Barite mineralization post-dated bedding-parallel stylolites, but pre-dated bedding-discordant stylolites and sulphide precipitation (except for the early diagenetic pyrite). The subhedral barite cement forms less than 5 vol.% of the total barite content. It consists of aggregates of fine-grained crystals (5 300 pm) randomly

distributed throughout the limestone (Fig. 9A). The bladed barite cement and the fracture-filling barite account for up to 9.5 vol.% of the total barite content. The barite cement consists of fine- to coarse-grained bladed crystals (300 pm to 1 cm long) (Fig. 9B), and rosette-like clusters (Fig. SC) in the void spaces of the limestone, with some replacement of the calcite cement. Shortly after barite mineralization, a sparry calcite cement filled the remaining void spaces of the limestone and the fractures (Fig. 9D). Under CL, the sparry calcite cement has a dull brown luminescence, and has S’“C and ai80 values of -2.6 to -13X%0 PDB and -13.6 to -14.2%0 PDB (Table 1 and Fig. 5), respectively. The fracture-filling calcite has a zoned dull brown to bright yellow luminescence, and S13C and S’*O values of -2.4 to -17.9%0 PDB and - 14.0 to -15.8%0 PDB (Table 1 and Fig. 5), respectively. Symmetrical banding in some fractures illustrates well this paragenesis. It shows bladed and blocky barite at the vein walls with well-formed scalenohedral terminations pointing towards the centre of the veins, followed inwards by coarse-grained sparry calcite (Fig. 10). The remaining void spaces within the fractures may be filled by sulphides, quartz, and bitumen (see Sect. 6.2.3). The S34S values of various types of barite all cluster in the range of f28.3 to +34%0, with a mean value of +30.6%0 (Table 2). These values suggest a marine sulphate source for sulphur because average

S. Paradis, D. Lavoie/Sedimentary Geology 107 (1996) 121-139

131

Fig. 8. Upper limestone unit. (A) Photomicrographof isopachous crust of palisade calcite. (B) Cathodoluminescencephotomicrograph of the syntaxial calcite cement showing its dull luminescence with internal brighter pyramidal bands. Scale bar = l mm.

~345 value of the Early Ordovician seawater was near +30%0 (Claypool et al., 1980; Holser, 1984). 87Sr/86Sr ratios of barite range between 0.70654 and 0.70781 with a mean value of 0.70689 (n = 15)

(Paradis et al., 1996). These values are lower than the Early Ordovician seawater value of 0.70907 (Burke et al., 1982) and than the Cambro-Ordovician shale value of 0.7176 ± 0.0024 (Doig et al., 1986).

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107 (1996) 121-1.W

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Geology 107 (1996) 121-139

Fig. 10. Bladed bark (Ba) at the vein wall with spany calcite (Ca) of ‘post-bar&e mineralization’ stage filling the remaining voids in the vein. Scale bar is 0.7 mm.

6.2.3. Late-mineralization events The relationship between sulphides, quartz, bitumen, and organic matter is sometimes ambiguous. It seems that migration of hydrocarbons along fractures, accumulation of hydrocarbons along stylolites, and precipitation of solid bitumen botryoids with coarse mosaic texture (anthraxolite) and sulphides were coeval and culminated shortly after the last phase of fracture-filling calcite cementation. However, hydrocarbon migration may have also started as early as the syntaxial calcite cementation (Fig. 7). The time interval between precipitation of the different sulphides is unknown but is believed to be short, starting with pyrite and sphalerite and ending with chalcopyrite and galena. Finally, tectonic stylolites crosscut all these features, suggesting that most of diagenesis and mineralization (barite and sulphides) occurred prior to the latest phase of tectonic thrusting associated with the Taconian orogeny. 7. Discussion The mineral paragenesis and stable isotope geochemistry link carbonate diagenesis to Ba-Zn-Pb

mineralization. Constraints on the nature of the fluids and the timing of the mineralization can therefore be addressed. 7.1. Fluid evolution Based primarily on the carbon isotope composition of the carbonates, it is suggested that the fluids circulating in the lower limestone unit were at one time significantly different from those recorded by the upper mineralized limestone. 7.1.1. The lower limestone unit The presence of vadose dissolution features and gravitational fabrics of calcite cement within the lower limestone unit suggest the occurrence of an early meteoric event restricted to the lower limestone unit (Lavoie, 1992). The S’*Opnn values of the vadose calcite cement and the soil pisoids are 1.5 to 4.7%0 lower than published Early Ordovician marine carbonate values of -6.2 to -6.7%60 PDB (Wadleigh and Veizer, 1992). The higher S’*opn~ value of the soil pisoids (Fig. 5) compared to that of the meteoric vadose cement suggest an evaporitive

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‘so-enrichment of near-surface trapped soil moisture (vadose water) compared to percolating meteoric waters (Lavoie and Paradis, 1993); a relationship common to arid settings (James and Choquette, 1990b). The near-marine 6’3Cpon values of meteoric carbonates of the lower limestone (Fig. 5) suggest a rockdominated diagenetic system consistent with the lack of plant or algae cover on the Early Ordovician land mass (Chaloner, 1970). The 6180pon and 613Cpon fields for the recrystallized limestone and the fracture-filling calcite of the lower limestone overlap. This could suggest alteration of marine limestone and precipitation of fracture-filling calcite from isotopically similar fluids (Fig. 5). The wide range of S180pon values (Fig. 5) supports an interpretation of fluid mixing. Because more than 75% of the 6’80pon values are lower than the meteoric vadose cements and soil pisoids, various end-member fluid compositions are possible, including various mixtures of seawater and meteoric fluids. The Early Ordovician marine S13Cpon values (see Fig. 5) of the recrystallized limestone and fracture filling calcite argues for the absence of isotopically-light (biogenic) CO? in the late fluids. The restricted occurrence of sulphides and solid bitumen suggest that minor quantities of H&rich volatile hydrocarbons migrated late through fractures and along stylolites. This precipitation suggests burial temperatures between 170”and 270°C (Robert, 1985). Such a temperature range is further supported by the high reflectance (2 6%) of the organic matter, the associated phyllosilicate assemblage, the composition of chlorite (Chagnon et al., 1992), and the 180-depleted values of the late fracture-filling calcite (S’80pou = -6.3 to -15.4%0 PDB). Assuming that the late fracture-filling calcite precipitated at temperatures ranging from 170” to 270°C the S180s~ow values of fluids would range from f4.2 to -+18.3%0 (equation of O’Neil et al., 1969), and the fluid 613Cpon values would range from -2.4 to +2.8%0 (equation of Deines et al., 1974). These S13Cpnnvalues are consistent with a derivation from Ordovician seawater (-2.0 to + 1.5%0; Wadleigh and Veizer, 1992) for the fracture fluids, but the S180s~ow values are not. The latter falls within the S180u20 range given for water of metamorphic origin.

Geology 107 (1996) 12/-I_
7.1.2. The upper bioclastic limestone unit The mixed crinoid-syntaxial calcite cement, the sparry calcite cement, and the fracture-filling calcite yield low 6180pon values of -13.7 to ---14.8rOc, -13.6 to -14.2%0, and -14.0 to -15.8%0, respectively (Fig. 5). These values are similar to those of the recrystallized limestone (- 11.4 to - 15.5%0 PDB) and to most (88% of the 6’80pon values) of the fracture-filling calcite (- 10.2 to - 15.4%0 PDB) of the lower limestone unit. The 6’80pon values of the mixed crinoid-syntaxial calcite cement, the sparry calcite cement, and the fracture-filling calcite of the upper limestone unit are on average -7%~ more depleted than published Early Ordovician marine carbonate values (Wadleigh and Veizer, 1992: Fig. 9). Such depleted values are typical of basinal brines (cf. Taylor, 1979), and can be explained either by exchange at elevated temperatures with seawaterderived or connate fluids during burial, or by exchange with ‘8O-depleted meteoric fluids, or even by a combination of both mechanisms. Discrimination between these mechanisms would require the use of another tracer such as R7Sr/86Srratio. The estimated 6’sOs~ow values of fluids responsible for the post-barite calcite precipitation in the upper limestone is constrained by BO-130°C primary fluid inclusion homogenization temperatures (Paradis et al., 1996). The values of S’80sMow of the fluids range from -4 to +2.8%0 (equation of O’Neil et al., 1969). These values are consistent with Ordovician seawater-derived basinal brines. The variable and 13C-depleted values of the crinoid-syntaxial (-3.9 to -15.0%0 PDB), sparry (-2.6 to -13..0%0 PDB), and fracture-filling (-2.4 to -17.%0 PDB) calcite cements likely reflect the addition of significant amounts of ‘“C-depleted carbon to the fluids. Making temperature corrections based on the primary fluid inclusion homogenization temperatures (see above), the 6’3Cpos values of the fluids range from -7.0 to -23.8%0 which are significantly lower than those of Ordovician seawater (Fig. 5), and implies highly 13C-depleted fluids. Similar 13C-depleted values are generally explained by thermal alteration of organic matter. When oxidized, thermogenic methane expelled from organic matter (6’“C = -35 to -50%0 PDB; Claypool and Kaplan, 1974) yields even more 13C-depleted CO2 (-35 to -llO%~, PDB; Raiswell, 1987), a process

S. Paradis, D. Lavoie/Sedimenrary

commonly invoked as a major source for 613Cdepleted carbonates. In our case, mixing of CI&derived CO? with marine-derived CO:! from the host limestone could result in 13C-depleted carbonates. However, from the measured 613C values, significant isotopic rock-fluid exchanges would be needed. Thermochemical sulphate reduction (Machel, 1987, 1989) releases CO:! less depleted in 13C (-10 to -25%0 PDB; Irwin et al., 1977; Hoefs, 1987) together with H2S in aqueous solution. Our data could be explained by thermochemical sulphate reduction because: (1) the abundance of sulphates in the entire succession, either as early or late precipitates; (2) the early bacterial-mediated sulphate reduction (Irwin et al., 1977) resulting in the formation of carbonate concretions (613C of -4.2 to -6.6%0 PDB; Fig. 5); and (3) the similarity of S13Cvalues released through thermochemical sulphate reduction and 613C values obtained from isotopically reset marine carbonates and fracture-filling calcite. Thermochemical sulphate reduction has also been proposed by TassC and Schrijver (1989) to explain the origin of sphalerite in Lower Ordovician carbonates of the southem Quebec Lowlands. i3C-depleted CO2 most likely originated from the organic matter in the underlying siliciclastic succession of the Upton Group and diffused, through fractures and along thrust planes, in the upper bioclastic limestone. As the deep-seated and low 13C COz- and HzScharged acidic fluids migrated upward into the upper bioclastic limestone, dissolution porosity was created. The critical role of COz-loaded brines has long been known for generation of secondary .porosity for hydrocarbon reservoirs (Faezel and Schatzinger, 1985). This secondary porosity permitted barite and minor pyrite (if ferrous iron was available) to precipitate in open void spaces. Ascending H&rich fluids were able to permeate the porous bioclastic limestone but cannot penetrate the impermeable rocks such as the shale and the mudstone-siltstone that overlay and underlie the bioclastic limestone. Therefore, fluid flow was most likely restricted to thrust planes and possible cross-cutting vertical fractures. The basinal fluids likely contained significant amounts of Ba+2. The low 87Sr/86Srratios of barite (0.70654 to 0.70781; Paradis et al., 1996) compare to Ordovician seawater (0.70907; Burke et al., 1982) indicate that a significant proportion of Sr in barite

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could have been derived from a source with low radiogenic 87Sr such as mafic volcanic and intrusive rocks of the Upion Group. The sulphur isotope values of barite (+28.3 to +34%0 CDT) are similar to the assumed Early Ordovician seawater average 634S value of +30%0 (Claypool et al., 1980; Holser, 1984), therefore suggesting that the sulphate sulphur was derived from coeval seawater sulphate, either directly from the ocean or indirectly from solutions which dissolved marine sulphate. The range of S34S values displayed by barite and sulphides suggests a common source of sulphur, being marine sulphate. Assuming that marine sulphate was the sulphur source, changes in S34S values would be primarily related to reduction of SOi- to H2S by bacterial or chemical action at or near the site of mineral precipitation. Because the temperatures for barite (40” to 9O“C), calcite (80“ to 130”(Z),and sulphide (>25O”C; Sassano and Procyshyn (1988)) precipitation have such a large range (40” to >25O”C) and increase with time, we ruled out bacterial activity and favoured chemical action as the main mechanism. Sulphide may be generated inorganically by thermochemical sulphate reduction, via reactions involving hydrocarbons at temperatures between 80“ and 130°C (Krouse et al., 1988). Such a mechanism often results in difference of approximately 14 to 24 per mil (e.g., Krouse et al., 1988) between S34Sso4 and S34Snzs values. Our sulphide values are depleted in 34S by 18-27 per mil with respect to barite, and are isotopically sufficiently light to account for reduced sulphur from the partial thermochemical reduction of marine sulphate. Precipitation of barite could have occurred in response to a drop in temperature accompanying mixing of two fluids at the depositional sites, i.e. a hot and reduced ascending CO2-, H$-, and Ba2+-charged basinal brines and the ambient cooler sulphate-rich fluids. However precipitation of barite could have also occurred without much change in temperature since it is extremely insoluble in presence of SO,. Other precipitation mechanisms, such as redox reactions, dilution of ligand activity, change in Eh and pH, and change in concentration of other species could have also been factors involved. Following barite precipitation, CaCOs -saturated fluids present in the various (fracture and dissolution void) pore spaces of the limestone precipitated cal-

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cite as a result of the increase in calcite saturation level (from CaCOs-dissolution) in already CaCOxsaturated hot brines. These very late fluids were strongly reducing, HzS-, hydrocarbon- and silicarich, and charged with metallic ion complexes. The precipitation of late calcite, solid bitumen (anthraxolite), sulphides, and quartz cement was more or less coeval. The mineralogical/metallic association and organic matter properties suggest moderately high temperatures of precipitation ( >2.50°C according to Sassano and Procyshyn (1988)) for these late mineral phases. On the other hand, the S13C data emphasize the importance of thermal degradation of organic matter. 7.2. Timing of mineralization

events

Cross-cutting relationships between fractures, bedding-parallel and bedding-discordant stylolites, and void-filling elements suggest a deep-burial setting. The temperatures recorded by the oxygen isotope and fluid inclusion data (Paradis et al., 1996) for the various calcite cements, and the textural/reflectance properties of the organic matter suggest a maximum burial depth of at least 7 km. HCroux and Bertrand (1991) also estimated a similar burial depth in their regional study on the maturation of organic matter which was inferred to be due to Taconian tectonic burial (Heroux and Bertrand, 1991; Bertrand and Dykstra, 1993). The emplacement of volcanic and gabbroic rocks within the Upton Group (Sassano and Procyshyn, 1988) should have increased the local geothermal gradient and created favourable conditions for the circulation of basinal brines, possibly channelled through the sedimentary pile along thrust planes, vertical fractures, and other conduits. Bedding-discordant stylolites cut through all calcite cements and mineralization, suggesting that most of the diagenesis, fluid circulation, and mineralization occurred prior to the final thrusting of the Taconian nappes. Other Lower Ordovician carbonate successions of the southern Quebec Appalachians have similar diagenetic histories. Coeval carbonate successions of the northeastern part of southern Quebec Appalachians differ in being devoid of barite and significant amounts of sulphides, but locally contain gas reservoirs (Htroux and Bertrand, 1991; Bertrand et al.,

Geology 107 (1996) 121-139

1992, 1993; Bertrand and Dykstra, 1993; Dykstra and Longman, 1995). These successions do not record significant open marine conditions nor were they associated with occurrences of contemporaneous volcanic and intrusive rocks (Bertrand et al., 1993). Higher thermal regimes in the southwestern area of southern Quebec are related to Early Cretaceous magmatic activity (185-250°C) centered in the Montreal area (Heroux and Tasse, 1990; Heroux and Bertrand, 1991). The increase in burial temperatures recorded by the Lower Palaeozoic successions in southern Quebec was also related to a greater post-Taconian thermal maturation effect imposed by a thicker Silurian-Devonian cover (Heroux and Bertrand, 1991). This, and changes in depositional facies, could explain the regional distribution of gas reservoirs in the less mature northeastern part of the Quebec Appalachians and the presence of barite and sulphide deposits in the more mature southwestern part. 8. Conclusions The diagenetic features associated with barite, zinc, and lead sulphide mineralization, are lacking in the lower limestone unit. The lower limestone was affected by an early meteoric event accompanied by evaporitive ‘8O-enrichment of soil waters, followed by recrystallization from a mixture of seawater and meteoric fluids in a rock-dominated system. The upper bioclastic limestone was affected by mixed marine- and organic-derived basinal brines. The upper limestone unit formed an excellent reservoir for precipitation of barite and sulphides. In the upper limestone unit, mineral paragenesis and stable isotopes of calcite cements of pre- and post-barite place important limits on the nature of carbonate diagenesis and mineralization: (1) Similar to the lower limestone succession, low S180 values of mineralized carbonates are typical of moderately high temperature seawater-derived basinal brines and deep-burial isotopic resetting. (2) The variable and low 6’3Cpon values of the upper limestone reflect the addition of significant t3C-depleted CO* to the basinal brines which most likely was derived from thermochemical sulphate reduction by organic matter. No evidence for this event was recorded in the lower limestone.

S. Pa&is,

D. Lavoie/Sedimentary

(3) Organic matter played a significant role in the mineralization process; it was abundant in the sedimentary succession hosting the mineralization, as testified by isotopically-distinct early (carbonate concretions) and late (calcite cement) precipitates. (4) Just prior to barite precipitation, organic matter contributed to sulphate reduction, liberating HzS for pyrite precipitation, and CO;?for limestone dissolution. Therefore, alteration of organic matter created acidic solutions that generated a secondary porosity; important for creating a suitable carbonate reservoir. (5) Barite precipitation was probably triggered by mixing of two different fluids (a hot and reduced COz, Hz!%, and Ba2+-charged basinal brines and cooler sulphate-rich fluids). Through time, as both the temperature of the ambient fluids increased and metal ions dissolved, physico-chemical conditions became favourable for sulphide precipitation. (6) The physico-chemical conditions suitable for barite and sulphide precipitation were probably best achieved prior to the latest phase of Taconian thrusting, in the Middle Ordovician. Acknowledgements We are extremely grateful to B. Ricketts, P.S. Mustard, and G.J. Simandl for critical reading of the original paper and providing constructive comments. B.C. Schreiber, T.K. Lowenstein, A.E. Fallick, E.W. Mountjoy, and W.J. Meyers also provided detailed and most useful comments on various parts of the paper. Drafting was done by Tonia Williams of the Geological Survey of Canada, Cordilleran Division. Robex Inc. allowed us to sample and study the drill cores of the Upton Ba-Zn-Pb deposit and provided pertinent information on the deposit. This is Geological Survey of Canada Contribution 28094. References Baldwin, A.B., 1973. Report recommending mineral exploration for 1973-74 in an ama located in the St-Lawrence Lowlands, Province of Quebec. Minis&e de I’Energie et des Ressources du Quebec, Service du potentiel mineral, GM-39067. Beaupo?, M., 1975. Stratigraphie et structure du ‘Complexe SaintGermain’ et de la partie frontale des Appalaches de Drummondville. Unpubl. M.Sc. thesis, Universite de Montreal. Bertrand, R. and Dykstra, J., 1993. Organic metamorphism and burial histories in the St. Lawrence Lowlands and in the

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