Cation Study of Fluid Inclusion Decrepitates in the ...uregina.ca/~chiguox/s/1998 Savard and Chi...

Economic Geology Vol. 93, 1998, pp. 920-931

Cation Study of Fluid Inclusion Decrepitates in the Jubilee and Gays River (Canada) Zn-Pb Deposits Characterization of Ore-Forming Brines*

MARTINE M. SAVARD t AND GUOXIANG CHI

Geological Survey of Canada-Qudbec, Centre G•oscientifique de Qudbec, 2535 Laurier, P.O. Box 7500, Sainte-Foy, Qudbec, Canada GIV 4C7

Abstract

An SEM-EDS instrument sensitive to C and O is used to measure cation ratios in individual fluid inclusion

decrepitates of main paragenetic phases from two Nova ScotJan Mississippi Valley-type deposits. The technique confirms that the analyses obtained for the carbonate hosts are not contaminated by substrate and permits measurements of small amounts of decrepitates. The technique has been used to investigate fluid compositions in inclusions of presulfide-stage dolomite and calcite, sulfide-stage sphalerite and calcites, and postsulfide calcite at the Gays River and Jubilee Zn-Pb deposits in Nova Scotia. The results indicate that Na and Ca are the two major cations. Minor amounts of K and Mg are also detected. NaC1 and CaC12 ranges for decrepitates from the two deposits are as broad as for modem mixed brines and thereby suggest that solution mixing partly controlled the palcobrine compositions. When compared to other Mississippi Valley-type deposits, the Gays River and Jubilee deposits are similar in NaC1-CaC12-KC1 composition to a group of Mississippi Valley- type districts including deposits from the Ozark region that are associated with large volume clastic sequences or palcoaquifers. Higher proportions of KC1 at Gays River and higher proportions of CaCls at Jubilee indicate that the mineralizing brines interacted with Horton Group clastic aquifers of different mineral compositions. The brines in the two areas might have originated from evaporation of seawater, but they were later modified by interaction with clastic rocks of different compositions.

Introduction

CONVENTIONAL fluid inclusion microthermometric tech-

niques allow estimation of fluid salinity; however, to document concentrations of chemical species of fluid inclusion solutes, it is necessary to use direct analytical methods, such as the leachate-ion chromatograph method (e.g., Crocetti and Holland, 1989) or the decrepitate-SEM-EDS method (e.g., Haynes and Kesler, 1987). Both methods have their advan- tages and shortcomings: the quantitative leachate method, unfortunately, gathers all types of inclusions (primary, secondary, and altered), whereas the decrepitate method, which can analyze decrepitates from individual inclusions, gives only semiquantitative results. Here we document for the first time semiquantitative results obtained with an SEM-EDS instrument equiped with a detector sensitive to light elements such as C and O. Detection of these elements allows recognition of substrate involvement when very small decrepitates are investigated in carbonates.

The Gays River and Jubilee Mississippi Valley-type deposits are located at the southern margin of the Maritimes basin (Fig. 1) and are hosted by carbonates of the Gays River and Macumber Formations, respectively. The two formations ap- pear at the same stratigraphic level at the base of the Windsor Group (Fig. 2). The Gays River deposit has been dated at ca. 300 Ma (Pan et al., 1993; Kontak et al., 1994). The Jubilee deposit has not yet been dated, but its metallogenic history suggests a genetic link with the Gays River deposit (Fallara et al., 1998).

The paragenetic phases at the Gays River and Jubilee deposits have been studied for their isotopic signals (Savard, 1996; Fallara et al., 1998; Savard and Kontak, 1998), fluid

Geological Survey of Canada contribution 1996184. Corresponding author: email, [email protected]

inclusions (Chi and Savard, 1995; Chi et al., 1995; Kontak, 1998) and their associated organic matter alteration and au- thigenic clay minerals (H•roux et al., 1994; Bertrand et al., 1998; Phagnon et al., 1998). The parent fluid chemistry of the paragenetic phases has been evaluated by conventional microthermometric methods, but the composition of the solutes needs further examination. In this paper, we use the SEM-EDS method to characterize the compositional evolution of fluids from pre-, syn-, and postsulfide stages in the two deposits. (See also the discussion on high-salinity brines in general by Chi and Savard (1997).)

The two main objectives of this article are to document and compare the major cation composition of decrepitares from primary inclusions in paragenetic phases from the Gays River and Jubilee deposits, and to determine whether the cation ratios show similarities with those obtained from other

Mississippi Valley-type districts and with those of modem brines in other basins.

Analytical Methods

Eight thin sections, previously studied for microthermome- try, were heated quieldy to 420øC to decrepitate the inclusions (quick heating produces a larger number of deerepirates). The sections were coated with gold and analyzed immediately after heating to avoid reerystallization of solids. Deerepirates occurring as isolated mounds were considered to have resulted from individual primary inclusions. Selected deerepitates were analyzed with a Jeo1840 secondary electron microprobe (SEM) equipped with a high purity germanium detector and a Norvar window for energy dispersive spectroscopy (EDS). Counts were integrated over a period of 100 s on K peaks for C, O, Na, Mg, C1, K, Ca, and S. Dead time of counts was 20 percent. A standardless semiquantitative program (S- Q) was used to calculate the weight percent concentrations of Na, Mg, C1, K, and Ca, ignoring the presence of C and

0361-0128/98/2007/920-1256.00 920

CATION STUDY OF JUBILEE & GAYS RIVER (CANADA) DECREPITATES 921

BRU TRIASSIC-JURASSIC ICK • LATE CARBONIFEROUS-PERMIAN

../EARLY CARBONXFEROUS ./111Ir DEVONIAN-CARBONIFEROUS •[IT]UPPER PROTEROZOIC-MIDDLE DEVONIAN/

//

50 km

usq,

OF CANSo

320 Ion GUf-BEC

MARITIMES BASIN

MAHONE BAY (AFTER BOEHNER et al. 1989) MAP AREA Horde and Barss 1975

FIG. 1. Location of the Gays River and Jubilee deposits with respect to the Maritimes basin.

O. The voltage was set at 10 kV so that contributions from the substrate due to excessive penetration were avoided, and so that contamination from the uppermost thin layers of decrepitate due to adsorption of moisture and other gases was negligible.

Surface contamination due to atmospheric exposure is manifested by the presence of C and O peaks in all the analyses. Analyses on pure NaC1 standards indicate that O and C are not derived from the inclusion-bearing crystals but are due to contamination in the uppermost thin layer. A consequence of this contamination is that the measured results are not in valence balance, i.e., C1- is deficient relative to cations, although this does not affect cation ratios. Tests on the NaC1 standards show that the contamination can be

greatly reduced by increasing the voltage from 7 to 15 kV. However, the increase in voltage (and therefore, depth of penetration of the electronic beam) increases the risk of involvement of the substrate. Repeated experiments indicate that for decrepitares larger than 4/am, a voltage of 10 kV usually avoids the involvement of the substrate and minimizes the effect of surface contamination (Fig. 3).

For decrepitares on sphalerite samples, a zero concentra- tion of S is used to indicate that the substrate is not involved

in the analyses. For decrepitares on calcite and dolomite samples, it is assumed that decrepitares on carbonates have the same level of C and O surface contamination as decrepitares on sphalerite samples; i.e., analyses of decrepitares on carbonate samples which have the same level of C and O peaks as sphalerite decrepitares are regarded as being devoid of substrate influence (Fig. 4).

Another limit on the validity of the analyses is the volatility of Na, Ca, K, and C1 at high beam densities. For a given current, the larger the raster area, the smaller the current density. Large raster areas are also desirable because they can average out chemical inhomogeneities in the decrepitates and counteract the effects of irregular topography (Fig. 5). However, an increase in raster area increases the risk of involvement of the substrate. As long as the substrate is not involved according to the criteria described above, raster areas should be as large as the size the decrepitate allows. For all analyses, the beam current was fixed at 0.1 nA, and the raster area was always larger than 0.2/am s in order to satisfy the condition of nonvolatility, i.e., current densities smaller than 0.5 nA//am 2 (Morgan and Davies, 1982).

The average diameters of analyzed decrepitates are 5.5, 5.5, and 5.2 and 5.0, 6.1, and 5.2/am for stage 1, 2, and 3 minerals from the Gays River and Jubilee deposits, respectively (Tables i and 2). The total diameter range is 3.5 to 10 /am. The semiquantitative method described above has been repetitively applied to different parts of a single decrepitate of 9/am from sphalerite in order to test the homogeneity and consistancy of results (sphalerite of sample 279, decrepitate 11; Table 2 and Fig. 6). The obtained results cluster in the CaC12-NaC1-KC1 and CaCls-NaC1-MgCI•. spaces (Fig. 6). This indicates that the effect of topographic relief and heterogene- ity is not significant on cation proportions.

Using a rejection threshold of 15 percent charge unbalance between Cl-and the sum of cations, the data rejected constitute 34 percent of the total set. This high level of rejection is probably due to the very small beam voltage (10 Kev) and the size

922 SAVARD AND CHI

A Mainland Nova Scotia SHUBEI•I•./•IE MLIS•;•IODOBOIT

BASIN

•'-- C•RET .,•[::::EOUS • B Pictou • '-

.... •v'v'•'vv •[• ß O •vv•vv --•-v•. • • v v v•v v v v•.

-•• F•.•VVVVV• • - H• • v v_.v •' , • GAYS RWER FM. ,

+ + ..... + +

Horto• . ß

B Cape Breton Island

'•'i••__• 1 PICTOU GROUP + CLIMBERLAND GROUP

AUTUNIAN

WESTPHALIAN • .....

NAMURIAN MABOU GROUP

ie Detachment) l

WINDSOR GROUP

(MACUMBER •M)

rOUnNASAN HORTON GROUP

; :: ?:• F[$$œTBROOKFM 17.• ß ..., (Acadian deformation)

380-400 My BASEMENT

FIc. 2. Stratigraphy of the upper Paleozoic sequence on mainland Nova Scotia near Gays River (A), and on Cape Breton Island near Jubilee (B).

of decrepitares (Tables 1 and 2) in comparison with those of other SEM-EDS studies (e.g., Haynes and Kesler, 1987).

Geology

Gays River setting

The Gays River deposit is located in central mainland Nova Scotia. It is hosted by Vis6an dolostones of the Gays River Formation which, together with the laterally equivalent Ma- cumber Formation, forms the basal carbonate succession of the Windsor Group on mainland Nova Scotia (Fig. 1). These carbonates were deposited during a rapid Vis6an marine in- cursion following the intracontinental post-Acadian rifting event which created the Maritimes basin (Boehner, 1984;

Schenk and Hatt, 1984; Lynch et al., 1998; Sangster and Savard, 1998; Fig. 1).

In the Shubenacadie and Musquodoboit sub-basins (Fig. 2A), the Gays River Formation rests on metawacke basement highs of the Cambro-Ordovician Megnma Group (Cambrian- Silurian). The laterally equivalent Macumber Formation overlies fiuviatile conglomerates and sandstones of the Fa- mennian-Tournaisian Horton Group (Fig. 2A). On mainland Nova Scotia, the fiuviatile clastic sediments of the Horton Group were mostly derived from the Megurea Group.

The Gays River carbonates are overlain by evaporites of the Carrolls Corner and Stewiacke Formations and, in the southwest portion of the Musquodoboit sub-basin, by silt- stones and sandstones of the Meaghers Grant Formation, which are also overlain by anhydrite of the Carrolls Corner Formation (Giles et al., 1979; Giles and Boehner, 1982). Based on the stratigraphic relationships in the studied area, which include the Windsor Group and remnants of the Car- boniferous (Namurian Mabou Group), a very conservative estimate of the maximum burial depth of the Gays River Formation is 1.5 km (Savard, 1996). Regionally, the maximum temperature reached in the burial environment was • 150øC, as based on Th of fluid inclusions in dolomites (Chi and Sa- yard, 1995) and on vitrinite reflectance (Ro) of kerogen in the dolostone (H6roux et al., 1994). In the area north of Gays River, the geothermal gradient has been estimated to be •85øC/km, using Ro values along a 1-km section through the Windsor Group (P. St.-Antoine and Y. H6roux, pers. commun., 1996).

The Gays River Formation is composed of abundant algal, coral, and bryozoan bafflestones, algal and bryozoan bind- stones, skeletal packstones and wackestones, and bituminous mudstones and wackestones which form carbonate bank and

interbank facies recognized throughout the Maritimes basin (Boehner et al., 1989). The Macumber Formation consists of thinly bedded skeletal wackestones and microbial mats representing deeper water equivalents of the Gays River For- mation (Lavoie and Sami, 1998). The Macumber Formation is mostly limestone in the area of investigation, but the Gays River Formation is entirely dolomitized. A connection between mineralization and dolomitization has been ruled out, because dolomitization took place in burial, prior to mineralization (Savard, 1996). The dolostone still had approximately 25 percent porosity before sulfide precipitation. These two facts signify that the dolostone does not represent an alteration by mineralizing fluids but constitutes a unit that resisted pore occlusion during burial. The preserved permeability later favored the passage of ore-forming fluids.

The deposit is located over a palcotopographic basement high separating the Shubenacadie and Musquodoboit sub- basins (Fig. 2A). At this locality the north-facing part of the carbonate banks has been replaced by fine-grained (< 10-20 •m) sphalerite and medium-grained galena to form the Gays River deposit. Reserves at the deposit are estimated at 2.4 Mr: 8.6 percent Zn and 6.3 percent Pb (Kontak, 1992). The high-grade ore part of the deposit occurs as a replacement of the dolostone, whereas low-grade ore fills preserved primary pores.

Isotopic and fluid inclusion microthermometric investigations of ore-stage calcite and sphalerite indicate precipitation


A 0,1 nA 7 kev B 0,1 nA 10 kev

Na

CI

CI

' CI

cO t cl

FIG. 3. Comparison of the C-O peaks at 7 kV (A) and 10 kV (B) for SEM-EDS tests on a salt standard (NaC1). The presence of the C-O diminishes at 10 kv. 0.1hA.

at high temperatures (150ø-250øC), interaction of a hot metal-rich brine with radiogenic continental clastic (and/or metasedimentary) rocks, and partial carbon buffering by the host dolostone (Chi and Savard, 1995; Kontak, 1998; Savard and Kontak, 1998).

Jubilee setting The Jubilee deposit is located in central Cape Breton Island

(Fig. 1). It is hosted by the Macumber Formation at the base of the Windsor Group, as described above. The thickness of the Macumber Formation on Cape Breton Island ranges from 10 to 50 m and comprises two main depositional facies, a lower micritic facies and an upper finely laminated facies mostly composed of microbial mats (Fallara et al., 1998; La- vole and Sami, 1998). The Macumber Formation conforma- bly overlies continental clastic rocks of the Horton Group (Devono-Carboniferous) and is in turn covered by thick (hundreds of meters) evaporites of the Carrolls Comer Formation, a part of the first transgressive-regressive cycle of the Windsor Group (Giles, 1981). The Horton Group is partly derived from and overlies the Fisset Brook Formation composed of bimodal extrusive volcanic rocks (rhyolite and basalts) and granitic plutons of diverse ages (see discussion).

Mineralization at the Jubilee deposit is contained within brecciated limestone of the Macumber Formation that has

been affected by hydrofracture brecciation mostly in its top unit (Fallara et al., 1998). The thickness of the complete Windsor Group has been estimated to be 2 km although numerous stratigraphic gaps have been documented on Cape Breton Island and interpreted to have resulted from exten- sional tectonism along the Ainslie detachment (Lynch et al., 1998). Two of these omissions are in the immediate area of the Jubilee deposit, the omissions and the hydrofracturing breeeia have been linked to the Ainslie detachment (Fallara et al., 1998). The Windsor Group has been later affected by the Alleghanian deformation that folded and faulted the en- tire sequence. Zonation in metal contents and isotope geochemistry also show that the postbreeeiation Jubilee and Road subvertical faults served as conduits for a metal-rich brine

expelled from the Horton aquifer (Fallara et al., 1998). The two faults are northwest striking and could be Alleghanian structures.

Reserves of the Jubilee deposit have been estimated to 0.9 Mt at 5.2 percent Zn and 1.4 percent Pb (Hein et al., 1993). Standard and fluid inclusion petrography, and isotope geochemistry indicate that hydrocarbons were present and acted as an agent for reduction of sulfates before, during, and after sulfide precipitation (Hein et al., 1988; Chi et al., 1995; Fal- lara et al., 1998).

The Gays River and Jubilee deposits share numerous char-

A 94-305-DOL-2 (4 (] o key)

Sa

B c• 217-sP-2 (6 g) Na (10 key)

FIG. 4. Comparison of the C-O peaks on sphalerite (A) and dolomite (B) decrepitates. Presence of C-O peaks in sphalerite suggests that the C and O cannot originate from the substrate, but from surface contamination. This minor contamination does not affect the cation ratios as discussed in the text.

924 SAVARD AND CHI

A 10 kv

B 10 kv WD15

F[c. 5. SEM photomicrograph showing the relief of individual decrepitates subsequently analyzed at 10 kV (results in Tables 1 and 2). A. Jubilee deposit, sample 256, sphalerite 7, decrepitate 10. B. Gays River deposit, sample 933, calcite 24, decrepitate 1.

acteristics with the suite of Irish carbonate-hosted deposits, such as sulfide precipitation from brines at temperatures up to 250øC (Chi and Savard, 1995; Chi et al., 1995; Chi et al., 1998). Fluid inclusion investigations indicate that (1) the temperature of regional diagenetic burial fluids around the Gays River deposit was higher than those around the Jubilee deposit (Chi and Savard, 1995; Chi et al., 1995, 1998); (2) hydrocarbons were rare at Gays River but abundant during mineralization at Jubilee; (3) both deposits show Tm and Th trends typical of mixing of two or more fluids, among which, one was heated (-•250øC), saline (-•25-30 wt % NaC1 equiv) and had Na/(Na + Ca) -• 0.7 to 0.8; (4) at both deposits oxygen and strontium isotope covariations suggest mixing of diagenetic background fluids with hot metal-rich fluids, and

34 6 S values indicate sulfur of marine affinities; and (5) comparison of the STSr/S6Sr of synsulfide carbonates and the Pb/ Pb in galenas suggests that the two deposits had distinctly different sources of metals (Sangster et al., 1998).

Paragenesis Prior to mineralization, carbonates of the Lower Windsor

Group in the Gays River and Jubilee sites underwent differ-

ent patterns of diagenesis which were controlled by their respective physical properties (Fig. 7). The porous and per- meable biolithic marine carbonates of the Gays River Forma- tion near Gays River underwent marine cementation and total dolomitization during burial (stage 1); their porosity remained near its initial value through these processes (Savard, 1996), whereas the fine limestone of the Macumber Formation on

Cape Breton Island became rapidly lithified through micro- crystalline cementation, compaction, and stylolitization (Sa- vard et al., 1996) and was later brecciated, cemented by fibrous calcite, and fractured (stage I; Fallara et al., 1998). Porosity-preserving replacement by dolomites, and pore-cre- ating brecciation or fracturing were determinant preore processes at Gays River and Jubilee, respectively. The ubiquitous fine layer of dolomite cement occluding less than 2 percent of pores at Gays River and the thick fibrous calcite at Jubilee are phases that can be used to characterize conditions that prevailed before sulfide precipitation. At both deposits, sphalerite and galena constitute the main ore minerals which are broadly coeval to anhedral calcites (stages 2 and II; Fig. 7). Associated barite and fluorite mainly postdate the sulfides. It is clear that some anhedral calcite around Gays River post- dated mineralization (stage 3), whereas most anhedral calcite seems to have been synmineralization at Jubilee, barite ap- pearing as the last paragenetic phase (stage II; Fig. 7). The following sections will present results obtained by SEM-EDS on the main paragenetic phases, i.e., on dolomite cement, sphalerite, and anhedral calcite from Gays River, and fibrous calcite, sphalerite, and anhedral calcite at Jubilee, in order to document chemical evolution of fluids at the two deposits.

Results

Gays River deposit

Results for dolomite (stage 1), sphalerite (stage 2), and anhedral calcite (stages 2 and 3) of the Gays River decrepitates are listed in Table i and plotted on ternary diagrams in Figure 8. NaC1 and CaC12 are the major components and they show a broad variation in abundance, whereas the amounts of KC1 and MgC12 are generally low and less variable (Fig. 8). Presulfide dolomite clusters at 80 wt percent NaC1 and 2 wt percent KC1, whereas sphalerite and syn- to postsulfide anhedral calcite show broad percentage (50-90 wt % NaCI and 2-15 KC1) ranges and the highest KC1 and CaCI• proportions (Fig. 8A). In CaCI•-NaC1-MgCI• space (Fig. 8B), sphalerite has systematically lower MgCI• proportions than calcite and dolomite, these carbonate phases having a simfiar MgC1.2 range (<8 wt % MgCI•). The paragenetic suite of minerals suggests increasing amounts of CaCI• and KC1 with time, mineralizing fluids being characterized by increases in these fractions and by broad chemical fields.

Jubilee deposit

Results for decrepitates from phases of the Jubilee deposit are listed in Table 2 and plotted on ternary diagrams in Figure 9. KC1 concentrations are very low for all the studied phases (Fig. 9A). Presulfide fibrous calcite concentrations cluster around 70, 20, and 10 wt percent NaC1, CaCI•, and MgCI•, respectively. In contrast, sphalerite and synsulfide anhedral calcite are always under 8 wt percent MgC12 and show varia-

CATION STUDY OF JUBILEE & GAYS RIVER (CANADA) DECREPITATES

T^BLE 1. SEM-EDS Analyses of Decrepitates from the Gays River Deposit

925

Bealn

Density (nM/.tm 2) Decrepitate diameter Sample no. Mineral • Stage

Concentrations

Ca Na Mg K C1 (wt %)

0.5 4 217 DOL 1 0.2 4 305 DOL 1 0.2 6 305 DOL 1 0.07 8 900 DOL 1 0.2 5 217 S 2

0.2 6 217 S 2 0.2 6 217 S 2 0.2 5 898 S 2 0.5 4 305 AC 3 0.3 10 305 AC 3 0.15 7 305 AC 3 0.2 5 305 AC 3 0.2 5 305 AC 3

0.2 4 305 AC 3

0.2 6 305 AC 3 0.3 3.5 888 AC 3 0.2 6 888 AC 3

0.15 8 888 AC 3 0.2 6 888 AC 3 0.3 5 888 AC 3

0.5 3.5 888 AC 3 0.3 4 888 AC 3 0.2 6 888 AC 3

0.15 8 898 AC 3

0.3 5 898 AC 3 0.3 4 898 AC 3 0.3 4 898 AC 3 0.3 4 898 AC 3

0.3 4 898 AC 3 0.5 4 898 AC 3 0.5 3.5 898 AC 3

10.6 31.8 0.9 0.4 56.3 6.7 34.1 0.6 0.7 58.8 5.9 34.8 1.8 1.0 56.5 4.0 35.9 0.6 2.1 57.4

16.6 23.5 0.6 2.1 57.2 11.9 27.9 nd 1.2 58.9 5.4 33.3 nd 7.6 54.8 1.9 36.3 nd 2.6 59.2 8.8 32.0 1.9 1.7 55.5 7.6 28.9 1.1 3.3 59.0

18.8 20.3 1.6 1.3 58.0

7.5 36.] 0.5 1.2 54.7 8.9 25.4 1.5 6.9 57.4 9.0 25.8 0.8 8.7 55.7

16.4 23.0 1.5 1.4 57.6

4.0 39.1 0.5 1.7 54.8 4.0 35.9 nd 1.8 58.2 6.6 35.5 1.0 2.4 54.6

5.5 34.8 0.7 4.3 54.7 7.0 34.4 nd 2.6 56.0 4.3 40.1 nd 1.9 53.6 3.3 40.1 nd 0.7 55.8 5.6 36.4 nd 1.7 56.3 3.6 38.1 0.5 1.9 55.9 5.8 36.8 nd 0.8 56.6 6.2 36.3 0.9 1.9 54.7 6.7 36.7 nd 1.2 55.4 5.6 36.6 nd 1.7 56.1 3.7 36.5 0.9 4.7 54.2 8.3 35.1 0.6 1.4 54.6

7.7 35.4 nd 1.9 55.1

• AC: anhedral calcite; Dol = dolomite; S = sphalerite; nd = not detected

tions between 40 and 90 wt percent and 10 and 60 wt percent for NaC1 and CaC12, respectively. Collectively, results for the paragenetie suite indicate a decrease in Mg and Ca concentrations with time. As illustrated by results on sphalerite and anhedral calcite deerepitates, the mineralizing system is characterized by broad NaC1-CaC12 ranges, the CaCl•-rieh end member being significantly higher than the presulfide fibrous calcite range.

Time trends and comparison of the Gays River and Jubilee deposit results

The paragenesis discussed earlier for the two deposits indi- cated that presulfide conditions can be studied using dolomite and fibrous calcite; mineralizing conditions can be revealed by sphalerite and anhedral calcite, with this last phase being also partly postsulfide at Gays River (Fig. 7). Diagrams of average concentrations per sample illustrate well the decrepitate chemical variation through time underlined in the for- mer section (Fig. 10). For the Gays River deposit, the system evolves from high Ca, low K, and high Mg proportions during dolomite precipitation (stage 1), to lower Ca, higher K, and lower Mg during mineralization and precipitation of some calcite (stage 2), with a final variation toward initial conditions during the precipitation of the syn- to postsulfide calcite (stage 3; Fig. 10). For the Jubilee deposit, the system evolves

from lower Ca and higher Mg proportions during precipitation of presulfide fibrous calcite (stage I), to higher Ca and lower Mg proportions during mineralization (i.e., lower Mg/ Ca ratios, stage II; Fig. 10A, B).

All the Gays River deerepitates are strikingly enriched in K relative to the Jubilee ones (Fig. 10A, B). MgCI• contents during mineralization are similar at the two deposits. A salient point is that Ca concentrations are relatively higher at Jubilee (Fig. 10A).

In summary, deerepitates of pre-, syn-, and postsulfide stages indicate significant variations in relative abundance of eations through time at the Gays River and Jubilee deposits. The main differences between the two deposits are the relatively higher K and lower Ca in the Gays River deerepitates. These eation characteristics can be discussed in terms of evo-

lution of brines involved during the Gays River and Jubilee mineralization.

Ore-Forming Brines--Discussion

Windsor cation composition compared to Mississippi Valley-type cation abundances

Results for Gays River and Jubilee decrepitates are plotted against data for Mississippi Valley-type deposits of the Pine Point, East Tennessee, and Ozark regions (Fig. 11). Cation

926 SAVARD AND CHI

TABLE 2. SEM-EDS Analyses of Decrepitates from the Jubilee Deposit

Beam

Density (nM•m 2) Decrepitate diameter Sample no. Mineral 1 Stage

Ca

Concentrations

Sa Mg (wt %/

c1

0.3 6 256 FC I 6.9

0.5 3.5 256 FC I 8.3 0.3 5 279 FC I 10.9 0.4 4 279 FC I 8.6 0.5 6 256 S II 5.8

0.5 4 256 S II 6.7 0.5 5 256 S II 12.9

0.5 5 256 S II 8.4

0.5 7 268-2 S II 18.9

0.5 6 268-2 S II 17.0

0.2 7 268-2 S II 22.9

0.5 3 279 S II 7.6

0.3 4.5 279 S II 22.5

0.2 4 279 S II 10.8 0.4 6 279 S II 15.6

0.3 4 279 S II 14.9

0.2 4 279 S II 12.5 0.2 7 279 S II 14.2

0.2 7 279 S II 11.3

0.3 9 279 S-r II 2.1

0.3 5 279 S II 17.0

0.15 9 279 S II 22.8

0.15 7 279 S II 21.3

0.3 6 279 S II 15.4

0.08 10 279 S II 20.7

0.5 3.5 256 AC III 14.8

0.4 4 256 AC III 19.4 0.2 6 256 AC III 20.6 0.2 7 256 AC III 2.6 0.25 4 256 AC III 14.7 0.2 8 256 AC III 17.1 0.3 4 256 AC III 19.0 0.1 9 279 S-r II 21.8 0.3 r II 21.2

r II 21.4

r II 20.3

r II 21.4

34.3

29.5

25.2

32.9 35.7

32.6

28.8

31.0

23.0

22.5

16.1

35.2

17.4 31.6

26.2

28.2

29.4 27.7

28.4

19.2

24.2

19.1

18.3

24.6

20.0 29.2

16.7

18.7

38.8

25.7

23.7

21.8

18.5

19.6

17.8

21.4

18.9

2.0

4.7

3.5

1.3

nd nd nd nd nd 0.5

nd 0.4

1.0

1.1

1.7

0.4 0.4

1.4

nd 0.4

0.7

nd 0.6

nd nd nd 0.4

1.8

1.2

nd 0.7

1.7

0.5

0.4

0.5

0.4

0.4

nd

nd nd 0.6

nd nd nd 0.3

nd nd 0.8

nd nd nd 0.7

nd 0.1

1.4

0.4

nd nd 0.4

0.6 0.9

nd 0.8

0.3

nd nd nd 0.4

nd nd 0.4

56.7 57.1

60.4 56.4

58.5

60.6

57.7

60.6 58.1

60.0

60.7

56.8

59.1

55.6

56.5

56.5

57.7

56.0

60.2

59.0

56.7

57.6

59.8

60.0

59.0

55.4 62.7

58.9

56.6 59.3

58.5

57.5

59.2

58.4

60.3

57.8 59.0

t AC = anhedral calcite; FC = fibrous calcite; S = sphalerite; r = repeat; nd = not detected

proportions of the Gays River decrepitares fall in a field com- parable with those for the East Tennessee district in the Appalachians and the Central Missouri and Viburnum Trend districts in the Ozark region in that they all show high KC1 and highly variable CaCI.2 concentrations (Fig. liB, C). De- crepitares from the Jubilee deposit compare with these from the Tri-State district setting, in terms of their low KC1 proportions (Fig. 11C). In general, the Gays River and Jubilee deerepirates in syn- to postsulfide minerals show CaCI.2 relative abundance lower than that for Pine Point equivalents (Fig. 11A) but similar to results for a group of North American deposits mineralized by brines which mostly interacted with elastic aquifers.

The Mississippi Valley-type host rocks are always carbonates, but the aquifers or fractured units that conducted mineralizing brines can be silieielastie rocks or carbonates. For the Mississippi Valley-type deposits discussed above, eation proportions of sulfide-stage deerepirates reflect brine compositions that were inherited through water-rock interaction during burial confinement and/or long distance migration. In

other words, brine eation ratios not only depend on the primary soume of fluids but also on aquifer compositions; if massive replacement and/or dissolution takes place during mineralization, host-rock composition will also influence the eation ratios (Viets and Leach, 1990; Hanor, 1994; Leach, 1994).

Qing and Mountjoy (1999,) suggest that the Pine Point mineralizing brines originated from the western margin of the Western Canada sedimentary basin and migrated more than 400 km in a Devonian earbonate aquifer. In the East Tennessee distriet, mineralizing brines from the shale-rieh Sevier basin (Kesler et al., 1989) traveled laterally through about 50 km of earbonates of the Lower Ordovieian Knox

Group that they dolomitized regionally (Montanez, 1994). In the Ozark region, ineluding the Central Missouri, Viburnum Trend, and Tri-State distriets, mineralizing brines were expelled from the Arkoma basin and migrated along hundreds of kilometers of the Cambrian Lamotte Sandstone and Paleo-

zoie earbonates-sandstones during the late Carboniferous- Early Permian rise of the Ouaehita Mountains (Leaeh and

CATION STUDY OF JUBILEE & GAYS RIVER (CANADA) DECREPITATES 99,7

CaCI 2

80

60

40

10 30/0 50/20 70/30 100/70

NaC1 Weight % MgC12/KCI

FIG. 6. Ternary plots of five repeated analyses for one large decrepitate in sphalerite (Jubilee deposit, sample 279).

Rowan, 1986; Viets and Leach, 1990). For the Gays River deposit which shows low Ca/Na, long interaction of the mineralizing brines with clastic rocks of the Horton Group has recently been proposed by Chi and Savard (1998), Fallara et

A Gays River Deposit Stage 1 Stage 2 Stage 3

Isopachous Calcite •

Replacive Dolomite

Dissolution Features ?• Dolomite cement

Micro-fracture Micro-breccia --• •

Sphalerite, Galena Methane ? ?

Liquid HC ?--- ? ? Fluorite, Barite

Silica ? ....... ? Anhedral Calcite

Stylolites •

B Jubilee Deposit Stage I Stage II

Pyrite • Fracture

Breccia

Dissolution Features ?

Fibrous Calcite --

HC generated/migrated

Marcasite/Pyrite • Sphalerite+Galena Chalcopyrite Anhedral Calcite

Stylolites • Barite •

FIG. 7. Paragenesis; HC = hydrocarbons. A. Gays River deposit. B. Jubilee deposit.

Car12

A

60

40

NaC1 8o 60 40 20 Weight % KC1

Dolomite, pre-sulfide ///•Sphalerite

nhedral calcite, syn- to post-sulfide

80

B

40

80 60 40 20

NaC1 Weight % MgC12

FIG. 8. Ternary diagrams of the most abundant cations present in the decrepitates of the main paragenetic minerals at the Gays River deposit. A. CaC12-NaC1-KC1. B. CaC1.2-NaC1- MgCI=.

al. (1998), Sangster et al. (1998), and Savard and Kontak (1998).

The significantly higher CaC12 concentrations for the Pine Point decrepitares are very likely due to longer term interaction with aquifer carbonates, whereas the low CaC12 proportions for the Ozark and Windsor deposits probably reflect brine migration in clastic aquifers. We suggest that for the discussed Mississippi Valley-type districts, the proportion of CaClz in the mineralizing brines increases with interaction with carbonate aquifers. For brines that interacted with elastic aquifers, differences in CaCls, NaC1, and KC1 proportions might be due to the different mineralogical composition of elastic rocks and the nature of the original brines (see below).

Comparison of Nova Scotian paleobrines to modern basinal brines

The composition of modern formation waters has been well documented for the Gulf Coast, Western Canada, and Illinois basins. Data compilation shows brines with distinct compositional fields (Fig. 12) which can be studied in terms

928 SAVARD AND CHI

CaCI2

A

. " ' ?.: '?" ¾' .5 NaCI 80 60 40 20 KC1

Weight %

Fibrous calcite Pre-sulfide []

A Sphalerite o ! \ Anhedral calcite

80 /" "':• 20 Syn-sulfide ' B

f. . .... NaC1 80 60 40 20 MgC12 Weight %

F]c. 9. Ternary diagrams of the most abundant cations present in the decrepitares of the main paragenetic minerals at the Jubilee deposit. A. CaC12-NaC1-KC1. B. CaC12-NaCl~ MgCL2.

of original composition and chemical modifications during interaction with aquifers. Carpenter et al. (1974) proposed that the high K contents of oil field brines in the Gulf Coast basin originated from evaporation of seawater which produced a brine later expelled from the Louann Formation (mostly halite). The K-Na relations obtained for the Western Canada sedimentary basin have been interpreted to result from aliagenetic reactions of a residual evaporitic brine that later mixed with meteoric waters (Connolly et al., 1990). In that basin, waters with the highest Na and Ca concentrations (brine end member) are in a carbonate aquifer, suggesting a local source of Ca. Similarly, the Illinois brine is interpreted to correspond to seawater evaporated to a level short of halite. The produced K- and Mg-rich brine, in burial, reacted with clays and limestone, became Ca-Na enriched, and mixed with an oil-rich brine and with fresh water (Stueber and Walter, 1991), the produced mixtures having the composition shown in Figure 12. Common processes for the three basins are seawater evaporation that created high K-Mg contents; Na-

Ca enrichment due to transformation during elastic and carbonate aliagenesis; and solution mixing that explain broad ranges of concentrations observed for K and Na, particularly for Na, the most abundant cation.

The Gays River mineralizing brine shows broad ranges and high K concentrations similar to those of the Gulf Coast basin brines, but its Na proportions show a broader range and are mostly higher (Fig. 12). Na proportions for the Jubilee mineralizing system spread over the combined range for the three modern basins; its very low K proportions compare with those of the Western Canada sedimentary and the Illinois basins.

Nature and source of the Nova Scotian palcobrines Fluid inclusions studied by mierothermometry all showed

salinities (15-30 wt % NaC1 equiv) significantly higher than that of seawater (Chi and Savard, 1995; Chi et al., 1995). The SEM-EDS study shows that the Nova Seotian palcobrines also have much higher CaC12 but lower NaC1, and even much lower MgC12, proportions than seawater (Figs. 10, 11). Other variations relative to seawater are that the Jubilee brines have lower KC1 proportions, whereas Gays River brines have higher KC1 ones (Figs. 10, 11).

Stage 1 deerepirates from the Gays River and Jubilee deposits show similar Ca and Na proportions, but their Mg and K proportions vary; the Gays River deerepitates have lower Mg and higher K. For the two deposits, the presulfide results cluster on all plots (Figs. 8-10). In contrast, synsulfide minerals record broad ranges of cation proportions, particularly Na (Figs. 8-12). These broad chemical variations very likely re-

m 0.6

• 0,4 .--

B

0.2 I II1 1

0,001 : :

0.001 0,02

0,15

Gays River: 1, 2, 3 Jubilee: I, II

32

0,04 0,06 0.08 0.1

o,1

0.05

I

0.001 II1¾1 - .

0.001 0,02

3

0.04 0,06 0,08

K/Ca+ Na+ Mg+ K (weight ratios)

0.1

FIG. 10. Plots of average results for each decrepitate coinparing the Gays River and Jubilee time sequences. The star represents seawater coinposition. A. For Ca proportions. B. For Mg proportions.


CaC12/• Gays River Deposit: 1,2, 3

/•. ;• ,.•ubilee Deposit: I, II

_ ••,••. , ]Pine P•nt •eposit

.)- . .. . :, . .

NaC1 •e•gm •o KC1

ast Tennessee Deposit

40 ••::•._ :.?phfle•te

0 ': '• " ' .' ':." NaCI •eight • • KC1

C•12• C /..•:.:• Oz•k Region_

/ 2 :••.•' . :• Cen•al Missore as • '• 22 '1 o Vib•num Trend IIc- •/¾::'......'....:......x...•.• TfisS•te

ß .. ' ..' ...

NaC1 % • • % KC1 Weight

',Ore'stage \ '\

,,.', ',{pine -p0int !9xeposit

FIG. 11. Ternary diagrams for comparison of cation proportions in fluid inclusions of Gays River and Jubilee deposits and selected Mississippi Valley- type deposits. The star represents seawater composition. A. Comparison with syn to postsulfide data of the Pine Point deposit, Canada, obtained by SEM- EDS (Haynes and Kesler, 1987). B. Comparison with syn to postsulfide data of the East Tennessee deposit, USA, obtained by SEM-EDS (Haynes and Kesler, 1987). C. Comparison with synsulfide data of deposits from the Ozark region, USA, obtained on leachares by atomic absorption spectrophotometry (Na, K, Ca, MR) and ion chromatography (C1) (Viets and Leach, 1990).

suit from solution mixing as inferred for modem brine compositions in the previous section.

It has been proposed that burial diagenetic brines, -80øC for Jubilee and - 150øC for Gays River, saturated the biolith- itc pores prior to mineralization and mixed with a metal-rich hydrothermal brine (•250øC) during mineralization (Chi and Savard, 1995; Chiet al., 1995; Savard and Kontak, 1998). The diagenetic and hydrothermal brines were expelled from different stratigraphic levels of the Horton Group where they resided prior to mixing (Chi and Savard, 1998; Savard and Kontak, 1998). Our data suggest that presulfide brines represent Na-rich aliagenetic solutions which later mixed with metal-rich brines--a K-rich brine at Gays River and a low Mg/Ca brine at Jubilee.

The MgC12 content during mineralization at Jubilee is similar to that at Gays River. A situation unexpected if in situ derivation of Mg by carbonate dissolution is invoked at the two deposits because their respective host rocks are limestone and dolostone. Most Mg is therefore brought by the brines. Evaporation of seawater at levels short of sulfates-halite can produce Mg-rich brines such as those expelled from evaporites of the Lower Windsor Group during burial.

Main chemical differences for synsulfide deerepitates of the Gays [liver and Jubilee deposits are the higher K and lower Ca proportions at Gays River which we interpret as evidence for distinct composition of mineralizing brines. This is most likely due to different mineralogieal compositions of the respective aquifers. The isotopic, fluid inclusion miero- thermometrie, and organic matter reflectance results indicate that the source for the metal-rich brines for the two deposits was in the Horton Group and significantly deeper than that of the Lower Windsor Group (H6roux et al., 1994; Chi and Savard, 1995; Chi et al., 1995; Fallara et al., 1998; Savard and Kontak, 1998). The Horton Group fluviatile units on mainland Nova Scotia near Gays River and on Cape Breton Island near Jubilee were deposited during the same time interval (Devono-Carboniferous), in similar successions (coarse- to shale- to medium-size grains), but their prove- nances are different. On mainland Nova Scotia, the Horton Group is mostly derived from metamorphic rocks of the Me- guma Group (Cambrian to Silurian). On Cape Breton Island, the sources of the Horton Group were diverse: Hadrynian, Cambrian, Ordovician, and Devonian granites and granodio- rites; Ordovician schists, amphibolite, gneiss, phyllite, and granite gneiss; and Devonian conglomerate, basalt, sandstone, tuff, andesitc, and siltstone. These lithological differences in basement rocks can possibly explain why the Gays River and Jubilee deposits show different K-Na-Ca compositional fields (Figs. 8-12), as well as their distinct Pb/Pb and S7Sr/S6Sr isotope ratios (see Sangster et al., 1998). However, no detailed study of mineralogical variations has been performed on the Horton Group units (B. Murphy, pers. commun., 1996), thus hindering further discussion.

Conclusions

Our SEM-EDS semiquantitative investigation of fluid in- elusion decrepitate chemistry for the paragenetie phases at the Gays River and Jubilee Zn-Pb deposits led to the following conclusions:

1. A technique sensitive to light elements and tests on salt

930 SAVARD AND CHI

B

A

0.15

+ 0.1

2 3

0.15

3

0.05

i O. 00i 3 0.4 0.05- ; 3 t

0.001- , a I ? • 0.001 0.2 0.4 0.6 0.8

Mg/Ca (weight ratios)

Brines

• Gulf Coast Basin • Illinois Basin

• Western Canada Sedimentary Basin Gays River Decrepitates: 1, 2, 3 Jubilee Decrepitares: a (stage I), b (stage II)

3 3

I I I

0.6 0.8

NaJCa+Na+Mg+K (weight ratios)

3

1 1.2

Fla. 12. Comparison of brine characteristics of Gulf Coast (Carpenter et al., 1974), Illinois basin (Stueber and Walter, 1991), and the Western Canada sedimentary basin (Cormoily et al., 1990) with the decrepitate ratios from the Gays River and Jubilee deposits. A. Ratios of K to sum of cations vs. Mg to Ca. Seawater composition falls off the graph, Mg/Ca = 3.14. B. Ratios of K to sum of cations vs Na to sum of cations. The star represents seawater composition.

standards, spha!edhte, and carbonates revealed surficial C-O contamination from air on all deerepitates studied, a phenom- enon never reported in previous SEM-EDS studies. The contamination can be reduced by choosing an optimal voltage of 10 Kev, high enough to diminish the C and O peaks but low enough to avoid piercing of deerepirates (avg size = 5/am). The technique shows that C-O contamination of carbonates never exceeds that of sphaledhte, certifying that the results on deerepirates do not involve matedhal from the host carbonate substrate.

2. SEM-EDS results show that the Gays River decrepitates of synmineralization phases contain higher KC1 fractions than their Jubilee counterparts.

3. Overall for their decrepitate cation proportions, the Windsor deposits share characteristics with Mississippi Val- ley-type deposits associated with elastic aquifers such as those of the Viburnum trend. We propose that Ca/Na ratios in Mississippi Valley-type brines can vary with the degree of brine interaction with carbonate aquifers.

4. Cation ratios vary through time at the two deposits. Gays River decrepitares show an increase in K concentrations from pre- to synsulfide stages, whereas Jubilee decrepitates show Ca enrichment.

5. The comparison of the mineralizing systems collectively suggests different mineralizing brine compositions for the Gays River and Jubilee deposits. In the Gays River area, the provenance for the fluviatile elastic rocks of the Horton Group was mostly metamorphosed marine sedimentary rocks, whereas on Cape Breton Island, the source was dominated

by plutonie rocks. We propose that Na-dheh brines largely composed of evaporated seawater were modified due to interaction with mineralogically distinct regional Horton aquifers which yielded different Ca-Na and K-Na contents.

Acknowledgments

Funding was provided by the Geological Survey of Canada, the 1992-1995 Canada-Nova Scotia Cooperation Agreement on Mineral Development, and the Magdalen basin Natmap program. Many thanks to Jean-Pierre Tremblay from Laval University for his help throughout the SEM-EDS investigations. The manuscript benefited from an early review by D. F. Sangster and from constructive comments by Economic Geol- ogy reviewers, S. E. Kesler and j. s. Hanor. Stimulating dis- cussions with Y. H6roux, A. Hamblin, B. Murphy, and D. Kontak are acknowledged.

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