Geochemical responses in peat groundwater over ... · posed of mantle olivine, pyroxene, and xenoliths of limestone. It also contains, ilmenite, and clinopyroxene, minor garnet and,

Geochemical responses in peat groundwater over Attawapiskatkimberlites, James Bay Lowlands, Canada and their application to

diamond exploration

Jamil A. Sader1,4,*, Keiko H. Hattori1, Julie M. Kong2, Stewart M. Hamilton3 &

Kerstin Brauneder1

1University of Ottawa, Earth Sciences Department, Ottawa, Ontario, K1N 6N5, Canada2

DeBeers Canada Inc, 65 Overlea Blvd., Toronto, Ontario, K4H 1P1, Canada3Sedimentary Geoscience Division, Ontario Geological Survey, Sudbury, Ontario, P3E 2G9, Canada

4Present address: Minerals and Metals Group, Vancouver, B.C., V6C 3E1, Canada*Corresponding author (e-mail: [email protected])

ABSTRACT: Peat groundwater compositions at depths of 0.4 and 1.1 m belowground surface in the Attawapiskat region of the James Bay Lowlands are evaluatedfor diamond exploration applications. Samples were collected along transects thattypically extended at least 200 m beyond the margins of Yankee, Zulu, and Golfkimberlites. Locations of upwelling groundwater usually occur at or near kimberlitemargins based on hydrogeological measurements and variations in peat groundwatergeochemical parameters (pH and EC are high, and the Eh is low relative toombrotrophic peat groundwaters). Concentrations of the kimberlite pathfindermetals Ni, Cr, light rare earth elements (LREEs), Ba, Mg/Ca, and alkalis arecommonly elevated at sample sites at or near kimberlite margins and wheregroundwaters are upwelling. The presence of elevated kimberlite pathfinders at thesesites suggests that fractures along the boundaries between kimberlites and limestoneformed during kimberlite emplacement provide dilation for upward movement ofgroundwater with elevated kimberlite pathfinder metals. Typically, Ni, Cr, LREE,and Ba behave similarly and thus high concentrations of these metals are found atsimilar locations along transects. On the other hand, locations of elevated alkalis andMg/Ca vary. The spatial variations among pathfinder metals in peat groundwatersare possibly due to geochemical processes in the peat, such as metal binding todissolved organic material, adsorption to insoluble organics or Fe-oxyhydroxides,and incorporation into secondary mineral precipitates, which can act to increase ordecrease metal solubility. The findings of this study are readily applicable in diamondexploration in wetlands elsewhere.

KEYWORDS: surficial geochemical exploration, groundwater geochemistry, kimberlite, diamond,James Bay Lowlands, metal migration, dispersion

SUPPLEMENTARY MATERIAL: Krigged water table data for Yankee and Zululocations are available at http://www.geolsoc.org.uk/SUP18488.

The James Bay Lowlands in northern Ontario, Canada repre-sents one of the largest continuous peat bogs on Earth with anarea of c. 300 000 km2 (Sjors 1963). In addition to the Atta-wapiskat kimberlite field, there are several kimberlites that makeup the Kyle Lake kimberlite field in the James Bay Lowlands. Itis likely that there are additional kimberlites that have not beendiscovered at these two fields. On a global scale, there havebeen numerous kimberlite discoveries in other northern regionssuch as Russia and Finland (Janse & Sheahan 1995; O’Brien &Tyni 1999; Lehtonen et al. 2005) where peat bog terrain iscommon (Frenzel 1983). Due to the lack of mineral soil atsurface, the most common exploration methods used in the

James Bay region are geophysical. However, kimberlites are notalways magnetic, and magnetic geophysical anomalies are notnecessarily kimberlite. To gain further information as towhether a geophysical response concealed by sediment covermay be kimberlite, surficial geochemical exploration in glaciatedterrains commonly utilize tills and soils (Mann et al. 1998;Cameron et al. 2004; McClenaghan et al. 2006; Hattori et al.2009; Sader et al. 2009) and recently peat (Hattori & Hamilton2008).

In wetlands, conventional mineral soil sampling is notfeasible in geochemical exploration due to the dominance ofsphagnum peat at surface. As the James Bay Lowland region is

Geochemistry: Exploration, Environment, Analysis, Vol. 11 2011, pp. 193–210 1467-7873/11/$15.00 � 2011 AAG/Geological Society of LondonDOI 10.1144/1467-7873/10-030

dominantly composed of water-saturated peat, peat groundwa-ters may be useful as a medium for surficial geochemicalexploration. Groundwater geochemistry has been shown toprovide information related to underlying rock types in wet-lands (Syrovetnik et al. 2004) and has also been used toeffectively vector to mineral deposits in a variety of settings(Leybourne & Cameron 2010). Anomalous geochemicalresponses in near-surface media are typically the result ofwater – ore interactions due to weathering processes at depth(Leybourne & Cameron 2006; Sader et al. 2007a).

This study was conducted to examine whether shallow peatgroundwater may be used to identify buried kimberlites inwetlands. The results present evidence to suggest that peatgroundwaters have metal anomalies due to underlying kimber-lites. Kimberlites easily undergo low temperature serpentiniza-tion (Sader et al. 2007a, b), which results in an unusualgroundwater geochemistry relative to waters flowing through ahost of other rock types (Leybourne & Cameron 2010). Thegeochemical contrast with waters whose origin is limestone orTyrell Sea sediment may be used for diamond exploration inwetlands. Geochemical results, coupled with hydrogeologicalparameters, are used to discuss metal transport mechanisms inwetlands.

LOCATION, GEOLOGICAL SETTING ANDMINERALOGY

The kimberlites in this study are located in the James BayLowlands c. 90 km west of the community of Attawapiskat andwithin 15 km of the DeBeers Victor diamond mine (Fig. 1a &b). These mid Jurassic (c. 170 Ma) kimberlites are emplacedinto Ordovician and Silurian limestone, dolostone, clastic sedi-mentary rocks, and Archean basement igneous and metamor-phic rocks (Norris 1993; Webb et al. 2004) (Fig. 2). Host rockto kimberlites at the bedrock surface is the Upper AttawapiskatFormation limestone (Fig. 2), which also locally outcrops in theform of bioherms up to 2 m above the surrounding groundsurface. Bioherms are reef cores composed of coral and skeletalremains of other marine organisms (Cowell 1983). Theygenerally represent locations of groundwater recharge, as bio-herms contain abundant void spaces and karstic textures.

A thin till layer (< 1 m in thickness) was deposited on theglacially eroded bedrock surface during the Quaternary. TyrellSea sediment (TSS) (2.1–21 m in thickness) is composed ofvarying fractions of silt and clay that are grey in colour. Small 10to 20 cm thick sand lenses and small pebbles are occasionallyobserved. This sediment was deposited between 10 and 5 Kafollowing the end of the last glaciation when the shoreline ofJames Bay (referred to as the Tyrell Sea during that period)extended inland c. 300 km west and southwest of its presentlocation. Isostatic rebound resulted in ground surface elevationincreases of 100–300 m since the retreat of the Laurentide icesheet in the Attawapiskat region (Shilts 1986). Peat (2.5–3.4 min thickness) is located at the surface and has been accumulat-ing since the retreat of the Tyrell Sea c. 5 000 years BP. The peatis dominantly composed of sphagnum and becomes progres-sively more decomposed with depth. All kimberlites are buriedby Quaternary sediment, with the exception of the Zulukimberlite, which outcrops at one location at the south margin.

The Yankee kimberlite is located in a shallow bowl-shapeddepression and bioherms are located southwest and northeastof the kimberlite (Fig. 3a). The Zulu kimberlite is located onthe east part of a lobate raised bog and there is a small biohermbeside the south margin of the kimberlite (Fig. 3b). Groundsurface at the Golf kimberlite is similar to that of Zulu, as theyboth slope gently from west to east. However, no bioherm

was noted in the areas adjacent to Golf (Fig. 3c). Theground surface at the Control site grades gently towards theAttawapiskat River c. 600 m to the north (Fig. 3d). Geophysical

Attawapiskat R.

Alb

any

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Alpha-1

Zulu

Yankee

Victor Mine Site

Control

b.

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Bravo-1

Attawapiskat Attawapiskat

kimberliteskimberlites

Cretaceous

Devonian

Silurian

Ordovician

Precambrian

Hudson Bay

James

Bay

Fig. 1. (a) Regional geology of the James Bay Lowlands; fromBellefleur et al. (2005); and (b) the locations of Attawapiskatkimberlites and the Control site in this study.

J. A. Sader et al.194

data (DeBeers, unpublished) suggest that there is no kimberlitein the vicinity of the Control site.

Attawapiskat kimberlite mineralogy and geochemistry

Detailed mineralogical descriptions of each kimberlite weremade by Sage (2000a). To summarize his work: Yankeekimberlite is hypabyssal with mantle and crustal xenoliths. Itcontains macrocrysts of garnet, ilmenite, phlogopite, clino-pyroxene, and olivine in no particular order of abundance.Diamond has not been found in this kimberlite. Zulu kimber-lite is diamondiferous and is composed of hypabyssal andbrecciated facies with limestone, basement, and mantle xeno-liths. It contains macrocrysts of olivine, phlogopite, ilmenite,and chrome diopside with minor garnet, chromite, and clino-pyroxene. Golf kimberlite is hypabyssal with xenocrysts com-posed of mantle olivine, pyroxene, and xenoliths oflimestone. It also contains, ilmenite, and clinopyroxene, minorgarnet and, minor chromite. It is unknown whether thekimberlite is diamondiferous or barren. The Alpha-1 andAlpha-1 North kimberlites are hypabyssal with limestone,basement, and mantle xenoliths. Olivine, pyroxene, phlogo-pite, ilmenite, minor chromite and minor chrome diopsidehave been identified. It is unknown whether the kimberlite isdiamondiferous or barren. The X-Ray kimberlite consists ofhypabyssal facies and contains limestone and other crustalxenoliths. The mineralogy includes olivine, ilmenite, chromediopside, and garnet, and the kimberlite is diamondiferous.The Bravo-1 kimberlite consists of hypabyssal facies withfresh and altered olivine, clinopyroxene, ilmenite, chromite,and rarely garnet. This kimberlite is diamondiferous. Carbon-ate, spinel, phlogopite, apatite, and perovskite are commongroundmass mineral in Attawapiskat kimberlites (Kong et al.1999; Armstrong et al. 2004).

The whole rock geochemical compositions of Attawpiskatkimberlites (DeBeers, unpublished data) are comparable toother Jurassic kimberlites along the Timiskaming fault such asthose from Kirkland Lake and New Liskeard (Sage 2000b) andother kimberlites worldwide (Mitchell 1986).

METHODS

Field procedures

Water sampling

Water samples were collected in the summer (August 14–23)and autumn (October 14–18) of 2007, and in the summer(early August) and autumn (late September) of 2006. Samplescollected in 2006 and 2007 are denoted by the prefixes 06 and07, respectively, and samples collected in fall are denoted by thesuffix F. Samples along Yankee, Zulu, and Golf transects aredenoted by the prefixes of Y, Z, and G, respectively. All watersamples from 2006 were reported by Brauneder (2007). Com-positions of water samples collected in 2007 are presented inTable 1.

Peat groundwaters were collected using piezometers with aninternal diameter of 19 mm, made of 1.5-m long white environ-mental grade polyvinyl chloride (PVC) or grey PVC. Piezom-eters function as a method to collect groundwater from adesired depth with minimal influence of waters from shallowerzones within an aquifer. In this study they were installed 1.1 mbelow ground surface (mbgs) into the peat at the Zulu, Yankee,and Control locations in 2007, and to 0.4 mbgs along thetransect at Golf and Control in 2006. Piezometers were pushedinto the peat with a loosely fitting plastic champagne cork at theend to prevent peat from entering the pipe while it was beinginstalled. The pipe was then pulled up c. 0.1 m so that the corkdid not impede water from flowing into the piezometer. Pi-ezometers were installed approximately every 25–50 m alongeach transect and at least 200 m beyond the kimberlite margins(where conditions permitted).

Monitoring wells were installed below the peat/TSS inter-face at each kimberlite location (two over the kimberlite andone outside the kimberlite margin) to collect deeper ground-water. Monitoring wells 07-MW-Y-10 and 11.5 at Yankee (Fig.3a), 07-MW-Z-15 and 17 at Zulu (Fig. 3b), 06-MW-G-10 and11 at Golf (Fig. 3c), 07-A-MW-11 and 12 at Alpha-1, and07-B-MW-01 and 03 at Bravo-1 were installed over the kim-berlites. Wells 07-MW-Y-01 at Yankee, 07-MW-Z-05 at Zulu,06-MW-G-01 at Golf, 07-A-MW-01 at Alpha-1, and 07-B-MW-02 at Bravo-1 were installed outside kimberlite margins.One well (07-MW-C-01) was installed at the Control site

Peat (3-4 m)Tyrell Sea Sediment

Lower Attawapiskat Fm.(mottled limestone and limey mudstone)

Ekwan River &Severn River Fm.(planar-bedded carbonates interbedded locally with calcareousshales and siltstones)

Red Head Rapids &Churchill River MudstonesBad Cache Rapids DolostoneBasal SandstoneArchean Igneous &Metamorphic Rocks

0

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Fig. 2. A schematic vertical section of rocks and sediment into which Attawapiskat kimberlites are emplaced. The kimberlite and host rocks areoverlain by a thin basal till layer (not shown), Quaternary Tyrell Sea sediment (12–4 Ka), and peat (4 Ka–present). Figure modified from Webbet al. (2004); geology from Norris (1993).

Water geochemistry and diamond exploration 195

(Fig. 3d). Each monitoring well was installed within 30 cm ofthe piezometer with the same identification (i.e. 07-MW-Z-17and 07-Z-17) with the exception of 07-MW-Y-11.5. Wells wereinstalled 70 cm below the peat/TSS interface except for07-MW-Z-15, which could only be advanced 13 cm below thepeat.

Groundwater samples were collected at a depth of c. 14mbgs from five exploration boreholes, All boreholes are casedthrough Quaternary units. They are located south of center atthe Zulu kimberlite (07-Z-07–12C) (Fig. 3b), c. 10 m outsideand up-gradient of the Yankee kimberlite margin (07-Y-07–7H)(Fig. 3a), and near the centers of Alpha-1 (06-A-BH-06),X-ray (07-X-07–014C) and Bravo-1 (07-B1–07-08C) kimber-lites. In addition, a spring discharging from limestone near theAttawapiskat River and the Control Site was sampled (Fig. 3d).

Field measurements

The water levels were recorded for all piezometers and moni-toring wells from Yankee and Zulu. The top of piezometers

and monitoring well casings were surveyed using a theodoliteand a surveying rod to determine the water table elevation, thegeneral direction of horizontal groundwater flow, and sites ofupwelling groundwater.

The pH, oxidation-reduction potential (ORP), electricalconductivity (EC), dissolved oxygen content (DO), and tem-perature were measured on-site with a Hanna HI-9828 multiprobe. The pH, DO, and EC probes were calibrated daily.Oxidation-reduction potential values have been corrected tothe standard hydrogen electrode (SHE) for waters at 10�C byadding 207 mV to the ORP values, and are reported as Eh.Water samples were filtered through 0.45-µm Sterivex-HVfilters (Millipore Corporation) into Nalgene high-density poly-ethylene bottles for cation and anion analysis. Samples fordissolved inorganic carbon (DIC) were collected in 40-mlbrown tinted borosilicate bottles with a silicone-Teflon septumcap. A polytetrafluoroethylene-rubber septum manufactured byChromatographic Specialties Inc. was inserted underneath theseptum cap to prevent DIC loss, as silicone is gas-permeable.

LegendPiezometer

Kimberlite

BiohermMonitoring Well

Exploration Borehole

a.

07-Y-0107-MW-Y-10

07-MW-Y-11.5

07-MW-Y-01

07-Y-07-7H

07-Y-17

07-Z-05

07-Z-3507-MW-Z-0507-MW-Z-17

07-MW-Z-15

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06-G-17

06-G-01

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06-MW-G-11

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06-C-01

06-C-08

07-MW-C-01

06-Spring

07-C-01

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d.

b.

c.

Fig. 3. Locations of piezometer transects, monitoring wells, and boreholes at (a) Yankee; (b) Zulu; (c) Golf; and (d) Control Site. Outlines ofkimberlites were determined by geophysics and drilling by DeBeers Canada.


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Tem

pºC

11.2

12.0

9.7

10.8

11.0

9.9

10.0

11.2

9.1

6.5

8.9

9.7

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9.6

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905.

605.

275.

465.

275.

415.

906.

156.

446.

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906.

216.

395.

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h(m

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257

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327

313

325

310

267

125

190

224

242

239

232

198

190

183

EC

(uS/

cm)

159

8838

4531

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229

168

280

271

154

399

379

303

349

DO

(mg/

L)

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571.

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000.

13bd

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290.

310.

57bd

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DIC

(mg/

L)

35.5

631

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26.7

220

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17.9

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25.3

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38.5

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56.4

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54.4

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Met

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Det

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imit

mg/

LC

aIC

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17.5

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38.7

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650.

580.

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563.

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2211

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0.37

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0.27

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0.43

0.00

0.50

2.51

2.62

2.48

1.78

2.65

2.22

1.63

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0.81

Na

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0.05

1.05

0.60

1.14

0.75

1.06

0.35

3.32

12.5

411

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20.9

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11.0

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25.5

130

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0.04

0.22

0.21

0.25

0.23

0.40

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0.27

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7114

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425

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18.7

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MS

315

0711

1151

362

226

113

852

623

524

910

4216

0987

722

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8827

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106

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240

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300

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2bd

bdbd

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8bd

bdbd

bdbd

bdbd

bdbd

Cr

ICP-

MS

0.02

0.10

0.10

0.24

0.13

1.14

0.40

0.25

0.32

0.13

0.08

0.20

0.13

0.07

0.21

0.08

0.10

Ba

ICP-

MS

0.02

7.60

3.66

3.32

3.29

7.80

6.31

20.7

249

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18.4

912

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15.8

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16.9

014

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12.6

214

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Ni

ICP-

MS

0.10

0.35

0.67

0.41

0.33

1.42

0.62

1.05

1.15

0.24

0.31

0.27

0.34

0.17

0.26

0.46

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Rb

ICP-

MS

0.00

50.

795

0.35

20.

605

0.14

90.

831

0.20

20.

497

2.63

82.

184

2.22

31.

831

3.43

51.

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Nd

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MS

0.00

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4


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Sam

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52.6

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52.6

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52.6

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52.6

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52.7

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52.7

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52.7

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52.7

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Tem

pºC

13.3

14.5

8.8

7.6

11.5

8.5

16.5

8.4

6.2

18.4

5.4

16.8

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127.

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6.77

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(mV

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326

310

124

420

927

927

660

192

210

237

224

212

102

175

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134

130

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3

EC

(uS/

cm)

481

474

524

377

448

248

285

440

410

409

475

407

411

479

304

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443

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0

DO

(mg/

L)

0.32

1.42

0.38

2.45

6.64

4.91

5.42

0.60

0.84

1.62

4.15

6.52

2.87

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1.95

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566.

827.

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90

DIC

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L)

68.3

376

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77.9

362

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70.9

961

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73.9

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64.6

457

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68.8

553

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56.4

112

6.16

32.0

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84.0

015

3.36

61.4

613

0.06

70.2

2

Met

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Det

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imit

mg/

L

Ca

ICP-

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0.03

110.

9411

2.39

85.6

525

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78.0

250

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3.17

102.

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6.19

112.

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3.89

43.6

282

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82.6

015

0.65

64.4

217

9.63

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80

Mg

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2.22

2.21

10.0

16.

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15.2

8na

7.91

8.63

6.03

5.86

4.27

3.98

3.09

2.49

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515

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10.8

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36

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0.29

0.58

0.55

0.68

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2.37

3.21

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11.9

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40

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27.6

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1.75

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118

117

294

3214

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8070

166

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180

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363

328

203

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612

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0.02

1.33

0.29

0.51

0.42

6.89

11.8

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1.21

0.91

1.65

1.16

0.22

0.24

2.09

0.80

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514

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0.36

0.43

0.28

0.23

Ba

ICP-

MS

0.02

8.83

15.5

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298.

6014

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13.0

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8.19

14.6

817

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015

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10.8

231

4.24

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15.4

331

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27.4

1

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MS

0.10

12.5

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6318

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20.9

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8625

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82.3

711

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9

Rb

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MS

0.00

51.

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1.39

63.

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4.26

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2.40

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40.

895

1.42

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0.74

60.

749

2.84

42.

161

0.78

70.

600

4.99

61.

686

3.82

11.

923

Cs

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0.02

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101

0.09

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0.36

70.

197

0.66

30.

599

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70.

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0.20

50.

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9.6

9.0

12.4

16.9

7.5

16.9

14.8

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048.

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373

299

444

1370

1237

312

799

104

252

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120

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612

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0.36

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55.

488

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0.00

220.

0011

0.00

710.

0091

Nd

ICP-

MS

0.00

30.

006

0.03

80.

030

0.40

20.

011

0.00

00.

028

0.03

2Sm

ICP-

MS

0.00

10.

004

0.01

60.

005

0.05

00.

002

0.00

10.

006

0.00

9

na=

not

anal

ysed

bd=

belo

wde

tect

ion


All water samples were kept cool in ice-packed coolers orrefrigerated until they were analysed.

Laboratory procedures

The elemental composition data of groundwaters weremeasured at the Geoscience Laboratories of the Ministry ofNorthern Development of Mines, Sudbury, Ontario, Canada.Waters were analysed for Ca, K, Mg, Na, and S using a Spectroinductively coupled plasma emission spectrometer (ICP-ES).Analysis of certified references FP83MI1 and FP83TE1 before,during, and after the run indicate a precision, calculated interms of relative standard deviation (RSD), of better than 5%for all metals except K (14%). The concentrations of Fe, Mn,Cr, Ni, Rb, Cs, Ba, and LREEs were determined using aninductively-coupled plasma mass spectrometer (ICP-MS).Analysis of certified reference SLRS-4 from the NationalResearch Council of Canada before, during, and after the runindicate a precision of better than 5% RSD for all metals exceptMn (7%) and Cs (7%). Prior to analysis, water samplesanalysed for cations were acidified to 1% concentration usingBaseline-grade HNO3 from Seastar Chemicals. Anions (Cl�

and SO42�) were determined using a Dionex ion chromato-

graph. The RSDs for internal references, which were includedduring the runs, are less than 5% for both Cl� and SO4

2�.Waters were analysed for DIC concentrations at the Universityof Ottawa G.G. Hatch Stable Isotope Laboratory using aFinnigan-Mat Delta Plus mass spectrometer. The 2 � analyticalprecision is � 0.002 ppm.

RESULTSHydrogeology and groundwater movement

The lateral velocity (v) of peat groundwater movement down-gradient was calculated using Darcy’s Law:

v = ( – Ki) ⁄ n

where K is the hydraulic conductivity of the peat, i is thegradient, and n is peat porosity. The calculation used a hydraulic

conductivity value of 0.001 cm/s based on measurements fromnorthern sphagnum bogs and spring fens (Chason & Siegel1986). This K value is in the upper range for the catotelum(peat zone deeper than 0.3–0.5 mbgs), as K typically rangesfrom 10�2–10�6 cm/s (Ingram 1983; Hoag & Price 1995;Price 2003) in this zone. An active peat porosity (n) value of 0.3was used based on measurements of the catotelum fromsphagnum peat bogs (Hoag & Price 1995, 1997).

Peat groundwaters along the Yankee transect flow fromapproximately SW–NE with a gradient (i) of �1.66 � 10�3.The water table is virtually horizontal between 07-Y-5.5 and 10(i = �1.46 � 10�5), and between 07-Y-12 and 15(i = �4.67 � 10�5) (Fig. 4a). The calculated velocity of peatgroundwater is 5.53 � 10�6 cm/s for the entire length of theYankee transect. However, the calculated velocity is signifi-cantly lower for the segment between 07-Y-5.5 and 10(4.82 � 10�8 cm/s), and for the segment between 07-Y-12and 15 (1.56 � 10�7 cm/s) along the transect. Upwelling ofdeep minerotrophic groundwater was detected at sites 07-Y-01,04, 08, 09 and 12 where the saturated zone is close to or abovethe ground surface. Additionally, the potentiometric surface ofmonitoring wells suggests upwelling in the vicinity of 07-MW-Y-01 and 07-MW-Y-11.5 (Fig. 4a). At Zulu, peat groundwaterflows from west to east with a horizontal gradient of�1.95 � 10�3 and is generally parallel to the ground surface(Fig. 4b). The calculated velocity of peat groundwater along thelength of the Zulu transect is 6.50 � 10�6 cm/s. Upwellingdeep groundwater is noted at 07-MW-Z-17 based on thepotentiometric surface in the monitoring well and likely extendsto the end of the probable transect.

At Golf, geochemical data are used to evaluate sites ofupwelling groundwater, as hydrogeological surveying was notconducted at this location. Elevated relative values of pH andlow Eh (Fig. 5c), and elevated Ca and EC (Fig. 6c) suggestupwelling of deep groundwater along the transect between sites06-G-08 (near the western margin of the kimberlite) to theeastern end of the transect. Elevated values of Ca, EC, andpH, and low values of Eh are commonly used to identify

Potentiometric Surface(Monitoring Wells)

met

ers

abo

ve s

eale

vel

0 100 200 300 400 500 600 700 800distance along transect (m)

78.5

79.0

79.5

80.0

80.5

met

ers

abo

ve s

eale

vel

WatertableGround Surface

9000 100 200 300 400 500 600 700 80087.0

87.5

88.0

88.5

89.0

89.5

MW-Y-01

MW-Z-17

MW-Z-15

MW-Z-05

MW-Y-11.5

MW-Y-10

Groundwater movement

b.a.

Upwelling groundwater

KimberliteBioherm Kimberlite

distance along transect (m)

07-Y

-01

07-Y

-02

07-Y

-03

07-Y

-04

07-Y

-06

07-Y

-07

07-Y

-08

07-Y

-09

07-Y

-10

07-Y

-11

07-Y

-12

07-Y

-13

07-Y

-14

07-Y

-15

07-Y

-16

07-Y

-17

07-Y

-5.5

07-Z

-05

07-Z

-07

07-Z

-09

07-Z

-11

07-Z

-15

07-Z

-17

07-Z

-21

07-Z

-23

07-Z

-25

07-Z

-27

07-Z

-29

07-Z

-31

07-Z

-33

07-Z

-35

07-Y

-19

07-Z

-13

Yankee Zulu

Fig. 4. Profiles of the ground surface, the water table, and potentiometric surfaces along the transects at (a) Yankee and (b) Zulu kimberlites.Sites with potentiometric surfaces greater than the water table indicate upwelling groundwater.


contrasts between upwelling minerotrophic and ombrotrophicgroundwaters in wetlands (Ingram 1983; Hill & Siegel 1991;Hoag & Price 1995). Regional drainage patterns of the area

surrounding Golf observed in aerial photography suggest thatregional groundwater movement is from west to east.

pH

Eh

0 100 200 300 400 500 600 700 800Distance (m)

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

pH

100

150

200

250

300

350

400

Eh

(m

V)

a.

KimberliteBioherm

07-Y

-01

07-Y

-02

07-Y

-03

07-Y

-04

07-Y

-06

07-Y

-07

07-Y

-08

07-Y

-09

07-Y

-10

07-Y

-11

07-Y

-12

07-Y

-13

07-Y

-14

07-Y

-15

07-Y

-16

07-Y

-17

Yankee

100 200 300 400 500 600 700 800Distance (m)

5.0

5.5

6.0

6.5

7.0

100

150

200

250

300

350

pH

Eh

(m

V)

b.

Kimberlite

0

Zulu

07-Z

-05

07-Z

-07

07-Z

-09

07-Z

-11

07-Z

-15

07-Z

-17

07-Z

-21

07-Z

-23

07-Z

-25

07-Z

-27

07-Z

-29

07-Z

-31

07-Z

-33

07-Z

-35

07-Y

-19

07-Z

-13

0 100 200 300 400 500 600 700 800Distance (m)

3.5

4.0

4.5

5.0

5.5

6.0

300

350

400

450

500

550

pH

Eh

(m

V)

c.

KimberliteGolf

06-G

-01

06-G

-02

06-G

-03

06-G

-04

06-G

-06

06-G

-07

06-G

-09

06-G

-10

06-G

-11

06-G

-12

06-G

-13

06-G

-14

06-G

-15

06-G

-16

06-G

-08

06-G

-05

06-G

-17

07-Y

-5.5

Fig. 5. Profiles of pH and Eh along transects over: (a) Yankee; (b)Zulu; and (c) Golf kimberlites. Typically, pH and Eh indicate sites ofupwelling groundwater and a shift from ombrotrophic to min-erotrophic peat groundwater conditions. Box plots represent themean, first and third quartile values for baseline ombrotrophic peatgroundwaters.

Electrical conductivityCa

a.

b.

c.

Ele

ctri

cal C

on

du

ctiv

ity

(µS

/cm

)

KimberliteBioherm

07-Y

-01

07-Y

-02

07-Y

-03

07-Y

-04

07-Y

-06

07-Y

-07

07-Y

-08

07-Y

-09

07-Y

-10

07-Y

-11

07-Y

-12

07-Y

-13

07-Y

-14

07-Y

-15

07-Y

-16

07-Y

-17

0 100 200 300 400 500 600 700 800Distance (m)

0

50

100

150

200

250

300

350

0

10

20

30

40

50

60

70

Ca

(mg

/L)

Kimberlite

07-Z

-05

07-Z

-07

07-Z

-09

07-Z

-11

07-Z

-15

07-Z

-17

07-Z

-21

07-Z

-23

07-Z

-25

07-Z

-27

07-Z

-29

07-Z

-31

07-Z

-33

07-Z

-35

07-Y

-19

07-Z

-13

0 100 200 300 400 500 600 700 800Distance (m)

0

50

100

150

200

250

300

350

400

0

10

20

30

40

50

60

70

Ca

(mg

/L)

Kimberlite

06-G

-01

06-G

-02

06-G

-03

06-G

-04

06-G

-06

06-G

-07

06-G

-09

06-G

-10

06-G

-11

06-G

-12

06-G

-13

06-G

-14

06-G

-15

06-G

-16

06-G

-08

06-G

-05

06-G

-17

0 100 200 300 400 500 600 700 800Distance (m)

0

25

50

75

100

125

150

0

5

10

15

Ca

(mg

/L)

Ele

ctri

cal C

on

du

ctiv

ity

(µS

/cm

)E

lect

rica

l Co

nd

uct

ivit

y (µ

S/c

m)

Yankee

Zulu

Golf

07-Y

-5.5

Fig. 6. Profiles of electrical conductivity and Ca along transects over:(a) Yankee; (b) Zulu; and (c) Golf kimberlites. Electrical conduc-tivity and Ca correlate well and indicate sites of upwelling min-erotrophic groundwaters. Box plots represent the mean, first andthird quartile values for baseline ombrotrophic peat groundwaters.


Geochemistry of groundwaters from kimberlite,limestone and Tyrell sea sediment

The pH values in water samples collected from explorationboreholes within Attawapiskat kimberlites vary from 7.5–8.5and the Eh varies from 23–272 mV. Concentrations of path-finder metals such as LREE, Ni, Cr, Ba, Mg, Rb, and Cs inkimberlite groundwaters are typically greater than those ingroundwaters collected from limestone (Table 1). On average,the majority of these pathfinder metals are also elevated inkimberlite borehole groundwaters relative to waters in TSS thatare directly above kimberlites. However, in TSS waters overkimberlites Ni and Cr have average concentrations of 26.5 and4.5 µg/l, respectively, which is twice the concentration inkimberlite borehole groundwaters (Table 1).

The pH values of groundwater in TSS vary between 6.5–8.5;however, at 06-G-MW-11F and 07-Y-MW-10 the pH values areas high as 9.25 and 9.05, respectively. The Eh values in TSSgroundwaters (average = 217 mV) are typically more elevatedcompared with Eh values in kimberlite groundwaters (average= 90 mV). Concentrations of LREEs, Ni, Cr, Ba, Mg, and Rbin TSS groundwaters are an average of 1.8, 1.7, 3.9, 2.8, 2.3, and1.2 times greater, respectively, over kimberlites compared tosamples collected outside their margins (Table 1). Additionally,the Mg/Ca weight ratios in TSS groundwaters are an average of3.9 times greater over compared to outside kimberlites. How-ever, Cs concentrations in TSS waters over kimberlites are only0.8 times the concentration of TSS waters outside kimberlitemargins.

Peat groundwater geochemistry

In peat groundwater samples, the pH values vary from 3.8–7.1and Eh values range from 125–512 mV (Fig. 5a, b, c). Thehighest pH and lowest Eh values are generally observed at sitesof upwelling minerotrophic groundwater along each transect.Of Yankee, Zulu, and Golf peat groundwaters, samples fromGolf have the lowest pH and highest Eh values. This may beexplained by the shallower collection depth of water at Golf(0.4 mbgs).

Ombrotrophic peat groundwaters are indicated by low EC(< 100 µS/cm) and Ca concentrations (< 8 mg/l) (Ingram1983; Hill & Siegel 1991; Hoag & Price 1995). Conversely,high EC values (> 100 µS/cm) and Ca concentrations (up to70 mg/l) suggest contributions of minerotrophic groundwatersand are observed at sites of groundwater upwelling along thetransects (Fig. 6a, b, c). High EC and Ca are accompanied byelevated DIC (Table 1). Dissolved organic carbon (DOC)concentrations vary between 3.30 and 158 mg/l, have a broadnegative correlation (r = �0.61) with EC, and are typically lowat sites where hydrogeological and geochemical measurementsindicate upwelling.

Baseline values are derived for metals (Ni, Cr, LREE, Ca,Mg, Mg/Ca, Ba, Rb, and Cs) and for pH, Eh, and EC frompeat groundwaters that were collected at 1.1 mbgs greater than200 m from kimberlite margins and from Control Site. As Golfpeat groundwaters were collected from a more shallow depth of0.4 mbgs, baseline values are those of waters collected 200 mbeyond the Golf, Yankee, Alpha-1 kimberlite margins andControl Site during 2006 sampling events (Brauneder 2007). Allwaters used to obtain baseline values are ombrotrophic withEC values of < 100 uS/cm.

Transition metals consisting of Ni, Cr, and LREE (La-Sm)are consistently elevated along transects at sites near kimberlitemargins. Although the Yankee transect shows four sites ofupwelling (07-Y-01, 04, 08–09, and 12), only site 07-Y-08 (westkimberlite margin) has significantly elevated Ni and Cr (Fig.

7a), and LREE (Fig. 8a) concentrations. These metals arebetween 1.5 and 2.5 orders of magnitude greater than thebaseline concentrations and are similar to concentrations

KimberliteBioherm

07-Y

-01

07-Y

-02

07-Y

-03

07-Y

-04

07-Y

-06

07-Y

-07

07-Y

-08

07-Y

-09

07-Y

-10

07-Y

-11

07-Y

-12

07-Y

-13

07-Y

-14

07-Y

-15

07-Y

-16

07-Y

-17

Yankee

Kimberlite Zulu07-Z

-05

07-Z

-07

07-Z

-09

07-Z

-11

07-Z

-15

07-Z

-17

07-Z

-21

07-Z

-23

07-Z

-25

07-Z

-27

07-Z

-29

07-Z

-31

07-Z

-33

07-Z

-35

07-Y

-19

07-Z

-13

Kimberlite Golf

06-G

-01

06-G

-02

06-G

-03

06-G

-04

06-G

-06

06-G

-07

06-G

-09

06-G

-10

06-G

-11

06-G

-12

06-G

-13

06-G

-14

06-G

-15

06-G

-16

06-G

-08

06-G

-05

06-G

-17

07-Y

-5.5

0 100 200 300 400 500 600 700 800Distance (m)

0.0

0.5

1.0

1.5N

i (µg

/L)

0.0

0.5

1.0

1.5

Cr

(µg

/L)

0 100 200 300 400 500 600 700 800Distance (m)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Ni (

µg/L

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Cr

(µg

/L)

0 100 200 300 400 500 600 700 800Distance (m)

0

10

20

30

40

50

60

70

Ni (

µg/L

)

0

1

2

3

4

5

Cr

(µg

/L)

NiCr

Fig. 7. Concentrations of Cr and Ni in peat groundwater alongtransects over: (a) Yankee; (b) Zulu; and (c) Golf kimberlites. Niand Cr are typically observed at sites of upwelling at kimberlitemargins. Box plots represent the mean, first and third quartile valuesfor baseline ombrotrophic peat groundwaters.


observed in groundwaters from exploration boreholes in kim-berlites (Table 1). Nickel, Cr and LREE are low at 07-Y-12(near the east kimberlite margin) (Figs 7a, 8a). Elevated Ni, Cr,and LREE concentrations in peat groundwaters from Zulu andGolf are typically 2–3 times greater than mean baseline values.At Zulu, Ni is elevated near the east kimberlite margin (07-Z-13to 19); however, Cr is only elevated at sites 07-Z-13 and to alesser extent at 07-Z-15 (Fig. 7b). Light REEs are significantlygreater than baseline concentrations at the east kimberlitemargin (07-Z-15 and 17; Fig. 8b). The location of Ni, Cr andLREE concentration profiles along the transect at Golf aresimilar to those of Zulu except that the highest concentrationsof these metals are located at the west margin of Golfkimberlite (between 06-G-04 and 08) (Figs 7c, 8c). However,elevated concentrations of Ni occur at 06-G-11 and at the eastmargin from 06-G-15 to 17 and elevated Cr is located at06-G-17 (Fig. 7c). At both Zulu and Golf, the highest metalconcentrations usually coincide with a small contribution ofupwelling minerotrophic water. Nickel, Cr, and LREE concen-trations are typically lower than baseline values at sites down-gradient at both Zulu and Golf, and at 07-Y-12 (Yankee) whereminerotrophic waters are more dominant. Collectively, Ni, Cr,and LREEs are referred to as ‘Group 1 metals’ due to thesimilarity of their spatial distribution along the transects in thisstudy.

Peat groundwaters along the Yankee transect show elevatedMg concentrations at 07-Y-08 and 11 (Fig. 9a). The Mg/Caratios are high at 07-Y-08 and between 07-Y-13 and 15, but arelow at 07-Y-11 (Fig. 9a). Elevated concentrations of Ca (Fig.6b, c), and Mg (Fig. 9b, c) observed along Zulu and Golftransects are consistent with upwelling minerotrophic ground-waters. However, the Mg/Ca ratios are highest at sites ofupwelling near kimberlite margins (Fig. 9b & c). Elevated ratiosare also detected at some sites where groundwater is notupwelling such as 07-Z-13 and 15 at Zulu and 07-G-02 to 05 atGolf.

Concentrations of Ba in peat groundwaters are 1–1.5 ordersof magnitude higher relative to the baseline concentrations at ornear Yankee and Zulu kimberlite margins. The Ba profile alongthe Yankee transect (Fig. 10a) is consistent with other alkalineearth metals. The profile is almost identical to that of Mg (Fig.9a), with elevated concentrations at 07-Y-08, 11 and 12.However, Ba is low at 07-Y-04 even though Ca is high. The Baconcentration profiles along at Zulu and Golf samplingtransects are not consistent with other alkaline earth metals, butare more similar to the profiles of Group 1 metals. Barium ishighest along the Zulu transect near the eastern margin of thekimberlite between sites 07-Z-17 and 21 (Fig. 10b). Along theGolf transect, Ba is elevated at the up-gradient margin ofthe kimberlite (between 06-G-06 and 09; Fig. 10c). Note thatthe Ba concentrations at Golf are 2 orders of magnitude greaterthan Yankee or Zulu. All waters collected from this samplingevent (summer 2006) have high Ba values. We have ruled outthe possibility of contamination, and analytical artifact. Becauseof these high concentrations, we have revised the baseline datafor Ba to reflect only waters collected during summer 2006 forconsistency.

Concentration profiles of alkali metals (Rb and Cs) in peatgroundwaters differ from those of Group 1 and alkaline earthmetals along transects and are typically displaced 50 to 100 mfrom sites of elevated Group 1 metals and Ba. The highestalkali metal concentrations at Yankee are at 07-Y-02, 08, and 09(Fig. 11a). Along the Zulu transect, alkalis are elevated at sitesof upwelling from 07-Z-19 to 29, but sharply decrease farthereast along the transect even though upwelling appears tocontinue (Fig. 11b). At Golf, the highest alkali concentrations

KimberliteBioherm

07-Y

-01

07-Y

-02

07-Y

-03

07-Y

-04

07-Y

-06

07-Y

-07

07-Y

-08

07-Y

-09

07-Y

-10

07-Y

-11

07-Y

-12

07-Y

-13

07-Y

-14

07-Y

-15

07-Y

-16

07-Y

-17

Yankee

Kimberlite Zulu

07-Z

-05

07-Z

-07

07-Z

-09

07-Z

-11

07-Z

-15

07-Z

-17

07-Z

-21

07-Z

-23

07-Z

-25

07-Z

-27

07-Z

-29

07-Z

-31

07-Z

-33

07-Z

-35

07-Y

-19

07-Z

-13

Kimberlite Golf

06-G

-01

06-G

-02

06-G

-03

06-G

-04

06-G

-06

06-G

-07

06-G

-09

06-G

-10

06-G

-11

06-G

-12

06-G

-13

06-G

-14

06-G

-15

06-G

-16

06-G

-08

06-G

-05

06-G

-17

07-Y

-5.5

0 100 200 300 400 500 600 700 800Distance (m)

0.00

0.05

0.10

0.15

0.20

0.25

LR

EE

(µg

/L)

0 100 200 300 400 500 600 700 800Distance (m)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

LR

EE

(µg

/L)

0 100 200 300 400 500 600 700 800Distance (m)

0

1

2

3

4

5

6

LR

EE

(µg

/L)

Fig. 8. Concentrations of LREEs (La-Sm) in peat groundwateralong transects over: (a) Yankee; (b) Zulu; and (c) Golf kimberlites.Light REEs have similar profiles to Ni and Cr and are typicallyobserved at the kimberlite margins. Box plots represent the mean,first and third quartile values for baseline ombrotrophic peatgroundwaters.


are observed at 06-G-05, in waters considered ombrotrophic(Fig. 11c). At Golf and Yankee alkali concentrations are onlyslightly higher than baseline values.

Sulphate concentrations are low at all sites (mean = 0.35,median = 0.3, max = 3.97 mg/l) (Table 1) and total sulphurconcentrations in waters are also low (mean = 0.4, max = 0.9,many samples less than the detection limit of 0.3 mg/l) (Table1). Chloride concentrations are low and range from 0.12–25.43 mg/l with mean and median concentrations of 3.6 and2.0 mg/l, respectively (Table 1).

DISCUSSION

Metal sources in peat groundwaters

There are only three possible sources of metals in the study areathat could contribute to elevated pathfinder metals in peatgroundwaters: (1) host rock (dominantly Attawapiskat Forma-tion limestone); (2) TSS; or (3) kimberlite. A limestone sourcefor the elevated metals can be discounted because the concen-trations of Group 1, Ba, Mg/Ca and alkali metals in peat, TSS,and kimberlite groundwaters are always greater than concen-trations in limestone groundwater (sometimes by more thanone order of magnitude). Additionally, the Upper AttawapiskatFormation limestone is typically much lower in minerals thathost high concentrations of Group 1, Ba, or alkali metals

(Norris 1993), especially compared to the mineral compositionof kimberlites (Mitchell 1986). Plots of kimberlite and lime-stone groundwaters from this study, together with ground-waters from Kirkland Lake kimberlites (Sader et al. 2007a) (Fig.12) highlight the differences in pathfinder metal concentrationsbetween kimberlite and limestone sources. The elevated path-finder metals observed in Attawapiskat and Kirkland Lakekimberlite groundwaters in slightly alkaline conditions(pH = 7–8.5) are due to the chemical weathering of olivine,pyroxene, and phlogopite in kimberlites (Sader et al. 2007a).Elevated Mg/Ca ratios are due to the serpentinization ofolivine, which often results in an Mg-HCO3

� waters inultramafic rock (Barnes & O’Neil 1969; Palandri & Reed 2004).In kimberlite waters that have circum-neutral pH (7–9) andbicarbonate alkalinity Mg/Ca ratios are typically 0.5–0.8 (Saderet al. 2007a) and are comparable to kimberlite waters in thisstudy (Fig. 12).

Elevated metal concentrations at or near the kimberlitemargins are most likely due to geochemical influences fromkimberlite rather than TSS. Of the TSS groundwaters, concen-trations of Group 1, Ba, and alkalis are 1.2 to 4 times greater(with the exception of Cs) for those waters collected overkimberlites versus those collected outside the margins. TheMg/Ca ratios in TSS groundwaters over kimberlites support akimberlite origin as they are c. 4 times greater than TSS

KimberliteBioherm

07-Y

-01

07-Y

-02

07-Y

-03

07-Y

-04

07-Y

-06

07-Y

-07

07-Y

-08

07-Y

-09

07-Y

-10

07-Y

-11

07-Y

-12

07-Y

-13

07-Y

-14

07-Y

-15

07-Y

-16

07-Y

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Yankee Kimberlite Zulu

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Fig. 9. Concentrations of Mg andMg/Ca in peat groundwater alongtransects over: (a) Yankee; (b) Zulu;and (c) Golf kimberlites. With theexception of Yankee, Mg indicateslocations of upwelling minerotrophicgroundwater. The broad elevatedMg/Ca peaks along transects suggestwaters rich in Mg relative to Ca may becoming from buried kimberlite. Boxplots represent the mean, first and thirdquartile values for baselineombrotrophic peat groundwaters.


groundwaters collected outside kimberlites and 10 times greaterthan ratios in groundwaters from limestone. Although there isa dolomite unit within the host Palaeozoic rock, it is 200 mbelow the Upper Attawapiskat Formation and is likely of little

KimberliteBioherm

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Fig. 10. Concentrations of Ba in peat groundwater along transectsover: (a) Yankee; (b) Zulu; and (c) Golf kimberlites. Barium istypically elevated only a kimberlite margins and their profiles differcompared with other alkaline earth metals (i.e. Ca and Mg) at Zuluand Golf. Box plots represent the mean, first and third quartilevalues for baseline ombrotrophic peat groundwaters.

KimberliteBioherm

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Fig. 11. Concentrations of Rb and Cs in peat groundwater alongtransects over: (a) Yankee; (b) Zulu; and (c) Golf kimberlites.Although alkalis at Golf are marginally elevated relative to baselinevalues at 06-G-08 the concentration was prominent relative to othersites along the transect. Box plots represent the mean, first and thirdquartile values for baseline ombrotrophic peat groundwaters.


influence on the shallow groundwater geochemistry in thisstudy.

Spatial distribution of kimberlite pathfinder metals inpeat groundwaters

Hydrogeological controls on vertical metal movement

The most likely pathway for the upward migration of path-finder metals from kimberlites is along kimberlite–limestonemargins. Group 1, Ba, Mg/Ca, and alkalis are commonly foundat peat groundwater sample sites that have evidence ofupwelling and that are also within 200 m of kimberlite margins.Conversely, upwelling at other sites such as near bioherms(07-Y-04 at Yankee), or sites that are greater than 200 m fromkimberlites did not usually indicate elevated pathfinder metals.

The location of upwelling deep groundwaters to peat nearthe margins of kimberlites could be due to elevated hydraulicconductivity in fractured rocks near the boundary betweenkimberlite and the host rock. During kimberlite emplacement invarious geological settings, the margins of kimberlite pipes andadjacent host rocks are commonly fractured due to the explo-sive release of volatiles (Mitchell 1986; Wilson & Head 2007).These fractures likely represent preferential pathways forupward groundwater movement from kimberlite. With respectto Attawapiskat kimberlites, the highly fractured zone sur-rounding the Victor kimberlite forms a ring up to 150 mwide and has a hydraulic conductivity of 5.79 � 10�3 cm/s(Hydrologic Consultants 2004). In contrast, non-fractured hostAttawapiskat Formation limestone had a moderately lower

lateral hydraulic conductivity (1.16 � 10�3 cm/s), and a muchlower vertical hydraulic conductivity (1.16 � 10�5 cm/s),(Hydrologic Consultants 2004). Additionally, the less commonoccurrence of elevated pathfinder metal concentrations directlyover kimberlites in this study could be explained by their lowerrelative hydraulic conductivities. The K values in several brec-ciated kimberlites such as Diavik in Canada (Kuchling et al.2000), and Letlhakane, The Oaks and Venetia in South Africa(Morton & Mueller 2003) are 4 � 10�5, 1.97 � 10�6,1.62 � 10�7, and 5.8 � 10�8 cm/s respectively.

Hydrogeological controls on lateral peat groundwater movement

Profiles of geochemical parameters (i.e. EC, pH, Eh) alongtransects do not have a characteristic dispersion plume patterndown-gradient, such as those observed in streams (Leybourneet al. 2003), or aquifers (Freeze & Cherry 1979). This suggestslimited lateral movement of metals within peat groundwatersand is consistent with low hydraulic conductivity of peat withinthe catotelum (Ingram 1983; Chason & Siegel 1986; Fraser et al.2001; Price 2003). The profiles likely indicate the presenceof mixing as peat groundwaters grade from dominantlyombrotrophic to minerotrophic. The portion of water fromdeep sources increases relative to ombrotrophic peat ground-water at sites along transects at Zulu (07-Z-17 to 29) and Golf(06-G-08 to 15).

The combination of the hydraulic conductivity of peat andthe very small water table gradient suggest that at the depth ofsample collection (1.1 mbgs), peat groundwater will migrate

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Attawapiskat Kimberlite WatersUpper Attawapiskat Fm. Limestone WatersKirkland Lake, Ontario Kimberlite Waters

7 8 9 10 11 12 13pH

Fig. 12. Comparison of Mg, Mg/Ca,Cr, Ni, Ba, Rb, and Cs in groundwatersfrom Attawapiskat kimberlites, UpperAttawapiskat Fm. limetone, andKirkland Lake kimberlites, Ontario.Metals from almost all Attawapiskatkimberlite water samples follow thesame trend as metals in lower pH (7–9)waters from Kirkland Lake kimberlites.Kirkland Lake kimberlite water datafrom Sader et al. (2007a).


laterally between 0.2–200 cm/year. It could also be less if thepeat freezes to sample depth during winter. There is a greaterpotential for metals to migrate laterally at Golf, as samples wereonly collected at 0.4 m, and may be in a zone of higherhydraulic conductivity (i.e. the acrotelm); however, adispersion-plume profile of geochemical parameters was notobserved.

Peat geochemical controls

It is likely that geochemical processes within the peat exertconsiderable controls on kimberlite pathfinder metals in peatgroundwaters and may explain the spatial variability betweenmetals and/or groups of metals along transects. Although it isassumed the suite of pathfinder metals originates from buriedkimberlite, their displacement relative to each other and relativeto sites of upwelling suggests adsorption, binding and mineralprecipitation may play a role in controlling their solubility inpeat groundwaters.

Variable DOC concentrations may influence metal concen-trations in ombrotrophic and minerotrophic peat groundwaters.Where DOC concentrations are elevated (low contributionsfrom upwelling groundwaters), pathfinder metals such asGroup 1 may preferentially bind to dissolved organic matter(DOM) and result in enhanced metal solubility. Metals boundto DOM are typically inhibited from adsorption or precipitationprocesses (Cornell & Schwertmann 2003). Conversely, wherewaters are strongly minerotrophic and DOC concentrations arelow, Group 1 metals may be less likely to form complexes withDOM. Decreases in metal-DOM complexes typically result ingreater concentrations of free ions, which could easily bescavenged from solution by adsorption or secondary mineralprecipitation (Tang & Johannesson 2005; Syrovetnik et al.2008). In this study, Group 1 metals are usually lower thanbaseline values at sites of strong upwelling such as 07-Y-12, atYankee, 07-Z-21 to 35 at Zulu, and 07-G-10 to 17 at Golf. Atthese sites, Group 1 metals may adsorb to Fe-oxyhydroxidesor be incorporated into their structure. Fe contents inAttawapiskat peat along Yankee and Golf transects (Hattori &Hamilton 2008) increase with increasing upwelling groundwatercontributions observed in this study.

In comparison to Group 1 metals, alkaline earth and alkalishave lower affinities to bind to dissolved organics, or to adsorbto insoluble organics (Stevenson 1994) and they have almost noaffinity to adsorb onto Fe-oxyhydroxides at pH values in thisstudy (Kinniburgh et al. 1976). Elevated Mg/Ca ratios aretypically observed at sites of other elevated pathfinder metals inthis study (Group 1, Ba, and alkali metals). Because both Mgand Ca behave similarly in peat groundwaters with respect toadsorption or binding to organic substances and becauseMg/Ca is a ratio and not an absolute metal concentration,Mg/Ca ratios may be less susceptible to variable geochemicalprocesses in peat.

Contrasts between peat and peat groundwatergeochemistry

The peat groundwater hydrogeological and geochemical datasuggest that metal anomalies in peat at Yankee and Golf(Hattori & Hamilton 2008) are the result of upward movementof deep groundwater from kimberlites. Peat samples of Hattoriand Hamilton (2008) were collected at the same sites as peatgroundwater samples of this study, but at 0.6 mbgs. Goodcorrelations were observed between the results of ammoniaacetate leach at pH 5 (AA5) of peat (Hattori & Hamilton 2008)and alkaline earth metals in peat groundwaters. As AA5

typically leaches metals weakly adsorbed and metals thatco-precipitate with carbonates, Mg/Ca ratios in peat and peatgroundwaters are positively correlated (r = 0.76).

Elevated LREEs, Ni, and Rb in peat (by AA5) and peatgroundwater correlated well at Yankee sites 07-Y-08 and 09.Light REEs are also high in concentration at similar locations inboth peat (AA5) and peat groundwaters at the Golf kimberlite.However, the elevated LREE and Ni concentrations in peat at07-Y-11 at Yankee and Ni at Golf are coupled with ground-water concentrations that are the same as or less than baselinevalues. The elevated metals in peat groundwaters are insteadobserved at locations where peat (AA5) concentrations are low.There are a number of possibilities as to why discrepanciesbetween the locations of elevated LREE, Ni, and Rb concen-trations in peat (AA5) and peat groundwaters exist at somesites. It is possible that they are related to the affinity of thesepathfinder metals to remain in solution, or to be removed fromsolution via precipitation or adsorption. It is also possible thatAA5 is not the optimum leach for some metals. The recoveryof Ni, LREEs, and Rb in AA5 compared to total peatconcentration was only 30, 4, and 17%, respectively (Hattori &Hamilton 2008), as AA5 is not favored to leach metals stronglyadsorbed to insoluble humic substances or Fe-oxyhydroxides.Differences in the spatial distribution of elevated Ni, LREEs,and Rb in peat (AA5) and peat groundwaters may also berelated to different sampling depths of media. Peat groundwa-ters were collected at 1.1 mbgs at Yankee and 0.4 mbgs at Golf.Redox conditions, metal concentrations, concentrations ofdissolved organic material, and the degree of peat humificationvary significantly with small changes in depth (Clymo 1983;Syrovetnik et al. 2004; Beer & Blodau 2007).

PEAT GROUNDWATERS IN GEOCHEMICALEXPLORATION

Application to exploration

Groundwaters from peat and other geological units are excel-lent media for geochemical surveys (Leybourne et al. 2003;Sader et al. 2007a; Leybourne & Cameron 2010). This studyshows that kimberlite pathfinder metals in peat groundwatersare likely reliant on the presence of upwelling groundwaterfrom kimberlites. Pathfinder metals in these groundwaterscontrast with dilute ombrotrophic peat groundwaters andgroundwaters that have upwelled from limestone or TSS. Thelimestone host rock in this study does not contain minerals thathost high concentrations of kimberlite pathfinder metals. Fur-thermore, at locations where the kimberlite host rock is igneousor metamorphic crystalline such as the Superior, the Slave, andthe Churchill cratons in Canada, kimberlite pathfinder metalsare typically released to groundwater more readily from kim-berlites due to the relatively rapid weathering of ultramaficminerals such as olivine and pyroxene (Sader et al. 2007a).Therefore, groundwaters influenced by kimberlite-water inter-actions are likely distinguishable from upwelling groundwatersthat passed through different rock types.

Large cratonic regions in the northern hemisphere have highpotential for the existence of undiscovered diamondiferouskimberlite occurrences (Janse & Sheahan 1995). As theseregions commonly have vast wetlands, shallow groundwater hasthe potential to be utilized as a geochemical exploration tool tomitigate the greater challenges involved in implementing moreestablished exploration methods such as geophysics. Otherintrusive bodies such as syenite and mafic dykes may showgeophysical features similar to kimberlites. Surficial water geo-chemical methods can assist by providing evidence as to


whether a geophysical anomaly is a kimberlite before thecommencement of drilling.

Survey design

This study suggests that sampling of groundwaters should becombined with a hydrogeological survey of the areas near apotential buried kimberlite body. A hydrogeological surveyprovides information on the flow direction of groundwatersand sites of upwelling deep waters. Installation of monitoringwells is useful to confirm upwelling and they are typically moreprecise at locating sites of upwelling in peat groundwaterscompared with the detection of variations in geochemicalparameters such as EC, Eh, and pH. For example, the07-Z-MW-17 (Zulu) monitoring well hydrogeological dataindicate upwelling; however, there are only low to moderategeochemical upwelling indicators in peat groundwaters at thatsite relative to sites farther east along the transect. Thedisadvantage of monitoring wells is that installation is physicallydemanding and time-consuming. As it is not feasible to installmonitoring wells at each site that a piezometer is installed at toidentify sites of upwelling, it is important to rely on variationsin geochemical parameters in conjunction with hydrogeology todetect groundwater upwelling. One alternative to the installa-tion of many monitoring wells would be to install piezometernests that are only within the peat zone. Hydrogeological datafrom piezometers beside each other at various depths in thepeat would permit the construction of flow nets and betterindications of peat groundwater movement.

Peat groundwater samples should be collected at least 200 mbeyond the suspected kimberlite margins where possible, asfractured host rock may extend as far as 150 m from themargin. If upwelling of deep groundwater is present, a transectmay be extended longer than 200 m to gauge possible geo-chemical differences in peat groundwaters from differentsources. Additionally, the data from samples > 200 m from thesuspected kimberlite body provide local baseline values. Peatand soil sampling well beyond the margin of targets was alsosuggested in exploration surveys for kimberlites in wetlands(Hattori & Hamilton 2008). Depending on site conditions, it islikely that grid sampling would provide an enhanced portrait ofpeat groundwater geochemistry associated with a potentialburied kimberlite.

We thank DeBeers Canada for providing financial and logisticsupport for the project. Ed Francisco, and Brad Wood wereparticularly helpful. The financial support was also provided by aNSERC Collaborative Research grant to Keiko Hattori. Fieldworkwas assisted by and Katherine Mellor. Jamil Sader acknowledges anOntario Graduate Scholarship and a student research grant from theSociety of Economic Geologists. Journal reviewers Mats Astrom andAgnete Steenfelt, and Editor Gwendy Hall provided thorough andinsightful reviews that greatly benefited this paper.

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Received 24 February 2010; revised typescript accepted 25 August 2010.


88.97 masl

87.53 masl

Water table elevation

Figure A-1. Peat groundwater flow direction at Yankee based on

kriging of water table data.

Figure A-1.

80.22 masl

78.73 masl

Water table elevation

Figure A-2. Peat groundwater flow direction at Zulu based on kriging

of water table data.

Figure A-2.

Documents

Geochemical responses in peat groundwater over ... · posed of mantle olivine, pyroxene, and xenoliths of limestone. It also contains, ilmenite, and clinopyroxene, minor garnet and,