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Geochemical mapping employing active and overbank stream-sediment, lake sediment and lake water in two areas of Newfoundland
Journal of Geochemical Exploration, 49 ( 1993 ) 123-143
123
Elsevier Science Publishers B.V., A m s t e r d a m
Geochemical mapping employing active and overbank stream-sediment, lake sediment and lake water in two areas of Newfoundland J.W. McConnell a, C. Finch a, G.E.M. H a l P and P.H. D a v e n p o r t a aNewfoundland Department of Mines and Energy, Geological Survey Branch, P.O. Box 8700. St. John's, Nfld. AIB 4J6, Canada bGeological Survey of Canada, 601 Booth St., Ottawa, Ont. KIA OE8, Canada (Received 7 December 1992; accepted after revision 15 June 1993 )
ABSTRACT Geochemical data from samples of active and overbank stream sediment in one area and from lake sediment and lake water in another are compared and evaluated. Samples of both types of stream sediment were collected from drainage basins 2-10 km 2 in area in a glaciated landscape and the < 63 /zm fractions were analyzed for 38 elements. In general, trace element distribution patterns are similar in the two media and reflect the chemistry of the underlying bedrock. Absolute concentrations are higher for most elements in the overbank sediment. For elements having concentrations near the analytical detection limit, such as Au, Mo and Pb, the overbank sediment provides more reliable data in those parts of the area with low background levels. In this area, suitable overbank material is more widespread and available for sampling than is active sediment. In some drainages, stream-sediment data are clearly contaminated by past mining activity but overbank data appear unaffected. Lake water was collected from another area where a lake sediment survey had been conducted eleven years earlier. The < 180/~m fraction of sediment was analyzed for 29 elements and the water for about 50 elements. Due to the contrasting nature of these two sample media, element distribution patterns are less similar than in the case of the two types of stream sediment. Nonetheless, elements which are hydromorphically dispersed and which are partitioned consistently between water and sediment (such as F, U, As, and rare earths ) do demonstrate a strong spatial correlation. As analytical detection limits for water continue to improve, the versatility of this medium in geochemical mapping will increasingly complement more conventional sampling media as well as providing an alternate medium in areas where sediment is unavailable.
INTRODUCTION
A problem to be faced in compiling geochemical data for large regions is whether one can meaningfully compare or combine data from different types of surveys in a given area. In Newfoundland, the opportunity arose to address this question using four types of sample media in two distinct areas (Fig. 1 ). In the Baie Verte/Springdale area a stream survey was conducted in which both active and overbank sediment were collected to obtain data to identify
0 3 7 5 - 6 7 4 2 / 9 3 / $ 0 6 . 0 0 © 1993 Elsevier Science Publishers B.V. All rights reserved.
124
j.w. MeCONNELLET AL. 52 °
~4. o
146 ° ~,7. o 60 °
58 °
56 °
INDEX
9a °
MAP
Fig. 1. Location o f survey areas.
drainage basins containing gold or base-metal mineralization. These two sets of data are evaluated and compared. The second area studied is along the south coast of Newfoundland between Bay d'Espoir and Fortune Bay, where a lake-water survey was conducted in 1991 over diverse bedrock terranes. A reconnaissance lake-sediment survey had been done over the same area 11 years previously that provided a data base of 33 elements with which to compare the newly acquired water data. STREAM S U R V E Y
Study area
The Baie Verte Peninsula has been a focus of mining since 1864. Numerous gold and base metal occurrences are known, several of which are currently being evaluated for possible production. The peninsula is divided tectonostratigraphically by the Baie Verte Line - - a structural break between the H u m b e r Zone to the west, dominated by schist, gneiss and granitoids, and the Dunnage Zone to the east, dominated by ophiolite, volcanic cover sequences and intrusive rocks. The geology of the area (after Colman-Sadd et al., 1990 ) and the sampled drainage basins are shown in Fig. 2. Most of the known min-
I0
C/'
Ultramafic rocks of ophiolite complexes
-~
Siliciclastic rocks
['",
Drainage
basin
Psammitic schist
Schist
-] Marine siliciclastic rocks -~ Submarine volcanic rocks; mostly mafic; incl ophiolite IT]
~]
STRATIFIED ROCKS Proterozolc to Carboniferous [~ Non-marine sandstone and shale -~ imodal subaerial volcanic rocks
Mafic intrusive rocks
~
Granitoid intrusions
Granite and syenite
Gabbro
Fig. 2. Geology and sampled drainage basins of the stream survey area (geology after Colman-Sadd et al., 1990).
I
Bole Verte Line
6
-~
Cambrian to Devonian
INTRUSIVE ROCKS
,--t m
:z
z
r~
>
m
o.<
,H
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bl
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126
j.w. McCONNELL ET AL.
eral occurrences are found in the Dunnage Zone - - particularly associated with ophiolitic and mafic volcanic rocks. The survey area between Green Bay and Halls Bay is underlain primarily by Cambrian ophiolitic rocks and Ordovician volcanic and epiclastic rocks (Kean and Evans, 1987, 1990). These units host several volcanogenic massive sulphide deposits including past producers and epigenetic gold occurrences. Pleistocene glaciation left most of the area with a thin covering of locally derived colluvium, patchy occurrences of glacial sediment or a thin till veneer (Grant, 1986; Liverman and Taylor, 1990 ). Active stream sediment and overbank material are likely derived from a combination of such glacial material and actively eroding bedrock. Reconnaissance surveys of organic lake sediment have been made over the entire island at a sample density of about 1 per 6 km 2. Samples from this area have been analyzed for 33 elements including Au, As, Sb and base metals (Davenport and Nolan, 1988; Davenport et al., 1989). These data were used in selecting areas for the stream survey.
Sampling methods Stream sites were sampled that had upstream catchments on the order of 2-10 km 2. Where possible, samples of overbank and active sediment were collected at each site, generally within 100 m of each other. A total of 143 samples of active sediment and 175 samples of overbank sediment were collected. Samples of both types were obtained at 121 sites. Of these sites, 19 were sampled in duplicate (site duplicates ). The streams have generally moderate to fast flow rates with most sediment ranging in size from gravel to small boulders. Where present, fine sediment was most often found in the lee, or around the base, of boulders and sampling was frequently time consuming. Approximately 200 to 300 g of < 300/~m active sediment were obtained at each site by wet-sieving in the field. Overbank sediment is characteristically fine-grained material deposited during flood conditions on flood plains or on low and level stream banks. Some researchers regard the geochemistry of these deposits as more representative of the upstream catchment basins than are deposits of active sediment (Ottesen et al., 1989). Sediment from such areas was sampled by using a spade to obtain a channel sample down the entire overbank profile. Collection took only three or four minutes. Thicknesses of the overbank layers ranged from 10 to > 100 cm. The average thickness was 35 cm and 6 sites had more than 100 cm of sediment. Deposits of overbank sediment were more frequently encountered than were deposits of suitably fine-grained active sediment.
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
127
Analytical methods The sediment samples were air-dried in the field, further dried in ovens at 60°C and then sieved to <63 #m (230 mesh). Both active and overbank sediment samples were analyzed for the following elements: Ag, As, Au, Ba, Br, Ce, Co, Cr, Cs, Cu, Dy, Eu, Fe, Ga, Hf, La, Li, Mn, Mo, Nb, Na, Ni, Pb, Rb, Sb, Sc, Sm, Sr, Ta, Tb, Th, U, V, W, Y, Yb, Zn and Zr. Some elements, notably Ba, Ce, Co, Cr, Cu, Fe, La, Mn, Mo, Ni, Pb, Sc and Zn were analyzed by more than one method. As a check on accuracy and precision, a standard sample of known composition and a sample duplicate (split) were included in each batch of 20 samples. The analytical methods comprised instrumental neutron activation analysis (INAA), inductively coupled plasma emission spectroscopy (ICP-ES), atomic absorption spectroscopy using a HF-HC1OaHC1 digestion (AA-total) and atomic absorption spectroscopy using an aqua regia digestion (AA-partial). The first three methods give total or near-total analyses, the last is partial. The analytical method is identified in the these data by the addition of suffixes to the element name: "1" for INAA, "2" for ICP-ES, "3" for AA-total and "4" for AA-partial (e.g. Ni4 identifies nickel analyzed by AA-partial ).
Results and discussion Summary statistics of data selected to illustrate a range of elements from the < 63/~m fractions of active and overbank stream sediment are presented in Table 1. To ensure comparability, only samples from the 121 sites represented by both sample media are included. Since examination of data populations indicated these to be positively skewed and more closely representative of log-normal than normal populations, geometric means and log standard deviations are given. Examination of the median values reveals some interesting trends in the two sample media. For seventeen of the 23 elements, the concentration in overbank sediment is higher than in active sediment. This enrichment in overbank sediment likely has two principal causes. The overbank material generally contains a greater proportion of finer-grained material than the active sediment (despite their both being < 63/~m) and hence would have more surface area with sites suitable for ion adsorption. Secondly, the concentration of Fe4 and Mn4 (aqua regia digestible; i.e. largely present as (hydr)oxides of iron and manganese ) in overbank sediment is nearly double that in active sediment. This condition would also have the effect of increasing the ion-adsorption capacity of this medium relative to active sediment. Note, however, that the median values of total iron (Fel) in the two media are very similar, 6.0% and 7.3%. Spearman (ranked) correlation coefficients between element content and iron content for active and overbank samples are shown in Fig. 3 (a and b).
128
J.W. McCONNELL ET AL.
TABLEI
Medians, geometric means, log standard deviations and ranges of data for < 63/~m active sediment (A.S.), and <63/~m overbank sediment (O.S.) ( n = 121 for each sample type; data in ppm unless otherwise noted) Element Median
Asl Aul a Ba2 Cd4 Cel Co4 Crl Cu2 Cu4 Fel b Fe4 b La2 Mn4 Mo2 Nb2 Ni4 Pb4 Rb2 Sbl Thl UI Y2 Zn4
Xg Mean
Log S.D.
Range
A.S.
O.S.
A.S.
O.S.
A.S.
O.S.
A.S.
O.S.
10.0 4.0 237 0.2 54 20 280 29 28 6.0 2.12 19 870 4 9 35 5 15 0.53 5,3 2.9 30 90
22.3 7.0 200 0.5 86 27 259 61 47 7.3 4.15 26 2150 7 4 30 18 19 0.40 6.1 3.3 32 74
11.0 6.6 219 0.3 46 19 339 28 27 5.8 2.09 20 1100 4.9 9.3 40 5.9 19 0.60 5.1 3.0 30 87
20.9 7.1 219 0.5 89 28 251 53 43 7.1 3.98 28 2090 8.3 4.2 35 19 20 0.46 5.6 3.5 31 78
0.54 0.95 0.28 0.32 0.35 0.27 0.44 0.39 0.37 0.13 0.17 0.25 0.48 0.16 0.34 0.39 0.36 0.30 0.32 0.28 0.31 0.20 0.25
0.55 0.44 0.29 0.45 0.44 0.41 0.35 0.48 0.48 0.17 0.25 0.34 0.69 0.26 0.40 0.45 0.38 0.32 0.31 0.34 0.42 0.23 0.30
0.2-435 <2-1550 39-1250 <0.1-3.0 5-350 2-116 10-6570 3-422 3-403 2.2-12.0 0.51-4.49 4-150 85-14000 2-29 1-50 4-590 1-96 2-85 0.13-16.2 1.1-26.0 0.5-18.0 6-121 18-438
1.1-535 <2-95 48-973 <0.1-12 5-727 2-250 41-2410 3-1416 2-1130 1.6-18.9 0.27-14.50 4-223 24-54800 2-80 1-33 2-317 1-541 2-132 <0.10-6.77 0.6-35.0 0.3-60.9 5-106 6-474
a data in ppb. b data in weight %.
When the elements are plotted in order of increasing correlation with Fe 1 some interesting trends emerge. Firstly, the active-sediment data have stronger correlations with both Fel and Fe4 than do the overbank data. Secondly, plotting in order of increasing Fe 1 correlation tends to group the elements by their affinities, i.e. lithophile elements such as Zr, Rb, Hf, Ba and Nb have negative correlations with total iron and plot at the left end of both figures. The middle range is dominated by chalcophile elements such as Cu, Zn, As and Sb and the upper range by siderophile elements such as Ni, Co and Sc. The effect of oxide scavenging can be estimated by the strength of the coefficients with Fe4. This effect can be seen to be stronger in active sediment than in overbank sediment where 13 elements in active sediment have coefficients
129
GEOCHEMICAL MAPPING EMPLOYING ACTlVE AND OVERBANK SEDIMENT AND WATER
1
0.9 0.8
0.70.60.50.4-
0.30.2 0.1 Confidence levels
0 -0.1 -0.2 -0.3 -0.4
[]
-0.5 -0.6
(a)
Z'r21RI~21TI~ llNb2~Sm ' ' llPb41Y'2 ' IZ 'n41M'o21C 'u41T;2 ] N'i41N'i 1 l scllFe4lFe2[ ' ' ' Ba2Hfl
[]
U1 L a 2 A u l S b l D y 2 C u 2 A s l M n 4 C d 4 C r l M n 2 C o 4 C o l Fel with Fel ('totol Fe) + with Fe4 ( ' p o r t i o l )
1
0.9 0.8
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
99% confidence levels<
-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 Z
~ Bo2Hfl
n Ul [3
4 oM 2Cu T~2 NI4 Nil S c l F e 4 F e 2 ( b ) Lo2Aul Sbl Dy2Cu2AslMn4Cd4Crl M n 2 C o 4 C o l F'el
with Fel ( t o t a l Fe) +
with Fe4 ( p a r t i a l )
Fig. 3. Spearman correlation coefficients of analyses of total iron ( F e l ) and aqua regia digestible iron (Fe4) with: (a) active sediment data and (b) overbank sediment data. (n = 121. )
130
j.w. McCONNELLET AL.
> 0.40 whereas only 9 coefficients in overbank sediment are > 0.40. However, the overall trends in the two sample media are similar. Cumulative frequency plots permit a visual comparison of the shape and nature of population distributions. Fig. 4 displays the distributions in active 9e 1000
90
80 7060504030 20
%0
2 X
go
98
80 7060504030 20
i0
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JO00908•
Cerium
Arsenic
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99 | 90 1000 Copper
90 70 5050 40 90 20
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9. )~
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Gold
I0
20 90 40 5060 70 80
90
9B ,
. . . . . . . . . . . . 10 20 3040506070 80 90
98 Z
: Lead
S £3 O-
/
' l'o ' e'o ~'o ,0 so 6o 70 e'o' 9o ' 9e x 98 Z go 80 7060504030 20 JO00 . . . . Molybdenum
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/o: ' ~0 90 4'0 eos0 73 eb' 90 90
Nickel
O0 7 0 6 0 5 0 4 0 3 0
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9B ~
1
90
00 7 0 6 0 5 0 4 0 9 0
20
10
2 x
! Zinc
too
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o
Fig. 4.
2
Cumulative frequency plots of Asl, Aul, Cel, Crl, Cu4, Mo2, Ni4, Pb4 and Zn4 in and overbank sediment. (Solid line i s a c t i v e s e d i m e n t and dotted line is overbank sediment).
active
131
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
1 .00
0.95
Strong correlations
0.90
0.85
0.80
0.75
0.70
0.65
(a) [
0.60
Tblt Bal
i
i
i
i
i
i
i
F
i
i
Sr21 scll R'bll Ba21 Zr21 U't I Nil' Ico41 sc21 LalJ Ni21 Ni41 or21 Hfl
Rb2
Tat
Crl
Sin1
Cu2
Ce2
Cel
La2
Thl
Be2
Sbl
As1
0.70
0.60
Weak to moderate correlations
0.50
0.40
0.30
ence
leve!
0.20
(b) 0.10
i
C'd211dn21
AU1
i
Fell
I~ln4
c'd,I
Fe2
Wl
i
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i
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i
i
i
[
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i
Ico21 zn21 Zn,I Coll Mo21 C,11 N'b21 L,2[
Ybl
TI2
Pb2
Pb4
Nal
Co4
Dy2
Idol
V2
Fig. 5. Spearman correlation coefficients of analyses of active sediment with overbank sediment
(n=12l).
t'/%
L
Aine
~ . , , ~
4
Mine
I~lt j, st
17
'0
O0
75
i
i90 ppm
0
(km)
,5
10
rig. 6. Nickel (Ni4) in active sediment. (See Fig. 2 for geologic legend. Contacts are shown as medium-weight lines; faults are heavy-weight. )
3oie Verte
Jr'~
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f
/
in active sediment
~
/
_
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'
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° ~ c ~
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60
110
185
5
446 ppm
0
(km)
5
10
in overbank sediment
Fig. 7. Nickel (Ni4) in overbank sediment. (See Fig. 2 for geologic legend. Contacts are shown as medium-weight lines; faults are heavyweight. )
L/~_I__
BeieVerteLine-
,/-% / %"~)
j~
)
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Ni
134
J.W.McCONNELLETAL.
and overbank sediment of As, Au, Ce, Cr, Cu, Mo, Ni, Pb and Zn. These illustrate a range of similarities and differences in geochemical responses in the two media. Nickel and Zn have very similar plots. The distribution curves of Ce, Cr, Cu and Pb have similar shapes but differ in absolute concentrations. Arsenic has a major population break ("A" in Fig. 4) at the 95th percentile in active sediment that is absent in the overbank data. Molybdenum has two distinct populations with a break-point ("B") at about the 75th percentile in the overbank data that is not apparent in the active sediment data. The absence of this feature in active sediment may result from most of the data being near the detection limit hence a large error in analytical precision for Mo could obscure the break. The largest difference in the two distributions is for Au. Nearly 90% of the overbank samples have detectible Au whereas it can be detected in only 55% of the active sediment samples. However, the upper range of Au values is an order of magnitude greater in the active sediment. One way of quantifying the cumulative effect of these various factors (such as grain size and amorphous Fe hydroxides/oxides) that contribute to the overall element content of samples is to calculate correlations between pairs of variables. Spearman correlation coefficients for 40 elements between active and overbank sediment are plotted in Fig. 5. Note that correlation coefficients are moderate to strong - - greater than 0.40 - - for all the elements but Cd2, Aul, Mn2 and Mn4. Coefficients > [0.40[ are significant at the 99.9% confidence level. Gold analyses are notoriously difficult to reproduce even within the same medium because of the nugget effect. It has been shown elsewhere that there is no significant correlation for Au analyses between site duplicate samples of either active sediment or overbank sediment in this area, although other elements are well correlated (McConnell, 1992 ). Manganese is largely controlled by varying E h / p H conditions rather than primary upstream metal contents. It is not apparent why Cd2 has poor correlation between the two media although its relatively high detection limit and strong correlations with Mn4 and Fe4 are possible reasons. Figs. 6 and 7 are coloured drainage-basin maps, overlain on a geologic base, that display the Ni (Ni4) contents of active and overbank sediments in sampled catchments. The colour intervals were selected by choosing inflection points on the cumulative frequency curves of the two media (Fig. 4). These correspond approximately to the 50th, 70th, 85th and 92nd percentiles in both cases. Sample sites on the maps are denoted by black dots; drainage basins from which only sample material of the other medium was obtained are uncoloured. The correlation coefficient for Ni4 between the two sample media is 0.82. Since Ni concentration in igneous rocks has an inverse correlation with degree of magmatic differentiation, Ni might be expected to reflect the bedrock geology of this mainly igneous terrane. The distribution patterns of Ni in the two figures are similar and do reveal variations in lithology. Most
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
13 5
distinctive is the trend of high values along the Baie Verte line in the western part of the map area - - an area predominantly underlain by mafic and ultramafic rocks. The area to the west of Green Bay is underlain predominantly by granitic rocks (Unit 11; see Fig. 2 for legend) and is characterized by low Ni values in overbank sediment. Due to a scarcity of material, few samples of active sediment were collected here. The Springdale Peninsula, located between Green Bay and Hails Bay, is underlain largely by mafic volcanic rocks (Unit 3 ). However the presence of higher Ni contents in both active and overbank sediments suggests that the northwest side, relative to the southeast side, is underlain by volcanic rocks of more mafic character. Geological mapping (Kean, 1984; Kean and Evans, 1987) indicates that the area is a composite of complex fault blocks of similar volcanic units that, in many cases, are difficult or impossible to distinguish in the field. The geochemical patterns may assist in unravelling the stratigraphic interpretation. These data demonstrate the different responses the two media have to the effects of mining-related contamination of the surface environment. Active stream sediment from the drainages downstream from the Rambler Mine railings ( C u - P b - Z n - A u ) and from a drainage basin adjacent to the ore-haulage road north of the Tilt Cove Mine (Cu-Au) are highly anomalous in Cu and Pb and, in one instance, Au and As. In contrast, analyses of the overbank sediment from the same sites all fall well within the local background (McConnell, 1992). The locations of the mines are shown in Figs. 6 and 7 although the Ni contents do not show the contamination effect as Ni is not an ore element at either mine. LAKE S U R V E Y
Study area
The survey area is located north of Fortune Bay and east of Bay d'Espoir and is divided into two contrasting geological terranes by the northeast trending Hermitage Bay Fault (Fig. 8 ). To the southeast, the older Avalon Zone is comprised predominantly of volcanic, sedimentary and intrusive rocks of Hadrynian age which are intruded by a Devonian high-silica granite. The Gander and Dunnage zones to the northwest of the fault are composed of clastic-sedimentary and volcanic rocks also intruded by granites. Numerous small mineral occurrences are known from the area including Sn and Mo in the high-silica granite and As, Au, Sb, Cu and Zn, mostly in the Dunnage Zone. These two terranes are also distinctive geochemically in the reconnaissance lake sediment data. Generally, the area to the northwest of the fault, relative to the area to the southeast, is higher in the elements As, Ba, Co, Cu, Cs, Ni, Sb and lower in F, U and REEs.
136
J.W. McCONNELL ET AL.
LJ
(3 "'..'ilj':" ~ 7
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6
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G E O L O G Y LEGEND 8
High silica granite
7
Granite
6
Siliciclastic sedimentary rocks
5 4
Felsic and mafic volcanic rocks Clastic sedimentary and volcanic rocks
3
Siliciclastic sedimentary rocks
2
Bimodal volcanic rocks
1 Granitoid intrusion
Fig. 8. Geologyand location of the lake surveyareas (geologyafter Colman-Saddet al., 1990). Sampling
methods
A total of 136 lake-water samples were collected in a single day by helicopter in the Bay d'Espoir-Fortune Bay area including 12 pairs of site duplicates. A sample was obtained by landing near the centre of the lake and immersing and filling a rinsed, 250 ml nalgene bottle. The water survey was conducted over the same area as the previous lake sediment survey although only 108 of the lakes were sampled for both media. Samples of organic, lake-bottom sediment were collected from the centres of lakes during the reconnaissance survey using a float-equipped helicopter and a procedure described by Friske and Hornbrook ( 1991 ). Analytical methods L a k e w a t e r s . Within 24 hours of collection and following analysis for pH and total dissolved solids, the samples were filtered through a 0.45 # m filter paper and acidified with 2 ml of nano-pure HNO3 to await further analysis. To date, 105 elemental analyses employing 8 different methods have been performed (Table 2 ). Several elements have been analyzed by more than one means. Method descriptions can be found in Finch et al. ( 1992 ). L a k e s e d i m e n t . Lake sediment samples were partially air-dried in the field then oven-dried at 40 ° C. This material was disaggregated with a mortar and
13 7
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
TABLE2
Methods of lake water analyses by participating laboratory. Laboratories: N D M E - - Newfoundland Department of Mines and Energy; M U N / C E R R - - Memorial University of Newfoundland/Centre for Earth Resources Research; G S C - - Geological Survey of Canada. Analytical methods: ICP-ES - inductively coupled plasma emission spectrometry; U S N - - ultra sonic nebulizer; MS - - mass spectrometry; H G - - hydride generation; IC - - ion chromatography; ISE - - ion selective electrode; AA - atomic absorption spectrophotometry; F - - flame N D M E Laboratory
MUN/CERR
G S C Laboratory
ICP-ES
USN-ICPES
ICP-MS
HG-ICP-MS
IC-ICP-MS
ISE
HG-AA
F-AA
Si Ca Mg Na K S Fe Mn
Li Be B A1 P Fe Sr Ba Ti
Li Be B Mg A1 Si P S C1 Ca Ti
Mn Fe Co Ni Cu Zn As Br Se Mo Ag
Cd Sn Sb I Cs Ba La Ce Hg Ti Pb
Se Te Bi Sb
REEs (i.e., La, Ce, Pr, Nd, Sin, Eu, Gd, Tb, Dy, Ho, Er, T m , Yb, Lu) A1 Co Cd Ti Ni Pb V Cu U Mn Zn Fe Y
F
As
Na K Ca Mg Mn Fe Zn
V
Ba
Bi
Cr
La
U
Cr Mn Co Ni Cu Zn Y Mo
pestle and sieved with a 180 #m (80 mesh) stainless-steel sieve. The finefraction was analyzed by three methods; by instrumental neutron activation
(INAA) for As, Au, Ba, Br, Ce, Cs, Eu, Hf, La, Rb, Sm, Sb, Sc, Se, Na, Ta, Tb, Th, U and Yb; by atomic absorption spectroscopy using a 4 M HNO31 M HC1 digestion (AA-partial) for Ag, Co, Cu, Fe, Mn, Ni, Pb and Zn; by fluoride-ion specific electrode for F following fusion of the sample with a flux of NaCO3/KNO3 and solution in 10% citric acid. Results and discussion Summary statistics of 17 elements for lake sediment and lake water are presented in Table 3. To ensure comparability, only data from sites sampled for both media and from sites with complete sets of both data are included. The elements chosen for inclusion are those that have detectable concentrations in at least 40% of the samples. Correlation coefficients between the two media for these elements are shown in Fig. 9. The elements Mn, Rb, Fe and Na are not significantly correlated between media whereas the elements U, As, Yb and F are the most strongly correlated (r >0.7). Maps of the distributions of this second group in sediment and water show the greatest similar-
27.6 150 100 14 2.1 194 6.57% 47 1660 0.30% 16 22 0.3 7.8 1.1 4.2 3.5
0.318 1.164 0.208 0.029 0.020 17 62 0.145 5.52 1.61 ppm 0.191 0.532 0.008 0.022 0.004 0.023 0.008
28.8 141 102 15.8 2.1 240 5.75% 45.7 1950 0.30% 20.4 19.1 0.43 8.4 1.5 10.2 4.3
Sed.
Sed.
Water
Mean
Median
0.355 1.202 0.186 0.039 0.023 29.5 60.3 0.117 6.17 1.66 ppm 0.170 0.513 0.011 0.022 0.004 0.047 0.010
Water
0.85 0.45 0.40 0.49 0.34 0.46 0.40 0.32 0.75 0.28 0.48 0.43 0.52 0.36 0.45 0.38 0.58
Sed.
Log S.D.
0.53 0.33 0.33 0.51 0.33 0.46 0.43 0.31 0.47 0.11 0.36 0.21 0.43 0.32 0.35 0.74 0.50
Water
0.2 25 14 1 0.2 40 0.41% 6.8 77 0.08% 1 2 0.02 1.3 0.2 0.6 0.2
1320 1400 651 164 10 3240 7.6% 225 77000 1.1% 841 160 17.6 58.1 16 735 67.1
0.102 0.321 0.012 0.012 0.006 10 10 0.010 0.158 1.06 ppm 0.008 0.119 0.005 0.002 0.0003 0.004 0.001
Min.
Min.
Max.
Water
Sed.
Range
9.287 25.322 0.766 1.128 0.554 220 493 0.877 79.32 6.54 ppm 0.852 2.233 0.352 0.080 0.017 1.298 0.120
Max.
Methods of water analyses: ( 1 ) ICP-MS: As, Ba, Ce, Co, Cs, Mn and Rb, (2) IC-ICP-MS: La, Pb, Sm, Tb, U and Yb, (3) ISE: F, (4) ICP-ES: Fe and Mn, (5) HG-ICP-MS: Sb.
As Ba Ce Co Cs F Fe La Mn Na Pb Rb Sb Sm Tb U Yb
Element
Medians, geometric means, log standard deviations and ranges of lake sediment and lake water data (n = 93 ). Sediment data in ppm and water data in ppb unless otherwise noted
TABLE 3 oo
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
13 9
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Mn Rb Fe Na PU Co La Cs Ba C e S m S b Tb F
Yb As U
Fig. 9. Spearman correlation coefficients of analyses of lake sediment with lake water (n = 93 ). (Method of water analysis: ( 1 ) ICP-MS: As, Ba, Ce, Co, Cs, Mn, and Rb; (2): IC-ICP-MS: La, Pb, Sm, Tb, U and Yb; (3) ISE: F; (4) ICP-ES: Fe and Na; (5) HG-ICP-MS: Sb. )
ity. The rare earth elements La, Ce, Sm and Tb also have quite strong correlations between concentrations in sediment and water. By comparing the medians (or means) in Table 3, it is apparent that there is a great range in the partitioning of elements between water and sediment. At one extreme the partitioning ratios of Fe and Na in water relative to sediment are less than 0.1 ( X 10 -s ) whereas the water/sediment ratio of fluoride is 8.8 ( X 10-5). The partitioning ratios of the medians are shown in Fig. 10. This variable partitioning must, in part, reflect the degree of similarity of the element distribution patterns in the two media. Where the dispersion of an element is largely geologically controlled, one would expect similar patterns in different media. However, the mobility of Fe, for example, is largely a function of Eh, being mobilized under reducing conditions and fixed as a hydrous oxide under the oxidizing conditions that are common in Newfoundland lakes. Since virtually all of the Fe is taken up in sediment in this form it is not surprising that there is no correlation between the two media. Fluoride on the other hand has a correlation coefficient of 0.8 and has very similar patterns in the two media. Antimony (Sb) has the second highest partitioning ratio and also is strongly correlated between the two sample types. As an example of the similarity of distribution patterns for elements with strong correlations between the two sample media, maps showing the distributions of contoured As data (r = 0.8 ) from lake water and lake sediment are
140
J.W. McCONNELLET AL.
4 3.5 3 2.5 2 1.5 1 0.5 [] I
0
J
L
I
I
I
Na Ire Co Ce Yb S m La Mr] Tb
t
I
d
~
I
I
I
I
O Ba Cs As Pb Rb Sb F
Fig. 10. Partitioning ratios ( × 105) between lake water and lake sediment (n = 93 ).
presented (Figs. 11 a and b). Similarities between the two maps are apparent. The western area in both maps has considerably higher contents of As than the east reflecting the contrasting geology - - the west being dominantly finegrained siliciclastic terrane and the east largely granitic. Arsenic in both media shows a northeast trend in the western area, again reflecting the regional geologic strike direction (Fig. 8 ). The map of As in water (Fig. 11 a) appears to provide more information in the eastern area than does the sediment map (Fig. 1 l b), and suggests that the western and southern boundaries of the granitic terrane have higher As levels than do the central and eastern portions. SUMMARY AND CONCLUSIONS
Stream survey
In this particular study area, overbank is the preferred sample medium because it is available at more sites, is easier to sample and, for most elements, provides a better geochemical contrast. Furthermore, past mining activity has contaminated some drainages in the area, resulting in contamination of the active sediment but not the overbank sediment (McConnell, 1992). Elsewhere, under different geological and physiographic conditions, active sediment may be preferable. In similar glaciated landscapes, where samples are from small catchments (2-10 km2), for most elements data from overbank and active stream sediment could be readily combined using an inter-medium levelling approach such as that of Garrett ( 1991 ).
3.21 1.00
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.
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.
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Zig. 11. Arsenic in lake water (a) and lake sediment (b) in the Bay d'Espoir area.
(a)
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142
Lw. McCONNELLETAL.
Lake survey
Lake sediment and lake water are clearly two very different sample media, with trace-element abundance levels being between 104 and 106 lower in water than sediment. Furthermore, comparison of spatial patterns between the two media is hampered by high detection limits for many elements in water relative to their rather low local background values in the study area: only 17 of the elements analyzed in both media have at least 40% of values in water above the analytical detection limit. Nonetheless, 11 of the 17 elements show moderate to strong spatial correlations (r > 0.4) between the sediment and water distribution patterns. With the current rapid progress in lowering analytical detection limits in water analysis, this number will no doubt grow. The spatial correlations between the two media for F, Yb, As and U are strong enough to permit levelling and integration of the results, and for Tb, Sb, Sm, Ce, Ba, Cs and La the distribution patterns reveal the same regional geochemical features. This implies that hydromorphic dispersion, followed by fixation in the lake sediment, is the dominant dispersion process for F, Yb, As and U, and is important for the remaining elements that show this spatial correspondence. The results also suggest that the geochemical patterns for many elements in lake water may be quite stable over time. Lake sediment provides a geochemical signal integrated over many years, whereas in this study the water data provide a picture of surface water chemistry for a single day. More work to determine seasonal variability in lake water chemistry is necessary, but this study suggests that surface water, together with drainage sediment, should be considered as a key sample medium for international geochemical mapping. ACKNOWLEDGEMENTS
We would like to thank Simon Jackson, Brian Fryer and Henry Longerich of the Department of Earth Sciences, Memorial University of Newfoundland, for their contributions to the ICP-MS analyses of the lake water and Larry Nolan of the Geological Survey Branch for producing the colour figures. The manuscript benefitted substantially from the comments of two anonymous reviewers.
REFERENCES Colman-Sadd, S.P., Hayes, J.P. and Knight, I., 1990. Geology of the Island of Newfoundland, Map 90-01. Newfoundland Department of Mines and Energy, Geological Survey Branch. Davenport, P.H. and Nolan, L.W., 1988. Gold and associated elements in lake sediment from regional surveys in the Botwood map area (NTS 2E). Newfoundland Department of Mines, Mineral Development Division, Open File 2E/563.
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
143
Davenport, P.H., Nolan, L.W. and Hayes, J.P., 1989. Gold and associated elements in lake sediment from regional surveys in the Sandy Lake map area (NTS 12H). Newfoundland Department of Mines and Energy, Geological Survey Branch, Open File 12H ( 1012). Finch, C., Hall, G.E.M. and McConnell, J.W., 1992. The development and application of geochemical analyses of water. In Current Research. Newfoundland Department of Mines and Energy, Geological Survey Branch, Report 92-1, pages 297-307. Friske, P.W.B. and Hornbrook, B.H.W., 1991. Canada's National Geochemical Reconnaissance programme. Trans. Inst. Min. Metall., Sect. B, 100: B47-B56. Garrett, R.G., 1991. Regional geochemical data compilation and map preparation, Labrador, Canada. J. Geochem. Explor., 39:91-116. Grant, D.R., 1986. Surficial geology of the Baie Verte Peninsula. Geological Survey of Canada, 1: 50,000 maps comprising 2E/13; 12H/7, 8, 9, 10, 15 and 16; 12I 1. Open File 1312. Kean, B.F., 1984. Geology and mineral deposits of the Lushs Bight Group, Notre Dame Bay. In Current Research. Newfoundland Department of Mines and Energy, Geological Survey Branch, Report 84-1, pp. 141-155. Kean, B.F. and Evans, D.T.W., 1987. Geological map of NTS 2E/12. Newfoundland Department of Mines, Mineral Development Division, Map 8707. Kean, B.F. and Evans, D.T.W., 1990. Geology and mineralization of the Lushs Bight Group Springdale Peninsula. In: Metallogenic framework of base and precious metal deposits, central and western Newfoundland. 8th IAGOD Symposium Field Trip Guidebook. Geological Survey of Canada, Open File 2156, pp. 130-136. Liverman, D. and Taylor, D., 1990. Surficial Geology of insular Newfoundland, preliminary version. Newfoundland Department of Mines and Energy, Geological Survey Branch, Map 90-08. McConnell, J.W., 1992. Conventional and overbank stream-sediment surveys on the Baie Verte and Springdale peninsulas. Newfoundland Department of Mines and Energy, Geological Survey Branch, Open File NFLD 2191, 35 pp. Ottesen, R.T., Bogen, J., Bolviken, B. and Bolden, T., 1989. Overbank sediment: a representative sample medium for regional geochemical mapping. J. Geochem. Explor., 32: 257-277.
123
Elsevier Science Publishers B.V., A m s t e r d a m
Geochemical mapping employing active and overbank stream-sediment, lake sediment and lake water in two areas of Newfoundland J.W. McConnell a, C. Finch a, G.E.M. H a l P and P.H. D a v e n p o r t a aNewfoundland Department of Mines and Energy, Geological Survey Branch, P.O. Box 8700. St. John's, Nfld. AIB 4J6, Canada bGeological Survey of Canada, 601 Booth St., Ottawa, Ont. KIA OE8, Canada (Received 7 December 1992; accepted after revision 15 June 1993 )
ABSTRACT Geochemical data from samples of active and overbank stream sediment in one area and from lake sediment and lake water in another are compared and evaluated. Samples of both types of stream sediment were collected from drainage basins 2-10 km 2 in area in a glaciated landscape and the < 63 /zm fractions were analyzed for 38 elements. In general, trace element distribution patterns are similar in the two media and reflect the chemistry of the underlying bedrock. Absolute concentrations are higher for most elements in the overbank sediment. For elements having concentrations near the analytical detection limit, such as Au, Mo and Pb, the overbank sediment provides more reliable data in those parts of the area with low background levels. In this area, suitable overbank material is more widespread and available for sampling than is active sediment. In some drainages, stream-sediment data are clearly contaminated by past mining activity but overbank data appear unaffected. Lake water was collected from another area where a lake sediment survey had been conducted eleven years earlier. The < 180/~m fraction of sediment was analyzed for 29 elements and the water for about 50 elements. Due to the contrasting nature of these two sample media, element distribution patterns are less similar than in the case of the two types of stream sediment. Nonetheless, elements which are hydromorphically dispersed and which are partitioned consistently between water and sediment (such as F, U, As, and rare earths ) do demonstrate a strong spatial correlation. As analytical detection limits for water continue to improve, the versatility of this medium in geochemical mapping will increasingly complement more conventional sampling media as well as providing an alternate medium in areas where sediment is unavailable.
INTRODUCTION
A problem to be faced in compiling geochemical data for large regions is whether one can meaningfully compare or combine data from different types of surveys in a given area. In Newfoundland, the opportunity arose to address this question using four types of sample media in two distinct areas (Fig. 1 ). In the Baie Verte/Springdale area a stream survey was conducted in which both active and overbank sediment were collected to obtain data to identify
0 3 7 5 - 6 7 4 2 / 9 3 / $ 0 6 . 0 0 © 1993 Elsevier Science Publishers B.V. All rights reserved.
124
j.w. MeCONNELLET AL. 52 °
~4. o
146 ° ~,7. o 60 °
58 °
56 °
INDEX
9a °
MAP
Fig. 1. Location o f survey areas.
drainage basins containing gold or base-metal mineralization. These two sets of data are evaluated and compared. The second area studied is along the south coast of Newfoundland between Bay d'Espoir and Fortune Bay, where a lake-water survey was conducted in 1991 over diverse bedrock terranes. A reconnaissance lake-sediment survey had been done over the same area 11 years previously that provided a data base of 33 elements with which to compare the newly acquired water data. STREAM S U R V E Y
Study area
The Baie Verte Peninsula has been a focus of mining since 1864. Numerous gold and base metal occurrences are known, several of which are currently being evaluated for possible production. The peninsula is divided tectonostratigraphically by the Baie Verte Line - - a structural break between the H u m b e r Zone to the west, dominated by schist, gneiss and granitoids, and the Dunnage Zone to the east, dominated by ophiolite, volcanic cover sequences and intrusive rocks. The geology of the area (after Colman-Sadd et al., 1990 ) and the sampled drainage basins are shown in Fig. 2. Most of the known min-
I0
C/'
Ultramafic rocks of ophiolite complexes
-~
Siliciclastic rocks
['",
Drainage
basin
Psammitic schist
Schist
-] Marine siliciclastic rocks -~ Submarine volcanic rocks; mostly mafic; incl ophiolite IT]
~]
STRATIFIED ROCKS Proterozolc to Carboniferous [~ Non-marine sandstone and shale -~ imodal subaerial volcanic rocks
Mafic intrusive rocks
~
Granitoid intrusions
Granite and syenite
Gabbro
Fig. 2. Geology and sampled drainage basins of the stream survey area (geology after Colman-Sadd et al., 1990).
I
Bole Verte Line
6
-~
Cambrian to Devonian
INTRUSIVE ROCKS
,--t m
:z
z
r~
>
m
o.<
,H
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126
j.w. McCONNELL ET AL.
eral occurrences are found in the Dunnage Zone - - particularly associated with ophiolitic and mafic volcanic rocks. The survey area between Green Bay and Halls Bay is underlain primarily by Cambrian ophiolitic rocks and Ordovician volcanic and epiclastic rocks (Kean and Evans, 1987, 1990). These units host several volcanogenic massive sulphide deposits including past producers and epigenetic gold occurrences. Pleistocene glaciation left most of the area with a thin covering of locally derived colluvium, patchy occurrences of glacial sediment or a thin till veneer (Grant, 1986; Liverman and Taylor, 1990 ). Active stream sediment and overbank material are likely derived from a combination of such glacial material and actively eroding bedrock. Reconnaissance surveys of organic lake sediment have been made over the entire island at a sample density of about 1 per 6 km 2. Samples from this area have been analyzed for 33 elements including Au, As, Sb and base metals (Davenport and Nolan, 1988; Davenport et al., 1989). These data were used in selecting areas for the stream survey.
Sampling methods Stream sites were sampled that had upstream catchments on the order of 2-10 km 2. Where possible, samples of overbank and active sediment were collected at each site, generally within 100 m of each other. A total of 143 samples of active sediment and 175 samples of overbank sediment were collected. Samples of both types were obtained at 121 sites. Of these sites, 19 were sampled in duplicate (site duplicates ). The streams have generally moderate to fast flow rates with most sediment ranging in size from gravel to small boulders. Where present, fine sediment was most often found in the lee, or around the base, of boulders and sampling was frequently time consuming. Approximately 200 to 300 g of < 300/~m active sediment were obtained at each site by wet-sieving in the field. Overbank sediment is characteristically fine-grained material deposited during flood conditions on flood plains or on low and level stream banks. Some researchers regard the geochemistry of these deposits as more representative of the upstream catchment basins than are deposits of active sediment (Ottesen et al., 1989). Sediment from such areas was sampled by using a spade to obtain a channel sample down the entire overbank profile. Collection took only three or four minutes. Thicknesses of the overbank layers ranged from 10 to > 100 cm. The average thickness was 35 cm and 6 sites had more than 100 cm of sediment. Deposits of overbank sediment were more frequently encountered than were deposits of suitably fine-grained active sediment.
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
127
Analytical methods The sediment samples were air-dried in the field, further dried in ovens at 60°C and then sieved to <63 #m (230 mesh). Both active and overbank sediment samples were analyzed for the following elements: Ag, As, Au, Ba, Br, Ce, Co, Cr, Cs, Cu, Dy, Eu, Fe, Ga, Hf, La, Li, Mn, Mo, Nb, Na, Ni, Pb, Rb, Sb, Sc, Sm, Sr, Ta, Tb, Th, U, V, W, Y, Yb, Zn and Zr. Some elements, notably Ba, Ce, Co, Cr, Cu, Fe, La, Mn, Mo, Ni, Pb, Sc and Zn were analyzed by more than one method. As a check on accuracy and precision, a standard sample of known composition and a sample duplicate (split) were included in each batch of 20 samples. The analytical methods comprised instrumental neutron activation analysis (INAA), inductively coupled plasma emission spectroscopy (ICP-ES), atomic absorption spectroscopy using a HF-HC1OaHC1 digestion (AA-total) and atomic absorption spectroscopy using an aqua regia digestion (AA-partial). The first three methods give total or near-total analyses, the last is partial. The analytical method is identified in the these data by the addition of suffixes to the element name: "1" for INAA, "2" for ICP-ES, "3" for AA-total and "4" for AA-partial (e.g. Ni4 identifies nickel analyzed by AA-partial ).
Results and discussion Summary statistics of data selected to illustrate a range of elements from the < 63/~m fractions of active and overbank stream sediment are presented in Table 1. To ensure comparability, only samples from the 121 sites represented by both sample media are included. Since examination of data populations indicated these to be positively skewed and more closely representative of log-normal than normal populations, geometric means and log standard deviations are given. Examination of the median values reveals some interesting trends in the two sample media. For seventeen of the 23 elements, the concentration in overbank sediment is higher than in active sediment. This enrichment in overbank sediment likely has two principal causes. The overbank material generally contains a greater proportion of finer-grained material than the active sediment (despite their both being < 63/~m) and hence would have more surface area with sites suitable for ion adsorption. Secondly, the concentration of Fe4 and Mn4 (aqua regia digestible; i.e. largely present as (hydr)oxides of iron and manganese ) in overbank sediment is nearly double that in active sediment. This condition would also have the effect of increasing the ion-adsorption capacity of this medium relative to active sediment. Note, however, that the median values of total iron (Fel) in the two media are very similar, 6.0% and 7.3%. Spearman (ranked) correlation coefficients between element content and iron content for active and overbank samples are shown in Fig. 3 (a and b).
128
J.W. McCONNELL ET AL.
TABLEI
Medians, geometric means, log standard deviations and ranges of data for < 63/~m active sediment (A.S.), and <63/~m overbank sediment (O.S.) ( n = 121 for each sample type; data in ppm unless otherwise noted) Element Median
Asl Aul a Ba2 Cd4 Cel Co4 Crl Cu2 Cu4 Fel b Fe4 b La2 Mn4 Mo2 Nb2 Ni4 Pb4 Rb2 Sbl Thl UI Y2 Zn4
Xg Mean
Log S.D.
Range
A.S.
O.S.
A.S.
O.S.
A.S.
O.S.
A.S.
O.S.
10.0 4.0 237 0.2 54 20 280 29 28 6.0 2.12 19 870 4 9 35 5 15 0.53 5,3 2.9 30 90
22.3 7.0 200 0.5 86 27 259 61 47 7.3 4.15 26 2150 7 4 30 18 19 0.40 6.1 3.3 32 74
11.0 6.6 219 0.3 46 19 339 28 27 5.8 2.09 20 1100 4.9 9.3 40 5.9 19 0.60 5.1 3.0 30 87
20.9 7.1 219 0.5 89 28 251 53 43 7.1 3.98 28 2090 8.3 4.2 35 19 20 0.46 5.6 3.5 31 78
0.54 0.95 0.28 0.32 0.35 0.27 0.44 0.39 0.37 0.13 0.17 0.25 0.48 0.16 0.34 0.39 0.36 0.30 0.32 0.28 0.31 0.20 0.25
0.55 0.44 0.29 0.45 0.44 0.41 0.35 0.48 0.48 0.17 0.25 0.34 0.69 0.26 0.40 0.45 0.38 0.32 0.31 0.34 0.42 0.23 0.30
0.2-435 <2-1550 39-1250 <0.1-3.0 5-350 2-116 10-6570 3-422 3-403 2.2-12.0 0.51-4.49 4-150 85-14000 2-29 1-50 4-590 1-96 2-85 0.13-16.2 1.1-26.0 0.5-18.0 6-121 18-438
1.1-535 <2-95 48-973 <0.1-12 5-727 2-250 41-2410 3-1416 2-1130 1.6-18.9 0.27-14.50 4-223 24-54800 2-80 1-33 2-317 1-541 2-132 <0.10-6.77 0.6-35.0 0.3-60.9 5-106 6-474
a data in ppb. b data in weight %.
When the elements are plotted in order of increasing correlation with Fe 1 some interesting trends emerge. Firstly, the active-sediment data have stronger correlations with both Fel and Fe4 than do the overbank data. Secondly, plotting in order of increasing Fe 1 correlation tends to group the elements by their affinities, i.e. lithophile elements such as Zr, Rb, Hf, Ba and Nb have negative correlations with total iron and plot at the left end of both figures. The middle range is dominated by chalcophile elements such as Cu, Zn, As and Sb and the upper range by siderophile elements such as Ni, Co and Sc. The effect of oxide scavenging can be estimated by the strength of the coefficients with Fe4. This effect can be seen to be stronger in active sediment than in overbank sediment where 13 elements in active sediment have coefficients
129
GEOCHEMICAL MAPPING EMPLOYING ACTlVE AND OVERBANK SEDIMENT AND WATER
1
0.9 0.8
0.70.60.50.4-
0.30.2 0.1 Confidence levels
0 -0.1 -0.2 -0.3 -0.4
[]
-0.5 -0.6
(a)
Z'r21RI~21TI~ llNb2~Sm ' ' llPb41Y'2 ' IZ 'n41M'o21C 'u41T;2 ] N'i41N'i 1 l scllFe4lFe2[ ' ' ' Ba2Hfl
[]
U1 L a 2 A u l S b l D y 2 C u 2 A s l M n 4 C d 4 C r l M n 2 C o 4 C o l Fel with Fel ('totol Fe) + with Fe4 ( ' p o r t i o l )
1
0.9 0.8
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
99% confidence levels<
-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 Z
~ Bo2Hfl
n Ul [3
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with Fel ( t o t a l Fe) +
with Fe4 ( p a r t i a l )
Fig. 3. Spearman correlation coefficients of analyses of total iron ( F e l ) and aqua regia digestible iron (Fe4) with: (a) active sediment data and (b) overbank sediment data. (n = 121. )
130
j.w. McCONNELLET AL.
> 0.40 whereas only 9 coefficients in overbank sediment are > 0.40. However, the overall trends in the two sample media are similar. Cumulative frequency plots permit a visual comparison of the shape and nature of population distributions. Fig. 4 displays the distributions in active 9e 1000
90
80 7060504030 20
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98
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00 7 0 6 0 5 0 4 0 9 0
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o
Fig. 4.
2
Cumulative frequency plots of Asl, Aul, Cel, Crl, Cu4, Mo2, Ni4, Pb4 and Zn4 in and overbank sediment. (Solid line i s a c t i v e s e d i m e n t and dotted line is overbank sediment).
active
131
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
1 .00
0.95
Strong correlations
0.90
0.85
0.80
0.75
0.70
0.65
(a) [
0.60
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i
i
i
i
i
i
i
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Weak to moderate correlations
0.50
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0.30
ence
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i
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Fig. 5. Spearman correlation coefficients of analyses of active sediment with overbank sediment
(n=12l).
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4
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rig. 6. Nickel (Ni4) in active sediment. (See Fig. 2 for geologic legend. Contacts are shown as medium-weight lines; faults are heavy-weight. )
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'
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185
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10
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Fig. 7. Nickel (Ni4) in overbank sediment. (See Fig. 2 for geologic legend. Contacts are shown as medium-weight lines; faults are heavyweight. )
L/~_I__
BeieVerteLine-
,/-% / %"~)
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134
J.W.McCONNELLETAL.
and overbank sediment of As, Au, Ce, Cr, Cu, Mo, Ni, Pb and Zn. These illustrate a range of similarities and differences in geochemical responses in the two media. Nickel and Zn have very similar plots. The distribution curves of Ce, Cr, Cu and Pb have similar shapes but differ in absolute concentrations. Arsenic has a major population break ("A" in Fig. 4) at the 95th percentile in active sediment that is absent in the overbank data. Molybdenum has two distinct populations with a break-point ("B") at about the 75th percentile in the overbank data that is not apparent in the active sediment data. The absence of this feature in active sediment may result from most of the data being near the detection limit hence a large error in analytical precision for Mo could obscure the break. The largest difference in the two distributions is for Au. Nearly 90% of the overbank samples have detectible Au whereas it can be detected in only 55% of the active sediment samples. However, the upper range of Au values is an order of magnitude greater in the active sediment. One way of quantifying the cumulative effect of these various factors (such as grain size and amorphous Fe hydroxides/oxides) that contribute to the overall element content of samples is to calculate correlations between pairs of variables. Spearman correlation coefficients for 40 elements between active and overbank sediment are plotted in Fig. 5. Note that correlation coefficients are moderate to strong - - greater than 0.40 - - for all the elements but Cd2, Aul, Mn2 and Mn4. Coefficients > [0.40[ are significant at the 99.9% confidence level. Gold analyses are notoriously difficult to reproduce even within the same medium because of the nugget effect. It has been shown elsewhere that there is no significant correlation for Au analyses between site duplicate samples of either active sediment or overbank sediment in this area, although other elements are well correlated (McConnell, 1992 ). Manganese is largely controlled by varying E h / p H conditions rather than primary upstream metal contents. It is not apparent why Cd2 has poor correlation between the two media although its relatively high detection limit and strong correlations with Mn4 and Fe4 are possible reasons. Figs. 6 and 7 are coloured drainage-basin maps, overlain on a geologic base, that display the Ni (Ni4) contents of active and overbank sediments in sampled catchments. The colour intervals were selected by choosing inflection points on the cumulative frequency curves of the two media (Fig. 4). These correspond approximately to the 50th, 70th, 85th and 92nd percentiles in both cases. Sample sites on the maps are denoted by black dots; drainage basins from which only sample material of the other medium was obtained are uncoloured. The correlation coefficient for Ni4 between the two sample media is 0.82. Since Ni concentration in igneous rocks has an inverse correlation with degree of magmatic differentiation, Ni might be expected to reflect the bedrock geology of this mainly igneous terrane. The distribution patterns of Ni in the two figures are similar and do reveal variations in lithology. Most
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
13 5
distinctive is the trend of high values along the Baie Verte line in the western part of the map area - - an area predominantly underlain by mafic and ultramafic rocks. The area to the west of Green Bay is underlain predominantly by granitic rocks (Unit 11; see Fig. 2 for legend) and is characterized by low Ni values in overbank sediment. Due to a scarcity of material, few samples of active sediment were collected here. The Springdale Peninsula, located between Green Bay and Hails Bay, is underlain largely by mafic volcanic rocks (Unit 3 ). However the presence of higher Ni contents in both active and overbank sediments suggests that the northwest side, relative to the southeast side, is underlain by volcanic rocks of more mafic character. Geological mapping (Kean, 1984; Kean and Evans, 1987) indicates that the area is a composite of complex fault blocks of similar volcanic units that, in many cases, are difficult or impossible to distinguish in the field. The geochemical patterns may assist in unravelling the stratigraphic interpretation. These data demonstrate the different responses the two media have to the effects of mining-related contamination of the surface environment. Active stream sediment from the drainages downstream from the Rambler Mine railings ( C u - P b - Z n - A u ) and from a drainage basin adjacent to the ore-haulage road north of the Tilt Cove Mine (Cu-Au) are highly anomalous in Cu and Pb and, in one instance, Au and As. In contrast, analyses of the overbank sediment from the same sites all fall well within the local background (McConnell, 1992). The locations of the mines are shown in Figs. 6 and 7 although the Ni contents do not show the contamination effect as Ni is not an ore element at either mine. LAKE S U R V E Y
Study area
The survey area is located north of Fortune Bay and east of Bay d'Espoir and is divided into two contrasting geological terranes by the northeast trending Hermitage Bay Fault (Fig. 8 ). To the southeast, the older Avalon Zone is comprised predominantly of volcanic, sedimentary and intrusive rocks of Hadrynian age which are intruded by a Devonian high-silica granite. The Gander and Dunnage zones to the northwest of the fault are composed of clastic-sedimentary and volcanic rocks also intruded by granites. Numerous small mineral occurrences are known from the area including Sn and Mo in the high-silica granite and As, Au, Sb, Cu and Zn, mostly in the Dunnage Zone. These two terranes are also distinctive geochemically in the reconnaissance lake sediment data. Generally, the area to the northwest of the fault, relative to the area to the southeast, is higher in the elements As, Ba, Co, Cu, Cs, Ni, Sb and lower in F, U and REEs.
136
J.W. McCONNELL ET AL.
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Fig. 8. Geologyand location of the lake surveyareas (geologyafter Colman-Saddet al., 1990). Sampling
methods
A total of 136 lake-water samples were collected in a single day by helicopter in the Bay d'Espoir-Fortune Bay area including 12 pairs of site duplicates. A sample was obtained by landing near the centre of the lake and immersing and filling a rinsed, 250 ml nalgene bottle. The water survey was conducted over the same area as the previous lake sediment survey although only 108 of the lakes were sampled for both media. Samples of organic, lake-bottom sediment were collected from the centres of lakes during the reconnaissance survey using a float-equipped helicopter and a procedure described by Friske and Hornbrook ( 1991 ). Analytical methods L a k e w a t e r s . Within 24 hours of collection and following analysis for pH and total dissolved solids, the samples were filtered through a 0.45 # m filter paper and acidified with 2 ml of nano-pure HNO3 to await further analysis. To date, 105 elemental analyses employing 8 different methods have been performed (Table 2 ). Several elements have been analyzed by more than one means. Method descriptions can be found in Finch et al. ( 1992 ). L a k e s e d i m e n t . Lake sediment samples were partially air-dried in the field then oven-dried at 40 ° C. This material was disaggregated with a mortar and
13 7
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
TABLE2
Methods of lake water analyses by participating laboratory. Laboratories: N D M E - - Newfoundland Department of Mines and Energy; M U N / C E R R - - Memorial University of Newfoundland/Centre for Earth Resources Research; G S C - - Geological Survey of Canada. Analytical methods: ICP-ES - inductively coupled plasma emission spectrometry; U S N - - ultra sonic nebulizer; MS - - mass spectrometry; H G - - hydride generation; IC - - ion chromatography; ISE - - ion selective electrode; AA - atomic absorption spectrophotometry; F - - flame N D M E Laboratory
MUN/CERR
G S C Laboratory
ICP-ES
USN-ICPES
ICP-MS
HG-ICP-MS
IC-ICP-MS
ISE
HG-AA
F-AA
Si Ca Mg Na K S Fe Mn
Li Be B A1 P Fe Sr Ba Ti
Li Be B Mg A1 Si P S C1 Ca Ti
Mn Fe Co Ni Cu Zn As Br Se Mo Ag
Cd Sn Sb I Cs Ba La Ce Hg Ti Pb
Se Te Bi Sb
REEs (i.e., La, Ce, Pr, Nd, Sin, Eu, Gd, Tb, Dy, Ho, Er, T m , Yb, Lu) A1 Co Cd Ti Ni Pb V Cu U Mn Zn Fe Y
F
As
Na K Ca Mg Mn Fe Zn
V
Ba
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Cr
La
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pestle and sieved with a 180 #m (80 mesh) stainless-steel sieve. The finefraction was analyzed by three methods; by instrumental neutron activation
(INAA) for As, Au, Ba, Br, Ce, Cs, Eu, Hf, La, Rb, Sm, Sb, Sc, Se, Na, Ta, Tb, Th, U and Yb; by atomic absorption spectroscopy using a 4 M HNO31 M HC1 digestion (AA-partial) for Ag, Co, Cu, Fe, Mn, Ni, Pb and Zn; by fluoride-ion specific electrode for F following fusion of the sample with a flux of NaCO3/KNO3 and solution in 10% citric acid. Results and discussion Summary statistics of 17 elements for lake sediment and lake water are presented in Table 3. To ensure comparability, only data from sites sampled for both media and from sites with complete sets of both data are included. The elements chosen for inclusion are those that have detectable concentrations in at least 40% of the samples. Correlation coefficients between the two media for these elements are shown in Fig. 9. The elements Mn, Rb, Fe and Na are not significantly correlated between media whereas the elements U, As, Yb and F are the most strongly correlated (r >0.7). Maps of the distributions of this second group in sediment and water show the greatest similar-
27.6 150 100 14 2.1 194 6.57% 47 1660 0.30% 16 22 0.3 7.8 1.1 4.2 3.5
0.318 1.164 0.208 0.029 0.020 17 62 0.145 5.52 1.61 ppm 0.191 0.532 0.008 0.022 0.004 0.023 0.008
28.8 141 102 15.8 2.1 240 5.75% 45.7 1950 0.30% 20.4 19.1 0.43 8.4 1.5 10.2 4.3
Sed.
Sed.
Water
Mean
Median
0.355 1.202 0.186 0.039 0.023 29.5 60.3 0.117 6.17 1.66 ppm 0.170 0.513 0.011 0.022 0.004 0.047 0.010
Water
0.85 0.45 0.40 0.49 0.34 0.46 0.40 0.32 0.75 0.28 0.48 0.43 0.52 0.36 0.45 0.38 0.58
Sed.
Log S.D.
0.53 0.33 0.33 0.51 0.33 0.46 0.43 0.31 0.47 0.11 0.36 0.21 0.43 0.32 0.35 0.74 0.50
Water
0.2 25 14 1 0.2 40 0.41% 6.8 77 0.08% 1 2 0.02 1.3 0.2 0.6 0.2
1320 1400 651 164 10 3240 7.6% 225 77000 1.1% 841 160 17.6 58.1 16 735 67.1
0.102 0.321 0.012 0.012 0.006 10 10 0.010 0.158 1.06 ppm 0.008 0.119 0.005 0.002 0.0003 0.004 0.001
Min.
Min.
Max.
Water
Sed.
Range
9.287 25.322 0.766 1.128 0.554 220 493 0.877 79.32 6.54 ppm 0.852 2.233 0.352 0.080 0.017 1.298 0.120
Max.
Methods of water analyses: ( 1 ) ICP-MS: As, Ba, Ce, Co, Cs, Mn and Rb, (2) IC-ICP-MS: La, Pb, Sm, Tb, U and Yb, (3) ISE: F, (4) ICP-ES: Fe and Mn, (5) HG-ICP-MS: Sb.
As Ba Ce Co Cs F Fe La Mn Na Pb Rb Sb Sm Tb U Yb
Element
Medians, geometric means, log standard deviations and ranges of lake sediment and lake water data (n = 93 ). Sediment data in ppm and water data in ppb unless otherwise noted
TABLE 3 oo
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
13 9
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Mn Rb Fe Na PU Co La Cs Ba C e S m S b Tb F
Yb As U
Fig. 9. Spearman correlation coefficients of analyses of lake sediment with lake water (n = 93 ). (Method of water analysis: ( 1 ) ICP-MS: As, Ba, Ce, Co, Cs, Mn, and Rb; (2): IC-ICP-MS: La, Pb, Sm, Tb, U and Yb; (3) ISE: F; (4) ICP-ES: Fe and Na; (5) HG-ICP-MS: Sb. )
ity. The rare earth elements La, Ce, Sm and Tb also have quite strong correlations between concentrations in sediment and water. By comparing the medians (or means) in Table 3, it is apparent that there is a great range in the partitioning of elements between water and sediment. At one extreme the partitioning ratios of Fe and Na in water relative to sediment are less than 0.1 ( X 10 -s ) whereas the water/sediment ratio of fluoride is 8.8 ( X 10-5). The partitioning ratios of the medians are shown in Fig. 10. This variable partitioning must, in part, reflect the degree of similarity of the element distribution patterns in the two media. Where the dispersion of an element is largely geologically controlled, one would expect similar patterns in different media. However, the mobility of Fe, for example, is largely a function of Eh, being mobilized under reducing conditions and fixed as a hydrous oxide under the oxidizing conditions that are common in Newfoundland lakes. Since virtually all of the Fe is taken up in sediment in this form it is not surprising that there is no correlation between the two media. Fluoride on the other hand has a correlation coefficient of 0.8 and has very similar patterns in the two media. Antimony (Sb) has the second highest partitioning ratio and also is strongly correlated between the two sample types. As an example of the similarity of distribution patterns for elements with strong correlations between the two sample media, maps showing the distributions of contoured As data (r = 0.8 ) from lake water and lake sediment are
140
J.W. McCONNELLET AL.
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presented (Figs. 11 a and b). Similarities between the two maps are apparent. The western area in both maps has considerably higher contents of As than the east reflecting the contrasting geology - - the west being dominantly finegrained siliciclastic terrane and the east largely granitic. Arsenic in both media shows a northeast trend in the western area, again reflecting the regional geologic strike direction (Fig. 8 ). The map of As in water (Fig. 11 a) appears to provide more information in the eastern area than does the sediment map (Fig. 1 l b), and suggests that the western and southern boundaries of the granitic terrane have higher As levels than do the central and eastern portions. SUMMARY AND CONCLUSIONS
Stream survey
In this particular study area, overbank is the preferred sample medium because it is available at more sites, is easier to sample and, for most elements, provides a better geochemical contrast. Furthermore, past mining activity has contaminated some drainages in the area, resulting in contamination of the active sediment but not the overbank sediment (McConnell, 1992). Elsewhere, under different geological and physiographic conditions, active sediment may be preferable. In similar glaciated landscapes, where samples are from small catchments (2-10 km2), for most elements data from overbank and active stream sediment could be readily combined using an inter-medium levelling approach such as that of Garrett ( 1991 ).
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Lake survey
Lake sediment and lake water are clearly two very different sample media, with trace-element abundance levels being between 104 and 106 lower in water than sediment. Furthermore, comparison of spatial patterns between the two media is hampered by high detection limits for many elements in water relative to their rather low local background values in the study area: only 17 of the elements analyzed in both media have at least 40% of values in water above the analytical detection limit. Nonetheless, 11 of the 17 elements show moderate to strong spatial correlations (r > 0.4) between the sediment and water distribution patterns. With the current rapid progress in lowering analytical detection limits in water analysis, this number will no doubt grow. The spatial correlations between the two media for F, Yb, As and U are strong enough to permit levelling and integration of the results, and for Tb, Sb, Sm, Ce, Ba, Cs and La the distribution patterns reveal the same regional geochemical features. This implies that hydromorphic dispersion, followed by fixation in the lake sediment, is the dominant dispersion process for F, Yb, As and U, and is important for the remaining elements that show this spatial correspondence. The results also suggest that the geochemical patterns for many elements in lake water may be quite stable over time. Lake sediment provides a geochemical signal integrated over many years, whereas in this study the water data provide a picture of surface water chemistry for a single day. More work to determine seasonal variability in lake water chemistry is necessary, but this study suggests that surface water, together with drainage sediment, should be considered as a key sample medium for international geochemical mapping. ACKNOWLEDGEMENTS
We would like to thank Simon Jackson, Brian Fryer and Henry Longerich of the Department of Earth Sciences, Memorial University of Newfoundland, for their contributions to the ICP-MS analyses of the lake water and Larry Nolan of the Geological Survey Branch for producing the colour figures. The manuscript benefitted substantially from the comments of two anonymous reviewers.
REFERENCES Colman-Sadd, S.P., Hayes, J.P. and Knight, I., 1990. Geology of the Island of Newfoundland, Map 90-01. Newfoundland Department of Mines and Energy, Geological Survey Branch. Davenport, P.H. and Nolan, L.W., 1988. Gold and associated elements in lake sediment from regional surveys in the Botwood map area (NTS 2E). Newfoundland Department of Mines, Mineral Development Division, Open File 2E/563.
GEOCHEMICAL MAPPING EMPLOYING ACTIVE AND OVERBANK SEDIMENT AND WATER
143
Davenport, P.H., Nolan, L.W. and Hayes, J.P., 1989. Gold and associated elements in lake sediment from regional surveys in the Sandy Lake map area (NTS 12H). Newfoundland Department of Mines and Energy, Geological Survey Branch, Open File 12H ( 1012). Finch, C., Hall, G.E.M. and McConnell, J.W., 1992. The development and application of geochemical analyses of water. In Current Research. Newfoundland Department of Mines and Energy, Geological Survey Branch, Report 92-1, pages 297-307. Friske, P.W.B. and Hornbrook, B.H.W., 1991. Canada's National Geochemical Reconnaissance programme. Trans. Inst. Min. Metall., Sect. B, 100: B47-B56. Garrett, R.G., 1991. Regional geochemical data compilation and map preparation, Labrador, Canada. J. Geochem. Explor., 39:91-116. Grant, D.R., 1986. Surficial geology of the Baie Verte Peninsula. Geological Survey of Canada, 1: 50,000 maps comprising 2E/13; 12H/7, 8, 9, 10, 15 and 16; 12I 1. Open File 1312. Kean, B.F., 1984. Geology and mineral deposits of the Lushs Bight Group, Notre Dame Bay. In Current Research. Newfoundland Department of Mines and Energy, Geological Survey Branch, Report 84-1, pp. 141-155. Kean, B.F. and Evans, D.T.W., 1987. Geological map of NTS 2E/12. Newfoundland Department of Mines, Mineral Development Division, Map 8707. Kean, B.F. and Evans, D.T.W., 1990. Geology and mineralization of the Lushs Bight Group Springdale Peninsula. In: Metallogenic framework of base and precious metal deposits, central and western Newfoundland. 8th IAGOD Symposium Field Trip Guidebook. Geological Survey of Canada, Open File 2156, pp. 130-136. Liverman, D. and Taylor, D., 1990. Surficial Geology of insular Newfoundland, preliminary version. Newfoundland Department of Mines and Energy, Geological Survey Branch, Map 90-08. McConnell, J.W., 1992. Conventional and overbank stream-sediment surveys on the Baie Verte and Springdale peninsulas. Newfoundland Department of Mines and Energy, Geological Survey Branch, Open File NFLD 2191, 35 pp. Ottesen, R.T., Bogen, J., Bolviken, B. and Bolden, T., 1989. Overbank sediment: a representative sample medium for regional geochemical mapping. J. Geochem. Explor., 32: 257-277.