Dunn Colin E. Biogeochemistry in Mineral Exploration

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Environment

Arctic tundra – dwarf birch (Betula nana and B. glandulosa), Labrador tea (Ledum

groenlandicum and L. palustre). Typically associated with very stunted black spruce

and willow (Salix spp.), and an understory dominated by crowberry (Empetrum

nigrum), bearberry (Arctostaphylos alpina) and blueberry (Vaccinium spp.).

Biogeochemical survey

Combined samples of twigs and leaves from dwarf birch were colle cted at 96

sample stations that traversed the Impala and Bighorn kimberlites. Figure 11-20

shows the locat ion of the orientation sampling programme with respect to geo-

graphic features.

After oven drying at 70 1C, leaves were separated from twigs and the leaf tissue

was milled to a ﬁne powder. Control samples were inserted and 1 g portions of each

sample were digested in nitric acid then aqua regia, and analysis was by ICP-MS for

66 elements.

A. Impala (Fig. 11-20a)

Figure 11-21 shows proﬁles of several elements with respect to the location of the

Impala kimberlite. The left column shows the north–south traverse, and on the right

are proﬁles along the west–east traverse, which was limited to the west by the pres-

ence of a lake.

These proﬁles for selected elements show that relative enrichment with respect to

the location of the kimberlite occurred either over the kimberlite or at its margins.

The boggy area in the east has given rise to increased uptake of Mn and Co with

metres

0 100 200

Lake

Kimberlite

Lake

a) Impala – Sample Locations

0 100 200

metres

Trail

Lake

Geophysical target

(kimberlite)

b) Bi

horn – Sample Locations

Fig. 11-20. Sample locations with respect to surface features and the Impala and Bighorn

kimberlites – Ekati Trend, NWT, Canada.

382 Case Histories

North to South Traverse West to East Traverse

Pb ppm

0.02

0.04

0.06

0.08

0.1

0 100 200 300 400 500 600 700 800

Kimberlite

0 100 200 300 400 500 600 700 800

Kimberlite

0 100 200 300 400 500 600 700 800

Kimberlite

0 100 200 300 400 500 600 700 800

Kimberlite

0 100 200 300 400 500 600 700 800

Kimberlite

Pb ppm

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Bog

Au ppb

Bog

Y ppm

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

Ce ppm

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Bog

Li ppm

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Li ppm

0.5

1.5

2.5

3.5

Bog

Mn ppm

200

400

600

800

1000

1200

1400

1600

Mn ppm

200

400

600

800

1000

1200

1400

1600

1800

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

Bog

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

Fig. 11-21. Proﬁles of element concentrations over the Impala kimberlite. Left column shows

the north–south traverse; right column the west–east traverse. Concentrations in dry leaves.

383Biogeochemistry in Mineral Exploration

coincident Au enrichment. In summary, the concentrations of elements with respect

to the kimberlite were as follows:



Strongest signatures – Pb, Au, REE, Li, Mn and Co.



Weaker response – Rb, Hg, Ba, Sn and S.



Slight response – Ca, Cs, Cu and Sb. There was possibly a weak response, too, in

Ta, but concentrations were close to the detection limit.

B. Bighorn (Fig. 11-20b)

Proﬁle plots of the data from two traverses over the Bighorn kimberlite show a

similar suite of elements to those at the Impala kimberlite that are enriched over or at

the margins of the kimberlite (Fig. 11-22).

Similarly to the Impala kimberlite, the two traverses over the Bighorn kimberlite

showed relative enrichment either over the kimberlite or at its margins. In summary

the concentrations of elements were as follows:



Strongest signatures –Au, Co, Fe, Mn, Mo, Pb, REE and Zn.



Weaker response – Ba, Hg, Li, S, Sn and Sr.



Slight response – Cs, Rb and Ta (Ta concentrations were close to detection).

Table 11-V compares the multi-element signatures at the two kimberlites.

South Africa: Kimberley

Geology

Volcanic and sedimentary rocks of the Precambrian Karoo Supergroup. The

Perdevlei diatreme intrudes through Palaeoproterozoic dolomites and is covered

by red soil and a calcrete cap varying in depth from 2 to 7 m. It is an intensely

carbonated, partly clay-degraded and oxidized volcaniclastic diatreme that has been

TABLE 11-V

Comparison of element signatures along traverses over the Impala and Bighorn kimberlites.

Elements in parentheses indicate weak signatures

Location Over and peripheral to

kimberlite (dominant)

Over and peripheral to

kimberlite (lesser)

Over and peripheral

kimberlite (minor)

Impala Au, Co, Mn, Li, Pb,

REE and Zn

Ba, Cs, Hg, Rb, S, Sn

and Sr

Ca, Cu, P, Sb and

Ta (Cd)

Bighorn Au, Co, Fe, Mn, Mo,

Pb, REE and Zn

Ba, Hg, Li, S, Sn and Sr Ca, Cs, (Cu), (P),

Rb, Sb and Ta

384 Case Histories

Pb ppm

0.05

0.1

0.15

0.2

0.25

0 100 200 300 400 500 600

Kimberlite

Pb ppm

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 100 200 300 400 500 600

Kimberlite

Au ppb

Fe %

0.002

0.004

0.006

0.008

0.01

0.012

0.014

Y ppm

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Ce ppm

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Mo ppm

0.05

0.1

0.15

0.2

0.25

Li ppm

0.2

0.4

0.6

0.8

Mn ppm

200

400

600

800

1000

1200

1400

Mn ppm

200

400

600

800

1000

1200

1400

Co ppm

0.2

0.4

0.6

0.8

1.2

1.4

1.6

Co ppm

0.5

1.5

Zn ppm

100

150

200

250

300

350

400

Zn ppm

100

150

200

250

300

350

400

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

0 100 200 300 400 500 600

Kimberlite

Fig. 11-22. Proﬁles of element concentrations in dry birch leaves from sites over the Bighorn

kimberlite. Left column shows the north–south traverse; right column the west–east traverse.

385Biogeochemistry in Mineral Exploration

mined and no longer in operation. The Welgevonden kimberlite has a calcareous

cover. The Kouewater kimberlite has oxidized iron-formation cover.

Environment

Warm, temperate, arid scrubland.

Biogeochemical surveys

Combined samples of twigs and leaves were collected from the following:



Perdevlei: ‘Vaalbos’ or camphor bush (Tarchonanthus camphoratus)from16sample

stations.



Kouewater: Black Thorn Acacia (Acacia mellifera) from 23 sample stations. Black

Thorn Acacia is also one of the most common plant species in the Kalahari.



Welgevonden: a shrub identiﬁed as ‘Hak en Steek’ (27 samples of an unknown

genus, possibly a member of the Asparagus family).

The locations of the samples selected for the orientation surveys are shown in

Fig. 11-23.

At Perdevlei, the stems and foliage of the vaalbos samples were combined and

milled to a ﬁne powder. At the other localities the foliage was separated from the

tough and ﬁbrous twigs and only the foliage was analyzed. A 1 g portion of each

milled sample was digested in nitric acid then aqua regia and the solution analysed by

ICP-MS for 66 elements.

Perdevlei (Calcrete)

No samples could be taken from over the kimberlite because it had been mined-

out. With respect to all samples colle cted along this line, relative enrichments of

elements occurred mostly at the northern rim of the mined-out kimberlite. This suite

of elements included Ca, Li, Na, Nb, Ni, P, Rb, S, Se, Sn and Ta. At the southern rim

of the open pit the elements with highest concentrations were Au, B and Mo.

Kouewater (Fe-Formation cover)

Element enrichments occurred at the margins of the kimberlite – especially REE

and Cs (Fig. 11-24). All REE exhibited the same patterns with the exception of Ce

that was more enriched than the other REE over the kimberlite, although it, too, had

higher concentrations at the kimberlite margins. The implication is that there was

some differentiation of Ce. Also enriched at the kimberlite margins were Co, Fe, Nb,

Ni and to a lesser degree Ca, B, Mn, Cu, Te, Th and Zn.

Welgevonden (Calcareous sediment cover)

Elements that exhibited the highest concentrations over the kimberlite in relation

to the surrounding terrain were Hg, Li, Mn, Mo, P, Rb, Sn and Ta. Ther e were

386

Case Histories

slightly depleted levels of Cs, Mg, Se and Sr over the margins of the kimberlite. Lead

and Nb were enriched at the northwest margin, whereas Ni, Zn and REE were

enriched to the southeast.

Expanded surveys – summary observations

Samples for the orientation survey comprised one third of the complete collection.

The purpose was to examine the element distribution patterns along one traverse

over each of the three kimberlites. The remaining samples were kept in reserve

pending the outcome of the initial results. This is a sound approach in that there is

Perdevlei

Sample Sites

0 100 200

Location of mined-out

kimberlite

metres

a) Vaalbos sites - calcrete

0 100 200

metres

Approximate subcrop

of kimberlite

Kouewater Vegetation Sample Sites

b) Black Thorn Acacia sites – Fe-formation cover

Welgevonden Vegetation Sample Sites

0 100 200

Approximate subcrop

of kimberlite

metres

c) ‘Hak en Steek’ sites – calcareous sediment cover

Fig. 11-23. Sample sites over three kimberlites in South Africa (Kimberley area). Orientation

survey locations.

387Biogeochemistry in Mineral Exploration

considerable expense involved in launching a sample collection programme, whereas

once in the ﬁeld there is relatively small additional cost in obtaining more samples.

Furthermore, since seasonal changes in plant chemistry are likely to occur it is

preferable to collect samples within a short time span (two to three weeks), obviating

any doubts with regard to anomalies being related to collections made later in the

season.

In light of the encouraging orientation survey results, the remainder of the com-

plete suite of 277 samples was prepared for analysis and analysed in the same manner

by the same laboratory.

In summary, the over all results indicated that approaching and over the

kimberlites there were consistent enrichments of Sr, Ca, Ni, REE and locally Cu, Hg,

Mo, Nb, Re, Se, Te and Zn. Elements that were notably enriched at the margins of

the kimberlites were Cs, Li, Pb, REE, Sn, Ta and Zr.

Thanks are extended to Dr. W. B. Coker and BHP Billiton for permission to

release data from the Ekati area and South Africa.

Canada: Buffalo Head Hills, North-Central Alberta

Geology

During the late Cretaceous, kimberlites were intruded throug h Phanerozoic strata

with contemporaneous marine clastic sediment deposition. Boyer et al. (2003c )

reported the following:

The Cretaceous (85 Ma) Buffalo Hills kimberlite province presently consists of thirty-

six bodies distributed over 250 square kilometres in northern Alberta, Canada. Of

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 200 400 600 800

Cs ppm

Kimberlite

0.5

1.5

2.5

0 200 400 600 800

Nd ppm

Kimberlite

NW S

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 200 600 800

Eu ppm

Kimberlite

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 200 600 800

Yb ppm

Kimberlite

NW S

400 400

Fig. 11-24. Kouewater – Cs and REE (represented by light [Nd], medium [Eu] and heavy

[Yb] REE) enrichments in relation to kimberlite beneath Fe-formation cover.

388 Case Histories

the thirty–six kimberlites identiﬁed to date all but one are classiﬁed as crater facies at

the level of outcrop or to the depth of the exploration drilling y (they) are predom-

inantly ﬁne-bedded olivine crystal tuff with coarser horizons rich in juvenile magma-

clastic volcaniclastic kimberlite. Xenocrysts include olivine, chromian pyrope garnet,

eclogitic garnet, spinel, enstatite, and chromian diopside. Diamond was recovered

from 24 of the kimberlites. The mesostasis is usually serpentine, carbonate minerals and/

or chlorite.

Exploration history

Kimberlites were ﬁrst identiﬁed during geophysical exploration for oil and gas in

the late 1990s. In 1997, Ashton Mining of Canada Inc., in conjunction with Alberta

Energy Company and Pure Gold Resources Ltd., discovered kimber lites on the

0.5

1.5

2.5

3.5

4.5

5.5

6.5

Au ppb

Max. 7.8

GOLD

in ash of

White Spruce

Top Stems

metres

Kimberlite

0 4000

K14

TQ155

BH225

K252

1.5

2.5

3.5

4.5

5.5

P %

Max. 6.49

PHOSPHORUS

in ash of

White Spruce

Top Stems

metres

Kimberlite

0 4000

K14

TQ155

BH225

K252

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

0.24

0.26

0.28

0.3

Se ppm

Max. 0.44

SELENIUM

in ash of

White Spruce

Top Stems

metres

Kimberlite

0 4000

K14

TQ155

BH225

K252

0.008

0.01

0.012

0.014

0.016

0.018

0.02

0.022

0.024

0.026

0.028

0.03

0.032

Te ppm

Max. 0.094

TELLURIUM

in ash of

White Spruce

Top Stems

metres

Kimberlite

0 4000

K14

TQ155

BH225

K252

Fig. 11-25. Elements in top twigs from white spruce – Buffalo Head Hills kimberlite cluster of

north-central Alberta.

389Biogeochemistry in Mineral Exploration

southeastern ﬂanks of the Buffalo Head Hills. Within the survey area the only out-

cropping kimberlite is K5 (Fig. 11-25). Post-emplacement sedimentary cover up to

130 m thick can be found over the remainder of the bodies. Despite the cover, some

of the kimberlites form subtle topographic highs as a consequence of preferential

erosion of the softer country rock. The kimberlites range in size from less than one

hectare up to 47 Ha, with circular to irregular shapes based largely on the outlines of

their magnetic anomalies (Boye r et al., 2003a,b,c).

Environment

Boreal forest. Mostly white spruce (Picea glauca); trembling aspen (Populus

tremuloides); and in the wetter areas black spruce (Picea mariana), willow (Salix

spp.) and tamarack (Larix laricina). The understory is dominated by alder and

Labrador tea.

Biogeochemical survey

Airborne tree top reconnaissance survey over the kimberlite swarm comprising the

Buffalo Hills Kimberlite Province, with semi-detailed sampling focusing on several

known kimberlites.

Two areas were surveyed in the autumn of 2000. This account deals with the

larger of these areas, where white spruce is the dominant species and represents

the only treetop sample medium discussed. The geochemical responses of other

vegetation species and tissues, and other sample media are discussed in Seneshen et al.

(2005).

Figure 5-3 (Chapter 5) shows the sample sites and the locat ions of known

kimberlites of which K5 is the only occurrence that outcrops in the area. The others

have a till cover ranging from 7 to 35 m in thickness. The original survey plan was to

use a helic opter to collect white spruce samples at 1 km intervals on an offset grid in

accord with the pattern shown in Fig. 5-3. However, budgetary constraints neces-

sitated that the num bers of samples were reduced to a 2 km grid within an area of

176 km

, resulting in a collection of 60 samples for the reconnais sance component of

the area surveyed. In addition, crews were asked to collect an additional ﬁve samples

over each of the six known kimberlites (K4, K5, K6, K7, K14 and TQ155) in order

to establish signatures that might be unique to the kimberlites. Including all ﬁeld

duplicates, a total of 120 treetops were collected.

Samples were air-dried for two weeks, and the needles then removed from the

twigs. The twigs were reduced to ash by controlled ignition at 475 1C for 24 h, the ash

dissolved in aqua regia and the solution analysed by ICP-MS. Results for this survey

are reported as concentrations in ash. The ash yield of the samples average 3.3% and

so data should be divided by 30 to obtain an approximate dry-weight equivalent

concentration.

390

Case Histories

Element concentrations were on average similar to the background concentra-

tions that would be expected for white spruce twigs. There were no unusually high

concentrations of any elements. However, a notable feat ure was that although con-

centrations remained low, trees growing over the locations of the buried kimberlites

yielded slightly elevated levels of several elements, especially Au, P, Se and Te

(Fig. 11-25). For Te, in particular, anomalous concentrations (with respect to the

whole dataset) occurred over all the known kimberlites. The Au values were barely

above the usual background values for Au in plant ash, but probably because of the

very low Au content of the Cretaceous clastic sediments that form the su rrounding

host rocks, there is sufﬁcient contrast for the kimberlites to emerge as slightly

enriched in Au. Presumabl y, the small traces of Se and Te represent a geochemical

association with the Au. Phosphorus enrichment with kimberlites is a common

association. Other elements were locally enriched over one or more kimberlites

(e.g. As, Ba, Cr, Nb, Ni, Pb, Rb, REE, Sr and V).

This biogeochemical survey comprised part of a much larger exploration programme

involving the comparison of many different geochemical sampling media. Of these, the

media that provided the best indications of deeply buried (i.e. >30 m) kimberlite pipes

were white spruce-top twigs, C-horizon, till, peat, and sub-peat sediments. The partial

extraction method of enzyme leach

provided a good signature from the C-horizon

soils (Seneshen et al., 2005).

Kimberlites – summary

The unusual abundance of many trace elements in kimberlites is striking (Dawson,

1980). Kimberlites in general are characterized by high content of (a) elements com-

patible with ultramaﬁc rocks – Mg, Cr, Ni, Co; and (b) ‘incompatible’ elements that

are considerably more concentrated than are usually found in ultramaﬁc rocks –

especially K, P, Zr, Nb, Sr, Ba, Rb, Cs and the light REE.

A biogeochemical study conducted over the crater-facies kimberlite at Sturgeon Lake,

Saskatchewan, pointed to the possible relevance of Cr, Mn, Nb, Ni, Rb and Sr distri-

bution patterns (Dunn, 1993). It was considered that Sr may be derived from the car-

bonates associated with the kimberlite. The Rb probably occurs in phlogopite from which

it is easily solubilized by weakly acidic groundwater and transported until it is absorbed,

through plant roots, and translocated to the aerial parts, providing a halo of enrichment

around and above a kimberlite that may be concealed by sediments.

Several subsequent studies have noted positive Rb and Sr signatures in various

types of vegetation growing over kimberlites, as demonstrated in some of the above

examples, and by McClenaghan and Dunn (1995), Dunn and McClenaghan (1996).

At this time it appears that the rather large list of elements that most commonly

are enriched in vegetation growing either over or marginal to kimberlites is, in

alphabetical order Au, Ba, Cs, Co, Hg, Li, Mn, Mo, Nb, Ni, P, Pb, Rb, REE , Se, Sn,

Sr, Ta, Te and Zn. The ultimate objective of discriminating barren from diamond-

iferous kimberlites remains a cha llenging goal.

391

Biogeochemistry in Mineral Exploration