Biogeographicalj Geobotanical chemical investigations

Biogeographicalj Geobotanical
and Biogeochemical investigations connected with
exploration for nickel-copper ores in the hot,
wet summerjdry winter
savanna woodland
.
envl ronment
Prof. Monica M. Core *, Ph.Do, B.Sc. (Visitor)
SYNOPSIS
The Empress nickel/copper deposit west of Gatooma in Rhodesia occurs within gabbro and amphibolite,
disposed at the contact of a granodiorite stock (Barebottom Hill) and the surrounding greenstones of the
Archaen Bulawayan System. The orebody has a pipelike form and a poor surface expression due to plantation of the area by Miocene (Africa) and later Victoria Falls erosion cycles. Barebottom Hill represents a
monadnock remnant in the geomorphological surfaces while traces of the once larger Empress orebody have
been removed.
Limitations in soil geochemistry necessitated both studies of the vegetation ecology and the biogeochemistry
of the area, both of which were anomalous. The use of geobotany was found to be restricted but biogeochemistry offers a most effective prospecting tool and was found to define the sub-outcrop of the ore-body more
precisely than soil geochemistry since these methods reflect and differentiate anomalies emanating from
bedrock mineralisation and those due to surface phenomena.
INTRODUCTION
The EmIJress nickel/copper
deposit occurs in level
terrain covered by open savanna woodland some thirty
miles west of Gatooma, Rhodesia (Fig. 1). It was known
by ancient people whose old workings were prospected in
1926-29. Subsequently the ore body was outlined by soil
geochemistry,
magnetic and electro-magnetic
surveys
and by diamond drilling.
The poor surface expression of the ore body coupled
with the problems of distinguishing between comparable
geochemical
anomalies over the ore body and over
unmineraIized
bedrock both in the Empress area and
elsewhere in Rhodesia suggested investigations aimed at
establishing the role of geobotany and biogeochemistry
in the search for nickel/copper
ores in the savanna
woodland environment.
Geobotanical reconnaissance
was undertaken
by the
author in May, 1966, when a new species of Barleria
growing only over the sub-outcrop of the nickel/copper
deposit and over oxidized copper bearing material on
Barebottom
Hill, one mile to the west, was discovered.
Follow-up
geobotanical
and biogeochemical
investigations were carried out under the author's direction
by Misses L. Coupland and J. Cudmore in March/April
1967 i.e. at the end of the rainy season when the grasses
could be readily identified and most trees and shrubs
were bearing fruit. Some small shrub and herb species,
including the Barleria Bp. nov., which flower after the
grasses have died down, were not apparent
at this
season, however, and some indicator species may have
been missed.
A list of the various species of plants mentioned in
the text is given in the Appendix for reference purposes.
*Professor of Geography,
Fig. 1-Regional
Bedford College, London University.
geology
of the area
Nickel mine.
around
Empress
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JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
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MAY1'971
199
GEOLOGY
The Empress
nickel/copper
deposit occurs within
gabbro or amphibolite
host rock of proterozoic
age
disposed at the contact of a granodiorite stock and the
surrounding
greenstones
of the Archaean Bulawayan
system which forms part of the Basement
Complex
(Figs. 1 and 2). A gradational Mixed Zone occurs between
the granodiorite and the gabbro. The stock, the Mixed
Zone and the gabbro together have an oval trumpet-like
form, inclined slightly to the north east, with dips of
about 65°. The ore body, which has a pipe-like form,
occurs at or close to the south western contact of the
gabbro and greenstones.
Andesitic dykes which are
post mineralization
and probably of Jurassic age cut
through all the rock units.
Nickel/copper
mineralization
occurs in both the
gabbro and the Mixed Zone, the ore body being defined
by a combined nickel and copper content of one per
cent and over. This occurs in the central portion of the
gabbro; in places it extends to the greenstone contact
but usually is separated by an intervening
low grade
section. At surface the ore body has a strike length
of 3000 ft and an average width of 70 ft, with a maximum
of 200 ft. It tapers in depth. The mineralization
comprises blebs, masses and minute veinlets of pyrrhotite
~
and chalcopyrite,
with pyrrhotite,
pentlandite,
the
only significant nickel bearing mineral, chalcopyrite,
pyrite and violarite as the most abundant minerals.
The nickel/copper
ore body has very little surface
expression. It does not produce a relief feature or a
conspicuous gossan. Over the level terrain it weathers to
a soft clayey decomposed rock with, in places, a single
layer of rounded boulders of more resistant material
beneath which weathering extends to a depth of 50 ft.
The only evidence of sub-surface mineralization
occurs,
rarely, where the boulders have cavities formed by the
leaching of sulphides on their outer weathered surfaces or
display fresh rock with disseminated
pyrrhotite
and
chalcopyrite in their centres. Occasional minute specks
of malachite are the only indications of copper mineralization in the clayey decomposed rock.
Over Barebottom
HilI, which forms a prominent
feature rising some 120 ft above the level terrain west
of the Empress ore body, the bald appearance caused by
the absence of trees coupled with abundant malachite
staining on the rock outcrops provide obvious evidence
of mineralization.
This, however, is confined to a capping
of oxidized copper bearing material within a zone of
altered gabbro which is underlain in turn by barren
altered lineated rock and greenl"tones. This oxidized
C&ruNtane
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200
MAY 1971
Nickel
JOURNAL
prospect.
of transects.
Geology
OF THE SOUTH
and
AFRICAN
location
INSTITUTE
OF MINING AND METALLURGY
material which was worked by ancient people from
open pits, may in fact represent a remnant of the main
ore body which was folded over, lip like, in a westerly
direction. Today greenstones underlie the surface between Barebottom
Hill and the main ore body and
there is no trace of mineralization
between the two.
Thus today, in the Empress area the paradoxical
situation exists whereby the nickel/copper ore body has
little surface expression while ~ remnant of its former
greater extent, now completely oxidized and valueless,
provides
obvious
evidence
of mineralization.
This
situation is readily explained by study of the physiography and geomorphology of the area.
PHYSIOGRAPHY
AND GEOMORPHOLOGY
Physiography
and geomorphology
influence, in turn,
the degree and nature of bedrock exposure, the extent of
overburden, the depth, properties and characteristics
of
the soils and the vegetation distributions.
Their study is
important
for our understanding
of the geochemistry,
geobotany and biogeochemistry
of the Empress area.
In the immediate vicinity of the Empress nickel/ copper
deposit the terrain is level to very slightly undulating
with an average height of 3400 ft above sealevel.
Barebottom
Hill is the only prominence. To the north
west, however, a pronounced
east facing escarpment
marks the edge of the Mafungabusi
plateau.
This
plateau, some 4 000 ft high and formed of Karoo and
Sijarira sediments and lavas, forms part of the Miocene
African surface whose formation under hotter and wetter
conditions
than those prevailing
today was accompanied
by intense leaching
and laterization.
This
surface formerly extended
over the Empress
area,
where, however, it has been largely destroyed by erosion
along the Umniati river and its tributaries
durinO' the
later and drier Victoria Falls planation cycle. Th~ has
exposed the underlying Basement rocks.
Barebottom
Hill represents
a partially
consumed
remnant
of the old surface and its cap of oxidized
copper/nickel
bearing material is a relic of the period
when the whole thickness of the ore body in that area
was subjected to intense leaching and oxidation. All
trace of the former cover has been removed ,probably
recently, from the Empress ore body which hence has
little surface expression today.
SOILS AND OVERBURDEN
Consequent
on the physiographic
evolution of the
landscape the soils are residual, immature
and with
little or no evidence of horizon development.
Being
derived from rock units with only minor mineralogical
differences, they show only minor differences of texture
and composition. Of red to reddish brown colour, they
tend to be clay loams ovcr diorite and sandy loams over
greenstones; over gabbro they are brown to dark brown
silt loams with, in places, a high percentage of rocks or
boulders. All have a remarkably low pH value of 5.0 to
6.0 for soils derived from basic parent material.
GEOCHEMISTRY
Soil geochemistry
effectively defined the Empress
nickel/copper ore body'! Values of 3 000 ppm nickel and
500 ppm copper outlined the ore body and those of
500 ppm and 100 ppm respectively limited the dispersion
areas for these metals. Maximum values of 8 000 ppm
nickel were obtained in places over the ore body.* The
analytical results, however, posed certain problems for
exploration over a wider area. High nickel values of up to
1 000 ppm in surface soil samples collected over unmineralized
serpentinite
and diorite in the Empress
area and much higher values exceeding 3 500 ppm
found in surface soils over unmineralized serpentinite in
several other localities in Rhodesia,
indicated
the
limitations
of surface soil sampling in the search for
nickel ore bodics. ** Profile soil sampling at Empress,
however, disclosed that whereas nickel values consistently increase with increasing depth from surface
into the weathered bedrock over the ore body, over
barren rocks they increase to the top of the weathered
bedrock and thereafter
decrease. While a promising
exploration
tool, profile sampling is, howevcr, costly.
Plant sampling which in effect examines the weathered
bedrock zone is cheaper and within the savarul'1 woodland environment
might be expected to yield results
complementary
to those obtained
from surface soil
sampling.
BIOGEOGRAPHY/GEOBOTANY AND
BIOGEO-
CHEMISTRY AS EXPLORATION TOOLS
The biogeographical/geobotanical
and biogeochemical
investigation had four main objectives. The first was the
recognition of either an anomalous plant community or
of indicator specics confined to the nickel/copper
ore
body which might assist the discovery of other similar
deposits elsewhere. The second was the establishment
of
levels of nickel and copper uptake by plants growing
over this type of ore body in the savanna woodland
environment.
The third, related to the second, was the
appraisal of biogeochemistry
as a more precise prospecting tool than geochemistry for locating the bedrock
mineralization
in the Empress environment.
The fourth
was a comparison
of the metal contents
of plants
growing over the Empress ore body and over Barebottom
Hill to see whether
biogeochemistry
could
differentiate
between the two comparable geochemical
anomalies which respectivcly
emanate from sulphides
in the ore body and from oxidized copper and nickel
bearing material overlying barren bedrock on Barebottom Hil1.
THE ROLE OF BIOGEOGRAPHY /GEOBOTANY
AS A GUIDE TO GEOLOGY AND BEDROCK
MINERALIZATION
In order to aohieve the objectives outlined above the
vegetation
was investigated
by means of a series of
suitably located belt transects along which geobotanical
data recording, soil and plant sampling were under-
*Values obtained in the 1957 geochemical survey.
**This problem has now been overcome by very recent advances
in interpretation - to the extent that drilling targets can now
be outlined over 'blind' deposits where no vegetation anomaly
exists (F. C. Bohmke, March, 1970.)
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
MAY 1971
201
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202
MAY 1971
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JOURNAL
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soil geochemistry,
AFRICAN
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ore body and adjacent
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
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soil geochemistry,
rock units.
relief and
MAY 1971
203
taken, supplemented
by ground reconnaissance
vegetation mapping over the intervening areas.
and
These studies established
that the characteristic
vegetation of the country in which the Empress nickel/
copper ore body is located is a savanna woodland
dominated by fairly closely spaced relatively tall trees
of the Brachystegia and Isoberlinia genera over the
higher ground of the old Mrican surface and by Colophospermum mopane trees along the valleys representing
the younger geomorphological
surfaces. This gives way,
however, to a very open woodland of small trees mostly
of the Combretum genus over the whole of the area
underlain by the nickel/copper ore body, by the gabbro
host rock and by rocks of the gradational mixed zone.
Within
this area small Dichrostachys
cinerea trees
distinguish the area underlain by the gabbro host rock
while a fairly dense growth of low Dalbergia melanoxylon
shrubs demarcates the ore body (Figs. 3 and 4).
Shrubs of the latter species occur sporadically over the
other bedrock units. In sharp contrast Colophospermum
mopane and Commiphora
mossambicensis
trees and
shrubs are largely confined to the greenstones
and
diorites.
Due to the similarity of the soils arising from the
similar mineralogical composition of the bedrock units
present there are no clear-cut
boundaries
between
vegetation
associations
characteristic
of individual
units such as occur in areas with highly contrasting ro(Jk
types. Nevertheless the perceptible changes in the form
of the vegetation and in the composition of the tree and
shrub layers, noted above, provide some guidance to the
bedrock geology and to the nickel and copper levels in the
soils. This may be illustrated by reference to Figs. 3 and
4 which record the geobotanical and geochemical data
along two of the transects run across the area. The most
notable features are the changes of species composition
which occur where the nickel and copper values in the
- 80 mesh fraction of the soils sampled at 4 to 6 in
depth, exceed 5 000 ppm," the association of Dichrostachys cinerea trees with the gabbro host rock and the
fairly dense growth of Dalbergia melanoxylon shrubs
over the ore body. The extension of the geochemical
anomaly and associated with it low pH values of 5 to
5.5 over the areas underlain by gabbro and mixed zone
rocks explains the absence of Colophospermum mopane
and Commiphora mossambicensis trees from these areas.
It also explains the contrast between the vegetation
over the ore body, gabbro host rock and mixed zone rocks
on the one hand and that of the barren diorite and
greenstones, on the other and likewise the absence of a
clearly defined vegetation anomaly confined to the ore
body. Marked differences in the composition of the grass
and herb layer between the anomaly and background
areas are also apparent.
Most notable are the sparse
cover and paucity of grass species other than Aristida
adscensionis over the ore body and gabbro host rock and
the restriction of Barleria Bp. novo and Celosia trigyna
to the actual ore body. These species are believed to be
.Values
gation.
204
from
the
MAY 1971
1967 geobotanicaljbiogeochemical
JOURNAL
the nickel/copper
indicators in this particular environment.
Thus while a clearly defined vegetation
anomaly
represented by a distinctive plant assemblage indicative
of mineralization
comparable
with those found over
copper ore bodies in other parts of Africa and over
copper and lead/zinc deposits in the Cloncurry and
Bulman areas of Australia3 is not present over the
Empress nickel/copper
ore body, nevertheless,
changes
in the form and composition of the vegetation provide
guides both to bedrock geology and to the nickel and
copper levels in the soil and thereby assist location of
the ore body.
N
t
Fig. 5-Barebottom
Hill. Geology
Transect 3.
and
location
of
By contrast striking changes of vegetation
feature
Barebottom Hill (Figs. 5, 6 and 7) where differences of
aspect, relief and drainage complicate evaluation of the
influence of geochemistry.
As the name suggests, its
summit, which is underlain by oxidized copper/nickel
bearing material within altered serpentinized
gabbro,
is virtually bare of trees and covered by a dense growth
of tall grasses and low shrubs while its slopes, underlain
by altered lineated rocks and greenstones, carry trees of
numerous species, mostly different from those of the
surrounding plateau, with below a dense undergrowth of
N
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ftet
Bearing
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1~1I8'
Fig. 7- Transect 3 showing the relationships
1-+25'
between plant distributions,
Bare Bottom Hill.
soil geochemistry,
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
relief and geology
MAY 1971
over
205
hrubs, herbs and grasses. The thick woodland on the
slopes is partly related to good soil drainage conditions
and partly to greater freedom from cattle grazing
compared with the level plateau. The differences of the
composition between the northern and southern slopes
suggest that aspect and variations
in soil character
associated with differences in the bedrock parent material are important
influences. Reference to Fig 7
suggests that the stunted form of the tree and shrub
species over the hill summit coupled with the cut-out of
the grass species characteristic
of the slopes, notably
Heteropogon contortus and their replacement
by Danthoniopsis viridis, is associated with an overall nickel!
copper geochemical anomaly in the - 80 mesh fraction
of the surface soil. This anomaly in which copper values
exceed 5 000 ppm and nickel values are between 1 000
ppm and 2 000 ppm extends over the slopes of the hill
where it is a surface drainage phenomenon
caused by
contamination
from old copper workings and has little
influence on tree growth. The Danthoniopsis viridis which
dominates and, in this area, is unique to the summit
vegetation is joined by Bulbostylis burchelli and Celosia
trigyna, which also occurred over the sub outcrop of the
Empress ore body, over the malachite stained rubble in
the vicinity of the old copper workings. Thus over
Barebottom Hill there is a marked geobotanical anomaly
which is associated with a surface geochemical anomaly
of similar order to that over the Empress ore body, but
due to the presence of oxidized copper bearing material
in altered gabbro from which drainage contamination
has spread over the slopes from old workings.
Stunted trees and shrubs and a virtually
monospecific grass layer are features common to the vegetation
over the Empress ore body and the summit of Barebottom Hill. Whereas only Dalbergia melanoxylon, as
dominant,
and Combretum hereroense are the only
common shrubs over the Empress
ore body, over
Barebottom
Hill they are accompanied by a variety of
other species including
some characteristic
of the
vegetation of the African land surface. Despite this and
due largely to the density of Danthoniopsis viridis the
vegetation
anomaly
over Barebottom
Hill is more
obvious than that over the Empress ore body. The
differences may be explained by the differing nature of
the geochemical
anomalies.
The particularly
strong
anomaly in the ground layer over Barebottom
Hill
suggests a connection with the oxidized copper bearing
material yielding readily available toxic minerals to
shallow rooting species, whereas the paucity of tree and
shrub species over the Empress ore body suggests a
relationship
with toxic conditions at dcpth emanating
from the Bulphide ore body and affecting particularly the
deeper rooting species.
THE ROLE OF BIOGEOCHEMISTRY IN LOCATING
BEDROCK MINERALIZATION
For the biogeochemical
investigation
Dalbergia melanoxylon, Combretum hereroense and C. ghasalense were
sampled along two transects across the Empress ore
body. Burkea africana and Combretum ghasalense together with two samples of C. hereroense and one each of
206
MAY 1971
JOURNAL
Dalbergia melanoxylon
and Colophospermum
mopane
were sampled along one transect across Barebottom Hill
and Dalbergia melanoxylon,
Colophospermum
mopane
and Brachystegia
boehmii were sampled
along one
transect over the area at the northern margin of the
diorite mass.
The results of the analyses show that both Dalbergia
melanoxylon and Combretum hereroense take up very
large quantities of nickel and quite large amounts of
copper where they grow over the ore body. Here, as may
be seen from Figs. 8 and 9 the nickel and copper contents of the plants define the sub-outcrop
of the ore
body more precisely than those in the surface soils.
""t,,,
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'"
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.
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.
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PPER
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ili"
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I
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~""'
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- .,"""","
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Fig. 8- Transect
1 showing the nickel, copper and chromium contents
of Dalbergia
melanoxylon
and Combretum
hereroense
and those in soils sampled at 4-6 inches at the
same sites over the Empress Nickel/Copper
ore body and
adjacent rock units.
Thus leaf samples containing 500 ppm to 1 350 ppm
nickel and between 20 ppm and 200 ppm copper and
twig samples containing 300 ppm to 900 ppm nickel and
20 ppm to 200 ppm copper define the sub-outcrop of the
ore body. Over the gabbro host rock west of the suboutcrop of the ore body, whereas the geochemical
anomaly continues with nickel and copper levels in the
surface soil is in excess of 5 000 ppm, the nickel and
copper contents in the plant leaves drop to less than
400 ppm and just over 10 ppm respectively and those
in the plant twigs to less than 300 ppm and less than
10 ppm respectively.
East of the sub-outcrop
of the
ore body, nickel and copper values in the leaves and
twigs of both plant species are high over the geochemical
anomaly over the mixed zone rocks. The ore body within
the gabbro host rock dips in this direction while mineral-
OF THE SOUTH
AFRICAN
INSTITUTE
OF MINING AND METALLURGY
Copper
content
oJ
(O.)DDlbetyia
50.
mel4MNyIM
.
10
5
GM IJMAmbrttum
(oIumn no
I
Z
!
4
!
I>
fJlrasaUnse
plant
Ill"
I..... '"
Qlmnt 9fOI'II>
I>f.yw
9"""Ik
tnh JIIIII'g.-h
0Ibri' ~k
Thorns
(4)
(Cl)
ca)
I
50 Copper conunt of Combrtlum hertrwnse
!1
~
10
5
I
~-j'.'~If'-~~
(b)
(/I)
Nw! "",Itn! Dj i;om
Mincrul
~
content
5.000
f::
I
ULLiJ
I
Iwtvtwnse
(b)
oj 5<111
LOCO
500
t:::
IO~
e--:/
./
/"
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~.",
/-'-------.
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5
I'
SE
0
2
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8
9
N.W
Fig. 9-- Transect 2 showing the nickel and copper contents of Dalbergia
melanoxylon,
Combretum
hereroense
and
Combretum ghasalense
and those of soils sampled at 4-6 inches at the same sites over the Empress nickel/copper
ore body and adjacent rock units.
ization occura also in the mixed zone, so that the higher
values in the plant s1mples m::ty be truly reflecting value;;;
at depth. Further
e::tst over the diorite values fall
sharply to around 10 ppm nickel and between 5 ppm
and 10 ppm copp<}r in the le::tves and twiga of both species.
Similar values occur also in s'l,mples of Dalbergia melarwxylon collected ne'u the northern margin of the diorite
mass where nickel levels in the surface soil are around
100 ppm.
Although both geochemical and biogeochemical values
were generally lower along the second transect over the
ore body the nickel and copper contents of the leaves
and twigs of Dalbergia melanoxylon
and Combretum
hereroense again defined the sub-outcrop of the ore body
more accurately than the values in the surface soil. The
Combretum ghasalense sampled over diorite in the background area contained around 10 ppm nickel and between 5 and 10 ppm copper in both leaves and twigs, the
actual values being closely related to those in the surface
soil.
The very high nickel values in the plant samples
collected over the ore body indicate that, in this environment, trees and shrubs take up this metal freely.
The contrast with background
values is very great,
greater than that between the anomalous and background geochemical values. The results indicate that the
JOURNAL
OF THE SOUTH
AFRICAN
INSTITUTE
.., ,I..,.
""'
'"'" ""',"'
'
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Fig. 10- Transect 3 showing the nickel, copper and chromium contents
of Combretum
hereroense,
Combretum
ghasalense,
Colophospermum
mopane,
Dalbergia
melanoxylon and Burkea africana and those of soils sampled
at 4-6 inches at the same sites over Barebottom
Hill.
OF MINING AND METALLURGY
MAY 1971
207
plants generally resist the uptake of copper to about
10 ppm provided values in the soil do not exceed 500
ppm beyond which uptake increases rapidly to levels of
200 ppm, a phenomenon found over ore bodies elsewhere. 4
At levels generally below 50 ppm and 10 ppm respectively the nickel and copper contents of Oombretum
ghasalense, O. hereroense and Burkea africana sampled
over Barebottom
Hill (Fig. 10) were very much lower
than those in Oombretum hereroense and Dalbergia
melanoxylon sampled in the vicinity of the Empress
ore body although the nickel and copper values defining
the geochemical anomaly were comparable
over the
transect lines. While comparison of results of different
species may be questionable the fact that levels in the
Oombretum hereroense sampled
on Barebottom
Hill
were lower than those in O. ghasalense and Burkea
africana suggests that overall biogeochemical values are
genuinely lower over Barebottom
Hill than over the
Empress ore body, reflect marked differences in metals
content at rooting depth and in bedrock and confirm
the Barebottom
Hill geochemical anomaly as a surface
phenomenon.
Thus the results of the biogeochemical investigation
suggest that plant sampling is a most effective exploration technique in the savanna woodland environment, capable not only of detecting the presence of a
nickel/copper
ore body but also of locating it more
precisely than surface geochemistry and of distinguishing
between a geochemical
anomaly
emanating
from a
nickel/copper
body and one produced by near surface
oxidized copper-bearing material.
ACKNOWLEDGEMENT
This investigation was carried out under a grant from
the Natural Environment Research Council for Investigation of Plant Indicators of Mineralization. The
Rio Tinto Zinc Company generously provided travel
and field support facilities in Rhodesia and made
available results obtained from mapping and drilling
operations. The author wishes to record her thanks to
both these organizations. She is particularly grateful to
Dr J. W. N. Sharpe, formerly of the RTZ Company, who
initiated the reconnaissance and actively supported the
project, to Mr D. Forbes for valuable geological assistance
in the field during the subsequent follow-up programme
and to Mr F. C. Bohmke for reading and commenting
on the script.
The plants were identified at the Salisbury and Kew
Herbaria where the assistance of Professor H. Wild
and Mr P. G. Taylor respectively, is gratefully acknowledged.
The diagrams have been drafted and produced by
Misses L. C. Coupland (now Mrs Gadd), E. Smith and
K. A. Keeble at Bedford College, University of London.
REFERENCES
I. SHARPE, J. W. N. 'The Empress nickel/copper
deposit,
Southern Rhodesia' in The Geology of some ore deposits in
Southern Africa. 1964. pp 497-508.
2. WILDING, 1. G. P. 'Investigations
of secondary dispersion of
nickel in Rhodesia and Zambia'. Ph.D. University of London,
1965.
3. COLE, M. M. and PROVAN, D. M. G. and TOOMS, J. S. 'Geobotany biogeochemistry
and geochemistry
in mineral exploration in the Bulman-Waimuna
Springs area, Northern
Territory,
CONCLUSION
An Assessment of the Roles of Geobotany and Biogeo.
chemistry as Prospecting Tools in the Hot Summer Wet
Winter Dry Savanna Woodland Environment
The treeless vegetation over the area bearing oxidized
material on Barebottom Hill contrasts so strongly with
the surrounding
woodland as to attract the attention
of any prospector.
The vegetation
anomaly over the
Empress ore body, however, is less obvious and probably
would be recognized only by the trained geobotanist.
The use of geobotany in the search for similar ore bodies
with equally poor surface expression in this particular
environment is thus pro ba bly restricted. Biogeochemistry
on the other hand offers a most effective prospecting tool.
In this environment the trees and shrubs growing over a
nickel ore body take up nickel freely during the hot
wet summer period of active growth and produce biogeochemical anomalies which may contrast more sharply
with background
values then geochemical anomalies
over the same area. More important,
the nickel and
copper contents
of the plants may define the sub.
outcrop of an ore body more precisely than those in the
surface soils while by virtue of reflecting metal values at
rooting depth they differentiate
between geochemical
anomalies emanating from bedrock mineralization
and
those due to surface phenomena.
208
MAY 1971
Australia'.
Trans.
Sect.
B.
Instit.
Min.
Vol. 77, 1968. pp B81-B104.
4. NwHoLLs, O. W., PROVAN, D. M. G., COLE, M. M.
J. S. 'Geobotany and geochemistry in mineral
in the Dugald River area, Cloncurry district,
Trans. Instit. Min. Metall. Vo!. 74, 1964-5. Part
799.
Metall.
and TOOMS,
exploration
Australia'.
H, pp 695-
APPENDIX
SPECIES
MENTIONED
IN TEXT
ACANTHACEAE
Bar/eria sp. novo
Dicliptera verticil/aris (Forsk) C.Ch. ~Hypoestes
Dychoriste monroii S. Moore
Hypoestes verticil/aris R. Br.
Justicia Ir.irlr.iana T. Anders.
Monechma debile (Forsk.) ~Justicia debilis
verticil/aris
R.Br.
AMARANT
ACEAE
Achyranthes aspera L.
BURSERACEAE
Commiphora mossambicensis
(Oliv.)
Engl.
COMBRET ACEAE
Combretum e/eagnoides Klotzsch.
C. ghasa/ense Engl. et Diels.
C. hereroense Schinz.
C. mossamb/censis
Klotzsch.
C. zeyheri Send.
COMMELlNACEAE
Aneilema hock;; de Wild
COMPOSIT AE
Aspilia mossambicensis (Oliv.) Wild.
Bidens pilosa L.
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
CONVOLVULACEAE
Ipomoea pileata Roxb.
I. sinensis (Desr.) subsp.
CYPERACEAE
Bulbostylis burchelli
belpharosepala.
C. B. Clarke.
GRAMINEAE
Andropogon gayanus Kunlh
Aristida adscensionis L.
Chloris virgata Swartz.
Danthoniopsis
viridis (Rend le) C. E. Hubb.
Digitaria nemoralis Henr.
Ergrostis cilianensis Link ex Lutati.
E. superba Peyr.
Hackelochloa granularis (L.) Kuntze.
Heteropogon contortus (L.) Beauv.
Loudetia flavida (Stapf.) C. E. Hubbard.
Panicum novemnerve Stapf.
Setaria pallide-fusca (Schumach.)
Sorghum versicolor Anderss.
Sporobolus panicoides A. Rich.
Tragus berteronianus Schult.
Urochloa mossambicensis
(Hack) Dandy.
LEGUMINOSAE
Acacia spp.
Brachystegia spiciformis Beuth.
Burkea afrikana Hook.
Cassia quarrel (Ghesq.) Steyaert.
Colophospermum
mopane J. Kirk ex Beuth.
Crotalaria cephalotes Steud ex A. Rich.
Dalbergia melanoxylon Guill. et Perr.
Dichrostachys cinerea (L.) Wight et Arn.
Dolichos daltonii Webb.
Indigofera colutea (Murmif) Merr.
Isoberlinia spp.
Peltophorum africanum
Tephrosia longipes Meisn.
T. purpurea (L.) Pers var pubescens Bak.
LlLIACEAE
Asparagus
sp. of A. aspergillus
LINEAE
Erythroxylum
zambesiacum
Jessop.
N. Robson.
MAL V ACEAE
Sida alba L.
PEDALlACEAE
Ceratotheca sesmoides End!.
POL YGALACEAE
Polygala erioptera
D.C.
RUBIACEAE
Borrera scabra (Schumach
et Thonn).
Disperma crenatum (Lindan) Milne Redh.
Xeromphis obovata (Hochst) Keay =Randia
SAPINDACEAE
Allophyllus rubifolius (Hocht ex A. Rich.) Engl.
SOLANACEAE
Solanum panduriforme E. May.
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
MAY "1971
209