HOLE EFFECTS IN DIAMOND CORES FROM
PALABORA
J.D.S.VIELER
I.CLARK
Department of Mining Engineering,
University of the
1 ABSTRACT.
Palabora Mining Company has
mined a large, low grade copper open pit in the
Transvaal Province RSA for more than 25 years. The ore-body lies within an alkaline complex
of intrusive pegmatoids. Residual fluids from more than one injection
of carbonatite magmas exploited fractures and veins
in the host rocks to deposit minerals of economic interest.
Initial geostatistical work on diamond cores indicated an
interesting potential application of the "Hole-Effect" model. This is most significant in the horizontal
direction and appears "damped" towards the vertical. The hole-effects have been
highlighted by the application of multiple indicator cut-off grades;
when samples from each of the major host rocks were isolated, hole-effects with
different cycle lengths were apparent on application of different indicators;
these may be signatures from the individual copper mineral deposition
"regimes".
Subsequent comprehensive analysis of historical blastholes has
led to the zonation of the ore-body by anisotropy and
grade. There is confidence that these
models will be applicable at depth because of the remarkable vertical
continuity of the lithology and mineralisation.
2
INTRODUCTION.
Palabora Mine is sited 500 Km north-east
of
3 THE
DEPOSIT AND THE SAMPLING DATA
3.1 Geological Summary.
The Palabora Igneous Complex shown in Figure 2, is the
result of multiple alkaline
intrusions (in order pyroxenite, syenite and ultrabasic pegmatoids) into Archean gneiss
country rock. The age of the
complex is generally assumed to be greater than 2060
million years.
Pyroxenite intruded first in a kidney-shaped stock 6,4 Km N-S by 2,6 Km E-W.
A peripheral corona of feldspathic pyroxenite was formed by
interaction with the gneiss. Numerous syenite plugs were forcibly injected
into the gneiss surrounding the main pyroxenite. This was followed by
an extended period of non-violent and partly metasomatic
activity, forming vertically disposed pegmatoid pipes
at three sites within the pyroxenite and causing fenitization of the gneiss at the external contacts.
At the central site only ("Loolekop" see
Figure 3), a dunite plug with some interstitial carbonatite was emplaced and then altered to foskorite. Banded carbonatite was then intruded at
the centre and in concentric bands, bearing some copper sulphides. Subsequent fracturing of the entire infilling
of the pipe and renewed igneous activity led to the intrusion of a dyke-like
body of transgressive carbonatite
at the intersection of two prominent zones of weakness. A divergent stockwork of transgressive
veins cutting across all the older rocks was also developed. The bulk of the copper mineralisation
accompanied this phase. Intensive post-carbonatite fracturing provided further channels for
residual sulphide-rich fluids to permeate the transgressive carbonatite and, to
a lesser extent, the older rocks.
Shearing, brecciation, plastic flow and recrystallization occurred as the mass cooled. Some sulphides were remobilised
and redeposited as valleriite
at low temperatures.
Much later, the entire complex was
invaded by barren dolerite dykes.
3.2 Lithologies and
Foskorite is composed of olivine, magnetite, apatite and
phlogopite in variable proportions.
Patches of calcite occur in addition to the carbonatite
veining from later intrusives. Apatite and baddeleyite
are abundant and economic but the magnetite is too high in titanium to be
saleable.
Banded carbonatite is a
magnetite-rich (20%) dolomitic sovite
characterised by crude allignment
of the magnetite in rudimentary layers concordant with the general concentric
structure. Early, low-grade copper
mineralisation accompanied this rock with bomite as
the dominant phase. Transgressive
carbonatite is similar without the crude
banding. It is clearly cross-cutting and intrusive to the other rocks. It contains low-Ti magnetite, the bulk of the
copper minerals (chalcopyrite, bomite, chalcocite and cubanite are
dominant) and urano-thorianite.
The Palabora carbonatites are
unusual and geochemically different from others,
especially the high copper content. The sulphide textures are indicative of exsolution
during cooling combined with reaction with late-stage fluids. The magnetite precedes the sulphides and its
formation probably reduced the oxygen fugasity of the
system, triggering sulphide precipitation. The sulphide
deposition is estimated to have started at a
temperature of 600 C, falling to 200 C for the valleriite.
Copper grades average 0.5% in the open pit and minimum
mining units can exceed 1% Cu.
Individual assays of diamond cores or blast-holes can record up to 10%
Cu.
3.3. Sampling and
A total of 137 surface diamond drill holes were drilled between 1956
and 1976 intersecting the ore-body at depths up to 1.2 Km below surface. These holes indicate a remarkable vertical
continuity of lithology, mineralogy and grade as shown in Figure 4. These cores
were logged for structure, lithology and assayed in 5
feet (later 1.5 metres) sections for copper.
Composites were assayed for subsidiary
minerals.
The long term ore reserves were
constructed by traditional methods from the diamond cores. Annual tonnage and grade experience factors are applied for long term planning. The chippings from production blast-holes in
the open pit are sampled and assayed.
The position and grade of every hole (250 000+) were recently captured
on computer and this is now routine. The
blast-hole assays are utilised for grade control and
short-term mine planning. Blast-hole
geological maps are produced of each bench, but carbonatites cannot be distinguished in chippings. A further 36 diamond holes (20 Km) are
currently being drilled from an exploration shaft within the open pit as part
of the underground feasibility studies.
All this information has recently been utilised for the development of experimental geostatistical
models. These models, in conjunction
with a comprehensive geological block model, will be used
for the construction of future ore reserves.
4 GEOSTATISTICAL STUDIES OF DIAMOND CORES.
4.1
Down-the-Hole Semi-Variograms.
The diamond cores were initially
studied by constructing down-the-hole semivariograms. It was apparent from an early stage that the
copper is closely associated with the carbonatite lithologies and that segmentation of the core by logged
rock-type would be necessary. The rocks
are interfingered and mixed throughout the deposit
and therefore only continuous cores of 50m or longer were
selected. This selection yielded
about one hundred relatively uncontaminated lengths of the three main
copper-bearing rocks, foskorite (FK),
banded carbonatite (BC) and transgressive
carbonatite(TC).
A semi-variogram of a length of trangressive
carbonatite (LK016TCI) is shown in Figure 5a.
The Nugget Effect is typically high, 0.6 of the total
variance. The experimental model fitted
is a HoleEffect with a cycle period of 32 metres, with evidence of a second hole at 64 metres. As the holes
are not as deep as the N.E., the model has been decayed to fit the data up to
40 metres.
Figures 5b to 5f are Indicator Semi-Variograms of the
same length of core.
Figure 5b (0.2% Cu) shows a smoothly rising
semi-variogram broken sharply at lags of 7, 20 and 45-50 metres. This type of semi-variogram is common for low
and high indicator values.
Figure 5c (0.6% Cu) shows well-developed holes at 32 and
64 metres (this cycle length corresponds to that of Figure 5a).
Figure 5d (0. 8 % Cu) clearly exhibits a different cycle
period of 15 metres. Figure 5e
(1.4% Cu) shows a generally rising but rather confused
picture.
Figure 5f (2.7% Cu) shows remnants of a smoothly rising
graph but is broken more frequently than in 5b.
An example of banded carbonatite
(LK122BCI) is shown in
Figure 6a. The experimental model fitted
is a Hole-Effect with a cycle period of 15 metres. In this case the
holes are deep and the model has not been decayed. This graph is somewhat confused between lags
of 20 and 40 metres.
Figures 6b to 6e are Indicator Semi-Variograms of the
same length of core.
Figure 6b (0.2% Cu) exhibits a cycle period of 15 metres
whereas Figure 6c (0.8%
Cu) clearly shows a cycle period of 19-20 metres.
Figure 6d (1.4% Cu) clearly exhibits a cycle period of
14 metres. Figure 6e (1.6% Cu) shows
evidence of a smoothly rising graph broken into "segments" of roughly
10 metres.
Similar examples can be found
in the foskorite cores.
All the above cycle distances are along apparent dips
and should be converted to the horizontal using the
dips of the individual holes.
For each of the major rock types, three and sometimes
more indicators have been found which yield cyclic
indicator semi-variograms of different periodic lengths. The interpretation is that these spacings are signatures of the different phases of
mineralisation. The infilling of the
pipe was fractured by intermittent activity of varying intensity. The major joints so-formed
were exploited by mineralised residual fluids,
depositing copper and other minerals in the planes of weakness. Where later joints encountered copper
deposited by a previous phase, remobilisation may
have occurred, giving rise to some of the small-scale textures described by
previous authors. The sharp breaks seen
especially in the low and high grade indicator
semivariograms may be a measure of late stage fracture spacing.
4.2 Global Diamond Core Semi-Variograms.
Nearly all the diamond drilling was aligned
perpendicular to strike. Figure 7 is a
plan view of the drilling pattern over the central high grade
core and Figure 8 is a section viewed from the East.
After classification of the entire suite by rock type,
global semi-variograms were run (Figures 9,10 and 11-the different symbols represent tolerances of 5, 10 and
15 degrees). These gave reasonable
semi-variograms in the North-South (Figure 9) and Vertical (Figure 10)
directions but there was an obvious lack of "infilling" in the EastWest direction (Figure I 1). Holes are evident at 80 & 160m
North-South and the Vertical range is still to be
established but exceeds 200m.
5 GEOSTATISTICAL STUDIES OF BLAST HOLES.
A comprehensive study of the blast-holes from the open
pit has been done.
The data was split into 100m by 100m cells
containing about 3 000 holes each. It
was found that local anisotropies exist which can be related to the
geology. There is a generally concentric
pattern following the banded carbonatites (see Figure
3), sharply broken by very strong radial anisotropies in the transgressive carbonatites. On the basis of
anisotropy and grade it was possible to divide the deposit into geostatistical
provinces.
Semi-variograms from the main E-W trending transgressive carbonatite area are shown in Figure 12.
These semi-variograms do not display the Hole Effects seen previously in
the cores as when large quantities of data are analysed, the Hole Effects
"counteract" each other and are smoothed out
towards Two-Component Sphericals.
6 DEFINITIVE MODELS FOR KRIGING.
The intention is to utilise
both the blast-hole models (corrected for the difference in support) and the
global diamond-core models to construct kriging systems. The blast-hole models will be particularly
useful in deriving the E-W parameters because of the lack of diamond drilling
in this direction (see Figure 11).
The underground reserves will be developed from the
current exploration drilling, and several other deep holes
which intersect the area below the projected bottom of the open pit.
7 CONCLUSIONS.
On a small scale, this copper deposit exhibits some
extraordinary geostatistical behaviour.
Almost every section of diamond core shows a Hole-Effect, sometimes
dominant but often "recessive" within a Spherical- or Linear-type
model.
As with most Hole-Effects, the behaviour disappears when
large quantities of data are analysed simultaneously and Spherical Models have been fitted to the extensive blasthole
records for the purposes of developing kriging systems.
Nevertheless, it is interesting that the rocks contain
evidence of individual phases within a complex mineralisation process.
8 SELECTED REFERENCES.
Aldous, R.T.H. Ore Genesis in Copper
Bearing Carbonatites, Ph.D. Thesis, Imperial College
of Science and Technology, University of London, 1980.
Journel, A.G and Froidevaux, R. Anisotropic Hole-Effect Modelling,
Proceedings of the 17th APCOM Symposium, 1982 pp 572-585.
Leroy, A.J. and Lill, J.W. Case Study of Palabora Mining Company Limited, Surface Mining
Symposium 1989.