Trans. geol. Soc. S. Afr.. 81 (1978), 227-228
"CARBON DIOXIDE AND WATER FROM METAMORPHIC REACTIONS AS AGENTS FOR
SULPHIDE AND SPINEL PRECIPITATION IN MAFIC MAGMAS"
by
S. A. DE WAAL
(Trans. geol. Soc. S. Afr. 80(3), 193-196)
DISCUSSION
by C. F. VERMAAK
I must congratulate Dr. de Waal on a very lucid and wellconceived paper and certainly one which gives food for
thought. I am especially enamoured by the attractive alternative which his model of the V1akfontein nickel pipes offers in contrast to my own. Perhaps he will allow me to
offer some facts which may not exactly fit his theory to
perfection, in order that he may ponder and possibly utilize these to make his theory even more acceptable.
Dr. de Waal envisages the faulting of the area to be the
pathway or conduit for his mineralizing fluids. I agree that
there can be no doubt regarding the control of faulting or
conjugate dilatent fracturing for the nickel mineralization. On the other hand, it should be pointed out that the
bulk of the faulting in the area is related to the emplacement of the Pilanesberg Alkaline Complex, from which
the faults radiate as a result. It follows that if the nickel
mineralization is controlled by faulting which is ca. 500
m.y. post-Bushveld, then some agent other than the Bushveld must be chosen to have produced the metamorphism
to have distilled water and CO2 from the Transvaal sediments. Naturally the Pilanesberg intrusion could have provided the necessary heat (and possibly even the necessary
volatiles) for this process but then one would have expected the incidence of the nickel pipes to be greatest
close to the Pilanesberg, with a lesser apportionment away
from it, but there is no evidence for such a distribution.
Naturally basification (i.e. silica removal from orthopyroxenites to produce olivine-rich rocks) by hot circulating
waters is no new concept and was naturally considered,
but I was also struck by the lack of a source of heat to produce this mechanism which would also explain the spatial
distribution of the nickel pipes in the Bushveld Complex.
As a result I chose the mechanism of overpressure along
the faults which obviously were a major control to the distribution of the nickel mineralization, suggesting that the
overpressure was caused by the "room problem" related
to the intrusion of the Pilanesberg Complex. This is in
keeping with the work of A vias and Sorensen (References,
see Yermaak, 1976. Econ. Geol. Y.71, pp. 284-286) who
suggest that overpressure along faults can produce temperatures far in excess of normal metamorphism. It was
also considered that pressure would enhance the removal
of silica from orthopyroxene to produce olivine, therefore
accommodating the pressure by producing a smaller-volumed mineral and rock in accordance with the Le Chateliers' principle. It would also favour the removal of large
atoms such as K and the combination with the silica would
produce the abundant graphic granite veins in the area although the amount of potash and phosphorus in those
rocks is problematical unless this was contributed by the
water as Dr. de Waal suggests.
After logging the many shafts, pits and boreholes during
the mapping of the area west of the Pilanesberg, the impression gained was that there were two seemingly unrelated events, namely an earlier "basification" or
GEOL 81/2 -
H
harzburgite formation along the faults and the later mineralizing event to produce the nickel pipes. In the final
analysis, however, these temporally separated events are
intimately linked in the overall mineralization process
slllce
(a) nickel, copper, chrome, platinoids, etc. were concentrated by the basification process along faults and
(b) the fault channels were utilized by fluids causing sulphurization of the preconcentrated metals.
This does not appear to conflict with the postulates of
Dr. de Waal except that he would have many of the reactions occur while the Bushveld was still fluid although
the evidence suggests that the faulting and mineralization
were post-Bushveld.
In the light of his postulates and those of McLean
(1969), Haughton and others (1974) and McLean and
others (1976), I wonder if Dr. de Waal would care to
speculate on the cause of the iron-enrichment of olivine
and chromite as one proceeds from the edge to the centre
of a nickel pipe?
c/o Geological Department.
J.CI. Co. Ltd ..
P.O. Box 231.
Johannesburg 2000.
AUTHOR'S REPLY TO DISCUSSION
R efl\' to C. F. Vermaak
I wish to express my appreciation to Mr. Yermaak for
the time he has spent to read my paper and for the effort
he has taken to submit his critique in writing to the Transactions. If this example is followed more often one could
expect that
I. authors of papers will know that at least somebody is
interested in their work and
2. authors will think twice before they publish something
that will land them in the deep end of the pool.
"And with these words of wisdom, allow me to try a breast
stroke to safer depths."
In reading Mr. Yermaak's critique I came to the conclusion that our main point of difference is with regard to the
age of the faulting, which, by all appearances, controls the
pipe mineralization. My interpretation of this issue is that
two major zones of floor instability existed during the time
of emplacement of the pipes and that these zones are outlined by the present-day distribution of the pipes. Reference to Figs. 3 and 4, and especially Fig. 6 of Yermaak
(1976) shows the one linear zone to extend from V1akfontein 207 JP in the south to the bpundary of Turflaagte
163JP and V1akfontein 164JP in the north. This zone of
pipes is parallel to, and roughly coincides with, the Rustenburg fault which at present appears to be a very old
fracture zone that originated prior to the intrusion of the
Bushveld Complex but remained active over a very long
span of geological time. The second zone of pipes is found
228
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
on the farms Bierkraal 134JP and Groenfontein 138JP,
again straddling two major faults parallel to and probably
related in age to the Rustenburg fault. These two zones
mark the general directions of floor instability but the
fracturing in the cumulate pile may still have been conjugate in nature. These fracture zones then connected the
carbonate-rich aureole with the magma chamber and thus
acted as conduits for the H2 0-C02 fluid. At this point I
may also explain that I expect partial melting along these
conduits to have taken place at levels normally deeper
than the actual level of pipe formation. This melting would
happen in partly consolidated cumulates so that by all appearances the phenomenon could be mistaken to be "post
Bushveld" as Mr. Vermaak put it. This phenomenon may
also explain the depth expression of the magnetic linear
lones.
To round off the picture, the Pilanesberg intruded and
in addition to possible thermal effects, also caused physical rejuvenation of existing faults and weak zones (basified
zones) and even new faulting. So, in fact, I advocate the
idea that the Pilanesberg contributed to, but is not solely
responsible for, the structural damage we see today; rather
what we see is the effect of weak zones in the earth's crust
that remained active from pre-Bushveld to post-Karoo
times. I do hope that this explanation clarifies my views on
the fault-control mechanism and I will now proceed to the
phenomenon of enrichment of iron in olivine and chromite as one proceeds from the edge towards the centre of
the pipes.
I think the solution lies in one or all of three possibilities. First, the sulphide is tapped from a more differentiated magma (higher level) and may finally become trapped
in cumulates that precipitated from a less differentiated
magma (lower level). Reaction between the sulphide and
the surrounding cumulate material is therefore expected.
Second, one should take into account that cooling of the
protopipe (molten sulphide body in partly consolidated
cumulate) took place very slowly and that during this process further reactions could be expected. In the Fe-S-O
system (Naldrett, 1969) it was shown that on cooling, excess oxygen dissolved in sulphide melts, finally extracting'
some iron to form magnetite (or wustite). I suggest that
this iron oxide, which expectedly was more abundantly released in the more massive parts of the sulphide pipes, partitioned into the olivine and chromite to cause the iron
enrichment. Third, according to the experimental work of
Holloway and Burnham (1972) the CO2-H 2 0 fluid will tend
to extract an iron-rich partial melt from the mafic rock.
Therefore, all three factors strongly indicate possible enrichment of iron in the oxide and silicate material towards
the centres of the pipes.
"Am I standing on dry land or is the dust I see only an
optical illusion'?"
REFERENCES
Holloway, F. R., and Burnham, C. W. (1972). Melting relations of
basalt with equilibrium water pressure less than total pressure.
J. Petrol., 13, 1-29.
Naldrett, A. 1. (1969). A portion of the system Fe-S-O between
900 and I 080 °C and its application to sulphide ore magmas. J.
Petrol., 10, 171-201.
Vermaak, C. F. (1976). Nickel pipes of Vlakfontein and vicinity,
Western Transvaal. Econ. Ceol., 71, 261-286.
National Institute for Metallurgy,
Private Bag X3015,
Randburg 2125.
Accepted for publication by the Society on 31.8.1978.