The polyphase history of gold mineralization seen in the Pietersburg greenstone belt is integrated with the geochemical and tectonic evolution of greenstone belts as a whole. The four distinct regional geological settings of gold mineralization are described

Dewit, M. J.

Jones, M. G.

English

NASA Technical Reports Server

N 86 -25 9 09
GEOLOGICAL EVOLUTION OF THE PIETERSBURG GREENSTONE BELT, SOUTH
AFRICA, AND ASSOCIATED GOLD MINERALIZATION; M.G. Jones, Department of
Geology, Imperial College, London and M.J. de Wit, Lunar and Planetary
Institute, Houston, Texas.
This poster presents current thoughts based on preliminary field work
carried out as part of a Ph.D. project, the aim of which Is to Integrate
the polyphase history of gold mineralization seen in the area with the
geochemical and tectonic evolution of the greenstone belt as a whole.
Gold mineralization is found in four distinct regional geological settings;
1. A first phase of gold mineralization was associated with early low
grade met amor phi sm and metasomatism of a 'greenstone basement' sequence of
serpentinites (metaperidotites). These are generally intrusive into a
series of BIF units, ferruginous shales and cherts. There are also
associated extrusive tholeiitic metabasalts and ocellular-bearlng
komatiitic basalts. The regional hydration which characterizes this early
metamorphism resulted in major chemical alteration of the basement and
large scale fluid movement, with migration of Fe, and Mg ions, SiC>2 and
possibly gold. Early shear zones (possibly represented by a now flat-lying
carbonate-fuchslte-gneiss horizon) may have facilitated this fluid
movement .
2. The basement sequence is unconformably overlain by a 'cover' of
coarse clastic sandstones and conglomerates which contain basement-derived
detritus. The conglomerates are often well sorted and graded and may
represent coarse turbidites. Placer-type pyrite and BIF clasts, both
containing minor gold values, are present In these cover rocks and hence a
second period of gold mineralization (reworking) is envisaged.
3. The older rock sequences and gold mineralization above were all
affected by a regional deformation event and it is the associated
structural traps which contain the most significant gold occurrences seen
in this greenstone belt. A well developed upright cleavage with a
predominantly NE-SW strike and three major composite shear zones (each
containing a number of tectonic breaks) are the main manifestations of this
deformation. Strain analysis in the shear zones has been carried out using
ocelli from the pillowed komatiitic basalts. The measurements indicate
that close to or within the shear zones the finite strain ellipsoid results
from a minimum of 50-70% flattening across the cleavage and 100 - 180%
extension along the main stretching lineation seam.
Antitaxial and composite extension veins have been recognized. The veins
contain fibrous crystals of quartz and calcite which plunge parallel to the
stretching lineation (as defined by stretched conglomerate and breccia
clasts lying in the cleavage plane). The veins are thus syn-kinematic with
this main deformation event. The orientation of the quartz fibres is
parallel to the incremental extension growth direction of the dilational
veins and so the stretching lineation is parallel to the kinematic movemnt
direction (approx. NW-SE when rotated to the horizontal). The veins are
formed by the crack-seal fibrous growth mechanism and semi-quantitative
strain analysis indicates clearly that the incremental strain ellipse (in
the X-Y plane) did not change orientation significantly during the
deformation event.
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Field evidence indicates that the shear zones were thrusts (SE over NW)
with both vertical and lateral components of movement. One of the shear
zones, the Synmansdrift shear zone is marked by an unusual chaotic breccia
which consists of white and brown-red banded chert and B1F clasts,
identical to the BIF from the basement, set in a red ferruginous shale-like
matrix. The clast content and size vary abruptly both across and along
strike and there is no well-defined bedding. The lithology can be traced
for 7km along strike and may be upto 100m in thickness but its upper
(southern) boundary is ill-defined as it grades over tens of metres into
conformably overlying but often highly deformed red shales and sandstones.
Hence the upper contact appears to be sedimentary although this has yet to
be confirmed by lithogeo chemistry. The lower contact is clearly tectonic
and an L-S tectonite fabric is well-developed. As well as small clasts,
the lower half of the breccia also contains extremely large (upto 100m long
x 20m wide) BIF inclusions which themselves are 'clasts' within the
lithology. These larger inclusions can be clearly seen to be tectonically
ground-up by a ' spalding-of f ' process which produces the smaller, often
euhedral, breccia clasts.
As a whole the unit constitutes a tectono-sedimentary melange which is
envisaged to have formed as a sedimentary wedge above a low dipping shear
zone (thrust) during horizontal shortening across the region. Large scale
movement of Fe ions, Si02 + Au occurred.
Gold mineralization is found in quartz + tourmaline veins associated with
various structural traps e.g. fold hinges and minor shear planes Including
ultracataclasites. In the vicinity of these traps pressure solution and
metamorphic segregation features are common which indicate fluid movement
and possible gold mobility from the deformed sediments (and possibly the
basement rocks) into the traps. This fluid migration may have occurred
early with respect to the deformation with the resultant veins being
subsequently slightly deformed and tectonically displaced probably later
but within the same deformational event.
4. A later porphyro bias tic overprint of gold-bearing arsenopyrite is
seen locally within the shear zones as well as porphyroblasts of ephesite
(a lithium-bearing brittle mica) and andalusite. These features seem to
indicate a later period of gold mineralization and 'static' metamorphism
probably related to granitic and pyroxenitic intrusions which provided a
heat source (and possibly fluids) for element mobility and mineralization
within the already deformed volcano-sedimentary pile.


Geological evolution of the Pietersburg greenstonebelt, South Africa and associated gold mineralization

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