'fUE OCCURHENCE OF VISInLE GOLD AT RANDFON'l'EIN ES'l'A'l'ES.
121
NOTE ON THE OCCURRENCE OF VISIBLE GOLD AT
HANDFONTEIN ESTATES.
(Read 26th July, 1937.)
By D. Kransdorff, M.Sc., A.M., Ph.D., M.A.I.M.E.
INTRODUCTION.
This paper is intended as a preliminary to a projected paper on
the relation of gold to certain structures at Randfontein Estates.
Some of t,he ma'tter descr~bed here has already been dealt with by
Prof. R. B. Young in his most excellent book, "the Banket," the
recapitulation being jvstified by some additional information obtajned,
a.nd by the emphasis it is intended to place on certain aspects. The
danger of drawing conclusions from unusual specimens is fully re2lized,
but it must be pointed out that in spite of the fact that Randfontein
Estates is a low grade mine, visible gold in the Leader (South) Reef
is a c')mrnon occurrence. In faet, owing to the exceptionally narrow
width :)f this reef, rich values over the channel width have to be
presGnt, to make it a,t all payable. It is. believed that owing to this
comparative abundance of v,isible gold, its relations may be taken
as typifying the behaviour of gold throughout the mine. The specimens
exhibited at the. presentation of this paper have been collected over
a period of many years, la;rgely by Mr. E. R. Roberts, the Chief
Surveyor of Randfontein Estates.
DISTRIBUTION.
Visible gold on the mine luts been found in most of the reefs
worked to date, 'liz.: Hock Tunnel Bastard (North Reef), West (Ma,in)
Reef, Leader (South) Beef, Johnstone (Livingstone?) Beef, East
(Bird?) Reef, Lindum and Horsham (Kimberley Reefs), and Black
Reef. Visible gold has also been reported in a quartz vein close to
the I~eader Ree;f, but this has not been seen by the author:
It might be mentiQned at this juncture that including Black Reef,
some 16 different reefs are being worked to-day at Randfonte,in. Altogether over 200 different reefs and small pebble conglomerates are
known to exist on the property,. Size of pebble, robustness of reef,
quantity or type of pyrites, and presence of carbon bear no ·invariable
relation to the values, but. it may be significant to ,record that all
the reefs worked are characterized by well-developed partings (bedding
faults) generally on the footwal!, but sometimes some distance below
the footwall, or even 011 the hanging wall. Bedding fault or parting
as distinct from lower angle thrust fault is used in the sense of
movement along bedding planes, such as. occurs during folding. This
feature has been stressed in recent descriptions of the origin of the
N. Rhodos,ian Copper deposits. It is, unfortunately, impossible to
always utilize these bedding faults in stoping, for particularly in the
Leader Reef, where the va,]ues are congregated on a footwall parting,
122
'rRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
the ground for a foot or more below t.he reef must be broken. In
contrast, the unpayable reefs are" frozen" on to their walls.
Some generalizations as to the vertical distribution of gold in
the vaxious reefs as revealed by sampling are placed here on record.
Black Ree/.-Practically all oxidised-values almost invariably richer
on the bottom cf the Bee£.
Johnstone Ree/s.-Top Beef-generaHy richest on the bottom. Middle
Beef-values likely to be found all over.
Bottom Beefgenerally richest on the top.
East Ree/s.-Top portion of reefs generally! by far the richest.
lf1ide Reef.-Occurs about 150 ft. stratigraphically above the Leader.
Values follow the "buckshot" pyrites ~and, which wanders
from top to bottom of the reef.
New Reef.-Occurs about 90 ft. stratigraphically above the Leader.
Values may be found anywhere in the ree£.
Leader Reef.-The footwall contact is generally the richest.
vfiest Reef.-'l'hE' best values are sometimes found on both hanging
wall and footwall contacts, but they are likely toO be found
anywhere,.
Lind1l1n Reef.-·Values generally on the footwall, in conjunction with
" buckshot" pyrites, but lat,ter sometimes leaves the reef, and
may enter the closely adjacent Upper Lindun-l Beet, whid·
becomes the payband.
H01sham Reef.-Values likely to be found all over.
This reef is
locally underlain by what is known a.s the Horsham shale, and
before the gold premium when the pay limit was much higher,
it was customary to stop development on this reef as soon
as the shale made its appearance, owing to the poor values
encounter.3d.
To date, all the Horsham stopes with shale
footwall have been low grade in contrast to many stopes with
good values where there has been no shale. It does not follow
however, that lack of shale means good values.
In v,iew of a statement in the literature by E. Homersham 1
that the mine plans at Bandfontein show the Main Beef Leader,
it is perhaps necessary to emphasize the iact that the Bandfcmtein
Leader Beef is definitely not the Main Beef Leader, but almost
certainly the South Beef. A discontinuous reef that may be correlated
with the }\1ain Beef Leader does occur above the" cab "-the local
equivalent of the Black Bar, but it almost jnvariably contains no
guld.
}\fANNER OF OCCURRENCE OF VISIBLE GOLD.
As has already been pointed out, visible gold is common in the
Leader Beef, which is the bottom member of a group of conglomerates
1 HOMERSHAM,
1931.
16.
E.: Trans. Geol. Soc. of S..4.., Annexure, Vol. XXXIV,
THE OCCURRENCE OF VISIBLE GOLD AT RANDFONTEIN ESTATES.
123
occurring some 180 ft. stratigraphically above the West or Main
Reef. \Vhile some of these upper closely associated conglomerates
are payable locally, as a rule it is only the Leader itself that contains
much gold. In fact it is surprising that a rQbust reef generally less
than 2 ft. above the Leader may be almost barren of gold. The
Leader itself probably averages well under 6 inches in width throughout the mine, and in the richest section-tha.t known as the South
Vertical, the average width is of the order of 2 inches. Frequently
pebbles may diRappear from the Leader, the penOlil line contact
remaining~ nevertheless often being highly payable over a stoping
width. A carbon streak associated with visible gold is typical of
this contact. Sometimes even where a distinct conglomerate exists,
the bedding fault may leave t.he bottom of the pebbles, and tra.veJ.'se
the footwall quartzites for some distance, before rejoining the base
Such a diverged part.ing may be marked
of the conglomerate band.
by visible gold, but it is generaJly very difficult for a, sampler to find.
Should the sa,mpler miss the parting an unpayable section may be
recorded, whereas in reality the section is h.ighly payable.
As Inentioned before, the presence of carbon may indicate high
values, but this is not always the case. Aga,jn some of the richest
specimens ever encountered contain little or no pyrites. Clearly both
ca:rbon and visilile gold have been deposited from solution, as is
showl:. by the Cl'y~talline habit of t,he gold, and by the occurrence
of br)th an branches from the main seam or contact. These branches
often ext.ended into cracks into certain pebbles, or in the surrounding
qUill tzjtes. OcC'asicnally the gold may spread to surround certain
of the pebbles. Of particular interest is the occurrence of gold as
numerous diversely oriented and connected seams in a true quartzite.
ThesE'; seams cIea,rJy do not represent cross bedding, but are due to
tr.e brecciation of the quartzit,e. Such bre.cciation is almost certainly
related to the movement that, has occurred along the main parting
01 Ledding fault.
In connection with the distance the gold may
sl'ref1.d from the rn8,in cont'1ct, the author has observed three seams
wi::h visible gold, the outer two being some 30 inches apart.
Cros~ bedding is actually very common both immediately above
and belo'w the Leadpr Reef. It is of importance to record tha,t no
gold has been found by tIle author in 01' adjacent to these planes
r8pl'CSenting cross beddin5' r:rhis is mther surprising in the case ot
cross bedding immediately above the Leader, where: some gold might
be expected, if the placer origin ,is correct.
Eyklence of replacement ~Jf pebbles (and matrix) by gold is often
very ovident. An interest,Ing spec.imen in the Handfontein Geological
1\I.u~'3e.um shows wpll-rounded ellipsoid!:tl pebbJes resting on a seam of
vislble gold, but with the bottom of the pebbles crenulated due to
rt-phl,o':ement by gold. The crenulated oottoms are in contrast to the
smooth outliD"es of the upper portions of the pebbles. Sometimes
124
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
similar ,veIl-rounded. pebbles may have perfectly :flat bases, this probably being due t.o the shearing along an immedia.tely underlying
bedding fault.'
In the pa,ragenetic sequence, the gold arJpears to be later than
thE: :::a,rbon, pellets of which it often completely surrounds. Individual
pellets 0-( carbon have bf:en observed under high magnification to
contain numerous reticulated veins of gold. SOIne rich occurrences
show the carborl as a thick seam up to ± inch thick, and rimmed
above and below by what may be described as thin seams of gold.
A few rich specimens of gold are liberally bespeckled with tiny
carhon pellets, which appear to be isolated from one another.
Occasionally the gold may be found smeuxed on fault planes,
cloBe to where ,t.hese faults mtersect the reef. It is not uncommon
to see fHr more gold on the fault planes than occurs in the reef
itself, this feature suggesting that the deposition of the gold postdabed the form,Htion of the fault. This evidence is strengthened by
the ct'ystalline nature of the gold on the fault planes, and by the
presence of the gold in seattered films. In this (lonnec·tion,
a 'f5pedmen worthy of description is one with a visible gold
sea,ln showing an ~bnorm& 1 development of gold at its intersection
with u, fault plane. The striatjons are wel] preserved by the gold;
anc~ narrow gold ve,ins branching off from tbe main seam into the
overlying pebbles and quartjjtes,paralIel these g'old preserved striations.
This suggests that the striations of the fault were formed at the
same time as the gold-bearing fraetures, and since -these latter ar~
undoubtedly pre-gold i.n age, it is likely that the gold on the fault
plan!?, is pre-faulting in age.
.
\Vhat does a:rpear to be certain .is tha,t the deposition of the
visible gold post-dates the movement alon6 the main parting or bedding fault. Sinre this bedding fault has most probably been formed
during the original rold~ng resnlting. in the Handfontein syncline, it
appears that the deposition of gold from solution occurred subsequent
to this folding, though probably only a relatively short time separa.ted
the two events. Occasionally the visible gold seam shows small drag
folds, which are also believed to be pre-gold deposit jon in age. The
author believes tha.t few- it any of the men at Randfontein would
dispute the ract that the rich gold occurrences are related to somewhat
disturbed ground-generally faults of insignificant throw. For instance,
an occurrence of \\1 est Reer, which shows no visible gold in the reef
itself, shows it nevertheless along a ra,ult plane that has displaced
the reef some small distance.
The Leader Reef or rather the immediate surrounding quartzites
are often characterised by breaking with a frosty surfa,ce due to an
abundance of sericite replacing the quartz grains. In general it may
be stated tha,t the richer the reef, t,he more obvious is t,he frosty
break:ng surface. Other quartzites particularly below the East zone
are al:3o characterised by an abundance of sericite, but the origin
THE OCCURRENCE OF VISIBLE GOLD AT RAND FONTE IN ESTATES.
125
is. c.learly due to the shearing of impure quartzites that may have
orIgmally been felspathic. The general appearance is quite distinct
from that of the Leader sericitization.
Rich Leader reef is also
?haracterised by what may be termed " live looking" pebbles. The
mtended analogy of this and the sericitization to wallrock alteration
is quite obvious.
All the features here described are perhaps compatible with the
present accepted theory of reprecipita,tion of placer gold, but there
is another featl]re present that appears to offer evidence of prime
importance, that is more difficult of explanation by this thelory.
Thisls the relation of gold distribution to faulting. It is a matter
thalJ has already been referred to in this paper, and it is hoped to
amplify this in a following paper.
1'here are faults of a great many ages present, and some of
the later ones undoubtedly have no bearing on the values. However
certain faults may be recognised as having a great influence. These
are predominantly of the low angle thrust Itype, and associated
pattern. Over a period of more than s.ix months, the author has had
oceasion to pass very frequelJtly through a Leader stope on the South
Vertical section of the mine, and invariably has been able to observe
up to about a, 2 foot stretch of visible gold on the portions of reef
uontiguous to a low angle thrust fault that has produced an overlap
of some 20 extra feet of reet. Away from the fault no visible gold
has been seen, except where other related faults occur. The stope
face during the period of observation has been advanced some 100 feet.
This fault is unlikely to be post gold in age, because it as too much
of a coincidence to expect a post gold fault to ha,ve followed the
centre of an exceptlOnally n:trrow patch o£ gold at least 100 feet long.
It appears more reasonable to the author to consider that the fault
had it, definite influence on the gold deposition. The inference is that
either the gold is hydrothermal in origin, or that the original placer
gold has migrated (considerably more than has been admitted by
placerists) to be deposiLed later a.t points where minor faulting has
occurred, a feature typical of numerous hydrothermal deposits. It
may be of intemst in this connection to mention tha,t Whitelaw2. in
his description of the 'l'arkwa-Abosso bankets refers to the relatIOn
between gold values and thrust faulting, though ,he has ignored this
feature in his argument to dispose of t,he hydrothermal theory.
In conclusi(ln, the author wishes to express his thanks to the
management of the Randfontein Estates Gold Mining Co., Ltd., for
permission to publish this paper, and particularly to Mr. E. R. Roberts,
the Chief Surveyor of Randfontein Estates, £or numerous helpful suggestic.ns.
2 'VHITELAW, O. A. L.: "The Geol. and Min. Features of the TarkwaAbosso Gold Field," Gold Coast Geol. Survey, Memor. No.1, pp. 5-31.