Pocket mineralisation is the name applied to podlike
bodies of coarse-grained rock which are the result
of alteration of sediments. In some areas pockets
are cassiterite bearing, and these pockets have been mined
at Rooiberg Mine.
Rooiberg Mine is one of four operating mines in the
Rooiberg Tin-field which is situated 64 km west of
Warmbaths, Transvaal, South Africa.
In general observations, many pockets were observed
to be different; therefore research was started to
discover any trends which may be useful to guide a mine
geologist as to where tin may be found. Although much
general research has been done on tin no one has attempted
to classify pocket types.
All the pockets studied are situated in the
Boschoffsberg quartzite member which is part of a sequence
of sediments consisting of feldspathic quartzite, arkosite,
shale and volcanic deposits. These deposits constitute the
pendant which is entirely surrounded by Bushveld granite.
In Rooiberg Mine itself a thickness of 150 m of
feldspathic quartzite can be observed, and all~the pockets
at Rooiberg occur in this unit.
The intrusion of the Nebo granite is believed to have
caused fracturing of the roof rocks (Stear 1976), and the
subsequent settling of the sediments caused much of the normal
faulting and gravity sliding which is seen at Rooiberg; this
intrusion and subsequent tectonic adjustments were
accompanied by recrystallisation of the sediments. All the pockets are associated with structural
features such as faults, joints, and even stylolites.
They can occur in many shapes, but are usually rounded.·
The macroscopic characteristics of pockets enable them
to be classified, as some are uniformly textured while
others have different types of halo development and
well-developed cores. There is also a considerable
variation in mineralogy.
By means of macroscopic and microscopic characteristics
of pockets, trends can be observed in their relation to the
fissure system. Along fissures there is a recognisable
sequence in the occurrence of the pocket types with respect
to the metallifero-us mineral content.
The pockets were also noticed to occur in a much wider
zone of sedimentary alteration than was originally
recognised.
All these factors were found to help a mine geologist
in determining the location of ore.
Nesvormige mineralisasie verwys na die peulvormige
afsettings van grofkorrelrige minerale wat die gevolg is
van verandering van die omslui tende sedimente. Op plekke
is die neste kassiterietdraend, en sulke neste word by die
Rooibergmyn ontgin.
Rooibergmyn is een van vier werkende myne in die
Rooiberg-tinveld wat 64 km wes van Warmbad, Transvaal,
Suid-Afrika, gelee is.
Algemene waarnemings het getoon dat die neste van mekaar
verskil en daarom is navorsing begin om enige neigings vas
te stel wat nuttig mag wees in die plaaslike opsporing van
tin. Alhowel daar al baie navorsing oor tin gedoen· is,
het niemand nog probeer om die nessoorte te klassifiseer
nie.
Al die neste wat bestudeer is, is in die Boschoffsbergkwartsiet
gelee, wat 'n gedeelte is van 'n opeenvolging van
sedimente wat uit veldspatiese kwartsiet, arkosiet, skalie
en vulkamese afsettings bestaan. Hierdie afsettings
vorm 'n dakhanger wat geheel en al deur Bosveldgraniet
omring word.
In die Rooibergmyn self bereik die veldspatiese
kwartsiet 'n dikte van 150 m en al die neste by Rooiberg
kom in bierdie eenhe1d voor. Die word gereken dat die
)
intrusie van .Nebograniet die breukvorming van die
dakgesteentes veroorsaak het, en die daling van die sedimente
na die indringing van-- die graniet het met baie van die
afskuiwings en swaartekragglyding wat by Rooiberg gesien word, gepaard gegaan. Die magmatiese 1ndringing en die
daaropvolgende tektoniese aanpassing is ook vergesel van
herkristallisering van die sedimente.
Al die neste is geassosieer met strukturele verskynsels
soos verskuiwings, naatgroepe en selfs stiloliete. Hulle
kan in baie vorms voorkom, maar gewoonlik is hulle afgerond.
Die makroskopiese kenmerke van die neste maak dit moontlik
om hblle te geklass1f1seer want party net 'n gelykmat1g
tekstuur terwyl ander verskillende tipes van kransontwikkeling
en goedontwikkelde kerns het. Daar is ook 'n aansienlike
variasie in die mineralogie.
Die makroskopiese en mikroskopiese kenmerke van die
neste openbaar bepaalde neigings wat in hulle verhouding met
die naatsisteem opgemerk word. Langs nate is daar 'n
herkenbare volgorde in die voorkoms van die tipe nes met
betrekking tot die inhoud van die metaalhoudende minerale.
Deur chemiese ontleding is dit ook vasgestel dat die
neste in 'n veel wyer sone van verandering in die
sedimentere gesteentes yoorkom as wat aanvanklik geblyk het.
Dit is vasgestel dat al hierdie faktore vir 'n
myngeoloog in sy allerdaagse werk van hulp is om erts te vind.
