Low-grade tin deposits occur within the apex of the Bobbejaankop
Granite intrusion at the Zaaiplaats Tin Mine. The finegrained
hood-facies of the Bobbejaankop Granite, called the
Lease Granite, seems to be intrusive into granophyric granite
of the Rashoop Granophyre Suite. However no clear evidence
could be found to support the general view that the Bobbejaankop
Granite itself is intrusive into the granophyric granite. Tin
mineralization is confined to the Bobbejaankop and Lease granites
where the following types of mineralization are developed:
low-grade disseminated orebodies in both granites;
highly enriched pipe-like metasomatic orebodies in both granites;
high grade greisenized orebodies in the Lease Granite.
The area was geologically mapped on a 1:1 000 scale. A series
of 5 boreholes was drilled on a straight line across
the intrusion. The borehole core was logged, continuously
sampled and analysed for Sn, W, Cu, Pb, Zn, Mo, As, Sb, Th,
U, Rb, Sr, Y, Ba, Zr, Nb, Ti, P, Mn, Fe, S, C0 2 and F. Selected
samples from the different granites and low-grade ore
in Bobbejaankop Granite were also analysed for major elements.
The analytical results indicate that the orebodies occur within
a definite zone conformable with the outlines of the intrusion
and characterized by anomalously high Sn values ( > 35
p.p.m.). Distribution patterns of major elements show no drastic
variatfons in the tin-rich zone. However, a significant
increase in Na and a decrease in K is present. The Na and K variations together with petrographic evidence indicate that
albitization has occurred. W, As, S, and F are usually enriched
within the tin-rich zone. There are no geochemical
haloes and no mineralogical or chemical zonations associated
with the tin-rich zone. Cassiterite is disseminated through
the tin-rich zone and occurs interstitially. These observations
indicate that the cassiterite of the low-grade tin deposits
has a primary magmatic origin. The orebodies are further
considered as locally enriched portions of the tin-rich
zone.
Geochemically .the Bobbejaankop and Lease granites are almost
identical, although the latter is slightly depleted in F and
slightly enriched in Nb. Their similarity indicates that they
probably crystallized from the same magma. Compared to both
these rocks, the granophyric granite is enriched in Ti, Fe, Mn,
P, Ba, Zn and Zr and depleted in Rb, Y, Nb, Th, U and F, which
indicates that the granophyric granite is less differentiated
than the Bobbejaankop and Lease granites. These sharp changes,
occurring exactly on the contact of the granophyric and Lease
granites, unquestionably indicate that the latter has not originated
through metasomatism of the granophyric granite. The
relatively high Nb, Y, W, Mo, Sn, Pb and Zn contents of the
Bobbejaankop and Lease granites indicate that they should be
classified as A-type granites.
The abundance of incompatible elements, volatiles and metasomatic
minerals together with the occurrence of vugs and miarolitic
cavities, suggest that the Bobbejaankop and Lease
granites are late differentiates that crystallized from a
fluid and volatile-enriched magma that intruded at a high level.
The Lease granite probably represents a quenched product
of fluids and volatiles that was concentrated at the top of
the Bobbejaankop Granite magma. The low-grade orebodies probably
originated through simple crystallization of a cooling
magma which was locally more highly enriched in fluids and volatiles,
due to the concentration of the latter beneath an impermeable
roof of crystallized granite.
Laegraadse tinafsettings kom voor in die boonste gedeelte van
die Bobbejaankopgranietintrusie by die Zaaiplaats Tin Myn.
Die fyne-korrelrige dakfase van die Bobbejaankopgraniet, die
Leasegraniet, blyk intrusief tB wees in die granofiriese graniet
van die Suite Rashoopgranofier. Oaar is geen getuienis
wat daarop dui dat die Bobbejaankopgraniet wel intrusief in
die granofiriese graniet is nie. Tinmineralisasie is beperk tot die
Bobbejaankop- en Lease graniete waar die volgende tipes mineralisasie
voorkom: laegraadse gedissemineerde ertsliggame
in beide graniete; hoogsverrykte metasomatiese, pypagtige
ertsliggame in beide graniete; ho~graadse greisenagtige ertsliggame
in Leasegraniet.
Die gebied is geologies op 'n 1:1 000 skaal gekarteer. 'n
Reeks van 5 boorgate is op 'n reguit lyn dwarsoor die intrusie
geboor. Die boorgatkern is beskryf, ononderbroke bemonster
en geanaliseer vir Sn, W, Cu, Pb, Zn, Mo, As, Sb, Th,
U, Rb, Sr, Y, Ba, Zr, Nb, Ti, P, Fe, Mn, S,
selekteerde monsters van Bobbejaankopgraniet
vir die vol'le reeks hoofe lemente.
CO2 en F. Geis
geanaliseer
Heelrotsanalises dui daarop dat die laegraadse ertsliggame voorkom in
'n bepaalde sone parallel aan die buitelyne van die intrusie en gekarakteriseer deur anomaal hoe Sn waardes ( > 35 d.p.m.) •
Verspreidingspatrone van hoofelemente toon geen drastiese
variasies in die tinryke sone nie, alhoewel 'n kleiner maar
tog betekenisvolle toename in Na en 'n afname in K voorkom.
Die Na en K variasies, tesame met petrografiese getuienis, dui daarop dat albitisasie voorkom. W, As, S en F is gewoonlik
verryk in die tinryke sone. Geen geochemiese halos en
geen mineralogiese of chemiese sonering word in assosiasie
met die tinryke sone aangetref nie. Kassiteriet kom gedissemineerd
en interstisieel voor in die tinryke sone. Hierdie
waarnemings dui daarop dat die laegraadse Sn-afsettings 'n
primer-magmatiese oorsprong het. Hulle word verder beskou as
lokaal verrykte gedeeltes van die tinryke sone.
Geochemies is die Bobbejaankop- en Lease graniete feitlik identies,
alhoewel laasgenoemde effens verarm is in F en effens
verryk is in Nb. Dit dui daarop dat die Bobbejaankopen
Lease graniete waarskynlik vanuit dieselfde magma gevorm
het. In vergelyking met bei9e hierdie graniete, is die granofiriese
graniet merkbaar veiryk in Ti, Fe, Mn, P, Ba, Zn
en Zr en verarm in Rb, Y, Nb, Th, U en F, wat daarop dui dat
die granofiriese graniet minder gedifferensieerd as die Bobbejaankop-
en Lease graniete is. Hierdie skerp veranderinge
kom presies op die kontak van die Leasegraniet met die granofiriese
graniet voor en dui onteenseglik daarop dat die
Leasegraniet nie gevbrm het deur metasomatose van die granofiriese
graniet nie. Die relatief hoe Nb, Y, W, Mo, Sn, Pb
en Zn inhoud van die Bobbejaankop- en Leasegraniete dui daarop
dat hulle as A-tipe graniete geklassifiseer behoort te
word.
Die hoe konsentrasie van onversoenbare elemente, vlugtige bestanddele
en metasomatiese minerale, asook die voorkoms van
kristal- en miarolitiese holtes, dui daarop dat die Bobbejaankop-
en Leasegraniete laat differensiate voorstel en gekristalliseer
het vanaf magma, lokaal verryk in vlugtige bestanddele.
Hierdie magma het waarskynlik ook vlak ingedring. Die Leasegraniet
verteenwoordig waarskynlik 'n kil-produk van vloeistowwe
en vlugtige bestanddele wat gekonsentreer is in die top
van die Bobbejaankopgranietmagma. Die laegraadse ertsliggame
het waarskynlik ontstaan d.m.v. kristallisasie volgens die gewone
kristallisasie volgorde van 'n afkoelende magma, wat in
hierdie geval lokaal verryk in vloeistowwe en vlugtige bestanddele
a.g.v. die konsentrasie van laasgenoemde onder 'n ondeurlatende
dak van gekristalliseerde graniet.
