A geochemical and petrographical investigation of the low-grade tin deposits in the Bobbejaankop granite at the Zaaiplaats Tin Mine

Abstract

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.