Geochemiese en mineralogiese verspreidingspatrone in die wandgesteentes van tindraende ertsliggame in die Zaaiplaatsmyn, Potgietersrusdistrik

Abstract

A mineralogical and geochemical investigation of the country rock around pipe-like ore bodies in the Lease Granite was undertaken to obtain a better understanding of the conditions of formation of the pipes. It was also hoped that the information gathered by the investigation would result in a more systematic approach of the exploration for such bodies. Tin mineralization is confined mainly to the Bobbejaankop Granite and Lease Granite. Three types of mineralization occur: firstly, disseminated ore mineralization in the Lease Granite and especially in the Bobbejaankop Granite, secondly, steeply to moderately inclinded mineralogically zoned pipe-like ore bodies in the upper regions of the Bobbejaankop Granite and in the Lease Granite, and thirdly, funnel-shaped lenticular ore bodies at the upper contact of the Lease Granite. The country rock around two mined out pipe-like ore bodies was investigated mineralogically and geochemically. It was found that certain trace elements, namely, Cu, W, As, Zn and Pb form characteristic halo patterns around ore body A, while the country rock around the ore body was subjected to secondary alteration as could be seen especially in the characteristic colour change as well as the sericitization of the feldspar minerals. It was also found that the shape and size of the haloes and the shape and size of the alteration halo in the country rock overlap partially. From this it was concluded that the two phenomena, namely alteration of the country rock and ore deposition are associated in space but did not necessarily take place at the same time. Halo patterns were also identified around the Lease Pipe but in this instance the haloes are more extensive. The patterns could, however, not always be determined exactly as the boreholes did not penetrate the country rock deep enough. It is, however, clear that certain trace elements such as Cu, W, As, Zn, Pb, Bi and Sb show fluctuations in the vicinity of the pipe, whereas major element distribution patterns do not show such variations. Furthermore, tin shows a significant halo around the Lease Pipe but not around pipe A. Only slight alteration of the country rock around the Lease Pipe was encountered. The fact that both pipes investigated show elemental zoning in the country rock, indicates that there was migration of the ore elements from the ore body to the granitic country rock. The alteration of the country rock around pipe A could probably be due to early preparatory fluids that migrated through specific zones, before the pulse of ore fluids was deposited. Three boreholes in study area 1 provided borehole core material across the contact between the Lease Granite and the Rashoop Granophyre. The pegmatite zone is not developed in this area. Characteristic variations were observed for Rb, sr, U, Th, Nb and Ti over the contact. These variations have been interpreted as a definite discontinuity in the deposition pattern of the abovementioned elements. According to the existing hypothesis on the genesis of the Bush= veld granitoids the Lease Granite is regarded as the late phase fluid of a granitic magma that was subjected to chilling or quenching during crystallization. One would therefore expect a definite discontinuity between the deposition pattern of the elements in the roof rocks (Rashoop Granophyre) which formed earlier. and the deposition pattern of the elements in the late phase aplitic granite (Lease Granite).•n Mineralogiese en geochemiese ondersoek is onderneem op die wandgesteentes rondom pypvormige ertsliggame in die Leasegraniet met die doel om tot •n beter begrip te kom oor die vormingstoestande van die pype. Daar word ook gehoop dat die inligting verkry uit die ondersoek sal lei tot •n meer sistematiese benadering van eksplorasie van sulke liggame. Tinmineralisasie is hoofsaaklik beperk tot die Bobbejaankopgraniet en Leasegraniet. Orie tipes mineralisasies kom voor, te wete eerstens gedissemineerde ertsmineralisasie in die Leasegraniet en veral in die Bobbejaankopgraniet, tweedens steilhellende, mine= ralogies gesoneerde, pypvormige ertsliggame in die boonste dele van die Bobbejaankopgraniet en in die Leasegraniet, en derdens plat lensvormige ertsliggame aan die boonste kontak van die Leasegraniet. Die wandgesteentes om twee uitgemynde pypvormige ertsliggame in die Leasegraniet is mineralogies en geochemies ondersoek. Daar is gevind dat bepaalde spoorelemente naamlik, Cu, W. As, Zn en Pb karakteristieke halopatrone om ertsliggaam A gevorm het, terwyl die wandgesteentes om die ertsliggaam sekondere verandering ondergaan het en uitdrukking vind in veral 'n kenmerkende kleur= verandering sowel as serisitisasie van die veldspaatminerale. Daar is ook gevind dat die vorm en omvang van die halo's en die vorm en omvang van die wandgesteenteverandering gedeeltelik oorvleuel. Die afleiding wat hiervan gemaak word is dat die twee verskynsels, naamlik wandgesteenteverandering en ertsafsetting wel ruimtelik geassosieer is. maar nie noodwendig op dieselfde tydstip plaasgevind het nie. Halopatrone kan ook om die Leasepyp geidentifiseer word alhoewel die halo's in hierdie geval baie meer uitgebreid vertoon. Die patrone kon egter nie altyd duidelik bepaal word nie omdat die boorgate nie die wandgesteentes diep genoeg gepenetreer het nie. Dit is egter duidelik dat bepaalde spoorelemente soos Cu, W, As, Zn, Pb, Bi en Sb wel fluktuasies in die omgewing van die pyp toon, terwyl die hoofelementverspreidingspatrone nie sulke variasies vertoon nie. Sn vertoon verder 'n betekenisvolle halo om die Leasepyp terwyl dit nie 'n halo om pyp A vertoon het nie. Daar is ook slegs geringe wandgesteenteverandering om die Leasepyp waargeneem. Die feit dat beide bestudeerde pype 'n elementsonering in die wandgesteentes vertoon, dui daarop dat migrasie van die erts= elemente vanaf die ertsliggaam na die granitiese wandge= steentes plaasgevind het. Die wandgesteenteverandering om pyp A is ook moontlik toe te skryf aan vroee voorbereidende vloeistowwe wat deur bepaalde sones migreer het, voordat die puls van ertsvloeistowwe afgeset is. Drie boorgate in studiegebied 1 het boorkernmateriaal verskaf oor die kontak tussen die Leasegraniet en die Rashoopgranofier. Die pegmatietsone is nie in hierdie gebied ontwikkel nie. Karakteri= stieke variasies is vir Rb, Sr, U, Th, Nb en Ti oor die kontak waargeneem. Hierdie variasies word as 'n definitiewe diskon= tinuiteit in die afsettingspatroon van bogenoemde elemente . geinterpreteer. In die bestaande hipotese oar die genese van die Bosveldgranitoide word die Leasegraniet beskou as die laatfase vloeistof van •n granitiese magma wat verkilling ondergaan het tydens kristal= lisasie. Hiervolgens sou verwag word dat •n duidelike diskonti= nuiteit sou bestaan tussen die afsettingspatroon van die elemente van die vroeggevormde dakgesteentes (Rashoopgranofier) en die afsettingspatroon van die elemente van die laatfase aplitiese graniet (Leasegraniet).