The development and control of pocket-mineralisation in the Rooiberg quartzites

Abstract

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.