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

[摘要] Pocket mineralisation is the name applied to podlikebodies of coarse-grained rock which are the resultof alteration of sediments. In some areas pocketsare cassiterite bearing, and these pockets have been minedat Rooiberg Mine.Rooiberg Mine is one of four operating mines in theRooiberg Tin-field which is situated 64 km west ofWarmbaths, Transvaal, South Africa.In general observations, many pockets were observedto be different; therefore research was started todiscover any trends which may be useful to guide a minegeologist as to where tin may be found. Although muchgeneral research has been done on tin no one has attemptedto classify pocket types.All the pockets studied are situated in theBoschoffsberg quartzite member which is part of a sequenceof sediments consisting of feldspathic quartzite, arkosite,shale and volcanic deposits. These deposits constitute thependant which is entirely surrounded by Bushveld granite.In Rooiberg Mine itself a thickness of 150 m offeldspathic quartzite can be observed, and all~the pocketsat Rooiberg occur in this unit.The intrusion of the Nebo granite is believed to havecaused fracturing of the roof rocks (Stear 1976), and thesubsequent settling of the sediments caused much of the normalfaulting and gravity sliding which is seen at Rooiberg; thisintrusion and subsequent tectonic adjustments wereaccompanied by recrystallisation of the sediments. All the pockets are associated with structuralfeatures such as faults, joints, and even stylolites.They can occur in many shapes, but are usually rounded.·The macroscopic characteristics of pockets enable themto be classified, as some are uniformly textured whileothers have different types of halo development andwell-developed cores. There is also a considerablevariation in mineralogy.By means of macroscopic and microscopic characteristicsof pockets, trends can be observed in their relation to thefissure system. Along fissures there is a recognisablesequence in the occurrence of the pocket types with respectto the metallifero-us mineral content.The pockets were also noticed to occur in a much widerzone of sedimentary alteration than was originallyrecognised.All these factors were found to help a mine geologistin determining the location of ore.

[发布机构] University of Pretoria