"99.96% would imply 0.04% impurity which include all impurities (all, including Sulphur). Hence, by deduction, S content would be less than 0.04%"

in addition, the feldspar and quartz were said to be the majority of the contaminants. although they didn't give the percentages, it sounded like they were the 'vast' majority. so even if we only give them 50%, they would still account for al least 0,02% of the contaminanation leaving the other 0.02% as the maximum that the sulfer contamination could ever possibly be, with a good chance it would be much less than even that.