Importance of concentration of gold in gold parting

I read following lines about gold parting:



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> Gold is not attacked by $\ce{H2SO4}$ or $\ce{HNO3}$, but copper and silver dissolve in them, when concentration of gold is less than $30\%$. If however, the gold content in sample is more than $30\%$, then copper and silver are also not attacked by acid of any strength. Hence, before acid treatment, impure sample is melted to add necessary amount of silver to reduce gold content to quartation.
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My question is why aren't silver and copper attacked by acids of *any strength* when concentration of gold exceeds $30\%$?


Also at the end it's written that:



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> Gold is separated and fused with borax and nitre and $100\%$ gold is obtained.
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Why do we fuse with borax and nitre $(\ce{KNO3})$ at the end? I don't see any need to do as gold won't react with either of them.


