Question about silver, gold, mining..and lack of them

[Sardaukar]

I seem to recall there was some shortage of coinage starting from 3rd century...and I remember vaguely that one reason was that many existing mines were exhausted. Mining techniques were not advanced enough to continue production, being one reason for shortage of rare metals.

Was this true ?

Have I been hallucinating with too much beer..or can someone point me to sources ?

Thanks in advance !