One hundred sixty-five years and only 1.3 tonnes? Okay...if the numbers you read are correct, then either the author had a bad day when he or she wrote that page, or the Romans really don't know how to mine (who wouldn't doubt this?). That value is little. Here's what I'm thinking. Did the Romans have a primitive version of the IRS? and if so, perhaps they under-reported their earnings from the mines, so as to not pay much in taxes to the SPQR.