I think I know where people are getting the 600 tonnes from. Basically if you balance the equations you get:
4CuFeS2 + 9.5O2 + 4SiO2 —> Cu2S + Cu2O + 7SO2 + 4FeSiO3
Cu2S + 2Cu2O —> 6Cu + SO2
These are what you got, but the top one is simplified and it’s the answer you would usually get doing it from experience as we do usually put O2 in fraction in the simplest eq and you’d balance O2 last as it has the most atoms here (so you’d do the CuFeS2, then FeSiO3 as it’s the second simplest, then balance the Si, then the S, then the O). Anyway, I think this would lead some students to believe it’s 4:6 without realising you need to go through the reaction twice to get the 2 molecules of Cu2O that you need so it’s actually 8:6