A sulphide ore like ZnS is first roasted into its oxide prior to reduction by carbon because:

The Gibbs free energy of most slphides are greater than tat for CS2. In fact, CS2 is an endothermic compound. Therefore, the ΔfGo of MxS is not compensated. So reduction of MxS is difficult. Hence it is common practice to roast sulphide ores to corresponding oxides prior to reduction.