Roasting is a stage in processing used to make highly refractory ores amenable to cyanidisation. Like BIOX, roasting effectively does in hours what natural weathering does over thousands of years – the oxidisation of sulphides.
As the name suggests, the crushed ore is heated to very high temperatures ( degrees C) and maybe done so under high pressure in an autoclave. The sulphides are converted to oxide products and the gold freed. The ore is then processed using CIP/CIL methods.
Roasting is a high energy and high cost process and so only the most refractory ores with high grades (currently around g/t Au) able to justify the expense are processed this way. Usually the material that is actually roasted in a concentrate rather than straight ore itself. The concentrate will have come from the flotation process.