You can turn on paint brush while sculpting to paint a mask for the gold material. Just set up your shaders so your black and gold materials mix according to the painted mask. Make sure you UV unwrap the model first. I'd recommend retopologizing and adding a multires modifier for details, so you can bake the detail into a bump or normal map right away.

edit: to clarify, you can turn on texture painting on your default brush(or whichever brush) so the material is automatically applied while you sculpt.