I assume that you are mostly interested in the antelope heads, which were probably produced by lost wax casting. Objects made from lost-wax casting have been found that are about 6000 years old, and lost-wax objects associated with the Aegean are found from ~1000BCE. The lost-wax process was widely used across the ancient world and continues to be widely used today.

Basically, the creator would begin by carving the desired object in solid wax. Next, they would create a mould using the solid wax model and plaster or other material. Now the (empty) mould would be used to create a hollow-wax cast by adding successive thin layers of wax to the mould. This hollow cast would be removed and used to form a fireproof mould (often clay-based). This fireproof mould would then be heated so the wax can drain out (be "lost"). Next, melted metal into the mould, and after it cools, remove it. The final casted metal piece will have the shape of the initial wax model used to create the mould. Since wax is a very easy to work material, it's possible to create very fine detail that is carried to the final product, and very easy to create many identical casts.