I don't now about the weight, but Alexander with (Ammon) horns is a coin that was minted, although it may not have been by ATG. Alexander himself tended to use Herakles' (Hercules) image on his coins, Zeus on the flip side. Later, he did mint some with his own image. BUT various Hellenistic rulers used Alexander's image on *their* coinage, notably both Lysimakos and Ptolemy, at least with the horns. If you can read Greek, you can probably look on the flip side and see what name is there. If it's a gold coin, it's probably a stater. I honestly don't know of any GOLD coins that used the horn image, but then, I'm not a numismatics specialist, just a Macedoniast. Below is a link to some images of coins. Perhaps you can find the one that looks like yours among them.