No more details are given other than it's a large disc of silver that's big enough to be noticed from afar, and it's just sitting there on the ground. It's certainly not a flying object. I think what St.Athanasius is describing here is the devil trying to lure St.Anthony into a minor love of material wealth by presenting him with this very large piece of silver. Basically satan tempted him by dropping $1,000,000 from the sky.
Sorry Jitpring, but I think you got carried away after reading "silver disc".
I'll make it up to you though... lol
I remember reading in one of the lives of the saints (and I think it was St.Ignatius Loyola) that after having finished praying at night, this glowing orb of light flew into his hermitage. It didn't display any kind of malicious behavior, but on the contrary, it made him feel full of love, peace, and joy... kind of like putting him into a sort of ecstasy. But he quickly grabbed his staff and started beating it and chasing it from his cave.
I've heard so many people give accounts of orbs like this and then afterwards associating them with extra-terrestrials/aliens. These people don't bother to drive them away, but lose themselves in the ecstasy experienced in it's presence. I heard one guy say that after he recited the Holy Name of Jesus the orb disappeared immediately. Now that tells you everything right there.