Haunted Gold Mine (again...)

Sorry if this looks like double posting, since I made a similar thread before, but I won't ask exactly the same question here.

What I do want to know is, what ability makes the haunted gold mine buildable upon gold mines? Is it the Blighted Gold Mine ability or something else? I looked at the data fields for the Melee Haunted Gold Mine and couldn't see anything extraordinary.

So, does anyone know why it can be placed on top of gold mines and nowhere else?