Currently only one mine per resource allowed.
Beside the common parameters from Map Object Format there are some additional parameters:
{
/// produced resource
"resource" : "mithril",
/// amount of resources produced each day
"defaultQuantity" : 1,
/// displayed name of mine
"name" : "name text",
/// displayed description of mine (for popup)
"description" : "description text",
/// Image showed on kingdom overview (animation; only frame 0 displayed)
"kingdomOverviewImage" : "image.def"
}