Leasing would be the ideal way to go for the landowner. However, the mineral rights purchasers want to secure the rights, in the event the landowner decides to sell the land, so they purchase them instead. Mineral rights are often done by corporations looking to make a profit.
When you purchase real estate, it comes with a "bundle of rights", i.e. mineral rights, riparian rights, the right to farm it, the right to develop it, etc.. When you choose to sell one of the rights in that "bundle", it often reduces the value of the property, or at least narrows the prospects for a potential buyer. I, for one, would never purchase a piece of land without the full bundle of rights. I would never purchase a piece that didn't come with mineral rights, for all kinds of reasons, but many people do.
The Sierra Club is notorious for buying up the development rights of cash strapped land owners, and farmers. The farmers think it's a good deal because they still own the land, and can use it as they always have. Then, when the owner needs to sell the land, it's almost worthless. Naturally the Sierra Club will purchase the land for pennies on the dollar. They now have the full bundle of rights to the land, and can sell it for huge profit. And, they do. It's predatory behavior.
To the original poster: If you merely own the mineral rights, then you may want consider weather it would be in your own best interest to go ahead and sell those rights, at this time. Although it might be the Chinese making the offer. If you own the land, I would never sell the mineral rights. Just my 2 cents...