I've ended up with the boundary of my quarry such that the corner Land Mark is at point (0,0), i.e. not in the same chunk as the quarry machine.
Will the quarry's chunk remain loaded?
Yes, it will. I did some experimentation, here is what
As you can see, the extra green dot hanging off the edge is the chunk with the quarry machine in, it is loaded on top of the area within the Land Marks.