I like the idea but, the carrier keeps getting stuck . I notice when this happened , rolling your face on the keyboard pans the camera to the right a little ,  and undoes the construction of the buildings.  I somehow ended up getting 5 ruby gems in the process.   

Great game idea , I would love to see more of this. 

Please insert the following code into one of your scripts  please. :)  

#quit game with ESC 

func _input(event):

    if event.is_action_pressed("ui_cancel"):

        get_tree().quit()

OR if you really want to be a godam mad man...

#stop players from quiting AltF4 or X button

func _notification(what):

if what == NOTIFICATION_WM_CLOSE_REQUEST:

get_tree().set_auto_accept_quit(false)

I liked the idea for this game a lot, really cool implementation of the "layered elements" challenge.

The simple art style works really well with the theme, it reminds me of NES games, my only problem with it is the different pixel sizes between the game and the UI, besides that it's amazing. I'm missing a bit of music or sound effects though. It doesn't need anything too crazy in that department.

Gameplay wise I found a few problems, but even then the game was really fun to play. First, the mining upgrades feel kinda useless and even counterproductive, since the bottleneck is always the rock carrier and not the miners. Even spending everything on speed and carry capacity, the rock pile never stopped increasing, which is a problem given the rocks use physics simulation. That's the second problem, if the pile becomes too big, the game begins to slow down a lot. Also the carrier seems to sometimes get stuck but I saw your comment saying you found that after the deadline which is ok.