Aluminum oxide does occur naturally in some iron ores. And the x-ray gun apparently doesn't distinguish between oxides, and metals in their pure form.

I sent a link of the pics to a friend of mine who is retired geologist and he said it looks like a slag heap to him or perhaps a lining of an old furnace, but I told him it covered a wide area and he said slag again. So I guess you need to find a local expert to settle this.