That is interesting. Odd that the coal; was not found in metallurgical use as these people had an extensive gold and copper (bronze) metallurgy.
At Tiahuanaco the builders had some kind of portable smelter, as evidenced by the metallic “clamps” which held (and still hold) stones together. The builders of the structures carved out bowtie-shaped slots across block boundaries, then poured in the molten metal, which cooled into that shape.