Considering the amount of heat and pressure - along with immense amounts of time - it takes for coal to form from fossil vegetative detritus and such, the likelihood of anything made of so soft a material as aluminum remaining intact is just impossible.
However, should a crevasse open, during an earthquake, when the phenomenom of liquifaction occurs, into the crust all the way between the surface and a coal seam, it could be possible for a human artifact to fall into the coal seam, and be compacted at relatively low pressure into that seam once the crevasse collapses.
If the depicted object really is made of aluminum, keep in mind that aluminum smelting is a very recent development in terms of human metalurgical technololgy. The aluminum pyrimdic cap on top of the Washington Monument was breath takingly expensive to make back in the 19th century - more valuable than solid gold, in fact.
So, back to the "gear gets into a coal seam", well, how about a more likely scenario? At a coal mining raw processing plant, in modern era Russia, a bit of equipment breaks and falls into a huge mass of just-ground coal, the gear and w/e sifts down to the bottom of that pile of coal, gets forgotton for a couple of decades, because they always top up the coal pile without bothering to scrape down to the last dregs, and over that time, moisture and pressure and the acidic nature of coal cause the gear bits to be cemented into the coal. Then one day, they finally scrape up that last very bottom layer of coal, and it gets sold, unnoticed within the order, to the Russian citizen who discovers - an ancient alien artifact! wow!