i had kinda wondered that myself until i was recently watching an episode of mine rescue, with the two mechanics from the hoffman mining show, this guy saves all the black sand and sells it in 5 pound bags, apparently its used in rare metal magnets? i think that's what he said in the episode

I had someone say that rose bushes like it. So I dumped a few pounds in our rose bushes and they really seemed to get a lot of new blooms. Not a scientific level finding..... But it did seem to work very well.

I put some classified, clean black sand into decorative glass bottles (that you can get from Hobby Lobby or Micheal's) and put them on shelves.

I used to get steel filings from a local brake drum shop to spread around my roses every couple years, they love the iron as it slowly disintegrates.
your magnetite tailings work the same way.