FWIW those are the ratios for the oxides themselves but the formations are not necessarily huge piles of pure oxides, if you go a bit lower to the "sources" section the lowest-concentrated formations viable for exploitation are
> Banded iron formations (BIFs) are sedimentary rocks containing more than 15% iron composed predominantly of thinly bedded iron minerals and silica (as quartz).
However that's only for post-industrial societies, at least if you have alternatives, as it requires churning through ridiculous amounts of materials.
When you don't have alternatives the ironsand article (which would be used in places with no good or accessible ore deposits e.g. japan, famously) quotes
> Sand used for mining typically had anywhere from 19% magnetite to as low as 2%.
though much like gold panning the ironsand would be sluice-separated to a concentration of 30-50% before it was further processed.
Most ironsands deposits are not considered financially exploitable to this day though, with the exception of NZ's where the iconic "black sand" beaches of north island are extremely rich in magnetite (up to 40%).
FWIW those are the ratios for the oxides themselves but the formations are not necessarily huge piles of pure oxides, if you go a bit lower to the "sources" section the lowest-concentrated formations viable for exploitation are
> Banded iron formations (BIFs) are sedimentary rocks containing more than 15% iron composed predominantly of thinly bedded iron minerals and silica (as quartz).
However that's only for post-industrial societies, at least if you have alternatives, as it requires churning through ridiculous amounts of materials.
When you don't have alternatives the ironsand article (which would be used in places with no good or accessible ore deposits e.g. japan, famously) quotes
> Sand used for mining typically had anywhere from 19% magnetite to as low as 2%.
though much like gold panning the ironsand would be sluice-separated to a concentration of 30-50% before it was further processed.
Most ironsands deposits are not considered financially exploitable to this day though, with the exception of NZ's where the iconic "black sand" beaches of north island are extremely rich in magnetite (up to 40%).