'iron-rich' is a modern English compound of 'iron' + the adjective-forming element 'rich'. 'iron' originates from Old English 'īsarn'/'īsern', from Proto-Germanic '*īsarną' (possibly borrowed from Proto-Celtic '*isarnon'), where the root meant 'iron' (the metal). 'rich' originates from Old English 'rīċe'/'rice', from Proto-Germanic '*rīkjaz', where the root meant 'powerful' or 'ruling' and later developed senses of wealth and abundance.

'iron' changed from Old English 'īsarn' (also written 'īsern') through Middle English forms to modern English 'iron'. 'rich' changed from Old English 'rīċe' to Middle English 'riche' and then to modern 'rich'. The compound 'iron-rich' is a straightforward modern formation combining these two elements to describe abundance of iron.

The individual elements kept their basic senses ('iron' = the metal; 'rich' = abundant/wealthy), and together they came to mean 'containing a large amount of iron' in modern usage.