I have found lots of information on ancient smelting and forging of metal, but precious little on how these ancient people actually found the metal to work with. Most information points to meteor iron and bog/swamp iron, which yielded very small amounts. How did they (or did they even) prospect for iron veins in the earth, well before the invention of things like metal detectors.
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People don’t use metal detectors today to prospect for iron ore. Iron is actually quite easy to find – it’s almost always in an oxyde or sulphite form, and the soil around deposits will reflect that by coloration. How did they know that red soil leads to iron? Because previously they used bronze, and before that copper and noticed that certain minerals indicate metal presence – copper appears in elemental but also other forms, and needed heating to figure out; once you discovered that certain rocks yield metal, you’d experiment with that other rocks that looks red, and the one that looks black etc etc etc.
Iron Ore is visible on the surface as red streaks due to the rust. Once ancient peoples recognized that these rocks could be smelted or slagged into iron then they kept eyes open for them.
Mining for iron in Europe goes back as far as Roman times. The Romans actively mined for minerals, but mines were relatively shallow due to the difficulties of extracting water from the tunnels.
Much of medieval era iron though was *bog iron* which required wading through swamps to find the iron rich rocks.
It is almost impossible for metalworking civilizations to start with iron. Copper is a far easier metal to start with because it can be accidentally smelted. Put a copper bearing rock in a hot campfire, and it will bleed copper.
In addition to being pretty easy to smelt, copper is just hard enough that it is able to make fairly decent tools on its own. So before the Bronze Age started, there was almost certainly a very small copper age.
The copper age taught people that some rocks give metals when heated, which caused them to start experimenting with hotter fires and trying a bunch of different types of rock. This eventually lead to the production of more metals like tin and eventually iron.
Most likely the reason Native Americans never develop metalworking beyond gold working was because they’re just doesn’t happen to be enough copper deposits near the surface to have got them started.
Iron ore is very easy to find on the surface. This is because of its distinct red colour. Just look for a red rock and you have found your iron ore mine. Alternatively look for red water as this water most likely have flowed through an iron ore vein.
The red color thing others have mentioned is good, but it’s also worth saying that sometimes you just needed to know where to go. Some little living things use iron as an important part of their food, and they live in bogs. They make new iron build up, so if you had a bog, you could go and find some to use every few years. The mineral is even called Bog Iron for this reason.
People like/need to travel around and sometimes they see rocks. Sometimes they are overwhelmed with the compulsion to pick those rocks up, look at them, and even show them to others. Travel by particular methods, like by boat or simply traveling alongside a river or stream will expose the traveler to a sampling of whatever rocks are found in the region.
I fish and i’ve found all kinds of interesting rocks including magnetite iron ore and Bog iron ore in the river. Magnetite is unusually dense and has a unique crystal structure, so it would be appealing even if you didn’t know it was ore. Bog iron is unusual and almost sounds metallic when you clank it against another rock.. I’ve found Coal too.
Eventually we got so wise as to be able to infer, by association, where certain rocks might be located.
For example, Serpentinite is very rich in Nickel and Chromium. Serpentinite also causes massive changes in ecosystems, forming what are called Barrens. Prospectors used the presence of certain types of plants as an indicator, with great success, to infer where those nickel and chromium sources might be found.
The first part of [this video](https://www.youtube.com/watch?v=0H-NCgWZZfw&list=PLu5jI_OEKlo-deJv0tlKmZ7ISGg_YHdCY&index=69) by Primitive Skills shows how to find promising rocks to extract iron from.
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