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This question is pretty straightforward. I would like to build an earth-like world in which humans don't have access to iron, or at least can't mine any prior to full scale industrialization. They can scavenge very small quantities from meteor impacts to make "magic" steel weapons, but I want to force them to rely on bronze and other materials for all daily use. I'd like to do this with a minimum of other changes to the geology and biology of the world.

My problem is that I don't know any way to prevent humans from mining iron other than making it extraordinarily scarce, and I don't know if this will have spiraling natural consequences.

First, would a planet with so little iron that it couldn't be mined develop differently, geologically speaking? (No nickel of course, for a start. But I'm fine with that.) Second, could I make iron that scarce without radically changing the terrestrial biology, for example by forcing my creatures to use a different less efficient mineral as an oxygenator?

Can you have an earth-like world with humans that don't have access to iron as a material?

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    $\begingroup$ (And before anyone pops in to make it the basis of a comment, I'll add that there was some limited iron mining in Mesoamerica. I also don't know how well the example of one landmass can be scaled up to a planet, geologically speaking. But it might be a place to start.) $\endgroup$ – Era Jan 23 '18 at 4:14
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    $\begingroup$ do you need it to be the entire planet or just a part of it, there were inuit tribes that only got iron from meteorites and many island nations had very little iron to work with. of course if they don't have iron the chances of them having any other metals is even less likely since they are rarer. $\endgroup$ – John Jan 23 '18 at 5:23
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    $\begingroup$ @John I'd strongly prefer the entire planet, or at least the entire planet accessible to humans. Does it follow that all metals must occur together? I thought only certain metals were linked do to natural processes? (i.e. iron and nickel.) $\endgroup$ – Era Jan 23 '18 at 5:36
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    $\begingroup$ @AlexP According to the wiki, primitive blast furnaces started in China before Christ. And until 4 centuries ago, steel was not that much used. Iron is brittle, and it took ages to take over bronze weapons and armors. And it only did it because it was cheaper, not because it was better. A world with more copper and tin could delay the introduction of the iron age - even if known - until after the Renaissance. $\endgroup$ – Rekesoft Jan 23 '18 at 15:02
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    $\begingroup$ Related: How many people do you need to kill to make a sword - TL;DR - it's about 4g of iron per person, so around 350 people per sword. $\endgroup$ – Sobrique Jan 24 '18 at 11:13
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You can still have similar iron content to Earth, without forming any useful deposits. We think that most of the iron deposits we're mining today come from a period with relatively low oxygen followed by a major release of free oxygen. The two main events on Earth correspond to the Oxygen catastrophe (when photosynthesizing organisms first appeared on a major scale) and the Snowball (when almost all photosynthesis stopped for a while, progressively deoxygenating the oceans).

The shorter the period of low-oxygen, the fewer large deposits of iron you'll have. So if photosynthesis developed earlier (plausible) and there was no snowball (plausible), you wouldn't get the massive banded iron formations, but there'd still be roughly the same amount of iron dispersed throughout the crust. This would make it very impractical for mining until you can process large amounts of earth efficiently. (Alternatively, you could have periods with very large weathering activity that would disperse the already formed deposits. This might be interesting if you want to explore the world with later technology - there would still be large deposits of iron, but the only ones surviving would be very hard to get to; under water, deep within mountains etc.)

It wouldn't prevent the formation of all kinds of deposits - only the gigantic banded iron formations, which we rely on heavily nowadays. But in earlier times, humans used iron from all sorts of places - volcanos, bacterial "lumps" etc. These are much trickier to handle, though (volcanos usually produce a lot of magnetite which is much harder to refine, the bacterial stuff is extremely low yield), which might be just enough to push the humans away from working with iron - it's hard enough with good ores. Deposits with lots of impurities like sulfur (including pyrite) are also very hard to process, since early human tech didn't have a good way of getting rid of the extra sulfur (which makes iron brittle and almost useless).

Another approach might be making "bronze deposits" more plentiful. The problem with bronze wasn't that it was a worse material compared to early iron/steel - it was quite a bit better in many ways. The problem was that you needed to get copper from one region, and tin from another - their geological formation tends to keep the deposits far apart. So bronze was associated with long-range trade, and when trade networks failed, so did bronze production - and people were forced to rely on iron, which was plentiful pretty much everywhere. But even on Earth, there are (and used to be more) deposits that are basically "bronze ore" (it might be how humans first developed bronze) - a mix of copper ore and arsenic in the right ratio that pretty much gives you bronze by accident. Make those plentiful on your world, and humans might never go for iron on a large scale. Meteoric iron would still be useful (it makes for great steel), but wouldn't lead humans to general iron processing anymore.

The tricky bit might be getting from this "stasis", though. One of the many things that got industrial revolution started was cheap iron and steel. If your civilizations never got through the early iron age, they might not get to the technology needed for large scale iron mining and processing. This is especially true if you go with the "good iron deposits are scarce and hard to access" route. But if you want to keep your story within the "stasis", you can ignore that.

A sideways approach might be making wood scarce. Before you know how to make coke (an industrial-age tech on Earth), coal is worthless for iron production - see "sulfur + iron = oops" above. As an extra bonus, mining large amounts of coal is tricky, since you need deep mines that are flooded easily. So if there's very little wood for charcoal production, iron would get even more expensive than it was on Earth. This might be the perfect solution for you - it keeps iron ores basically the same as on Earth, without having them reasonably usable. It makes meteoric iron pretty awesome, without having it lead to large-scale iron processing. It keeps all the resources there, ready to be used by a more advanced civilization, without making them useful to the "low-tech" (just like aluminum is extremely plentiful in the crust, but painful to refine). Another bonus is that this wood deficiency can be just a tiny time period on geological scales (so no major effects on life and geology outside of the forests), but cover the whole of your society's existence. It might be the result of a climate change, or some major parasite the woodlands didn't adapt to yet, or even an older civilization that destroyed the resources and itself :P

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    $\begingroup$ I like the idea of pushing bronze instead of nerfing iron, it feels much more natural, knowing the problem of long distance trade. Heck, you just need one good copper and tin deposits to happen to be close together by chance. I wouldn't worry about getting out of stasis, though: Why do we produce metals like aluminum, titanium, wolfram, neodym, and such? Because every single one has properties no other metal on earth has. Any advancing technology will eventually get to the point, where it's just worth it to investigate ways to produce steel for its supreme strength, no matter the difficulties. $\endgroup$ – cmaster Jan 23 '18 at 22:43
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    $\begingroup$ The biggest problem with bronze was that they... kinda ran out. It was easier to make than iron, produced better results with less knowledge. The average distances between copper and tin/arsene sources grew so large that it was the end of the bronze age. The metallurgy of iron and pig iron was well known early, quite probably more well known than today. I don't have the references by hand but I have read in several books that the arab metalworking in the early b.c's could produce good irons and even steels. (since we now have blast furnaces we don't need the old methods of refining.) $\endgroup$ – Stian Yttervik Jan 24 '18 at 8:43
  • $\begingroup$ The point was; they probably knew about iron and steel much earlier than the end of the bronze age, they just preferred bronze. $\endgroup$ – Stian Yttervik Jan 24 '18 at 8:44
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    $\begingroup$ just curious how civilization would ever advance to the point of "efficient large scale earth moving" with out iron. Unless you have some other replacement metal, how are you ever going to develop the tools to move the earth, much less large scale cities with skyscrapers and such. $\endgroup$ – alemus Jan 24 '18 at 15:45
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    $\begingroup$ @alemus Skyscrapers are total red herrings. They're no pre-requisites of advanced technology, and they're rare even today. You don't need iron for moving earth, even at pretty large scales. Again, the main reason iron was used so much is it's ubiquity and relatively low cost. You'd use iron where it was economical, and other materials where not. Iron is versatile, yes, but not irreplacable - make other materials cheaper and/or more abundant and iron is no longer the "king". $\endgroup$ – Luaan Jan 24 '18 at 18:42
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My problem is that I don't know any way to prevent humans from mining iron other than making it extraordinarily scarce, and I don't know if this will have spiraling natural consequences.

Humans require iron in their bodies - think blood. We don't function without it and inadequate supplies are medically dangerous, so in practical terms humans could not live on such a world. A species with some different biochemistry might.

Without iron being relatively common in surface layers humans can't function here and to have it reasonably common in the ground requires that it will appear in large-scale deposits at some point that are at least possible to mine.

So off the bat, no iron or exceptionally rare iron and humans is a non-starter.

You'll need to move to humanoids with a different biochemistry.

First, would a planet with so little iron that it couldn't be mined develop differently, geologically speaking? (No nickel of course, for a start. But I'm fine with that.)

Short answer yes, long answer, probably unlikely.

Yes, most things are possible if they're exceptional cases on the edge of the bell curve.

Unlikely because planets like Earth are made of material from nebular clouds (we think) which cannot avoid being Iron-rich (or more precisely, cannot avoid having expected relative abundances of elements due to the way they would be created). Once iron is in there (and nickel is also going to be), you will get planets with Iron and it's likely to form in the surface as a result of geological activity.

Second, could I make iron that scarce without radically changing the terrestrial biology, forcing my creatures to use a different less efficient mineral as an oxygenator?

Not in my view.

But as I'm also suggesting you can't avoid Iron anyway, there's really no need.

I would like to build an Earth-like world in which humans don't have access to iron, except for what can be scavenged from meteor impacts, or at least not before industrialization. This would make steel a sacred, "magic" metal taken from the sky, and force them to rely on bronze for daily use.

Although Iron, Copper, and Steel have been known for thousands of years, copper and iron alloys were the primary metals in normal use for most of that time.

Steel, although known of, was something like a rare and very expensive metal reserved for only the most important things: military weapons, vital tools. It would be like perhaps Titanium is today.

The problem you have is that humans are curious. If they find any substance they will immediately start trying to break it apart, mix it with other stuff and who knows what. They'll find Steel by accident or design and after that, because they are not great at keeping secrets, this will spread. Steel is going to appear.

In pre-industrial times to make steel (specifically) such a special thing for people you simply need to make it hard to get good iron easily. That could easily mean that the easiest deposits of quality iron were e.g. from a meteor and simply not easily mined.

Or you could simply have the natives incorrectly associate the iron deposit they work with a chance meteor or comet that "led them to it". Nothing like a good random astronomical event to play with the human mind's tendency to find connections when there really are not any.

You could make the discovery of the process of making steel an inspiration from the Gods. Perhaps the process itself was discovered by or controlled by a powerful religious organization - that would allow them to control it, limit it, and of course to label it "sacred". Maybe the dominant religion bans it and it becomes a cult secret. There are many ways to work this.

Even greed: I discover how to make steel. Do I (a) rush out and tell everyone or (b) keep it a family/clan/religious secret and try and make a lot of money and power from this knowledge? And if history tells us anything it's that (b) is the chosen option for humans faced with this "dilemma".

So I would suggest looking at social engineering and psychology to make steel this big deal in your world, rather than making a lot of implausible changes to force Iron to be so rare. Politics, religion, greed - these are much more powerful ways to alter the world than trying to rewrite physics and biology.

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  • $\begingroup$ Hi, thanks for your answer. Re: "That could easily mean that the easiest deposits of quality iron were e.g. from a meteor and simply not easily mined." Does that mean it's plausible to have an generally earth-like world with enough iron to get the iron atoms into living creatures, but still have all the mineable iron deposits be inaccessible to mining? Is there something that could lead to this? $\endgroup$ – Era Jan 23 '18 at 5:22
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    $\begingroup$ StephenG mentioned the bell curve. Just have the larger deposits be in more inaccessible places like steep mountains, bottoms of lakes, etc. Nothing impossible, just improbable. $\endgroup$ – jaxad0127 Jan 23 '18 at 6:40
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    $\begingroup$ You can play with the history of your people too. Have legends of an ancient advance race with magical material (steel), which in reality mined basically all of the good iron over the continent and lost it all for whatever reason. Maybe there is plenty of their steel around but it's an alloy that requires high temperature to manipulate, leaving it inaccessible until industrialization. Like aluminum or titanium, plentiful but hard to purify/manipulate until industrialization. $\endgroup$ – jaxad0127 Jan 23 '18 at 6:41
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    $\begingroup$ @jaxad0127 or that ancient race used all the mineable iron to build a giant spaceship to leave the planet… $\endgroup$ – Holger Jan 23 '18 at 9:58
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    $\begingroup$ Just a thought, but maybe you could consider a history where a large planetary impact stripped the planet of its iron? For instance, the moon has only 3.5% iron compared to earth's 35% iron because the impact that created the moon was largely from the outer layers of the planet as opposed to the iron rich core. I.e. you put your setting on the moon. Not sure of the biology of life developing there, but at least geologically, it's plausible to have a iron poor geology. $\endgroup$ – Sandy Chapman Jan 23 '18 at 13:25
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You could make Iron difficult to mine if humans were not the the first intelligent life on the planet if most of the easily accessible ore was mined out (and perhaps transported to space). Large or easily mined iron will not exist, this will require a very advanced precursor race. However this will make other materials scarce as well. You will not find many fossil fuels or any other metals on such a planet either, so this will probably not work. However...

There was a period in history where steel/iron tools mostly came from meteorites. Making steel is tricky,especially the first step, smelting, but many iron meteorites don't need to be smelted, since they are already low in impurities. They do not need to be smelted first and smelting is the hard part, most of the rest of making iron tools is fairly easy and already understood. Of course this is only a temporary condition and will not last long, technology matches ahead and people will keep trying once they know how useful it is.

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  • $\begingroup$ it would have to be transported off world - its easier to get iron from scrap iron than from iron ore $\endgroup$ – jk. Jan 23 '18 at 13:22
  • $\begingroup$ Assuming it is still scarp and has not rusted away to nothing. $\endgroup$ – John Jan 23 '18 at 16:07
  • $\begingroup$ rust is basically iron that has turned back into iron ore though en.wikipedia.org/wiki/Goethite it doesnt destroy the iron $\endgroup$ – jk. Jan 23 '18 at 19:08
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    $\begingroup$ It does however make it harder to recover, dispersed iron evenly spread through the soils would not be minable. $\endgroup$ – John Jan 23 '18 at 23:44
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or at least can't mine any prior to full scale industrialization. They can scavenge very small quantities from meteor impacts to make "magic" steel weapons

Sure. The easily minable ore just needs to be:

  1. too deep underground for current mining technology to reach, or
  2. bound in minerals with high "thermodynamic barriers", or
  3. in high-quality ores that are in places where people don't live.

Sure, animals need iron for hemoglobin, but not that much. It can be in soils but not in mineable quantities.

I don't know any way to prevent humans from mining iron other than making it extraordinarily scarce

That's perfectly reasonable. Apparently you edited your Q, since whereas "no iron" is infeasible... "hard to get to in usable quantities" is perfectly reasonable.

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  • $\begingroup$ Do note that medieval humans used very sparse "deposits" of iron as well - but only after iron was already in common use. It might be plausible that without a pre-existing iron infrastructure, noöne would even try exploiting those. $\endgroup$ – Luaan Jan 23 '18 at 9:39
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Once again, economics saves the day.

You can have a planet with plenty of iron, but the iron is in dirt instead of in ore. What's the difference between dirt and ore? Ore has enough iron in, and is accessible enough, that you can mine it at a profit. Dirt doesn't.

(This carries the amusing consequence that ore can turn into dirt, and dirt into ore just by the value of the commodity, or the technology used to get it, changing or improving.)

In short, you only mine any mineral if it's worth it. So if your iron was distributed evenly throughout the bedrock then insane amounts of expensive effort would need to be made to process enough rock to get any iron - ie: you have dirt, not ore.

Your iron could be found in high concentrations, but only in incredibly inhospitable environments.

All you need to do is find any way to make the cost required to extract it greater than the value of having it.

You might argue that your people don't use money, don't have an economy - that doesn't matter at all. The time (and resources, and risk) that someone spends trying to process dirt as if it was ore could be spent doing something else. If spending that time doing something else provides more value to the individual (or tribe) than spending time extracting iron from topsoil - they will stop bothering to extract iron from topsoil.

You made one iron sword twice as good as a bronze one? And it took you fifty times as long and 5 guys died achieving it? We'll stick with bronze! (And you're fired.)

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  • $\begingroup$ I like this answer. Humans didn't move from bronze to iron because iron was better, but because iron was far more accessible to most areas of civilization than the combination of copper and tin required to make bronze. Large quantities of both ensure that bronze remains far cheaper than iron to develop. $\endgroup$ – Michael W. Jan 24 '18 at 15:32
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I don't think you can even get to be an Earthlike planet without producing mineable iron ore deposits. Most of the iron ore we mine comes from banded iron formations https://en.wikipedia.org/wiki/Banded_iron_formation which were mostly created as part of the Great Oxygenation Event. Without a GOE, there's no oxygen in the atmosphere, and hence the planet isn't Earthlike.

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    $\begingroup$ But do these iron formations need to be easily accessible to humans? $\endgroup$ – RonJohn Jan 23 '18 at 7:18
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    $\begingroup$ You're putting cart before the horse here. Banded iron formations were caused by GOE, not GOE caused by iron formations. You can have GOE without forming iron deposits. $\endgroup$ – user28434 Jan 23 '18 at 8:55
  • $\begingroup$ @RonJohn: Yes, the deposits are going to be accessible, At least, I can't think of any sort of geological activity that would make all of them inaccessible. $\endgroup$ – jamesqf Jan 23 '18 at 18:57
  • $\begingroup$ @user28434: While I'm not a geochemist, I don't see how you could get a GOE without creating iron formations in the process. You're going to have iron dispersed in the planetary crust, just as a matter of basic astrophysics. That iron is soluble: when the first photosynthetic organisms start making oxygen, it will react with the dissolved iron and be deposited as a band of iron oxides. More iron will dissolve, more oxygen will be produced to react with it, and more bands will be laid down until the dispersed iron has all reacted. Then the oxygen content of the atmosphere can increase. $\endgroup$ – jamesqf Jan 23 '18 at 19:03
  • $\begingroup$ "I can't think of any sort of geological activity that would make all of them inaccessible." Some possibilities: #1 Post-Pre-Cambrian sedimentation covers them. #2 Erosion washes them into the sea. #3. Tectonic subduction. #4 Flood basalts like the Siberian Traps. $\endgroup$ – RonJohn Jan 23 '18 at 19:35
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Consider planetary formation. Countless asteroids smash into each other and form a big super-heated molten rock. Naturally the heavier elements (metals) sink towards the core over time, which is why Earth has a (mostly) iron core. As the impact frequency slows and the planet eventually cools and starts to solidify, later impacts will tend to deposit their minerals close to the surface, resulting in a fairly abundant supply of minerals in the upper planetary layers.

If you want fewer surface metals, make it so that fewer asteroid collisions occurred on this planet after it cooled. Perhaps it was formed relatively late by two or more large planetoids colliding into one super-heated mass, so that by the time the surface cooled, relatively few asteroids remained. I imagine this would also similarly reduce the abundance of other metals, but that may be something you can work with.

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A combination of poor ores like taconite and lack of fuel to process it will do. It could be that most banded iron formations in your world are flooded by very shallow seas due to glacial melting long ago. An advanced society could dig the tunnels on the coast to mine the iron but they would need iron to build the tools to mine more iron.

Also, don't forget that without iron it will be more difficult to build ships like galleons due to the difficulties of woodworking and many parts of the ship being made of iron and that your agriculture will suffer without iron tools like plowshares using iron on critical parts.

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A Recently terraformed generic world, minus the iron bacteria.

The modern concentrated forms of iron comes from biological activities: banded iron formations comes from the great oxygenation event from an acidic starting ocean PH, so iron could precipitate into banded iron formations; bog iron comes from autotrophic iron bacteria which oxidizes ferrous ions in bog water.

If the plants were strong enough in capturing/holding iron, or if the bacteria did not develop matallorespiration (the process that reduces ferric ions to ferrous ions) ,(the plants would acquire iron with an acid, from rocks.), then there would be ether a lack of soluble iron in the water, or the lack of ability to highly concentrate the dissolved iron in freshwater, eluding it to the seas. Also, if the grains of sand is too fine/containing excess chromium/titanium/aluminum, any bog iron will be unusable for bloomeries (because it won’t Melt)

For any planet that have been lifeless/waterless, only recently terraformed, through either cometary water(which is unusually oxidative because of UV radiation and cosmic rays, therefore lacking soluble iron) or through seeding with life, there would be no great oxygenation event coupled with dissolved iron, too little time to form a deposit/the terraformed forgot to introduce iron bacteria/introducing plants that locks up iron from the bogs/no bogs could form at all from the planetary terrain, there will be no concentrated iron ores over 3%, but would not be deficit in iron or other metals.

As for the culture, it can be from a failed colonization effort, or the world might be abandoned by some other civilizations, or the world is populated through interplanetary panspermia of some kind; while some evolutionary accident prevented the development of metallorespiration in bacteria. Or the colonists were not volunteered, and the world is to be designated as an (later abandoned) agricultural world; hence the intentional deprivation of any workable iron/metal deposits other than ones that could be accessed via space age technology, for the imperial empire that sent the culture there.

Metals for biochemistry or planetary magnetic field? Yes.

Metals that becomes accessible after major industrialization? Yes.

Metals that can be smelted with primitive technology, therefore producing an Iron Age culture? No.

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