I am assuming that resources will be sourced from nearby locations in the solar system (with the Moon as first choice) for three reasons:
The cost of getting anything in bulk up from Earth is very high
Environmental: a wish to expand mankind’s resources, not deplete the Earth further.
Anything beyond Mars is expensive, long round trip and energy consumption.
This section is based on current knowledge of Moon and Mars geology, which is still quite limited. The odds are that more resources will be discovered than are currently known.
Iron: actually easier than on Earth. There appears to a lot of metallic iron in the lunar regolith, apparently of meteoric origin. Just sort through regolith with a magnet. If not, 15% of lunar maria (the flat regions) are iron oxide by weight, similar to iron ores on earth. Nickel and chromium ores seem plentiful as well, to make steel alloys and stainless steel. Lunar maria rocks contain a high concentration of manganese, the main alloying element used in steelmaking.
Aluminium: the usual ore on Earth (bauxite, essentially aluminium oxide) is not present on the moon. However anorthite (calcium aluminium silicate) is one of the most common rocks on the Moon. It can be smelted to its constituent metals using the FFC Cambridge Process, or some variant. As a by product, you also get calcium (to make lime, and hence mortar and concrete) and silicon (for solar panels). Calcium is an excellent electrical conductor, cannot be used on earth because it oxidises rapidly, but could work in the vacuum of space. The Cambridge process is under development to find a better way to produce titanium metal, where earth ores are plentiful, but the current process is expensive and polluting. Ilmenite, the main titanium ore, has been found on the Moon.
What the Moon is apparently short of is base metals (copper, lead, zinc). There are substitutes e.g. aluminium for electrical uses, nickel or chromium instead of zinc galvanising for corrosion protecting steel. There may be resources on Mars, we don’t really know yet.
O’Neill suggested that oxygen could be liberated from the oxides in the lunar regolith. That is indeed the case, but an easier alternative may be the huge amount of carbon dioxide present in the dense Venusian atmosphere. Venus may have gravity close to Earth, but all a collector vessel has to do is skim through the atmosphere, collect the carbon dioxide (CO2) rich air (efficient storage would then freeze it, as dry ice) and skim out again – little extra energy (ΔV) would need to be expended.
If you can separate the oxygen from CO2, you also get carbon or a carbon based organic (feedstock for plastics and other chemicals). You also get sulphur from the sulphuric acid in Venusian air. Splitting CO2 takes quite a bit of energy – yet plants do it all the time, through photosynthesis. Natural photosynthesis (e.g using vats of algae) is slow and will not work with high concentrations of CO2. However artificial photosynthesis is the subject of considerable research, potentially to provide solar power and to treat CO2 rich flue gases from fossil fuel power stations.
Flue gases typically contain 10-15% CO2 while Venusian atmosphere is 97% CO2. This actually could make an artificial process more efficient, and power input from solar cells would be cheap. The end products sought, from CO2 and water input, would be oxygen and an organic product, probably an alcohol (such as methanol) or organic acid (such as acetic acid). The latter could form the basis for the numerous carbon based products used in everyday life, notably plastics. It should be noted that the oxygen needed is almost all for the initial load for the habitat - with enough plant life inside it, normal photosynthesis should maintain all or nearly all the oxygen content of the atmosphere.
Water is the big one. Humans live in a totally water dependent environment. In a reasonably humid environment, 25% or more of the weight of topsoil is water, plus lakes, streams, drinking and washing, and a suggested water jacket around a habitat as part of radiation protection. Water would be efficiently recycled, but there is an initial load required. There is plenty of water in the Solar System, but most of it is either on Earth (costly and environmentally damaging to source from) or in the outer regions, Asteroid Belt or beyond, requiring long and costly journeys. Several of the moons of Jupiter and Saturn have sub surface oceans which together contain more water than in the Earth’s oceans - but they may also for that reason contain life. There is some on the Moon, but current evidence suggests not much.
Fortunately there seems to be masses of water on Mars, in polar ice caps or subsurface. Where there is water, there may be life, and this may include microbial life on Mars, although we have not discovered any yet. If so, we would have to make sure that any water extraction would not damage it. As with oxygen, water is mainly an initial load, after a water cycle will maintain it within the habitat.
The most problematic essential element is nitrogen, 78% of an earth like atmosphere. While OK in a medical emergency, breathing pure oxygen is bad for health over a protracted period, and would be a huge fire risk. In any case, you need nitrogen to feed nitrogen-fixing bacteria in soil, essential for plant growth, and to produce artificial fertilisers. As with water, nitrogen is plentiful on Earth and in the outer solar system, not on the Moon, and amounts on Mars (as either gas or nitrates) do not seem plentiful, according to current knowledge. The best source seems to be Venus – only 3% of the atmosphere, but such is the high pressure of the atmosphere, that is more nitrogen than in the Earth’s atmosphere.
Elements that we need in trace amounts are potassium, phosphorus, sodium and chlorine (the latter two as common salt, sodium chloride). Rare on the Moon, but seemingly plentiful on Mars.
For building materials, you can make glass from regolith, and fused regolith makes a glassy building brick. Regolith plus lime and crushed lunar rock would probably make an acceptable concrete.
Meteorites – And Security
Small scale ecosystems