The correct environment for Iron Ore is archaean to protorozoic age (older than 2 billion years old...) former shallow shelf seas. These are essentially ancient examples of hydrothermal fluids (hot waters) expelled from cooling oceanic crust. Earth's greatest examples of economic iron ore were formed in rythmic bands of haematite and magnetite (iron oxide minerals) and silica formed when hot Fe2+ rich fluids rose from anoxic oceans into surface waters which actually contained oxygen formed by the beginning of evolution of cyanobacteria (blue green algae), which were the first things to evolve to use photosynthesis, thus converting earths atmosphere from CO2 rich and deadly to oxygen rich as it is today. The interaction with oxygen oxidised the reduced fluids from Fe2+ to Fe3+ becoming insoluble as iron oxides. The earth's iron resources are the result of a one time only geological circumstance (oxyatmoinversion) which will never happen again. When we use it up, its gone.
Gold forms in numerous environments, again, often old.
3 main types of gold deposits exist in nature:
1) Conglomerate hosted gold
40% of the gold ever mined is from the Witwatersrand in South Africa.
This formed as a modified placer type deposit. Tiny amounts of gold from mountain ranges (explained in a moment) were eroded from river systems which eventually deposited heavy gold in energetically slow environments when the river system could no longer carry it. This was then upgraded and concentrated by hot waters, and potentially a meteor impact.
The second major type of gold forms in orogenic zones. When geological plates collide, huge pressures and temperatures occur, forming mountain ranges. The heat concentrates minute amounts of gold which collects in weak zones in mountains.
The third type of gold deposits are known as epithermal. Again, hot waters are key, in this case coming off volcanoes, these are acidic, hot, and Cl- rich and scavenge gold from within the local rocks. When these hot fluids rise towards the surface they can dump the gold in economic concentrations in a variety of ways: Either through:
Boiling: The pressure reduction caused by the fluids rising towards the surface results in the exolution of trapped gases. This exerts a strong cooling effect, destabilising the complexes moving the gold, causing it to be dumped in one place.
Interaction with other fluids: Other surface waters can also exert a cooling effect doing the same thing as described above.
Wall rock interaction: If these acidic fluids come into contact with limestones, or other chemically different rocks, the neutralisation of the fluids by the carbonates also causes the fluid complexes to dump the gold.
