Minerals from the oxidation zones of lead, zinc and copper deposits
A warm climate at the end of the Cretaceous and beginning of the Tertiary, combined with rainfall followed by evaporation, facilitated the alteration of the near-surface part of most deposits in Europe and North Africa. This is often referred to as the iron cap, a zone rich in iron oxides resulting from the oxidation of iron-bearing sulfides.
In this zone, sulfides are oxidised and replaced by iron, manganese hydroxides, and oxides.
Metals recombine with anions present in the solution: carbonate, phosphate, vanadate, molybdate, and arsenate. The result is the formation of brightly-coloured, self-morphing crystals that grow unhindered in water, sometimes in evaporation.
These are often the most sought-after minerals for collectors, but they provide information only on this oxidation stage and not on the formation conditions of the primary deposit.