Deposits formed from weather of pre-existing mineralizaiton
– Supergene enrichment occur relatively near the surface by weathering
– Supergene processes include meteoric water circulation with concomitant oxidation and chemical weather.
– The meteoric waters oxidize the primary (hypogene) sulphide ore minerals and redistribute the metallic ore elements.
– Supergene enrichment and deposits occur at the base of the oxidized portion of an ore deposit, at and below the water table (reduction).
– Metals that have been leached from the oxidized ore are carried downward by percolation groundwater, and react with the hypogene sulphides at the supergene-hypogene boundary.
– The reaction produces secondary sulphides with metal contents higher than those of the primary ore. In particular chalcocite Cu2S, covelite CuS, digenite Cu9S5