Supergene deposits form through oxidation, dissolution, and reconcentration of metals near the Earth's surface by descending water and gases. Metals are leached from the upper part of sulfide deposits and redeposited below the water table, increasing the metal content in the primary ore zone. This process of supergene enrichment can create spectacularly rich deposits. An example is seen at the Inspiration orebody in Arizona, where the average copper content increased from 1% to up to 5% through supergene processes.

1. SUPERGENE DEPOSITS
ORE GEOLOGY

2. SUPERGENE DEPOSITS
Supergene is a term used to describe sub-surface processes
and their products formed by the activity of descending water
and gas.
Supergene metal deposits form when common rock types
exposed at or near the Earth’s surface and undergo oxidation,
dissolution and reconcentration of the metals.

3. The term is applied more commonly to the enrichment of sulphide
deposits but it has been extended by many workers to include similar
processes affecting oxide or carbonate ores and rocks such as those of
iron and manganese.
In supergene sulphide enrichment the metals of economic interest are
carried down into hypogene (primary) ore where they are precipitated
with a resultant increase in metal content.
In the case of iron and manganese ores it is chiefly the gangue material
that is mobilized and carried away to leave behind a purer metal deposit.

4. Zone of Oxidation
When a sulfidic ore body is brought close to the earth’s surface through
erosion, it will also get easily affected by the agents of chemical
weathering.
Surface waters percolating down the outcrops of sulphide orebodies
oxidize many ore minerals and yield solvents that dissolve other minerals.
The sulfur is expected to get oxidized to sulfate ion.
Pyrite is almost ubiquitous in sulphide deposits and this breaks down to
produce insoluble iron hydroxides (limonite) and sulphuric acid.

5. Copper, zinc and silver sulphides are soluble and thus the upper part of a
sulphide orebody may be oxidized and generally leached of many of its
valuable elements right down to the water table. This is called the zone of
oxidation.
The ferric hydroxide is left behind to form a residual deposit at the surface
and this is known as a Gossan or Iron hat.
As the water percolates downwards through the zone of oxidation because it
is still carbonated and still has oxidizing properties, It may precipitate
secondary minerals, such as Malachite and Azurite.

7. The leached and oxidized zones may not be without economic important as
ores.
One of the world's largest open pit gold-silver mines, Pueblo Viejo,
Dominican Republic is developed in the oxidized zone of sulphide protore.
Another example is the world's only germanium-gallium mine, the Apex
Mine, Utah. At this locality germanium and gallium have been concentrated
in the secondary iron oxides-material.
It is very probable that there are more such orebodies awaiting discovery.

8. ZONE OF ENRICHMENT
The bulk of the dissolved metals stays in solution until it reaches the
water table below which conditions are usually reducing.
This leads to various reactions that precipitate the dissolved metals and
result in the replacement of primary by secondary sulphides.
At the same time, the grade is increased and in this way spectacularly
rich bonanzas can be formed.

9. This zone of supergene enrichment usually overlies primary mineralization,
which mayor may not be of ore grade.
Reaction occur in this zone of enrichment which lead to secondary
enrichment in mineral in the Primary ore are :-

10. It is thus imperative to ascertain whether newly discovered near surface
mineralization has undergone supergene enrichment, for, if this is the case, a
drastic reduction in grade may be encountered when the supergene enrichment
zone is bottomed

11. Supergene enrichment has been important in the development of many porphyry copper
deposits and a good example occurs in the Inspiration orebody of the Miami district, Arizona.
The unenriched ore averages about 1% Cu and consists of pyrite, chalcopyrite and
molybdenite.
Supergene enrichment increased the grade up to as much as 5% in some places.
Chalcocite is the main secondary sulphide, having replaced both pyrite and chalcopyrite.

12. Condition of Deposits
• Considerable time for the evolution of significant secondary
mineralization.
• Water table be fairly deep
• Ground level is slowly lowered by erosion, such phenomena are
restricted to non-glaciated land areas.
• Highly pyritic primary mineralization can produce more sulphuric acid
during weathering giving rise to more efficient leaching of copper
than in less pyritic deposits

14. Summary
The net effect of these supergene processes is to move metal
ions from the leached zone to the enriched zone, increasing
the concentration there to levels higher than in the
unmodified primary zone, possibly producing a deposit worth
mining.