The rrost fundamental revision of concepts in metallogeny over the last 30 years has been the re - interpretation of pyritic and polyrretallic• sulphide ore deposits as deposits of the syngenetic volcano - sedimentary type. One is disturbed by the proximity •sj.de by side in the same for:ma.ti• on but at different stratigraphic levels, of auriferous and polyrretallic - sulphide ore deposits.
Furtherm::,re gold is often a byproduct of the latter deposits, and sulphides are carrron in gold deposits. For example pyrite and pyrhotite are present in both types of deposit which always occur in cl,ose relation to sediment• ary rocks of a chemical nature such as jasper, carbonate and ferruginous formations. On the basis of pertinent observations, canadian, Australian, South Afri.,,pan & French authors proposed a syngenetic volcano- sedimentary- m::::del for exclusively auriferous deposits without significant develop-rent of pyrite. It is why this group is called volcano sedimentary group.

This group contrasts in other classifications, with deposits in re• lation to subvolcanic intrusions, often porphyritic in nature. Such depo• sits contain gold generally associated with other minerals such as copper, lead, zinc and silver. This second group is called plutono-volcanic type or epithermal type.

It is not always easy to distinguish between these.two large groups, especially when dealing with deposits inclosed in ancient terrains that have undergone significant metanorphism. If the volcano sedimentary group is predaninantly stratifonn, at the level of underground feed, it can in• clude discordant stringer zones. In contrast, hydrothermal circulation around porphyritic intrusions in carbonate levels can lead to stratifonn or skarn replacer.ent.

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