Supergene Enrichment- How Natural Processes have Transformed Low-Grade Copper Mineralization into Economic Ores

In “What is a Porphyry Copper Deposit?”, I discussed the general relationships of the primary alteration and mineralization that are found within porphyry copper systems. Also known as “hypogene ” alteration and mineralization, these assemblages are produced by the circulation hydrothermal solutions (hot water) as a porphyry copper is formed at depth. Today’s article will discuss “supergene ” alteration and mineralization assemblages, which are formed when these sulfide-bearing deposits are exposed to near-surface, oxidizing solutions (groundwater) as they are exhumed by erosion and exposed to weathering. The simplified cross-section shown below, represents a vertical slice (like cutting a slice of cake) through the shallow levels of an idealized porphyry copper system that has been exposed by erosion. Summarizing the primary (hypogene) alteration and mineralization observed in porphyry copper systems, the low grade potassic core is located at the center of the system. It is characterized by quartz, potassium feldspar, biotite and anhydrite with minor amounts of sulfide-bearing minerals. Phyllic alteration assemblages contain quartz and sericite with abundant pyrite and lesser amounts of copper-bearing sulfides. Chlorite, epidote and calcite with minor amounts of pyrite are typically found in the outer propylitic zone. Straddling the boundary between the low grade potassic core an