Engineering Consequences of Supergene Processes in Porphyry Copper Systems

Abstract Supergene sulfide enrichment in porphyry copper systems of the cordillera results in a modified but distinct mineralogy that is the product of redox reactions, fluid flow, vegetation, and bacterial activity. The supergene effects produce a vertically zoned mineralogical profile that is superimposed on a laterally zoned hypogene profile. The supergene profile is made up of layers of rocks in which original lithology, mineralogy, and physical state have been modified by igneous intrusion, cycles of uplift, tectonic dislocation, weathering, changes in levels and shape of the water table, and by episodes of climate variability. Whereas the nature of the supergene-modified strata can be generalized on the basis of a century of research and evolving exposure that allow modeling of the nature of the process, the variability of age, setting, weathering, and lithological differences among the population of systems may impose very different characteristics of such generalized profiles. The dominating process involves water and the product of its reaction with the mineralized K-Al-Si rocks. Of concern here is production of acid. The corrosion of minerals by acid along the fractures of stockworks results in rock volumes for which original properties of mass, coherence, and physical-geophysical properties are changed. The processes of copper solution and reprecipitation into enriched blankets are controlled by redox boundaries and the availability of reduced sulfur. In the precipitation process, copper is believed to replace iron in sulfides with no net change in density or sulfide mineral content. The oxide minerals mined for leaching are commonly zoned in the profile and the oxidized column above copper enrichment loses sulfur and retains iron. The stockwork that served as a control of hypogene mineralization also acts as a control of permeability in the supergene processes. It too is modified with the result that rock masses are weakened by a solution of minerals along fractures, phenomena that affect comminution and pit-slope stability, as well as changing many electrical properties that are related to coherent conduction or resistance. All of these effects are of engineering consequence.