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.
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Supergene Environments, Processes, and Products
At least five altered and mineralized porphyry centers related to the cooling of a polyphase Eocene intrusion occur within a 25-km2 "pampa"-type area in the southwestern sector of the Chuquicamata district in northern Chile. These deposits take place 1 to 2 km apart as discrete porphyry "columns" covered by postmineral, poorly consolidated Miocene sedimentary rocks. Such copper oxide and sulfide deposits were discovered and evaluated by drilling done by Codelco from 1996 through 2007 during a brownfield exploration program, driven by the necessity to replace and increase leacheable ore consumed by the Chuquicamata and Radomiro Tomic operations. During this program a resource of more than 20 million metric tons (Mt) Cu was discovered, including 6 Mt Cu of oxide, mixed and secondary sulfide ore, representing one of the largest supergene copper resources discovered worldwide during the last 10 years.
Despite their close location and their genetic relationship to a single, polyphase intrusion mineralization event, the five porphyry centers display contrasting host-rock and structural framework as well as different hypogene alteration and ore mineral assemblages. This picture reaches high levels of complexity because of the different levels of exposure of the mineral systems, resulting from primary emplacement processes and post-mineral faulting. These hypogene features and the effect of landscape and climate evolution controlled supergene alteration, thus generating different profiles in each specific porphyry center. The key controlling factors in the supergene overprint are discussed on the basis of their relationship to ore and gangue mineralogical abundance and occurrence, assemblage distribution, geochemical response, and the broad geologic setting.
As exploration for covered porphyry copper deposits in the southwestern sector of the Chuquicamata district progressed, numerous lessons were learned about the origin of supergene profiles and the analysis and use of supergene effects and their products as a guide for exploration. These lessons, which include geological and geochemical criteria among others, are discussed in the context of the appraisal of the mineral potential of copper oxide-mixed-secondary sulfide blankets and underlying sulfide protore.