Downward-percolating oxidizing meteoric waters are the single most important factor in the processes that transform primary, hypogene, sulfide protore to secondary, supergene, nonsulfide ore. Faults provide the secondarily enhanced permeability pathways for these fluids and thereby exert a fundamental control over the formation of many—if not most—supergene deposits. Under the relatively low confining lithostatic pressures at or near the earth's surface, fault structures are commonly highly permeable and allow selective weathering and related supergene processes to occur more quickly and reach more deeply than local exhumation and erosion, the greatest threats to any supergene ore deposit subsequent to its formation.
On encountering sulfides, meteoric waters charged with atmospheric oxygen initiate a metal fractionation process involving the liberation and acidic mobilization of soluble base metals, the residual enrichment of insoluble materials in situ, and the progressive reprecipitation of metals at some distance from their source. This fractionation process is fundamentally controlled by the differential solubility of the metals involved and by the hydrodynamic behavior of the supergene meteoric fluids. In turn, the migration of these fluids depends on the permeability of the rocks, which is predominantly fault- and fracture-controlled. Finally, precipitation and fixation of the supergene ore minerals depend on the type and reactivity of available host rocks and the surface area available for reaction.
The contribution of fault and fracture zones is arguably more important to the genesis of supergene zinc deposits than for other supergene base metal deposits, such as Cu-enrichment blankets, because of zinc's particularly high solubility and mobility in the supergene environment.
Several common examples of fault control can be seen in supergene zinc deposits, including the following:
Faults that simply provide access for oxidizing meteoric fluids to hypogene sulfide ores and convert them into supergene ore in situ or near the fault zones.
Faults that juxtapose sulfide protore source rocks and highly reactive trap rocks (e.g., carbonate rocks) in such a way that oxidation, remobilization, and reprecipitation occur in close proximity, although not in situ.
Fault blocks of impermeable lithotypes can act as hydrologic barriers to supergene ore fluids transporting dissolved base metals away from their sulfide sources and cause “ponding” or diversion of these fluids away from potential reactive hosts. The first increases residence time and consequently, fluid-wall rock interaction, whereas the second can create dispersion halos or even force metal-bearing groundwaters to surface, where they can form springs. Both processes can create metal halos detectable by exploration programs.
Fault systems (especially transpressive wrench systems) that fracture large volumes of rock on all scales can increase both permeability of the fractured rock and increase reactive surface area to an exceptional degree. Locally, such pervasive fracture patterns can provide the ground preparation necessary to turn relatively unreactive host rocks such as impermeable metasiliciclastic or volcaniclastic rocks into trap rocks. In some cases (e.g., Skorpion) this can make these shattered rocks more favorable than more reactive adjacent carbonates to which the fluids have more limited access.
Only faults and fault breccias have the extent to allow ultradeep meteoric oxidation and supergene mineralization to penetrate more than 1,000 m below surface, in situations where this is generally considered to be well below the groundwater table.
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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.