Peter K. M. Megaw, 2009. "Evaluation of Oxidized Pb-Zn-Ag Carbonate Replacement Deposits of Mexico in Light of Supergene Zinc and Residual Lead Enrichment Processes", Supergene Environments, Processes, and Products, Spencer R. Titley
Download citation file:
The majority of Mexico’s carbonate replacement deposits were discovered in outcrop over 100 years ago and most were oxidized to significant depths. Oxidized mantos and chimneys were diligently exploited as continuous compact bodies of silver-rich cerussite and anglesite, while ignoring then-valueless underlying or surrounding irregular, high-grade (>20% Zn) bodies of mixed Fe-Mn-Zn oxides, smithsonite, hemimorphite, willemite, and hydozincite. Recent development of effective solvent-extraction metallurgy to recover zinc from zinc oxide minerals has renewed interest in these bypassed bodies, but in most cases little was recorded of their tonnage, grade, or morphology. However, an understanding of original sulfide orebody limits, composition, morphology, and leaching processes can provide a first-order indicator of the potential zinc content and mineability of a given deposit prior to mounting a full-scale exploration effort.
Figures & Tables
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.