Application of Mass-Balance Modeling of Sources, Pathways, and Sinks of Supergene Enrichment to Exploration and Discovery of the Quebrada Turquesa Exotic Copper Orebody, El Salvador District, Chile

摘要

A computerized district-scale copper mass-balance analysis at the El Salvador porphyry copper deposit was completed, using all available assay data to address the genesis of exotic copper deposits and their relationship to the enrichment blanket. Of special importance was identifying the controlling geologic factors useful in exploration for defining prospective corridors leading to undiscovered exotic ores to extend mine life, and providing an estimate of the mass of copper most likely to be discovered. In principle, our metal sources, pathways, and sinks approach is analogous to the oil field strategy of identifying hydrocarbon source rocks, migration pathways, and final reservoirs, except that here we discover these component parts of the system and their spatial linkages, not by organic biomarkers but rather by geochemical mass-balance calculations involving assays, densities, and spatial geometry, which define volumes of the interrelated subsystems. A district geochemical model was created on the Vulcan three-dimensional GIS program consisting of the protore, enrichment blanket, and leached capping in terms of grade and bulk-rock density distribution. The copper assay, density, and volume mass-balance equations from Brimhall et al. (1985) were programmed in Vulcan and solved in a two-step computational procedure. The first step is an approximation to set a rough position of the effective original top of protore containing significant values of copper This is necessary because some of the leached cap has been eroded. The second step incorporates relict sulfide mineralogy in the existing portion of the leached cap to verify and refine the position of the preerosional surface of contributory protore mineralization from which oxidation mobilized significant copper The numerical protore model reflected primary copper-grade zoning calculated from assays at the base of the enrichment blanket and accommodated lateral variation by subdividing the protore rock volume into a bundle of 50-m~2 vertical columns. Grade values in each column of the bundle were then projected upward through the higher reaches of the deposit into the leached capping. Mass balance during coupled leaching and enrichment was computed on each column separately. Overall flux of copper in each column was computed to determine whether all of the copper liberated from the leached cap was fixed in the blanket column below as a balanced geochemical profile or whether some of it escaped, as well as to ascertain the magnitude of the copper lost from the negative flux zones. Where the flux is zero, all of the copper extracted from the leached capping was reprecipitated in the blanket as secondary sulfide mineralization. Over much of the weal extent of the blanket copper fixation was indeed nearly perfect. Sulfide mineral textures in these areas show extensive replacement of the primary by secondary sulfides as rims and along cracks. However; two sizable separate regions were identified as negative flux or source zones totaling 2.3 million tons (Mt) of copper where the flux was negative, indicating that fixation of copper released from the leached cap was quite imperfect in these areas of the enrichment blanket. Here, primary sulfides are hardly replaced by chalcocite, indicating the passage of significant copper out through the blanket. Hence, these zones were interpreted as clearcut cases of source zones for copper that continued to migrate downward and laterally beyond the Suits of the enrichment blanket and out into the surrounding hydrologic flow regime. Factors identified heft contributing to the imperfect fixation, limited replacement of primary sulfides, and escape of copper include: (1) high structural permeability along latite dike and/or fault systems serving as conduits, (2) fluid movement inferred to be so rapid as to minimize the residence time required for chalcocite replacement of primary sulfides, and (3) locally unreactive sericite-kaolinite alteration