Dating technics applied to supergene copper deposits

Abstract

International audienceConstraining the timing of geological events is a key issue in many Earth Sciences related studies. Over the last 15 years, technicalimprovement associated with the development of data modeling provided new perspectives due to greater analytical precisions,the ability to measure low contents and better interpretation of complex histories by combining different dating technics. In theAtacama Desert, supergene processes formed world-class ore deposits that are mined because of their high Cu-contents. Thesupergene mineralization are mainly found closely to porphyry-Cu deposits or in a distal location as exotic Cu-deposits. The mainproblem with most of these exotic deposits is that they are often buried under sediments and therefore hard to find. The formationof such deposits is proposed to be related to specific climatic periods and relief conditions. However, many uncertainties remainregarding the emplacement ages of these deposits. These data are of prime importance for the exploration of new deposits andto constrain the paleoenvironmental conditions needed for their formation. Therefore, we have carried out a study integratingthermochronology (fission tracks and (U-Th)/He) and geochronology (LA-ICP-MS U-Pb). We applied thermochronology onporphyry-Cu deposits and modeled the data set to constrain their exhumation history. Our results, compared to previouslypublished supergene oxide mineralization dating, indicate that supergene mineralization occurred ~10 Ma after the main periodof porphyry-Cu exhumation during a stage of slow landscape denudation. This appears to be a required condition for efficientsupergene Cu-enrichment in arid context. However, this study rose the question about the accuracy of supergene Cu-enrichmentdating previously defined using K-Ar and Ar-Ar dating on associated minerals (not directly on the mineralization itself). Therefore,we attempted to date directly Cu ores (pseudomalachite and Mn-rich chrysocolla from Chuquicamata) using the in-situ U-PbLA-ICP-MS method. The data obtained define a concordant date at about 20 Ma, coherent with supergene ages already knownin the Atacama Desert, and demonstrating, for the first time, that supergene Cu mineralization can be dated by the U-Pb method.Our studies demonstrate that the combination of different dating technics and modelling is a powerful tool that can contribute topropose a metallogenical model for the formation of supergene exotic Cu mineralization