Dating technics applied to supergene copper deposits

Constraining the timing of geological events is a key issue in many Earth Sciences related studies. Over the last 15 years, technical improvement 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 the Atacama Desert, supergene processes formed world-class ore deposits that are mined because of their high Cu-contents. The supergene mineralization are mainly found closely to porphyry-Cu deposits or in a distal location as exotic Cu-deposits. The main problem with most of these exotic deposits is that they are often buried under sediments and therefore hard to find. The formation of such deposits is proposed to be related to specific climatic periods and relief conditions. However, many uncertainties remain regarding the emplacement ages of these deposits. These data are of prime importance for the exploration of new deposits and to constrain the paleoenvironmental conditions needed for their formation. Therefore, we have carried out a study integrating thermochronology (fission tracks and (U-Th)/He) and geochronology (LA-ICP-MS U-Pb). We applied thermochronology on porphyry-Cu deposits and modeled the data set to constrain their exhumation history. Our results, compared to previously published supergene oxide mineralization dating, indicate that supergene mineralization occurred ~10 Ma after the main period of porphyry-Cu exhumation during a stage of slow landscape denudation. This appears to be a required condition for efficient supergene Cu-enrichment in arid context. However, this study rose the question about the accuracy of supergene Cu-enrichment dating 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-Pb LA-ICP-MS method. The data obtained define a concordant date at about 20 Ma, coherent with supergene ages already known in 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 to propose a metallogenical model for the formation of supergene exotic Cu mineralization.