Multi-approach study (preliminary essays) to understand the paleoenvironmental and paleoclimatic conditions related to the genesis of exotic Cu-deposits, northern Chile

Late Cenozoic (<35 - >10 Ma) secondary Cu-sulfides and, eventually, exotic Cu-deposits are considerably common within the Atacama Desert. These deposits are supposed to have a strong relationship with the paleoenvironmental and paleoclimatic conditions from that time. A study of the Tesoro Basin attempts to understand these connections using two proxies: clay mineral analysis and magnetic properties. Results obtained from both techniques are compared and related to the stratigraphic record previously determined in this area. Three main geological units are distinguished: Zanja perimetral gravels, Quebrada Los Arrieros gravels and Quebrada Los Arrieros fine-grained deposits. Clay mineral analysis results (fig. 1) help to confirm the paleoenvironmental meaning of the different facies associations located in the Quebrada Los Arrieros gravels. Variations in smectite, kaosmectite and illite contents can be explained as changes in the ability of being transported and selected in the sedimentary system. A priori, smectite crystals are smaller and form low-density agglomerates. They are probably able to be in suspension for a relatively longer time than the other clay minerals. Thus, some kind of preferential selection is expected during transportation of (detrital) clays. The magnetic properties (fig. 2) are the key to understanding the hydrologic variations through time in the Quebrada Los Arrieros fine-grained deposits. Correlation between magnetic properties and facies associations suggest that the hydrologic regime controls the amount of detrital minerals vs. neoformed ferromagnetic minerals. Higher values of magnetic susceptibility are correlated to strata with finer particles, which are interpreted as the result of higher water table periods. These results, combined with the stratigraphic record and the different paleosols observed, suggest a trend of formation commencing with a relatively humid period that progressively changes to a period of dessication. It remains unanswered as to whether Cu-exotic ores are formed throughout a geologic particular period (tectonic or climatic) or they are only dependent on specific geomorphologic conditions that can be determined from detailed sedimentary studies. This preliminary study has yielded a promising approach that can be used to precisely define the paleoenvironmental and paleoclimatic conditions throughout the Late Cenozoic stratigraphic record in the Atacama Desert. This knowledge is necessary to accurately answer the previous questions concerning the genesis of Cu-exotic ores.