Andean uplift and climate evolution in the southern Atacama Desert deduced from geomorphology and supergene alunite-group minerals

Abstract

Supergene alunite group minerals from the Late Eocene El Salvador porphyry Cu district, the El Hueso epithermal gold deposit and the Coya porphyry Au prospect located in the Precordillera of Northern Chile (~ 26 to 26° 30´ Lat. S) have been dated by the 40Ar/ 39Ar method and analyzed for stable isotopes. These data support published geomorphologic and sedimentologic evidence suggesting that the Precordillera in the Southern Atacama Desert had been uplifted as early as the late Eocene and, thus, significantly prior to the Altiplano which attained its high elevation only in the late Miocene. The oldest supergene alunite from the Damiana exotic deposit at El Salvador was dated at 35.8 ± 1 Ma and yielded a δD (VSMOW) value of −74‰ which indicates elevations of the Precordillera near El Salvador of at least 3000 m in the Late Eocene. In contrast, Miocene supergene alunite from El Salvador, El Hueso, and Coya have less negative δD signatures reaching values as high as −23 to −25‰ at El Hueso and El Salvador between about 8.2 and 14 Ma. Late Miocene to Holocene supergene alunite, jarosite and natroalunite ages are restricted to El Hueso and Coya located near 4000 m above sea level in the Precordillera, roughly 1000 m higher than the present elevation of El Salvador. The δD values of samples younger than ~ 5 Ma vary between −57 and −97‰. The complex evolution of the δD signatures suggests that meteoric waters recorded in supergene alunite group minerals were variably affected by evaporation and provides evidence for climate desiccation and onset of hyper arid conditions in the Central Depression of the southern Atacama Desert after 15 Ma, which agrees well with published constraints from the Atacama Desert at 23–24° Lat. S. Our data also suggest that wetter climatic conditions than at present prevailed in the latest Miocene and early Pliocene in the Precordillera. The new and previously published age constraints for El Salvador indicate that supergene mineralization at the Damiana exotic Cu deposit occurred periodically over 23 Ma in a locally exceptionally stable paleohydrologic and geomorphologic configuration.