Abstract

Our current research focus on the link between supergene processes, geomorphological evolution and the origin of copper deposits in arid to hyperarid climates in the Atacama Desert of Northern Chile since Eocene to present.. The continuous uplift during the Incaic tectonic phase has allowed the unroofing of copper mineralized porphyries favoring the overprinting of supergene alteration to hypogene mineralization producing oxidation and leaching of primary sulfides. At the same time, subplanar surfaces developed in the Precordillera and Central Depression between southern Perú and south of Atacama Desert in response of the uplift since Miocene. This geomorphological structures were conserved to the present due to low erosion rates associated to the hyperaridification process that affected the north of Chile since Middle Miocene. The study of supergene profiles and well preserved subplanar surfaces will help to understand how the processes of uplift, denudation and climatic conditions will help to the process of supergene alteration in the north of Chile since the Eocene. Specifically, the Precordillera between ~26° y 27° Lat. S. ) is characterized by the occurrence of several porphyry copper deposits that have been submitted to supergene alteration processes since 36 Ma ago (e.g. El Salvador and Potrerillos porphyries) and epithermal deposits (Silica El Hueso and Coya, among others), all showing a good outcrop exposition of their supergene profiles which it is an exceptional opportunity to carry on this research. Previous studies in the selected area propose that the geomorphological evolution is characterized by the formation of pediplanation surfaces that alternate with incision episodes of the landscape. Thereby, it has been identified 2 pediplanation surfaces over summits between each period of incision previous to 17 Ma, and other surface developed before 10 Ma earlier than the incision of the current drainage systems (fig. 1. until 1000 m of incision), although, studies inferred that there is a synchronicity among the formation of pediplanation surfaces and development of supergene profiles, these research do not present a geochronological resolution which allows to probe this synchrony, nor detailed studies of the supergene alteration profiles which can be related with a stage of the landscape evolution, that is to say, with the stage of pediplanation or incision. This research seeks to address the following question: how the supergene alteration is affected by factors which model the landscape of the Precordillera in the Atacama Desert between ~26° y 27° Lat. S.. In other words, how the landscape evolution (uplift and weather) influences the distribution and temporality of supergene mineralization in the area. We propose that the supergene alteration is related to incision process which produce a slow and constant decrease of the water table, and exposition of the sulfide minerals to the oxidative environment. As a result, in the upper part of the profile are the oldest ages of alteration, and near to the basin incision are the youngest ages of the profile (fig. 2). Accordingly, to the success of this research it is propose the mineralogy study in supergene profiles and detailed textural analysis of the alteration to understand the events sequence and its temporality in the region with emphasis in the El Hueso-Coya area which is a poorly studied zone. Also, the association between hanged surfaces (pediplains) that are described in the literature whit alteration zones which are observed in field will allow us to interpret how the supergene alteration and enrichment develops and preserves in the selected study area and what is its relationship with the landscape modeling at regional scaleand long term (since Eocene). To determine the age of the alteration events 39Ar/40Ar geochronology will be useon minerals from thr alunite group (e.g. alunite and jarosite). Additionally, the presence of sulfur, hydrogen and oxygen in the crystallographic structure of these minerals will allow the use of stable isotopy to recognize the context of the supergene alteration and enrichment (paleoenvironment, uplift history, paleoaltitude of the area, fluid provenance, among others). This study provides an exciting opportunity to increase our knowledge on the genesis of supergene enrichment profiles and exotic deposits and their relationship with the landscape evolution in arid to hyperarid conditions, knowledge that may be use in the exploration of new copper resources in the Precordillera.