Carlos E. Ulriksen

Geological mapping at a scale of 1:100,000 of an area over 3,000 square kilometres, the examination of 165 mineral occurrences and ore deposits, and 13 new K/Ar and 4 new ⁴⁰Ar/³⁹Ar dates form the basis for this thesis.

Paleozoic rocks metamorphosed to greenschist grade, folded into WNW-ESE trending folds and intruded by post-tectonic Permian (259 Ma ⁴⁰Ar/³⁹Ar) granitoids constitute the crystalline basement of the Cifuncho-Cerro del Pingo study area. The Andean Orogen was overprinted onto this basement and onto remnants of relatively undeformed Permo-Triassic volcanic and sedimentary formations.

In the western part of the area, a marine, fossiliferous and partially volcanic Lower Jurassic (Hettangian to Sinemurian) sequence, intruded by a Lower Jurassic (186 Ma ⁴⁰Ar/³⁹Ar) granitoid pluton is unconformably overlain by over 3,000 m of mafic to silicic volcanics of Middle to Late Jurassic age. The above formations were intruded by small porphyry plutons (undated) and by swarms of mafic dykes during the Late Jurassic (142 Ma, K-Ar whole rock).

East of the Atacama Fault System that divides the area marine to continental sediments and volcanics were deposited during the Early Cretaceous (Neocomian) and were intruded by granitoid plutons during Early to early Late Cretaceous times (124 Ma to 96 Ma).

The major longitudinal Atacama Fault System has affected all formations. Over 3,000 m of vertical displacement (west block down) can be deduced for pre-Cretaceous times and the fault has displaced Neogene gravels. Slickensides indicate some late component of strike-slip motion.

Dyke swarms and the tectonic style of the study area attest to a prevailing extensional regime which was probably imposed by subduction-related tectonism since the Jurassic. Compressive tectonism is mainly a Late Tertiary phenomenon at this latitude.

A Lower Jurassic metallogenic unit (Au, Ag, Co, Cu, Pb, Zn) occurs mainly nearest to the coast, and is followed inland by veins and disseminations of Cu of probable Middle to Upper Jurassic age. A Lower Cretaceous metallogenic unit (Fe, apatite, Cu, Mn, barite) occurs in the east is genetically associated with Cretaceous magmatism and in part structurally related to the Atacama Fault System. Cenozoic ore deposits occur farther inland outside the area.

This work confirms both the existence of an Early Jurassic magmatic episode and the eastward younging of magmatic and metallogenic units previously suggested for a more southerly transect of the Central Andes.

Keywords:
Pages: 236
Supervisor: Marcos Zentilli