Tertiary Magmatic Evolution of the Maricunga Mineral Belt in Chile

Abstract

Late Oligocene-Early Miocene volcanic activity along the magmatic front of the south Central Andes in Northern Chile was associated with the formation of epithermal gold-silver deposits in the Maricunga mineral belt (28° S to 26° S). Magmatic activity began at approximately 26 Ma and lasted until ∼ 6 Ma, when the volcanic front migrated eastward. The evolution of the centers within the belt reflects crustal thickening associated with the dramatic tectonic changes in the Central Andes that occurred as the subducting Nazca plate changed shape and caused modifications in the overlying crust and lithospheric mantle. Among these changes were the cessation of volcanism in the “flat-slab” to the south and the uplift of the Puna plateau to the east. Within this setting, Maricunga-belt magmatism can be viewed in five magmatic-tectonic episodes. The first was a Late-Oligocene to Early-Miocene (26 to 20 Ma) episode, during which volcanic complexes erupted over a moderately dipping subduction zone through a ∼ 45-km-thick crust in the north and a crust of ∼ 35- to 40-km thickness in the south. Centers included the large andesitic to dacitic Cerros Bravos stratovolcano in the north and dome complexes in the south. More alkaline lavas (Segerstrom basalts) erupted in the back-arc. Most dome complexes were altered hydrothermally as widespread epithermal gold-silver mineralization occurred. The second episode in the Early Miocene (20 to 17Ma) represented a virtual volcanic lull during a period of compressional deformation and crustal thickening. The third period in the Middle Miocene (17 to 12 Ma) was marked by the eruption of voluminous stratovolcanic complexes all along the belt, and ended with the emplacement of “gold porphyries” during a second mineralization event. As with the domes hosting the Early Miocene mineral deposits, the mafic residual mineral assemblage associated with the Mid-Miocene “gold porphyries” was dominated by amphibole. In both cases, these mineralization events were followed immediately by periods of crustal thickening. During the fourth period in the Early to middle Late Miocene (11 to 7 Ma), a radical change in the distribution of centers occurred, as activity became concentrated at the dacitic Copiapo volcanic complex. Magmatic chemistry at this time is consistent with significant crustal thickening after 12 Ma. The last magmatic period in the Late Miocene to Pliocene (7 Ma to 5 Ma) was marked by small volumes of activity at the southern end of the belt. Erupted magmas included rhyodacites from the Jotabeche caldera and glassy mafic andesitic to andesitic flows along fault zones. These magmas have trace-element chemical signatures (e.g., extreme La/ Yb ratios and high Na20 and Sr concentrations) that are virtually unique in the Central Andes. Their characteristics are best interpreted as being the result of the equilibration of these magmas with a high-pressure garnet-bearing residual assemblage in a cooling mantle wedge over a shallowing subduction zone and in a thickened garnet-granulitic to eclogitic facies crust. Crustal thickness reached more than 60 km in the Jotabeche region. On a regional scale, the Maricunga belt is cut by the WNW-striking Valle Ancho fault zone, which projects beneath the Copiapo center. This important structural discontinuity, across which a change in the 87Sr/86Sr isotopic ratio occurs, could mark the northern limit of the Chilenia terrane that was accreted to Gondwana in the Late Paleozoic.