Metallogeny, structural evolution, post-mineral cover distribution and exploration in concealed areas of the northern Chilean Andes

Abstract Mineral exploration of prospective areas concealed by extensive post-mineralization cover is growing, being very complex and expensive. The projection of rich and giant Paleocene to early Oligocene porphyry-Cu-Mo belts in northernmost Chilean Andes (17.5–19.5°S) has major exploration potential, but only a few minor deposits have been reported to date, due to the fact that the area is largely covered by post-mineral strata. We integrate the Cenozoic stratigraphic, structural and metallogenic evolution of this sector, in order to identify the most promising regions related to lesser post-mineral cover and the projection of different metallogenic belts. The Paleocene to early Eocene metallogenic belt extends along the Precordillera, with ca. 30 km wide, and includes porphyry-Cu prospects and small Cu (±Mo-Au-Ag) vein and breccia-pipe deposits. Geochronological data indicate an age of 55.5 Ma for an intrusion related to one deposit and ages from 69.5 to 54.5 Ma for hydrothermal alteration in one porphyry-Cu prospect and largest known Cu deposits. The middle Eocene to early Oligocene porphyry belt, in the Western Cordillera farther east, is associated with 46–44 Ma intrusions. It is estimated to be 40-km wide, but is largely concealed by thick post-mineral cover. The youngest Miocene to early Pliocene metallogenic belt, also in the Western Cordillera, is well-exposed and includes Au-Ag epithermal and polymetallic veins and manto-type deposits. The Oligocene-Holocene cover consists of a succession of continental sedimentary and volcanic rocks that overall increase in thickness from 0 to 5000 m, from west to east. These strata are subhorizontal in the west and folded-faulted towards the east. Miocene gentle anticlines and monocline flexures extend along strike for 30–60 km in the Precordillera and were generated by propagation of high-angle east-dipping blind reverse faults with at least 300–900 m of Oligocene bedrock offset. The thickness of cover exceeds 2000 m in the eastern Central Depression, whereas it is generally less than 1000 m in the Precordillera along the Paleocene to early Eocene porphyry-Cu belt and it can reach locally up to 5000 m in the Western Cordillera, above the middle Eocene to early Oligocene belt. In the studied Andean segment, the Miocene to early Pliocene metallogenic belt is superimposed on the Paleocene to Oligocene belts in a 40–50 km wide zone. This overlap may be explained by an accentuated migration of the magmatic front, from east to west, since ca. 25 Ma, as a consequence of subduction slab steepening after a period of magmatic lull and flat subduction from ca. 30–35 to 25 Ma. The identified areas of lesser cover thickness are prone to exploration for concealed deposits, especially along the projection of major porphyry-Cu-Mo belts.