Cenozoic magmatism and mineralization in the Sierras Pampeanas of San Luis, Argentina

San Luis Tertiary metallogenic belt is located in the Sierras Pampeanas of San
Luis Province, Argentina, in the southeastern extreme of the Pampean flat-slab segment.
Volcanic centers and related hydrothermal mineralization were generated on
the inner side of an arc environment as a consequence of the subduction zone shallowing
between 27° and 33° S. Mineralization and volcanic rocks occur within a westnorthwest-
trending magmatic belt which is roughly perpendicular to the main reverse
faults that constitute the western boundary of the San Luis range. The volcanic rocks,
which intrude and overlie Precambrian-Paleozoic basement, crop out from La Carolina
in the west to El Morro in the east, becoming progressively younger toward the
east. The volcanic rocks comprise andesites, dacites, latites, and trachytes emplaced in
four volcanic centers: La Carolina, Cañada Honda-Cerros Largos, Cerros del
Rosario, El Morro. Mesosilicic magmas belong to normal to high-K calc-alkaline and
shoshonitic types showing at both regional and local scales a K enrichment with
decreasing ages.
Several mineral deposits have a close spatial and temporal linkage with Tertiary
volcanism and are hosted by both basement and volcanic rocks. The La Carolina district
has low-sulfidation base-metal-sulfide-rich, gold and silver epithermal deposits.
Both veins and disseminated/stockworks are associated with a dome-diatreme complex.
Cañada Honda has the Diente Verde gold-rich porphyry copper deposit and several
carbonate, base-metal-sulfide-rich, gold-silver low-sulfidation epithermal veins.
Cerros del Rosario District has low-sulfidation carbonate veins.
K-Ar ages demonstrate that the volcanic events and associated mineralization
took place between late Miocene and late Pliocene immediately prior to, and during
the final stages of the slab flattening between 27° and 33° S. Volcanic activity began at
10–11 Ma and ended at 1.9 Ma. K-Ar data indicate that the hydrothermal system was
probably short-lived at La Carolina while in Cañada Honda the hydrothermal system
lifespan might be restricted to 2.6 m.y. Volcanic rocks and associated mineralization
formed 700 km east from the Chilean trench for about 8.6 m.y. and over a west-east
distance of 80 km suggesting a swift eastward shift of the volcanic front. Two magmatic
pulses and associated period of hydrothermal mineralization accompanying the
eastward volcanic migration are distinguished: 10–11 to 6.3 Ma in the west and 6.4 to
1.9 Ma in the east. Metal-sulfide dominated deposits in the west and carbonate dominated
deposits in the east of the belt indicate an eastward-shallowing level of erosion
and decreasing mineralization age.
The San Luis metallogenic belt reflects the extraordinary broadening of the magmatic
arc in the flat-slab segment at 33° S and resembles the situation of the Bajo deLa Alumbrera gold-rich porphyry at 27° S. Both have similar geochemical signatures
and were emplaced in arc-transverse magmatic trends located just in the south and
north transitions of the flat-slab segment where the subduction angle changes. The
transversal attitude with regard to the magmatic arc (particularly the 80 km San Luis
belt) could be taken as a strong evidence to suggest discontinuities or structural weakness
zones along 27° and 33° S that led to the magma ascent and controlled the formation
of hydrothermal mineralization.