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The San Luis Gold Belt of Argentina in a Regional Metallogenic Context
Nilda E. Urbina
Universidad Nacional de San Luis. Ej. de los Andes 950. 5700 San Luis. Argentina.
urbina@unsl.edu.ar
In the Central Andes, the segment between 27º and 33º S is particularly important because it
contains the major Tertiary epithermal and porphyry type mineralizations in Argentina. These
mineralizations are associated with compressive tectonism and calc-alkaline magmatism that
migrated abruptly eastward from late early Miocene as a consequence of the shallowing of
subduction angle. The metallogenic relevance of this Andean segment was first noticed with regard
to the location of the Chilean Maricunga and El Indio belts, and the gold-rich porphyry copper Bajo
de la Alumbrera in Argentina. Between 27º 30´ and 31º 30´ S, more than 500 Km along the high
Argentinian Andean Cordillera, numerous findings have been made in the 90s that now constitute
interesting prospects and deposits. This narrow belt contains mainly high sulfidation gold
epithermal prospects and deposits with minor low sulfidation types as well as gold-copper porphyry
mineralizations, that are considered to be the southern extension in Argentina of the Maricunga belt,
and the Argentinian counterpart of the El Indio belt. All these new Argentinian prospects together
with Chilean Maricunga and El Indio belts constitute a unique and continuous narrow belt of early
to middle Miocene age. With the eastward migration of volcanic activity in the flat-slab subduction
segment, other important districts occur in: Nevados del Famatina district with Cu-Mo-Au
porphyry and La Mejicana Cu-Au high sulfidation epithermal deposit; Farallón Negro district with
Bajo de la Alumbrera gold-rich porphyry copper, Farallón Negro-Alto de la Blenda low sulfidation
Au-Ag-Mn epithermal veins, and the easternmost Agua Rica Cu-Au-Mo porphyry and associated
Cu-Au high sulfidation epithermal vein system; and the San Luis metallogenic belt with La
Carolina district which hosts low sulfidation base metals sulphide-rich, gold and silver epithermal
deposits and prospects and Cañada Honda district with the Diente Verde gold-rich porphyry copper
and low sulfidation base metals sulphide-rich and gold epithermal deposits and prospects. In this
context, the San Luis metallogenic belt reflects the extraordinary broadening of the magmatic arc in
the flat slab segment at 33º S and resembles the situation of Farallón Negro district at 27º S. Both of
them are associated with high K calc-alkaline to shoshonitic magmas and exhibit enrichment in
LILE and LREE indicating an arc signature for active subduction-related magmas. The geochemical
affinity between San Luis and Farallón Negro trace-elements content is made evident in
overlapping fields of both suites of rocks in plots of Ba/La versus La/Yb and La/Sm versus Sm/Yb.
Both show similar low La/Yb ratios indicating a hornblende-dominated, garnet-free source and arc-
like Ba/La ratios (most higher than 20). The low abundance of HFSE indicating low degree of
HFSE fractionation is reflected in plots of light (La/Sm) versus heavy (Sm/Yb) REE ratios. La/Ta
ratios > 25 showing clear arc affinities for San Luis belt are however higher than those of Farallón
Negro. As a whole, the geochemical features of San Luis metallogenic belt and Farallón Negro
district are closely similar to each other suggesting that both were probably generated at similar
magma conditions. On the other hand, the unusual emplacement of Farallón Negro district and the
San Luis metallogenic belt far from the Chilean trench as well as the orientation of igneous districts
transverse to the orientation of the Andean magmatic arc at 27º and 33º S (particularly the 80 km
northwestern-striking San Luis belt), can be taken as a strong evidence to suggest discontinuities,
instabilities or zones of structural weakness. These zones controlled both magma ascent and
associated hydrothermal alteration and mineralization. Thus, the Farallón Negro and San Luis arc-
transverse magmatic trends by themselves constitute lineaments in the overriding plate that
probably reflect major crustal inhomogeneities. This model provides a plausible explanation for the
setting of both Farallón Negro district and San Luis belt, particularly for the latter which represents
the easternmost and youngest Tertiary volcanic and metallogenic event in the flat-slab segment.
