1
Acta geológica lilloana 30 (1): 122, 30 de junio de 2018
Petrography, geochemistry and geochronology of San
Jorge porphyry Cu-Au deposit, Mendoza, Argentina.
Constraints for the timing of magmatism and
associated mineralization
ä
Abstract The San Jorge porphyry Cu-Au deposit, Argentina, integrates the Paleozoic to
Early Jurassic metallogenic belt recognized in the Southern Andes. San Jorge is currently the
only deposit considered economically viable due to its supergene enrichment. Previous studies
have reported Middle to Upper Permian ages (257-270 Ma) for the intrusion-mineralization
processes. Granite porphyry, two granodiorite porphyries (G1 and G2) and an alkali-feldspar
granite dike were recognized. These granites intrude sedimentary rocks (Yalguaráz Formation)
that hosts the main mineralization. Two tourmaline alteration events occurred prior to the
potassic episode. They are overprinted by phyllic alteration that has an elongated shape with
potassic mineralized cores. The mineralization is linked to the potassic alteration in the sedi-
mentary sequence and in the granite porphyry that has the highest Cu contents (1% to 3%).
The phyllic alteration has pyrrhotite, arsenopyrite and minor chalcopyrite. Digenite, chalcocite
and covellite appear in the supergene; malachite and brochantite in the oxide; and goethite,
hematite and jarosite in gossan zones.
The granitic porphyry, granodiorite porphyries G1 and two potassic alteration biotites are
of Early Permian age. This event produces the potassic alteration and mineralization and is
15 Ma older than the ages determined by other authors. The granodiorite porphyry G2 and
the alkali feldspar granite dike are Upper Permian in age. The results suggest that the evolu-
tion of the western margin of Gondwana began in late Carboniferous and continued during
the Permian period. In this Paleozoic to early Jurassic metallogenic belt, La Voluntad Cu-Mo
porphyry is the oldest deposit (early Pennsylvanian, Garrido et al. 2008). Mineralized and no
mineralized San Jorge granites (Early Permian-Late Permian) integrate the intrusive event of
the Choiyoi Group.
Keywords: San Jorge copper-gold deposit-petrography-geochemistry-
40
Ar/
39
Ar and U-Pb
geochronology.
ä
Resumen Petrografía, geoquímica y geocronología del pórfido Cu-Au San Jorge,
Mendoza, Argentina. Límites para el magmatismo y la mineralización asociada. El depósito
de pórfido Cu-Au de San Jorge, Argentina, integra el cinturón metalogénico del Paleozoico
Mirta M. GARRIDO
1
, Laura E. GRECCO
2
, María V. GONZALEZ
1
,
Cecilia M. PAVON PIVETTA
1,2
1
Departamento de Geología. Universidad Nacional del Sur (UNS). Bahía Blanca. Argentina.
Correo electrónico: mgarrido@criba.edu.ar, violeta.gonzalez@uns.edu.ar, cecilia.pavon@uns.edu.ar
2
Instituto Geológico del Sur (INGEOSUR). Depar tamento de Geología (UNS). Bahía Blanca. Argentina.
Correo electrónico: mlgrecco@criba.edu.ar
ä Ref. bibliográfica: Garrido, M. M., Grecco, L. E., González, M. V. y Pavón Pivetta, C. M. (2018).
Petrography, geochemistry and geochronology of San Jorge porphyry Cu-Au deposit, Mendoza, Argen-
tina. Constraints for the timing of magmatism and associated mineralization. Acta geológica lilloana
30 (1): 1-22
ä Recibido: 11/11/17 Aceptado: 27/03/18
ä URL de la revista: http://actageologica.lillo.org.ar
ä Algunos derechos reser vados. Esta obra está bajo una Licencia
Creative Commons Atribución No Comercial Sin Obra Derivada 4.0 Internacional.
D . O . I . : h t t p s : / / d o i . o r g / 1 0 . 3 0 5 5 0 / j . a g l / 2 0 1 8 . 3 0 . 1 / 1
2
M. M. Garrido et al.: Petrography, geochemistry and geochronology of San Jorge porphyry...
al Jusico Temprano, reconocido en los Andes del Sur. San Jorge es actualmente el único
depósito considerado económicamente viable debido a su enriquecimiento supergénico. Estu-
dios previos dieron edades del Pérmico Medio a Superior (257-270 Ma) para los procesos
de intrusión-mineralización. Se reconocieron pórfidos de granito, dos pórfidos de granodiorita
(G1 y G2) y un dique de granito alcali-feldespático. Estos granitos intruyen rocas sedimentarias
(Formacn Yalguaráz) que alberga la principal mineralizacn. Dos eventos de turmalinizacn
ocurrieron previos al episodio potásico. Están sobreimpuestos por una alteración fílica que
tiene una forma elongada con núcleos mineralizados posicos. La mineralización está ligada
a la alteracn potásica en la secuencia sedimentaria y en el pórfido de granito que tiene los
mayores contenidos de Cu (1% a 3%). La alteración fílica contiene pirrotina, arsenopirita y
menor calcopirita. Digenita, calcosina y covelina aparecen en la zona supernica; malaquita
y brochantita en la de óxidos; y goethita, hematita y jarosita en las zonas gossan. El pórfido
granítico, el pórfido de granodiorita G1 y dos biotitas de alteración potásica son de edad
Pérmico inferior. Este evento produce alteración potásica y mineralización y tiene 15 Ma más
que las edades determinadas por otros autores. El pórfido de granodiorita G2 y el dique de
granito álcali-feldestico son de edad Pérmico superior. Se propone un mínimo de 2 Ma de
edad para la duración del sistema hidrotermal.
Los resultados sugieren que la evolución del margen occidental de Gondwana comenzó a
finales del Carbofero y continuó durante el período Pérmico. En este cinturón metalogénico
del Paleozoico al Jurásico Temprano, el pórfido La Voluntad Cu-Mo es el depósito más antiguo
(Pensilvaniano temprano, Garrido et al., 2008). Los granitos mineralizados y no mineralizados
de San Jorge (Pérmico Temprano-Pérmico Superior) integran el evento intrusivo del Grupo
Choiyoi.
Palabras clave: Depósito de Cu-Au San Jorge, petrograa, geoquímica, geocronología de
40
Ar/
39
Ar y U-Pb.
INTRO D UCTION
Porphyry copper deposits of similar age
from the Andes in western South America
have been grouped into a series of metal-
logenic belts. These NS belts are a few km
wide and several km long and occur parallel
to the Pacific margin. The San Jorge por-
phyry Cu-Au deposit, Argentina, is one of
13
porphyry Cu systems in the Paleozoic to
Early Jurassic metallogenic belt (Fig. 1) rec-
ognized so far in the Southern Andes (Camus
2003; Sillitoe and Perelló 2005). San Jorge is
currently the only porphyry Cu-Au deposit in
the Paleozoic Belt considered economically
viable. R
esources are estimated at 194.5 Mt
with 0.48% Cu, and 0.21 g/t Au with a cut-
off grade of 0.3% Cu (Compañía Minera San
Jorge-Coro Mining Corporation 2008). The
supergene enrichment gives an economic
possibility horizon to this deposit.
The purpose of this paper is to provide
new geological information on the deposit
and to better constrain the age of the San
Jorge porphyry system using more reliable
and robust isotopic systems.
The new geological information enables
a detailed petrologic-geochemical study on
the magmatism linked to the mineralization
in an attempt to understand its tectonic for-
mation environment. The granite porphyry
and alkali feldspar dike rocks analysed in
this work are comparable to those described
by Williams et al.
(19
99) as granodiorite
porphyry and the andesitic porphyry dike
respectively. Here we use U-Pb zircon dat-
ing to determine the age of crystallization
of intrusive bodies and a precise
40
Ar/
39
Ar
technique on magmatic and hydrothermal
biotite to better estimate the duration of
the magmatic-hydrothermal process. Previ-
ous geochronological studies on the deposit
have reported Middle to Upper Permian
ages. A magmatic biotite from a granodiorite
stock yielded a K-Ar age of 270±4 Ma (Sil-
litoe 1
977),
whereas a hydrothermal biotite
from a porphyritic andesite dike was dated
using the K-Ar method at 266 Ma. Ad-
ditionally, sericite from a quartzite sample
yielded a K-Ar age of 257±5 Ma (Williams
et al. 1
999).