Porphyry Copper Metallogenesis Along the Middle Eocene

UNIVER SIDAD DE CONCEPCIÓN
DEPARTAMENTO DE CIENCIAS DE LA TIERRA
10° CONGRESO GEOLÓGICO CHILENO 2003
Porphyry Copper Metallogeny of the Middle Eocene-Early Oligocene Arc of Western South
America: Relationships with Volcanism and Arc Segmentation
Constantino Mpodozis1 and José Perelló2
Sipetrol. Vitacura 2736, Santiago, Chile (cmpodozis@sipetrol.cl); 2Antofagasta Minerals. Ahumada 11, Of. 602. Santiago, Chile
1
There is consensus that segmentation was characteristic of the middle Eocene-early Oligocene arc of western South America,
between central Peru and southern Chile and Argentina. For example, it is apparent that magmatism was discontinuous along the
arc and that volcanism, in particular, was important in certain segments, whereas in others it did not occur at all. While in central
Peru Incaic volcanic activity was significant (Lower Tacaza Volcaanics), the segment between southern Peru (south of the
Abancay Deflection) and latitude 28ºS in northern Chile was characterized by overall subdued volcanism. Middle Eocene to early
Oligocene magmatism in this segment is restricted to a narrow belt south of Cuzco (Andahuaylas-Yauri batholith: 48-32 Ma; Anta
Fm: 38-30 Ma) and to a string of Eocene (45-32 Ma) plutons and porphyry stocks that extend for >800 km along the Precordillera
and Cordillera de Domeyko, between 20° and 28° S. Clusters of intrusive rocks at Quebrada Blanca-Collahuasi, Chuquicamata-El
Abra, Telégrafo-Centinela, Escondida, Sierra Exploradora, and Potrerillos are characteristic of this segment and are separated by
relatively amagmatic zones. Coeval volcanism was minor and restricted to a few dome centers and related pyroclastic rocks as at
Cerro Casado (44-37 Ma) in the area of Telégrafo-Centinela and Quebrada La Majada (37-35 Ma) at Sierra Exploradora. Timeequivalent, synorogenic continental sedimentation occurred in transtensional and contractional basins all along the arc, from
southern Peru (San Jerónimo and Puno Gps), through western Bolivia (Lower Tiwanaku, Berenguela, Turco and Potoco Fms), to
northern Chile (Sichal and Loma Amarilla Fms) and northwestern Argentina (Geste and Quiñoa Fms). Importantly, this segment
contains many giant porphyry copper and porphyry-related skarn systems (Tintaya and Antapaccay, Peru; Collahuasi, El Abra,
MM, and El Salvador, Chile), including two of the world´s largest deposits at Chuquicamata and Escondida.
Middle Eocene to early Oligocene (45-36 Ma) volcanism seems to have increased significantly south of 27º, being amply
distributed along the Chile-Argentina border east and southeast of Maricunga (Astaburuaga Fm) and further into Argentina,
between 28° and 30°S at Río Blanco-Macho Muerto and Valle del Cura regions (Peña Negra Fm; Valle del Cura tuffs).
Magmatism on the Chilean side, where erosion has been much more severe, is represented by a series of scattered porphyry stocks
emplaced in Paleozoic basement units (Bocatoma unit in the El Indio area; 39.5-31 Ma). As opposed to its northern counterpart,
this segment is characterized by minor, small-size porphyry copper mineralization (Loica; Apolinario), although the larger La
Fortuna cluster (~35 Ma), located at the transition with the northern segment, is an exception. Further south of latitude 32º, in
central Chile, Eocene volcanism has not been positively identified, although vertebrate paleontological evidence together with a
late Eocene age of 36 Ma obtained from the basal parts of the Coya-Machalí (=Abanico) Fm suggest that at least part of this thick
volcanic sequence may have started accumulating earlier than previously thought. Southernmost Eocene arc lavas, represented by
the Colipilli volcanics (44-39 Ma) in the Andes of Neuquén, Argentina, may constitute the extension of the basal Coya-Machalí
horizons, thereby confirming widespread volcanic activity during the middle to late Eocene. This southern segment of the arc is
significantly barren of porphyry copper mineralization.
Despite geological uncertainties and exploration maturity between segments, it is apparent that giant porphyry copper formation
along the arc took place only in those segments characterized by overall subdued coeval volcanism. By analogy with modern arcs,
along-the-arc magmatic segmentation during the middle Eocene to early Oligocene is primarily attributed to the changing
geotectonic regimes associated with varying subduction configuration. Fission track studies of barren and mineralized stocks of
the northern segment indicate high exhumation rates in response to rapid surface uplift and concomitant erosion, in turn, in
response to the high-strain conditions imposed by the Incaic orogenesis. This setting is further interpreted to have been the
product of slab flattening along the entire segment, presumably in response to accelerated convergence rates between the Farallon
and South America plates. Increasing subaerial volcanism further south required the presence of more relaxed crustal conditions,
perhaps in response to a gradually steepening subducted slab. Under conditions of severe crustal thickening associated with Incaic
deformation in southern Peru and northern Chile, the magmas responsible for porphyry copper genesis were ultimately generated
at depths of ~50 km. Shortening of the upper crust would have impeded rapid magma ascent and venting, and favored efficient
magmatic evolution and fluid storage in large, confined magma chambers. Similarities with giant porphyry copper formation of
the Paleocene-early Eocene belt in southern Peru are also apparent.
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