Tectonic Setting of Andean IOCG deposits: Upper crustal strike

XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
S9_043
Tectonic setting of IOCG deposits in the Central Andes:
Strike-slip-dominated deformation
Cembrano, J.1, Garrido, I.1, and Marquardt, M.1
(1) Minería Activa S.A., AV. Américo Vespucio Sur 340, Santiago, Chile.
cembrano@gmail.com
Introduction
A key question regarding the evolution of convergent margins is the nature of the link
between regional tectono-magmatic processes and ore deposit formation [1]. Although
several authors have proposed general metallogenic models for the genesis of IOCG
deposits [2, 3] only a few of them have somehow addressed the key role played by coeval
crustal deformation in the process of transport and emplacement/precipitation of
mineralizing fluids [4, 5, 6]. In this work we further assess the link between magmatic
arc tectonics and the nature, spatial distribution, and mineralization patterns of IOCG
deposits by looking at several case studies distributed throughout the Coastal Cordillera
of northern Chile.
Tectonic and geologic framework
The Coastal Cordillera of Northern Chile is characterized by widespread outcrops of
dioritic to monzodioritic plutonic rocks ranging in age from ca. 190 to 100 Ma [7, 8].
They intrude a thick pile of partly coeval volcanic and sedimentary rocks formed in an
intra-arc and/or back-arc environment, mainly represented by the Jurassic La Negra
Formation and the Early Cretaceous Chañarcillo and Punta del Cobre Groups [9, 10].
Plutonic and volcanic rocks are emplaced within and/or cut by regional-scale, arc
parallel, ductile to brittle shear zones that have been loosely regarded as the Atacama
fault system and its precursors.
The Atacama Fault System (AFS) is the most important structure of the Central Andes
forearc [7, 11]. It extends for ca. 1,000 km between Iquique (21ºS) and La Serena (30ºS).
The large-scale geometry of the AFS was formed during the late Jurassic and Early
Cretaceous towards the end of a Jurassic magmatic event that dominated most of the
present-day Coastal Cordillera. During the formation of the AFS, large brittle structures–
1
XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
more than 60 km in length– were formed by sinistral strike-slip movements [12, 13].
Some of the NS-striking master faults and subsidiary NW-striking splay faults are
organized into strike-slip duplexes that occur at various scales from regional to local [14,
15].
The geometry, kinematics and timing of deformation of the AFS are well known at the
regional-scale [7, 8, 11, 13, 14]. Although the kinematics and timing of deformation
varies along-strike, most authors agree that intra-arc sinistral transtensional to sinistral
transpressional deformation occurred during arc construction at least from 160 to 110 Ma
along most of the present-day Coastal Cordillera from Antofagasta to Vallenar. Absolute
age determinations of ductile deformation document coeval sinistral and dip-slip normal
displacement at around 130 Ma and sinistral displacement around 125 Ma [7, 8, 16].
Strike-slip tectonics, coeval with arc magmatism during the Late Jurassic–Early
Cretaceous, has been attributed to highly oblique subduction and strong plate coupling
following an increase in plate convergence rate. The Atacama Fault system sensu stricto
started at ca. 125 Ma when arc magmatism ceased at the present-day Coastal Cordillera
and progressive cooling led to more localized, sinistral strike-slip brittle deformation
expressed as NS and NW-striking fault networks.
Tectonics and mineralization
IOCG deposits throughout the Coastal Cordillera of northern Chile from Taltal to La
Serena share a remarkable structural pattern: they almost invariable occur in close spatial
and temporal association with NNW to WNW-striking sinistral strike-slip fault zones
(e.g. Candelaria, Carola, Manto Verde, Teresa de Colmo) with only a few occurrences
along NE-trending structures (e.g. Cerro Negro). Most of the NW-striking structures can
be regarded as second-order faults splaying off the margin-parallel master faults of the
AFS. However, a significant number of these faults are kinematically unrelated to the
AFS, showing evidence of long-lived displacement that pre-dates and outlasts that of the
AFS [8].
Iron-oxides and copper sulphides/oxides ore bodies consist of massive/banded veins and
mantos, hydrothermal breccia bodies, and vein-parallel striated brittle faults. As a whole
they form a structural mesh of primary and secondary discontinuities hosting variable
amounts of copper ore [17]. Kinematic indicators within the NW-striking fault-veinbreccia systems in most -if not all- of the IOCG deposits of the Coastal Cordillera of
northern Chile, suggest that mineralization was concomitant with sinistral and sinistraltranstension [4, 5, 18, 19, 20].
2
XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
The model
IOCG deposits in the Coastal Cordillera of northern Chile are closely spatially and
temporally associated with the transition from intra-arc ductile to brittle sinistral
deformation at ca. 124 Ma with a peak at about 118 to 105 Ma [18, 21, 22]. Cu-bearing
hydrothermal fluids associated with locally high-K magmatism channelled through NNW
and NW-trending old and newly created fractures that acted as Riedel shears and tension
cracks within an overall sinistral, NNE-trending, deforming arc-system. Some regionalscale Riedel shears seem to sole out from the Central Valley shear zone, a belt of
transpressional deformation, east of the AFS, which was active from ∼110 to 80 Ma.
Both Riedel shears and tension fractures not only served as channel-ways for fluid
migration but may have triggered episodic mineral precipitation either by fluid pressure
fluctuation and/or by mixing between fluids from different sources (e.g. basinal,
magmatic, meteoric). The process of IOCG mineralization likely occurred under
progressively more brittle conditions representing the last stages of hydrothermal activity
associated with a waning magmatic arc in the present-day Coastal Cordillera at ca.110
Ma.
References
[1] Richards, J., Boyce, A., Pringle, M. (2001) Geologic evolution of the Escondida area, northern
Chile: a model for spatial and temporal localization of porphyry Cu mineralization. Economic
Geology, vol. 96, 271-305.
[2] Barton, M., Johnson, D. (1996) Evaporitic-source model for igneous-related Fe oxide-(REECu-Au-U) mineralization. Geology vol. 24, 259-262.
[3] Hitzman, M. (2000) Iron Oxide-Cu-Au Deposits: What, Where, When, and Why; in Porter,
T.M. (Ed.). Hydrotermal Iron Oxide Copper-Gold y Related Deposits: A Global Perspective,
Australian Mineral Foundation, Adelaide, 9-25.
[4] Bonson, C., Grocott, J., Rankin, A. (1997) A structural model for the development of Fe-Cu
mineralization Coastal Cordillera, Northern Chile (25º15´-27º15´). VIII Congreso Geológico
Chileno 1997, vol. III, 1608.
[5] Sanhueza, A., Robles, W., Cembrano, J., Orrego, M., Pavez, A. (2000) Origen tardimagmático
de la magnetita en el distrito Manto Verde y su asociación a magmatismo sintectónico y
exhumación rápida en un duplex extensional de la Zona de Falla de Atacama. Actas del IX
Congreso Geológico Chileno, Puerto Varas, vol. II, 161.
[6] Arevalo, C., Grocott, J., Martin, W., Pringle, M., Taylor, G. (2006) Structural Setting of the
Candelaria Fe Oxide Cu-Au Deposit, Chilean Andes (27º30' S). Economic Geology, vol. 101, 1–
24.
[7] Scheuber, E.,Gonzales, G. (1999) Tectonics of the Jurassic-Early Cretaceous magmatic arc of
the north Chilean Coastal Cordillera (22°-26°S): A story of crustal deformation along a
convergent plate boundary. Tectonics, vol. 18, 895-910.
3
XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
[8] Grocott, J., Taylor, G. (2002) Magmatic arc fault systems, deformation partitioning and
emplacement of granitic complexes in the Coastal Cordillera, north Chilean Andes (25°30´S to
27°00´S). Journal of the Geological Society, London, vol. 159, 425-442.
[9] Godoy, E.,Lara, L. (1998) Hojas Chañaral y Diego de Almagro, Región de Atacama. Servicio
Nacional de Geología y Minería, Mapas Geológicos N. 5-6, 1 mapa escala 1:100.000, Santiago.
[10] Oliveros, V. (2005) Étude geochronologique des unités jurassiques et Crétacé Inférieur du
Nord du Chili (18º30'-24ºS, 60º30'-70º30'W): Origine, mise en place, altération, métamorphisme
et minéralisations associées. Thesis, Université de Nice-Sophia Antipolis and Departamento de
Geología, Universidad de Chile, Santiago, p. 305.
[11] Brown, M.; Diaz, F., Grocott, J. (1993) Displacement History of the Atacama Fault System,
25°-27°S, Northern Chile. Geological Society of American Bulletin, vol. 105, 1165-1174.
[12] Hervé, M. (1987) Movimiento sinistral en el Cretácico Inferior de la Zona de Falla de
Atacama al norte de Paposo (24ºS), Chile. Revista Geológica de Chile, vol. 31, 37-42.
[13] Scheuber, E., Andriessen, P. (1990) The kinematic and geodynamic significance of the
Atacama fault zone, northern Chile. Journal of Structural Geology, vol. 12, 243-257.
[14] Taylor, G.K.; Grocott, J.; Pope, A., Randall, D.E. (1998) Mesozoic fault systems,
deformation and fault block rotation in the Andean forearc: a crustal scale strike-slip duplex in the
Coastal Cordillera of northern Chile. Tectonophysics, vol. 299, 93-109.
[15] Cembrano, J., González, G., Arancibia, G., Ahumada, I. Olivares, V., Herrera, V. (2005)
Fault zone development and strain partitioning in an extensional strike-slip duplex: A case study
from the Mesozoic Atacama fault system. Tectonophysics, vol. 400, 105-125.
[16] Scheuber, E., Hammerschmidt, K., Friedrichsen, H. (1995) 40Ar –39Ar and Rb–Sr analyses
from ductile shear zones from the Atacama fault zone, northern Chile: the age of deformation.
Tectonophysics, vol. 250, 61–87.
[17] Sibson, R. H. (1996) Structural permeability of fluid-driven faultfracture meshes: Journal of
Structural Geology, vol. 18, 1031–1042.
[18] Vila, T., Lindsay, N., Zamora, R., (1996) Geology of the Mantoverde copper
deposit, northern Chile: A specularite-rich hydrothermal-tectonic breccias related to the Atacama
fault zone: Society of Economic Geologists Special Publication 5, p. 157–170.
[19] Correa, A. (2000) Geología del yacimiento de Fe-Cu Teresa de Colmo, Región de
Antofagasta, Chile. Actas 9th Congreso Geológico Chileno, vol. 2, 102–106.
[20] Orrego M, Zamora R (1991) Manto Ruso, un yacimiento de cobre ligado a la Falla de
Atacama, norte de Chile. Actas 6th Congreso Geológico Chileno, vol. 1, 174–178.
[21] Mathur, R., Marschik, R., Ruiz, J., Munizaga, F., Leveille, R.A., Martin, W. (2002) Age of
mineralization of the Candelaria Fe oxide Cu-Au deposit and the origin of the Chilean iron belt,
based on Re-Os isotopes. Economic Geology, vol. 97, 59–71.
[22] Pop, N., Heaman, L., Edelstein, O., Isache, C., Zentilli, M., Pecskay, Z., Valdman, S., Rusu,
C. (2000) Geocronología de las rocas ígneas y los productos de alteración hidrotermal
relacionadas con la mineralización de Cu-Fe (Au) del sector Adriana-Carola-Cobriza (parte este
del distrito Punta del Cobre-Candelaria), en base a dataciones U-Pb en circo´ n, 40Ar/39Ar y KAr. Actas 9th Congreso Geológico Chileno, vol. 2, 155–160.
4