Download Tectonic Setting of Andean IOCG deposits: Upper crustal strike

XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
S11_058
IOCG-type deposits in north-central Chile:
A case study and implications for exploration
Marquardt, M.1, Cembrano, J.1, Siña, A.1 and Garrido, I.1
(1)Minería Activa S.A., AV. Américo Vespucio Sur 340, Santiago, Chile.
[email protected]
Introduction
IOCG-type deposits have been widely described for the Coastal Cordillera of northern
Chile [1], where they appear to have a strong spatial and temporal relationship with the
Atacama fault system [2, 3, 4, 5]. In contrast, only a few IOCG deposits have been
reported and examined in detail south of La Serena, where there are a number of such
occurrences [6, 7, 8]. This contribution presents and discusses the geology and structure
of the Toro District, located east of the city of Illapel, in the internal domain of the
Coastal Cordillera of this part of the Central Andes.
Geology
The Toro District emplaced within Early Cretaceous andesitic volcanic unit, interbedded
with volcaniclastic rocks. This unit is part of the Cretaceous Quebrada Marquesa Fm.,
which, at the regional scale, is intruded by coeval/younger mid-Cretaceous plutonic rocks
range from monzodiorite to tonalite, some of them magnetite-bearing [9]. Field mapping
shows that the wallrocks strike NNW and dip shallowly to the east, exhibiting a minimum
thickness of 350 m. The most important lithology corresponds to a 200 m-thick
volcaniclastic breccia unit, which hosts most of the copper mineralization in the Toro
area. Two regional, NS-striking, subparallel strike-slip faults flank the project area to the
west and east respectively: the El Espino and Manquehua faults. These master faults are
linked by a series of NW-striking, second-order, sinistral strike-slip faults making up a
duplex structure that largely dominate the local structure (Fig. 1-a).
Mineralization and alteration
Copper oxide and sulphide mineralization and hydrothermal alteration are strongly
spatially associated with the Las Lajas-Herminia fault-vein system (LHVS; Fig. 1-b); a
1
XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
1.5 km-long by 0.2 km-wide structural mesh of NW-striking faults, fault-veins and
hydrothermal breccia, located at a dilatational jog between two overstepping, NNWstriking, sinistral faults. Mineralization occurs as copper oxides, copper pitch/wad and
limonite largely related with quartz and specularite; sulphide relics (chalcopyrite and
minor pyrite, subordinate gold) are widely distributed at surface outcrops. Bornite has
been locally recognized.
Main alteration paragenesis associated with mineralization breccias within the LHVS
consists of strong, locally pervasive chlorite (±sericite±epidote±quartz). Quartz-sericiteclay associations occur marginally around the main mineralized breccia body, along with
jarositic limonite suggesting shallow-level leaching. Surrounding the described
mineralization and alteration there is a widespread epidote-chlorite-hematite
(±albite±calcite) alteration all along the LHVS, fading away from it.
Chip sampling from mineralized and/or hydrothermally altered rocks of the Toro District
consistently yields copper grades values over 0.1%, including numerous samples from
apparently barren rock. About one third of the samples exhibit copper grades over 1%
and as high as 5%; preferentially localized at both extremes of the target zone, as in Las
Lajas and Herminia mines. These results, along with the field-supported structural model
of a high-fluid-flow dilatational jog strongly suggest the possibility of having a sulphidebearing mineralized breccia at depth (1-1.5% Cu). Significant gold contents, ranging
from 0.1 to 0.7 g/ton have been found marginally around the main mineralized breccia
body and along numerous peripheral veins and fault-veins.
Conclusions
Ore deposits at Toro District, consisting of veins-fault-veins, breccias bodies and
widespread hydrothermal alteration zones are closely spatially and temporally associated
with subvertical, NNW and NW-oriented second-and-third-order strike-slip faults that
splay off regional NS-trending, sinistral master faults (Fig. 1). Mineralization was
synkinematic with early to mid-Cretaceous (?) sinistral and sinistral-transtensional
deformation. This tectono-magmatic setting is similar to that described for these type of
deposits in the Coastal Cordillera of northern Chile and therefore has strong implications
for exploration within a new Andean segment (e.g. IOCG: El Espino; Cu-Ag: Tres
Valles).
Acknowledgements
We would like to thank Jorge Gonzalez and Roberto Naranjo for assistance in the field.
Alejandra Vilches helped preparing the figures and maps. This project was conducted
while the authors worked for Central Andes Resources.
2
XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
References
[1] Sillitoe, R. (2003) Iron oxide-copper-gold deposits: an Andean view. Mineralium
Deposita, N°. 38. p. 787–812.[2] 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, p. 1608.
[3] 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, p. 161.
[4] 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, pp. 1–24.
[5] Cembrano, J., Garrido, I., Marquardt, M. (this congress) Tectonic Setting of IOCG
deposits in the Central Andes: Strike-slip-dominated deformation. XII Congreso
Geólogico Chileno, Santiago, 2009.
[6] Lopez, G.P., Van Treek, G., Travisany, V., Richard, M., Hitzman, M., Nelson, E.
(2005) Calcic alteration related to iron oxide-copper-gold (IOCG) mineralization, El
Espino prospect, central Chile: Window to the World, Program with Abstracts,
Geological Society of Nevada Symposium 2005, Reno/Sparks, Nevada, May 2005, p. 62.
[7] Lopez-Orrego, G., Van Treek, G., Travisany, V., Hitzman, M., Nelson, E., Richard,
M. (2006) Regional alteration in the El Espino iron oxide copper gold district, coastal
Cordillera of northern Chile: Geological Society of America, Abstracts with Programs, v.
38, no. 7, p. 373.
[8] Correa, A. (2003) El Espino, un nuevo depósito del tipo Fe-Cu (Au) en Chile.
Comuna de Illapel, Región de Coquimbo. Tesis de Magister, Universidad Católica del
Norte, Antofagasta, Chile, p. 69.
[9] Rivano, S., Sepúlveda, P. 1991 Hoja Illapel. Servicio Nacional de Geología y
Minería, Carta Geológica de Chile, No. 69, 132 p., 1 mapa escala 1:250.000.
3
XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
-------------------------Coastal Cordillera------------------
a
Manquehua
Fault
b
El Espino
Fault
NW-striking, sinistral
strike-slip faults and
splays
Target zone
Figure 1: (a) Regional structural framework for the IOCG-type deposits along the Coastal
Cordillera of north-central Chile south of La Serena. (b) Inset shows the LHVS within the
Toro District, mineralized breccias bodies are represented in light green.
4