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Short Papers – IV South American Symposium on Isotope Geology
NEW U-Pb AGES FOR HOST ROCKS, MINERALIZATION AND ALTERATION OF IRON
OXIDE (Cu-Au) DEPOSITS IN THE COASTAL CORDILLERA OF NORTHERN CHILE
Gelcich, S.H.1; Davis, D.W.2 and Spooner, E.T.C.3
1. SERNAGEOMIN (Chile) - University of Toronto, Ontario, Canada. gelcich@geology.utoronto.ca
2. Earth Science Department, Royal Ontario Museum, Toronto, Ontario, Canada
3. Department of Geology, University of Toronto, Ontario, Canada
Keywords: Iron oxide deposits, U-Pb dating, Coastal Cordillera Northern Chile, Manto Verde, Carmen
INTRODUCTION
The Coastal Cordillera of northern Chile is a very
fertile province in terms of Fe-oxide deposits, including
Kiruna type and hydrothermal systems rich in Fe, Cu and
Au. One of the main metallogenic features of the Coastal
Cordillera in northern Chile is the Chilean Iron Belt
(CIB), a 600 km N/S belt that includes ~ 50 iron deposits
(Nystrom & Henríquez, 1994). These deposits are of
massive magnetite–apatite type; they occur as veins,
irregular and pseudo stratiform bodies and are always
hosted in volcanic and/or plutonic suites that record
magmatic activity in the early Andean Stages (JurassicEarly Cretaceous). A number of hydrothermal iron oxide
Cu-Au deposits are spatially related to the pure iron
deposits; e.g. Manto Verde (Vila et al., 1996; Zamora &
Castillo, 2001). The age of the mineralization and related
alteration is poorly constrained.
The Coastal Cordillera in the area of Chañaral (Fig. 1)
has been the subject of recent studies including: regional
geology (Godoy & Lara, 1998: Lara & Godoy, 1998);
geochronology (Dallmeyer et al., 1996; Gelcich et al.,
2002); structural geology and batholith architecture
(Brown et al., 1993: Grocott & Taylor, 2002). This work
presents new preliminary U-Pb ages of intrusive rocks
and related mineralization in two areas in the Coastal
Cordillera of Chañaral: Manto Verde and Carmen (Fig.1).
METHODS
U-Pb analyses were carried out at the Royal Ontario
Museum. Zircons were dissolved in teflon bombs
according to methods described in Krogh (1973, 1982).
Apatites and titanites were dissolved in Savillex vials. U
and Pb were separated using HCl for zircon analysis
Krogh (1973) and HBr for titanite and apatite (Corfu,
1988). Precise ages on Mesozoic minerals must rely on
the 206Pb/238U-decay system. All errors are given at 2
sigma levels. Current Pb blank levels of less than 1
picogram permit precise analyses of fractions consisting
of one (zircons) or several grains (titanite and apatites).
The isotopic composition of initial Pb was estimated
using the model of Stacey & Kramers (1975). Titanite
and apatite ages are especially sensitive to the inferred
initial Pb isotopic composition. Further analyses are
required on low U/Pb coexisting mineral phases in order
to get a better estimation of the common lead component.
RESULTS AND DISCUSSION
New ages have been obtained in the Manto Verde
(Cu-Au) and Carmen (Fe) areas. The new record includes
U-Pb zircon ages for spatially associated plutons and
minor stocks and U-Pb mineralization ages from titanite
(Manto Verde) and apatite (Carmen).
CARMEN MINE
Carmen is an irregular magnetite body elongated in a
west-east direction, with a length of about 550 m, a width
of between 30 and 50 m and a depth greater than 250 m
(Henríquez et al., 1991). It is hosted by volcanic rocks of
the La Negra Formation affected by contact
metamorphism related to the intrusion of the early
Cretaceous Sierra Aspera pluton (131.3 ± 0.4 Ma (Fig.
2a); Gelcich et al., 2002). Mineralization in Carmen
consists of magnetite with variable contents of apatite and
actinolite. This mineral suite forms massive and breccia
type bodies (Henríquez et al, 1991). Apatite from a distal
magnetite-apatite dyke located 500 m south of the main
pit in Carmen was dated at 129.8 + 3.0 Ma (Fig. 2b); this
is the first reported dating of apatite associated with
massive magnetite ore in the CIB. The 3-m wide dyke is
subvertical and has a general NS strike. It shows some
banded intergrowth textures of magnetite and wedgeshaped apatites. The dyke has fine grained borders, and
magnetite stockworks developed as halos in the volcanic
host rock. Zircon from a mineralized quartz diorite stock,
in the northern pit of Carmen Mine was dated at 130.6 +
0.3 Ma (Fig. 2c). These two new ages are in the range of
zircon U-Pb ages in the Sierra Aspera pluton (Gelcich et
al., 2002).
MANTO VERDE
Manto Verde is the largest and the main economic
deposit in the area of Chañaral. Cu mineralization is
present mainly as oxides, products of oxidation of
original Cu sulphide hypogene mineralization in
specularite-rich hydrothermal breccias. At deeper levels
the main iron oxide in association with Cu sulphide
mineralization is magnetite (Zamora & Castillo, 2001).
Cornejo et al. (2000) inferred a series of metasomatichydrothermal stages for the hydrothermal alteration
assemblages at Manto Verde, starting with potassic
metasomatism, followed by hydrothermal magnetitesulphide, hematite-sulphide stages, and a later carbonate
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Short Papers – IV South American Symposium on Isotope Geology
phase. The host rock in Manto Verde is mainly composed
of andesitic lavas of the La Negra Formation that are
intruded by a number of small dykes. These rocks define
a structural block limited by the central and eastern
branches of the Atacama Fault System. The Manto Verde
structural Block is bounded on the west by the Las Tazas
North Pluton (134.1 + 0.4 Ma; Gelcich et al., 2002) and
on the east by the Sierra Dieciocho Pluton (129.6 + 0.3
Ma, Fig. 2d; Gelcich et al., 2002). A primary zircon U-Pb
crystallization age of 128.9 + 0.6 Ma (Fig. 2e) was
obtained for a quartz monzodioritic to granodioritic dyke
(felsic intrusive or “pseudo aplite” in the terminology of
Zamora & Castillo, 2001).
Figure 1. Geological map of the Coastal Cordillera of Chañaral, with the location of Manto Verde and Carmen.
The main plutons are labeled. From Godoy & Lara, 1998; Lara & Godoy, 1998.
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Short Papers – IV South American Symposium on Isotope Geology
Figure 2. U-Pb concordia diagrams. Carmen ages: A) zircon age of the Sierra Aspera pluton (Gelcich et al., 2002); B) apatite age
from magnetite-apatite south dyke; c) quartz dioritic host rock in Carmen north pit. Manto Verde ages. D) zircon age of Sierra
Dieciocho Pluton (Gelcich et al., 2002); E) zircon age of the “Felsic Intrusive”; F) titanites associated with potassic alteration of the
“Felsic intrusive”. Apatite and titanite ages should be considered preliminary, see text for discussion.
This unit shows pervasive potassic alteration (Stage I
in the Cornejo et al., 2000, model), including titanite as a
major phase. According to Cornejo et al. (2000) titanite
represents the alteration product of primary ulvöspinel,
with a consequent liberation of Fe to the hydrothermal
fluids, which finally precipitated as iron oxides
accompanying the main sulphide mineralization. Titanites
were dated giving a concordant age of 126.4 + 0.5 Ma
(Fig. 2f). U-Pb data from this area define a closely spaced
succession of thermal events. They indicate that
mineralization in Manto Verde is most probably related
to the cooling and differentiation of the Sierra Dieciocho
pluton.
Both massive magnetite-apatite (Carmen) and
Hydrothermal Fe-Cu-Au (Manto Verde) deposits show
clear temporal relationships to stocks or dykes that
intruded volcanic cover sequences. These later intrusive
units represent more differentiated phases that can be
temporally and compositional related to the main plutonic
units in the area. Thus, Fe oxide mineralization in the
coastal Cordillera can be genetically related to the
extensive Jurassic to Early Cretaceous plutonism of the
Coastal cordillera.
ACKNOWLEDGEMENTS
We acknowledge Richard Zamora and Boris Castillo
for access to samples from Manto Verde, and
Sernageomin for their field logistics. Special thanks to
Mr. Bob McRae for his essential financial support.
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