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Miner Deposita (2006) 41: 246–258
DOI 10.1007/s00126-006-0055-9
ARTICLE
L. E. Ramírez .C. Palacios .B. Townley .
M. A. Parada .A. N. Sial .J. L. Fernandez-Turiel .
D. Gimeno .M. Garcia-Valles .B. Lehmann
The Mantos Blancos copper deposit: an upper Jurassic
breccia-style hydrothermal system in the Coastal
Range of Northern Chile
Received: 26 April 2005 / Accepted: 6 February 2006 / Published online: 22 April 2006
#Springer-Verlag 2006
Abstract The Upper Jurassic Mantos Blancos copper
deposit (500 Mt at 1.0% Cu), located in the Coastal Range
of northern Chile, displays two superimposed hydrother-
mal events. An older phyllic alteration probably related to
felsic magmatic–hydrothermal brecciation at ∼155 Ma, and
younger (141–142 Ma) potassic, propylitic, and sodic
alterations, coeval with dioritic and granodioritic stocks
and sills, and dioritic dikes. Main ore formation is
genetically related to the second hydrothermal event, and
consists of hydrothermal breccias, disseminations and
stockwork-style mineralization, associated with sodic
alteration. Hypogene sulfide assemblages show distinctive
vertical and lateral zoning, centered on magmatic and
hydrothermal breccia bodies, which constitute the feeders
to mineralization. A barren pyrite root zone is overlain by
pyrite-chalcopyrite, and followed upwards and laterally by
chalcopyrite-digenite or chalcopyrite-bornite. The assem-
blage digenite–supergene chalcocite characterizes the cen-
tral portions of high-grade mineralization in the breccia
bodies. Fluid inclusions show evidence of boiling during
the potassic and sodic alteration events, which occurred at
temperatures around 450–460°C and 350–410°C, and
salinities between 3–53 and 13–45 wt% NaCl eq.,
respectively. The hydrothermal events occurred during
episodic decompression due to fluid overpressuring,
hydrofracturing, and sharp changes from lithostatic to
hydrostatic conditions. Sulfur isotope results of hypogene
sulfide minerals fall in a narrow range around 0 per mil,
suggesting a dominance of magmatic sulfur. Carbon and
oxygen isotopic data of calcites from propylitic alteration
suggest a mantle-derived carbon and oxygen isotope
fractionation due to low-temperature alteration.
Keywords Cu mineralization .Upper jurassic .
Coastal range .Northern Chile
Introduction
This paper presents the results of a comprehensive and
updated study of the Mantos Blancos ore deposit, in the
Coastal Range of northern Chile (Fig. 1). Pre-mining
resources of this deposit are estimated at 500 million metric
tons with 1.0% Cu, of which 200 million tons were
extracted between 1960 and 2002 (Maksaev and Zentilli
2002). The remaining ore reserves stand at 142 million tons
with 0.86% Cu, and a resource of 156 million tons with
0.89% Cu (Anglo Base Metals Report, May 2003).
The Coastal Range is host to Upper Jurassic to Lower
Cretaceous copper deposits of volcanic-hosted strata-
bound type, and Cretaceous, generally heavily eroded
porphyry-type systems, which constitute a NS-trending
metallogenetic province (Camus 2003). The volcanic-
hosted strata-bound ore bodies are mainly associated with
hydrothermal breccia feeder structures, in which the
hydrothermal breccias contain at least 50% of the economic
Editorial handling: R. King
L. E. Ramírez (*).C. Palacios .B. Townley .M. A. Parada
Departamento de Geología, Universidad de Chile,
P.O. Box 13518-21, Santiago, Chile
e-mail: lramirez@cec.uchile.cl
Tel.: +56-2-9780233
Fax: +56-2-6963050
A. N. Sial
NEG LABISE Department of Geology,
Federal University of Pernambuco,
C. P. 7852,
Recife-PE, 50.732-970, Brazil
J. L. Fernandez-Turiel
Institute of Earth Sciences J. Almera, CSIC,
Sole i Sabaris,
08028, Barcelona, Spain
D. Gimeno .M. Garcia-Valles
Faculty of Geology, University of Barcelona,
Marti i Franques,
08028, Barcelona, Spain
B. Lehmann
Institut für Mineralogie und Mineralische Rohstoffe,
Technische Universität Clausthal,
Adolph Roemer Strasse 2 A,
38678 Clausthal-Zellerfeld, Germany
mineralization and the highest ore grades. The hydro-
thermal breccias are coeval with barren and generally
incipiently altered stocks and sills of mainly dioritic
composition, and are intruded by late mineralization
dioritic dikes.
Sulfide mineralization consists of chalcocite, digenite,
bornite, chalcopyrite, and pyrite related to sodic hydro-
thermal alteration (Palacios 1990; Wolf et al. 1990). Most
of these deposits are relatively small, with resources
between 10 to 50 million tons grading 1% Cu (Espinoza et
al. 1996). The porphyry-copper-type mineralization is
associated with granodioritic porphyries and hydrothermal
breccias, in which the hypogene mineralization consists of
chalcopyrite, pyrite, and minor bornite and molybdenite,
and occurs coeval with potassic and phyllic alteration
(Camus 2003).
The Mantos Blancos ore body, located 30 km NE of
Antofagasta, was described in the past as disseminated
copper mineralization in a bimodal rhyolite–andesite
sequence by Chávez (1985), but, in general, has been
considered as a strata-bound Cu deposit in recent reviews
(Espinoza et al. 1996; Maksaev and Zentilli 2002). No
detailed studies have been performed since 1985, when
Mantos Blancos comprised a series of open pits and
underground mines. During the past 20 years, the mine has
been transformed into a large open-pit operation, which
now provides much better geological exposures and more
detailed information.
The aim of this paper is to present new data on the
geology, hydrothermal alteration and mineralization, fluid
inclusions, and stable isotopes, and to discuss the
metallogeny and origin of the deposit.
Tectonic and geologic setting
During the Jurassic to Early Cretaceous, a subduction-
related magmatic belt was established along the present
Coastal Range of northern Chile. It is represented by a
7,000-m thick basaltic to andesitic volcanic pile (La Negra
Formation) and granitic to dioritic plutonic rocks. The
volcanic sequence evolved with time from an initial stage of
tholeiitic affinity to a calc-alkaline composition (Palacios
1984; Rogers and Hawkesworth 1989; Pichowiak et al.
1990; Kramer et al. 2005). Based on radiometric age data
and paleontological arguments, the extrusive event oc-
curred between the Lower Jurassic to the Oxfordian
(Rogers and Hawkesworth 1989; Gelcich et al. 2004;
Kramer et al. 2005). The Jurassic volcanic pile was
deposited without significant relief building, indicating
considerable crustal subsidence, probably related to crustal
thinning in an extensional setting (Dallmeyer et al. 1996;
Maksaev and Zentilli 2002).
The intrusive rocks, also of calc-alkaline composition,
include granites, tonalites, granodiorites, and diorites of
Lower Jurassic to Early Cretaceous age (200–130 Ma;
Scheuber and Gonzalez 1999; Oliveros 2005). Tectonic
evolution of the Coastal Range during the Jurassic is
interpreted in terms of coupling and decoupling between
the subducting oceanic and overriding continental plates
(Scheuber and Gonzalez 1999). From 195 to 155 Ma, an
intra-magmatic belt was widespread, spatially related to the
north–south trending, sinistral strike–slip dominant Ataca-
ma Fault Zone. However, at the end of Jurassic time, due to
foundering of the subducting plate, subduction rollback,
and decoupling, an east–west-trending extensional regime
developed. At the end of the Jurassic to the Early
Cretaceous, seismic coupling of the subducted plate is
suggested by the return of the sinistral strike–slip style of
deformation (Scheuber and Gonzalez 1999).
Geology of the deposit
Rock units recognized within the Mantos Blancos ore
deposit consist of a rhyolitic dome and its magmatic–
hydrothermal breccias, intruded by dioritic and granodi-
oritic stocks and sills. The dioritic and granodioritic stocks
Fig. 1 Geological map of the Coastal Cordillera, Northern Chile,
and location of the Mantos Blancos ore deposit (star) and the Upper
Jurassic volcanic-hosted copper deposits (diamonds). In grey are the
Middle to Upper Jurassic volcanic rocks of the La Negra Formation,
crosses represent Jurassic plutonic rocks. Modified after Maksaev
and Zentilli (2002)
247
locally grade upwards into magmatic–hydrothermal brec-
cias. These rock units are all mineralized to variable
degrees. Late mafic dikes crosscut all previously men-
tioned rock units and are essentially barren. All the above
rock units are informally grouped as the Mantos Blancos
Igneous Complex (MBIC; Fig. 2). The local structural
framework at deposit scale is characterized by three groups
of faults: 1) NE- and NW-trending subvertical faults with
evidence of sinistral and dextral movements respectively,
2) NS / 50–80° W normal faults, and 3) NS / 50–80° E
normal faults.
The MBIC consists of the following major rock units:
Rhyolitic porphyry dome
The central part of the deposit consists of a rhyolitic dome
(Figs. 2and 3). The dome structure is partially preserved in
the open-pit walls, but its geometry has been roughly
defined from drill core logs and samples of the early stages
of exploitation of the ore deposit (Chávez 1985), and later
lithological modeling. Due to pervasive alteration, the
contacts between different internal flows are very difficult
to observe; however, near-horizontal and vertical flow
laminations are typical, varying between 1 to 4 cm in
thickness. West of the pit, the felsic dome is intercalated
with felsic tuffs and andesitic lava flows, and is intruded by
dioritic and granodioritic sills. The rhyolitic dome consists
of a rhyolite porphyry with fragments of corroded quartz
and feldspar phenocrysts (1–5 mm) in an intensively
altered felsic groundmass.
Rhyolitic magmatic–hydrothermal breccia system
Several sub-vertical monomictic and matrix-supported
rhyolitic magmatic and hydrothermal breccia bodies, have
been recognized within the felsic dome intrusion (Figs. 2
and 3). They consist of irregular bodies, about 100 to 250 m
in vertical extent, and semi-oval to circular sections, 50 to
100 m in diameter. The matrix is composed of rhyolitic rock
flour with intense alteration and disseminated sulfide
minerals (Fig. 4a). The fragments are altered, irregular in
shape, poorly sorted, and vary in size between 1 cm and
several meters. In the centre of the ore deposit, the rhyolitic
magmatic and hydrothermal breccias are intruded by late
dioritic to granodioritic magmatic–hydrothermal breccias.
Fig. 2 Geological map of the Mantos Blancos ore deposit
248
Bimodal stock and sill system
The rhyolite dome is intruded by a subvolcanic complex of
porphyritic dioritic and granodioritic stocks and sills. At
least five gently dipping sills of both rock types occur in the
mine, varying in thickness between 10 and 50 m. The
feeder relationship between the stocks and sills has been
locally observed (Fig. 3). The granodiorite porphyry is
composed of 10 to 30% phenocrysts of hornblende,
plagioclase, quartz, and biotite, in a groundmass of quartz,
feldspars, biotite, and hematite microlites. The diorite
porphyry has 5 to 10% pyroxene and minor amphibole
phenocrysts in a groundmass of fine-grained pyroxene,
plagioclase, and magnetite. In both rock types, the
porphyritic texture grades to aphanitic near the intrusive
margins. The diorite porphyry has millimeter-size amyg-
dules filled with quartz and quartz-sulfide. Mutual intrusive
relationships between both granodioritic and dioritic rocks
are common, and enclaves of one in the other have been
frequently observed. The dioritic enclaves show convolute
to flame-like contacts (Fig. 4b) with the host granodiorite,
whereas, the granodioritic enclaves exhibit sharp or
brecciated contacts with the surrounding diorite. Back-
veining between the two lithological types is also observed.
Recent
40
Ar/
39
Ar data on amphibole provide ages of
142.18±1.01 Ma for the granodiorite, and 141.36±0.52 Ma
for the diorite (Oliveros 2005).
Dioritic to granodioritic magmatic–hydrothermal
breccia system
Two polymictic and matrix-supported pipe-like magmatic–
hydrothermal breccias hosted within the rhyolitic dome, at
the top of some dioritic and granodioritic stocks and
spatially related with NS-trending faults, are recognized
(Figs. 3and 4c−e). The central and largest breccia body is
crosscut by at least three metric-size sills; two dioritic and
one granodioritic in composition. The breccias form near-
vertical bodies, with a vertical extent of about 700 m, and
diameters between 100 and 500 m. It is likely that these
bodies did not reach the upper levels of the ore deposit, as
they were not observed and described in the earlier study
by Chávez (1985). The upper part of the breccia pipes
exhibit hydrothermal characteristics as evidenced by the
presence of a matrix mainly composed of hydrothermal
gangue and ore minerals. The breccia consists of altered
angular and subrounded fragments of the rhyolitic dome
and the granodioritic and dioritic porphyries. They are
poorly sorted and range in size from 1 cm to 15 m.
Downwards in the breccia bodies, magmatic features are
progressively evident, with granodioritic fragments in an
altered and mineralized dioritic matrix, as well as dioritic
fragments in a granodioritic matrix (Fig. 4f).
Mafic dyke swarm
Intruding all the rock units in Mantos Blancos deposit,
partially altered late-ore dioritic dikes were emplaced. They
are subvertical and have orientations preferentially NNE,
and subordinate NS–NNW. The dikes are 1 to 12 m wide
and represent about 15% of the total rock volume in the
deposit. They exhibit porphyritic texture, composed of 10–
25% phenocrysts of altered plagioclase, amphibole, and
minor pyroxene, in a very fine-grained groundmass of
feldspar, amphibole, and minor biotite and magnetite. An
40
Ar/
39
Ar date on amphibole from a late-mineral dike in the
mine is 142.69±2.08 Ma of age (Oliveros 2005).
Hydrothermal alteration and mineralization
Two hydrothermal events have been recognized, based on
the superimposition of alteration minerals and relationship
between different stages of veinlets. The first event is
represented by the rhyolitic magmatic–hydrothermal
brecciation hosted by the rhyolitic dome. The second
event, which represents the main stage of mineralization, is
hosted mostly within the dioritic to granodioritic mag-
matic–hydrothermal breccias, dioritic sills, and the rhyo-
litic dome, and may be genetically associated with the
intrusion of dioritic and granodioritic stocks.
Elevation (m.a.s.l)
Ore grade > 0.5% Cu
0 1 Km
cp-py
cp-py
cp-bor
cs-dig
E
W
1.000
800
600
400
200
0
cp-py
cp-py cp-bor cs-dig
cp-py
cp-py
cp-bor
cp-py
py
py
py
cp-py
py py
py
cp-py
Fig. 3 E–W profile of the Mantos Blancos ore deposit. For symbols, and location of profile see Fig. 2
249
First hydrothermal event
The first hydrothermal event is characterized by the
assemblage chalcopyrite, bornite, pyrite, quartz, and ser-
icite. This assemblage occurs: 1) disseminated in the matrix
of irregular and sub-vertical bodies of rhyolitic magmatic–
hydrothermal breccias, 2) planar veinlets, 3) disseminated
within the rhyolitic dome and in fragments of the
hydrothermal breccias, and 4) as isolated crystals or as
rim assemblages within and on quartz phenocrysts of the
rhyolitic dome. In the rhyolitic magmatic–hydrothermal
breccias, chalcopyrite and bornite are the most abundant
sulfides. Around these bodies the sulfides are chalcopyrite
and pyrite. The phyllic veinlets contain the sulfide minerals
as open space filling within fractures, and often display
weak alteration halos of sericite and quartz. Due to the
intense and widespread superimposition of the main
(second) hydrothermal event, it was not possible to
establish the extent and intensity of this first event. It
probably extended to all rocks of the rhyolitic dome. An
Fig. 4 Photographs of: arhyo-
litic magmatic-hydrothermal
breccia, bdioritic enclave with-
in the granodiorite showing
convolute contacts, c,d, and
edioritic to granodioritic mag-
matic-hydrothermal breccias in
which hydrothermal features
dominate, fdioritic to grano-
dioritic magmatic-hydrothermal
breccia with dominating mag-
matic features, and gpebble
dike
250
40
Ar/
39
Ar age on sericite from this first hydrothermal event
yields an age of 155.11±0.786 Ma (Oliveros 2005).
Second hydrothermal event
The main hydrothermal alteration and mineralization event
at Mantos Blancos is centered on the dioritic to
granodioritic magmatic–hydrothermal breccias and is
considered syngenetic with both breccia formation and
emplacement of the granodioritic and dioritic stocks and
sills. The mineralized zone extends discontinuously for
3kminanE–W direction, has a width of up to 1 km and
depth of 600 m. The hypogene mineralization occurs
between the elevations of 720 and 450 m asl. (Fig. 3).
Primary mineralization developed mainly within and
around the magmatic–hydrothermal breccia pipes, yet the
ore deposit exhibits a discontinuous lateral ore grade
distribution. The highest Cu grades occur within the
breccias with lateral zoning to progressively lower
concentrations. This fact suggests that the magmatic–
hydrothermal breccia pipes served as the feeder bodies of
the main mineralization.
In the second hydrothermal event, the early alteration
stage was potassic and propylitic, followed by sodic
alteration. The potassic and propylitic mineral assemblages
are centered on the dioritic to granodioritic magmatic–
hydrothermal breccias, affecting all lithologies of the
deposit. These alteration types developed pervasively,
disseminated, filling amygdules within the dioritic sills,
and as weak halos around flame-like veinlets that crosscut
the first generation phyllic veinlets in the rhyolitic dome.
The potassic alteration is characterized by K-feldspar,
quartz, tourmaline, biotite–chlorite, magnetite, chalcopy-
rite, digenite, and minor pyrite (Fig. 5). Relicts of K-
feldspar, tourmaline, and biotite are observed in most
locations, suggesting that potassic alteration was initially
widespread, but was subsequently overprinted and ob-
literated by later alteration stages. Dioritic and granodiorit-
ic sills, that contain amygdules filled with quartz, chlorite,
digenite, chalcopyrite, and traces of K-feldspar and
tourmaline, intruded the magmatic–hydrothermal breccias.
Propylitic alteration occurs extensively in the whole
deposit, affecting all of the rocks (including sills and
dikes), and overprinting and obliterating the potassic
alteration assemblage. It occurs as disseminations and
veinlets of quartz, chlorite, epidote, calcite, albite, sericite,
hematite and minor chalcopyrite, galena, and pyrite. These
minerals also fill amygdules within dioritic sills and dikes.
Laterally, propylitic alteration consists of quartz, chlorite,
epidote, and pyrite, forming a ring around the orebody at
least 2 km wide. From elevations of 600 m to the upper part
of the deposit, a swarm of N 25–30° E striking and sub-
vertical pebble-dikes have been observed. These pebble-
dikes are 10- to 20-cm thick and consist of rounded
fragments of the rhyolitic dome, dioritic and granodioritic
rocks, set in a matrix of quartz, epidote, calcite, galena, and
pyrite (Fig. 4g).
Both potassic and propylitic alterations were followed
by sodic alteration, containing albite (replacing feldspar),
hematite, pyrite, chalcopyrite, and Ag-rich digenite, with
minor amounts of quartz. This mineral assemblage is very
extensive, centered on the magmatic and hydrothermal
breccias, and occurs as disseminations, cavity fillings, and
sharp veinlets. Sodic alteration and mineralization affected
all lithological types between elevations of 500 m to the
surface and spatially coinciding with the current commer-
cial ore zone. Above the elevation of 500 m, the dioritic
sills that intruded the magmatic–hydrothermal breccias
exhibit intense stockwork with a sodic alteration mineral
assemblage. As the syn-mineralization granodioritic and
dioritic stocks and sills have been dated at 142.18±1.01 and
141.36±0.518 Ma (Oliveros 2005), respectively, and a late-
ore dike yields an age of 142.69±2.083 (Oliveros 2005),
the age of the main hydrothermal event is constrained
between 141 and 142 Ma.
Supergene oxide mineralization has been mined, with
only patches of atacamite, chrysocolla, and malachite
remaining. This supergene mineralization was described in
detail by Chávez (1985). Although he reported primary
chalcocite (late within the hypogene assemblage), our data
indicate the presence of only secondary chalcocite (Fig. 6).
The secondary sulfides are mainly chalcocite (forming
zones of high-grade copper mineralization centered over
the magmatic–hydrothermal breccia bodies, with bornite–
digenite), and weak layers of covellite, together with
cuprite-native copper and tenorite.
Fluid inclusion studies
Fluid inclusion studies were carried out on quartz crystals
of the second hydrothermal event. Samples include quartz
crystals from potassic, propylitic, and sodic veinlets, and
HYDROTHERMAL EVENTS
MINERALS First Second
Phyllic Potassic Sodic Propylitic
Quartz
Sericite
K-feldspar
Biotite
Tourmaline
Chlorite
Albite
Epidote
Calcite
Pyrite
Magnetite
Hematite
Chalcopyrite
Bornite
Digenite
Galena
Magmatic and Rhyolitic Dioritic and granodioritic
hydrothermal dome and stocks and sills, brecciation
events brecciation and dike intrusion.
Fig. 5 Hypogene mineral assemblage of the hydrothermal events at
the Mantos Blancos ore deposit
251
from potassic and propylitic amygdules of the dioritic sills
and stocks. A total of 23 samples were taken from the
central part of the deposit (Fig. 7), from which 153
microthermometric measurements of primary inclusions
were done. Vertical sampling extends to a depth of 850 m.
Heating and freezing experiments were conducted on a
Linkam THMS600 stage for homogenization temperatures
(T
h
) up to 450°C and on a Linkam TS1500 stage for T
h
above 450°C. The uncertainty for heating runs is about
±2°C at 400°C.
Three fluid inclusion types were recognized, following
the classification scheme of Nash (1976): I (liquid-
dominant inclusions without halite daughters), II (vapor-
dominant inclusions without halite daughters), and IIIb
(vapor-dominant inclusions with halite daughters). All
fluid inclusions types have mostly rounded shapes and
ranged from 5 to 15 μm. No evidence was observed for
either liquid CO
2
or clathrate formation, freezing point
depression measurements rule out the presence of signif-
icant CO
2
. Apparent salinities are reported in weight
percent NaCl equivalent (wt% eq.), based on the halite
solubility equation for halite-saturated inclusions and on
the final ice-melting temperature for halite-undersaturated
inclusions (Bodnar and Vityk 1994). The fluid inclusion
microthermometric data are presented in Table 1and Fig. 8.
The highest temperatures were measured in types II and
IIIb inclusions trapped in quartz from veinlets of the
potassic alteration assemblage within the matrix of the
magmatic–hydrothermal breccia at elevations between 239
and 260 m. The type-II inclusions homogenize between
550 and 608°C and have salinities of 9.9 to 10.1 wt% NaCl
eq., whereas, the IIIb-type inclusions have T
h
values
between 530 and 590°C and salinities ranging from 52 to
74 wt% NaCl eq. The coexistence of both types of
Fig. 6 Microphotographs of
adigenite relict in chalcocite,
band cdigenite with hematite
flakes replaced by chalcocite,
dchalcocite with inclusions of
hematite flakes, echalcopyrite
replaced by covellite (blue), and
fnative copper in cuprite (red
internal reflections in grey) with
replacement rim of tenorite
S
Elevation (m)
1.000
900
800
700
600
500
300 600 m0
400
300
200
100
0
N
cp-py
cp-cs-dig
cp-dig
cp-py
cp-py
cp-py
cp-py
Ore grade > 0.5% Cu
Q-103
CP-1-15
CP-1-2
CP-1-22
Q-10 Q-7
Q-8
Q-3
Q-4
Q-5
Q-6
Q-2
Q-9
Q-100 Q-101
Q-102
Q-12Q-13
Q-11
Q-1
Q-1-1
Q-105
Q-104
Fig. 7 N–S profile of the Mantos Blancos deposit showing the location samples used in the fluid inclusions study. For symbols, and location
of profile, see Fig. 2
252
Table 1 Microthermometry data of fluid inclusions from the second hydrothermal event
Sample Elevation
(m.a.s.l.)
Size
(μm)
Th (L-v)
(°C)
Th (Halite)
(°C)
%L %V
(%in)
%
Halite
Tm (ice)
(°C)
Salinity (wt%
NaCl equiv)
Remarks N° of
inclusions
Q-1 239 5–8 601±7 24±9 76±9 −6.5±0.5 9.9±0.7 Veinlets of K-assemblage
in MHB
5
239 5–9 500±20 580±10 10±5 30±4 60±5 71±3.0 5
Q-104 247 8–10 505±15 20±5 20±5 −18.0±2 19.4±3.0 Veinlets of K-assemblage
in sill of dioritic porphyry
9
Q-1-1 260 5–10 564±14 550±20 23±8 77±8 −6.7±0.8 10.1±1.0 Veinlets of K−assemblage
in MHB
5
260 5–10 490±10 20±10 20±10 60±10 62±10.0 7
Q-105 260 8–10 465±12 19±6 81±5 −15.0±3.5 18.5±3.0 Veinlets of K-assemblage
in sill of dioritic porphyry
5
Q-2 684 5–10 390±12 449±20 11±4 51±6 38±4 52.4±1.6 Veinlets of K-assem-
blage in MHB
5
6–10 462±8 15±10 85±10 −1.5±0.5 2.5±0.8 3
Q-3 684 5–8 404±6 464±6 10±2 50±10 40±8 53.5±0.5 Veinlets of K-assem-
blage in MHB
3
10 455±6 10±5 90±5 −2.0±1 3.3±2.5 2
Q-100 720 5–10 413±13 20±10 80±10 −19.4±1.4 22.2±10 Amygdules filled by K-
assemblage in dioritic sill
5
Q-101 720 10–15 380±15 25±10 75±10 −19.4±1.4 22.1±10 Amygdules filled by K-
assemblage in dioritic sill
5
Q-4 696 8–10 302±16 349±26 15±6 50±5 35±8 42.2±1.9 Veinlets of Albitic as-
semblage in matrix of
MHB
6
8–10 357±23 10±6 90±6 9.9±0.9 13.9±1.1 5
Q-5 696 8 349±20 349±20 6±5 60±10 35±5 42.3±1.6 Veinlets of Albitic as-
semblage in MHB
2
8–15 346±6 9±3 90±7 −9.4±1.2 13.4±1.4 5
Q-6 696 7–10 362±8 10±5 90±5 13.2±1.8 Veinlets of Albitic
assemblage in MHB
5
Q-7 708 7–10 356±11 8±2 92±2 −9.7±1.2 14.0±1.4 Veinlets of Albitic
assemblage in MHB
5
Q-8 720 8–10 376±25 413±2 10±4 50±2 40±6 47.8±0.3 Veinlets of Albitic
assemblage in MHB
3
5–15 351±23 10±5 90±5 −8.8±1.8 12.6±2.2 3
Q-9 720 8 371 423 8±2 50±4 42±5 48.7 Veinlets of Albitic
assemblage in MHB
1
8–10 313±15 11±7 89±4 −8.5±1.0 12.3±1.3 5
Q-103 768 5–10 358±3 75±10 25±10 −12.5±5.0 15.3±2.5 Veinlets of K-assem-
blage in sill of dacitic
porphyry
6
Q-10 720 8–10 301±1 90±5 10±5 7.1±0.1 10.6±1.0 Veinlets of Propylitic
assemblage in sill of
dioritic porphyry
2
CP-1-22 760 8–12 218±25 65±8 35±8 −19±6.8 20±2.4 Amygdules in dioritic
porphyry filled by Pro-
pylitic assemblage
11
Q-11 780 8–15 269±11 70±10 30±10 −6.6±0.6 9.8±0.9 Veinlets of Propylitic
assemblage in RPD
4
Q-12 780 7–12 249±5 68±12 32±12 −7.9±1.3 12.0±2.4 Veinlets of Propylitic
assemblage in RPD
5
Q-102 792 8–10 335±5 90±4 10±6 −10.5±0.5 14.5±0.5 Veinlets of Propylitic
assemblage in sill of
dioritic porphyry
2
253
inclusions within the same growth zone of a quartz crystal,
is considered as indicative of deposition from boiling
fluids. In these brines, T
h
(halite) values are at least 60°C
greater than T
h
(l−v) values in the same samples (Fig. 9).
Fluid inclusion observations of samples from potassic
alteration assemblages at an elevation of 684 m also display
evidence of boiling: Type-IIIb inclusions have T
h
values
between 449 to 464°C and salinities between 52.4 and 53.5
NaCl eq., and co-exist with vapor-rich type-II inclusions
(with T
h
between 462 and 415°C, and salinities between
2.5 and 3.3 wt% NaCl eq.). Also in these brines, T
h
(halite)
values are at least 65°C greater than T
h
(l-v) values in the
same samples. Quartz crystals from potassic alteration
assemblage in amygdules and veinlets from sills in the
diorite contain type I and II inclusions. In these samples, T
h
values decrease systematically with an increase in elevation
(from an average of 515°C at 360 m to 365°C at 720 m). In
contrast, salinities remain relatively constant (19–22 wt%
NaCl eq.). Fluid inclusions associated with propylitic
alteration assemblages have been measured in samples
from elevations of 720 to 816 m. They correspond to type-I
inclusions, in which T
h
values vary between 340 and 150°C
and salinities between 9 and 22 wt% NaCl eq.
Fluid inclusions in quartz related to the sodic assemblage
were difficult to measure due to the limited amounts of
albite-bearing quartz veinlets. Fluid inclusions in quartz
obtained from these veinlets in the matrix of the magmatic-
hydrothermal breccia at elevations between 696 and 768 m,
are mainly of types II and IIIb. Evidence of boiling has
been recognized at elevations of 696 to 720 m asl, in which
both types of inclusions coexist in growth zones of similar
hydrothermal quartz crystals. The brines have T
h
values
between 349 and 423°C and salinities ranging between 42
and 48 wt% NaCl eq., whereas, the vapor-rich-two phase
inclusions have T
h
values between 313 and 364°C and
salinities between 13 and 14 wt% NaCl eq. Brines in the
Sample Elevation
(m.a.s.l.)
Size
(μm)
Th (L-v)
(°C)
Th (Halite)
(°C)
%L %V
(%in)
%
Halite
Tm (ice)
(°C)
Salinity (wt%
NaCl equiv)
Remarks N° of
inclusions
Q-13 792 8–10 247±3 70±5 30±5 −6.5±0.5 9.8±0.7 Veinlets of Propylitic
assemblage in RPD
6
CP-1–15 816 7–11 187±35 65±10 35±10 −8.8±5.6 12±5.1 Amygdules in dioritic
porphyry filled by Pro-
pylitic assemblage
6
CP-1–22 816 6–9 318±15 80±10 20±10 −10.1±1.3 14.1±1.2 Veinlets of Propylitic
assemblage in dioritic
porphyry
2
Th (L+v) Liquid-Vapor homogenization temperature, Th (Halite) halite dissolution temperature, Tm (ice) melting temperature of ice, % L,V,
Halite abundance of phases at room conditions, MHB magmatic and hydrothermal breccia, RPD rhyolitic porphyry dome
Table 1 (continued)
200
Salinity ( wt% NaCl eq.)
250 300 400 500350 450 550 600
20
10
30
40
50
60
70
Potassic alteration
Sodic alteration
Propylitic alteration
Th (˚C)
Fig. 8 Homogenization temperature vs salinity of fluid inclusions
Fig. 9 Halite dissolution temperature versus liquid-vapor homog-
enization temperature of boiled fluid inclusion samples from
potassic and sodic alteration
254
same sample exhibit halite dissolution temperatures greater
than the vapor homogenization temperatures.
Stable isotope studies
Sulfur
Seventeen sulfide samples from the second hydrothermal
event were analyzed for δ
34
S at the Scientific-Technical
Services of the University of Barcelona. Sulfide samples
were separated mechanically to obtain splits with 50–80 μg
of sulfur. Between 100 and 300 μg of pure sulfide were
mixed with V
2
O
5
(1:1), homogenized and packed into
high-purity tin cups. The sulfur isotopic composition was
analyzed using a Continuous Flow-Isotope Ratio Mass
Spectrometry (CF-EA-IRMS). Samples were combusted in
an elemental analyzer (Carlo Erba EA 1108) connected to a
Finnigan MAT Delta C gas mass spectrometer via a
Finnigan MAT Conflo II interface. Results are expressed in
the per mil notation relative to the international Vienna-
Canyon Diablo troilite (VCDT) standard. The reproduc-
ibility of measurements was ±0.3‰. The δ
34
S values of 11
samples of pyrite, five samples of chalcopyrite, and one
sample of digenite are reported in Table 2and Fig. 10. All
samples were taken in the central part of the deposit,
between elevations of 450 and 780 m asl. The analyzed
sulfides exhibit δ
34
S values ranging from −5 to 1.2 per mil,
with a mean value of −1.4‰and a standard deviation of
1.8‰. Results are similar to those previously reported by
Sasaki et al. (1984) and Vivallo and Henriquez (1998).
Pyrite shows the widest sulfur isotope range in comparison
to the Cu-sulfides, and the variation is independent of
alteration types or host rock lithology (Fig. 10).
Carbon and oxygen
Eighteen calcite samples were analyzed for δ
13
C and δ
18
O
at the stable isotope laboratory (LABISE) of the Depart-
ment of Geology, Federal University of Pernambuco,
Brazil. CO
2
gas was extracted from micro-drilled powder,
in a high-vacuum line after reaction with 100% orthophos-
phoric acid at 25°C for 1 day. CO
2
released, after cryogenic
cleaning, was analyzed in a double inlet, triple collector
SIRA II mass spectrometer. Results are reported relative to
PDB, in per mil notation. The uncertainties of the isotope
measurements were better than 0.1‰for carbon and 0.2‰
for oxygen, based on multiple analyses of an internal
laboratory standard (BSC). Values of δ
13
C and δ
18
Oof
calcite samples from propylitic alteration stage (of the
second hydrothermal mineralization event) are reported in
Table 3and Fig. 11. All samples were taken in the central
part of the deposit, between elevations of 172 and 900 m
asl. The carbon isotope values of calcites vary between
−4.37 and −6.71‰, whereas, the δ
18
O values fluctuate
between 13.08 to 23.49‰.
Discussion
Based on available radiometric ages and geological
observations described in this study, the Mantos Blancos
ore deposit was formed by two superimposed Upper Jurassic
hydrothermal events. The older event occurred at ∼155 Ma,
coeval with the rhyolitic magmatic–hydrothermal breccia-
tion and phyllic alteration. The younger event represents the
main hydrothermal mineralization (∼141–142 Ma) and is
genetically related to dioritic and granodioritic stocks and
sills and coeval magmatic–hydrothermal brecciation. Prob-
ably, both hydrothermal events contributed to extensive but
irregularly distributed ore grades of hypogene mineraliza-
Table 2 Sulfur isotope of sul-
fides from the main hydrother-
mal event at the Mantos Blancos
ore deposit
a
Hydrothermal alteration stage
associated with the analyzed
sulfide
b
Host rock of the sulfide
MHB Magmatic Hydrothermal
Breccia
Sample no. Mineral δ
34
S
CDT
(‰) Hydrothermal alteration
a
Lithology
b
M-25 Pyrite −2.0 Propylitic Granodiorite
CPM-54 Pyrite −1.9 Potassic Diorite
CP-122 Pyrite −2.6 Sodic Diorite
CPM-53 Pyrite −4.0 Propylitic Rhyolitic dome
M-3 Pyrite 1.2 Propylitic MHB
M-4-A Pyrite 0.7 Propylitic MHB
BC-708 Pyrite −0.1 Potassic MHB
P-2-1 Pyrite −0.3 Potassic MHB
C-684 Pyrite −1.1 Potassic MHB
N-684 Pyrite −1.2 Potassic MHB
M-24 Pyrite −5.0 Propylitic MHB
M-25 Chalcopyrite −2.1 Propylitic Granodiorite
CPM-54 Chalcopyrite −0.5 Potassic Diorite
CPM-54a Chalcopyrite −2.0 Potassic Diorite
CPM-53 Chalcopyrite −4.5 Potassic Rhyolitic dome
BC-708 Chalcopyrite −1.3 Potassic MHB
CPM-54a Digenite −3.2 Potassic Diorite
255
tion. High-ore-grade mineralization is restricted to the upper
part of the magmatic–hydrothermal breccias from the second
hydrothermal event. The radiometric ages for the two
hydrothermal events reported by Oliveros (2005) agree with
previous
40
Ar/
39
Ar (total gas in albite) and whole rock Rb–Sr
(errorchrons in strongly altered samples) radiometric ages
(150–146 Ma; Munizaga et al. 1991; Tassinari et al. 1993).
The younger event is characterized by three types of
alteration and mineralization: an early potassic, a propyl-
itic, and a late sodic stage. The potassic and propylitic
alteration stages occurred coeval with dioritic and
granodioritic porphyry stock intrusions, magmatic–hydro-
thermal breccias and late sill and dike emplacements. The
late sodic alteration that developed centered around the
magmatic–hydrothermal breccias, associated with intense
fracturing and brecciation (including in the sills) and the
main mineral deposition. The ore grade, alteration, and the
copper sulfide mineral zoning indicate that the magmatic–
hydrothermal breccia bodies represent the feeders to the
hydrothermal system. The hydrothermal activity, was
followed by the intrusion of a dioritic dike swarm. An
indication of local subsidence is the common occurrence of
sills intruded by vertical dikes as part of the same magmatic
event. Because the magmatic pressure must exceed the
least main horizontal stress and the tensile strength of the
rock cover to form discordant intrusions, these intrusive
PDB
Fig. 11 δ
13
C(‰)vsδ
18
O(‰) diagram showing the distribution of
calcites from the Mantos Blancos ore deposit. Fields and arrows
after Taylor et al. (1967) and Keller and Hoefs (1995)
Table 3 C and O isotope analyses (‰) of calcites from the Mantos
Blancos ore deposits
Sample
18
O
SMOW
(‰)
18
O
PDB
(‰)
13
C
PDB
(‰)
56-585 14.98 −15.40 −6.16
56-590 17.42 −13.04 −6.69
VB-1 18.74 −11.71 −5.50
97-230 23.49 −7.14 −6.58
VB-2 17.60 −12.86 −5.36
06-268 13.27 −16.44 −5.13
06-335 15.87 −14.54 −6.27
BC-1 13.91 −16.44 −5.13
33-200 16.72 −13.71 −6.91
33-257 20.81 −9.75 −5.72
33-288 19.87 −10.66 −4.37
33-298 13.08 −17.25 −6.02
DV-1 14.59 −15.78 −5.09
1-14B 16.51 −13.92 −6017
696-41 13.88 −16.47 −6.17
1-14C 16.68 −13.75 −5.42
CPM1-21 16.85 −13.60 −4.75
Fig. 10 δ
34
S(‰) values of sulfides from the main hydrothermal
event at the Mantos Blancos ore deposit (a). Diagrams band cshow
the types of alteration and host rock, with which the sulfides are
related
256
relationships between sills and dikes are an indication that
sufficiently thick magmatic overburden was progressively
formed to produce a change of the least principal stress
from vertical to horizontal (Parada et al. 1997). As this sill–
dike relationship has been observed at Mantos Blancos, it is
suggested that the tectonic setting during mineralization
corresponded to a local extensional regime, probably
related to a transtensional faulting within the Atacama
Fault System.
Evidence of boiling associated with potassic alteration
has been found in samples up to an elevation of 684 m asl.
At this elevation, fluid inclusions T
h
values exceed 450°C.
At such temperatures, rocks in the hydrothermal system
behave in a ductile manner: with strain rates smaller than
10
−14
/s, rocks of dioritic or granodioritic compositions
behave quasiplastically, making brittle fracturing difficult
and allowing fluid pressure to approach lithostatic values
(Fournier 1991,1999). As a consequence, the magmatic–
hydrothermal breccias most likely did not reach the
paleosurface, and the hydrothermal system mostly formed
at lithostatic pressure. The hydrothermal fluids within the
magmatic–hydrothermal breccias evolved along a cooling
trend, as indicated by the fluid inclusion data in quartz of
the propylitic assemblage.
The emplacement of dioritic and granodioritic sills
crosscutting the magmatic–hydrothermal breccias at dif-
ferent levels, sealed the hydrothermal system, over-
pressured the fluids, hydrofractured the rocks, and
produced the sodic boiling. The thermodynamic evolution
of brine into the field of gas+solid salt at 350–400°C
(conditions under which sodic alteration associated boiling
occurred), has important implications regarding the con-
centration of HCl that may be transported when and if
steam escapes into the overlying rocks. Fournier and
Thompson (1993) noted an abrupt increase in the concen-
tration of HCl° in steam when NaCl begins to precipitate at
pressures below 300 bars. This increase occurs because
hydrolysis reactions that produce HCl° and NaOH by the
reaction of NaCl with H
2
O become important only at
pressures sufficiently low for halite (and probably also
NaOH) to precipitate (Fournier and Thompson 1993). In
addition, an order of magnitude higher than HCl° concen-
tration is obtained at comparable pressures and tempera-
tures when quartz is present. This occurs because quartz
reacts with NaOH to form albite at the expense of K-
feldspar or plagioclase (Fournier and Thompson 1993).
The limited amounts of quartz-bearing albite veinlets in the
deposit support this model.
In addition, as fluids migrated away from the early heat
source (the magmatic–hydrothermal breccias) and down a
thermal gradient, K-feldspar was the stable alteration
mineral, as reflected by potassic alteration. The reverse
reaction operated when fluids migrated away from a
second heat source (intrusion of sills), conditions under
which the albite stability field expanded at the expense of
K-feldspar (Hezarkhani et al. 1999; Simmons and Browne
2000). Both processes probably occurred at Mantos
Blancos, in which the entire evolution points to a prograde
(potassic and propylitic)–retrograde (sodic) hydrothermal
sequence. These results can be interpreted as boiling events
and associated decompression occurring episodically due
to fluid over-pressuring, hydrofracturing, and sharp chang-
es from lithostatic to hydrostatic conditions.
The sulfur isotopic results from hypogene sulfides
suggest a largely magmatic source for sulfide sulfur and
indicate a co-genetic relationship for the analyzed sulfide
minerals. C–O isotopes in fresh calcite crystals reported in
this paper suggest C of magmatic origin, probably of
mantle provenance (Cartigny et al. 1998), and fractionation
of O following the trend of low-temperature alteration
caused by magmatic–hydrothermal fluids.
Acknowledgements This study was funded by a FONDEF
(CONICYT, Chile), grant DO1-1012, awarded to the authors and
the Mantos Blancos division of Anglo American Chile. Permission
for publication was granted by the University of Chile, the Chilean
Government, and AngloAmerican Chile. We thank the Mantos
Blancos mine geology staff, especially to Jorge Pizarro, with whom
we had the pleasure of working. Special acknowledgement to Jens
Wittenbrink for his constructive comments to the manuscript. Finally,
this paper was improved through the valuable reviews of Shoji
Kojima, Robert King and Larry Meinert.
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