Northern Chile

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doi: 10.1111/rge.12129 Resource Geology Vol. 67, No. 2: 197–206
Original Article
Geological and Geochemical Characteristics of the Intrusion-
Related Vein-Type Gold Deposits in the El Morado District,
Coastal Cordillera, Northern Chile
Shoji KOJIMA, Iván SOTO, Milenka QUIROZ, Paulina VALENCIA and Iván FERNANDEZ
1 Departamento de Ciencias Geológicas, Universidad Católica del Norte, Chile
Abstract
El Morado is the only auriferous district in the north Chilean coastal Cordillera, and has breccia-associated
Au–Fe–Cu vein-type gold deposits, such as Beatriz and Lilianita. In addition to geological and mineralogical
descriptions, geochemical characterization of the two vein deposits, and fluid inclusion and oxygen isotope
analyses of the Lilianita quartz vein were performed with the objective to elucidate their ore-forming
characteristics.
The two deposits are hosted in Late Jurassic to Early Cretaceous dioritic intrusions, and are characterized by
the oxidized mineral association of electrum (Au 75 Ag 25 to Au 85 Ag 15 ), chalcopyrite, hematite and magnetite.
Gold content of vein samples weakly correlates with Ag contents, but is not correlative with Cu contents. Fluid
inclusion data of the vein quartz indicate relatively high-trapping temperature (290 to 340°C) and low salinity
(3.2 to 13.1 wt% NaCl) conditions with low-pressure boiling evidence. The δ 18 O values of the corresponding
quartz samples are in a narrow range of +11.1 to +12.5 ‰. These data combined with the fluid inclusion
thermometric data suggest that the quartz-mineralizing fluid with δ 18 O values between +4.9 and +6.2 ‰
was derived from primary magmatic water with lesser amounts of low-temperature surface water, such as
seawater or meteoric water. All these results show that the gold deposits in the El Morado district are
intrusion-related oxidized veins, which formed from magmatic-hydrothermal fluids at relatively lowpressure
shallow conditions, compared to the Cu-rich iron oxide–copper–gold (IOCG) vein deposits in the same
province.
Keywords: El Morado district, Lilianita gold deposit, coastal Cordillera, intrusion-related oxidized vein, fluid
inclusions, oxygen isotope compositions.
1. Introduction
The north Chilean coastal Cordillera (22°S to 24°S) is a
famous province that has many Manto-type and iron
oxide–copper–gold (IOCG) vein-type copper deposits
(Boric et al., 1990; Espinoza et al., 1996; Sillitoe, 2003).
Noticeably, within the Cordillera, only the El Morado
district has intrusion-hosted gold-bearing quartz vein
deposits, such as Beatriz and Lilianita. Detailed mineralogical
and geochemical studies of these gold deposits
in the El Morado district have not yet been performed,
although several metallogenetic studies were made for
the Manto and vein-type IOCG deposits (e.g., Kojima
et al., 2003; Tristá & Kojima, 2003). Furthermore, it is
Received 26 July 2016. Revised 17 October 2016. Accepted for publication 13 November 2016.
Corresponding author: S. Kojima, Departamento de Ciencias Geológicas, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta,
Chile. Email: skojima@ucn.cl
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S. Kojima et al.
not clearly shown which type of gold deposit the
Beatriz and Lilianita correspond to, nor how the gold
deposits of the El Morado district were generated in
the coastal Cordillera. This study presents geologic
and geochemical features of the Beatriz and Lilianita
gold deposits, particularly in relation to the chemical
data of the two gold–quartz veins, as well as fluid
inclusion and oxygen isotope data of the Lilianita
gold–quartz vein, to elucidate metallogenic characteristics
of the El Morado gold district and to estimate the
origin of the Lilianita vein-forming ore fluid.
2. Outline of geology and the deposits
The eastern part towards the Mejillones Peninsula in
northern Chile is composed of the three districts,
Naguayán, El Desesperado and El Morado (Fig. 1),
where more than 30 small copper mines are known
(Boric et al., 1990). These mines have generally highgrade
copper averages up to 4 wt% Cu, and have been
exploited since the 1850s (Arce, 1930). Among these,
only the district El Morado (23°16.5´S, 70°21´W), which
is located approximately 40 km north of Antofagasta
City, has small vein-type gold deposits. Since the late
19th century, these deposits were exploited, and had a
maximum 75 g/t Au and an average grade of 20 g/t
Au in their prosperous period (Boric et al., 1984, 1990).
After the Second World War the gold deposits were
developed intermittently, but are closed now.
The El Morado district is mainly composed of
thick Jurassic andesite strata (termed the La Negra
Formation), Late Jurassic to Early Cretaceous granitic
intrusions, Cretaceous conglomeratic sediments
(termed the Caleta Coloso Formation) and Cenozoic
alluvial covers (Fig. 1). The La Negra Formation
consists mainly of monoclinal andesitic to basaltic
andesite lavas with lesser amounts of tuff breccias,
sandstone and limestone (García, 1967; Boric et al.,
1990; Kojima et al., 2003). The Late Jurassic to Early
Cretaceous granitic intrusions are plutonic complexes
of holocrystalline gabbroic to granitic units of the
magnetite-series and calc-alkaline suite (Ishihara et al.,
1984; Fernandez, 2004). The Cerro Fortuna intrusion
body covering the El Morado district is variable in composition,
comprising of diorite and tonalite to granodiorite
units (González, 1996). The hornblende–biotite
diorite to quartz diorite unit as the host rock was dated
Fig. 1 Simplified geologic map around the El Morado district of the coastal Cordillera in Antofagasta Region (from Boric
et al., 1990; Ordenes, 2002).
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El Morado vein-type gold deposits
at a Late Jurassic age of 153 ± 4 Ma, using the K–Ar
method on biotite (Cortés et al., 2007). Furthermore,
several age data of the associated microdioritic dikes
and dioritoid stocks are obtained as follows (Maksaev,
1990; Scheuber, 1994; González, 1996; Cortés et al.,
2007): 148.5 ± 1 Ma (Ar–Ar method on hornblende),
148 ± 4 Ma, 145 ± 3 and 138 ± 3 Ma (K–Ar method on
biotite), and 147 ± 6 Ma and 145 ± 5 Ma (K–Ar method
on amphibole), which correspond to latest Jurassic
to Early Cretaceous ages. The microdioritic dike is
traversed by lower zone of the Beatriz vein, and this
means that the vein mineralization occurred after the
latest Jurassic dike emplacement. The host rocks,
particularly on the hanging wall side, experienced
Ca–Mg metasomatic propylitic alteration with epidote,
chlorite and actinolite, as well as argillic alteration and
silicification which only occur locally and adjacent to
the principal veins.
The structural framework of the district is dominated
by two major faults, the Aeropuerto and Mititus
Faults, which are developed in parallel to the Atacama
Fault running in a NNE direction. Both faults are
considered as sinistral strike-slip faults formed during
transtensional oblique subduction initiated at the
Jurassic to Early Cretaceous ages (e.g., Scheuber &
Andriessen, 1990). In addition, the Mitutus Fault indicates
a recent reactivation as a normal fault, but the
Aeropuerto Fault has no indication of such a feature.
The Beatriz and Lilianita veins, which are the larger
gold deposits in the district, are hosted in the Cerro
Fortuna dioritic unit of Late Jurassic to Early Cretaceous
age. The two deposits are composite veins,
including the dioritic gangue rocks. The Beatriz vein
has a width of between 1.3 and 2.6 m to a depth of at
least 35 m from the surface, displaying a N32°W trend
with 49°NE dip at a lower zone (331 m high) and a
N20°E trend with 79°SE dip at an upper zone (355 m
high). The Lilianita vein has a width of between 0.8
and 2 m to a depth of at least 25 m, and a N26°Wtrending
orientation with 81°NE dip. Both veins represent
a strike length of more than 100 m and possibly up
to 500 m, and have several modes of hydrothermal
breccias. Their vein mineralization, based on modes
of occurrences and microscopic observations, is divided
into the following five stages: (i) early stage; (ii)
main stage; (iii) late stage; (iv) late barren stage; and
(v) post-mineralization supergene stage (Fig. 2). The
early stage is characterized by voluminous milkyquartz
vein with sericite and its breccias, including
small amounts of hematite, magnetite, rutile and chalcopyrite.
Among them, rutile is probably a mineral of
Fig. 2 Paragenetic sequence of the El Morado vein
mineralizations.
host rock residue, because this also occurs in the host
rock as an accessory mineral. The main stage is represented
by crystalline quartz matrix with an intense
brecciation of earlier-stage veins, with varying abundances
of such constituent minerals as quartz, sericitic
clay, Fe oxides and jarosite. The vein mineralogies
of the two deposits are very similar, containing ore
minerals of hematite, electrum, chalcopyrite and
magnetite. Electrum occurs as discrete grains with size
of <190 μm in the vein quartz. Small amounts of quartz
veinlets with coarse-grained (0.1–2 mm) specular hematite
occur in the late stage, and are cut by translucent
quartz veinlets as the late barren stage. The postmineralization
stage is characterized by supergene alteration
of primary iron minerals into jarosite, goethite
and hematite. In the Beatriz vein an appreciable difference
in metallic mineral abundance is represented in
the aforementioned two levels. In the upper zone, the
metallic minerals are relatively abundant, and small
amounts of atacamite and chrysocolla occur as
secondary copper oxides.
3. Samples and analytical methods
3.1 Sample selection
A total of 51 vein samples that were collected from the
main drifts and discarded ore heaps, were first studied
under the reflected-light polarizing microscope to
examine paragenetic relationships of ore minerals. On
the basis of these observations, samples were selected
for vein chemistry, fluid inclusion, and oxygen isotope
studies. Among the 51 samples, 21 representative specimens
(Beatriz: B1 to B11, Lilianita: A1 to A10) were
supplied for bulk chemical analyses. Among the 10
Lilianita specimens, three samples (A4 to A6) were
used for SEM-EDS analysis of electrum grains.
Microthermometric analyses were made for seven
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S. Kojima et al.
Lilianita samples (Nos. ML-02, 03, 05, 06, 07, 08, 09) of
the main-stage crystalline quartz, which have primary
fluid inclusions of sufficient sizes for measurements.
Furthermore, six quartz specimens except ML-08 were
also provided for oxygen isotope analysis. Crystalline
and pure portions of these quartz specimens were
carefully separated, and finely powdered in an agate
mortar.
3.2 Analytical methods
Major element (Al, Fe, Mg, Ca, Na, K, Ti, and P), and 32
minor and trace element (Ag, As, Au, Ba, Be, Bi, Ce,
Cd, Co, Cr, Cu, Hf, La, Li, Mn, Mo, Nb, Ni, Pb, S, Sb,
Sc, Sn, Sr, Ta, Th, U, V, W, Y, Zn, and Zr) analyses of
principal vein-bands in the Beatriz and Lilianita gold
deposits (21 samples) were made using an inductively
coupled plasma-mass spectrometer (ICP-MS) at ACME
Analytical Laboratories (Vancouver, Canada). Each
sample was treated for 1 hour in HCl–HNO 3 –HF–
HClO 4 solution for each 0.5 g sample. The composition
of electrum grains in the Lilianita samples was semiquantitatively
verified using an Oxford Inca EDS
system (Oxford, Concord, MA, USA) combined with
a JEOL JSM6360LV-type scanning electron microscope
(SEM; JEOL, Akishima, Japan) at the Universidad
Católica del Norte (UCN), Chile.
The thermal analyses of primary inclusions in quartz
were carried out to obtain their homogenization
temperature (Th) and salinity values, using a Linkam
THMSG600-type heating-freezing stage equipped with
TMS 93-LNP programmable controllers (Linkam
Scientific Instruments, Surrey, UK) at UCN, Chile.
All samples were prepared as doubly polished thin
sections approximately 0.3 mm thick. Heating rates
of 1 to 2°C/min and 0.1 to 0.6°C/min were adopted
for the Th and ice melting temperature measurement,
respectively. The salinity equivalent to NaCl wt%
was obtained using the equation involving a degree
of freezing point depression (see Table 1 in
Bodnar, 1993).
Oxygen was extracted from 5 mg samples at 550–
600°C according to the conventional BrF 5 procedure
of Clayton and Maeda (1963). Isotope compositions
were analyzed on Themo-Finnigan Delta plus XP
Continuous-Flow Isotope-Ratio Mass Spectrometer
(CF-IRMS) at Queen’s University, Canada. All δ 18 O
values are given in units of permil (‰) relative
to VSMOW international standard, with a precision
of ±0.1 ‰.
4. Analytical results
4.1 Geochemistry of quartz veins and electrum
Chemical compositions of the vein specimens are
summarized in Table 1. The Au contents of the analyzed
samples are generally below the lower limit of
detection, however several samples report detectable
Au, with the maximum concentration of 17.6 ppm
(C11) and 2.7 ppm (A6) from the Beatriz and Lilianita
specimens, respectively (Table 1). In these Au-rich
samples the correlating Ag contents yield 0.6 ppm
and 7.4 ppm in the two samples, respectively, and the
Ag content is weakly correlated with Au contents of
the Beatriz and Lilianita veins. However no positive
correlation between the Au and Cu contents is contrastively
displayed in the analyzed samples. In the
Au-detectable samples of the Lilianita vein (A4 to A6)
the Au and Cu contents exhibit a negative correlation.
Noticeably, the Cu contents are high in the two veins,
ranging up to 5.57 wt% at Beatriz, but with a low Au
value of 0.1 ppm. At the Beatriz vein the contents of
Au, Ag, Cu, Pb and Zn at the upper zone (BU samples
in Table 1) are generally higher than those of the
lower zone (BL samples in Table 1). The Fe-rich samples
with abundant hematite are generally enriched in
elements as Mn, Co, As, V and W. It is known that
these elements are all detected in hematite as a principal
mineral from iron oxide–copper–gold (IOCG)
deposits (Carew, 2004). As suggested earlier, the quartz
veins contain host rock residue as rutile. Thus the
high contents of such rock-forming elements as Na, K
and Ca may be considered as a result of host rock
assimilation.
The SEM-EDS examination of three Lilianita samples
represents normalized chemical compositions of electrum
in a range of Au 75 Ag 25 to Au 85 Ag 15 . No other
Au- or Ag-bearing minerals except for electrum were
observed in all the samples. Thus the electrum compositions
are conformable with the aforementioned
feature that the Au content is weakly correlated with
Ag contents in the Au-detectable samples.
4.2 Microthermometry and oxygen isotope
compositions
Primary fluid inclusions in transparent quartz of the
main-stage which coexists with Au–Cu minerals, were
examined including the viewpoints of necking and
leaking range. All inclusions examined are of twophase
liquid–vapor (L–V) and vapor–liquid (V–L)
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El Morado vein-type gold deposits
Table 1 Chemical compositions of selected major (%), minor and trace elements (ppm) of the Beatriz and Lilianita veins
Sample A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11
Location LL LL LL LL LL LL LL LL LL LL BL BL BL BL BL BL BL BU BU BU BU
Al wt% 9.44 9.43 9.37 9.49 5.54 11.07 7.56 8.75 8.87 8.71 8.05 1.23 0.51 3.54 9.41 8.06 7.86 8.27 8.40 6.82 2.25
Fe wt% 5.71 5.66 5.43 5.93 23.70 4.51 6.71 5.18 5.60 5.45 2.79 10.71 0.96 5.15 1.81 1.77 1.81 2.52 3.40 7.15 9.89
Mg wt% 2.33 2.26 2.20 1.06 0.30 0.62 1.71 2.10 2.14 2.08 2.57 0.15 0.32 1.72 1.96 2.49 3.28 2.54 1.83 1.33 0.44
Ca wt% 5.08 5.07 5.00 1.56 0.19 1.81 0.42 5.88 4.77 4.77 7.27 0.11 34.97 8.79 8.39 7.57 9.01 7.88 1.50 1.33 1.92
Na wt% 2.68 2.82 2.65 2.49 0.18 2.15 0.12 2.37 2.65 2.55 4.38 0.12 0.06 0.10 4.92 4.85 4.11 3.91 0.30 0.46 0.59
K wt% 1.25 0.94 1.27 1.88 2.21 1.65 2.79 1.12 1.24 1.30 0.18 0.29 0.08 0.61 0.21 0.14 0.21 0.21 3.06 2.17 0.12
Ti wt% 0.533 0.454 0.459 0.551 0.158 0.380 0.296 0.354 0.546 0.494 0.734 0.034 0.031 0.111 0.893 0.648 0.695 0.727 0.331 0.439 0.010
P wt% 0.091 0.080 0.093 0.033 0.094 0.030 0.071 0.055 0.091 0.083 0.083 0.015 0.015 0.026 0.165 0.139 0.164 0.107 0.135 0.049 0.013
Au Ppm <0.1 <0.1 <0.1 1.2 0.4 2.7 <0.1 <0.1 <0.1 <0.1 <0.1 0.2 <0.1 0.6 <0.1 <0.1 <0.1 0.2 0.1 0.5 17.6
Ag ppm <0.1 <0.1 <0.1 0.3 1.1 7.4 0.2 <0.1 <0.1 <0.1 <0.1 <0.1 0.1 0.1 <0.1 <0.1 <0.1 0.1 0.1 0.3 0.6
Cu ppm 93.8 63.8 94.0 397 814 294 1490 101 102 107 19 718 195 1549 99.3 13.7 7.1 528 55709 10355 27634
Pb ppm 10.0 6.6 7.3 7.6 112.5 20.6 15.5 9.9 5.9 6.4 0.8 3.5 5.6 2.5 1.4 1.1 1.2 1.8 5.0 3.4 14.8
Zn ppm 65 53 78 99 140 274 414 91 64 68 16 30 6 91 18 15 17 34 101 85 32
As ppm 6 6 6 13 72 7 19 4 6 7 4 52 2 8 4 2 2 4 4 12 43
Sb ppm 0.7 0.7 0.6 0.8 1.6 0.6 1.0 0.8 0.6 0.6 0.2 1.8 0.1 1.7 0.2 <0.1 <0.1 0.5 1.4 3.0 3.1
Sc ppm 26 25 26 21 15 20 20 24 25 26 19 4 1 8 29 24 28 27 26 22 4
Ba ppm 219 203 262 187 361 201 627 240 230 263 33 20 18 43 30 25 104 61 185 117 9
Rb ppm 57.3 39.5 44.8 146.3 63.8 73.2 60.0 38.8 60.5 56.4 2.5 8.6 2.2 19.1 4.2 2.0 1.9 3.6 87.7 63.0 2.4
Li ppm 6.1 6.9 7.6 6.8 3.3 5.5 24.0 11.4 6.4 6.8 3.2 10.1 1.6 18.4 5.5 3.2 4.5 9.8 17.4 9.2 11.7
Cr ppm 26.6 29.3 23.1 27.6 28.7 20.7 29.8 29 28.4 25.3 32 17.7 1.6 91.3 13 26.2 29.1 33.4 73.3 61 6.9
Y ppm 22.6 23.5 23.3 4.0 5.1 4.1 23.2 21.5 23.9 24.0 21.8 3.1 16.9 15.8 35.6 49.9 53.7 31.6 12.9 8.6 2.8
Sr ppm 401 392 379 299 353 369 49 335 357 361 359 36 125 38 368 323 368 370 42 144 58
Ni ppm 7.9 8.4 8.1 8.2 3.7 4.2 17.1 8.4 7.8 7.7 19.7 11.7 6.3 11.2 17.0 8.1 14.2 14.1 15.0 18.8 6.8
Be ppm 2 <0.1 <0.1 <0.1 1 <0.1 1 1 1 1 1 <0.1 <0.1 <0.1 1 1 1 1 1 1 <0.1
Co ppm 47 41 34 41 66 24 421 34 34 97 26 198 14 33 48 29 22 25 71 12 78
Hf ppm 0.8 0.9 0.7 0.9 0.3 0.8 0.5 0.8 0.8 0.9 1.0 <0.1 <0.1 0.2 2.5 2.7 2.4 2.0 1.4 1.3 <0.1
Nb ppm 3.8 3.2 3.5 3.6 0.8 2.9 2.2 2.8 4.5 4.2 4.5 0.1 0.1 0.6 6.3 6.6 6.1 4.9 2.2 3.3 0.1
Sn ppm 1.7 1.3 1.3 1.4 0.7 1.2 1.1 1.3 1.5 1.4 0.3 0.1 <0.1 0.4 1.7 1.8 1.2 0.8 0.9 0.7 0.1
Ta ppm 0.3 0.2 0.2 0.4 0.1 0.2 0.2 0.2 0.3 0.3 0.3 <0.1 <0.1 <0.1 0.3 0.5 0.6 0.3 0.2 0.2 <0.1
Th ppm 11.9 9.6 8.6 9.7 4.6 6.7 9.0 11.2 10.8 8.9 2.4 0.1 0.1 0.4 5.4 11.6 6.2 2.2 2.8 3.2 0.1
U Ppm 2.8 2.8 1.6 19.1 13.5 11.2 8.4 1.6 2.2 1.4 0.3 0.6 0.1 0.7 2.0 2.0 0.9 0.8 80.2 1.3 5.1
V ppm 237 248 229 272 514 203 205 197 226 224 249 153 29 191 179 213 217 263 265 316 163
W ppm 175.8 131.0 90.0 64.6 98.0 45.9 161.7 97.1 103.0 >200 94.0 >200 9.5 2.0 196.8 126.9 94.5 121.8 3.5 3.8 145.1
Zr ppm 15.7 14.7 12.7 13.6 5.9 14.5 9.7 17.1 13.2 15.6 16.8 0.4 0.1 7.1 62.5 55.5 55.5 49.5 32.7 28.2 0.5
Mo ppm 2.1 1.5 2.8 5.0 73.6 3.6 25.3 2.3 2.1 2.1 0.4 7.0 1.8 3.7 0.2 0.1 0.5 0.4 1.6 1.8 5.1
Mn ppm 1082 909 1108 189 193 205 4691 1500 1114 1054 388 1308 5331 2418 405 246 400 633 3227 579 1772
Cd ppm 0.1 0.1 0.1 <0.1 0.4 0.1 0.1 0.2 <0.1 0.1 <0.1 0.7 0.7 0.3 <0.1 <0.1 <0.1 0.1 0.3 <0.1 0.2
Bi ppm 0.1 0.1 0.1 0.1 28.4 0.1 2.0 0.1 0.1 <0.1 <0.1 3.4 0.1 0.7 <0.1 <0.1 <0.1 <0.1 0.1 1.1 215.4
La ppm 15.1 13.3 14.1 3.4 9.8 7.3 12.5 13.5 14.6 14.2 6.2 1.5 9.7 4.9 16.6 18.0 16.8 9.3 4.5 15.6 1.4
Ce ppm 35 30 30 7 19 16 31 28 32 30 15 3 18 10 48 63 52 22 18 26 2
Abbreviations: LL, Lilianita; BL, Lower zone of Beatriz (331 m high); BU, Upper zone of Beatriz (355 m high)
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S. Kojima et al.
without any solid phases, and their coexistence is
locally observed as the evidence of boiling phenomenon
(Fig. 3). Sizes of the inclusions generally range from
3to20μm, occasionally reaching about 100 μm. No
liquid CO 2 or clauthrate are observed in any inclusions
studied under the microscope.
The homogenization temperatures (Th) of 111 measurable
inclusions range widely from 202 to 399°C,
with the majority between 290 and 340°C (Fig. 4). The
V–L inclusions display relatively higher Th (324 to
399°C), compared with those of the L–V inclusions
(202 to 380°C), suggesting that vapor–liquid immiscibility
occurred during the primary inclusion capturing.
The NaCl equivalent salinities are fairly low, ranging
from 3.2 to 13.1 wt% NaCl (Fig. 4). As a general
characteristic there is neither a positive nor a negative
correlation between the Th and salinities.
The δ 18 O values of the six vein-quartz samples from
the Lilianita deposit are in a narrow range of +11.1 to
+12.5 ‰, displaying fairly limited isotopic compositions
(Table 2).
Fig. 4 Histograms of homogenization temperature
(above) and salinity equivalent to NaCl wt% (below)
for fluid inclusions from the Lilianita deposit.
5. Discussion and remarks
5.1 Origin of ore fluid
The Lilianita primary fluid inclusions show evidence of
boiling, and thus the Th values obtained can be regarded
as the mineralizing temperatures. Accordingly, the δ 18 O
values of mineralizing fluids were calculated using
the fractionation factor (α) of oxygen isotope between
quartz and water (Matsuhisa et al., 1979):
1000 ln α = 3.34(10 6 /T 2 ) – 3.31,
Table 2 δ 18 O values of the Lilianita quartz samples and
those of mineralizing fluid calculated
Sample
no.
Av. Th
(°C)
Th range (No.
of inclusions)
δ 18 O quartz
(‰)
δ 18 O fluid
(‰)
ML-02 342 314–362 (9) 11.7 6.2
ML-03 315 234–367 (16) 12.5 6.2
ML-05 321 286–350 (8) 11.7 5.6
ML-06 320 297–367 (14) 11.6 5.4
ML-07 294 202–352 (21) 12.2 5.1
ML-09 321 270–358 (15) 11.1 4.9
Fig. 3 Photomicrographs of (A) liquid-rich two-phase inclusions and (B) coexistence of liquid-rich and vapor-rich inclusions
from the Lilianita deposit. The scale bars are 20 μm.
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El Morado vein-type gold deposits
and the average Th values of the corresponding
quartz specimens. According to the experimental
results of Horita and Wesolowski (1994), the oxygen
fractionation between water and vapor is very minute
in high-temperature hydrothermal conditions. Thus,
the boiling effect in the oxygen isotope fractionation
was not considered here.
As stated earlier, some gaseous inclusions exhibit
vapor/liquid immiscibility during inclusion capturing.
Some of these inclusions have Th values higher than
about 370°C (Fig. 4), which are not indicative of real
Th data. Thus, all the Th values higher than 370°C were
excluded in calculating the average Th values. The δ 18 O
values of mineralizing fluids are listed in Table 2, which
suggests that the δ 18 O values of mineralizing fluid
(+4.9 to +6.2 ‰) overlap the lower range relative to
the common δ 18 O range of +6.0 to +10 ‰ for primary
magmatic water (e.g., Cambell & Larson, 1998; Hoefs,
2009). Figure 5 shows a correlative relation between
the δ 18 O and Th values of the mineralizing fluids,
suggesting slight dilution of a low-temperature and
low-δ 18 O surface water with deep-seated magmatic
water. Indeed, we can obtain δ 18 O water ≈ 0.0 ‰ at
T = 25°C, using the least squared equation of δ 18 O water
(‰) = 0.019 T (°C) –0.52 (Fig. 5).
The oxygen isotope composition of meteoric water in
Late Jurassic to Early Cretaceous ages, when the gold
deposits of the El Morado district were formed, has
not yet been defined. Nonetheless, we can estimate
the probable δ 18 O range under the following
assumptions:
(1) The altitude and latitude, which highly affect δ 18 O
composition of meteoric water, were not significantly
different in the mineralization age (Late Jurassic to
Early Cretaceous) to the present period.
(2) The “amount effect” of rainfall is not considered
here, because there is no information on Late Jurassic
to Early Cretaceous climates of northern Chile.
Thus, the δ 18 O variation due to the amount effect is
assumed to be included in the estimated δ 18 O range
of meteoric water.
(3) The oxygen isotope ratios of meteoric water
can be inferred from the Miocene Precordillera data
(δ 18 O= 5.5 to 7.5 ‰, Agemar et al., 1999) and the
present precipitation data of northern Chile
(δ 18 O= 3.5 to 9.0 ‰, Squeo et al., 2006; Herrera &
Custodio, 2014).
On the basis of these assumptions, it can be estimated
at a probable range of δ 18 O= 4to–8 ‰ for
the Late Jurassic to Early Cretaceous meteoric water.
Meanwhile the δ 18 O composition of oceanic seawater
during geologic history is a subject of controversy.
Muelenbachs (1998) insisted that the δ 18 O composition
of the oceanic seawater is buffered to a constant δ 18 O
values near 0 ‰ (VSMOW) by hydrothermal and
weathering processes at mid-oceanic ridges. However,
Wallmann (2001) modeled a strong 18 O-depletion in
seawater in Paleozoic ages. Even if the Wallmann’s
model is adopted for the δ 18 O composition of seawater,
only <0.5 ‰ depletion is generated in Late Jurassic to
Early Cretaceous seawater. Thus the δ 18 O composition
of seawater in the mineralization ages could be
approximated to ~0 ‰.
The δ 18 O ranges of all the fluid reservoirs in discussion,
which were possibly related to the El Morado
gold mineralization, are summarized in Figure 6. The
oxygen isotopic range of the mineralizing fluids is
explained by mixing of magmatic water with the
seawater or meteoric water. Such a fluid mixing may
have played an important role in the Au deposition.
5.2 Mineralization characteristics of the El
Morado gold vein deposits
Sillitoe and Thompson (1998) summarized intrusionrelated
Au-vein deposits in the world, representing
Fig. 5 Correlation between fluid δ 18 O values and trapping
temperatures of ore-fluid, based on homogenization
temperatures of fluid inclusions in the Lilianita
quartz vein.
Fig. 6 Summary of oxygen isotopic compositions of
fluids related to the formation of the Lilianita deposit.
See text for detailed explanations.
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203

S. Kojima et al.
several principal types based on their geochemical
associations. The Au-vein deposits of Mesozoic coastal
provinces in Northern Chile, which were generated in
association with highly oxidized calc-alkaline intrusions
emplaced at Cordilleran margins above active
subduction zones, are characterized by Au–Fe oxide–
Cu association. Pluton-related vein-type gold deposits
similar to the El Morado Au deposits occur in the
Mantos de Punitaqui (30°50´S, 70°15´W) of the
Coquimbo district. These deposits also comprise Au,
Fe oxides and Cu minerals characterized as Early
Cretaceous “intrusion-related oxidized vein” deposits
(Sillitoe, 1991; Sillitoe & Thompson, 1998). Thus, the
Mantos de Punitaqui and also Cu-rich IOCG vein
deposits in the coastal Cordillera of the Antofagasta
region are comparable to the El Morado gold deposits
in their mineralization characteristics.
Primary fluid inclusions in the Lilianita quartz-vein
deposit exhibit a boiling phenomenon, which is
inferred to have occurred at 290 to 340°C. Applying
the measured salinities at the temperature range to
the experimental results of Bischoff and Pitzer (1989),
a fairly low-fluid pressure of less than about 140 bars
(~14 MPa) could be estimated for the gold–quartz
mineralization. It is known that a low-pressure boiling
system prefers Au to partition to the brine phase in the
case of saline fluids (Simon et al., 2005). Polyphase
saline inclusions coexisting with vapor-rich inclusions
are reported in the Mantos de Punitaqui gold mineralization
(Sillitoe & Thompson, 1998), and thus it is likely
that a low-pressure boiling was a significant trigger for
formation of the gold-partitioned saline fluids. The
mineralizing fluid responsible for the Lilianita ore
formation is not associated with saline phases, and so
it is not certain whether the low-pressure boiling could
be an effective factor for the Au concentration. By
contrast, the Cu-rich IOCG deposits of the same region
have high-salinity polyphase (39–68 wt% NaCl) and
high Th (401–560°C) fluid inclusions in quartz veins,
and the boiling phenomenon is not observed (Tristá &
Kojima, 2003). In general, high-temperature hypersaline
solution enhances chloride complexing in copper
transport under magmatic-hydrothermal conditions
(e.g., Liu & McPhail, 2005). This phenomenon suggests
a great Cu-transport capacity of the high-Th hypersaline
fluids related to the Cu-rich IOCG deposits.
Currently, the main trigger of the Au concentration of
gold deposits in the El Morado still remains uncertain,
but it can probably be implied that the P–T–X
condition is an important factor for the formation of
Au-enriched magmatic-hydrothermal solutions.
The Th and salinity data also support an alternative
idea that gold deposits in the El Morado are shallow
manifestations of the Cu-rich IOCG-type deposits.
The high-temperature and hypersaline fluids related
to the formation of the IOCG would have been highly
diluted by low-temperature and low-salinity surface
water to produce the El Morado mineralizing fluid.
However, such a large-scale mixing is not deduced
from the present oxygen isotope data of the vein
quartz. It is therefore suggested that the El Morado
gold mineralization is initially distinct from the Cu-rich
IOCG mineralization in terms of fluid source.
A similar Au–Fe oxides–Cu vein-type deposit
named India Coya (27°36´S, 70°23´W), which is hosted
in a Cretaceous granodioritic intrusion of the Copiapó
district, has been recently exploited (Osorio et al.,
2015; Cortés, 2016). Unfortunately, no fluid inclusion
and isotope data have yet been presented for the
vein deposit, and so further geochemical studies
including fluid inclusion research are required for
the deposit to compare with gold deposits in the
El Morado.
6. Conclusions
1. Only the El Morado district has quartz vein-type
gold deposits (Beatriz, Lilianita) in the north Chilean
coastal Cordillera, which are hosted in a Late Jurassic
dioritic intrusion of the magnetite-series and calcalkaline
suite.
2. Gold and silver mineralization of the deposits is
characterized by the occurrence of electrum (Au 75 Ag 25
to Au 85 Ag 15 ). Other ore minerals include chalcopyrite,
specular hematite and magnetite, suggesting the
intrusion-related oxidized vein-type deposit denominated
by Sillitoe and Thompson (1998).
3. The analyzed Au contents of the veins weakly
correlate with Ag contents, but are not correlative with
Cu contents.
4. Fluid inclusion data exhibit a relatively hightemperature
(290 to 340°C) and low salinity (3.2 to
13.1 wt% NaCl) ranges at fairly low-pressure (< ~
140 bars) boiling conditions.
5. The δ 18 O values of quartz-vein samples from the
Lilianita deposit are in a narrow range of +11.1 to
+12.5 ‰. Combining these data with the Th data, the
δ 18 O values of the mineralizing fluids are estimated
to be between +4.9 and +6.2 ‰. The δ 18 O range implies
that the mineralizing fluid originated from primary
magmatic water with a small amount of low-
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El Morado vein-type gold deposits
temperature surface water, such as seawater or
meteoric water.
6. The estimated fluid δ 18 O data of the Lilianita deposit
exclude the idea that the El Morado gold deposits
are shallow manifestations of the Cu-rich IOCG-type
deposits in the same region.
Acknowledgments
Part of the expenses of this study was covered by the
vice-rectory of Universidad Católica del Norte, to
which we express our sincere thanks. We would like
to thank Dr. Andrew Menzies who improved this
manuscript, and Drs. Y. Watanabe, Y. Morishita and
R. Takahashi for their critical reading of the manuscript.
Thanks are due to Leonel Jofre of Universidad Católica
del Norte who completed the original figures used in
this article.
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