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Journal of Geochemical Exploration 89 (2006) 124 – 128
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Hydrothermal alteration and fluid inclusion study of the Lower
Cretaceous porphyry Cu–Au deposit of Tiámaro,
Michoacán, Mexico
Carlos Garza-González a,⁎, Antoni Camprubí b , Eduardo González-Partida b ,
Germán Arriaga-García a , Fernando Rosique-Naranjo a
a
Facultad de Ingeniería, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México D.F., Mexico
b
Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Carretera 57 km. 15.5,
76023 Santiago de Querétaro, Qro., Mexico
Received 11 August 2005; accepted 4 November 2005
Available online 23 March 2006
Abstract
The Tiámaro deposit in Michoacán state has been dated as Lower Cretaceous (Valanginian), though most of the porphyry deposits
in central Mexico were dated or have an attributed Eocene–Oligocene age. The host rocks belong to a volcanoplutonic complex
overlain by red conglomerates. These rocks were intruded by pre-Valanginian plutonic and hypabissal rocks. Propylitic, phyllic, and
argillic alteration assemblages developed, and their superimposition draws the evolution of the deposit. Stage I is represented by
propylitic assemblages, stage II contains the main ore forming stockworks and both phyllic and argillic assemblages, and stage III
contains late carbonatization assemblages. The obtained temperatures and salinities from inclusion fluids are low for a porphyry-type
deposit, but we interpret that the known part of the deposit represents the shallow portion of a bigger deposit. The evolution of
mineralizing fluids draws a dilution trend of brines from “porphyry-like” to “epithermal-like” stages. The richest ore zone is roughly
located between the 300 and 350 °C isotherms, though unnoticed resources may occur at depth.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Tiámaro; Mexico; Porphyry Cu–Au; Hydrothermal alteration; Fluid inclusions
1. Introduction
In south-central Mexico up to 16 porphyry type
deposits have been identified. They can be generally
distributed into three ranges of age: (1) associated with
the Laramide orogeny, as La Sorpresa in Jalisco, (2)
Eocene–Oligocene, as La Verde, Inguarán and Tumbiscatío in Michoacán, and (3) Miocene–Pliocene, as
Ixtacamaxtitlán in Puebla, Santa Fe and Tolimán in
⁎ Corresponding author.
E-mail address: [email protected] (C. Garza-González).
0375-6742/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.gexplo.2005.11.046
Chiapas. The formation of porphyry type deposits in
northwestern Mexico is generally associated with
Laramide tectonomagmatic activity (75–50 Ma; Clark
et al., 1982; Damon et al., 1983), but in central Mexico
the deposits that have been dated, or have an attributed
age, mostly formed during the Eocene–Oligocene
(Valencia-Moreno et al., 2006). Two deposits, however,
were recently reported to have formed between Upper
Jurassic and Lower Cretaceous: El Arco in Baja
California (164.1 ± 0.4 Ma, Re–Os; Valencia et al.,
2004), and Tiámaro in Michoacán (140 to 131 Ma, U–Pb
and Ar/Ar; Garza-González et al., 2004: A. Iriondo,
C. Garza-González et al. / Journal of Geochemical Exploration 89 (2006) 124–128
2005, pers. commun.). These ages will widen exploration efforts to previously neglected areas, especially in
southern Mexico. Estimated grades for the porphyry
Cu–Au deposit of Tiámaro are 0.60% Cu and 0.1 g/t Au
for 500 Mt (Garza-González et al., 2004). Despite the
economic importance of porphyry type deposits in
Mexico, these deposits still lack systematic studies on
their mineralogy and fluid chemistry, and the metallogenic provinces that contain them also need further
regional characterization.
This paper is the first attempt to characterize the
temperature and composition of the mineralizing fluids
and their evolution in the Tiámaro porphyry copper
deposit, by means of fluid inclusion microthermometry.
2. Regional geology
The Tiámaro porphyry copper deposit is located in
the northeastern part of the Michoacán state, ∼155 km
125
SW of Mexico City, within the Sierra Madre del Sur
(SMS) and close to the Trans-Mexican Volcanic Belt
(TMVB). In this zone are found the northernmost outcrops of intrusive Cretaceous rocks of the SMS (Fig. 1).
The host rocks belong to a pre-Valanginian volcanoplutonic complex (Garza-González et al., 2004) that
includes porphyry andesites, subvolcanic breccias,
aglomerates, and calcalkaline dacite flows on top, all
overlain by red conglomerates. These rocks were
intruded by dioritic and tonalitic plutonic and hypabissal
rocks. These rocks can be characterized as primitive
tholeiites formed in an island arc setting, from their trace
element composition (Garza-González et al., 2004). The
above complex was intruded by the Tuzantla batholith
(Fig. 1), whose composition varies from quartz
monzonite to granodiorite with calcalkaline affinity,
close to the compositional field of adakites. These rocks
have yielded U–Pb ages in zircon of 132.3 ± 1.4 and
131.0 ± 1.1 Ma, respectively (Garza-González et al.,
Fig. 1. Geological map of the Tiámaro area, Michoacán. Key: SMO = Sierra Madre Occidental, SMS = Sierra Madre del Sur, TMVB = Trans-Mexican
Volcanic Belt.
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C. Garza-González et al. / Journal of Geochemical Exploration 89 (2006) 124–128
2004). Another part of the porphyry deposit occurs
within a tonalitic stock, and in adamellitic, tonalitic, and
granitic hypabissal bodies that intrude the volcanoplutonic complex. This second set of intrusive rocks shows
geochemical and geochronological evidence for a
genetic link with the Tuzantla batholith. There is a late
generation of monzonitic intrusives, but they are barren.
The above rocks are unconformably overlain by Tertiary
and Quaternary volcanic and sedimentary rocks.
3. Structure of the deposit
The Cu–Au mineralization, mostly found as stockworks, is associated with the tonalitic stock in the east,
and with adamellitic and granitic–tonalitic hypabissal
bodies in the west (Fig. 1). About 70% of the ore bodies
are hosted by the above rocks, and the rest are hosted by
microdiorites of the volcanoplutonic complex.
Propylitic, phyllic, and argillic alteration assemblages
were identified in the deposit, and potassic alteration was
identified in the northern part of the Tuzantla batholith.
The evolution of the deposit can be characterized by
three major hypogenic alteration stages. Stage I, or early
hypogenic alteration, basically consists of propylitic
alteration, found either as a selective alteration of mafic
minerals, or as a chlorite–epidote–calcite–sericite
association in veinlets within a microdiorite intruded
by tonalite in the central part of the deposit. Such
alteration, however, is commonly pervasive and represented by the association of chlorite, epidote, carbonates,
Fig. 2. Above: Geological section of the Tiámaro deposit, showing the distribution of isotherms after Th of fluid inclusions, the ranges of Tmi, the
general distribution of alteration assemblages,and the position of the tonalite stock. Below: Correlation of average Th and Tmi obtained in fluid
inclusions in different associations of the deposit, in order of formation in the legend. Key: Th = temperature of homogenization, Tmi = temperature of
ice melting.
C. Garza-González et al. / Journal of Geochemical Exploration 89 (2006) 124–128
pyrite, sericite, various clay minerals ± actinolite, found
in the southern part of the deposit. It contains deep
mineralized zones as stockworks with early chalcopyrite–pyrite and late bornite where phyllic alteration is
superimposed on propylitic alteration.
Stage II, or intermediate hypogenic alteration,
contains both phyllic and argillic alteration assemblages.
Most of the copper ores are associated with phyllic
alteration, as pyrite–chalcopyrite ± bornite stockworks.
In tonalites, microtonalites and microdiorites, phyllic
alteration is usually pervasive and consists of quartz,
sericite and pyrite (± chlorite and clay minerals), and a
conspicuous subordinate carbonatization. Phyllic alteration also occurs as abundant sericite stringers and
veinlets, as massive sericite associations replacing
plagioclase fenocrysts and microcrystals, and abundant
quartz–chlorite–pyrite–chalcopyrite ± bornite veinlets.
A sericite sample from phyllic alteration in the host
microgranite at the El Rey mine (Tiámaro area) yielded
a 40Ar/39Ar age of 140 ± 5 Ma (A. Iriondo, 2005, pers.
comm.). The similarity between this age and the U–Pb
ages obtained in the Tuzantla batholith, and their
closeness in space suggests that both sets of intrusive
rocks are part of the same event and that they may have
formed from the same magmas. The argillic alteration
assemblage is superimposed on the phyllic assemblage,
and occurs at depth between the propylitic and phyllic
assemblages, at the contact zone between tonalites and
microdiorites and between microdiorites and microtonalites. It generally consists of illite–smectite and other
clay minerals, chlorite, calcite, and epidote. This assemblage contains a pyrite–chalcopyrite stockwork
grading up to 2730 ppb Au.
Stage III, or late hypogenic alteration, consists of
quartz ± sphalerite veinlets and two carbonatization
stages, the first of them associated with the last occurrence of chalcopyrite in the evolution of the deposit,
and the second carbonatization stage is barren and may
represent the final waning stage of hydrothermal activity
in the deposit.
4. Fluid inclusions
A microthermometric study of fluid inclusions was
carried out on 23 samples from ore zones (stockworks),
25 samples from phyllic and argillic alteration zones
(stage II), and 16 samples from late calcite (stage III),
obtained from both surface exposures in mine workings
and drill cores. The analyzed minerals were quartz and
calcite. We analyzed only inclusions hosted by minerals
lacking evidence for recrystallization. Primary, secondary, and pseudosecondary inclusions were found, which
127
are two-phase at room temperature, liquid-rich inclusions with no daughter minerals. The fluid inclusions are
mostly 5 to 15 μm in size, but range up to 50 μm.
S(NaCl,KCl) + L + V fluid inclusions, typical for most
porphyry type deposits, were not found at the Tiámaro
deposit, probably because drilling has not yet reached
either any zone with boiled-off paleofluids or the
deepest part of metal ores. This deposit also exhibits a
prominent late “epithermal” event that might allow us to
trace a complete evolution for mineralizing fluids.
The results are shown in Fig. 2. In stockworks,
homogenization temperatures (Th) range from 295 to
350 °C, and ice melting temperatures (Tmi) from − 7 to
− 16 °C. These correspond to calculated salinities that
range from 10.5 to 19.5 wt.% NaCl eq. (using the
equation for the H2O–NaCl system of Bodnar, 1993),
and an average pressure of 145 bars. In phyllic and
argillic assemblages, Th ranges from 110 and 295 °C, Tmi
from − 5 to − 11 °C, and salinities from 7.8 to 15 wt.%
NaCl eq. In late calcite veinlets (stage III), Th ranges
from 101 to 220 °C, Tmi from − 3 to − 11 °C, and
salinities from 5 to 15 wt.% NaCl eq., for an average
pressure of 1.3 bars. Thus, the evolution of mineralizing
fluids shows a dilution of brines from “porphyry” to
“epithermal” stages, more noticeable regarding minimum or average values than the variation ranges.
Microthermometric data from stage II display a bimodal
distribution, but only due to the distribution of
temperatures of homogenization, and there is no clear
evidence for mixing at any stage. The richest ore zone is
located roughly between the 300 and 350 °C isotherms
(Fig. 2), but that perspective is expected to change as
deeper zones of the deposit are drilled and studied.
5. Conclusions
The porphyry Cu–Au deposits at Tiámaro, Michoacán, Southern Mexico, is hosted by a pre-Valanginian
volcanoplutonic complex, and is due to the intrusion of
mainly tonalitic hypabissal rocks dated as ∼132–131 Ma
(U–Pb) and 140 ± 5 Ma (Ar/Ar) that have a geochemical
signature similar to that of adakites. As porphyry-type
deposits in south-central Mexico were dated or have
attibuted Eocene–Oligocene ages, the ages obtained in
Tiámaro may necessarily change the exploration strategies for this type of deposits in the region.
The evolution of the deposit can be defined by the
superimposition of alteration assemblages: stage I
corresponds to propylitic alteration, stage II to phyllic
and argillic alteration, and contains the main ores in
stockwork zones, and stage III corresponds to late
carbonatization.
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In stockworks, homogenization temperatures (Th)
range from 295 to 350 °C, ice melting temperatures (Tmi)
from − 7 to − 16 °C, and salinities from 10.5 to 19.5 wt.%
NaCl eq. In phyllic and argillic assemblages Th ranges
from 110 and 295 °C, Tmi from − 5 to − 11 °C, and
salinities from 7.8 to 15 wt.% NaCl eq. In late calcite
veinlets (stage III), Th ranges from 101 to 220 °C, Tmi
from − 3 to −11 °C, and salinities from 5 to 15 wt.%
NaCl eq. These data suggest that the known part of the
deposit may only be its upper portion, and more
resources may be found at depth, that are related to
more saline and hotter fluids, indicating more typical
temperatures for the environment of formation of a
porphyry-type deposit.
Acknowledgements
This work was funded through the research project
IN103703 granted by DGAPA-PAPIIT-UNAM. We also
thank the staff at the Tiámaro mine for their kind
assistance, and Juan Tomás Vázquez, from the Centro de
Geociencias for the elaboration of thin sections. We also
gratefully acknowledge the critical reviews of Steven
Kesler and an anonymous referee.
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