The Altar Porphyry Cu

49 Pages·2014·3.14 MB·English

by  Dilles, John

The Altar porphyry Cu-(Au-Mo) deposit (Argentina): a complex
magmatic-hydrothermal system with evidence of recharge processes
Maydagan, L., Franchini, M., Chiaradia, M., Dilles, J., & Rey, R. (2014). The Altar
porphyry Cu-(Au-Mo) deposit (Argentina): A complex magmatic-hydrothermal
system with evidence of recharge processes. Economic Geology, 109(3),
621-641. doi:10.2113/econgeo.109.3.621
10.2113/econgeo.109.3.621
Society of Economic Geologists
Accepted Manuscript
http://cdss.library.oregonstate.edu/sa-termsofuse
The Altar porphyry Cu-(Au-Mo) deposit (Argentina):
a complex magmatic-hydrothermal system with evidence of recharge processes

Laura Maydagána,b,c*, Marta Franchinia,b, Massimo Chiaradiad, John Dillese, Roger Reyf

a CONICET; b Departamento de Geología y Petróleo, Facultad de Ingeniería, Universidad
Nacional del Comahue, Buenos Aires 1400, 8300 Neuquén, Argentina.
c Departamento de Geología, Universidad Nacional del Sur, San Juan 670, 8000 Bahía
Blanca, Argentina
d Department of Mineralogy, University of Geneva, Rue des Maraîchers 13, 1205 Geneva,
Switzerland
e College of Earth, Ocean and Atmospheric Sciences, Oregon State University,
104 CEOAS Admin Bldg, Corvallis
f Peregrine Metals Ltd. Santa Fe (Oeste) 117, Piso 5, Edificio Derby, Ciudad San Juan,
Argentina
*Corresponding author: [email protected]
Abstract
Altar (31º 29’ S, 70º 28’ W) is a large porphyry Cu-(Au-Mo) deposit with associated
epithermal Au-(Ag-Cu) veins located in the Cordillera Principal of SW San Juan Province
(Argentina). Altar is a complex magmatic-hydrothermal system formed from several
magmatic and hydrothermal pulses during the middle-late Miocene. New LA-ICPMS U-Pb
ages in zircons from the Altar porphyries indicate four discrete events of intrusions over an
extended magmatic life time of ca. 3 m.y. It comprises a pre-mineralization porphyry (11.75 ±
0.24 Ma), three mineralized porphyries (11.62 ± 0.21 Ma and 11.68 ± 0.27 Ma, 11.13 ± 0.26
Ma, 10.35 ± 0.32 Ma) related to hydrothermal breccias, two post-mineralization intrusions,
and a post-mineralization breccia (8.9 ± 0.4 Ma). The three mineralized porphyries
(porphyries 2, 3 and 4) were emplaced within ~0.7-1.3 m.y.
Amphibole phenocrysts from the porphyries crystallized from oxidized magmas (fO =
2
NNO +1 to +2) at temperatures of 780 to 850°C and pressures between 0.9 and 1.8 kbar
corresponding to depths of ~4-7 km. Anorthite and Fe- rich rims in the plagioclase
phenocrysts suggest that the magmatic chambers were episodically recharged by a less
evolved magma. The middle-late Miocene intrusions are interpreted to have been derived
from a deeper and relatively large magmatic reservoir that supplied magmas to smaller
chambers located in the upper crust. The focused magmatic output to shallow levels during a
period of a few million years in the Altar area has been a main requirement in the formation
of this large porphyry copper deposit.
Introduction
Detailed studies of porphyry Cu-Au-Mo systems have highlighted the complexity of
their magmatic-hydrothermal evolution which comprises successive phases of intrusion,
hydrothermal fluid release, and metal deposition (e.g., Skewes and Stern, 1996; Proffett,
2003; Harris et al., 2004, Seedorff et al., 2005; Vry et al., 2010; Longo et al., 2010; Stern et
al. 2011). These complex systems can be developed episodically over time spans that may last
up to several million years. During this period magmas and parental plutons associated with
the porphyries may experience processes such as recharge or mixing (e.g., Keith, 1997;
Hattori and Keith, 2001; Maughan et al., 2002; Halter et al., 2004; Audétat and Pettke, 2006;
Chiaradia et al., 2009a; Chiaradia et al., 2012). These processes influence the metal budget
and evolution of the magmatic-hydrothermal system.
Altar (31º 29’ S, 70º 28’ W) is a large porphyry Cu-(Au-Mo) deposit with associated
epithermal Au-(Ag-Cu) veins (802 Mt @ 0.42% Cu and 0.059 g/t Au, with a cut-off grade of
0.30% Cu equivalent, Table 1; Peregrine Metals Ltd., 2010). It is located in the Cordillera
Principal of SW San Juan Province, Argentina, which corresponds to the southern portion of
the Andean flat-slab segment (28-33ºS). The world-class porphyry Cu-Mo deposits of Los
Pelambres and Pachón occur 25 km to the south (Fig. 1). The importance of this region has
been highlighted by the recent discovery of the porphyry Cu districts of Piuquenes, Rincones
de Araya, Quebrada de la Mina, Yunque, Los Azules, and Los Bagres (Fig. 1; Mpodozis and
Cornejo, 2012).
Maydagán et al. (2011) presented the first geological mapping and petrographic and
geochemical studies of the Altar magmatic rocks. The Altar upper subvolcanic suite which
intruded volcanic rocks of the lower volcanic complex is genetically linked to the porphyry
Cu-(Au-Mo) and epithermal Au-(Ag-Cu) veins. The lower volcanic complex is part of an
early Miocene magmatic arc formed above a normal dipping subduction zone whose magmas
underwent fractional crystallization and crustal contamination. The upper subvolcanic suite
represents a late-middle Miocene magmatic arc formed above a shallow subduction zone
(Maydagán et al., 2011).
This study describes the Altar subvolcanic porphyritic stocks, presents constraints on
duration of magmatism, and processes that occurred in parental magma chambers during the
evolution of the magmatic-hydrothermal system. Our work is based on a detailed petrographic
study coupled with analyses of mineral chemistry, new LA-ICPMS U-Pb dating and Sr, Nd,
and Pb radiogenic isotopes of the different porphyritic intrusions.

Tectonic setting
Altar is located in the southern portion of the flat-slab segment (~27°-33° 30’S) of the
Central Andes. In this region, the Nazca plate is subducting nearly horizontally beneath the
South American plate at ~100 km depth (Gans et al., 2011). The flat-slab segment presents a
smooth transition to the north, toward the Central Volcanic Zone (CVZ), and an abrupt
transition to the south, to the Southern Volcanic Zone (SVZ; Fig. 1; Cahill and Isacks, 1992;
Anderson et al., 2007; Gans et al., 2011). Several studies have documented the Miocene to
recent evolution of the flat-slab segment (Allmendinger et al. 1990; Kay and Abbruzzi, 1996;
Kay and Mpodozis, 2002). During the Early Miocene (27–20 Ma) this segment had a slab
geometry similar to that currently observed in the normal-slab segment at 35ºS, and a crustal
thickness of 35-40 km (Kay and Abbruzzi, 1996; Kay and Mpodozis, 2002). The shallowing
of the subduction zone progressed from Middle to Late Miocene (20–5 Ma). It was
accompanied by crustal thickening, subduction of the Juan Fernández ridge (e.g., Yáñez et al.,
2001), a substantial decrease in the astenospheric wedge thickness and eastward migration,
and broadening of the arc (Kay et al., 2005). Cessation of magmatic activity over the Miocene
flat-slab occurred at 5 Ma. At this time, magmatism occurred in the back-arc, in the Farallón
Negro, Pocho and San Luis magmatic centers (Kay and Mpodozis, 2002).

Local Geology
The Altar area was poorly known geologically until the mid-1990s, when CRA
Exploration Argentina outlined a broad zone of alteration and mineralization. Between 1999
and 2003, Río Tinto tested this target with 2,841 m of diamond drilling. Since 2005, Peregrine
Metals Ltd. has completed 140 diamond drill holes (55,688 meters). Recently, the project has
been acquired by Stillwater Mining Company, which is conducting a comprehensive drilling
campaign, metallurgical testwork program, environmental baseline study, and preliminary
economic evaluations for a copper-gold mining and concentrating project at Altar.
Maydagán et al. (2011) presented the first field mapping and data on magmatic rocks of
the Altar region. The authors grouped the igneous rocks into two sequences: the early
Miocene lower volcanic complex and the middle-late Miocene upper subvolcanic suite. The
lower volcanic complex is an early Miocene arc in which mantle-derived magmas evolved at
low pressures through plagioclase- and pyroxene-dominated fractional crystallization and
assimilation of crustal rocks (AFC; Maydagán et al., 2011). The Altar porphyry Cu-(Au-Mo)
deposit is partially hosted and related to the upper subvolcanic suite, a series of porphyritic
stocks, dikes, and breccias that intruded the lower volcanic complex in the central and east
ridges of the Altar district (Fig. 2). Magmas of the upper subvolcanic suite require a
hornblende-bearing residual mineral assemblage that is interpreted to reflect their higher
water contents (Maydagán et al., 2011).

Analytical Techniques and Sampling Methodology
Over 200 samples representatives of the different porphyritic rocks and breccias of the
upper subvolcanic suite were collected from surface exposures and drill core from the Altar
region. The samples were studied by transmitted and reflected light petrography at the
Universidad Nacional del Comahue (Neuquén, Argentina) and twenty samples were selected
for analysis.
Three radiometric ages of the subvolcanic stocks were determined by U-Pb LA-ICPMS
on zircons at the PCIGR- Pacific Center for Isotopic and Geochemical Research at the
University of British Columbia (Canada). Another radiometric age of the late breccia was
determined by U-Pb LA-ICPMS on zircons at the Arizona LaserChron Center, University of
Arizona, U.S.A. Details of the dating methods and results of LA-ICPMS analyses of zircons
from the porphyries and late breccia are provided in digital appendices A5-A10.
Major, trace and rare earth elements were analyzed by inductively coupled plasma-
emission spectrometry and ICP mass spectrometry (Group 4A-4B combined package) at
Acme Analytical Laboratories Ltd., Canada (Table 4). Seven samples were analyzed for Sr
and Nd isotopes and ten for Pb isotopes at the Mineralogy Department at the University of
Geneva (Switzerland) following the method of Chiaradia et al. (2009a; Table 5).
Chemical compositions of magmatic minerals (plagioclase (n = 74), ulvöspinel (n = 17),
amphibole (n = 20), biotite (n=16), and magnetite (n = 8) from different subvolcanic
intrusions were determined with electron microprobe at three different laboratories with some
samples analyzed in duplicate at more than one laboratory. The full data set of microprobe
analysis is provided in the digital appendices A1-A4.

Altar subvolcanic intrusions
At least five porphyritic intrusions and three breccias have been recognized in the Altar
district. They are distinguished based on their textures, phenocryst abundances, related veins,
degree of alteration and mineralization, and cross-cutting relationships (Fig. 2; Table 2).
Hydrothermal alteration and Cu-(Au-Mo) and Au-(Ag-Cu) mineralization in the project are
associated spatially and temporally with the porphyries and breccias emplacement. A late
undeformed volcanoclastic breccia contains fragments of both the lower volcanic complex
and upper subvolcanic suite. It crops out in the northern edge of the district over an area of
~0.2 km2 (Fig. 2).

Pre-mineralization and syn-mineralization subvolcanic plutons and magmatic-hydrothermal
breccias
Porphyry 1 crops out on the eastern ridges of the district at elevations of 4000 m, where
it has intruded tuffs of the lower volcanic complex and is cut by Au-(Ag-Cu) epithermal
veins (Fig. 2, Table 2). It is the largest porphyry with a surface area of ~1 km2. Phenocrysts
of plagioclase (30-60 vol. %, 0.1-7mm), edenite (2-15 vol. %), accessory spinel minerals
(―magnetite-ulvöspinel solid solution‖, 1-1.5 vol. %), apatite, and zircon are set in a fine-
grained (<40 μm) quartz + feldspar + magnetite groundmass (Fig. 3a). Some plagioclase
phenocrysts have compositional zoning documented by cores of oligoclase (An ) with
28-32
FeO concentrations between 0.10 and 0.18 wt. percent (Fig. 4; digital appendix A1), middle
sections of oligoclase (An and An ), and an outer edge (light colored; Fig 4a) of
31-34 21-33
andesine (An ) that also exhibits an increase in FeO (0.39 to 0.45 wt.%) content. Other
39-42
crystals show alternating zoning patterns with cores of labradorite (An ) and margins of
64
andesine (An ; Figs. 4b; digital appendix A1). Porphyry 1 has been affected by weak to
39
moderate propylitic alteration and, at the contact with the epithermal veins, by intense argillic
and phyllic alteration.
Porphyry 2 crops out in the east valley over an area of ~0.5 km2 (Fig. 2) where it has
intruded porphyry 1 and the volcanic host rocks. Porphyry 2 has undergone phyllic alteration
(quartz + white mica + tourmaline; Fig. 3b). The porphyritic texture has been preserved with
relict phenocrysts of plagioclase (50 vol. %, 0.4-3 mm) and zircon set in a groundmass that
has been altered to microgranular quartz aggregates (100-200 μm). Porphyry 2 has been cut
by a stockwork of early quartz veinlets (2-10 vol. %) and contains Cu-(Au-Mo)
mineralization below the oxidation zone.
Polymictic matrix supported breccia 1 was intersected by drill-holes in the east valley
(Fig. 2), in the contact between porphyry 2 and the volcanic wallrocks. The breccia consists of
subrounded fragments (10-70 vol. %) of porphyry 1, porphyry 2, tuff, quartz veins, and
plagioclase crystals in a matrix consisting of mud-sized fragments. It has been affected by
phyllic alteration and weak Cu-(Au-Mo) mineralization. At shallow levels it has cut porphyry
2 and presents an hydrothermal cement of quartz + tourmaline + pyrite (2-10 vol. %; Fig. 3c).
Porphyry 3 crops out on the central ridge (Fig. 2) where occurs as a stock (~0.3 km2)
and related dikes that cut tuffs of the lower volcanic complex. Porphyry 3 contains
phenocrysts of plagioclase (45-50 vol. %), edenite (3-10 vol. %), biotite (1 vol. %) and
accessory quartz, magnetite, and zircon set in a coarse-grained (100-200 μm) hydrothermal
quartz + feldspar + biotite groundmass (Fig. 3d). Most of the phenocrysts of plagioclase
analyzed have oscillatory compositional zoning with cores of andesine (An ) and
39
intermediate zones of oligoclase (An ; Fig. 5; digital appendix A1). An increase in the
25-31
anorthite molar content (An ) in the outer edge of the phenocryst is accompanied by an
31
increase in FeO (0.25 wt. %; Fig. 5a). Porphyry 3 contains 2-10 volume percent quartz veins
and has undergone moderate potassic, sodic, and locally intense phyllic alteration and
moderate Cu-Au mineralization.
Porphyry 4 crops out on the southern portion of the central ridge (Fig. 2) where it has
intruded the volcanic wallrocks. Phenocrysts of plagioclase (60-70 vol. %) have been altered
to hydrothermal albite (An ; digital appendix A1) and accessory zircon occurs in a
1-4
groundmass altered to an aggregate (100-400 µm) of hydrothermal quartz. Porphyry 4 has
been affected by pervasive potassic alteration at depth and phyllic alteration at shallow levels.
It has less groundmass compared to porphyry 3 and a higher proportion of quartz veinlets
(10-25 vol. %; Fig. 3e). Porphyry 4 has the highest copper grades of the district and contains
Mo and Au mineralization.
Polymictic clast- to matrix-supported breccia 2 crops out in the central ridge as an halo
around porphyry 4. It also occurs as dikes that have intruded porphyries 3-4 and the volcanic
wallrocks. This breccia contains sub-rounded fragments (55 vol. %) of porphyry 3, porphyry
4, tuff, and quartz veins set in a mud-sized matrix. It has been affected by intense phyllic
alteration, quartz veining (10-25 vol. %; Fig. 3f), and contains Cu-Au mineralization. At
shallow levels in the central ridge the breccia presents an hydrothermal cement of quartz +
tourmaline + pyrite (20 vol. %) and cuts porphyry 4.

Post-mineralization subvolcanic plutons and magmatic breccia
Several post-mineralization intrusions have been recognised based on crosscutting
relationships, low metal contents, and weak veining and alteration.
An andesite porphyry crops out on the east and north-east ridges and has intruded
porphyries 1 and 2 (Fig. 2). This rock has phenocrysts of plagioclase of variable size (40-70
vol. % ), amphibole (1-7 vol. %), accessory quartz (1 vol. %), and fine-grained opaques
(<1 mm, magnetite) in a microcrystalline (0.025-0.05 mm) quartz + feldspar groundmass
(Fig. 6a). This porphyry differs from the other porphyries because of the different size of the
plagioclase phenocrysts and the finer grained groundmass with higher proportion of
plagioclase. It is weakly quartz-veined (1-2 vol. %) and has been affected by moderate
potassic and propylitic alteration. This intrusion is barren.