1-s2.0-S0169136824001318-main

Ore Geology Reviews 167 (2024) 105998
Contents lists available at ScienceDirect
Ore Geology Reviews

journal homepage: www.elsevier.com/locate/oregeorev
The application of PRISMA hyperspectral satellite imagery in the

delineation of distinct hydrothermal alteration zones in the Chilean Andes:
The Marimaca IOCG and the Río Blanco-Los Bronces Cu-Mo
porphyry districts
Anna Sorrentino a, Rita Chirico b, *, Francesca Corrado a, Carsten Laukamp c, Diego Di Martire a,
Nicola Mondillo a, d
a
Department of Earth, Environment and Resources Sciences, University of Naples Federico II, Naples, Italy
b
Department of Geosciences, University of Padua, Padua, Italy
c
CSIRO Mineral Resources, Kensington, WA, Australia
d
Natural History Museum, London, UK
A R T I C L E I N F O A B S T R A C T
Keywords: This study provides an overview of the application of hyperspectral imagery acquired by the Italian satellite
Hyperspectral analysis mission “PRecursore IperSpettrale della Missione Applicativa” (PRISMA), for mapping hydrothermal and su
PRISMA pergene mineral alteration zones associated with different Cu deposits in the Chilean Andes. Study areas include
Porphyry copper deposits
the Marimaca Copper Project in the Naguayán district (Antofagasta Province), hosting Late Jurassic – Early
Iron oxide-copper–gold
Mineral mapping
Cretaceous iron oxide-copper–gold mineralizations and the Río Blanco-Los Bronces district (Santiago Region)
known for its Late Miocene – Early Pliocene copper-molybdenum porphyry deposits. These mineral systems
exhibit hydrothermal alteration haloes extending over several kilometers, with upward and outward mineral
zonation.
To characterize the surface-exposed alteration, the satellite imagery was processed using a multiple feature
extraction workflow targeting the relative abundances and compositions of specific supergene and hydrothermal
alteration minerals, including Fe-oxides and hydroxides (hematite-goethite), phyllosilicates (micas-kaolinite-
chlorite), hydroxyl-bearing sulfates (alunite-natroalunite-jarosite) and epidote. The results reveal a zonation
from proximal sulfates (natroalunite to alunite) and Al-rich white mica (ranging from 0.3 km-wide for the area
identified to the east of the Marimaca deposit to 1 km-wide in correspondence of the Los Sulfatos deposit,
respectively) to distal Al-poor white mica (0.2 to 0.5 km-wide, respectively) and an outer chlorite-epidote zone.
In the Marimaca Project area, the structurally-controlled alteration evolves west to east from chlorite-rich to
phengitic white mica (Al-poor). A NNE-SSW trending supergene leached cap covers a 6 km-long and up to 2 km-
wide area from north to south. Our study documents how spaceborne hyperspectral imaging spectroscopy can
support mineral exploration by enabling non-invasive reconnaissance mapping of the outcropping rocks,
providing specific targeting information for follow-up field surveys and drilling campaigns.
1. Introduction 2021). The present study interprets data from the Italian Space Agency’s
(ASI) PRISMA mission, which is part of a new generation of VNIR-SWIR
During the past decade, the hyperspectral remote sensing technology hyperspectral satellite sensors that have not yet undergone a compre
has been established as a robust tool for the interpretation of outcrop hensive evaluation for geological mapping and mineral exploration. In
ping mineral assemblages and lithological units, with important impli particular, the PRISMA data are here used for mapping distinct alter
cations for the mineral exploration and ore deposits characterization ation haloes associated with porphyry Cu and Iron Oxide-Copper-Gold
(Taranik and Aslett, 2009; Bedini, 2017; Peyghambari and Zhang, (IOCG) deposits in the Chilean Andes. The hydrothermal wall-rock
* Corresponding author.
E-mail address: rita.chirico@unipd.it (R. Chirico).
https://doi.org/10.1016/j.oregeorev.2024.105998
Received 5 February 2024; Received in revised form 19 March 2024; Accepted 20 March 2024
Available online 21 March 2024
0169-1368/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
A. Sorrentino et al. Ore Geology Reviews 167 (2024) 105998
alteration zones characterizing these deposit types (e.g., Sillitoe, 2010), and the iron oxide zonation associated with IOCG deposits have also
that are commonly used for vectoring toward the cores of the mineral been investigated through remote sensing in recent times (Corriveau,
ized systems, are highly suitable targets for satellite-based remote 2007; Mount Isa Inlier, Laukamp et al., 2011). Several applications of
sensing surveys, due to their diagnostic mineral compositions and often reflectance spectroscopy to the study of IOCG deposits mainly focused
large (kms wide) alteration haloes. on the characterization of the 2200 nm absorption feature, to discrim
Porphyry Cu(-Mo) and IOCG deposits represent the two most sig inate potassic alteration patterns, as well as defining variable intensity
nificant sources of copper in the Chilean Andes, followed by stratabound degrees of sericitic alteration associated with ore forming processes (e.
Cu(-Ag) ores, also known as manto-type Cu(-Ag) mineralization, and g., variation of aluminum content of phengite in Olympic Dam IOCG
massive magnetite deposits (Zentilli, 2022). These deposits are geneti deposit, Tappert et al., 2013), and on the identification of opaque phases
cally associated with subduction-related Andean arc magmatism (Silli for detecting “reduced” rock types (e.g., Starra area, Mount Isa Inlier,
toe, 2003; Sillitoe and Perelló, 2005; Maureira et al., 2022). Porphyry Cu Cudahy, 2016).
(-Mo) deposits are S-rich ores consisting of veins and disseminations of Literature regarding lithological mapping and geological application
chalcopyrite, bornite ± molybdenite and pyrite (e.g., Sillitoe, 1972, using the recently launched (2019) PRISMA hyperspectral satellite is
2010; Richards, 2009). IOCG deposits have been first described by quickly growing up, intending to evaluate and validate PRISMA for
Hitzman et al. (1992) and have become research objectives over the past various geological purposes, for example: (1) remote identification of
few years. IOCGs differ in some important characteristics from porphyry CO2 emissions (Romaniello et al., 2021, 2022) and (2) methane emission
Cu(-Mo), being mainly Fe-oxide rich (magnetite and/or hematite > 10 points (Guanter et al., 2021); (3) identification and mapping regolith
%) deposits, with chalcopyrite and minor bornite (±Au, Ag) as economic and supergene and hydrothermal alteration minerals (e.g., the Cuprite
minerals. Their origin is mostly considered to be related to the migration deposit in Nevada, Bedini and Chen, 2020; the Jabali deposit area in
of “oxidized”, acid and sulfide-poor hydrothermal metal-bearing fluids Yemen, Chirico et al., 2022; the Rocklea Dome area in Western
through structural lineaments (faults and/or shear zones) and to the Australia, Laukamp, 2022; the Ondoto carbonatite complex in Namibia,
precipitation of metals into “reduced” traps (Williams et al., 2005; Kopackova-Strnadova et al., 2023), and (4) lithological mapping (e.g.,
Groves et al., 2010; Chen, 2013; Richards and Mumin, 2013; Kreiner and the Makhtesh Ramon national park area in Israel, Heller Pearlshtien
Barton, 2017). et al., 2021; Laukamp, 2022). Nonetheless, the PRISMA data remains
The application of multispectral and hyperspectral remote sensing unexplored with respect to the mineralogical characterization of
technologies to the exploration of porphyry Cu-Mo deposits is long- porphyry-Cu deposits and IOCG mineralization.
established, and in the past, it was the focus of many studies, most of During the past decade, the availability of VNIR-SWIR imaging
them intended for mapping the hydrothermal host-rock broad alteration spectroscopy data from space has significantly increased. Several orbital
patterns characteristic of this deposit-type (Bedini, 2017; Peyghambari hyperspectral remote sensing missions involving various space agencies
and Zhang, 2021, and references therein; Portela et al., 2021). Several are in operational phase, for instance the ASI’s PRISMA, the German
studies have been conducted using multispectral data from the space Aerospace Center’s (DLR) DLR Earth Sensing Imaging Spectrometer
borne missions Landsat, Sentinel-2, WorldView-3 (WV-3) to map the (DESIS) and Environmental Mapping and Analysis Program (EnMAP), as
alteration mineral distribution yielding satisfactory results (Abrams well as the Hyperspectral Imager Suite (HISUI) of the Japanese Ministry
et al., 1983; Pour and Hashim, 2012; Chen et al., 2021; Ishagh et al., of Economy, Trade, and Industry (METI) and the NASA’s Earth Surface
2021). Notably, the free-of-charge Japanese Advanced Spaceborne Mineral Dust Source Investigation (EMIT) missions, while others are in
Thermal Emission and Reflection Radiometer (ASTER) mission aboard advanced stages of development.
the US TERRA platform (https://terra.nasa.gov/), which features a 14- The new generation of hyperspectral imaging sensors, including
band spectral resolution spanning from Visible Near (VNIR) – Short PRISMA, are notable for their high spectral resolution, which includes
wave (SWIR) to the Thermal Infrared (TIR) regions, facilitated a hundreds of spectral bands covering the VNIR-SWIR range. The DLR’s
comprehensive analysis of alteration minerals and their distribution (e. EnMAP system is characterized by 235 bands in the 420–2450 nm range
g., Cudahy et al., 2008, Pour and Hashim, 2012; Cudahy, 2016; Chen and 30 m GSD, while NASA’s EMIT has 280 bands ranging from 380 to
et al., 2021; Ishagh et al., 2021). However, multi-spectral systems with 2500 nm with a 60 m GSD, and the Japanese HISUI offers up to 20 m/
SWIR-2 spectral resolutions > 40 nm, such as ASTER, are insufficient for pixel VNIR-SWIR products with a mean spectral resolution around 11
remote detection of < 20 nm wavelength changes, rendering hyper nm. In contrast to the first hyperspectral VNIR-SWIR satellite sensor,
spectral data a more suitable alternative (Cudahy, 2016). For example, Hyperion, the signal-to-noise ratio of the new generation of hyper
by using the AVIRIS airborne hyperspectral data, Berger et al. (2003) spectral satellite sensors is much higher, vastly improving the ability to
detected sericitic, argillic and propylitic mineral alteration zones asso characterize mineral species and mineral composition. The abundance
ciated with the Red Mountain and Sunnyside deposits in Arizona. By of spectral information provided by these new sensors enables the
using Hyperion EO-1 spaceborne data, Zadeh et al. (2014) could map the collection of continuous data across the entire spectral range, in contrast
alteration systems associated with porphyry deposits, despite the mod to multispectral sensors, which sample discrete wavelength intervals.
erate spatial resolution (30 m/pixel) and low signal-to-noise ratio of the This capability allows for improved discrimination of surface features
sensor. Kokaly et al., (2017a) and Graham et al. (2018) through a compared to the multispectral sensors mentioned earlier. For example,
multiscale approach from field-based and laboratory-level spectroscopy PRISMA is valuable for the identification and geochemical character
to airborne hyperspectral imaging (HyMap; HyVista Corporation), per ization of several minerals which are common products of both super
formed characterization and mapping of the alteration zones occurring gene and hydrothermal alteration associated with porphyry-Cu(-Mo)
at the Orange Hill and Bond Creek Porphyry Cu-Mo deposits (Alaska). and IOCG ore-type systems (Sillitoe, 2010), because these phases exhibit
These authors focused on the identification of spectrally dominant diagnostic absorption features in the VNIR-SWIR wavelength region
minerals, like white micas, chlorite, clays and sulfates, paying special covered by the satellite system. These minerals are: Fe-oxides and hy
attention to the 2200 nm absorption feature and its wavelength position. droxides, micas (muscovite and phengite), clays such as kaolinite and
In this latest specific case study, the shifting to longer wavelengths pyrophyllite, chlorite-epidote, and sulfates like alunite and jarosite (e.g.,
observed at airborne viewing scale gave the authors critical information, Spatz, 1996; Tangestani and Moore, 2002; Berger et al., 2003; Bishop
resulting in the discrimination between the white mica associated with and Murad, 2005; Mars and Rowan, 2006; Riley et al., 2007; Pour and
the porphyry deposit formation (long wavelength, Al-poor or “phen Hashim, 2012; Bedini, 2017; Laukamp et al., 2021; Thiele et al., 2021,
gitic”) compared to the Al-rich white mica associated with plutonic and among others).
volcanic arc rocks not affected by magmatic-hydrothermal fluids. The present study aims at: 1) evaluating the capability of PRISMA
Similarly to the previous cases, the hydrothermal alteration patterns satellite hyperspectral imagery for mineral exploration, by inspecting its
2
performances in alteration mineral mapping, since it can represent a The methodology that we used is based on the combination of single
rapid and cost-effective mean for gaining and revealing similar infor feature extraction indices (or band ratios) into multiple feature extrac
mation from regional to district scale; 2) identifying the alteration as tion workflow applied to the PRISMA hyperspectral scenes with the
semblages for vectoring to the mineralized bodies, possibly defining new specific objective of: 1) mapping large footprint alteration signatures
exploration targets in the considered areas suitable for follow-up in (km scale), by extracting white micas, chlorite (and epidote), kaolin
vestigations. Two test areas were chosen: (1) the Marimaca Copper group minerals and sulfates (alunite and jarosite) and Fe-oxides and
Project area (Costal Cordillera, Antofagasta Region, northern Chile), and hydroxides relative abundances, and 2) identifying compositional vari
(2) the Río Blanco-Los Bronces copper-molybdenum porphyry district ation for white micas, and evaluating compositional variations for
(Central Andes, Santiago Region) (Toro et al., 2012; Piquer et al., 2015). alunite.
The areas were selected as test sites mainly because of the almost total
absence of vegetation cover and, above all, the well-known, well- 2. Regional geology
exposed and extended hydrothermal upward and outward zonation
patterns displayed by the host rocks in the surroundings of orebodies. The Central Andes of Chile are characterized by four longitudinal
The Río Blanco-Los Bronces Cu-Mo Porphyry district was considered as a metallogenic belts that are the host for a broad spectrum of economically
“validation site” for the present study, since the detailed mineralogical Cu deposits and prospects, containing also great quantities of other el
mapping of the hydrothermal alteration patterns has been published ements such as Fe, Au, Pb-Zn, Ag, or Mn (Sillitoe, 2003; Sillitoe and
(Irarrazaval et al., 2010; Toro et al., 2012), and allowed to test the ac Perelló, 2005). Each belt developed progressively from west to east,
curacy of the satellite mineral maps - this makes not strictly necessary to during distinct metallogenic periods, as a result of the eastward migra
have validation ground samples. For the Marimaca deposit area, on the tion of arc magmatism (Sillitoe et al., 1982; Beckinsale et al., 1985; Clark
other hand, published alteration mineral distribution maps are quite et al., 1990a; Noble and McKee, 1999; Gendall et al., 2000; Sillitoe and
limited, therefore a few descriptions of the geological features and the Perelló, 2005; García et al., 2017; Cabello, 2021). The epochs span from
mineralization characteristic, distribution and composition occurring in Middle - Late Mesozoic on the Pacific coast to Miocene - Early Pliocene
the exploration reports (Kalanchey et al., 2020) were used to cross-check along the eastern border of the orogen, at the Chile - Argentina boundary
the satellite maps. (Figs. 1a and 2a) (Sillitoe, 2003; Sillitoe and Perelló, 2005; Cabello,
Fig. 1. (a) Location of PRISMA image covering the Río Blanco-Los Bronces study area, main porphyry copper deposits, metallogenic belts and epochs are indicated
(modified from Sillitoe, 1988; Sillitoe and Perelló 2005); (b) Simplified geological map of the Río Blanco-Los Bronces Porphyry Cu-Mo district (modified from Toro
et al., 2012) showing only the area corresponding to the one covered by PRISMA. Coordinate Reference System: WGS 84 / UTM zone 19S.
3
Fig. 2. (a) Location of PRISMA image covering the Marimaca Copper Project area, IOCG, manto-type deposits, metallogenic belts and epochs are indicated (modified
from Sillitoe, 1988; Sillitoe and Perelló 2005); (b) Simplified geological map of the Marimaca study area (modified from Cortés et al., 2007) showing only the area
corresponding to the one covered by PRISMA. Coordinate Reference System: WGS 84 / UTM zone 19S.
2021). of the Chilean porphyry Cu (-Mo, -Au) deposits (e.g., Cerro Colorado,
The Mesozoic belt extends semi-continuously along the Coastal Spence, Chuquicamata, Esperanza, Escondida; Río Blanco–Los Bronces,
Cordillera of northern Chile with an average thickness of about 30 km El Teniente; Ossandón et al., 2001; Rowland and Clark, 2001; Bouzari
and hosts one of the world’s major IOCG belts (including deposits like and Clark, 2002; Cotton III, 2003; Maksaev et al., 2004; Deckart et al.,
Marimaca, Candelaria-Punta del Cobre and Mantoverde; Marschik and 2005; Padilla-Garza et al., 2005; Sillitoe and Perelló, 2005), along with
Fontboté, 2001; Benavides et al., 2007) (Fig. 2a), these deposits are small Cu (±Mo-Au-Ag) veins and tourmaline breccia-pipes, occurred
spatially associated with minor Cu-poor massive magnetite mineraliza from the Paleocene to Early Pliocene during relatively short time spans
tion, manto-type Cu (e.g., Mantos Blancos and El Soldado; Wilson et al., (lasting about 10 – 20 m.y.; Fig. 1a).
2003; Ramírez et al., 2006) and subordinate porphyry-Cu deposits (e.g.,
Andacollo; Llaumett et al., 1975; Munizaga et al., 1985). The formation 2.1. The Río Blanco-Los Bronces district
of IOCG deposits took place under extensional and transtensional tec
tonic regimes, developed in response of retreating convergent margin The giant Río Blanco-Los Bronces Porphyry Copper-Molybdenum
(slab roll-back) and high-angle subduction of the Phoenix plate. During district is located approximately 50 km northeast of Santiago. The dis
the Early Cretaceous, maximum crustal thinning occurred, causing a trict falls within the easternmost metallogenic belt of Late Miocene to
peak in the development of IOCGs accompanied by greater metal en Early Pliocene age and covers an area of ~ 15 km2 (Fig. 1a) (Toro et al.,
richments (Sillitoe, 2003; Chen, 2013). From the Late Cretaceous, 2012). The Río Blanco deposit is an underground mine owned by the
compressive tectonic pulses inverted most of the structures (fault and Andina Division of the state-owned Codelco-Chile, whereas the Los
back-arc basin), probably triggered by the final opening phases of the Bronces deposit is an open pit mine held by the Anglo American Sur. In
Atlantic Ocean and the decrease of the subduction angle at the Chilean 2009, the overall production amounted to 448150 t Cu and 2163 t Mo
trench (Mpodozis and Ramos, 1989; Larson, 1991; Ladino et al., 1997), (Deckart et al., 2014).
causing intense shortening and thickening of the upper crust. This The porphyries are hosted by volcanic and volcanoclastic rocks of the
resulted in a drastic reduction in the formation of IOCG, massive Abanico and Farallones Formations (~23 – 17 Ma; Fig. 1b), deposited,
magnetite and manto-type deposits, and creating favorable conditions respectively, during the phases of opening and subsequent inversion of
for the genesis and development of porphyry-Cu systems. The formation the Abanico Basin (Piquer et al., 2015). These formations were intruded
4
by numerous plutonic rocks of Miocene to Pliocene age, like the large geological model and the sulfide potential under the Marimaca deposit
San Francisco batholith (~16 – 8 Ma; Fig. 1b), a granodiorite-dominated and in the surrounding areas. The results of the aeromagnetic survey,
complex extending at surface over an area of at least 200 km2 (Warnaars conducted over an area of 2 x 2 km, show a large magnetic anomaly
et al., 1985; Toro et al., 2012). The major Cu(-Mo) mineralization was extending at least 700 m depth below the mineralized area and that is
associated with the emplacement of several porphyry intrusions controlled by west–northwest-trending fault systems (Kalanchey et al.,
centered on the east side of this batholith, whose position was controlled 2020).
by closely spaced N-NW-trending basement faults defining structural The local geology consists mainly of Jurassic volcanic and intrusive
corridors (Warnaars et al., 1985; Toro et al., 2012). The copper-iron- rocks, overlaying sporadic outcropping of the oldest Devonian meta
molybdenum sulfides occur as quartz-vein stockworks and dissemina sedimentary formation (Sierra del Tigre) (Fig. 2b) (Cortés et al., 2007).
tions as well as vertically continuous hydrothermal tourmaline breccia The first evidence of the establishment of a subduction-related
complex (Warnaars et al., 1985; Serrano et al., 1998; Ricardo et al., magmatic belt is the emplacement of thick basaltic to andesitic volca
1999; Skewes et al., 2002; Deckart et al., 2005; Irarrazaval et al., 2010). nic sequence of the La Negra Formation (Sinemuriano – Titoniano;
The mineral system exhibits, at district-scale, a typical vertical and Fig. 2b), which evolved over the time from tholeiitic to calc-alkaline
lateral hydrothermal zonation pattern with remnants of high sulfidation affinity (Kramer et al., 2005; Cortés et al., 2007). The Lower Jurassic –
and/or advanced argillic assemblages at shallow levels, surrounded by Early Cretaceous plutonic rocks, also of calc-alkaline affinity, comprise
sericite-illite, whereas K-silicate alteration assemblages are mainly diorite, monzonite and monzodiorite, with lesser gabbro, quartz
present in association with chalcopyrite-bornite and chalcopyrite-pyrite monzonite and metadiorite bodies (Fig. 2b) (Cortés et al., 2007). The
at depth (Sillitoe and Perelló, 2005). Following breccia formation, intrusions belonging to the Naguayán Plutonic Complex (~154 Ma) and
magmatic activity was characterized by late mineral dacite porphyry the associated dyke swarms of gabbro through rhyodacite composition,
intrusions culminated in the post-mineral intrusive and extrusive La characterized by variable orientation ranging from oldest to youngest,
Copa rhyolite breccia complex (Fig. 1b) (Irarrazaval et al., 2010). from northeast–southwest to north– south, to northwest–southeast,
The Los Bronces – Río Blanco area was affected by at least three display close relationship with the Fe Oxide-Cu-Au mineralizing systems
porphyry-related hydrothermal alteration-mineralization and associ in the area (Fig. 2b) (Cortés et al., 2007; Kalanchey et al., 2020). Alluvial
ated hydrothermal-breccia events (Irarrazaval et al., 2010). The breccia and marine deposits of Mio-Pliocene to Quaternary age overlie all the
cement displays a vertical hydrothermal zonation, varying from quartz, mentioned units (Fig. 2b) (Cortés et al., 2007). The main tectonic
sericite, tourmaline with pyrite > chalcopyrite, in the upper levels, to structure is represented by the Atacama Fault Zone (AFZ), which is an
biotite, K-feldspar with chalcopyrite, bornite > pyrite, in the deeper arc-parallel wrench fault system extending for > 1000 km along the
levels (Toro et al., 2012). Coastal Cordillera (Scheuber and Andriessen, 1990). To the west, the
The Los Sulfatos area is centered on an extensive igneous to Naguayán Banded Fracture Belt (NBFZ) forms a wide zone of sub-
magmatic-hydrothermal breccia complex, related to the La Paloma and parallel faults and fractures (around 15 km-long and 3 km-wide) with
Los Sulfatos main porphyry centers (Irarrazaval et al., 2010; Toro et al., trend north–south to north–northeast, dipping at 40–60◦ to the east or
2012). The hydrothermal breccia complex is cemented by tourmaline southeast, to which is associated the emplacement of pre-mineral mafic
near surface and by biotite at depth. At shallow levels, the cement dykes and, subsequently, that of the mineralization (Kalanchey et al.,
breccia is distinctly affected by sericitic alteration and contains chal 2020).
copyrite – pyrite, minor tennantite-tetrahedrite, galena, sphalerite and The Marimaca deposit has affinities mainly with vein-style IOCG
ankerite that make up high-grade copper bodies (on average ~ 3––15 % deposits and subordinately with “manto-type” mineralization styles. The
Cu). The porphyry centers at Los Sulfatos and La Palomas are also mineralization is structurally controlled by fractures of the NBFZ and
characterized by peripheral chlorite alteration partially overprinting the displays a hydrothermal ore style, occurring as massive orebodies, veins,
potassic alteration in the core of the system. Remnants of advanced stockworks and breccias. The mineral assemblage consists mainly of
argillic haloes (kaolinite, alunite) are also preserved (Irarrazaval et al., chalcopyrite, as the major copper-bearing mineral, moderate to minor
2010). pyrite, minor bornite, covellite and primary chalcocite (Kalanchey et al.,
The Río Blanco-Los Bronces district is characterized by secondary 2020; Oviedo, 2022). The emplacement of hypogene orebodies is pre
enrichment and leached capping with presence of Fe-oxides and hy ceded and accompanied by hydrothermal alteration, characterized by
droxides, limited to a restricted area, whose shape and depth suggest regional calcic–sodic metasomatism with albite replacing plagioclase
that the supergene processes are related to the present vadose-water and actinolite and magnetite replacing mafic minerals. Further alter
downflow due to snowmelt during summer months and are still active ation minerals range from chlorite and epidote to sericite and hematite
(Warnaars et al., 1985; Sillitoe, 2005). in the proximal zone to precious metals, following the common alter
ation pattern proposed by Hitzman et al. (1992) and subsequently
2.2. The Marimaca Copper Project modified by Sillitoe (2003), Williams et al. (2005) and specifically by
Kreiner and Barton (2017) for sulfur-poor IOGC systems. Towards the
The Marimaca Copper Project is located in the Antofagasta Province, east of the eastern limit of the main mineralized area, the so-called
approximately 45 km north of Antofagasta and 1250 km north of San “hanging wall alteration front” controls the mineralization toward the
tiago, within the Mesozoic Coastal Cordillera in the Naguayán district top of the parallel-fractured monzonite and diorite units and dykes-
(Fig. 2a). The geographic UTM coordinates of the center of the area are related orebodies and marks the beginning of a prominent alteration
374,820 E and 7,435,132 S (Zone 19S, Datum WGS84) and the zone is characterized by more hematite than magnetite, a well-developed
characterized by minimal vegetation cover. The project is held 100 % by argillic halo and intense albitization. The feeder zones close to the
the Marimaca Copper Corporation, the concession covers an area of 742 hanging wall alteration display white clay (albite–sericite) and minor
km2, subdivided in two packages: the Marimaca area (625 km2) and the chlorite-hematite haloes (Oviedo, 2017; Kalanchey et al., 2020; Oviedo,
Iván area (116 km2). 2022). Towards west of the mineralization, the “footwall alteration”
The Marimaca mine was active from the 1990 s to the mid-2000 s, consists of more actinolite and magnetite, chlorite haloes and variable
through underground mining and small-scale artisanal pits that have degrees of albitization (Oviedo, 2017, 2022).
produced around 100 Mt of copper oxides between 1 and 2 % of Cu. In The Marimaca area has undergone supergene oxidation resulting in
2022, the estimated mineral resources (measured plus indicated) were the formation of an enriched zone of secondary sulfides, overlaying the
140 Mt at 0.48 % Cu (Oviedo, 2022). Since 2016, the mining company primary mineralization, and of a limonitic leached cap which comprises
has been reassessing historical drilling and conducting new drilling, Fe oxy-hydroxides (hematite and goethite), deriving from the alteration
geological mapping, sampling and geophysical surveying to define the of pyrite and magnetite, with minor clays and gypsum, and occurrences
5
of jarosite as haloes surrounding the northwest-trending faults. The 1985; Morris et al., 1985; Burns, 1993; Cudahy and Ramanaidou, 1997).
oxide zone is exposed on surface and extends around 150 – 300 m in Three other processes, instead, cause the absorption features in the
depth (Kalanchey et al., 2020). SWIR region (e.g., Laukamp et al., 2021): (1) first overtones of funda
mental stretching vibrations of hydroxyl groups (2νOH) in hydroxyl-
3. Data and methods bearing minerals or a combination band of the OH-related stretching
fundamental and a first overtone of a H2O-related bending vibration
3.1. The PRISMA hyperspectral data: Characteristics and processing (2δH2O); (2) combinations (ν + δOH) of hydroxyl-related fundamental
stretching (νOH) and bending vibrations (δOH), and (3) CO3-related
The hyperspectral PRISMA scenes covering the Marimaca and Río combinations (e.g., 2ν3 + ν1 CO3) or overtones (e.g., 3ν3CO3) of funda
Blanco-Los Bronces-Los Sulfatos districts were acquired and down mental stretching vibrations.
loaded from the PRISMA mission website (PRISMA data portal - https: Looking at the specific minerals of interest, white micas (e.g.,
//www.prisma.asi.it; accessed on 2 February 2022) at respectively (1) muscovite, phengite, paragonite) and Al-bearing smectites (e.g., mont
14:58:05 on 26 January 2022 (UTC), and (2) at 14:43:24 a.m. on 17 morillonite, beidellite) comprise a wide and variable group of minerals
February 2022 (refer to Table A1 for the acquisition conditions of the characterized by the main absorption feature at ~ 2200 nm (Al-OH)
analyzed scenes). The Level 2C products, already corrected for the at (Laukamp et al., 2021; Meyer et al., 2022). The position of this feature is
mospheric effects (Bottom-Of-Atmosphere reflectance; see product assumed to be proportional to the octahedral cation composition of
specifications for more details at www.prisma.asi.it) (ASI, 2020), were white mica and Al-smectites, following the coupled octahe
analyzed by several pre-processing and processing steps carried out dral–tetrahedral Tschermak exchange (Mg, Fe2+)VI + SiIV ↔ AlIV + (Al,
using ENVI 5.6.2 software with the aim of obtaining hyperspectral Fe3+)VI (Velde, 1965; Duke, 1994; Laukamp et al., 2021; Meyer et al.,
mineral maps (Fig. 3). The VNIR and SWIR bands were first adjusted to 2022). The exchange of divalent and trivalent Fe, Mg and/or Al in the
improve geolocation accuracy, then corrections and filters were applied octahedral layer (M− site) results in variations of the M− OH bond
for orthorectification, cross-track illumination and smoothing. Subse length, causing a shift of the 2200 nm absorption feature towards shorter
quently, pixels that most likely do not contain the target minerals (e.g., (Al-rich) or longer (Al-poor) wavelengths. The investigation of this shift
areas characterized by alluvial-colluvial cover), and areas affected by can give useful information regarding the type and intensity of the
shadows or belonging to non-geological materials (mainly vegetation, alteration, as well as for defining pH/geochemical gradients of the
snow and man-made features), were masked and excluded (Fig. 3). More metallogenetic fluids (Halley et al., 2015; Wang et al., 2017; Meyer
comprehensive details of the employed methodology can be found in the et al., 2022). Similarly, variations in the position of the feature at around
Appendix. 2160 nm, at either ~ 2159 or ~ 2170 nm, allow for the characterization
of kaolinite and alunite, respectively, associated with the evaluation of
the presence and shifting of features at ~ 1480 (OH bonds) and ~ 1760
3.2. The multiple feature extraction method applied to the PRISMA
nm (SO2- 4 bonds) characteristic of sulfates (like alunite). The shift in this
hyperspectral images
secondary absorption feature at 1480 nm is due to the contrasting bond
length/strength of K+ and Na+ linked to OH groups. Specifically, an
The VNIR-active minerals absorptions are mainly caused by elec
absorption feature around 1470 nm suggests the presence of alunite,
tronic processes, like the Charge Transfer Feature (CTF) and the Crystal
while an absorption nearing 1495 nm suggests the presence of
Field Absorption Feature (CFA) in, for example, iron oxides (Curtiss,
Fig. 3. Schematic workflow showing the pre-processing steps and the multiple feature extraction method used in this study.
6
natroalunite (Clark et al., 1990b; Thompson et al., 1999; Kerr et al., 4. Results
2011; Arbiol and Layne, 2021). This characteristic provides information
about the temperature of formation and can help in vectoring toward the 4.1. PRISMA mineral maps
center of the mineral system (Laukamp et al., 2021; Bishop and Murad,
2005; Stoffregen and Cygan, 1990). 4.1.1. The Río Blanco-Los Bronces Porphyry Cu-Mo district
These characteristics of absorption features can be determined and Fig. 4 reports the results of the spectral mapping obtained by means
characterized from reflectance spectra through the application of spec of the feature-extraction band ratios proposed in this study. In Fig. 4a the
tral indices and band ratios. The hyperspectral products obtained using areas characterized by the presence of the Al-OH feature at 2200 nm,
the band ratio method allow the visualization of wavelength position corresponding to Al-sheetsilicates, such as white micas, kaolinite and
variation (chemical composition) and absorption depth (absolute in smectites, are displayed with increasing relative abundance represented
tensity) of diagnostic features shown along the spectral signatures of by a gradient from blue to red colors.
rock-forming and alteration minerals defining each pixel of the hyper The zones defined by deeper 2200 nm feature (in yellow to red
cube acquired in the study area. The minerals features considered for colors), which mean higher relative abundances, occur over a wide area
this study for mineral mapping are referenced in Table 1 (Burns, 1993; in correspondence of the major mineralized deposits at Ortiga, Los
Cudahy and Ramanaidou, 1997; Laukamp et al., 2021 and references Sulfatos and Río Blanco-Los Bronces (Fig. 1b), as well as in other minor
therein). areas in the northern and the eastern portions covered by the PRISMA
We mapped the distribution of (1) epidote and chlorite, (2) white image. As illustrated in Fig. 4b, pixels characterized by the presence of
mica, (3) kaolinite, (4) pyrophyllite, (5) alunite, and (6) Fe oxides and the 2250 nm feature, corresponding to the association chlorite +
hydroxides. These hyperspectral mineral maps were lately interpreted as epidote, are distributed in several zones over the entire area covered by
“alteration zones” following the hydrothermal and supergene alteration PRISMA, generally appearing as outer rims of the areas outlined in the
models available for the study areas. The presence/absence of a 2200D map (Fig. 4a). Well defined geometries mainly occur in the Los
distinctive absorption feature was highlighted using a multiple feature Sulfatos area, as well as in distal areas north and south from Ortiga. Both
extraction workflow and thresholds in order to separate each mineral areas were better characterized by applying the 2200W, the 2160D and
ogical phase. Table 2 provides the expression for each index, constructed the 1480D indices (Figs. 4c, d and e), which describe the wavelength
using band combinations and ratios for mapping several target minerals shift of the 2200 nm feature and the depth of the 2160 nm and the 1480
based on their diagnostic spectral feature. A detailed description of the nm absorptions, respectively. The former, shown in Fig. 4c, defines a
spectral indexes construction and minimum thresholds definition is concentric pattern ranging from lower values (blue colors) in the center
provided in the Appendix, together with the high-resolution electronic – indicating Al-rich sheetsilicates characterized by the feature at around
versions of the PRISMA spectral mineral maps. 2191–2199 nm – evolving to higher values (from green, yellow to red
The hyperspectral mineral mapping performances are assessed by colors) in the outer zones – due to the absorption occurring at around
building up ROIs on PRISMA imagery over selected areas where the 2199–2206 nm.
target alteration mineral is defined by the PRISMA-derived mineral The highest values of the 2160D index (red colors in Fig. 4d) suggest
mapping. The spectra were, then, compared to the USGS reference that the Ortiga area is predominantly composed of kaolinite and alunite,
spectra (Kokaly et al., 2017b) representative of each target phase both defined by a characteristic doublet consisting of a major feature at
downsampled to the PRISMA hyperspectral resolution after continuum around 2206 nm related to a minor feature at 2160 nm and 2170 nm,
removal to normalize the spectra. respectively. Based on the 1480D index results, some areas are charac
terized by higher relative abundances of alunite (red colors in Fig. 4e).
On the contrary, the absence of the feature at 2160 nm at Los Sulfatos
(except for very limited areas; Figs. 4d and 5), suggests that white micas
are the main Al-sheetsilicates composing the altered rocks of the area.
White micas in the Los Sulfatos area evolve from Al-rich (i.e., muscovite)
in proximity of the main orebody to Al-poor white micas (or Al-rich
Table 1
VNIR electronic processes and SWIR 2-active vibrational modes typically observed in rock-forming and alteration minerals and their wavelength positions (expressed
in nm).
Mineral Group Mineral Species Assignment of Wavelength (nm) References
Absorption
Lower Limit Upper Limit (nm)
(nm)
di-oct. sheet silicate muscovite, phengite, ν + δ (M)2OH 2185 ([VI]Al- 2215 ([VI]Al-poor) (Vedder and McDonald, 1963)
paragonite rich)
kaolin group ν + δ M2OHo 2159 (Frost and Johansson, 1998)
ν + δ M2OHi 2209
pyrophyllite ν + δ MnOH 2166 (Zhang et al., 2005)
sulfate alunite 2νMnOH, ν + 2δH2O 1473 (K-rich) 1491 (Na-rich) (Bishop and Murad, 2005)
ν + 2δMnOH 1762 (Na-rich) 1764 (K-rich) (Bishop and Murad, 2005, Chang et al.,
2011)
ν + δ MnOH 2172 (Bishop and Murad, 2005)
sulfate jarosite 2νMnOH, ν + 2δH2O 1471 (Bishop and Murad, 2005)
tri-oct. sheet silicate clinochlore, ν + δ M2OH 2248 2261 (McLeod et al., 1987; Bishop et al., 2008)
chamosite,
ripidolite (Mg-rich) (Fe2+-rich)
sorosilicate epidote group ν + δ MnOH 2250 (White et al., 2017)
Fe-oxides and goethite, hematite, CFA 870 (hematite) 930 (goethite), 910 (Cudahy and Ramanaidou, 1997; Crowley
hydroxides jarosite (jarosite) et al., 2003)
Notes: Lower and upper wavelength positions only given for absorption bands where related compositional changes are discussed in this study (see Laukamp et al.,
2021 and references therein for further details).
7
Table 2
Band ratios used for extracting the relative intensity and wavelength position of the target minerals’ absorption features. Method based on Laukamp (2022).
Mineral Abundance Composition PRISMA depth PRISMA Lower Upper Masks
(intensity) band (wavelength) band (intensity) band wavelength band stretching stretching
ratio ratio ratio ratio limit limit
Fe (oxyhydr-) oxides 900D (B44 + B17SWIR)/ 2.16 2.20

abundance (FOA) B57
Al-sheetsilicates 2200D (B126 + B135)/ 2.05 from 2.10 to
B131 2.23
Al-sheetsilicate 2200W (B129/B133) 0.93 1.04 2200D > 2.054
Chlorite + Epidote 2250D (B134 + B139)/ 2.05 2.10
B137
Kaolin group 2160D (B123 + B131)/ from 2.025 to 2.13
B126 2.05
Kaolinite-Alunite/ 2160W B125/B126 0.95 1.05 2160D > 2.025
Pyrophyllite
Alunite/Jarosite 1480D (B51 + B56)/B52 2.25 2.30 2160D > 2.025–2.05 (for
alunite) 900D > 2.16 (for
jarosite)
Alunite 1480W B52/B53 0.89 0.93 2160D > 2.025 1480D >
2.26
Alunite 1760D (B76 + B82)/B80 2.18 2.20 2200D > 2.054 2160D >
2.025
phengite; defined by a feature centered at 2199–2206 nm) in distal is shown. Most white micas-bearing pixels to the south of the mineral
areas. As shown in Fig. 5, the Los Sulfatos deposit area is characterized ized deposit, are characterized by values higher than the established
by zoned geometry consisting of local concentric pattern that is defined threshold of ~ 1.04 (green to red colors), thus displaying a longer Al-OH
predominantly by Al-rich white micas (~1 km wide zone) in the center, absorption wavelength position close to 2206 to 2220 nm, meaning that
and, to a letter extent, by kaolinite(+alunite) (few pixels), followed by a the Al-sheetsilicates composition varies mainly from intermediate to Al-
concentric outer zone (~500 m wide) of moderate-wavelength white poor white mica (“phengitic”). A small area located ca. 6 km east from
mica, and then a more distal (~2 km away) chlorite + epidote zone the main project, hereafter “new target”, is characterized by a shorter Al-
exposed (at least 1 km wide) on at one side (west) of the white mica OH absorption wavelength position close to 2190–2200 nm (dark blue
proximal alteration zonation. zones in Fig. 6c), indicating that the Al-sheetsilicates composition is
However, the precise spatial pattern of the alteration zonation in this predominantly Al-rich (i.e., in the area white mica and kaolinite, as well
area is compromised by extensive transported cover and/or deep shade as Al-rich phengite, prevail). High values of the 900D (corresponding to
at the time of satellite image acquisition. The Ortiga area, instead, is Fe-oxides to hydroxides relative abundances) have been revealed both in
defined by a widespread zone with kaolinite and alunite mixture the Marimaca Copper Project area (including in the Mercedes, Roble and
(2160–2170 nm) with locally higher alunite contents (1480 nm), Cindy prospects) and in the new target area (red colors in Fig. 6d). In
evolving to variably extended white micas and chlorite + epidote general, it is feasible to differentiate hematite from goethite by
alteration zones – determining a zoned NW-SE trending alteration halo analyzing the shift around the 900 nm absorption feature using the
covering over 200 km2 (Fig. 4). The Río Blanco deposit area, although Hematite-Goethite ratio. However, in this study, their discrimination
partially covered by alluvial and/or colluvial sediments, is typified by was not possible. This could be due to either the pronounced mixing of
high relative abundances of Al-sheetsilicates, consisting mainly of white these two mineral phases or the predominance of goethite at the surface.
mica and/or Al-rich phengite as indicated by the 2200W compositional It may also be attributed to the see-saw pattern superimposed on the
index, whose values shift from intermediate compositions towards the longer VNIR wavelength range, or it could be a combination of all these
Al-poor end-member (pixels from green to red colors, Fig. 4c). The factors.
kaolinite and alunite phases are absent in the surrounding area of this A more detailed view of the Marimaca Copper Project (Fig. 7)
deposit, as indicated by the 2160D, as their values do not exceed the highlights zones with relatively high values of the 2200D ratio (corre
lower threshold, except for a spot ~ 3 km SE of the main Río Blanco sponding to the occurrence of Al-sheetsilicates) surrounding the main
deposit, surrounded in the outer zone by high values of Al-poor white mineralized area, mostly located to the east of the eastern limit of the
mica (Fig. 4d). In the same area, only a few pixels, towards the northeast mineralization (that is known as “hanging wall alteration front”, see
and southwest, are characterized by high relative abundance values of Kalanchey et al., 2020) (Fig. 7a).
chlorite and epidote (Fig. 4b). As previously shown in Fig. 6c, these areas are characterized by in
termediate to longer wavelength white micas, occurring in spots east of
4.1.2. The Marimaca district the Marimaca Copper Project areas (green to yellow colors), likely
In Fig. 6a, red colors correspond to areas characterized by high indicating a more phengitic composition. On the contrary, higher values
2200D values, which refer to relatively higher abundances of Al- of the 2250D index mainly characterize sporadic but widespread por
sheetsilicates (white micas and/or kaolin group minerals). The out tions (“haloes”) to the west of the main mineralized area (Fig. 7b).
lined pixels extend over a wide area of more than 30 km2, occurring in Relatively higher abundances of ferric iron (oxy-) hydroxides (Fig. 7c)
correspondence with the known Cu-mineralization occurrences. The characterize the central portion of the Marimaca Copper Project,
zone grades west- to southward to areas characterized by chlorite + including the Marimaca deposits and the other minor prospects (from
epidote relatively higher abundances as indicated by pixels with high north to south Cindy, Roble, Mercedes, Tarso and Sierra; Fig. 2b). In
2250D values (red colors in Fig. 6b). Fig. 7d, the results regarding the 1480 nm feature depth index (1480D),
The two defined areas are separated by a white mica + chlorite only performed on selected pixels with high 900D, are shown. The areas
transitional zone occurring in between, which is characterized by a corresponding to the Roble, Cindy and Emilia prospects are character
predominance of white mica over chlorite + epidote. In Fig. 6c, the ized by higher 1480D values, indicating higher relative abundances of
2200 nm wavelength-position band ratio (2200W), only performed on jarosite in those areas compared to the Marimaca Copper Project zone.
those pixels with 2200D values higher than the lower threshold defined, The linear, NNE-trending arrangement of high pixels with high (red)
8
(caption on next page)
9
Fig. 4. Mineral maps of feature-based spectral indices applied to the PRISMA L2C scene of the Río Blanco-Los Bronces district: (a) 2200 nm feature depth (2200D)
map, minimum threshold value at 2.05; (b) 2250 nm feature depth (2250D) map, minimum threshold at 2.05; (c) 2200 nm feature wavelength (2200W) map, masked
for 2200D > 2.05; (d) Kaolin group relative abundance (2160D) map, minimum threshold value at ~ 2.03; (e) Alunite relative abundance (1480D) map, masked for
2160D>~2.03. D is the depth (intensity) of the spectral absorption feature (i.e., relative abundance), W is the wavelength position (i.e., composition). All the maps
are masked for Miocene – Quaternary alluvium and colluvium deposits, indicated as white areas with black stripes. The white areas indicate the pixels masked out for
the terrain shadows, the occurrence of vegetational and snow cover, as well as areas where the relative abundance does not exceed the minimum threshold value.
Dashed purple boxes show the location of Fig. 5. Lower-case letters indicate the corresponding PRISMA spectrum in Fig. 9. The box in the bottom right corner shows
the position of the PRISMA imagery. Background World Imagery data sources: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, AeroGRID, ING,
and the GIS User Community. *Data from Toro et al. (2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
Fig. 5. Magnified mineral maps of feature-based spectral indices applied to the PRISMA L2C scene of the Los Sulfatos deposit area: (a) 2200 nm feature depth
(2200D) map, minimum threshold value at 2.05; (b) 2250 nm feature depth (2250D) map, minimum threshold at 2.05; (c) 2200 nm feature wavelength (2200W)
map, masked for 2200D > 2.05; (d) Kaolin group relative abundance (2160D) map, minimum threshold value at ~ 2.03. All the maps are masked for Miocene –
Quaternary alluvium and colluvium deposits, indicated as white areas with black stripes. The white areas indicate the pixels masked out for the terrain shadows, the
occurrence of vegetational and snow cover, as well as areas where the relative abundance does not exceed the minimum threshold value. Lower-case letters indicate
the corresponding PRISMA spectrum in Fig. 9. *Data from Toro et al. (2012).
1480D values can be ascribed to instrument striping and does not (+alunite), which are both characterized by broad absorption features at
represent a geological feature. around 2160 nm (2172 and 2166 nm, respectively), whereas lower
Fig. 8 shows in more detail mineral maps corresponding to the 2160W values occurring in pixels along the rims of the body suggest a
2200D, 2250D, 2160D, 2160W, 2200W, 1480D, 1760D, 1480W and greater relative abundance of kaolinite (for which one of the diagnostic
900D ratios, which can be used for vectoring towards the so-called “new spectral feature occurs at shorter wavelength, i.e. 2159 nm). The anal
target” occurring just a few kilometers (~6 km) east from the Marimaca ysis of the wavelength position of the Al-OH absorption feature is shown
Copper Project. The area is characterized by a circular zoned pattern in the 2200W map in Fig. 8e. The PRISMA-derived map displays very
highlighted by high 2200D values (Al-sheetsilicates relative abun low 2200W values at the center of the ore body, suggesting occurrence
dances; Fig. 8a). To the east, the area is characterized by higher 2250D of kaolinite and Al-rich white mica. The varying position of the feature
values, suggesting the presence of distal chlorite + epidote (Fig. 8b). The at around 2200 nm, which from the core zone outwards shifts to longer
2160D map (kaolin group minerals abundances and pyrophyllite/ wavelengths (from 2199 to 2214 nm), evidences a varying composition
alunite; Fig. 8c) reports higher values in the center of the area. As shown grading outward to an Al-poor (“phengitic”) white mica.
by the 2160W map in Fig. 8d, a shift to longer wavelengths Both the features at ~ 1480 nm and ~ 1760 nm, characterize alunite
(~2167–2170 nm) is recorded in the core zone of the area, potentially (Tab. 1). The wavelength position of the OH + H2O-related alunite ab
corresponding to a higher amount of either alunite or pyrophyllite sorption feature is shown in the 1480W map in Fig. 8h. The results show
10
Fig. 6. PRISMA-derived mineral maps of the Marimaca Project area, obtained by applying the feature-based spectral indices; (a) 2200 nm feature depth (2200D)
map, minimum threshold value at 2.05; (b) 2250 nm feature depth (2250D) map, minimum threshold value at 2.05; (c) 2200 nm feature wavelength (2200W) map,
masked for 2200D > 2.05; (d) Ferric Oxides Abundance (900D) map, minimum threshold at 2.16. All the maps are masked for Mio-Pliocene to Quaternary deposits,
indicated as white areas with black stripes. The white areas indicate the pixels masked out for the terrain shadows and the vegetational cover, as well as areas where
the relative abundance does not exceed the minimum threshold value. Dashed purple boxes refer to the main areas of interests (magnifications in Figs. 7 and 8).
Lower-case letters indicate the corresponding PRISMA spectrum in Fig. 9. The box in the bottom left corner shows the position of the PRISMA imagery. Background
World Imagery data sources: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, AeroGRID, ING, and the GIS User Community. *Data from Cortés
et al. (2007). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
11
Fig. 7. Magnified PRISMA-derived mineral maps on the main mineralized area of the Marimaca Project, obtained by applying the feature-based band ratios; (a)
2200 nm feature depth (2200D) map, minimum threshold value at 2.05; (b) 2250 nm feature depth (2250D) map, minimum threshold value at 2.05; (c) Ferric Oxides
Abundance (900D) map, minimum threshold value at 2.16; (d) Jarosite relative abundance (1480D) map, masked for 900D > 2.16. All the maps are masked for Mio-
Pliocene to Quaternary deposits, indicated as white areas with black stripes. The white areas indicate the pixels masked out for the terrain shadows and the
vegetational cover, as well as areas where the relative abundance does not exceed the minimum threshold value. Deposit number is reported in Cortés et al. (2007).
*Data from Cortés et al. (2007); Kalanchey et al. (2020); Oviedo (2017, 2022).
higher values in the core zone, corresponding to Na-rich alunite char 2OH at ~ 2200 nm) appears well defined; although smaller, the same
acterized by the main feature shifted at longer wavelength at ̴ 1491 nm, feature is well visible also in ROIs containing other minerals, like
compared to the rims (1480 nm). The area is widely characterized by chlorite and epidote (e.g., Fig. 9k), indicating a complex assemblage of
high 900D values (Fig. 8i), with highest values in correspondence of the alteration minerals. Moreover, as shown in Fig. 9e the displacement of
main kaolinite/alunite-bearing zone and of the areas with higher 2200D the center of the 2200 nm feature, from 2191 to 2200 nm, is well visible
(Al-poor white mica-muscovite) and 2250D values (Figs. 8a to h). at the PRISMA resolution. The chlorite and epidote main spectral ab
sorption feature (ν + δMnOH at ~ 2250 nm) is also sharp (Figs. 9d, f, h, j
and k) accompanied by a feature at ~ 2340 nm, and a deepening of the
4.2. Evaluation of the PRISMA performances through the analysis of the
absorption is shown by the spectra in Figs. 9d and h, referring to the
spectra
ROIs collected in the areas with highest 2250D values (see Figs. 4 and
6b).
The spectra derived from the selected ROIs (in black dotted lines),
A shallower feature is shown in the ROIs g, k and l (Figs. 9f, j and k)
created on the investigated PRISMA L2C VNIR-SWIR scenes are reported
collected in the Los Sulfatos and the new target areas (Figs. 5b and 8b).
in Fig. 9 and compared with USGS spectral library spectra of the target
The spectrum l, moreover, defines an area characterized by a mixture of
minerals (version 7; Kokaly et al., 2017b). Most of the PRISMA spectral
white mica and chlorite, as both the features at 2200 nm and 2250 nm
signatures look very similar and consistent with the USGS spectral li
are present. The ROIs picked up in the areas characterized by high
brary. In the PRISMA spectra deriving from high-2200D ROIs (e.g.,
values of the 2160D band ratio (Figs. 4d, 8c and d), report spectral
Figs. 9e and g), the main absorption feature of Al-sheetsilicates (ν + δ(M)
12
(caption on next page)
13
Fig. 8. Magnified PRISMA-derived mineral maps on the new target area obtained by applying the feature-based band ratios; (a) 2200 nm feature depth (2200D) map,
minimum threshold value at 2.05; (b) 2250 nm feature depth (2250D) map, minimum threshold value at 2.05; (c) 2160 nm feature depth (2160D) map, minimum
threshold value at ~ 2.03; (d) Kaolinite-alunite compositional variation based on the position of the absorption feature between 2159 nm (kaolinite) and 2167 nm
(alunite) (2160W), masked for 2160D>~2.03; (e) 2200 nm feature wavelength (2200W) map showing the wavelength shifting around the 2200 nm feature (2191 nm
to 2214 nm) meaning the presence of Al-rich (white mica, kaolinite) and Al-poor (phengite) sheetsilicates, masked for 2200D > 2.05; (f) 1480 nm feature depth
(1480D) map, masked for 2160D>~2.03; (g) 1760 nm feature depth (1760D) map, masked for 2160D>~2.03; (h) Alunite compositional variation based on the
1480 nm wavelength (1480W) map, masked for 1480D > 2.25; (i) Ferric Oxides Abundance (900D) map, minimum threshold value at 2.16. Lower-case letters and
black boxes (from o to s) indicate the corresponding ROIs-derived PRISMA spectrum in Fig. 12. Lower-case letters and pink boxes indicate the corresponding 6-pixels-
ROI-derived PRISMA spectrum in Figs. 9d-n. All the maps are masked for Mio-Pliocene to Quaternary deposits, indicated as white areas with black stripes. The white
areas indicate the pixels masked out for the terrain shadows and the vegetational cover, as well as areas where the relative abundance does not exceed the minimum
threshold value. *Data from Cortés et al. (2007). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of
this article.)
signatures comparable to those of kaolinite and/or alunite (Figs. 9a, b, c, interpretation of remote sensing over unknown areas it could be
l and m). The characteristic doublet absorption features fixed at ~ 2160 considered that the mineralization and the alteration characterizing the
and ~ 2209 nm (ν + δ Al2OHo and ν + δ Al2OHi, respectively) are well porphyry-Cu systems are generally confined to cylindrical porphyry
represented in the PRISMA-derived spectra, accompanied by combina stocks, whereas IOCGs have less focused alteration footprints, with
tion bands typical of kaolin group minerals, located at around 2330 nm haloes that could extend for several kilometers in association with sec
and 2380 nm (Figs. 9b and m). Moreover, differently from the spectra in ond- to lower-order splays of major faults zones, intrusive contacts, or
Fig. 9a collected in the Ortiga area, a shift to longer wavelengths permeable stratigraphic layers, (Meyer et al., 2022). Sulfur-poor IOCG
(~2167–2170 nm) characterizes the center of the system for the new systems are commonly characterized by the absence of acidic alteration
target area, potentially dominated by alunite, as displayed by the alunite minerals (like alunite) and show a more widespread occurrence of ser
spectral signature in Fig. 9l, as already shown in the 2160W, 1480D and icite and chlorite (Kreiner and Barton, 2017).
1760D maps in Figs. 8d, f and g. Finally, the spectrum collected in the
areas characterized by high Fe-oxides and hydroxides relative abun 5.1.1. The Río Blanco-Los Bronces district
dances, resulting from the application of the band ratio (900D) in the In the Río Blanco-Los Bronces district, alteration zones characterized
Marimaca Copper Project, were plotted against the spectral signatures of by distinct occurrence of alunite-kaolinte, sericite, biotite, chlorite-
gossan. Both the CTS and CFA features (at ~ 500 and 910 nm) related to epidote, have been already documented and mapped (Irarrazaval
the presence of Fe-hydroxides (goethite) are well represented in the et al., 2010; Toro et al., 2012), providing useful information for vali
PRISMA-derived spectra. Although very shallow, the Fe3+ 908 nm ab dating the satellite hyperspectral data. In the PRISMA district map, the
sorption feature (band 57 of the PRISMA VNIR cube), is quite well Al-OH 2200 nm feature (2200D) is highlighted in a large zone where the
represented in the PRISMA-derived spectrum in Fig. 9i. San Francisco batholith outcrops (Fig. 4a). High 2200D values
discriminate areas with relatively higher abundances of Al-
5. Discussion sheetsilicates, such as white mica, kaolin group minerals and/or Al-
smectite, which are characterized by the presence of a distinct spectral
5.1. Interpretation of the hydrothermal alteration footprint related to feature at 2200 nm. In order to better discriminate alteration minerals
IOCG and porphyry-Cu(-Mo) mineral occurrences derived from PRISMA assemblages and zones, a lower threshold of 2.05 for the 2200D index
mineral maps was defined, then the 2200 nm feature (2200W) composition band ratio
(Fig. 4c) was applied to distinguish Al-rich white mica and kaolinite-
The Andean IOCGs and porphyry Cu(-Mo) deposits are distinct bearing areas from Al-poor white micas (phengite) and Al-smectite.
mineral systems and not genetically related (Pollard, 2000; Lang and The information extracted from these two indices and from the identi
Thompson, 2001; Sillitoe, 2003): the former are Fe-oxide rich (pre fication of the area characterized by the presence of the 2160 nm feature
dominantly hematite and magnetite), and have volumetrically extensive (kaolin group and alunite relative abundances; Fig. 4d) enabled to
high-temperature alteration zones in which the fluid migration requires outline areas where the latter minerals outcrop, guiding the interpre
a significant physicochemical gradient, especially in terms of REDOX tation of the alteration zones distribution (Figs. 10a and b).
conditions, whereas, the latter are Fe-sulfide rich and have narrower More specifically, it was possible to discriminate areas with white
high-temperature alteration zones. Nonetheless, the PRISMA results micas (feature at 2200 nm but not at around 2160 nm) from kaolinite-
presented here show in both the deposit types the occurrence of wide and alunite-rich zones (both feature at 2200 nm and 2160–2175 nm,
alteration zones, with distinct mineral patterns, directly related to the characterized by variable intensities), corresponding in this case to the
magmatic-hydrothermal genesis (Barra et al., 2017). This is an impor sericitic and advanced argillic alteration alteration zones, respectively
tant point in the frame of remote sensing-based mineral exploration, (Irarrazaval et al., 2010; Toro et al., 2012). Looking in detail at the Los
because: (1) the similar genetic conditions could allow a comparison Sulfatos area (enlarged map in Fig. 5), the results obtained show that the
between these two deposit types in terms of alteration mineral assem white mica characterizing the sericitic alteration halo exhibits a
blages and zones; (2) the presence of large-scale alteration features is displacement of the position of the absorption at 2200 nm, from 2191
crucial for satellite remote sensing application and district-scale explo nm in the proximity of the center to 2199 nm outward (Fig. 5c). Several
ration, since available spaceborne sensors are commonly characterized authors have outlined that variation in white mica composition can
by a spatial resolution of around 30 m, and (3) the alteration minerals reflect either alteration type or intensity, or fluid geochemistry, as well
characterizing the aforementioned zones (e.g., white micas, kaolin as it can be a proxy for the occurrence of high-grade ores (van Rui
group minerals, sulfates - like alunite, chlorite, epidote and biotite) are tenbeek et al., 2005; Harraden et al., 2013; Tappert et al., 2013; Halley
very well characterized over the SWIR wavelength region (around 2200, et al., 2015; Meyer et al., 2022). The wavelength position of this feature
2160 and 2170, and 2250 nm, respectively). is mainly controlled by the Tschermak exchange (AlIVAlVISiIV VI
-1 (Mg,Fe)-1;
It is known that remote sensing investigations require to be validated Velde, 1965; Duke, 1994), which is also controlled by pH (Wang et al.,
through field data. Thus, in order to test the performances of PRISMA in 2017). This considered, the highlighted Tschermak exchange could
mineral mapping, we used detailed field-derived data from literature suggest the evolution from the more acidic and high-temperature
and technical reports for supporting the remote-based interpretations. alteration mineral core zone to the distal lower-temperature neutral to
For discriminating between the two deposit-types directly through acidic alteration mineral zone. Looking at the existing data in the Los
14
Fig. 9. Comparison between USGS representative spectra (Kokaly et al., 2017b) and PRISMA-derived L2C spectra (see Figs. from 4 and 8 for the location of the pixels
indicated with lowercase letters). The respective assignment of each absorption feature (electronic and vibrational processes) is indicated.
15
Fig. 10. (a) Geological mapping of the hyperspectral mineralogical associations resulting from the band depth and wavelength ratios applied to the PRISMA L2C
scene of the Sulfatos area; (b) Schematic section crossing the main mineralized area, on which is reported our interpretation of the distribution of hyperspectral
alteration minerals in depth compared with the alteration section reported by Toro et al. (2012). *Data from Toro et al. (2012). Mineral abbreviations: Anh =
anhydrite; Bo = bornite; Bt = biotite; Chl = chlorite; Cp = chalcopyrite; Ep = epidote; Kfs = K-feldspar; Gy = gypsum; Mt = magnetite; Py = pyrite; Qz = quartz; Ser
= sericite; Spec = specularite.
16
Sulfatos area (Toro et al., 2012), we see that the results are coherent high 2200D index values (Fig. 6a). As discussed above, since it can
with the mapped alteration zones and that the area characterized by indicate either the presence of Al-rich white mica (e.g., muscovite) or Al-
white mica absorption feature at 2191 nm perfectly fits with the contour poor white mica (e.g., phengite) (reflectance spectrum in Fig. 9g), the
of the modeled mineralization (dotted line with ≥ 0.5 % Cu in Fig. 10). index 2200W was performed for 2200D-pixel values > 2.05 (Fig. 6c).
Even in the proximity of the Río Blanco deposit, areas with high When occurring at shorter wavelengths, the former absorption position
relative abundance of Al-sheetsilicates have been identified, primarily indicates the presence of muscovite, which has higher Al amounts in the
characterized by an intermediate to Al-rich composition (feature posi octahedral site. Our results show that the white mica characterizing the
tion at around 2206 nm), suggesting the occurrence of sericite. studied area has the main absorption feature at around 2206 nm in
The intensity of the 2250 nm absorption feature (2250D band ratio) proximity of the Marimaca Copper Project (Fig. 6c), while values
was used for detecting the chlorite-epidote-bearing rocks, in this case varying from 2199 to 2206 nm are shown in a wider area occurring
mapped as characteristic for propylitic alteration zone (Irarrazaval et al., towards ESE, corresponding to the sericitic alteration described by
2010; Toro et al., 2012). The higher 2250D values are mainly concen several authors (e.g., Oviedo, 2017; Kalanchey et al., 2020; Oviedo,
trated in zones surrounding the reported deposits and in the NNW 2022) (Figs. 6a and c). Although reflectance spectroscopy can discrim
portion of the entire investigated area (Figs. 1b, 4 and 5). Toro et al. inate between white micas based on their aluminum content (e.g.,
(2012) describe strong and widespread Na-Ca-Fe metasomatism Tappert et al., 2013; Meyer et al., 2022), in this case, the use of tradi
(actinolite-tremolite-scapolite-chlorite-epidote-apatite-biotite and local tional analytical techniques on hand specimens would have been useful
pyrite-chalcopyrite) affecting the volcanic and intrusive rocks, as well as for confirming these observations.
an assemblage of chlorite-magnetite-epidote-calcite overprinting the In the same area, the absence of kaolin group minerals, as well as
late-magmatic breccias at El Plomo and San Manuel in the southwestern sulfates like alunite, characteristic of advanced argillic alteration, was
part of the Ortiga Area. The Los Sulfatos area is bordered by alteration confirmed considering the missing features at around 2160 and 2170
zones which from the center outward are composed of chlorite- nm, resulting in low values of the 2160D index. The lack of these min
(epidote)-biotite, chlorite-epidote-specularite-pyrite and chlorite- erals typical of acidic alteration mineral facies can be explained by
(epidote) (Irarrazaval et al., 2010; Toro et al., 2012). The PRISMA considering Marimaca a sulfur-poor IOGC system (Kreiner and Barton,
reflectance spectra, corresponding to the pixels with 2250D index higher 2017).
values, show absorption features characteristic of chlorite and epidote at From our study, it results also that the hydrothermal mineral pattern,
around 2250 nm. The bulk of the chlorite composition (chlorite Mg#) is then, progresses to chlorite-sericite and/or chlorite-epidote alteration
interpreted to be intermediate Fe – Mg chlorite based on the shift of the southward, as shown by the 2250D mineral map in Fig. 6b, which re
Mg/Fe – OH bond at wavelengths ranging around ~ 2250 – 2253 nm. ports the presence of the ν + δM3OH (with M = Mg, Fe, Al) feature
The presence of epidote in the area is confirmed by its minor diagnostic characteristic of chlorite and epidote. Based on our interpretation, the
absorption features at around 1550 nm and 1830 nm, which are due to presence of higher 2250D values in the westernmost zone of the study
2νΟН (Laukamp et al., 2021). These absorption features are typically area (Fig. 6b), closer to the Marimaca Project area, could suggest either
easy to separate from hydroxyl-related overtones in other minerals (e.g., chlorite-biotite alteration or chlorite-epidote in background Ca-Na
chlorite) and do not overlap with atmospheric absorptions in the 1400 regional alteration developed around the Marimaca IOCG system
and 1900 nm wavelength regions (White et al., 2017). However, as (Figs. 6b and 7b), as reported by the field study of Kalanchey et al.
observed in this study area, these minor absorption features are diluted (2020). The Ca-Na regional alteration in the area is shown mainly by
or completely lost when epidote occurs as secondary phase in mineral albite and actinolite replacing mafic minerals, however, chlorite and
mixtures with chlorite, posing a challenge for their mapping. epidote in spots are also common (Oviedo, 2017, 2022). However, none
In zones with higher values of 2160D index results in the Ortiga area of these minerals could be recognized in the area or separately mapped
(Fig. 4d, spectra in Figs. 9a, b and c), the presence of the feature at for two main reasons: (1) SWIR 2 spectral absorptions significantly
around 2160 nm to 2170 nm refers to the presence of kaolinite and overlap for these commonly co-occurring mineral groups (Laukamp
sulfates like alunite. These are common minerals occurring in the et al., 2021, and references therein), such as chlorite and epidote; (2)
advanced argillic alteration zone of porphyry-Cu(-Mo) systems, the so- their spectral separability can be performed at SWIR-2 longer wave
called lithocaps which can reach > 1 km in thickness if unaffected by lengths, i.e., at around 2320–2340 nm and between 2377 and 2390 nm,
notable erosion. The available geological map (see Fig. 1b) reports an which are covered by PRISMA bands, but have not considered for pro
area with a well-developed advanced argillic alteration zone (Toro et al., cessing due to their low SNR and thus resulting in noisy outputs.
2012) nearby the Ortiga deposit, validating our observations. The Focusing on the main mineralized area (PRISMA-derived mineral
spectral signatures collected in the area show a mixed contribution of maps in Fig. 7 and interpretation map in Fig. 11), the results show
both kaolinite and alunite considering only the feature at ~ 2160 nm, chlorite (+biotite + epidote) and sericite in less extended haloes sur
making it challenging to clearly distinguish a zonation from the rounding the center of the deposit area, which, instead display Fe3+-
advanced argillic alteration core, dominated by alunite, evolving to bearing minerals (hydroxides and sulfates – jarosite), that can be
kaolinite. Alunite, however, is also characterized by a strong absorption interpreted as leached cap. Looking for comparison at the field data
at around 1470–1480 nm (Bishop and Murad, 2005). Therefore, the mapped by the mining company (Kalanchey et al., 2020), the major
1480D index was employed for discriminating the areas where alunite is alteration features that control the mineralization, the so-called
present in higher abundances compared to kaolinite (Fig. 4e). In the Los “hanging wall” and “footwall” alteration fronts (the former indicated
Sulfatos area, only few pixels are characterized by high values of the with a red line in Figs. 7 and 11, hereafter “eastern” and “western”
2160D index, allowing the mapping of limited areas with both alunite mineralization limits), occur respectively, toward the top and the bot
and kaolinite (Figs. 5d and 10a). Indeed, Toro et al. (2012) describe tom of the parallel-fractured diorite and monzonite units and the dykes-
outcropping remnants of advanced argillic in the Los Sulfatos area, related mineralization. Based on field and drill-core observations, the
occurring as veins and ledges at higher elevations (>4600 m; Fig. 10b), “hanging wall” front defines the eastern limit of the mineralized area
supporting our observations. and the beginning of a sericitic alteration zone in form of haloes and
evolving to the east, accompanied by minor chlorite alteration
5.1.2. The Marimaca district (Fig. 11a). The western limit, instead, is defined mainly by actinolite-to-
At regional scale, the most prominent feature highlighted through magnetite alteration accompanied by variable degrees of chlorite
the mineral mapping performed using PRISMA data, is a widespread replacement (Oviedo, 2017, 2022). The latter field-based observations
area superimposed on the granodioritic rocks of the Naguayán Plutonic agree with the results obtained from the PRISMA mineral mapping
Complex, characterized by the occurrence of sericite, as evidenced by performed in this study (Figs. 7 and 11). The area, in fact, is highlighted
17
scene of the Marimaca Project area. The country rocks distribution is modified from Cortés et al. (2007), deposits names indicated by numbers are reported in the
notes of the geological map (Cortés et al., 2007). Mineralization distributions, the main prospects of the Marimaca Project and the mineralized area eastern limit
location derive from Kalanchey et al., (2020 – NI 43 101 Marimaca Project) and Oviedo (2017). (b) Schematic section reporting the possible alteration scheme and
mineralization geometry. Notes = Qz: quartz; Ser: sericite; Chl: chlorite; Ep: epidote; Fe-Ox: Fe-hydroxides; Cu: copper. *Data from Cortés et al. (2007); Kalanchey
et al. (2020); Oviedo (2017, 2022).
by non-continuous zones with relatively high 2200D values towards the shown in the northern part of the area of interest, mainly in corre
east of the Marimaca Project (Figs. 7a and 11) and others characterized spondence of the Roble, Cindy and Emilia prospects (together with
by high 2250D index values (higher chlorite-epidote relative abun higher 2200D index values – interpreted as sericitic alteration – as well
dances), towards the west (Figs. 7b and 11) and in correspondence of N- as zones of high 2250D values – implying chlorite (+epidote) alteration)
to-NNE trending mafic dykes (Oviedo, 2017, 2022). (Fig. 11a). Jarosite is defined by a deep absorption feature at ̴ 1470 nm
In addition, a well-developed supergene leached cap dominated by (Bishop and Murad, 2008; Tab. 1). The application of the 1480D index
Fe3+-oxy-hydroxides and sulfates (i.e., goethite, hematite, jarosite), highlights areas with higher relative abundances of jarosite mainly in
clays and copper oxides (Kalanchey et al., 2020) was identified in the correspondence of the Roble and Cindy prospects (Fig. 7d), in agreement
near-surface levels towards the central part of the Marimaca project with the Marimaca Copper Corp. announcements (Marimaca Copper
area. Goethite, hematite and jarosite are also reported to occur in form Corp, 2023 -https://marimaca.com/regulatory-news/), while in the
of haloes in correspondence of fractures and fault zones. The PRISMA- Marimaca project area it appears to be composed predominantly of
derived mineral mapping performed by applying the 900D feature- goethite (Fig. 7c).
extraction index was able to well delineate this wide supergene cap, in All the information occurring in technical reports joined together
accordance with that reported by the mining company. At Marimaca, with the ones obtained by the remote sensing approach, i.e., different
the Fe-Ox-bearing zones are developed mainly above the mineralized combination of mineralization styles, characteristics and the location of
area and around the NNW-SSE trending fault reported by the Chilean alteration zones in the field and from satellite observation, suggest a
Geological Survey (Cortés et al., 2007) (Figs. 7c and 11a). The same is deposit scheme likewise other IOCGs described in the Coastal Cordillera
18
with intermediate characteristics in-between magnetite- and hematite- in some zones may refer to a higher abundance of chlorite + biotite
rich deposits (as reported by Sillitoe, 2003; Williams et al., 2005; (+epidote) and characterize the outcropping potassic alteration zone
Chen, 2013; Barra et al., 2017, and references therein; Kreiner and typically associated with IOCGs.
Barton, 2017). The sericitic alteration represents the shallowest and the Potassic and sericite–chlorite alteration zones are typically
most widespread alteration facies observed in the Marimaca area. Its mineralization-related (Williams et al., 2005; Chen, 2013). Chlorite +
presence indicates either the uppermost manifestation of concealed biotite(+epidote) and, locally, Ca-alteration are associated with the
deposits or can occur in areas proximal to orebodies. Several minor Cu- magnetite IOCG subtype, which is related to the earlier stages of the Cu-
deposits are reported in the area by the Chilean Geological Survey mineralization. In contrast, sericitic or K-feldspar-chlorite alteration is
(Cortés et al., 2007). The presence of a more prominent 2250 nm feature generally associated with the hematite subtype. The predominance of
scene of the new target area and location of ROIs (o to s in Fig. 8); (b) Schematic section reporting the possible alteration scheme, mineralization location and related
pH values relative to alteration minerals stability (modified from Hedenquist and Arribas, 2022). Qz: quartz; Alu: alunite; Kln: kaolinite; Ser: sericite; Chl: chlorite;
Ep: Epidote. *Data from Cortés et al. (2007); (c) (o-s) PRISMA-derived reflectance spectra showing the feature wavelength position moving toward of the centre of the
mineral systems.
19
magnetite over hematite in the western limit area, nearby a potassic altered rocks, or partly or fully overlie and hide the porphyry deposit
alteration zone (Ca-metasomatism), is described by the reports, as well itself (Sillitoe, 2010, and references therein). The zoned mineral pattern
as delineated by higher chlorite-biotite-epidote relative abundances in resulting from the spectral mineral mapping may, therefore, reflect
the 2250D spectral mineral maps shown in this study (Fig. 7b). At the shallower levels of the typical vertical zonation described in the litera
same time, the eastern limit is described to be dominated by hematite ture for lithocaps (Sillitoe, 2010), as graphically described in the sche
over magnetite, nearby a sericitic alteration zone delineated also in the matic section in Fig. 12b.
present study by PRISMA imagery processing. The hematite-rich IOCG Alternatively, the shown pattern could represent a minor, indepen
types are described in literature associated with sericitic to sericitic- dent and distal expression of the wider porphyry- or IOCG intrusion-
chloritic alteration (Chen, 2013). This outward-to-upward transition related system, or be part of a now eroded and/or partly covered
can perhaps mirror a laterally-to-vertically zoned IOCG deposit, from larger advanced argillic zone (like for example Red Mountain, Northern
“deeper-intermediate” magnetite-rich to “shallower” hematite-rich Patagonia; Berger et al., 2003), which could occur alongside porphyry-
facies. Cu centers or above large IOCGs of the Coastal Cordillera (see Sillitoe
New target area - Through the PRISMA hyperspectral mineral map and Perelló, 2005; Chen, 2013). An important indicator for buried and
ping, the minerals typical of advanced argillic, sericitic and propylitic partly weathered mineralization occurrences is the presence of high
hydrothermal alteration zones, were delineated in an area about 6 km 900D values (and locally high 1480D values), thus indicating high Fe3+-
east of the Marimaca Copper Project. This area, to the best of our bearing mineral phases (i.e., goethite, jarosite) typically found in limo
knowledge, is not referred in any previously published reports of pub nitic leached caps associated with Cu deposits (Taylor, 2011). In the new
lications, suggesting that it may be either a minor, undervalued or a still target area, the abundance band ratio (centered at 913 nm; e.g., Lau
not-identified target (Figs. 8 and 12). Moreover, minerals common in kamp, 2022; Chirico et al., 2022) allows to map a quite extensive zone
supergene leached caps covering all three hypogene alteration zones likely characterized by the presence of goethite + jarosite likely over
were also observed. In detail, the distribution of the mineral features lying buried sulfides (Fig. 12).
results in a concentric pattern characterized by a core dominated by
alunite, pyrophyllite, or alunite + pyrophyllite (red pixels in the 2160W 5.2. Comparison with other spaceborne optical sensors
map in Fig. 8d, and blue to red pixels in the1480D and 1760D maps in
Figs. 8f and g, reflectance spectrum in Fig. 9l), surrounded by a Previous studies have successfully demonstrated the applications of
kaolinite-dominated rim, as shown by the spectrum in Fig. 9m, char optical satellite imagery for mineral exploration at regional scale.
acterized by a shorter wavelength position of the OH-related feature at Nonetheless, more comprehensive remote-based assessments of mineral
around 2160 nm. Spectra from o to q in Fig. 12c show the predominant compositions typically rely on costlier airborne hyperspectral surveys (e.
occurrence of kaolinite and alunite. The high content of white mica g., Berger et al., 2003, by using AVIRIS airborne data; Kokaly et al.,
shown by the remaining edge pixels in the 2200D map (Fig. 8a) is 2017a, by using HyMap data) as opposed to other multispectral satellite
possibly related to enhanced sericitization of feldspars of diorite to applications, even though the valuable design of sensors like ASTER or
monzodiorite rocks outcropping in the area. Within the sericite zone, the WV-3, incorporating specific spectral bands sensitive to key alteration
white micas observed apparently register a shift of the 2200 nm ab types, has facilitated the mapping of potentially valuable mineral in
sorption feature from the center outward, from ~ 2199 nm (high-Al formation for exploration since early 2000 s. As also demonstrated by
white micas) to ~ 2206 nm (low-Al white micas) (Fig. 8e and spectra r Laukamp (2022), PRISMA spaceborne hyperspectral imagery provides
and s in Fig. 12), likely mirroring chemical composition modifications. A mineral mapping capabilities, in some cases, comparable to hyper
more widespread area defined by high 2250D values is delineated spectral airborne imagery, although with reduced spatial resolution
outside of those described above, likely indicating the occurrence of (PRISMA VNIR-SWIR = 30 m/pixel; common airborne sensors 7 m/pixel
sericite + chlorite. With regards to alunite, Bishop and Murad (2005) up to 1 m/pixel). The reason is the intrinsic hyperspectral character of
and Chang et al. (2011), described a shift to longer wavelengths of the PRISMA (171 SWIR bands at SNR ≥ 100 at wavelengths larger than
1480 nm feature (from around 1480 nm to 1495 nm) correlated with the 2000 nm, Cogliati et al., 2021), compared to the coarser spectral reso
increasing NaK− 1 exchange vector. Chang et al. (2011) described alunite lution of other commonly used satellite multispectral sensors, such as
1480 nm feature shifting to longer wavelengths closer to the intrusive ASTER and WV-3 (Zhang et al., 2016; Chen et al., 2021). Previous
center due to higher Na and lower K relative content in the alunite. This studies have shown that ASTER SWIR bands can separate mineral groups
is related to the temperature of formation (Bishop and Murad, 2005) associated with advanced argillic (alunite, kaolinite, dickite, pyrophyl
and, therefore, it can be used to vector towards the center of the lite), phyllic (sericite and illite) and propylitic alterations (carbonate,
mineralization. The variation of the 1480W index shown in Fig. 8h can epidote and chlorite) (Abrams and Yamaguchi, 2019). Several studies
therefore be interpreted in this way. have shown that WV-3 allows detailed mineral and lithological mapping
It must be pointed out that the new target area is bordered by (Kruse and Perry, 2013). The results of band rationing applied to WV-3
Miocene to Quaternary alluvial-colluvial cover, which prevents satellite VNIR–SWIR data showed that it allows mapping and differentiating
remote mapping further alteration in the surrounding areas, therefore, chlorite + epidote assemblages, as well as kaolinite and montmorillonite
does not allow to verify the continuation of the suggested mineral because of its finer spatial and spectral resolutions in SWIR region,
zonation. In any case, the PRISMA mineral mapping of the new target compared to ASTER data (Salehi and Tangestani, 2020). Indeed, the
area show, in plan view, a concentric alteration pattern characterized by remote detection of < 10 to 20 nm wavelength changes, such as the
a core dominated by Na-alunite, alunite (or pyrophyllite), coupled with precise measurement of white mica Tschermak substitution at the 2200
a transitional (kaolinite + white mica) to a muscovite to Al-rich phengite nm feature and the alunite K-Na composition at the 1480 nm feature,
outer rim. All of them occur above rocks perhaps characterized by ser necessitates a spectral resolution of <=20 nm. Multispectral systems
icite + chlorite and chlorite + epidote occurrence. with SWIR-2 spectral resolutions exceeding 40 nm, like ASTER and WV-
As pointed out by Sillitoe (2003), extensive zones of sericitic or even 3, are ill-suited for this purpose, making hyperspectral data a more
advanced argillic alteration may either hide buried IOCGs or suggest suitable choice (Cudahy, 2016).
their presence nearby and be also associated with lithocaps overlying The PRISMA imagery demonstrates the potential of hyperspectral
porphyry-Cu systems (Sillitoe, 2010). Lithocaps can be very large (>10 satellite data for mineral exploration with regards to accurate mineral
and locally up to 100 km2) and wider than the underlying/related mapping, which is a key factor for successful mineral exploration.
porphyry-Cu ore IOCG deposit (Sillitoe, 2003, 2010). However, most of Regarding its performances, even after the latest upgrades on both the
them observed in the field represent erosional relicts, that can either L1 and L2 processors performed by ASI, aimed to solve errors and in
occur alongside porphyry-Cu deposits, and therefore over the propylitic crease the overall data quality (see PRISMA Latest News on https:
20
//www.prisma.asi.it), the striping effect affecting several SWIR bands is Acknowledgments

still present. However, the “fluctuating” pattern observed by previous
authors (e.g. Laukamp, 2022; Chirico et al., 2022) in the VNIR wave This paper was produced in the frame of the Ph.D. projects of Dr R.
length region when mapping the Fe-oxides and hydroxides, perhaps Chirico (Ph.D. scholarships Complementary Operational Program - POC
caused by the applied spectral resampling during the smile correction “Research and Innovation 2014–2020”) and A. Sorrentino (Ph.D.
processing within the L1 processor (Cogliati et al., 2021), has been scholarships of the Programma Operativo Nazionale - PON “Research
significantly reduced. Looking toward the future, the upcoming PRISMA and Innovation 2014–2020”), co-financed by the Italian Ministry of
second generation mission, along with other hyperspectral missions in University and Research and European Social Fund (ESF) 2014–2020;
operational phase and announced for the next few years, hold promise Supervisor: Professor Nicola Mondillo. Part of the work was performed
for advancing capabilities in this field. for the M.Sc. thesis of F. Corrado. The work was partly funded in the
framework of I_ProMoNaLISA project by the University Research
6. Conclusions Funding Program-Line A of the University of Naples Federico II, Scien
tific Responsible: Professor Diego Di Martire. The authors are grateful to
The PRISMA satellite hyperspectral imagery was analyzed for testing the Ore Geology Reviews Editor-in-Chief Professor Huayong Chen, and
its performances in the identification of outcropping hydrothermal and to the Associate Editor Dr D. Müller and the anonymous reviewer, for the
supergene alteration zones related to IOCG and porphyry-Cu(-Mo) ore constructive comments and suggestions that have significantly
deposits in the Chilean Andes at two different test sites: the Marimaca improved the quality of the manuscript.
Copper Project in the Naguayán district (Antofagasta region) and the Río Data access and data availability.
Blanco-Los Bronces district (Santiago region), with a focus on the Los Project carried out using PRISMA Products, © of the Italian Space
Sulfatos deposit. PRISMA can be used for this purpose because hydro Agency (ASI), delivered under an ASI License to use by visiting prisma.
thermal and supergene alteration minerals exhibit distinct and diag asi.it.
nostic spectral signatures within the wavelength ranges detected by the On request, high-resolution images of the PRISMA mineral maps for
sensor, serving as a supportive tool for identifying and mapping target the investigated areas can be provided.
mineral phases. In exploration phases, it may aid in defining zonation
based on the mineral associations and their respective compositional Appendix A. Supplementary data
variability, laying the groundwork for guiding subsequent ground-based
investigations. Alteration minerals that are optically active in the VNIR Supplementary data to this article can be found online at https://doi.
region, such as supergene products like Fe-oxides/hydroxides and sul org/10.1016/j.oregeorev.2024.105998.
fates (jarosite), as well as in the SWIR range, such as hydrothermal
minerals associated with advanced argillic (Na-rich alunite, K-rich- References
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Ore Geology Reviews

The application of PRISMA hyperspectral satellite imagery in the

Fe (oxyhydr-) oxides 900D (B44 + B17SWIR)/ 2.16 2.20

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//www.prisma.asi.it), the striping effect affecting several SWIR bands is Acknowledgments

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