Unveiling the hydrothermal mineralogy of the Chapi Chiara gold prospect, Peru, through reflectance spectroscopy, geochemical and petrographic data

Unveiling the hydrothermal mineralogy of the Chapi Chiara gold prospect, Peru, through reflectance spectroscopy, geochemical and petrographic data

    Unveiling the hydrothermal mineralogy of the Chapi Chiara gold prospect, Peru, through reflectance spectroscopy, geochemical and petr...

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    Unveiling the hydrothermal mineralogy of the Chapi Chiara gold prospect, Peru, through reflectance spectroscopy, geochemical and petrographic data Thais Andressa Carrino, Alvaro Penteado Cr´osta, Catarina Labour´e Bemfica Toledo, Adalene Moreira Silva PII: DOI: Reference:

S0169-1368(14)00177-2 doi: 10.1016/j.oregeorev.2014.07.012 OREGEO 1281

To appear in:

Ore Geology Reviews

Received date: Revised date: Accepted date:

22 April 2014 10 July 2014 16 July 2014

Please cite this article as: Carrino, Thais Andressa, Cr´ osta, Alvaro Penteado, Toledo, Catarina Labour´e Bemfica, Silva, Adalene Moreira, Unveiling the hydrothermal mineralogy of the Chapi Chiara gold prospect, Peru, through reflectance spectroscopy, geochemical and petrographic data, Ore Geology Reviews (2014), doi: 10.1016/j.oregeorev.2014.07.012

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ACCEPTED MANUSCRIPT Unveiling the hydrothermal mineralogy of the Chapi Chiara gold prospect, Peru, through reflectance spectroscopy, geochemical and petrographic data

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Corresponding author. Institute of Geosciences/University of Campinas R. João Pandiá Calógeras, 51 Zip code: 13083-970 Campinas-São Paulo Phone: +55 (19) 35215120 Email: [email protected]

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Author’s names and affiliations Thais Andressa Carrinoa1 Alvaro Penteado Cróstaa* Catarina Labouré Bemfica Toledob1 Adalene Moreira Silvab2

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Institute of Geosciences/University of Campinas R. João Pandiá Calógeras, 51 Zip code: 13083-870 Campinas-São Paulo Email: [email protected] b1

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Institute of Geosciences/University of Brasília Campus Universitário Darcy Ribeiro S/n, Asa Norte, Brasília - DF Zip code: 70910-900 Phone: +55 (61) 3107-6969 Email: [email protected] Institute of Geosciences/University of Brasília Campus Universitário Darcy Ribeiro S/n, Asa Norte, Brasília - DF Zip code: 70910-900 Phone: +55 (61) 3107-6984 Email: [email protected]

ACCEPTED MANUSCRIPT Unveiling the hydrothermal mineralogy of the Chapi Chiara gold prospect, Peru, through

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reflectance spectroscopy, geochemical and petrographic data

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ABSTRACT

Southern Peru contains important epithermal Au-Ag (± base metals) deposits, such as Canahuire,

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Tucari, Santa Rosa, Caylloma, Shila and Paula. The Chapi Chiara gold prospect is located in this

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region and is part of a paleo-stratovolcano of the Upper Miocene-Pliocene. The hydrothermal alteration of the prospect was characterized based on spectroradiometric data, geochemistry and

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petrography. The mineralogical data, interpreted based on reflectance spectroscopy, were spatialized using the sequential indicator simulation technique for producing probabilistic maps

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of alteration. The inner part of the paleo-stratovolcano (SW sector) is marked by three main cores of advanced argillic alteration (AAA) (quartz – alunite supergroup minerals - kaolinite - dickite ± topaz ± pyrophyllite ± diaspore) associated with topographic highs. The AAA1 core is

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surrounded by argillic alteration (quartz - illite - paragonitic illite - smectite ± pyrite) and

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propylitic alteration (quartz - plagioclase - chlorite - calcite - epidote - smectite ± kaolinite ± pyrite ± chalcopyrite ± magnetite). The central sector of the prospect, situated in the NE flank of the paleo-stratovolcano, is characterized by hydrothermal breccias structured towards N65E. The mineral phases comprise quartz - kaolinite - smectite ± barite ± dickite, and abundant pyrite, sometimes with traces of As. Anomalous geochemical values of Ag, As, Bi, Hg, Se, Sb and Te coincide with high gold contents in this sector of the prospect. Jarosite and goethite are evidence of a subsequent supergene event. Based on the mineralogical characterization, we conclude the existence of a high sulfidation epithermal system in Chapi Chiara. Hypogene minerals of higher temperature in the SW sector of the prospect, such as diaspore, pyrophyllite and topaz in the AAA zone, and epidote in the propylitic alteration zone, can reveal that the system is currently in 2

ACCEPTED MANUSCRIPT a relatively deep erosion level, suggesting its proximity in relation to the interface between a deep

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epithermal system and a mesothermal system.

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Keywords:

Epithermal system, reflectance spectroscopy, sequential indicator simulation, mineralogy,

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geochemistry, southern Peru.

1. Introduction

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Peru is characterized by a series of metallogenetic belts oriented parallel to the Andean Cordillera. These belts were formed in metallogenetic times ranging from Paleozoic to Cenozoic

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(de la Cruz, 1998). The origin of the mineralization is due to orogenesis associated with the formation of the Andean Cordillera which, in turn, favored the occurrence of metallogenetic processes for the concentration of metals, with the formation of deposits such as the epithermal-

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and porphyry-type ones (Clark et al., 1990).

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The belts responsible for most of the Peruvian gold production are the Epithermal Au-Ag Belt of Miocene, which crosses the entire cenozoic volcanic domain of the Western Cordillera, and the Epithermal Au-Ag Belt of Miocene-Pliocene, restricted to the south-central part of the Peruvian Cordillera (12°30’S – 18°00’S) (Fig. 1A) (Acosta et al., 2008; Quispe et al., 2008; Carlotto et al., 2009). In southern Peru, magmatism and metallogenetic belts were controlled by NW-SE, E-W and N-S fault systems. The epithermal Au and/or Ag (± base metals) deposits are characterized by ages between ~18 and ~4 Ma. Examples of low to intermediate sulfidation deposits include Caylloma (~18 Ma - Echavarria et al., 2006), Selene (~14 Ma – Palácios et al., 2004; Acosta et al., 2008), Shila and Paula (~10 Ma - Cassard et al., 2000; Chauvet et al., 2006), Arcata (~5 Ma 3

ACCEPTED MANUSCRIPT Candiotti et al., 1990), Canahuire (Santos et al., 2011); examples of high sulfidation deposits include Poracota (~14 Ma - Sarmiento et al., 2010), Santa Rosa (~7 Ma) and Tucari (~4 Ma)

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(Barreda et al., 2004 in Carlotto et al., 2009; Loayza et al., 2004), as well as the Cerro Millo

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prospect (~11 Ma - Hennig et al., 2008) (Fig. 1A).

The high sulfidation systems are preferentially located in the extreme southern Peru, near

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the boundary of Puno and Moquegua provinces (Fig. 1A), and are related to the volcanism of the

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Upper Miocene-Pliocene. The volcanic events are temporally associated with the Quechua compressive phases of the Andean Orogeny (Benavides-Cáceres, 1999).

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In the Quechua Phase I (~17 Ma), deformational events occurred in southern Peru, registered by open folds and reactivation of faults formed during the Peruvian (84-79 Ma: NW-

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SE faults) and Incaic (59-22 Ma: NE-SW, E-W and NW-SE faults) compressive phases in preexisting sedimentary and volcanic rocks (Benavides-Cáceres, 1999). After this phase, erosion events and a shallow lacustrine environment predominated. In this episode, between ~11 and 8

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Ma, sedimentary rocks (e.g., limestone, sandstone, mudstone, siltstone), interlayered with lava

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and tuffs of trachyandesitic, dacitic and rhyolitic composition, were formed (Palácios, 1995; Sánchez and León, 1995; Benavides-Cáceres, 1999). These rocks comprise the Maure Group and the Capillune Formation, both occurring in the study area (Fig. 1B). Between the compressive phases Quechua II (~8-7 Ma) and III (~5-4 Ma), the volcanic sequence of the Barroso Group was formed. It is characterized by andesite and trachyandesite, in addition to tuffs with similar composition and, less commonly, with rhyolitic composition (Marroco and Del Pino, 1966; Palácios, 1995; Sánchez and León, 1995; Benavides-Cáceres, 1999). Rhyolitic tuffs are predominant in the Sencca Formation, which is interpreted as a contemporary unit to the Barroso Group (Sánchez and León, 1995). Current records of wellpreserved paleo-stratovolcanoes, as in Cerro Millo, are evidence of volcanic activity from this 4

ACCEPTED MANUSCRIPT period. The migration of the Barroso magmatic arc towards the Peru-Chile trench resulted in the current active volcanism to the west of the high sulfidation epithermal systems above mentioned

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(Benavides-Cáceres, 1999; Mamani et al., 2010).

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The Chapi Chiara gold prospect, recently investigated by Gold Fields Inc. and Vena Resources Inc., is located next to Canahuire, Tucari and Santa Rosa deposits, as well as San

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Antonio de Esquilache (an old mine exploited for Cu, Pb, Ag, Zn and Au since colonial times)

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(Canchaya and Aranda, 1995) and Cerro Millo (a gold-bearing high sulfidation epithermal system) (Hennig et al., 2008) (Fig. 1A and Table 1).

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Chapi Chiara is the focus of this study, which is based on the integrated analysis of reflectance spectroscopy, geochemical and petrographic data. The goal is to determine the

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mineralogy and chemistry of the hydrothermal alteration, the type of hydrothermal system, in

2. Materials

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order to assess its potential for gold, with a particular focus on the level of erosion of the system.

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We used 671 reflectance spectroscopy measurements of rocks acquired with the TerraSpec™ spectroradiometer, manufactured by Analytical Spectral Devices (Fig. 1B). This instrument has a set of 512 silicon detectors in the spectral range from 350 to 1000 nm (VNIR) and two other sets of InGaAs detectors related to the spectral range from 1000 to 1800 nm (SWIR1) and from 1800 to 2500 nm (SWIR2), amounting to 2,151 channels (TerraSpec® Explorer, 2011). A field of view of 2 cm in diameter was employed for the measurements.

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ACCEPTED MANUSCRIPT In addition, we conducted petrographic analysis of 37 thin sections of rock samples collected during the field stage in September 2011 (Fig. 1B) to allow the characterization of the

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hydrothermal mineralogy, thus complementing the reflectance spectroscopy measures.

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Elemental analyses related to 506 rock samples (Fig. 1B) were obtained in the ALS Chemex Laboratory (Peru) through the inductively coupled plasma mass spectrometry, after aqua

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regia digestion. Ag, As, Au, Bi, Hg, Mo, Sb, Se, and Te data were selected for this study.

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Semiquantitative chemical microanalyses were conducted using Zeiss LEO 430i and Zeiss EVO MA15 scanning electron microscopes (SEM) coupled with an energy dispersive X-ray

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spectrometer (EDS). The operating conditions included accelerating voltage and current flow of approximately 20 kV and 3 nA, respectively.

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Additionally, digital elevation model (DEM) data generated from the ASTER sensor (ASTER GDEM), with spatial resolution of 30 m, were obtained on the website http://gdem.ersdac.jspacesystems.or.jp/. These data were used for generating the three-

3. Methods

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dimensional representation of the alteration systems presented in this paper.

3.1. Spectroradiometric data The spectroradiometric data were analyzed visually and also with the software The Spectral Geologist Professional (TSG PRO), version 7.1.0.019, using the algorithm The Spectral Assistant (TSA) for comparison of the spectral curves of the rock samples with the spectral curves from the reference library. The most abundant mineral phases recognized were: alunite group minerals, comprising alunite (or K-alunite), natroalunite (or Na-alunite) and alunite with intermediate composition, as well as kaolinite, dickite, jarosite, goethite and smectite.

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ACCEPTED MANUSCRIPT Additionally, illite, topaz, diaspore, chlorite, epidote, ammonium-bearing mineral, pyrophyllite and Ca-alunite (huangite) were also identified.

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To generate maps showing the spatial distribution of alteration minerals, we used the

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sequential indicator simulation (SIS) technique. The SIS is a stochastic technique that preserves the statistics of the original data, including the probabilistic reproduction of the variability of the

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regionalized variable, respecting the local probability distribution function (pdf) of the original

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data and the semivariogram. The latter is necessary for investigating the spatial continuity of the variable studied in preferred directions (anisotropy) or in all directions (isotropy) (Goovaerts,

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1997).

It is considered that the simulation of a categorical attribute z in N nodes uj' of a grid

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conditional to the data of the indicator´s variable I(zk), with k = 1, ..., K, follows these steps (Goovaerts, 1997):

(1) on each node u', the parameters (mean and variance) of the cumulative distribution function

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F{(u'; zk │ (n))} are determined using the indicator kriging technique based on the semivariogram

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model (previously produced) of the indicator´s variable. The conditional information n comprises the original data of the indicator´s variable and those previously simulated {z(l) (uj’); j = 1, ..., N}; (2) from this process, the simulated value z(l)(u’) is obtained, which is added to the data set; (3) then, the next node is proceeded, repeating steps 1 to 2. In this case study, the following mineral phases, previously interpreted from the reflectance spectroscopy, were selected by the amount of sample points: kaolinite (102), alunite group minerals (100), smectite (191), dickite (48) and jarosite (65). For each of these minerals, indicator´s variables [0, 1] were generated, where 1 means presence of the investigated mineral phase at the sampling points and 0 means absence of alteration or presence of other hydrothermal minerals. This relationship is shown in Eq. 1: 7

ACCEPTED MANUSCRIPT I ( z k )  1,

if z k  investigat ed mineral phase

0,

if z k  investigat ed mineral phase

, (1)

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where I(zk) is the indicator´s variable and zk is the mineral phase of the given k-th data (k = 1, ...,

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671).

From the generation of experimental semivariograms in two directions (anisotropy)

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(Table 2) and their mathematical modeling (Table 3), we proceeded to produce 500 realizations

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(simulated images), considering a grid with cell size of 50 m. Based on the 500 realizations, we produced the image of the mean, indicating a probability between 0 and 100% of occurrence of

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the mineral indicator´s variables.

The images of the mean of the 500 realizations were analyzed through probability cut-offs

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greater than, or equal to 25, 30 and 40%, and the mineral classes were combined on pixel-bypixel basis and spatialized in a georeferenced information system (GIS). These results allowed to perform a regional probabilistic analysis of potential areas for the occurrence of alunite group

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minerals, kaolinite, dickite, smectite, jarosite and their mixtures (Fig. 2).

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Alunite spectral data were also used for interpreting occurrences of alunite bearing some K and/or Na, considering the occurrences of samples marked by the pure end-member (K- or Naalunite), of mixtures of both Na- and K-endmembers in the same sample, as well as occurrences of alunite with intermediate compositions (Swayze et al., 2014). Thus, two indicator’s variables were produced, associated with: (a) sampling points with K-bearing alunite [1], and sampling points with absence of alteration or presence of other hydrothermal minerals [0]; (b) sampling points with Na-bearing alunite [1], and sampling points with absence of alteration or presence of other hydrothermal minerals [0]. Next, the stages of semivariogram production (Table 2) and modeling (Table 3) were employed to generate 500 simulated images for each indicator’s variable, and to calculate their mean images. The images of the probability of occurrence greater 8

ACCEPTED MANUSCRIPT than or equal to 25% of K-, and of Na-bearing alunite were combined in GIS, and the result is shown in Fig. 3, integrated with elevation data from ASTER GDEM and representative sampling

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points of the hydrothermal alteration zones.

3.2. Geochemical data

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Geochemical data comprising Ag, As, Au, Bi, Hg, Mo, Sb, Se, and Te, commonly

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employed in the study of hydrothermal deposits (White and Hedenquist, 1995; Hedenquist et al., 2000), were statistically analyzed using the principal component analysis (PCA) technique

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(Hotelling, 1933).

PCA allows the determination of uncorrelated linear combinations of the geochemical

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data set. Thus, specific information can be separated from different targets by using eigenvectors. These comprise weighted and linear combinations of the original geochemical data in the principal components (PCs) that are calculated by algebraic methods to compute the

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variance-covariance matrix (Mather, 2004). Eigenvector analysis is essential for the interpretation

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and selection of the PC containing the geochemical information of interest. The evaluation of the eigenvector signs, in turn, allows the understanding of how this information will be shown (high or low values of the scores generated). The PCs are presented in descending order of variability, and the last component is characterized by its lower contribution to explain the total variability of the original data. This variability is called eigenvalue (Hair et al., 1998). Table 4 shows the eigenvector matrices and the eigenvalues of nine PCs generated from 184 and 322 geochemical sampling points available, respectively, in the central and SW sectors of the Chapi Chiara prospect.

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ACCEPTED MANUSCRIPT 4. Results and discussion The mineral probability maps shown in Figs. 2 and 3 depict the spatial distributions of the

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most abundant hydrothermal mineral phases recognized in Chapi Chiara. The analysis of these

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maps allows the following hydrothermal alteration zones to be recognized: (a) SW sector, characterized by large proximal alteration cores marked by advanced argillic alteration (AAA)

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(alunite and kaolinite group minerals); (b) central sector, marked by the predominance of typical

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argillic alteration minerals (smectite and kaolinite); (c) eastern sector, also devoid of large proximal alteration, characterized by outcrops of fresh volcanic rocks (e.g., CC-05 and CC-08

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points).

4.1. Fresh volcanic rocks

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All these sectors are characterized in detail and discussed below.

Andesite outcrops with a low degree of alteration are visible in the eastern sector of the

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prospect (Fig. 2) and reflect the occurrence of eruption processes that generated lava domes and

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flows. The andesites are porphyritic, leucocratic and, occasionally, have preserved flow structures (Fig. 4A) and multiple fractures. These rocks are composed predominantly by subeuhedral and euhedral phenocrysts of clinopyroxene and plagioclase, wrapped by fine-grained groundmass formed mostly by plagioclase (Fig. 4B). Extensive outcrops of rhyolitic tuff also occur close to porphyritic andesite. These rocks are fine-grained and white in color, sometimes with gradual planar-parallel stratification characterized by volcanic fragments, such as pumice, which can exceed 5 cm in diameter (lapilli tuff). This increase in granulometry may have been derived from the selection process, by gravity, of the pyroclastic fragments thrown into the air during explosive magmatic eruptions, or

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ACCEPTED MANUSCRIPT it can be associated with the deposition on an aquatic environment that, in the present case, could be a lake, if these rocks belong to the Maure Group.

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Ruiniform structures produced by wind erosion are also notable (Fig. 4C), and were

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4.2. Hydrothermal alteration zones in the SW sector

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previously observed in this region by Marocco and Del Pino (1966).

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The SW sector is part of the extinct volcanic center (Fig. 3), where advanced argillic,

4.2.1. Advanced argillic alteration zone

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argillic and propylitic alteration zones were identified, as described below.

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Three main AAA cores were identified in the SW sector of the Chapi Chiara prospect, from the integration of the simulated mineralogy and ASTER GDEM data (Fig. 3). The paragenesis is characterized by the abundance of hypogene minerals of the alunite group (K-

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alunite, Na-alunite and alunite with intermediate composition), dickite and kaolinite (Figs. 5, 6A

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and B). Less commonly, it also contains topaz in mixture with alunite group minerals and/or pyrophyllite and dickite (samples CC-56, 107665, 107669, CC-33 and CC-34 – Figs. 5B, 6C, 7A), Ca-rich alunite (e.g., sample CC-43 – Fig. 6A), APS (aluminum-phosphate-sulfate) minerals (samples CC-37 and CC-32 – Figs. 5C and D), and diaspore (sample 108707 – Fig. 7B). The concentration of this mineralogy predominantly in higher topographic sectors, i.e. in excess of 5000 m.a.s.l. (cf., Fig. 3), may reflect the greater resistance of residual quartz in relation to erosional processes (Hedenquist et al., 2000). Alunite group

minerals

are characterized by the

general

chemical

formula

AM3(SO4)2(OH)6, where A is often a monovalent cation (e.g., K+, Na+) and M is a trivalent

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ACCEPTED MANUSCRIPT cation (Al3+). The structure is also formed by SO 24 tetrahedrons and MO2(OH)4 octahedrons (Bishop and Murad 2005; Bayliss et al., 2010). The outcrops are marked by oxidation (goethite)

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and alunite crystals are fine-grained and white in color, fully or partially fulfilling vugs or

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forming the groundmass in hydrothermal breccias, as noted in sample CC-56 (Fig. 5A).

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In spectroradiometry, these minerals are diagnosed mainly through absorption features located at approximately 1370-1510 nm (derived from the vibrational process of the O-H bond),

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and at 2172, 2210-2215 and 2325 nm as a result of the vibrational process of the Al-OH bond. In addition, there is also an absorption feature next to 2400 nm due to the vibrational process of the

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S-O bond (Hunt, 1979; Bishop and Murad, 2005; Cloutis et al., 2006) (Fig. 6A and B). The discrimination between K-alunite and Na-alunite is done by observing the shifts of

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the absorption features, in the order of a few nanometers, with features shifted toward longer wavelengths as Na contents increase (Thompson et al., 1999; Pontual et al, 2008b; Chang et al., 2011; Swayze et al., 2014). For this purpose, the spectral range 1370-1510 nm is used because

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the absence of influence of possible overlaps with absorption features from other minerals. The

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absorption features are located at approximately 1434 and 1496 nm in the case of Na-alunite (e.g., sample C-56 – Fig. 6C), while the features centered at approximately 1430 and 1479 nm indicate the presence of K-alunite (e.g., samples CC-25, CC-33, CC-34, 107665, CC-43 – Figs. 6A and B) (Thompson et al., 1999; Pontual et al., 2008b; Kerr et al., 2011; Swayze et al., 2014). According to Stoffregen and Cygan (1990), Na-alunite may be associated with formation conditions of higher temperatures in relation to K-alunite. Mixtures of Na- and K-alunite were identified (e.g., sample CC-56 – Fig. 6C), besides alunite with intermediate composition, recognized from the absorption feature located in the intermediate wavelength, as reported by Swayze et al. (2014). Intermediate absorptions, such as

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ACCEPTED MANUSCRIPT at ~1486 nm, are observed in samples CC-55, CC-37 and 107669 (Figs. 6A and 7A). Less commonly, the occurrence of huangite was verified, based on the characteristic absorption

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features located at ~1370, 1510 and 2377 nm (Fig. 6A), thus reflecting a Ca-rich composition as

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described by Swayze et al. (2014) in Cuprite. Additionally, rare APS minerals coexist with phases of alunite group minerals in a same crystal (Figs. 5C and D).

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Crystals of the alunite supergroup mineral phases are tabular, euhedral to subeuhedral.

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Their sizes vary from approximately 50 to 150 μm when coexisting with topaz (Fig. 5B), a mineral that is indicative of high temperature (above ~250-260 °C) and low pH conditions (White

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and Hedenquist, 1995; Thompson and Thompson, 1996; Hedenquist et al., 2000). In other parts of the prospect, alunite is marked by tabular crystals of approximately 10 up to ~70 μm (cf., Figs.

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5C and D).

Topaz appears predominantly as anahedral aggregated crystals. They have variable sizes, between approximately 3.5 and 35 μm, and are locally disseminated in the interstices of alunite

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crystals (Fig. 5B). This mineral is also discriminated by spectroradiometric data. Two intense

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absorption features, arising from the vibrational processes of the O-H bond, are clearly diagnostic of topaz and occur at approximately 1403 and 2085 nm (Pontual et al., 2008b; Kerr et al., 2011) (Figs. 6C and 7A).

Restricted occurrences of pyrophyllite were identified based on the intense absorptions features centered at ~1394 and ~2168 nm, arising from the vibrational processes of the O-H and Al-OH bonds, respectively, and at ~2320 nm (Al-OH) (Pontual el al., 2008b; Kerr et al., 2011). Pyrophyllite mixed with alunite and dickite is indicated by the sharp absorption feature located at 2168 nm (e.g., samples CC-37 and 107669 - Fig. 7A). Pyrophyllite is formed under conditions of acidic pH and temperatures above ~200-250 up to 350 °C (Thompson and Thompson, 1996).

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Another mineral formed under high temperature conditions (>200-230 °C) and acidic pH is diaspore (White and Hedenquist, 1995; Thompson and Thompson, 1996; Hedenquist et al.,

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2000), identified locally in the AAA zone through the broad absorption feature located between approximately 1600 and 2250 nm (sample 108707 - Fig. 7B). In this spectral range, there is a

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pronounced absorption feature centered at approximately 1800 nm, derived from the vibrational processes of the O-H bonds (Hunt and Ashley, 1979; Pontual et al., 2008b).

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Dickite and kaolinite, mixed or not with alunite group minerals, are also observed (Figs.

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6A and B). The presence of these minerals is mainly indicated by the double absorption features centered at ~1383 and ~1414 nm (dickite) and at ~1395 and ~1414 nm (kaolinite), caused by the

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vibrational processes of the O-H bond. Other main features appear at ~2180 and ~2206 nm (dickite) and at ~2160 and ~2206 nm (kaolinite), resulting from the vibrational process of the AlOH bond (Hunt, 1979; Pontual et al., 2008b) (Fig. 6B). Kaolinite is formed under temperature

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conditions below ~200 °C, while the genesis of dickite occurs under higher temperature conditions (~200 to 250 °C). For both minerals, pH conditions around 3 to 4 are required

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(Thompson and Thompson, 1996).

4.2.2. Argillic alteration zone Restricted occurrences of minerals such as smectite and illite (e.g., sample CC-52) and paragonitic illite (e.g., samples CC-24 and 106928) were recognized in the lower topographic relief sector (~4930 m.a.s.l.), around the AAA1 core (Fig. 3). This mineral paragenesis is formed under pH conditions tending to neutral and temperature below ~200 °C (smectite) and between ~200 and 300 °C (illite) (White and Hedenquist, 1995; Thompson and Thompson, 1996; Hedenquist et al., 2000).

ACCEPTED MANUSCRIPT The mineralogy of the argillic alteration zone is marked by absorption features derived from the vibrational processes of the O-H bond and/or molecular water (illite, smectite) at ~1400

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and ~1900 nm, and by features centered at ~2205 (illite and smectite), ~2195 (paragonitic illite),

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and at ~2330-2340 and ~2440-2445 nm (paragonitic illite/illite), resulting from the vibrational process of the Al-OH bond (Fig. 8A) (Pontual et al., 2008b).

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Sample CC-52 is a typical example of the argillic alteration zone, composed by an

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association of smectite, illite and quartz replacing plagioclase (Fig. 8B). Disseminated pyrite, characterized by subeuhedral to anhedral crystals with variable size, between approximately 18

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and 50 μm, is also observed (Fig. 8C). Supergene processes resulted in incipient jarosite from the weathering of pyrite, as verified in the semiquantitative chemical analysis by SEM-EDS, marked

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by the presence of K, Na (possible natrojarosite) and O (Fig. 8C). This characterization was also corroborated by the diagnostic absorption feature of jarosite centered at ~430 nm (Fig. 8C), derived from the electronic transition process of ferric iron ions (Hunt and Ashley, 1979; Bishop

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and Muray, 2005; Pontual et al., 2008b). In addition, oxides such as ilmenite also appear as trace

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constituents of these rocks.

Occasional ammonium-bearing minerals (e.g., samples 106928 and CC-24) occur in the argillic alteration zone. The presence of the ammonium is indicated by absorption features centered at approximately 1411, 1914, 2010-2020 and 2112-2118 nm (Fig. 8A) As previously mentioned in literature (Pontual et al., 2008b; Soechting et al., 2008), the features centered at ~2020 and ~2118 nm are not ambiguous with other minerals, allowing the accurate identification of ammonium in rock samples. In the case of samples 106928 and CC-24, the features centered at ~2195, ~2330 and ~2443 nm are related to the occurrence of paragonitic illite, and the absorption features of smectite are verified mainly by the deep and asymmetric feature at ~1900 nm

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ACCEPTED MANUSCRIPT (absorption feature of molecular water). In addition, buddingtonite was also identified to the north, outside the prospect area, as shown by the spectral curve of sample 105744 in Fig. 8A.

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Ammonium ( NH 4 ) has an ionic radius similar to that of K+ and can replace it in the

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structure of minerals such as feldspar, micas, jarosite, alunite, among others (Ridgway et al.,

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1990; Krohn et al., 1993; Godeas and Litvak, 2006). The source of ammonium is generally related to the interaction of hydrothermal fluids (meteoric and magmatic components) with

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organic matter-bearing sedimentary rocks, and at temperature conditions of at least 150 °C for the

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release of NH 4 in the solution (Phoebe Hauff personal communication in Godeas and Litvak, 2006).

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Gold values greater than 0.050 ppm, reaching a maximum value of 0.194 ppm, occur locally in the SW sector of the prospect and are accompanied mostly by high contents of Ag, Hg, Mo and Sb, whose inter-elementary relations were synthesized through statistical analysis with

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the PCA technique. PC1 explains 42.362% of the information contained in the original geochemical data (Table 4), and the association of Au, Ag, Hg, Mo and Sb is revealed through

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positive and high eigenvectors values (>0.7). As shown in Fig. 9, the highest gold values, shown by the high score values of PC1, are associated mainly with occurrences of argillic alteration, close to the advanced argillic alteration cores.

4.2.3. Propylitic alteration zone Propylitized rock outcrops are sparse (Fig. 10A) and, as the argillic alteration zone, are located in areas of lower topography in relation to the advanced argillic alteration core (AAA1) (Fig. 3). Petrographic and spectroradiometric analyses allowed the identification of calcite and chlorite replacing pyroxene from the parent rock (andesite) (Fig. 10B), plagioclase altered to

16

ACCEPTED MANUSCRIPT calcite (Fig. 10C), and prismatic pseudomorphs of plagioclase completely replaced by epidote (~0.35 and 0.65 mm) (Fig. 10D). Magnetite (Fig. 10B) and disseminated subhedral to anahedral

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pyrite, with rare inclusions of chalcopyrite (Fig. 10E), are also observed. Smectite and kaolinite

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were identified mainly by spectroradiometry (Fig. 11). In general, this mineral paragenesis tends to be formed from hydrothermal fluid of intermediate to low temperature and pH tending to

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neutral (White and Hedenquist, 1995; Hedenquist et al., 2000). Apatite is also present as a

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secondary mineral, representing a relict of the original volcanic rock (Fig. 10B). Chlorite is diagnosed by the intense absorption feature centered at ~2254 nm arising from

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the vibrational process of the Fe-OH bond, in addition to the feature at approximately 2348 nm, resulting from the vibrational process of the Mg-OH bond (Pontual et al., 2008a, b) (Fig. 11).

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These spectral characteristics are confirmed by the chemical analysis by SEM-EDS, which reveals a relative composition of iron-magnesium chlorite in sample CC-26 (Fig. 10B). Three absorption features appear at the interval between 725 and 1135 nm, arising from

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the electronic transition process of ferrous iron ions that are present in the chlorite structure

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(Hunt, 1979; Pontual et al., 2008b). These features are located at 725-741, 907-925 and 11101132 nm (Fig. 11).

The main absorption features of smectites appear at approximately 1410 (due to vibrational process of the O-H bond and water molecules), 1900 (asymmetric absorption derived from water molecules) and between 2200-2212 nm (derived from vibrational processes of the AlOH bond) (Hunt, 1979; Pontual et al., 2008b). Kaolinite was identified by the subtle double absorption features at ~1400 and ~2200 nm, in the same way as in the samples from the advanced argillic alteration zone (Fig. 11). The most intense absorption feature of the carbonates is derived from the vibrational process of the C-O bond and is centered at ~2300-2350 nm (Gaffey et al., 1986), a wavelength 17

ACCEPTED MANUSCRIPT very similar to that of chlorite. Moreover, the carbonates have a low absorption coefficient, and they are spectrally dominated by other minerals that also have absorption features in the same

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wavelengths (Pontual et al., 2008b).

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In the case of epidote, the main absorption features are located at ~2240 and ~2335 nm, which are also similar to the chlorite features. The hydrothermal epidote is formed in temperature

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conditions that exceed 200-230 °C and pH close to neutral, and the size and prismatic shape

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indicate an increase of the formation temperature (White and Hedequist, 1995; Thompson and Thompson, 1996; Hedenquist et al., 2000). These general descriptions are compatible with the

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textural aspects observed in thin section (photomicrography of sample CC-31A – Fig. 10D).

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4.3. Hydrothermal alteration in the central sector The central sector of the Chapi Chiara prospect is located in the NE flank of the paleostratovolcano (Fig. 3) and is characterized by occurrences of hydrothermal breccias associated

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with N65E structures. This region is marked by altitudes between 4948-4962 m.a.s.l. (e.g.,

(Fig. 12).

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samples CC-14 and CC-15) and 5015-5060 m.a.s.l. (e.g., samples CC-16, CC-17 and CC-18)

Unlike the SW sector of the prospect, there is a remarkable geochemical association synthesized in the first principal component (Table 4). PC1 explains 47.942% of the information of the original geochemical data and represents the association of high contents of Au with high contents of Ag, As, Bi, Hg, Sb, Se, and Te, as indicated by the respective eigenvectors which are positive and greater than 0.er6. The PC1 scores plotted on the alteration map of the central sector of the prospect follow the N65E structure of two main outcrops of hydrothermal breccias (Fig. 12). The high concentration of gold ranges from 0.0500 to 0.3220 ppm.

18

ACCEPTED MANUSCRIPT The outcrops of the hydrothermal breccias are characterized by lengths of up to ~100 m and width of a few meters. They consist of fragments of altered rocks rich in quartz and clay

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minerals. Vuggy texture is occasionally present, characterized by the typical bordering of

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subhedral recrystallized quartz (Fig. 13A and B). The cavities are filled by very fine-grained quartz, clay minerals, and subhedral to anhedral pyrite crystals with variable sizes (~5 to >300

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μm) (Fig. 13C). The clay minerals comprise kaolinite and smectite, which are recognized through

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absorption features located at the region of 1400 and 2200 nm, and at 1900 nm (in the smectite case) (Fig. 14A), and confirmed by petrography and semiquantitative chemical analyses (e.g.,

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samples CC-18 and CC-14 – Figs. 13D and F). Less defined absorption features centered at 1900 nm may indicate the presence of water in the form of inclusions in silica, as well as a broad

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feature in the 2200 nm region (Pontual et al., 2008a) that attenuates the absorption feature intensities of kaolinite, smectite and jarosite, as verified in the Fig. 14A (spectra of samples CC16, CC-17 and CC-18).

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Pyrite, sometimes featuring trace amounts of arsenic, was found abundantly and its

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irregular shape and textural variation may reflect the occurrence of more than one hydrothermal event. The occurrence of supergene process is indicated by the presence of jarosite in small amounts derived from the weathering process of pyrites, as shown by the diagnostic absorption features at ~430 and ~905-940 nm (derived from electronic transition process of ferric iron ions) (Fig. 14A), at ~1470 and ~2210 nm (derived from vibrational process of the O-H bond), at and ~2270 nm (derived from the vibrational process of the Fe-OH bond) (Hunt and Ashley, 1979; Bishop and Murad, 2005; Cloutis et al., 2006; Pontual et al., 2008b) (Fig. 14A). This evidence was corroborated by semiquantitative chemical analyses carried out on pyrites of samples CC-18 (Fig. 13D) and CC-14 (Fig. 13E).

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ACCEPTED MANUSCRIPT Dickite mixed with kaolinite was locally recognized in sample CC-15, through the double absorption features located at 1400 and 2200 nm spectral regions (Fig. 14B), besides barite (e.g.,

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sample CC-14 – Fig. 13F).

4.4. Discussions on the hydrothermal system

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The paleo-stratovolcano of which the Chapi Chiara prospect is part was possibly formed

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between 11.00 and 4.00 Ma (relative age range of volcanism in the Cerro Millo prospect and the formation of the Tucari deposit - Table 1), with concomitant hydrothermal activity. In this same

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period, Tucari and Santa Rosa deposits were formed, which, unlike the older deposits located northwest (e.g., Caylloma, Selene, Shila, Paula), are of the high sulfidation epithermal type, with

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similarities of host rock (context of the Maure and/or Barroso groups) and hydrothermal zoning in relation to both, the Chiara Chapi and the Cerro Millo prospects (cf., Fig. 1A and Table 1). The hypogene mineralogy verified on Chapi Chiara is composed of residual quartz,

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alunite supergroup minerals, dickite, kaolinite (± topaz, pyrophyllite, diaspore) (advanced argillic

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alteration) present in three main cores associated with topographic highs. In the distal parts, there is the presence of illite and smectite (argillic alteration), and of chlorite, calcite, epidote and clay minerals (propylitic alteration). In the case of the mineralogy of the argillic alteration, there was also the formation of ammonium-bearing minerals due to the interaction of the fluid(s) with sedimentary rocks containing organic matter. These rocks possibly belong to the Maure Group and/or the Capillune Formation. Reactive hypogene fluids (high temperature and acidic pH) favored leaching processes, leaving only residual silica (vuggy quartz). This was followed by the successive decrease in reactivity of the fluids with the host rocks, as indicated by the advanced argillic alteration minerals, passing to argillic and propylitic alteration in the distal parts. This type of mineral 20

ACCEPTED MANUSCRIPT zoning is characteristic of high sulfidation epithermal events, marked by the presence of oxidized S. These events take place in continental arc environments and are related to the emplacement of

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calc-alkaline magma at a depth that contributes with heat, fluids, volatiles and metals (Hedenquist

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et al., 2000; Sillitoe and Hedenquist, 2003). In the epithermal model, the initial hypogene fluid comes from the magma degassing process, with the release of oxidized magmatic volatiles (e.g.,

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SO2, HCl, H2S) that are, in general, hot and with low salinity. With the rise to shallower levels,

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these volatiles are absorbed by meteoric fluids favoring reactions that form sulfuric (H2SO4) and sulfidric (H2S) acids at temperature conditions ≤400 °C (disproportionation reaction of SO2), and

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dissociation reactions of predominantly acidic species (HCl, H2SO4) at temperatures ≤300 °C, producing a fluid with low pH (<2) and high temperature (~300 °C) (Arribas, 1995; White and

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Hedenquist, 1995; Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003). The leaching process of magmatic apatites by the initial reactive fluid may have been the fundamental mechanism mainly for the release of P necessary for the formation of APS minerals present in the advanced

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argillic alteration zone (Stoffregen and Alpers, 1987).

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The generation of secondary porosity and permeability (vugs) was required for the passage of late fluid(s) associated with the origin of minerals such as pyrites that fill these cavities (e.g., Fig. 13C). This type of fluid is characterized, in general, by low contribution of magmatic volatiles, which makes it less oxidized and acid (Arribas, 1995; Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003), although a more specific characterization in the study area was not attempted. As currently verified in the central sector of the prospect, hydrothermal breccias structured toward N65E represent favorable exploitation targets as suggested by the occurrence of dozens of gold anomalies in rock (with Ag and other related metals) associated with abundant pyrite filling cavities (Figs. 13C, D, and E).

21

ACCEPTED MANUSCRIPT After the termination of the Barroso volcanism, with consequent migration of the volcanic arc to the west, erosion started to take place, generating the current geomorphologic configuration

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of the paleo-stratovolcanoes (e.g., Fig. 3). With the increase of local tectonic stability and

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occurrence of weathering, supergene minerals were originated, comprising goethite, and jarosite arising from the weathering activity in pyrite.

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A possible genetic association of high sulfidation epithermal system with the porphyry

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system at depth, such as observed in the mining districts of Yanacocha (Northern Peru) and Pueblo Viejo (Dominican Republic) (Chang et al., 2012), can be inferred for Chapi Chiara. This

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possibility has already been proposed by Hennig et al. (2008) for the Cerro Millo prospect (Table 1). Additionally, located between Chapi Chiara and Cerro Millo, there is San Antonio de

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Esquilache, placed in another contemporary paleo-stratovolcano. This target is characterized by the current exposure at surface of diorite associated with occurrences of Cu, Zn, Ag and other metals (Canchaya and Aranda, 1995), revealing a context of deeper erosion (Fig. 1A). Thus, San

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Antonio de Esquilache represents the most concrete evidence of the potential for mineralization

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associated with diorite at levels below those currently exposed in Chapi Chiara and Cerro Millo.

5. Conclusions

Chapi Chiara prospect is located in a paleo-stratovolcano of the Upper Miocene-Pliocene and comprises a high sulfidation epithermal system. Porphyritic andesite, tuff and lapilli tuff of rhyolitic composition comprise the main volcanic rocks identified at this prospect. Three main cores of advanced argillic alteration are characterized, comprising quartz, alunite supergroup minerals (mainly K-alunite, Na-alunite and alunite with intermediate composition), kaolinite group minerals and more restricted occurrences of topaz, pyrophyllite and diaspore. In the distal parts of the system, this mineralogy tends to argillic and propylitic 22

ACCEPTED MANUSCRIPT alteration types. In the central sector of the prospect, the mineral paragenesis is composed of quartz - kaolinite - smectite ± barite ± dickite and abundant pyrite, related to hydrothermal

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breccias structured towards N65E, containing the highest gold levels recorded, among 0.0500 and

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0.3220 ppm, accompanied by anomalies of Ag, As, Bi, Hg, Sb, Se and Te. The occurrence of a supergene event is revealed by the presence of incipient jarosite, after the weathering of pyrite, in

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addition to goethite.

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The superficial display of minerals formed under high temperature conditions in the SW sector, such as topaz, diaspore, pyrophyllite and epidote, lean towards a deep level into the

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epithermal system, marked by the proximity of the epithermal-mesothermal domain limit (Corbett and Leach, 1998). This context is consistent with the possible proximity of dioritic

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intrusion(s) near the surface, as shown by the current exposure of intrusions at the surface in the contemporaneous and analogous paleo-stratovolcano of San Antonio de Esquilache. Thus, considering the likely intense erosion that might have taken place at Chapi Chiara, the potential

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for metals might be reduced in this high sulfidation epithermal context.

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The use of the reflectance spectroscopy technique favored a swift, albeit effective, mineralogical identification of argillic and advanced argillic alteration assemblages. In the case of the propylitic zone, different absorption coefficients of the minerals and the fact that the absorption features of chlorite, epidote and carbonates at the spectral range of ~2200-2350 nm occur very close in the spectrum, did not allow a clear spectral distinction between these minerals. Thus, this mineralogical discrimination was resolved through petrographic analyses and scanning electron microscopy. On the other hand, in the case of mineralogy easily diagnosed through the interpretation of reflectance spectra (Figs. 2 and 3), this case study makes it clear that the novel application of the stochastic simulation technique is a seemly option for the spatialization of mineralogical-spectral 23

ACCEPTED MANUSCRIPT data. Thus, the use of reflectance data combined with the SIS technique allowed us to explore the regional discrimination of predominant compositional variation of alunite, something that would

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not be feasible with routine spectral processing of multispectral remote imaging, due to the fact

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that the main diagnostic features on alunite composition are associated with small shifts of the absorption wavelength, in the order of a few nanometers. In the remote imaging case, a large

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number of spectral bands (hyperspectral imaging), combined with high spatial resolution and

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high signal/noise ratio are required to resolve the alunite composition with adequate accuracy.

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Acknowledgments

The authors would like to thank Gold Fields Inc., specially geologists Francisco Azevedo, Teresa

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Guevara, Shirley Custodio, Auri Morro Muñoz and Mariela Reyes, for supporting the development of this project. We also acknowledge the Institute of Geosciences and the Faculty of Mechanical Engineering at the University of Campinas for providing access to analytical

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facilities, in particular Dr. Erica Martini Tonetto, for the acquisition of SEM-EDS data, and the

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Espaço da Escrita – Coordenadoria Geral da Universidade Estadual de Campinas - for the translation services provided. T.A. Carrino acknowledges the State of São Paulo Research Foundation (FAPESP) for providing a doctoral scholarship (grant 2011/00106-8). A.P. Crósta and A.M. Silva thank the National Council for Scientific and Technological Development (CNPq) for the respective research grants. We also express our appreciation to Dr. Franco Pirajno for his editorial handling of this paper, and to an anonymous reviewer, whose comments and suggestion contributed for improving a previous version of the manuscript.

24

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deposit, Chucapaca Project, southern Peru. In: Barra, F., Reich, M., Campos, E., Tornos, F. (Eds.), Let's talk ore deposits. SGA 2011: Proceedings of the 11th Society for Geology Applied to

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Mineral Deposits Biennial Meeting; 2011 Sep 26-29; Antofagasta, Chile. p. 1-4.

Sarmiento, J.C., Castroviejo, R., Tassinari, C., Vidal, C.R., 2010. Inclusiones fluidas e isótopos

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de plomo em Chipmo y Poracota, región minera de Orcopampa – Peru, implicâncias para la exploración. CPG 2010: Resúmenes Extendidos del XV Congreso Peruano de Geología; 2010

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Sep 27-Oct 1; Cusco, Peru. Lima: Sociedad Geológica del Peru; 2010. p. 644-49.

Sillitoe, R.H., Hedenquist, J.W., 2003. Linkages between volcanotectonic settings, ore fluid

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compositions, and epithermal precious metal deposits. Soc. Econ. Geol. Newsl., Spec. Publ. 10,

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315-343.

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Stoffregen, R.E., Cygan, G.L., 1990. An experiment study of Na-K exchange between alunite and aqueous sulfate solutions. Am. Mineral. 75, 209-220.

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ACCEPTED MANUSCRIPT Stoffregen, R.E., Alpers, C.N., 1987. Woodhouseite and svanbergite in hydrothermal ore deposits: products of apatite destruction during advanced argillic alteration. The Can. Mineral.

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Swayze, G.A., Clark, R.N., Goetz, A.F.H., Livo, K.E., Breit, G.N., Kruse, F.A., Sutley, S.J.,

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Snee, L.W., Lowers, H.A., Post, J.L., Stoffregen, R.E., Ashley, R.P., 2014 Mapping advanced

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argillic alteration at Cuprite, Nevada, using imaging spectroscopy. Econ. Geol. 109, 1179-1221.

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Thompson, A.J.B., Thompson, J.F.H., 1996. Atlas of alteration. A field and petrographic guide to hydrothermal alteration minerals. Alpine Press Limited, Vancouver.

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Thompson, A.J.B., Hauff, P.L., Robitaille, A.J., 1999. Alteration mapping in exploration:

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application of short-wave infrared (SWIR) spectroscopy. Soc. Econ. Geol. Newsl. 39, 15-17.

White, N.C., Hedenquist, J.W., 1995. Epithermal gold deposits: styles, characteristics and exploration. Soc. Econ. Geol. Newsl. 23, 9-13.

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ACCEPTED MANUSCRIPT Figures captions

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Fig. 1. Location of the Chapi Chiara prospect in relation to the main epithermal Au-Ag belts of

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southern Peru (A) (modified from Carlotto et al., 2009). A simplified geological map of Chapi

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Chiara is shown in (B) (Rodríguez et al., 2000), including the location of the rock samples analyzed for reflectance spectroscopy, petrography, semiquantitative mineral chemistry (SEM-

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EDS) and whole rock geochemistry (determination of Ag, As, Au, Bi, Hg, Mo, Sb, Se, and Te

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contents using ICP-MS).

Fig. 2. Regional maps of mineral phases generated from the images of the mean of 500

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realizations for kaolinite, dickite, alunite group minerals, jarosite and smectite, considering the following probabilities of occurrence of these minerals: ≥25% (A), ≥30% (B) and ≥40% (C). The SW and central sectors of the prospect are indicated, as well as examples of fresh volcanic rock

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samples (CC-05 and CC-08).

Fig. 3. Simulated image of the alunite composition (probability of occurrence ≥25%) integrated with the digital elevation data from ASTER GDEM and rock sampling points related to the advanced argillic (AAA) (black lines), argillic (red lines) and propylitic (white lines) alteration zones. The highest gold occurrences and the geomorphology of the paleo-stratovolcano are also indicated.

Fig. 4. Field aspects (A) and thin section (transmitted light/crossed nicols) of porphyritic andesite (sample CC-08) showing plagioclase (Pl) and clinopyroxene (Cpx) phenocrysts surrounded by a

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(C).

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Fig. 5. Hydrothermal breccia in the advanced argillic alteration zone (A), and SEM image of tabular alunite crystals and small aggregated crystals of topaz in sample CC-56 (B); SEM images

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of alunite supergroup minerals in samples CC-37 (C) and CC-32 (D) are also shown. Alu =

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alunite; APS = aluminum-phosphate-sulfate minerals; Dck = dickite; Qtz = quartz; and Tpz =

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topaz.

Fig. 6. Spectra of advanced argillic alteration zone in the range between 1400 and 2200 nm

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showing: (A) the main absorption features of alunite, including the indication of the main shifted absorption wavelengths depending on the K- and Na-composition; (B) diagnostic features of dickite and kaolinite; and (C) absorption features of topaz mixed with K-alunite and/or Na-

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alunite. Dickite, kaolinite, K-alunite, Na-alunite and topaz spectra from Clark et al. (2007) are

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shown for comparison.

Fig. 7. Spectral characterization of: (A) samples CC-37 and 107669 marked by absorption features of pyrophyllite mixed with alunite, topaz and dickite; and (B) sample 108707 marked by spectral features in the visible-near infrared (VNIR) associated with goethite, and short-wave infrared (SWIR) spectra showing absorption features related to diaspore. Pyrophyllite and diaspore spectra from Clark et al. (2007) are shown for comparison.

Fig. 8. Spectral curves characterized by smectite, illite, paragonitic illite and ammonium, in addition to buddingtonite observed north of the Chapi Chiara prospect (A). Photomicrograph 35

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ilmenite; Ilt = illite; Py = pyrite; and Qtz = quartz.

Fig. 9. SW sector of the Chapi Chiara prospect shown in the simulated mineralogy image

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(probability of mineral occurrence ≥25%), integrated with the scores of PC1 that summarize

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geochemical values. The black rectangle shows the area of occurrence of argillic alteration (cf., Fig. 3) associated with higher levels of gold in rock (from 0.0500 to 0.1940 ppm) and of Ag, Hg,

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Mo and Sb.

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Fig. 10. General aspects of the propylitic alteration zone: (A) outcrop of propylitized andesite; (B) SEM image of calcite, and chlorite and its semiquantitative compositional estimation; (C) photomicrographs (transmitted light/crossed nicols) of plagioclase altered to calcite, and (D)

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plagioclase altered to epidote; (E) SEM image of pyrite with inclusions of chalcopyrite. Ap =

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apatite; Cal = calcite; Ccp = chalcopyrite; Chl = chlorite; Ep = epidote; Mag = magnetite; Py = pyrite; and Qtz = quartz.

Fig. 11. Spectral curves of the propylitic alteration zone, with predominance of spectral features of chlorite.

Fig. 12. Central sector of the Chapi Chiara prospect shown in the simulated mineralogy image (probability of mineral occurrence ≥25%), integrated with geochemical data, with the highest values represented by the highest PC1 scores.

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clay mineral and pyrite are shown, respectively, in (B) (transmitted light/crossed nicols) and (C)

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(transmitted light/parallel nicols). Supergene jarosite is recognized in semiquantitative chemical analyses of pyrites from samples CC-18 (D) and CC-14 (E). Barite is also associated with the

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mineralogy of these hydrothermal breccias (F). Brt = barite; Kln = kaolinite; Py = pyrite; Sme =

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smectite; and Qtz = quartz.

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Fig. 14. Representative spectra of the central sector of the Chapi Chiara prospect: diagnostic absorption features of kaolinite (~1400, 2160, 2206 nm), smectite (~1900, 2210 nm) and jarosite

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(~430, 1470, 2210, 2270 nm) (A), and, locally, of dickite (~1383, 1414, 2180, 2206 nm) mixed

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with kaolinite (~1395, 1414, 2160, 2206 nm) (B).

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ACCEPTED MANUSCRIPT Table 1 Synthesis of the geological characteristics of some high sulfidation epithermal deposits and

Core of the massive silica and vuggy quartz; quartzalunite zone occurs in the distal parts

Predominance of andesite, trachyandesite, dacite, latite and pyroclastic rocks of Maure and/or Barroso groups (11,00 ± 0.50 Ma – Ar-Ar dating of biotite from andesite to dacite lava with porphyritic texture)

Core of advanced argillic alteration (vuggy quartz and alunite), passing to argillic (kaolinitesmectitequartz) and propylitic (chloritecalcite-epidotequartz) alterations. Possible porphyry-type system at depth

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10.80 ± 0.90 Ma (Ar-Ar dating of hypogene alunite)

Main gangue minerals

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Pyrite and enargite. Moraine and colluvium of the southern part show gold content of 2 g/t (higher content than of primary origin:1g/t)

Quartz, sulfates and clay minerals

Barreda et al. (2004 in Carlotto et al., 2009); Loayza et al. (2004)

Pyrite

Quartz, sulfates and clay minerals

Hennig et al. (2008)

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~7.00 Ma (Santa Rosa deposit) and ~4.00 Ma (Tucari deposit)

Barroso Group rocks (~8.00 – 6.00 Ma): paleostratovolcanoes marked by trachyandesite and dacite domes. CondoromaCaylloma fault system and minor E-W faults favored the emplacement of these rocks

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Cerro Millo prospect (Au (Cu, Ag))

Hydrotermal alteration

Main ore mineral(s)

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Host rock(s)

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Santa Rosa and Tucari deposits (Au)

Mineralization age

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Deposit or prospect

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prospect associated with volcanic rocks of the Maure and/or Barroso Groups in southern Peru.

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ACCEPTED MANUSCRIPT Table 2 Parameters selected for the semivariogram generation of the most abundant mineral phases

Alunite group minerals Smectite Dickite Jarosite

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K-bearing alunite

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Na-bearing alunite

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15 17 20 20 17 15 20 20 17 15 20 18 18 18

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Width tolerance (m) 10 10 20 20 10 15 25 25 20 20 15 20 20 20

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90 Az. 180 Az. 50 Az. 320 Az. 90 Az 180 Az. 280 Az. 190 Az. 60 Az. 150 Az. 45 Az. 135 Az. 270 Az. 360 Az.

Kaolinite

Lag size (m) 200 200 160 150 250 250 160 150 200 210 165 155 150 160

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Azimuth Lags

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Indicator´s variable

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identified in the 671 spectroradiometric data. Angular tolerance 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45° 45°

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Alunite group minerals

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Smectite

Model Range (Max./Med/Min.) (m) Nugget Spherical 2754/2244/0 Spherical 2992/2244/0 Nugget Spherical 1425/487/0 Spherical 3750/3750/900 Nugget Spherical 4250/4250/2422 Spherical 3315/2252/0 Nugget Spherical 4000/1680/0 Spherical 4000/4000/0 Nugget Spherical 3400/1530/0 Spherical 3400/3400/1020 Nugget Spherical 2070/1759/1449 Spherical 2028/869/869 Nugget Spherical 1663/1188/0 Spherical 1900/1533/1188

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Kaolinite

Structure 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

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Indicator´s variable

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mineral phases identified in the 671 spectroradiometric data.

Dickite

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Jarosite

K-bearing alunite

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Na-bearing alunite

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Sill 0.045 0.080 0.020 0.050 0.020 0.120 0.050 0.010 0.150 0.015 0.003 0.050 0.012 0.010 0.014 0.013 0.090 0.070 0.021 0.001 0.040

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geochemical data associated with the central and SW sectors of the Chapi Chiara prospect.

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Central sector of the Chapi Chiara prospect (n = 184) Au Bi Hg Mo Sb Se Te Eigenvalue (%) PC1 0.812 0.624 0.737 0.413 0.725 0.823 0.710 47.942 PC2 -0.206 -0.184 0.198 0.303 -0.516 -0.336 0.362 11.516 PC3 -0.229 0.227 -0.423 0.795 0.107 0.024 -0.103 10.757 PC4 -0.081 0.642 -0.005 -0.097 -0.020 -0.152 0.114 7.599 PC5 0.272 -0.269 -0.015 0.215 0.187 0.040 -0.265 7.109 PC6 -0.109 0.162 0.201 -0.047 0.108 -0.221 -0.483 5.389 PC7 0.103 0.030 0.405 0.216 -0.180 -0.023 -0.094 4.184 PC8 -0.284 -0.118 0.173 0.026 0.325 -0.094 0.140 3.001 PC9 0.249 -0.006 -0.070 0.022 0.109 -0.356 0.095 2.503 SW sector of the Chapi Chiara prospect (n = 322) Ag As Au Bi Hg Mo Sb Se Te Eigenvalue (%) PC1 0.855 0.340 0.832 0.049 0.744 0.776 0.899 0.428 0.354 42.362 PC2 -0.457 0.204 -0.400 0.168 0.337 -0.020 -0.073 0.710 0.489 14.468 PC3 -0.009 0.458 -0.095 0.821 -0.143 -0.005 0.168 -0.051 -0.362 11.941 PC4 0.076 -0.743 0.121 0.521 0.105 -0.117 -0.001 0.042 0.162 9.950 PC5 -0.004 0.238 0.107 0.139 -0.111 -0.117 -0.045 -0.466 0.678 8.813 PC6 0.127 0.145 0.171 -0.019 0.379 -0.557 -0.127 0.008 -0.110 6.092 PC7 0.105 0.040 0.200 -0.014 -0.381 -0.171 -0.006 0.298 0.071 3.567 PC8 0.007 0.049 0.164 0.067 0.038 0.178 -0.371 0.032 -0.023 2.295 PC9 0.163 0.004 -0.126 0.005 -0.007 0.002 -0.052 0.013 0.023 0.512

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As 0.674 0.079 0.123 -0.454 -0.477 0.258 -0.115 -0.086 0.061

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Ag 0.626 0.560 -0.128 0.117 0.335 0.210 -0.315 -0.054 -0.081

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Graphical abstract

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ACCEPTED MANUSCRIPT Research highlights

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We interpreted the hydrothermal mineralogy of the Chapi Chiara prospect

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Mineralogy interpreted from spectroradiometry was modeled through sequential indicator simulation

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Advanced argillic, argillic and propylitic alteration zones were identified

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Supergene mineralogy is marked by goethite and jarosite

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Topaz, pyrophyllite and diaspore may indicate a deep high sulfidation epithermal system

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