# Nanoscale characterization of chrysocolla, black chrysocolla and pseudomalachite from supergene copper deposits of Atacama Desert of northern Chile

Zia Steven Kahou, Anne-Magali Seydoux-Guillaume, Pierre-Marie Zanetta, Stéphanie Duchêne, Stéphanie Brichau, Eduardo Campos

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## To cite this version:

Zia Steven Kahou, Anne-Magali Seydoux-Guillaume, Pierre-Marie Zanetta, Stéphanie Duchêne, Stéphanie Brichau, et al.. Nanoscale characterization of chrysocolla, black chrysocolla and pseudomalachite from supergene copper deposits of Atacama Desert of northern Chile. The American Mineralogist, 2024, 10.2138/am-2024-9300. hal-04871340

HAL Id: hal-04871340

https://hal.science/hal-04871340v1

Submitted on 7 Jan 2025

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Distributed under a Creative Commons Attribution 4.0 International License
1 **Revision 2**

2 **Nanoscale characterization of chrysocolla, black chrysocolla and pseudomalachite from supergene copper deposits of Atacama Desert of northern Chile**

4 Word count: 6255

5 Zia Steven Kahou¹*, Anne-Magali Seydoux-Guillaume², Pierre-Marie Zanetta², Stéphanie

6 Duchêne¹, Stéphanie Brichau¹,³, Eduardo Campos⁴

7 1. Géosciences Environnement Toulouse, Université de Toulouse, CNRS, IRD, CNES, UMR 5563, 14 avenue E. Belin, F-31400, Toulouse, France

9 2. Centre National de la Recherche Scientifique (CNRS), Université Jean Monnet (UJM) Saint-Étienne, Laboratoire de Géologie de Lyon : Terre, Planètes, Environnement (LGL-TPE), UMR5276, F-42023, Saint-Etienne, France

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3. Department of Geology, FCFM, Universidad de Chile, Plaza Ercilla 803,

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Santiago, Chile

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4. Departamento de Ciencias Geológicas, Universidad Católica del Norte, Avenida

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Angamos 0610, Antofagasta, Chile

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*Corresponding author: zia-steven.kahou@univ-lorraine.fr

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**Abstract**

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We present the first textural and chemical characterization at nanometer scale of chrysocolla

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[(Cu₂ₓAlₓ)H₂ₓSi₂O₅(OH)₄.nH₂O], black chrysocolla (a Mn-rich variety of chrysocolla) and

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pseudomalachite [Cu₅(PO₄)₂(OH)₄] from two distinct supergene copper deposits from

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Atacama Desert of northern Chile. These minerals are the most common copper minerals

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found in the supergene deposits associated with copper porphyries from Atacama Desert.

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However, the lack of nanoscale morphological information prevents a deeper understanding

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of their formation process. Nanoscale characterization using Transmission Electron
Microscope (TEM) imaging allows to address new founding on the structural states of chrysocolla, black chrysocolla and pseudomalachite offering valuable insights into their genesis. Chrysocolla and black chrysocolla are not single crystals but assemblages of Cu nanoparticles embedded in an Si-rich amorphous matrix. Scanning TEM (STEM) images reveal that chrysocolla consists of rounded Cu-rich nanoparticles embedded in an amorphous matrix while black chrysocolla consists of rounded Cu-rich nanoparticles with few needle-shaped Mn-rich particles, all embedded in an amorphous matrix. The richness in nanoparticles defines a layering that mimics the colloform texture observed in optical microscopy. In contrast, pseudomalachite is a massive polycrystalline mineral consisting of juxtaposition of large nanocrystal grains of ~ 500 nm. The STEM-EELS (Electron Energy Loss Spectrometry) spectra show that copper in chrysocolla and black chrysocolla is in a reduced state. This suggest that chrysocolla and black chrysocolla forms under reducing conditions, probably just below the water table or, an alternative hypothesis, that water table oscillation allows cyclical precipitation of Cu⁰-rich nanoparticles and oxidized copper-rich silicate. Conversely, pseudomalachite crystallization requires oxidative conditions. The oxidation state variations from chrysocolla (Cu⁰) to pseudomalachite (Cu²⁺), certainly happens during episodic switch of the water table linked to tectonic event or climatic changes. The findings have also implications for the U-Pb dating of supergene copper deposits since black chrysocolla and pseudomalachite can incorporate important U-content. The different structural states of the three minerals may explain their different behavior regarding U and Pb mobility and therefore the preservation of the U-Pb chronometric signal.

**Keywords:** Chrysocolla; Black chrysocolla; Pseudomalachite; Transmission electron microscope; Nanoscale characterization
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**Introduction**

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Supergene copper minerals (SCM) are a prime target for exploration companies in Atacama Desert because of their high copper grade and because copper is easily recovered by industrial processes compared to sulfide copper minerals. SCM result of the alteration of primary sulfides (i.e. chalcopyrite, pyrite, bornite) under oxidizing conditions within the vadose zone of a porphyry copper deposit (Chávez 2000). Chrysocolla (Ccl) and, to a lesser extent, pseudomalachite (Pmlc), are the most abundant copper minerals found in the supergene column of porphyry copper deposits (PCD) from Atacama Desert of Northern Chile (Sillitoe and McKee 1996; Chávez 2000; Sillitoe 2005, 2010). *Chrysocolla* has been first described by Foote and Bradley (1913) as gelatinous copper-rich precipitate containing cupric oxide, silica and water. Sun (1963) and Newberg (1967) have shown, by chemical analysis and X-ray diffraction (XRD), that chrysocolla is a mixture of an unidentified cryptocrystalline copper-phase embedded in an amorphous silica hydrogel. More recently, Farges et al. (2007) analyzed seven samples of chrysocolla from Peru and Congo using synchrotron imaging and redefined chrysocolla as a mesoscopic assemblage of spertiniite Cu-(OH)₂, amorphous silica and water, and questioned the validity of the existence of chrysocolla as a unique mineral species. In the last decade, this interpretation has been further challenged by XRD and vibrational spectroscopy (Frost et al. 2012; Frost and Xi 2013; Pinget 2016; Dold et al. 2022) which highlighted the lack of diffraction patterns, typical of amorphous materials. Few studies have investigated Cu-bearing assemblages and have provided rare images at these analytical scales, but none have focused on chrysocolla (Pellino et al. 2024). *Black chrysocolla* (B-Ccl) was first described by Schwartz (1934) as a mixture of copper and manganese oxides, silicate and water. Throop and Buseck (1971) have defined black chrysocolla as a manganese-rich variety of chrysocolla, based on the similarity of the XRD patterns and chemical composition form both species. However, little is known about

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the structural state of chrysocolla and black chrysocolla which could provide valuable information on their formation processes, particularly regarding precipitation, dissolution and alteration phenomena. In contrast to their structures, the chemical compositions of chrysocolla and black chrysocolla are well defined. Both minerals are made of a copper, silica and water, with additional manganese for black chrysocolla. The quantitative evaluation of minerals by scanning electron microscope (QEMSCAN) carried out by Campos et al. (2015), Menzies et al. (2015) and Pinget (2016) also showed that chemical composition of both chrysocolla and black chrysocolla fluctuates at micrometer scale. *Pseudomalachite* is a copper-bearing phosphate which, compared to chrysocolla and black chrysocolla, has a well-defined crystalline structure (Ghose 1963; Shoemaker et al. 1977). According to Magalhaes et al. (1986) and Crane et al. (2001), pseudomalachite forms, compared to libethenite and cornetite, from solutions of lower dissolved phosphate, at lower pH and lower $H_2PO_4$ activity, in the basal part of the supergene profile.

The nature of the chrysocolla phase remains a subject of debate, with no prior studies focused on characterizing its nanoscale structural state or the layout of its amorphous and cryptocrystalline phases. Similarly, nanoscale textural characterization of pseudomalachite has been scarcely explored. Furthermore, the prevailing opinion is that chrysocolla, black chrysocolla and pseudomalachite form under oxidizing conditions, above the water table (Chávez 2000). Yet, based on a detailed textural and geochemical modelling study, Dold et al. (2022) suggest instead that chrysocolla and black chrysocolla could form under reducing or near-neutral conditions. Accordingly, our study aims to investigate the chemistry and the mineral assemblages of black chrysocolla, chrysocolla and pseudomalachite sampled from two copper deposits from Atacama Desert of Northern Chile at the nanoscale, to better understand their formation conditions.

## Samples
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Black chrysocolla, chrysocolla and pseudomalachite have been sampled from the Mina Sur and Damiana copper deposits (see Kahou et al. 2021b). Their textural and chemical features have been extensively described in Kahou et al. (2021a, b) and are briefly summarized in the following.

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At microscopic scale, chrysocolla appears as millimetric bands of light-green color under plane polarized light, coating black chrysocolla or sedimentary clasts (Fig. 1A-D). Black chrysocolla (also known as copper pitch, Fig. 1A-B) appears as reworked angular clasts (Pincheira et al. 2003; Pinget 2016; Kahou et al. 2021a) and shows concentric banding texture with a black to brownish color under plane polarized light. Pseudomalachite exhibits pale green to dark green color bands with low pleochroism, coating both chrysocolla bands and black chrysocolla clasts (Fig. 1A). Pseudomalachite also appears as filling the available empty spaces within the clasts (Fig. 1C-D).

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## Methods

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The nanoscale study was carried out on three FIB-prepared foils extracted from two thin sections shown on Fig. 1. These are representative of black chrysocolla from Mina Sur (foil 1), chrysocolla from Damiana (foil 2) and pseudomalachite from Damiana (foil 3). Black chrysocolla and chrysocolla samples were cut perpendicular to the colloform layer while pseudomalachite sample was cut in random orientation (Fig. 1). The TEM foils were prepared using a Thermo Fisher Scientific FEI 125 Helios Nanolab 600i focused-ion beam scanning electron microscope (FIB/SEM) hosted by MANUTECH USD platform, in Saint-Etienne (France). Before extraction of the sample, the area is coated with Pt layer and thick carbon coat. A 30 kV 10 nA Ga-ion beam is used to cut out the foil (15 × 10 × 1 µm). The foil is then lifted-out and deposited on a Cu-half grid, followed by thinning using an ion beam with progressively decreasing voltage and current (30 kV, 16 kV, and 8 kV; 10 nA to a few
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pA), until reaching a thickness of ~ 100 nm thickness. It is then “cleaned up” at low voltage
(2 kV) to remove the surface layer.

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Nanoscale characterizations were performed with a Cs-corrected TEM (NeoARM200F
Cold FEG) using a 200 kV voltage, operated by the Consortium Lyon Saint-Etienne de
Microscopies (CLYM) hosted within the Hubert Curien Laboratory, Saint-Etienne (France).
Conventional TEM images and diffraction patterns were obtained using a CMOS camera
Gatan Rio (4kx4k). In scanning TEM (STEM) mode, images were acquired simultaneously
with two annular detectors (bright and dark field) from JEOL. Chemical maps were acquired
with a SDD CENTURIO-X wide-angle energy dispersive X-ray (EDX) spectrometer from
JEOL. Electron energy loss spectrometry (EELS) data (spectra and spectrum images) were
acquired with a Quantum ER from Gatan©. The Quantum ER is a Gatan Imaging Filter (GIF)
equipped with a Merlin EELS detector consisting of 4 Medipix3 chips for a total 1024x256
hybrid pixels. The experimental conditions for the EELS analysis are as follows; the
microscope was operated with a STEM condenser aperture of 25µm, a camera length of 8 cm
and a collection angle (β) of 25 mrad. Spectrum images (100x100 pixels) of a ~ 60x60 nm
field of view were acquired with an EELS entrance aperture of 5 mm, a 0.36 eV/channel
dispersion and a dwell time of 50 ms. Two frames were summed to increase the signal to
noise ratio (live time ~ 16 min). Single spectra of C K, O K, Cu L3,2 and Si K edges were
acquired by scanning small (~ 5x5 mm) areas to avoid spectral shift, using an EELS entrance
aperture of 2.5 mm, a 0.11 eV/channel dispersion and a 100 ms dwell time limiting beam
damage and allowing over 1000 spectra to be summed for a live time of ~1 min. The low loss
signal containing the zero-loss peak (ZLP) was acquired after each analysis by removing the
shift applied on the drift tube to probe core loss edges.

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Spectra were processed using personal Python scripts based on HyperSpy (de la Pena et
al. 2017). The energy position of the low-loss and high-loss spectra was calibrated via zero-
loss peak (ZLP) centering added to the applied drift tube. The average energy resolution calculated from the full width-half maximum intensity of the ZLP over all standard spectra is ~0.9 eV for 0.36 ev/ch dispersion ~0.6 eV for 0.11 ev/ch dispersion. The π* peak of the C K-edge at 285 eV obtained with the same energy dispersion was used for energy calibration. An inverse power law model (AE-r), where E is the energy loss and A and r are constants, was used to subtract the background. Principal component analysis (PCA) was performed on the spectral images to reduce noise. The data set was decomposed into two new data sets: one with the dimension of the signal space, known as factors (only the main factors are retained), and the other is the map representing the intensity of the factors, also known as loadings. For individual spectra, low amplitude frequencies were removed by FFT filtering.

The analysis of the minerals under the electron beam proved to be challenging. Despite the use of a liquid nitrogen cold trap and a plasma cleaner to limit the deposition of volatile elements during acquisition, high contamination and carbon build-up occurred at high magnifications thus limiting high-quality images acquisition. In STEM mode, the acquisition of small fields of view required the use of extremely short dwell times to ensure a low electron dose. Consequently, denoising processing was required to extract data with a relatively low number of counts.

## Nanoscale characterization

### Chrysocolla

Bright field (BF)-TEM images of FIB2 highlights two bands with nanoparticles: one band with numerous large (> 10 nm) nanoparticles (Band 1 in Fig. 2A) and one band with scarce small (< 10 nm) nanoparticles (Band 2 in Fig. 2A). In both bands (Fig. 2C, D), dark (i.e. with Bragg contrasts) rounded nanoparticles (label c) embedded in a homogenous grey amorphous matrix (label a) are observed. Multigrain selected area electron diffraction pattern
171 (SAED; Fig. 2B) from a domain in Band 1 reveals four rings arising from the presence of the
172 crystalline nanoparticles. Lattice planes from this pattern are measured at 2.17 Å, 1.88 Å,
173 1.32 Å and 1.14 Å which correspond to the main reflections of a native Cu crystal structure.
174 Observations in annular dark field (ADF)-STEM mode show the two bands with
175 nanoparticles from different size-distribution (Fig. 3A). The representative region of interest
176 selected in Band 1 shows dense, rounded nanoparticles embedded in an amorphous, lower
177 density material (Fig. 3B). High resolution images show that nanocrystals have different
178 orientations (Fig. 3C-D). Furthermore, some atomic columns appear brighter than others
179 suggesting that the mean atomic number (Z) within certain columns is higher, i.e., indicating
180 the presence of atoms with higher density than Cu (Fig. 3C-D). The decomposition in
181 principal components of the STEM-EELS spectrum imaging evidence two entangled phases
182 (Fig. 4). A copper-rich nanoparticle phase embedded in an amorphous silica (Fig. 4A-D). The
183 extracted spectra from the spectrum image demonstrate that the oxygen and silicon K edges
184 exhibit null intensities in the nanoparticles (Fig. 4C-D). The single CuL<sub>2,3</sub> spectrum (higher
185 energy dispersion) acquired on a nanoparticle (black Ccl curve, Fig. 4B) has been compared
186 with CuL<sub>2,3</sub> spectra from Cu standard with various oxidation state, Cu, CuO and Cu<sub>2</sub>O (Keast
187 et al. 2001). The Cu nanoparticle spectrum in Ccl closely match the spectrum of the standard
188 with a Cu<sup>0</sup> oxidation state (Fig. 4B).

189 **Black chrysocolla**

190 BF-TEM images of FIB1 (Fig. 5A) reveal tiny diffracting domains, which appear
191 mostly as dark spots, but sometimes also as fiber-like structures (Fig. 5A). Higher resolution
192 BF-TEM images (Fig. 5B-D) reveal the presence of small, (5-10 nm in size) rounded
193 diffracting crystals (dark dots, label c) embedded in an amorphous matrix (label a in the
194 homogenous grey domain in Figure 5C-D). Moiré patterns due to interference of diffracting
195 crystal lattice planes that are overlapping, can also be observed (Fig. 5C). Associated Fast
196 Fourier Transform (FFT) image of the nanoparticles (Fig. 5D) allows measurements of lattice
197 plane distances (d1 = 3.41 Å; d2 = 2.41 Å; d3 = 2.08 Å) and are consistent with native Cu.

198 Low magnification ADF images (Fig. 6A) exhibits fiber-shaped domains within black
199 chrysocolla foil (arrows). In ABF mode (Fig. 6B), the crystalline domains are clearly visible
200 while in ADF mode (Fig. 6C), the Cu nanoparticles show higher density than the amorphous
201 matrix. EDS mapping (Fig. 6E) and integrated chemical profile across a fiber (Fig. 6F)
202 reveals that the fiber-shaped domains are rich in Mn. The STEM-EELS spectrum image
203 acquired in the B-Ccl sample gave similar results than in the Ccl (Fig. 4) with the presence of
204 copper-rich nanoparticles embedded in an Si-rich amorphous matrix (the EELS-map is not
205 included to avoid redundancy). A single CuL<sub>2,3</sub> spectrum acquired on a nanoparticle (B-Ccl
206 curve, Fig. 4B) was also compared to Cu standard (Keast et al. 2001). The Cu nanoparticles
207 in B-Ccl also exhibit a Cu<sup>0</sup> oxidation state (Fig. 4B).

208 Pseudomalachite

209 The FIB foil (FIB3) is made of polycrystalline assemblage with different orientations
210 (Fig. 7A). One SAED pattern in [32-1] zone axis from one grain (circled in Fig. 7A) is shown
211 in Figure 7B and confirms the identification of pseudomalachite. The SAED pattern taken
212 from the area circled in Figure 7B reveals two kinds of phases. Most of the spots (1-5 in Fig.
213 7D) can be assigned to Pmlc, and the very thin ring (6 in Fig. 7D) to the nanoparticle
214 agglomerated in the dendritic clusters. Observations in STEM mode (ABF in Fig. 8A and
215 ADF in Fig. 8B) exhibit the progression of the nanoparticles from the surface of the thin
216 section into the crystal through to the grain boundaries. Close the surface, the nanoparticles
217 are more concentrated (Fig. 8A). The decomposition in principal components of the STEM
218 EELS spectra imaging highlights two phases (Fig. 8C). The pseudomalachite grain and the
219 nanoparticles agglomerated in the dendritic clusters. EELS spectra extracted from the
220 datacube (Fig. 8D, square box in Fig. 8C) show that the nanoparticles are rich in Cu and
depleted in O. The EELS spectra extracted from the spectrum image (square box in Fig. 8C) in the pseudomalachite, revealed the presence of pre-edge signal at the OK edge, corresponding to the presence of OH. The comparison of the CuL<sub>2,3</sub> edge of the sample to Cu standards demonstrates that Cu main oxidation state is Cu<sup>0</sup> in the nanoparticles and CuO as the main oxidation state in the pseudomalachite (Fig. 8F).

## Discussion

### Chrysocolla as a two-phase mixture

The BF-TEM images provide textural evidence that chrysocolla is a two-phase mixture of nanoparticles embedded in an amorphous Si-rich matrix. According to Newberg (1967), chrysocolla precursor is a gelatinous silica polymer. Newberg (1967) also argued that the presence of copper ions in solution (i.e., Cu<sup>2+</sup>) inhibits amorphous silica formation in the depositional environment. However, the STEM images have shown that rounded nanoparticles concentrate Cu are closely entangled within an amorphous silica matrix. Chrysocolla is therefore an assemblage of Cu nanoparticles embedded in a Si-rich amorphous matrix. This mixture is concurrent with the recent work of Dold et al. (2022), which shows 'humps' on the chrysocolla X-ray diffractogram, typical of X-ray amorphous material. Moreover, using XAFS, Farges et al. (2007) assigned the Cu nanophase to spertiniite while Frost and Xi (2013), using vibrational spectroscopy, rejected the presence of spertiniite in chrysocolla structure. Our data identifies Cu<sup>0</sup> as the main oxidation state of the Cu nanoparticles in the chrysocolla phase and OH hydroxyls are not detected in the EELS spectrum. Those observations invalidate the hypothesis of spertiniite as the Cu-bearing nanoparticles. Furthermore, the alternation of bands (Fig. 2A) characterized by variable size of nanoparticles mirrors the colloform texture of chrysocolla recognized by Kahou et al. (2021b) from optical and SEM images. This colloform texture suggests chemical variation of
245 the mineralizing fluids which leads to the gradual precipitation over time of these
246 nanoparticle layers. Moreover, the banding texture could also be the result of the variation of
247 both the composition of the mineralizing fluids and biogenic processes (i.e., episodically
248 anaerobic microbial population) as mentioned by Nelson et al. (2007) to explain chrysocolla
249 formation from Mina Sur deposit.

250 **Black chrysocolla, a Mn-rich variety of chrysocolla**

251 Structural and chemical information demonstrates that black chrysocolla consists of
252 two kinds of nanocrystals: fiber-shaped nanoparticles, which concentrate Mn, and rounded
253 nanoparticles of copper metal Cu⁰. Because of XRD patterns indistinguishable from
254 chrysocolla, Throop and Buseck (1971) defined black chrysocolla as a Mn- and/or Fe-rich
255 chrysocolla. Considering our results, black chrysocolla is indeed a Mn-variety of chrysocolla.
256 Furthermore, sequential XRD led Dold et al. (2022) to define black chrysocolla (called
257 copper pitch by these authors) as a co-precipitation of chrysocolla, birnessite (a
258 phyllomanganate from Mn-oxide minerals family defined by Holland and Walker (1996)),
259 and/or an amorphous Mn oxide phases. However, our data provide nanometer scale textural
260 evidence of the absence of birnessite and amorphous Mn oxide phase.

261 **A crystalline structure for pseudomalachite**

262 For *pseudomalachite*, the studies carried out by Berry (1950), Ghose (1963), and
263 Shoemaker et al. (1977), based on XRD analysis, showed a robust crystal structure. Our
264 diffraction patterns confirm the crystalline state of pseudomalachite. Our results are also in
265 agreement with the work of Krivovichev et al. (2016) who showed using XRD and DFT
266 simulation that crystal structure of pseudomalachite is made of Cu (Cu²⁺) and P polyhedra.
267 To the best of our knowledge, the present study provides the first EELS reference spectra for
268 pseudomalachite and could serve as a reference work. Pseudomalachite shows as well a
269 superficial transformation highlighted by the presence of copper nanoparticles clusters with
270 random orientations in dendrite-like structures, progressing inward mostly through the grain
271 boundaries (see Fig. 8A-B). The STEM EELS spectra of these nanoparticles assign them to
272 Cu<sup>0</sup>. In fact, dendritic clusters might form through an oxidation process during superficial
273 alteration (Jébrak and Marcoux 2008), through lattice reaction diffusion model (Chopard et
274 al. 1991) or nanoparticle nucleation and aggregation in ore formation (Saunders and Burke
275 2017). However, as the progression of these dendrites occurs from the surface of the sample
276 (thin section) towards the interior, we rather believe that their formation is linked to a
277 preparation artifact of the thin section.

278 **Implications for chrysocolla, black chrysocolla and pseudomalachite formation**

279 In supergene Cu-deposits such as those studied here, low-pH copper (and manganese)
280 solutions originate from the oxidation of copper sulfides (Newberg 1967; Münchmeyer
281 1996). These copper- and manganese-rich solutions are then neutralized by silica-rich altered
282 host rocks or downward gravels basin (Sillitoe 2005; Dold et al. 2022). For pseudomalachite,
283 copper oxidation state is 2+ (CuO), in connection with precipitation under near neutral to
284 oxidizing conditions. According to Magalhaes et al. (1986) and Crane et al. (2001),
285 *pseudomalachite* formation is driven by the pH and phosphate activity. At low pH, low
286 copper, and high phosphate ion activities, pseudomalachite precipitate in the lower part of the
287 supergene column, *i.e.* under neutral to oxidizing conditions, just above the water table. The
288 oxidation state of Cu in pseudomalachite therefore confirms these oxidizing conditions. For
289 chrysocolla and black chrysocolla, the mixture of amorphous silica and Cu-rich
290 nanocrystalline particles depicts formation of these phases from Cu- and Si-saturated
291 solutions, concurrent with interpretations of Sun (1963) and Newberg (1967). However, our
292 study proves that the oxidation state of copper in both chrysocolla and black chrysocolla is 0
293 (Cu<sup>0</sup>), *i.e.*, in the metal form (Fig. 4), which could mean that chrysocolla forms under
294 reducing conditions, most probably just below the water table. The long-accepted hypothesis
295 that the chrysocollas (i.e., chrysocolla and black chrysocolla) precipitates only under
296 oxidizing conditions is therefore challenged (Mortimer et al. 1977; Münchmeyer 1996;
297 Chávez 2000). The presence of Cu⁰ in chrysocolla, also supported by the study of Dold et al.
298 (2022), claims for re-interpretation of the context of formation of these deposits. Another
299 explanation could be that oscillations in the water table level during chrysocolla precipitation
300 could allow the incorporation of copper nanoparticles into its structure. The two-phase
301 configuration evidenced by the TEM studies provides a possible explanation for the presence
302 of Cu⁰ in chrysocolla. Recognizing that Cu⁰ is hosted by nanoparticles enclosed in the
303 amorphous hydrous matrix of chrysocolla, we suggest that the amorphous phase crystallized
304 above the water table in the condition typical for supergene copper minerals, but that the
305 nanoparticles crystallized below the water table, due to water table oscillation. This process
306 would lead to an oscillatory zoning between nanoparticle-rich and nanoparticle-poor layers,
307 such as observed in TEM images (Fig. 2A and 3A).

308 Moreover, the studied sample from Mina Sur deposit (see Fig. 1A) shows an alternation
309 between chrysocolla and pseudomalachite bands at microscopic scale. This alternation has
310 been explained by Kahou et al. (2021b and references therein) by slight changes in Si and P
311 activities as well as pH conditions (Fernández-Mort et al. 2018). We tentatively explain this
312 alternation by an oscillation of the water table at a larger timescale, where pseudomalachite
313 would form in stages where the water table is low and samples are permanently maintained in
314 oxidized conditions, and chrysocolla would form when the water table rises and samples are
315 submitted to episodic changes between oxidized and reduced conditions. Chrysocolla and
316 pseudomalachite bands alternation may happen during episodic balance or recharge of water
317 table related to seasonality, tectonic event or climatic variations.

Implication for U-Pb dating of Cu-rich supergene minerals
In an attempt to date Cu-rich supergene minerals by LA-ICPMS analysis, Kahou et al. (2021a) have shown that pseudomalachite keeps U and Pb in its structure, while black chrysocolla experienced post-growth U and/or Pb mobility. Here, we suggest that the ability of pseudomalachite to preserve the U-Pb chronometric signal is related to its crystalline structure. By contrast, for chrysocolla and black chrysocolla, the particular assemblage of copper nanoparticles mixed with an amorphous silica gel seems to have important consequences on the opening of the chronometric systems. Although we do not know where the U and Pb atoms are located (either in the copper nanoparticles or in the amorphous matrix), it is reasonable to think that the concentration and mobility of these two elements can be significantly different depending on the phases (copper nanoparticles or amorphous matrix) in which they are concentrated. Consequently, oscillations of the water table could lead to post-growth mobility of U and/or Pb resulting in element fractionation and hazardous dating results.
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Acknowledgments

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AMSG thanks S. Reynaud (Lab Hubert Curien, Saint-Etienne, France) for FIB-
preparation, and the CLyM (Consortium Lyon St-Etienne de Microscopie) for access to the
TEM NeoARM in Saint-Etienne (France). AMSG and PMZ acknowledge funding for a direct
electron detector from the French National Research Agency (ANR), project ANR-22-CE29-
0019 (PI M. Bugnet, MATEIS, France), and from the LabEx iMUST (ANR-10-LABX-0064),
LabEx MANUTECH-SISE (ANR-10-LABX-0075) and EUR MANUTECH SLEIGHT
(ANR-17-EURE-0026) of the University of Lyon under the "Investissements d'Avenir" and
"France 2030" investment plans set up by the French government. For open access, the
authors have applied a CC-BY public copyright license to any Author Accepted Manuscript
version arising from this submission.

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This work was supported by the program funding of the Institut Carnot ISIFoR (project Dali ref. n° 450808 and Focus ref. n° 450906), the TelluS Program of CNRS/INSU and ANID (Chilean National Research of Development Agency) by means of the FONDECYT Regular project NO. 1220062. We acknowledge the LMI Copedim, an IRD research program, and the Université Paul Sabatier (Toulouse, France) as well as Universidad Católica del Norte (Antofagasta, Chile). Finally, we want to gratefully acknowledge the Society of Economic Geologists Foundation for the Hugh McKinstry fund (SRG 20-15) received by Zia Steven Kahou.
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References

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Berry, L.G. (1950) On pseudomalachite and Cornetite. American Mineralogist, 35, 365–385.

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**Fig. 1** Thin sections microphotographs and SEM images showing the petrography of black chrysocolla (B-Ccl), chrysocolla (Ccl) and pseudomalachite (Pmlc) from Mina Sur (A, B) and Damiana (C, D). (A) Plane-polarized light photographs showing black chrysocolla clast coated by chrysocolla and pseudomalachite thick-size bands. White box indicates the location of BSE image. (B) BSE images showing the location of FIB foils prepared in B-Ccl (FIB1) (C) Plane-polarized light photograph showing chrysocolla band coating sedimentary clasts and pseudomalachite filling empty spaces. White box indicates the location of BSE image. (D) Secondary electron images showing the location of the three FIB foils prepared in Ccl (FIB2) and Pmlc (FIB3).

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**Fig. 2** BF-TEM (A, C-D) images and SAED (Selected Area Electron Diffraction) pattern (B) of the FIB foil cut in chrysocolla bands (FIB2; Fig. 1B). (A) At low-magnification BF-TEM image reveals two bands: one with larger nanoparticles (Band1) and the other with smaller nanoparticles (Band2). Note that the black dots correspond to diffracted materials (nanoparticles). (B) SAED pattern from zone in Band 1. Experimental distances (dhkl) from diffraction rings 1 to 4 are measured at 2.17 Å, 1.88 Å, 1.32 Å and 1.14 Å respectively. (C-D) BF-TEM images at high-magnification for Band 1 (c) and Band 2 (c) embedded in an amorphous matrix (a). See text for further explanation.

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**Fig. 3** STEM-ADF images from chrysocolla nano-phases. (A) Low magnification image showing the two bands with nanocrystals from different sizes. (B) Higher magnification showing rounded nanocrystals with higher density (c) embedded in an amorphous matrix from lower density material (a). (C-D) High resolution STEM-ADF images showing nanocrystals with different orientations. Note that for some of them single atoms are visible (e.g. nanoparticle in the bottom-left corner in C) and some seems to be denser than the others (appear brighter).

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Fig. 4 STEM-EELS (Electron Energy Loss Spectroscopy) analyses performed in nanoparticles from chrysocolla and black chrysocolla (FIB2 and FIB3). (A) Principal component analysis representing intense Cu L3,2 edge in the EELS spectrum extracted from a spectrum image (0.36 eV/ch) acquired next to a hole in the section. Cu nanoparticles clusters are imaged in Fig. 5. (B) Cu L2,3 edge single spectra (0.11 eV/ch), corrected from the background, and identifying Cu0 as the main oxidation state for the Cu-rich nanoparticles in both Ccl and B-Ccl (Keast et al. 2001). Reference spectra from Cu, CuO and Cu2O are indicated for comparison (Ewels et al. 2016). (C, D) Extracted spectra after denoising, showing the intensity of Si K, O K and Cu L3,2 edge. Cu is exclusively distributed within the nanoparticles; Si and O within the amorphous matrix.

Fig. 5 Transmission Electron Microscope (TEM) images in Bright-Field (BF) and High-Resolution (HR) modes of the FIB foil from black chrysocolla (FIB1; Fig. 1C). (A) Low-magnification BF-TEM image reveal the fine crystallinity of the sample. Note the tiny crystals (dots and needles-like) showing Bragg contrasts (black). (B-D) BF-TEM image shows rounded nanoparticles (c) embedded in an amorphous matrix (a). Fast Fourier Transform (FFT) image of the image in (D). Experimental distance (dhkl) 1 to 4 measured from FFT (d1 = 3.40 Å; d2 = 2.98 Å; d3 = 2.45 Å; d4 = 2.08 Å).

Fig. 6 Scanning TEM (STEM) images and Energy Dispersive Spectrometry (EDS)-chemical map (E) and profile of the nano-phases from black chrysocolla. (A) Low magnification Annular Dark-Field (ADF)-STEM images of the zone identified in Fig. 2A. Arrows indicate needle-shaped domains. (B-C) Annular Bright (B) and Dark (C) Field (ABF/ADF)-STEM images from the same area with a zoom in one needle-shape crystal (arrow) embedded in the amorphous silicate. ABF mode indicate the structural state of the nanophases (crystalline domains appears darker) and ADF their density (denser domains appears brighter). (D-E) STEM-ADF image of one needle associated with the composite image of Si Kα and Mn Kα
maps. Mn is exclusively enriched in the needles and Si in the matrix. (F) Integrated chemical profile from area shown in (D) performed across the needle. Note the presence of Mn exclusively in the needle with enrichment in Cu and O and depletion of Si.

**Fig. 7** BF-TEM images (A and C) and SAED patterns (B and D) in pseudomalachite (Pmlc). (A) Low magnification image of the FIB foil cut in Pmlc (FIB3: Fig. 1B). The sample is polycrystalline as shown by the grains with different orientations (various Bragg contrasts corresponds to different orientations of the grains) (B) SAED pattern in [32-1] zone axis in one grain (circled in Fig. 7A) confirmed pseudomalachite identification. (C) A zoom in the Pmlc close to the surface revealed the presence of nanoparticles clusters, dendrite-like structures, progressing into the sample. Note that these clusters are only present in the first two microns and absent in the rest of the sample deeper in. (D) SAED pattern from area shown in C. Most of these spots (1-5 in white) can be assigned to Pmlc, and the very thin ring (6 in yellow) to the nanoparticles agglomerated in clusters.

**Fig. 8** STEM-ABF (A) and ADF (B) images in pseudomalachite. Note that the dendritic clusters seem to progress into the Pmlc and are more abundant close to the surface. (C) Two first main component intensity map after decomposition of the signal (PCA). The spatial distribution of the two phases (pseudomalachite and Cu nanoparticles) can be identified based on the colorscale. (D, E) Extracted spectra after denoising (0.36 eV/channel) showing the O K edge and the Cu L<sub>3,2</sub> edge energies. O K pre-edge (black arrow) in pseudomalachite demonstrates the presence of OH radicals in the crystal. (F) Cu L<sub>2,3</sub> edge single spectra (0.11 eV/ch), corrected from the background, and identifying Cu<sup>0</sup> as the main oxidation state for the Cu-rich nanoparticles and Cu<sup>2+</sup> as the main oxidation state for the pseudomalachite. Reference spectra from Cu, CuO and Cu<sub>2</sub>O are indicated for comparison (Ewels et al. 2016).
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