1. Introduction

Epithermal deposits have been widely studied by several authors (e.g., Hedenquist et al., 2000; Simpson et al., 2001; Sillitoe and Hedenquist, 2003;  Camprubí et al., 2003; Camprubí and Albinson, 2006; Holley et al., 2016). These deposits are formed by hydrothermal fluid at a temperature between 100 to 320 °C , commonly 1 km below the water table (Camprubí et al., 2003). Based on the ore mineralogy, these types of deposits are classified into three end member types: high-sulfidation (HS), intermediate-sulfidation (IS), and low-sulfidation (LS) (e.g., Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003).

Specifically, LS epithermal deposits are formed by boiling fluids that are concentrated in permeable zones of the upper earth crust, analogous to geothermal active systems (Sillitoe, 2015) which are generated during discharge of a spectrum of fluid types under varied conditions. The ore-forming solutions (near neutral pH chloride waters) are mainly compounded by meteoric water which descends to deep areas through permeable zones, leaching the host rock and enriching the fluid in elements such K⁺ (Simmons et al., 2005; Warren et al., 2007) with a nil to small and variable component of magmatic water. Later on these solutions are heated, e.g., by an intrusive body or volcanic complex, producing its ascent to the surface through permeable zones and structures (Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003; Camprubí and Albinson, 2006). Subsequently, the complex ions solubility decreases as a result of the fluid rising to a shallower environment, precipitating minerals at the moment of boiling and subsequent drop in temperature, inside structures, forming veins filled by copper and silver sulfosalts, silver sulfides, halides, electrum, native gold, and silver, accompanied by hydrothermal minerals as adularia, quartz, K-bearing clays, and carbonates (Simmons et al., 2005). El Peñón Au-Ag deposit, located in the Paleocene metallogenic belt in the Antofagasta region, Chile (Fig. 1; Camus, 2003), is an example of a LS epithermal deposit. This deposit is characterized by gold and silver mineralization hosted in “bonanza-type” veins and hydrothermal breccias, both filled by quartz-adularia-carbonate alteration minerals (Sillitoe, 1993) that are hosted in Lower Cretaceous to Early Eocene volcanic rocks of rhyolitic to andesitic composition (Zuluaga, 2004).

 

Fig. 1. Location of the El Peñón deposit and simplified geological map of the district. Modified from Zuluaga (2004) and Warren et al. (2007).

The mineralizing hydrothermal fluids of LS deposits generate the alteration of the wall-rock usually by hydrolysis, hydration-dehydration, decarbonization, silicification and oxide-reduction reactions (Guilbert and Park, 2007). The intensity of this metasomatism is a consequence of the rock reactivity and the time of contact. The thermodynamic disequilibrium, resulting from the water-rock interaction, results in the dissolution of minerals, changes to the rock texture and formation of new mineral phases which are stable in the new hydrothermal conditions (Einaudi et al., 2003; Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003; Warren et al., 2007).  According to Giggenbach (1988), near the veins, in LS systems, a potassium metasomatism accompanied by silica deposition occur due to the rise of the fluids with precipitation of adularia, quartz, and carbonates, in addition to the dissolution of minerals containing Ca, Mg and Na. In the outwards areas far from the mineralized veins (until ~50 m) a hydrogen metasomatism dominates at a condition of sub-boiling temperatures (Warren et al., 2007). Furthermore, in the upper  part of the system, the presence of steam-heated CO₂-rich waters, and acidity raises lead to the formation of calcite and Al-bearing minerals such as kaolinite and minor alunite. In distal areas, where the meteoric water recharges the system, a Mg- and Ca- metasomatism occurs producing the dissolution of K-bearing minerals during the heating of the infiltrated fluids, enriching the fluid in K⁺, while Mg²⁺ and Ca²⁺ precipitate forming calcite and chlorite in the altered rock. Moreover, albite can be formed by minor Na metasomatism, if this mineral already exists in the wall rock, it is preserved. These reactions generate a series of alteration haloes, highlighting an assemblage quartz + calcite + adularia ± illite in the veins and near to them. This alteration is the one that contains the mineralization. Then moving away from the vein (until ~50 m), this alteration gradually changes to illite, illite-smectite layers, smectite, usually with pyrite and carbonates. Finally, a propylitic alteration that includes quartz, albite, chlorite, calcite, pyrite, and epidote dominate at distal part of the system (>50 m; Camprubí and Albinson, 2006; Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003; Simmons et al., 2005; Warren et al., 2007).  

The mineral phases that compose the alteration haloes are essential for the prospection of these types of deposits, being adularia, a potassium feldspar of low temperature, one of the minerals of most interest. Nevertheless, the recognition of this phase in hand specimens and thin sections could be difficult and problematic, especially if the adularia coexists with orthoclase, or other potassium feldspar minerals, which could be wall rock mineral components and not necessarily be part of the hydrothermal process. Therefore, the use of an analytical mineralogical method could be a feasible solution for the recognition of this mineral. Hence, the objective of this study is to develop a methodology to identify and quantify the adularia presence in the deposit by using X-ray diffraction (XRD) analysis. This will be accomplished by the detailed study of an adularia diffractogram pattern and comparing it with a series of diffractograms generated from samples collected in a cross section to the Aleste vein, one of the largest veins in the El Peñón deposit. In addition, the data will use to elaborate a cross section with the distribution of this mineral and other alteration phases around the veins of the deposit, emulating the adularia cross section recognized in the veins of Golden Cross deposit, New Zealand (Simpson et al., 2001). This study will provide a method for identifying adularia, and a deeper understanding of the behavior of this mineral, together with other alteration minerals, in the epithermal environment, and thus facilitate the work for geologists in the prospection of LS deposits.

2. Geological settings

The El Peñón deposit is located in the Paleocene-Eocene Metallogenic belt located to the west of Cordillera de Domeyko and in the Central depression of Chile (Fig. 1; Boric et al., 1990). This belt has an extension of ~1,300 km from the southern Peru to central-north of Chile (~29° S) and 30 to 50 km width, hosting several Au-Ag epithermal and porphyry copper deposits (Boric et al., 1990; Marinovic et al., 1995). 

2.1. Geology of the deposit

The El Peñón deposit (Fig. 1) is formed by volcanic rocks and rhyolitic domes intruded by a series of Upper Cretaceous to Eocene intrusives (Cabello, 1999; Pérez et al., 1999; Cornejo et al., 2003; Matthews, 2009; Zuluaga, 2004; Warren et al., 2004, 2007). The stratified units are fundamentally volcanic rocks deposited in continental environments which include: (i) Lower Cretaceous andesitic-dacitic volcanic sequence; (ii) Upper Cretaceous pyroclastic rocks sequence; and (iii) Paleocene-Eocene andesitic-basaltic lavas and rhyolitic domes (Zuluaga, 2004). These sequences are intruded by Upper Cretaceous to Eocene intrusive bodies of different compositions such as dioritic-monzodioritic, andesitic-monzonitic and dacitic-rhyolitic rocks (Zuluaga, 2004).

 The main structures of the district are two subparallel regional faults. The first is the Dominador Fault, with NNE direction and east dip, which delimits the Upper Cretaceous rocks with Paleocene-Eocene units to the west of the district. The second one is the NS-direction, and subvertical dip La Mula Fault that separates Paleogene rock units to the east of the district. These structures have been described as normal faults forming a graben where is the El Peñón deposit, nevertheless, transcurrent activity of these faults have been informed. The N-S structural domain is hosting Au-Ag bearing veins in its central block. To the north, the mineralized block is delimited by N40E faults (Zuluaga, 2004; Donoso, 2012).

2.2. Au-Ag ore and hydrothermal alteration at El Peñón

The El Peñón gold and silver ore is hosted in bonanza veins, hydrothermal breccias, and stockworks filled with quartz, adularia, and carbonates (Fig. 2). The Northern block hosts the Aleste and Bonanza veins, two of the most important veins in the district. These veins have north-south extensions of 1.5 km, and ~0.5 to 3.5 m width. Additionally, the veins tend to be wider at depth and form irregular bodies associated with intercalations of veins and hydrothermal breccias with different gold and silver grades (Donoso, 2012). In both the Central and Northern blocks (Fig. 2), the hydrothermal activity that gave rise to these veins is related to the last three pulses of five magmatic events recognized in the area. The ages of mineralization events are 52.47±0.12, 52.02±0.10 and 51.27±0.17 Ma determined by K-Ar dating (Matthews, 2009).

 

Fig. 2. Map of distribution of the main veins of the El Peñón deposit. The A-A’ line over Aleste vein shows the position of the cross section studied in this work (7,305,100 m N).

The gold and silver minerals are occluded or associated with base metal sulfides (Pb, Zn, Cu, Fe), Cu and Ag sulfosalts, Ag sulfides, halides, or as electrum (Donoso, 2012). The veins are made of quartz-adularia-carbonate, such as calcite, rhodochrosite, ankerite, and siderite (Zuluaga, 2004; Warren et al., 2008; Donoso, 2012). The upper zone of the veins is oxidized and characterized by the association of fluorite, limonite, cerussite, smithsonite, azurite, brochantite, chlorargyrite, bromargyrite, argentojarosite, native gold and silver. Below this is the mixed zone with hemimorphite, Ag-galena, acanthite, freibergite, proustite, miargyrite, sphalerite, polybasite, electrum, bornite, chalcocite and covelline. Finally, in the deepest part is the primary sulfide zone with pyrargyrite, tetrahedrite, galena, sphalerite, chalcopyrite, pyrite, and gold (Donoso, 2012).

The hydrothermally altered wall rocks around the main veins change from intense to weak altered moving some meters away from the core of the vein. Its mineral association includes quartz, adularia, albite, illite, illite-smectite interlayered, smectite, chlorite, carbonate, and pyrite. The veins present an assemblage of crustiform and colloform banded quartz, adularia, platy calcite (lattice texture) and pseudomorphs quartz; a few meters from the core of the vein, adularia alters to illite as do the wall rocks minerals, then illite-smectite, and finally to smectite, always accompanied by quartz (quartz, calcedony and opal), and carbonate minerals. Furthermore, far from the nucleus of the veins, a weak to intense propylitic alteration characterized by chlorite, calcite, and albite assemblage can be observed, additionally, Zuluaga (2004) extend this alteration at deep (~300 m) with a propylitic association of quartz, potassium feldspar, micas, chlorite, pyrite, calcite, and minor epidote at temperatures above 250 °C. Furthermore, near the surface, and distal areas of the system, an argillic alteration characterized by an assemblage of illite, smectite, kaolinite, calcite and calcedony, derived from CO₂-enriched fluids can be observed (Zuluaga, 2004; Warren et al., 2007). Above the water table, acid-sulfate waters produce an alunite, kaolinite, and amorphous silica mineral assemblage. Finally, siliceous sinter was formed when the fluids reached the surface in the last state of the hydrothermal system evolution (Hedenquist et al., 2000; Zuluaga, 2004; Sillitoe, 2015).

3. Methodology

3.1. Sample preparation and XRD analyses

The profile selected for this study crosses the Aleste Vein and has an extension of 1.2 km (Fig. 2). The selected samples within the studied section, consist of 152 specimens, from 9 drill-cores (Fig. 3). All the samples were analyzed at the XRD/XRF laboratory at Universidad Católica del Norte. The samples were pulverized in an agate mortar and sieved through a 270 Tyler mesh (74 µm). Then, 2 g of the powder were used for the XRD analyses. The analyses were conducted in a Siemens D5000 scintillation diffractometer operating at 30 mA and 40kV, using Cu Kα1 radiation. The scan range was 3 to 70° of 2θ with a step size of 0.020° (2θ) per step time of 1 second. All the samples were measured with the same parameters in order to conserve experimental conditions. The Powder Diffraction File database (PDF-2), from the Centre for Diffraction Data (ICDD) was used for mineral identification. The semi-quantification of the multimineral diffractograms was bases in the Rietveld refinement using the Total Pattern Analysis Software (TOPAS). This software uses the crystal structures, lattice parameters and crystallographic data of each mineral for the iteration.  semi-quantitative iteration parameters are shown in Supplementary material 1.

 

Fig. 3. Simplified geology and informal lithological units in the A-A’ cross-section in figure 1. Drill holes and the location of the samples used in this study are also shown. PGA: andesites and dacites; FBR: banded fluidal rhyolite; XTB: litho-crystalline tuff; COL: coluvio.

3.2. Adularia identification

An adularia crystal from Ag-Au epithermal veins from the Guanajuato Mining District, Mexico (Gross, 1975), was used as a standard to find the pattern that would permit the identification of this mineral in the studied samples diffractograms. The Guanajuato adularia was supplied by the Yamana Gold Company, no further information about it was provided. This specimen was analyzed by XRD to obtain a monomineralic diffractogram (Fig. 4A), the mineral was identified by using the high sanidine pattern from Bernic Lake Manitoba, Canada from the PDF-2 (code: 80-2106; Ferguson, et al., 1991). The mineral from which the high sanidine pattern was made is a mineral component of hydrothermal veins and massive quartz druses from the Buck Claim Pegmatite in the southeast of Manitoba, Canada (Lenton, 1979; Černý and Champman, 1984; Ferguson et al., 1991). This standard indicates a hydrothermal adularia crystal with a crystallographic structure of high sanidine content that is extremely pure (Na-poor), where Si and Al are completely disordered in their crystallographic structures. Once this standard pattern was determined, the following hypothesis was proposed, the El Peñón adularia is crystallographic and chemically the same as the Guanajuato adularia used as the standard. Consequently, both diffraction patterns are considered equal. Then, a high sanidine pattern was stablished to detect peaks that differentiate adularia from other potassium feldspars.

 

Fig. 4. A. Standard adularia diffractogram with the assignedhigh sanidine pattern. B. Diffractogram between 2θ=30° to 2θ=65° showing the 4 peaks, and their d-spacing (number above the peak), used to recognize adularia. The similarity between both high Sanidine and Orthoclase patterns can be observed.

3.3. Adularia content quantification

XRD analysis provides a semi-quantitative measure of abundance of the mineral phases present in certain diffractograms. Nevertheless, when a series of quantifications of the same pattern are run by TOPAS software some similitudes in the abundance values can be observed. Therefore, in order to determine if the semi-quantification content of adularia and orthoclase are possible in the same sample, and obtaining a percentage not far from the total abundance of K-feldspar, the following three iteration models were made when adularia was detected: 1) Semi-quantification considering orthoclase as a representative of K-feldspar groups (excluding adularia); 2) Semi-quantification considering adularia as a representative of K-feldspar groups (excluding orthoclase); and finally, 3) Semi-quantification considering both orthoclase and adularia.

These three iteration models determine if the similarity in adularia and orthoclase patterns generate abundance problems in the quantification of the total K-feldspar and provide abundance values of each mineral in the sample. Thus, the first two iterations must be similar in order to determine the total abundance of K-feldspar. The third iteration differentiates the percentage of abundance between both minerals. If the K-feldspar total abundance in the third iteration does not coincide with the other two, a second revision of the diffractogram is done to check for confirmation of the presence of both minerals, or to discard one of them. To see an example of this iteration method, consult Supplementary material 2.

The mineralogical profiles were created with the data from iteration (3), when both adularia and orthoclase were present in the diffractogram. Iteration (1) or (2) was used when only one of these mineral phases was detected. As a result, only one semi-quantification data per sample was obtained (Supplementary material 3).

4. Results

4.1. Adularia detection and semi-quantification in XRD diffractograms

The adularia and orthoclase patterns in the PDF-2 database yielded four useful secondary peaks (Table 1 and Fig. 4B) that differentiate adularia from orthoclase in the diffractograms of the studied samples. The differences of the secondary order peaks in both patterns are due to variations in the adularia crystallinity, and/or presence of trace elements in its crystallographic structure, caused by differences in the geological environments at the moment of mineral precipitation (Colville and Ribbe, 1968; Stewart and Ribbe, 1969; Ferguson et al., 1991).

The mineral quantifications show that adularia has a wide range of abundance. The results of the semi-quantification of this mineral together with orthoclase in the same diffractogram agree with the results of the other two quantifications mentioned in the methodology. However, some values of adularia could be slightly overestimated in some samples due to the TOPAS software uses the main peak areas in the diffractogram to calculate the relative abundance of each phase; a large part of these areas are composed of the main peaks of both minerals which are almost locate in similar positions.

4.2. Mineral distribution

4.2.1. Adularia

The distribution of adularia around the Aleste vein is shown in figure 5. This cross section shows that adularia has lithological control in its spatial distribution, predominantly related to the rhyolitic volcanic rock (FBR unit in Fig. 5), and it is restricted in the other rock units. There is a secondary structural control in its distribution associated with the veins. Near the surface is usually absent. Moreover, X-ray diffraction analyses indicated that adularia is associated with high contents of quartz, which is consistent with the quartz-adularia alteration within and near the veins. In addition, this mineral can be accompanied by sericite, illite, calcite, siderite, rhodochrosite, ankerite, pyrite, chlorite, and kaolinite.

 

Fig. 5. Distribution of adularia in the A-A’ cross section over the Aleste vein. The spatial distribution of adularia shows a strong correlation with rhyolite (FBR unit), and a secondary structural control related to veins. Lithologic units as in figure 3.

The semi-quantitative contents of adularia obtained from the quantification of the diffractogram were plotted in the Aleste vein cross section (Fig. 6). This section shows a strong correlation between an adularia abundance between 25 and 40% with gold mineralized zones in the veins.

 

Fig. 6. Comparison between adularia distribution (above) and gold-silver veins (below) in the cross section over the Aleste vein. There is a distinct relationship between the higher adularia concentrations and the gold mineralized zones in the veins.

4.2.2. Sericite and illite

These two minerals represent the quartz-sericite-illite and argillic alteration zones of the ore deposits, respectively. The cross section in figure 7 shows the semi-quantitative distribution of sericite and illite, from the vein core to the outward zones. These minerals are abundant near the surface and to a depth of ~150 m, in some areas until ~250 m. The distribution of these minerals coincides with areas of low content or no adularia (Fig. 7A). In figure 7A, the sericite cross section shows this mineral above the adularia halo (quartz-adularia alteration). On the other hand, illite is distributed mostly near the surface, less than ~100 m depth (Fig. 7B). At a depth of ~80 m a transition zone between these minerals, that represent the grading from the quartz-sericite-illite to argillic alterations, is observed. The XRD analyses show that sericite is associated with quartz, illite, kaolinite, calcite, ankerite, siderite, gypsum, pyrite, chlorite, and a lesser amount of montmorillonite. Alternatively, Illite is accompanied by kaolinite, chlorite, hematite, anhydrite, calcite, siderite, ankerite and magnesite, and trace amounts of montmorillonite, jarosite, natrojarosite, pyrite and alunite.

 

Fig. 7. Distribution of sericite (A) and illite (B) in the Aleste vein cross section.

4.2.3. Kaolinite

A kaolinite semi-quantitative distribution is shown in figure 8A. The highest values of this mineral are observed in the central east part of the cross section with semi-quantitative contents over 8%. The XRD analyses show two mineral associations that accompanied kaolinite: i) quartz, carbonates, montmorillonite, sericite and/or illite, and ii) chlorite, carbonates, gypsum, hematite, montmorillonite, heulandite, and jarosite assemblages.

 

Fig. 8. Distribution of kaolinite (A) and chlorite (B) in the Aleste vein cross section.

4.2.4. Chlorite

This mineral represents the propylitic alteration which is present in the distal zones of the epithermal system, and areas related to andesitic dikes with a high content of Na-, Mg- and Ca-bearing minerals that induce the formation of chlorite, calcite, and epidote as alteration minerals. Chlorite distribution is shown in figure 8B. The highest values of this mineral is where adularia is scarce or absent.

5. Discussion 

5.1. Mineral identification and semi-quantification by a XRD method

X-Ray diffraction proves to be useful in the recognition of minerals phases in hydrothermally altered rocks. In the case of adularia, the challenge is to identify this mineral in the diffractogram when it coexists with other K-bearing feldspars, mainly orthoclase and microcline. This study identified four secondary peaks in the diffractogram of the standard mineral, which can be useful to distinguish the presence of adularia in multimineral diffractograms of the studied samples. Despite, not further information of the Guanajuato adularia used as standard was provided by Yamana company, this specimen is from the Ag-Au epithermal veins of the Guanajuato mining district (Gross, 1975; Solé et al., 2021), which make this mineral suitable for this study. Even with this, it is known that the diffractograms of the same mineral can show differences in peaks distribution related to the ordering degree of the crystallographic structure and contents of trace elements (Colville and Ribbe, 1968; Stewart and Ribbe, 1969; Ferguson et al., 1991). Although the geological environments are similar between both the standard and the El Peñón adularias, could be some shift in the position of some peaks. So, we recommended for these types of studies, to obtain pure adularia samples from low sulfidation deposits located close to the prospection areas. Thus, would be ensured that the geological setting, the mineral geochemistry, and crystallographic structure would be closer to the mineral in search, and therefore, the standard diffractogram can be more precise, increasing the chance of success in the mineral prospection stage.

Adularia semi-quantification shows a very good correlation between the adularia content in a range between ~25 to 40% and the gold mineralized areas. Indicating that zones with this range of adularia abundance are favorable areas for gold prospection. However, the TOPAS software shows to be limited when two K-feldspars are quantified in the same diffractogram, which could overestimate the adularia content in detriment to the abundance of orthoclase. This problem could be solved by the use of an updated software version that focuses its quantification by using the secondary peaks of the minerals. Even with this limitations, this mineralogical study shows to be useful for the prospection of low sulfidation gold deposits.

5.2. Identification of low abundance clay minerals

Regarding the low abundance of clays, as smectites, these minerals cannot be efficiently identified by X-Ray diffraction. This problem is due to the high percentages of other minerals present such as quartz and carbonates, and the range in which the diffractometer measures (2θ=3°-70°) that causes the concealment of low abundance clay peaks by the background noise. Thus, we propose the XRD oriented aggregates study (Poppe et al., 2001), which focuses on the detection of low percentages of minerals by intensifying their peaks in the diffractogram. This type of analysis facilitates the development of detailed studies of the behavior of low-abundance minerals in the alteration haloes in the epithermal environment to be used as a proxy for the prospection of low sulfidation veins.

5.3. Hydrothermal alteration haloes definition

Alteration halos commonly described in low sulfidation epithermal deposits (e.g., Hedenquist et al., 2000; Simmons et al., 2005; Camprubí and Albinson, 2006; Warren et al., 2007) are clearly identifiable in the results of the XRD mineralogical analyses presented in this work. The changes observed in the hydrothermally altered wall rocks moving away from the internal zones with adularia to sericite, illite, illite-smectite, and finally, to kaolinite, expressing the hydrothermal fluid evolution from a high temperature, and near neutral pH solution in the mineralized zone, to a low temperature, and slightly acid-solution at the boundaries of the system (Hedenquist et al., 2000).

5.3.1. Adularia-quartz-carbonates-gold association

This K-feldspar is commonly found associated with low sulfidation epithermal deposits, its crystallographic structure changes from a maximum microcline to disordered high sanidine (the latter used in this study) according to the physicochemical conditions at the time of formation (Ferguson et al., 1991). Usually, adularia precipitate in low-sulfidation veins as fine-grained euhedral rhombic crystals, sub-rhombic, tabular, and pseudo-acicular, accompanied by colloform to crustiform quartz, platy calcite, and ore minerals (Dong and Morrison, 1995; Simmons et al., 2005). This mineral is often associated with quartz and carbonates, this association is interpreted as the product of boiling of the hydrothermal fluid at around 1 to 2 km deep below the water table (Simmons et al., 2005). Thin section descriptions of samples of the El Peñón deposit agree with the descriptions made by Simmons et al. (2005), showing rhombic adularia crystals associated with quartz and carbonates (Fig. 9). Based on the study by Dong and Morrison (1995), the adularia shape suggests a prolonged boiling that produced a drop in temperature (<220 °C) which is faster than the loss of CO₂, triggering the dissolution of previously formed carbonates in the earliest steps of the boiling at high-CO₂ fugacity and the replacement of them by adularia and quartz. Additionally, in prolonged boiling, the loss of H₂S induces the precipitation of gold from bisulfide complexes (Au(HS)₂^-, according to the following equation (Dong and Morrison,1995):

8Au(HS)^2-(aq)+4H₂O(aq)+6H⁺(aq)→8Au⁰+15H₂S (aq)+SO₄^2-(aq)          (1)

These processes explain the association between gold and the quartz-adularia alteration in the mineralized zones in the El Peñón veins. In addition, the paragenesis of adularia, siderite, rhodochrosite, ankerite, calcite, quartz and sulfide observed in the XRD analyses confirm the observations by Bissig et al. (2007) who reported the relationship between these minerals and the gold mineralization in the El Peñón veins, based on descriptions and elemental analysis (ICP-AES) in carbonate minerals.

 

Fig. 9. Pseudo rhombic adularia crystals from quartz-carbonate-adularia alteration in the Aleste vein. ad: adularia.

5.3.2. Sericite and illite

These minerals are the product of plagioclase and K-feldspar alteration by hydrothermal fluids with a slightly acidic pH, and temperatures between ~150  and 350 °C, in the epithermal environment (Hedenquist et al., 2000). Sericite is a term that refers to a fine-grained aggregate of white micas, as muscovite, that are commonly a product of feldspar alteration. In low-sulfidation epithermal environments, this mineral is the product of the feldspar alteration due to temperature- and pH-decrease of the post boiling activity of the hydrothermal fluid (Dong and Morrison, 1995). Instead, illite is a clay with similar geochemical composition to sericite, but with a smaller grain size and deficit of potassium in its structure. This fine-grained mica helps to interpret the temperature variation in the system; thus, it is useful as an indicator of the zones where the hydrothermal fluid passed, decreasing in temperature (under 300° C; Hedenquist et al., 2000).

The occurrence of sericite in XRD analyses of samples from veins in the El Peñón deposit was previously described only by Pérez (1999), other studies do not describe this mineral in the district, being illite the only K-bearing mineral in addition to adularia in the El Peñón veins (e.g., Cabello, 1999; Robbins, 2000; Zuluaga, 2004; Warren et al., 2004, 2007; Bissig et al., 2007; Donoso, 2012). Sericite detection in XRD analyses is associated with the appearance of Muscovite M1 pattern (Zadina, 1982) from the PDF-2 database in the diffractograms obtained. In the analyses, the presence of sericite and illite could be due to the potassium content in the crystal, variation in the structural order and variation in their granulometry, or just product of alteration of the first for the second mineral phase. Potassium-rich minerals, such as sericites, dominate in the areas closest to the ore, varying to clays with higher silica content towards the margins of the system, with clay minerals such as illite and illite-smectite (Rosenberg, 2002). Thus, in the El Peñón, the K-bearing minerals vary from sericite near the core to illite and finally, smectite at the boundary of the system, in accordance with the transition of K-metasomatism, pH and temperature changes in low-sulfidation epithermal environments (Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003; Simmons et al., 2005; Warren et al., 2007). The distribution of these minerals can clearly be seen in figure 7. Indeed, the distribution and semi-quantitative abundance of potassic minerals reflex the evolution of the fluids from the vein and core of the epithermal system to the outer zones, represented by the precipitation of adularia in the mineralized zones changing to sericite, later to illite in the outer areas. Indicating that the fluids moved upward through the structures, with decreasing temperature and increasing acidity (e.g., pH 4 to 5) as it moves away from the mineralized zones (Fig. 10).

 

Fig. 10. Schematic distribution of K-bearing alteration minerals in the A-A’ Aleste vein cross section, based on the semi-quantitative amount obtained by XRD for each mineral. Adularia content = >20%, sericite content = >10%, illite content = >7%. Some minor amounts of these minerals can overlap the other haloes. For more information about the semi-quantitative amount of each mineral see figures 6, 7 and 8.

5.3.3. Kaolinite

This mineral is the product of the alteration of micas, plagioclase, and K-bearing minerals by hydrolysis reactions (Hemley and Jonas, 1964), being part of the argillic and advanced argillic alterations. In low sulfidation epithermal veins, this mineral is associated with steam-heated and supergene environments, the last one associated with oxidation of sulfide minerals after the hydrothermal event (Hedenquist et al., 2000; Simmons et al., 2005; Warren et al., 2007).

In the studied samples, it is possible to recognize two mineral associations that accompany kaolinite: i) the mineral assemblage of kaolinite, quartz, carbonates, montmorillonite, sericite and/or illite. This mineral association indicates a genesis related to steam-heated CO₂-rich fluids, coming from the boiling zone, with a pH ~4 to 5 in a low O₂-availability environment, which creates intermediate argillic haloes below the water table and in the margins of the hydrothermal system (Hedenquist and Arribas, 2021). Instead, ii) the mineral association of kaolinite with carbonates, gypsum, hematite, jarosite, alunite, heulandite and montmorillonite and minor amounts of relict pyrite reflects a post-hydrothermal supergene alteration at low temperature (~40 °C) and an oxidizing environment. This alteration overprinted the hydrothermal alterations above the water table.

5.3.4. Chlorite

This mineral represents the distal propylitic alteration zone of the epithermal system. It can also be observed in andesitic dikes with a high content of Na-, Mg- and Ca-bearing minerals that induce the formation of chlorite, calcite, and epidote as alteration minerals. Chlorite is abundant in low-sulfidation deposits to the detriment of epidote which is common at a deeper environment. The cross section shows that the propylitic alteration is far from quartz-adularia veins and represents the margins of the epithermal system.

6. Summary and Conclusion

This study confirms that the X-ray diffraction method is reliable for the determination of adularia in multi-mineral diffractograms, demonstrating that this technique can be useful for targeting new mineralized veins in low sulfidation epithermal deposits. The spatial distribution of the alteration minerals in the studied cross section at El Peñón define the typical alteration halos described in Au-Ag low sulfidation epithermal deposits. In the veins from El Peñón deposit the following statements can be made:

The method to identify adularia and other minerals by using XRD demonstrated its usefulness for the definition of new prospection targets at El Peñón.

Adularia occurs mainly 100 m below the surface with great extension and abundance in the rhyolitic rock unit (informally called Riolita Fluidal-Bandeada unit (FBR)) and restricted in intermediate composition rocks. This mineral is closely correlated with the Au ore zones in the Aleste, Esmeralda and Borde W veins, being adularia semi-quantitative abundance between 25 to 40% a good indicator of gold mineralization.

Sericite and illite occur mainly above 100 m depth in areas where adularia is scarce or absent. The distribution of these K-bearing minerals together with adularia show the evolution of the hydrothermal fluid, and the structural control in the fluid flow, represented by a high content of adularia in the mineralized core changing to sericite, and illite to shallow areas, these minerals are always associated with a high content of quartz, carbonates, and lesser amounts of smectites, and pyrite.

Kaolinite is related to both argillic alterations caused by steam-heated fluids and to advanced argillic alteration associated with supergene alteration processes. Thus, the minerals associated with kaolinite are important in determining if the prospecting for low sulfidation epithermal veins is being conducted in the right environment.

Chlorite is indicative of propylitic alteration. This alteration usually is far from the mineralized areas, so this mineral is a reliable indicator of the margins of the hydrothermal system. This mineral can be useful in limiting the prospecting areas.

Acknowledgements
This work was financed by the Yamana Gold Company, El Peñón mine division. F. Álvarez is greatly acknowledged for his technical assistance with the X-ray diffraction analyses and lab work at Geochemical lab of the Universidad Católica del Norte. We are also grateful to the exploration division of El Peñón deposit for invaluable logistic support and for providing the samples for this study. Thanks to Dra. E. Fonseca, and Dr. W. Vivallo which provided reviews that greatly improved the manuscript.

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Supplementary material 1

Basic instrumental parameters for the iterative method used for the semi-quantification of mineral contents, in whole rock samples, used in the XRD/XRF laboratory, at Universidad Católica del Norte.

 

Supplementary material 2

The mineral content determinations was achieving following three semi-quantifications:

A) Semi-quantification considering orthoclase as a representative of K-feldspar groups (excluding adularia).
B) Semi-quantification considering adularia as a representative of K-feldspar groups (excluding orthoclase).
C) Semi-quantification considering both orthoclase and adularia.

The use of these three iterations is to determine if the TOPAS software is able to semi-quantify the content of adularia and orthoclase in the same sample, obtaining a percentage not far from the total abundance of K-feldspar.

Below is an example of these three semi-quantification in one of the diffractogram of a sample analyzed by XRD (Table S2; Fig. 1). It shows the results of each quantifications, the similarities in the iterations indicate that both minerals are in the sample and the semi-quantification is good, thus iteration c) is able to be used.

TABLE S2. SEMI-QUANTIFICATIONS REALIZED IN SAMPLE SPA-0011-229 IN BASE OF THE METHODOLOGY PROPOSED.

 

FIG. S1. Diffractogram of sample SPA-0011-229 with all the mineral phases detected.

 

Supplementary material 3

TABLE S3. X-RAY DIFFRACTION ANALYSES AND SEMIQUANTIFICATION OF THE PHASES PRESENT IN EACH SAMPLE. THE 100% CORRESPOND TO THE TOTAL OF CRYSTALLINE PHASES NOT COUNTING TRACE MINERALS (< 2% OF ABUNDANCE) AND THE AMORPHOUS PORTION OF THE ROCK.