Romina Lucrecia Lopez Steinmetz

Stefano Salvi

Carisa Sarchi

Carla Santamans

Lorena Cecilia Lopez Steinmetz
email: lucrecialopezsteinmetz@hotmail.com

Lucrecia Romina

López Steinmetz

Lorena Cecilia López Steinmetz

Lithium and Brine Geochemistry in the Salars of the Southern Puna, Andean Plateau of Argentina

Keywords: lithium triangle, salt pan, Central Andes, hydrochemistry

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Introduction

Within the past few years, it has become apparent that the global need for an energy transition from fossil fuels depends not only on our capacity to come up with a renewable source of energy, but also on our ability to store it. The success of Li-ion batteries, with early applications in mobile electronics and now increasingly in electric and hybrid vehicles, has caused batteries to become the prime source of Li consumption, worldwide. This has spurred a renewed interest on lithium as a resource, to the point that it is now considered one of the most critical materials of our times (e.g., Hache et al., 2018;[START_REF] Olivetti | Lithium-Ion Battery Supply Chain Considerations: Analysis of Potential Bottlenecks in Critical Metals[END_REF][START_REF] Ghadbeigi | Performance and resource considerations of Li-ion battery electrode materials[END_REF]. One of the principal sources of lithium is the mineral spodumene (along with petalite and lepidolite), found mainly in highly evolved pegmatites, with Australia being its largest producer (mainly from Greenbush; [START_REF] Kesler | Global lithium resources: relative importance of pegmatites, brine and other deposits[END_REF]. Processing, however, is carried out mostly in China, which is also the world's largest consumer of this metal (Sun et al., 2017).

Another abundant source of lithium is the dissolved salt in modern brine deposits found in evaporated altitude lakes, known as salars. The Andean plateau, located between Bolivia, Chile and Argentina (also known as the lithium triangle), hosts numerous salars, and the region contains more than 50% of the global Li resources [START_REF] Munk | Lithium brines: A Global Perspective[END_REF]. This form of lithium is more easily processed than hard rock, causing Chile to become the largest exporter of the finished product, lithium carbonate (Sun et al., 2017;Hache et al., 2018). This resource represents therefore a strategic supply of lithium for the western world.

The largest known Andean salar is the giant Uyuni salar (10,580 km 2 ) in Bolivia [START_REF] Ericksen | Geology and resources of salars in the central Andes: U.S. Geologial Survey[END_REF][START_REF] Risacher | Quaternary geochemical evolution of the Salar of Uyuni and Coipasa[END_REF], although this country is not a producer.

Second in size is the Salar de Atacama in Chile, which, with its 3,000 km 2 at mean grades of 1,400 mg L -1 , is the most important lithium resource in the world (Ide and Kunasz, 1989;[START_REF] Kunasz | Lithium resources[END_REF]. Argentina also has an enormous potential for lithium brine resources, with sixteen of the remaining major salars located in this country's portion of the Andean plateau, known as the Puna region (Fig. 1). The Puna plateau is divided into northern and southern parts.

The Northern Puna (located between ca. 22-24°S) hosts the salars of Guayatayoc, Salinas Grandes, Olaroz, Cauchari, and Jama, which range in size between 25 and 280 km 2 and contains brines ranging between about 80 and 1,000 mg L -1 in mean Li + concentrations [START_REF] López Steinmetz | Lithium-and boron-bearing brines in the Central Andes: exploring hydrofacies on the eastern Puna plateau between 23° and 23°30[END_REF][START_REF] López Steinmetz | Northern Puna plateau-scale survey of Li brine-type deposits in the Andes of NW Argentina[END_REF][START_REF] García | Characterization of lithium cycling in the Salar de Olaroz, Central Andes, using a geochemical and isotopic approach[END_REF]. Among these salars, Olaroz and Cauchari have the highest Li + grades (means of 1,014 mg L -1 and 860 mg L -1 , respectively), and are presently being mined. Although the twelve salars in the Southern Puna plateau (south of 24°S; Fig. 1) constitute a huge resource , only the Salar de Hombre Muerto is being currently operated. All of these salars have been staked for mining rights, however very little is known of the brine hydrochemistry of the Southern Puna's salars, except that mean Li + grades of 520 and 400 mg L -1 have been reported for Hombre Muerto (Garret, 2004;[START_REF] Kesler | Global lithium resources: relative importance of pegmatites, brine and other deposits[END_REF]Godfrey et al., 2013) and Rincón [START_REF] Toledo | Evapofacies halítica en el Salar del Rincón, Departamento Los Andes[END_REF], respectively, and an isotopic data set is available for the Salar de Centenario and Ratones [START_REF] Orberger | Stable isotopes (Li, O, H) combined with chemistry: powerful tracers for Li origins in Salar deposits from the Puna region[END_REF]. This manuscript reports the outcome of a large effort to obtain data from the 12 major salars scattered throughout the very remote Southern Puna region. It is the result of a two-year regional exploration campaign carried out by the CONICET and the Institute of Geology and Mining of the Jujuy University of Argentina. Along with the information previously reported by López Steinmetz (2017) and López [START_REF] López Steinmetz | Northern Puna plateau-scale survey of Li brine-type deposits in the Andes of NW Argentina[END_REF] on the northern Puna salars, its aim is to contribute to knowledge of the hydrochemistry of the Andean salars, and to complete the first broad survey of Li-bearing salars hosted on the Argentine plateau. This study concludes the Li survey and brines chemistry characterization of Andean salars, offering for the first time an overview of the true potential of the Andean Lithium Triangle.

Geology of the Study Area

The Southern part of the Puna plateau extends from 24°S to 26°30'S and is bounded by major volcanic peaks that mark the Chilean-Argentinian border, such as the Llullaillaco (6,723 m a.s.l.), Socompa (6,031 m a.s.l.), and Rincón (5,594 m a.s.l.), and by the Luracatao Range to the east (Figs. 2 and 3;cf. Alonso et al., 1984). The salars in this study are located in orographic depressions along NE-SW trending ranges that formed during uplift of the Andes. The main geology consists of Paleozoic basement units; these include: i) Ordovician metasediments (Copalayo, Caucota, and Falda Ciénaga formations;Turner, 1960;[START_REF] Aceñolaza | El Ordovícico de la Puna saltocatamarqueña: 5th Congreso Geológico Argentino, Actas[END_REF], submarine volcanic rocks (Schwab, 1973;Coira, 1979), riodacites, granites (Eastern Puna Eruptive Belt, Taca-Taca, Macón, Arita, and La Casualidad;[START_REF] Méndez | Faja Eruptiva de la Puna oriental: 5th Congreso Geológico Argentino[END_REF][START_REF] Méndez | Geología de la región Noroeste. Argentina: Dirección General de Fabricaciones Militares[END_REF][START_REF] Méndez | Geología de la región Noroeste. Argentina: Dirección General de Fabricaciones Militares[END_REF]; ii) Devonian marine sandstones (Salar del Rincón Formation); iii) Carboniferous continental sandstones (Cerro Oscuro Formation); and iv) Permian marine limestones (Arizaro Formation; [START_REF] Aceñolaza | El Neopaleozoico de la Puna: su fauna y correlación con áreas vecinas[END_REF][START_REF] Donato | Geología del Devónico y Neopaleozoico de la zona del Cerro Rincón, Salta, Argentina: 4th Congreso Geológico Chileno[END_REF][START_REF] Galli | Estudios sedimentológicos en los depósitos carboníferos y pérmicos del borde occidental de la Puna[END_REF].

Middle Eocene to upper Pliocene continental sedimentary rocks (Pastos Grandes Group;Turner, 1960;[START_REF] Alonso | Ocurrencia, Posición Estratigráfica y Génesis de los Depósitos de Boratos de la Puna Argentina[END_REF][START_REF] Alonso | Giant evaporite belts of the Neogene central Andes[END_REF] are exposed along the Calalaste and Los Colorados Ranges (Fig. 2A). Large areas of the Southern Puna are covered by upper Oligocene -Pliocene dacites, andesites, ignimbrites, and Quaternary basalts (Salfity et al., 1984;[START_REF] Alonso | Geología y metalogenia de la Puna[END_REF][START_REF] Jordan | Cenozoic stratigraphy and basin tectonics of the Andes Mountain[END_REF][START_REF] Montero López | Late Miocene ignimbrites at the southern Puna-northern Sierras Pampeanas border (∼27° S): stratigraphic correlation[END_REF]Maro et al., 2017).

Quaternary fine-grained sediments of alluvial origin cover the planes between the mountain ranges. Salt pans form on low-laying areas because of the extreme climatic conditions that prevail in these high-altitude endorheic basins (cf. López Steinmetz and Galli, 2015).

Methods

Between 2014 and 2017, 6 expeditions were run to the Southern Puna of Argentina in order to sample residual brines from the twelve major salars in this region. These are the Salar de Centenario (CE), Diablillos (DI), Hombre Muerto (HM), Incahuasi (IN), Pastos Grandes (PG), Pozuelos (PZ), Ratones (RA), Antofalla-Botijuelas (AN), Arizaro (AR), Pocitos-Quirón (PC), Rincón (RI), and Río Grande (RG, Figs. 2 and3). Precise sample location is provided in Table A1 of the Appendix. Most samples were collected in pits that were hand-dug in the salars, at depth intervals of 40 to 90 cm, plus a number that were taken directly from brine pools (samples CE1, CE2, DI3 to DI5, PG1 to PG3, PG5, PZ1, PZ5, PZ7, PZ8, AN1, AN2, PC1 to PC4, and RI 5). Samples were stored in pre-cleaned, 2-L polyethylene bottles and kept at 5°C in the field. No acidification was employed after sample collection. Samples were transported directly to the laboratory after collection.

All samples collected in the twelve salars were analyzed at the Agua de Los Andes S.

A. laboratory (Jujuy, NW Argentina) and the resulting data are shown in Table 1. Values for pH and electrical conductivity were determined with a Hanna HI2314 multi-range conductivity meter. The calibration of the meter involved several steps. 1) Verification of the temperature reading; 2) calibration of the conductivity meter using a 1,000-µS/cm calibration standard, to obtain a conductivity cell constant of 5.0 +/-0.5; and 3) calibration of the pH meter using buffer solutions for pH 7 and 4, to obtain a slope comprised between 55 and 60 mVs. Salinities are expressed as the total dissolved solids (TDS; mg L -1 ), which were obtained by employing a conversion factor of 0.50 on the measured electrical conductivity. This conversion factor corresponds to the ratio observed between the measured electrical conductivity of samples and their total analytical ionic concentrations (i.e., the sum of measured cations and anions). The major ion composition was determined following the procedure outlined by [START_REF] Clesceri | Standard Methods for the Examination of Water and Waste Water[END_REF]. Concentrations of calcium (Ca 2+ ) were obtained by titrating with a 0.01 M EDTA solution for the former, while those of magnesium (Mg 2+ ) were calculated by subtracting the calcium concentration to the total hardness. Titration with a 0.01-N sulfuric acid solution was employed to determine carbonate and bicarbonate ion (HCO3 -and CO3 2-) concentrations, using methyl-orange and phenolphthalein, respectively, as end-point indicators. Chloride (Cl -) was measured by titration using the argentometric method. Concentrations of sulfate (SO4 2-) were determined by turbidimetry on the precipitate obtained by mixing it with BaCl2 in an acetic acid medium. Atomic Emission Spectroscopy was employed to analyse sodium (Na + ), potassium (K + ) and lithium (Li + ) while boron (B) was determined by colorimetry using a carmine reactive.

Ion balance values were ≤10% in all cases (cf. Fig. A1 and Table A2, Appendix).

Li-Bearing Salars of the Southern Puna

Salar de Centenario (CE, east)

Centenario is a 7-km 2 playa lake located at 3,816 m a.s.l., between the Los Colorados, Pastos Grandes, and Luracatao Ranges (Fig. 2). Centenario also includes 70 km 2 of saline muddy shore zones, which are fed by the Singuel, Ciénaga Grande, and Copalayo rivers.

Brines collected in the Salar de Centenario are alkaline, with pH values comprised between 7.50 and 7.70, and TDS ranging between 26,090 and 48,590 mg L -1 (Table 1).

According to their major chemical composition, brines are of the Na + / Cl --SO4 2-type, with high SO4 2-, relatively abundant K + and Mg 2+ , and are depleted in Ca 2+ (Fig. 4). No correlation between major ions has been identified, with the exception of sample CE4, whose Na + :Cl -ratio ~1:1 reflects equilibrium solubility of halite (Fig. 5).

The Li + concentration in the Salar de Centenario ranges between 25 and 377 mg L -1 , with a mean Li + concentration of 141 mg L -1 . Positive correlations could exist between the Li + concentrations and those of K + , B, and Mg 2+ (Fig. 6). The mean Li:K ratio is 0.37 while the mean Li:Mg ratio is 0.20 (Table 2). Boron concentrations range between 277 and 581 mg L -1 , and the mean Li:B ratio is 0.97.

Salar de Diablillos (DI, east)

Diablillos is a 4-km 2 playa lake located at 4,032 m a.s.l., between the Los Colorados, Luracatao and Pastos Grandes Ranges (Fig. 2). Muddy shore zones covering about 29 km 2 at the mouth of the Diablillos river, to the east, are also considered as part of the Salar de Diablillos.

The Diablillos brines are alkaline, pH is 8.00, and TDS values range between 26,350 and 80,735 mg L -1 (Table 1). According to their major chemical composition, brines are of the Na + / Cl --SO4 2-type, with high SO4 2-, with relatively abundant K + and Mg 2+ (Fig. 4). The ~1:1 ratio between Na + and Cl -in samples DI3 and DI4 indicates the dissolution of halite. Brines are also characterized by SO4 2-> Ca 2+ > CO3 2-+HCO3 -(Fig. 5).

The Li + concentration in the Diablillos's brines ranges between 47 and 357 mg L -1 , with a mean Li + concentration of 180 mg L -1 . The mean Li:K ratio is 0.26 and the mean Li:Mg = 0.26 (Table 2). Boron concentrations are up to 808 mg L -1 , with a mean Li:B ratio of 0.86.

Salar de Hombre Muerto (HM, east)

The Salar de Hombre Muerto is located at 3,970 m a.s.l., between the Aguas Calientes, Calalaste, Los Colorados, and Pastos Grandes Ranges, at the foot of the Ratones volcano (5,252 m a.s.l.) (Vinante and Alonso, 2006). Hombre Muerto is by far the largest salar among those of the eastern part of Southern Puna. It spreads between 66°53'24" -67°13'30"W and 25°13'48" -25°30'36"S (Fig. 2), and includes a 230 km 2 salt pan in the west, a 65 km 2 lagoon in the eastern side, and a 245 km 2 playa domain that mainly develops in its northern reaches, for a total area of 540 km 2 . The main tributaries are the Ratones river to the north, and the Los Patos river, sourced at the foot of the Galán volcano (5,919 m a.s.l.), which reaches this salar from the southeast.

Brines from the Salar de Hombre Muerto are also alkaline, with pH values ranging between 7.80 and 8.20, and TDS ranging between 31,215 and 205,022 mg L -1 (Table 1).

According to their major chemical composition, they are of the Na + / Cl -type, with relatively abundant K + (Fig. 4). The concentrations Ca 2+ and SO4 2-in sample HM4 correspond to a stoichiometric ratio of 1:1, a value that suggest dissolution of gypsum (Fig. 5).

Hombre Muerto contains the highest values of lithium in Southern Puna, with a mean Li + concentration of 570 mg L -1 for a range between 85 and 1,013 mg L -1 . The highest grades are found in the middle part of the salar. The mean Li:K ratio is 0.69, and the mean Li:Mg ratio is 0.92 (Table 2). Boron concentrations range between 377 and 1,490 mg L -1 , and the mean Li:B ratio is 1.90 (Fig. 6).

Salar de Incahuasi (IN, east)

Incahuasi is a playa lake with an elongated shape oriented north-south. It is situated between the Calalaste and Morada mountain ranges, at an altitude of 3,269 m a.s.l. It is the southernmost salar in the Puna region, spreading between 67°39'03" -67°40'35"W and 26°18'44" -26°28'30"S (Fig. 2). Its southern edges are covered by more than 33 km 2 of dune fields. This salar includes two distinctive zones; north of 26°23'29"S is a saline, episodically flooded part, covering about 18 km 2 , while the 20 km 2 to the south consist of a saline muddy playa lake. The salar is mostly fed by the Cortaderas river, coming in from the northeast.

Brines from the Salar de The Incahuasi are slightly alkaline, with a pH ranging between 6.70 and 7.30. TDS values range between 48,220 and 107,340 mg L -1 (Table 1). According to their major chemical composition, brines are of the Na -Mg 2+ / Cl -type, with high Mg 2+ , and depleted in Ca 2+ (Fig. 4). Major ratios are characterized by Cl -> Na + , Ca 2+ > SO4 2-, and Ca 2+ > CO3 2-+HCO3 -(Fig. 5).

At Incahuasi, the Li + content in brines range between 22 and 145 mg L -1 , with a mean of 95 mg L -1 . The highest concentrations of Li occur in the middle part of its saline, floodable zone in the north. Brines from Incahuasi contain the highest concentrations of magnesium among all of the salars that are located in the eastern part of the Southern Puna (Fig. 6). K + and Mg 2+ concentrations do not correlate with Li + concentrations (Fig. 6); the mean Li:K ratio is 0.09 while the mean Li:Mg ratio is 0.01 (Table 2). A positive correlation may exist between B and Li + (Fig. 6). Boron concentrations range between 114 and 570 mg L -1 , and the mean Li:B ratio is equal to 0.62.

Salar de Pastos Grandes (PG, east)

The Salar de Pastos Grandes is located at 3,780 m a.s.l. between the Los Colorados, Pastos Grandes, and Japeler Ranges, between 66°43'48" -66°39'18"W and 24°31'30" -24°36'18"S (Fig. 2). Pastos Grandes has a triangular shape and the salt pan zones cover 27 km 2 , to which must be added a 4.80 km 2 saline lake, located at its north-eastern margin. The main tributaries are the Corral Colorado and Pastos Grandes rivers in the north, and the Ciénaga Redonda river arriving from the east.

Brines from Pastos Grandes are alkaline, with pH values ranging between 8.00 and 8.30, and TDS ranging between 154,030 and 226,600 mg L -1 (Table 1). According to their major chemical composition, brines are of the Na + / Cl -type, with relatively abundant Mg 2+ , and depleted in Ca 2+ (Fig. 4). When comparing these brines with those from the other salars, the Pastos Grandes shows the highest concentrations of CO3 2-and HCO3 -. Near stoichiometric trends between the Ca 2+ : CO3 2-+ HCO3 -and Ca 2+ : SO4 2-ratios likely reflect equilibrium solubility of Ca-Mg sulfates and Ca-carbonates (Fig. 5). However, the fact that the linear trend defined by the Mg 2+ : SO4 2-data deviates from a 1:1 ratio reflects that Mg salts, like epsomite (MgSO4 7H2O), are not in equilibrium with the brine.

The Li + concentrations in the Pastos Grandes's brines range between 305 and 857 mg L -1 , with a mean of 483 mg L -1 . The highest Li grade zone in the eastern part of the salar. The mean Li:K ratio is 0.82 while the mean Li:Mg ratio is 0.19 (Table 2). Boron concentrations range between 300 and 812 mg L -1 , and the mean Li:B ratio is 2.24.

Salar de Pozuelos (PZ, east)

The salar de Pozuelos is a 75-km 2 salt pan with an oval shape along a NE-SW direction.

It is located at 3,663 m a.s.l. at the interior of the Los Colorados Range (Fig. 2). The main tributary is the Pozuelos river to the north.

Brines from the Salar de Pozuelos are alkaline, with pH values ranging between 7.80 and 8.60, and TDS ranging between 205,243 and 298,574 mg L -1 (Table 1). According to their major chemical composition, brines are of the Na + / Cl -type, with relatively abundant K + and Mg 2+ (Fig. 3). Major ratios are characterized by Cl -> Na + , SO4 2-> Ca 2+ , and Ca 2+ > CO3 2- +HCO3 -. Non-stoichiometric ratios were observed combining different major ion pairs (Cl -> Na + , SO4 2-> Ca 2+ , and Ca 2+ > CO3 2-+HCO3 -), suggesting that the dominant ion concentrations are independent of salt equilibrium solubility (Fig. 5).

In the Pozuelos's brines, Li + concentrations range between 75 and 940 mg L -1 with a mean of 401 mg L -1 . Grades increase towards the central part of the salt pan. Stoichiometric 1:1 ratios are observed between Li + and K + in samples PZ5 and PZ6 (Fig. 6 A). The mean Li:K ratio is 0.69, and the mean Li:Mg ratio is 0.43 (Table 2). Boron ranges between 202 and 2,102 mg L -1 , and the mean Li:B ratio is 1.26.

Salar de Ratones (RA, east)

Ratones is a north-south stretched playa lake located at 3,822 m a.s.l. between the Colorados and Pastos Grandes -Luracatao Ranges. The playa lake surface covers only 3 km 2 , with most of the salar consisting of 47 km 2 of saline muddy shore zones, spread between 66°47'10" -66°50'54"W and 25°02'42" -25°11'42"S (Fig. 2). Main tributaries are the Singuel and Quebrada Honda rivers.

Brines from the Salar de Ratones are alkaline and moderately saline, TDS values are between 28,080 and 82,580 mg L -1 , and pH range between 7.50 and 7.80 (Table 1). According to their major chemical composition, brines are of the Na + / Cl --SO4 2-type, with relatively abundant K + and Ca 2+ (Fig. 4).

The Li + concentration in the Ratones's brines range between 88 and 260 mg L -1 , and the mean Li + concentration of the three samples collected in the salt pan zone is 158 mg L -1 . The Li + , K + and Mg 2+ mean ratios are: Li:K = 0.38, and Li:Mg = 1.24 (Table 2). Boron concentrations range between 312 and 509 mg L -1 , and the Li:B ratio = 1.06.

Salar de Antofalla-Botijuelas (AN, west)

Salars in the western part of the Southern Puna are relatively large. The Salar de Antofalla-Botijuelas covers some 655 km 2 with its elongated shape stretching N-S for more than 100 km. This salt pan is at an altitude of 3,330 m a.s.l., between 25°24'54" and 26°06'54"S at ca. 67°42'W (Fig. 3). The salar is delimited by the Calalaste and Antofalla Ranges and main tributaries are the Archibarca, Antofalla, Aguas Calientes, and Aguas Dulces rivers. There are abundant thermal springs around this salar, especially in the northern and central western borders of the salar (Botijuelas thermal sources), and samples AN1, AN2, and AN3 correspond to thermal waters sampled at temperatures of 31.50 °C.

Brines from the Salar de Antofalla-Botijuelas are only slightly alkaline, with pH values comprised between 7.40 and 7.80, and TDS ranging between 9,110 and 184,570 mg L -1 (Table 1). According to their major chemical composition, brines are of the Na + / Cl --SO4 2-type, with relatively abundant Ca 2+ and low Mg 2+ (Fig. 4).

The Li + concentrations in this salar increase northwards, ranging between 46 and 615 mg L -1 , with a mean of 209 mg L -1 . Positive trends parallel to 1:1 are observed between the Li + concentrations and those of K + and Mg 2+ (Fig. 6); the mean Li:K ratio is 0.90, and the mean Li:Mg ratio = 0.64 (Table 2). Boron concentrations range between 415 and 878 mg L -1 , and the mean Li:B ratio is 0.83.

Salar de Arizaro (AR, west)

Arizaro, also in western Southern Puna, is the largest salar in the Puna plateau and in Argentina altogether. This salt pan covers an area of 1,708 km 2 , between 67°18'33" -68°02'11"W and 24°20'42" -25°05'44"S (Fig. 3). Arizaro is located at an elevation of 3,474 m a.s.l., and is bounded by the Archibarca and Antofalla ranges and by the volcanic peaks that define the Argentinian border with Chile.

Brines from the Salar de Arizaro are alkaline, with pH values comprised between 7.30 and 7.40, and TDS ranging between 38,865 and 308,175 mg L -1 (Table 1). According to their major chemical composition, brines are of the Na + -K + / Cl --SO4 2-type, with relatively abundant Mg 2+ (Fig. 4). In addition of being the largest salar of the Argentinean plateau, Arizaro contains the highest concentrations of K + , B, and SO4 2-of the Southern Puna. No obvious correlation could be observed among data for major ions (Fig. 5). A Mg 2+ : SO4 2-linear trend parallel to the 1:1 ratio indicates some degree of equilibrium solubility for Mg salts, like epsomite (MgSO4 7H2O). However, trends parallel to the Na + and SO4 2-axes suggest the presence of other salts that are not in equilibrium with the brine.

The Li + concentrations in the Salar de Arizaro range between 52 and 465 mg L -1 , with an increase towards the south-central part. Mean Li + concentrations for the whole salar are of 191 mg L -1 . The mean Li:K ratio is 0.24 while the mean Li:Mg ratio is 0.16 (Table 2). Boron concentrations range between 380 and 3,382 mg L -1 , with a mean Li:B ratio of 0.28.

Salar de Pocitos-Quirón (PC, west)

The Salar de Pocitos-Quirón consists of a 50-km 2 saline system at an elevation of 3,663 m a.s.l. It is elongated north-south and lays between the Los Colorados and Calalaste Ranges (Fig. 3). A 10-km long alluvial fan at the mouth of the Los Colorados river separates the Salar de Pocitos to the north from the Quirón playa lake to the south. The Pocitos salt pan zone covers 28 km 2 and spreads between 67°00'18" -67°03'36"W and 24°22'30" -24°27'30"S. The Quirón playa lake has a surface of 422 km 2 and spreads between 24°15'18" and 24°46'12"S. In addition to the Los Colorados river, another important tributary is the Agua Blanca river that emerges at the foot of the Quevar volcano (6,130 m a.s.l.).

Brines from the Salar de Pocitos-Quirón are alkaline, pH values range between 7.80 and 8.30, and salinity is high, with TDS ranging between 29,480 and 299,217 mg L -1 (Table 1).

According to their major chemical composition, brines are of the Na + -Ca 2+ -Mg 2+ / Cl --SO4 2- type (Fig. 3). The Pocitos-Quirón brines contain the lowest concentrations of Li + , K + , Mg 2+ , and B among all salars in the Southern Puna (Fig. 6).

Li + ranges between 22 and 101 mg L -1 , with the highest grade zone occurring in the central part of the Pocitos salt pan. The mean Li + concentration is 57 mg L -1 , the mean Li:K ratio is 0.26 and the mean Li:Mg ratio is 0.17 (Table 2). Boron concentrations range between 60 and 208 mg L -1 . A strong positive correlation between Li + and B is observed along the stoichiometric ratio 1:1 (Fig. 6 B), and the mean Li:B ratio of these brines is 1.37.

Salar de Rincón (RI, west)

The Salar de Rincón is a playa lake located at 3,725 m a.s.l., at the foot of the Tultul (5,270 m a.s.l.), Rincón (4,530 m a.s.l.), and Incahuasi (5,690 m a.s.l.) volcanoes, next to the borders between Northern and Southern Puna and the Chile-Argentina border (Fig. 3). The Salar de Rincón spreads between 66°57'54" -67°10'46"W and 24°03'54" -24°12'23"S, covering a surface of 250 km 2 . Main tributaries to this salar include the Catua and Huaytiquina rivers, both coming in from the north, causing a 170-km 2 saline muddy shore zone that spreads up to 23°55'48"S.

Brines from the Salar de Rincón are alkaline, with pH ranging between 8.60 and 8.80, and TDS values comprised between 140,265 and 284,650 mg L -1 (Table 1). According to their major chemical composition, brines are of the Na + -K + / Cl --SO4 2-type, with relatively abundant Mg 2+ (Fig. 4). Major ion concentrations are more elevated in the central part of the salar. Samples along the Na + and Cl -stoichiometric 1:1 ratio suggest equilibrium solubility of halite. Other major ionic ratios are characterized by SO4 2-> Ca 2+ and Mg 2+ , and Ca 2+ > CO3 2- +HCO3 -(Fig. 5). Linear trends parallel to the SO4 2-axis indicate that the concentrations of Ca 2+ and Mg 2+ are not controlled by equilibrium solubility of salts (Fig. 5).

Lithium ion concentrations in the Rincón's brines range between 112 and 651 mg L -1 , with the highest-grade zone occurring in the central part of the salar, and the mean Li + concentration being 281 mg L -1 . Positive linear trends are observed between Li + and Mg 2+ (Fig. 6); the mean Li:K ratio is 0.25 while the mean Li:Mg ratio is 0.24 (Table 2). Boron concentrations range between 1,405 and 2,098 mg L -1 and the mean Li:B ratio is 0.45.

Salar de Río Grande (RG, west)

The Salar de Río Grande is a 176-km 2 elliptically-shaped saline system, oriented northsouth, occurring at an altitude of 3,668 m a.s.l. between 68°07'22" -68°12'07"W and 24°59'24" -25°06'18"S (Fig. 3). The surface of the salt pan zone covers 56 km 2 , with remaining of the salar consisting of a 120 km 2 saline muddy playa lake that extends to the south of the salar, to 25°10'55"S. The main tributary is the Río Grande river, which contains some hydrothermal springs in its southernmost headwaters.

Brines from the Salar de Río Grande are highly saline and characterized by TDS values between 199,070 and 284,605 mg L -1 . These brines are neutral to slightly alkaline, with pH ranging between 7.00 and 7.20 (Table 1). According to their major chemical composition, brines are of the Na + / Cl --SO4 2-type, with similar proportions of K + , Mg 2+ and Ca 2+ (Fig. 4).

Río Grande contains the highest concentrations of Ca 2+ and SO4 2-of the Southern Puna (Fig. 5 B). Linear trends parallel to the SO4 2-axis indicate that the concentrations of Ca 2+ and Mg 2+ are not controlled by equilibrium solubility of salts (Fig. 5).

The concentrations of Li + range between 139 and 692 mg L -1 , with the highest Li grade measured in the central part of the salar. The mean Li + concentration is 396 mg L -1 . Positive linear trends, parallel to the Li + axis, suggest that there is no obvious correlation among Li, K + , and B. However, a stoichiometric linear trend is observed between Li + and Mg 2+ (Fig. 6). The mean Li:K ratio is 0.72 while the mean Li:Mg ratio is 0.33 (Table 2). Boron concentrations range between 303 and 545 mg L -1 , the mean Li:B ratio = 2.81.

Discussion

Classification of brines in the regional context

There are a number of common features that can be distilled from the major chemistry of the brines investigated in this study (Fig. 4). The first observation is that they are all rich in Na + and Cl -and that, at least for most brines, concentrations of K + (e.g., HM, RA, AR, RI) and Mg 2+ (e.g., IN, PG, AR) are relative important. There are also important differences among the brines of these salars. For instance, DI, PC and AN show wide variations in the proportions of most cations, while RG, RI, IN, HM, and CE have relatively constant values across the salar.

In general, with the exception of RA, salars located in the western side of the Southern Puna are richer in Ca 2+ than those in the eastern side, which in turn, are relatively enriched in Mg 2+ .

The concentration of SO4 2-in western salars is generally higher than that in the eastern side, representing intermediated values between the latter (lower SO4 2-) and the salars from Northern Puna studied by López Steinmetz et al. ( 2018) (higher SO4 2-). Another remarkable difference between Southern and Northern Puna is a clearly higher concentration of Mg 2+ in the southern salars. These ionic patterns could be related to the characteristics of lithologic substrates. The basement rocks differ between the southern and northern portion of the Puna plateau: while Ordovician metasedimentary sequences predominate in the north, a metamorphic-magmatic basement (biotite gneiss, felsic granulites) prevails in the south [START_REF] Lucassen | The early Palaeozoic high grade metamorphism at the active continental margin of West Gondwana in the Andes (NWArgentina/N Chile)[END_REF]Bahlbourg et al., 2016;[START_REF] Egenhoff | Chemical and isotopic composition of lower to upper Ordovician sedimentary rocks (Central Andes /South Bolivia): implications for their source[END_REF]. Cenozoic andesitic and dacitic ignimbrites, associated with Miocene -Pliocene caldera complexes, largely cover the northern part of the Argentine Puna (particularly between 22°30' -23° S), whereas they are less abundant towards the south [START_REF] De Silva | Altiplano-Puna volcanic complex of the central Andes[END_REF][START_REF] Caffe | Petrogénesis of Early Neogene magmatism in the Northern Puna; implications for magma genesis and crustal processes in the Central Andean Plateau[END_REF]Schnurr et al., 2007;[START_REF] Kay | Regional chemical diversity, crustal and mantle sources and evolution of central Andean Puna plateau ignimbrites[END_REF]Grosse et al., 2017Grosse et al., , 2018;;[START_REF] Montero López | Late Miocene ignimbrites at the southern Puna-northern Sierras Pampeanas border (∼27° S): stratigraphic correlation[END_REF]. Conversely, although mafic volcanism does occur in the North, the Southern Puna is characterized by a large number of Miocene-to-Quaternary mafic monogenic volcanoes (285 centers; [START_REF] Haag | Geomorphology and spatial distribution of monogenic volcanoes in the southern Puna plateau[END_REF], which are typically formed of Mg-rich basaltic andesites (Maro et al., 2017a). They are characterized by more primitive compositions than those in the north (Maro et al., 2017b). Lavas with more alkaline compositions (i.e., basaltic trachyandesites with near shoshonite compositions) are rare across the Puna, and occur mostly towards the eastern side of this region [START_REF] Maro | Primer registro de lavas shoshonı´ticas en Puna norte, el cerro Barro Negro: 2 Simposio sobre Petrología Ignea y Metalogénesis Asociada[END_REF].

It is thus possible that the higher Mg 2+ concentrations in the Southern Puna's salars are a consequence of weathering (and alteration?) of mafic rocks, whereas the sulfate increase in the Northern Puna probably derives from the Ordovician metasedimentary sequences. This is consistent with observations by López Steinmetz (2017) concerning the Guayatayoc salar area, who suggested that weathering of certain sections of the Ordovician basement (Santa Rosita Formation;Turner, 1960;[START_REF] Moya | Lower Ordovician in the Southern part of the Argentine Eastern Cordillera[END_REF] made of marine mudstones, fine-grained silty sandstones, and shales (Buatois and Mángano, 2003) caused releasing of abundant SO4 2- [START_REF] López Steinmetz | Lithium-and boron-bearing brines in the Central Andes: exploring hydrofacies on the eastern Puna plateau between 23° and 23°30[END_REF].

The data presented above indicates that the chemistry of most brines evolves through the sulfated pathway, with only a minor portion of samples passing through the calcic-sulfated way (Hardie and Eugster, 1970;Fig. 7). Among our samples, the brines from IN stand out by having the highest concentration in Mg 2+ (Figs. 4A and5C), while the remaining samples evolve through the calcic pathway, given that they are Ca 2+ and Mg 2+ rich with respect to CO3 2- and SO4 2-. Based on the above observations, one can outline a relationship between the chemistry of brines that evolve through the calcic pathway and the local geology (Fig. 2 A): i) the high concentrations of Mg 2+ that characterize brines from Incahuasi are probably related to the Cerro Oscuro Formation (consisting of continental red beds; [START_REF] Galli | Estudios sedimentológicos en los depósitos carboníferos y pérmicos del borde occidental de la Puna[END_REF], which outcrops in the ranges just east of this salar (note that there are no monogenic volcanoes -i.e., Mg-rich basaltic andesites -just next to Incahuasi; Fig. 2); ii) the ionic proportions of Ca 2+ + Mg 2+ > CO3 2-+ SO4 2-generally correspond to samples that were collected adjacent to outcrops that dominated by Quaternary sedimentary and volcanic rocks. In contrast, salars having brines that evolve through the sulfated pathway are adjacent to basement sequences consisting mostly of intrusive and volcanic units. This strongly suggests an igneous source for the sulfur, although some SO4 2-was probably released from weathering of metasediments.

In the five western salars, the decreasing order of average salinity is AR ≥ PC ≥ RG ≥ RI ≥ AN (Fig. 8). The concentrations of Ca 2+ , Na + , K + , B , and Li + decrease according to [RG, RI, AR] ≥ [AN, PC]. In the seven eastern salars, the decreasing order of average salinity is PZ

≥ PG ≥ HM ≥ IN ≥ DI ≥ RA ≥ CE.
In addition to salinity, the magnitude of Na + , Li + , Cl -, and B concentrations can be summarized by [PZ, HM, PG] ≥ [RA, DI, CE, IN]. Although this pattern is valid, there are local variations and there is no recognizable concentration pattern for the remaining ions. According to the above, there would be no spatial (i.e., geographic) correlation with the ionic concentrations of these elements nor with Li + . Nonetheless, as mentioned previously, the concentrations of SO4 2-, and in a lesser extent also those of Mg 2+ , do follow spatial patterns.

Li concentration patterns

Variations in the grades of Li in the brines of these salars can be assessed by plotting Li + concentration against salinity and other conservative solutes such as Na + in the brines.

Figure 9 shows a general correlation for these parameters. This pattern is consistent with the incompatible geochemical behaviour of Li, i.e., it becomes enriched in the residual brine during salar evolution, a proxy of which is an increase in salinity. Another remarkable correlation can be observed between maximum Li + grades measured in brines and salar size (Fig. 10). This pattern is common to all of the salars studied in Argentina, including those from the Northern Puna [START_REF] López Steinmetz | Northern Puna plateau-scale survey of Li brine-type deposits in the Andes of NW Argentina[END_REF]. The best correlations can be observed among salars with surfaces smaller than about 400 km 2 . These show two positive trends: i) the Guayatayoc -RI -Salinas Grandes -HM trend, represented by a regression line with a linear correlation (R; Pearson correlation coefficient) of 0.80; and ii) the IN, RA, JA, DI, CE, RG, PG, PZ, OL, and CA salars, which follow a linear trend characterized by R = 0.84. This feature suggests a relationship between Li + grade and salar size, in addition to, of course, other major determining factors that have been pointed out by [START_REF] Munk | Lithium brines: A Global Perspective[END_REF], such as the existence of Li supplying sources, hydrological isolation, aridity, etc. A clear trend between Li + grade and salar size exists for the smaller salars, while those of AR, AN and PC do not show as good a correlation, most likely because their surfaces are much larger (up to three orders of magnitude), over diluting the effect. Accordingly, larger salars would be expected to contain higher Li + grade brines than smaller ones, which provides a useful criterion for the surveying of brine-type deposits.

The correspondence between the maximum Li + grades and salar sizes could be indicative of the importance of basin scale processes such as rock weathering, isolation and evaporation. In this sense, it is important to note that it has been recently shown that brines in Atacama formed in a near surface setting, through geochemical processes including lowtemperature weathering, evaporation, formation of transition zone brines (i.e., those with conductivity ranging between of 5 and 150 mS/cm), and ultimately halite crystallization to drive Li concentration (in the brine) to the highest levels [START_REF] Munk | Hydrogeochemical fluxes and processes contributing to the formation of lithium-enriched brines in a hyper-arid continental basin[END_REF]. Moreover, recent studies reported that Li content and isotopic composition in the Cenozoic volcanic rocks of the Central Andes can be explained by assimilation of the Paleozoic basement, and that volcanic and Paleozoic basement rocks are considered to be the primary source of Li in the Andean plateau [START_REF] García | Characterization of lithium cycling in the Salar de Olaroz, Central Andes, using a geochemical and isotopic approach[END_REF][START_REF] Meixner | Lithium concentrations and isotope signatures of Palaeozoic basement rocks and Cenozoic volcanic rocks from the Central Andean arc and back-arc[END_REF]. These elements may help understanding the lack of spatial pattern in the regional Li distribution. The fact that Li is sourced from a lithologic sequence that is widespread across the entire Puna would rule out a geographic control. Rather, consistently increasing Li grades with salar size define a pattern that depends on basin scale processes, on geomorphology and, perhaps, also on the age of the basin (i.e., older basins would have more time than younger ones to stock the products of weathering and evaporation).

The Li-bearing Southern Puna salars in an Andean context

It is commonly assumed that brines enriched in Li also have high concentrations of K and B. In Li brine-type deposits, K is typically obtained during Li recovering processes via brine evaporation, and borate mines are widespread in Andean salars. Positive trends between Li + -K + and Li + -B have been observed in CE, IN, PZ, AN, and PC (Fig. 6). Similar patterns have been also reported in salars of the Northern Puna. Furthermore, positive correlations might also exist between Li + -Mg 2+ and K + -Mg 2+ in CE, AN, RI, RG, and the Northern Puna's salars of Olaroz, Cauchari, Salinas Grandes, and Jama. These features could suggest common sources and a similar geochemical behavior for these ions.

Boron isotopic studies reported in the literature for the Puna region suggest the existence of two distinctive sources of B, one associated with volcanic activity controlling borate precipitation in western Puna salars, and another one related to weathering of basement rocks, prevailing in the eastern Puna (Kasemann et al., 2004). Accordingly, given the better correlation between Li + and B in the eastern Puna salars, we suggest that some of their Li + and B may have been provided by weathering of the Ordovician basement. Furthermore, Peralta Arnold et al.

(2017) reported isotopic data (δ 18 O and δD) coupled with high lithium concentrations in mature thermal waters (between 4 to 80 mg L -1 ) and in waters recycling evaporites (up to 600 mg L -1 ), which revealed that rock weathering is likely a significant suppling process for these salars.

A comparison between the concentrations of TDS-normalized ions (Figs. 8 and 11) allows assessing the relative abundance of Li + and other ions with respect to salinity (i.e., brine evolution and concentration through evaporation). Our data reveal that, relatively to those of their respective neighboring salars, brines from HM, DI, AR and IN are proportionally richer in K + (Fig. 11D), those from AR and AN are richer in B (Fig. 11 F), and the CE, PG, and HM brines are the richest in Li + (Figs. 8 B, and 11). IN and, to a lesser extent, AR and DI, also contain the highest TDS-normalized Mg 2+ grades in the Southern Puna (Fig. 11). The Salar de Arizaro, the largest salar of the Argentine Puna, has a mean Li + :Mg 2+ of only 0.16, similar to that of Atacama in Chile (Fig. 12, Table 2). It is important to note that reference values of the mean Li + :Mg 2+ ratio in operating Li brine-type mines of the Andean plateau are highly variable: 0.16 in Atacama, 0.89 in Olaroz, and ~0.90 in Hombre Muerto [START_REF] Ericksen | Geology and resources of salars in the central Andes: U.S. Geologial Survey[END_REF][START_REF] Risacher | Quaternary geochemical evolution of the Salar of Uyuni and Coipasa[END_REF]Godfrey et al., 2013;[START_REF] López Steinmetz | Northern Puna plateau-scale survey of Li brine-type deposits in the Andes of NW Argentina[END_REF]; see Fig. 12 and Li + :Mg 2+ ratios summarized in Table 2). Therefore, in operative terms, there is no strict threshold values to be considered as an acceptable Li + :Mg 2+ ratio. Quite the reverse, Li grades define the major criteria to operate a brine-type project (Fig. 12). Additionally, and subsidiary to Li grades, the attractiveness of a Li brine-type mining project will also be reliant on the Li + :Mg 2+ ratios and salar size and shape, as larger and more near-circular salt pans are better able to support long-term extraction than smaller ones or those having elongated shapes (Houston et al., 2011). Moreover, the resource estimate of a salt pan considers the aquifer geometry and the area of the concession, as deposits having a higher storage (i.e., the scale of the recoverable reserve of brine) provide, in general, more economic interest than smaller ones to develop Li brine-type mining (Gruber et al., 2010;[START_REF] López Steinmetz | Water legislation in the context of lithium mining in Argentina: Resources Policy[END_REF].

The Southern Puna remains a very remote and difficult to access region, due to poor roads conditions. Because of this, the location of salars in the Southern Puna is a constraint for operations compared with those located in the northern part, where roads and distances are suitable. Nevertheless, comparing Li grades, mean Li + :Mg 2+ ratios, and salar sizes can provide a rapid evaluation of the relative mining potential of these salars with respect to Li extraction (Fig. 12). Applying these considerations to the salars that are not being operated, the most interesting targets in Southern Puna are the Salar de Arizaro (the largest salt pan of the Argentine Puna), Antofalla-Botijuelas (which has interesting Li:Mg ratios and a large surface, though its elongated shape could be a constraint), and Pastos Grandes, Pozuelos, and Rincón (which have encouraging Li grades, interesting sizes, and relatively easy access). These salars embody interesting perspectives in a regional context.

Conclusions

This study reports the results of a regional geochemical survey of Li brines, covering the Southern Puna of Argentina. Brines from the twelve major salars in this region are characterized by mean Li + concentrations varying by up to one order of magnitude, from 57 to 570 mg L -1 , with mean Li:Mg ratios ranging from 0.01 to 1.24.

Brines are of the Na + / Cl --SO4 2-type and are relatively rich in K + and Mg 2+ . Data presented in this contribution reveal likely correlations between the concentrations of Li + , K + , B, and Mg 2+ , suggesting common sources and similar geochemical behavior for these ions. No geographical pattern for the Li + concentrations has been observed. However, salar size could be related to maximum Li + grade of brines. Larger salars would then be expected to contain higher Li + grade brines than smaller ones, making salar size a useful criterion for surveying brine-type deposits.

The comparison between mean Li + grades, Li + :Mg 2+ ratios, and salar size provides an assessment of the Li potential offered by a number of salars in the Souther Puna: Arizaro because of it dimension, Pastos Grandes, Pozuelos, and Rincón due to Li grades and accessibility, and Antofalla-Botijuelas on account of its Li:Mg ratios. In addition to these salars, Río Grande and Salinas Grandes have a remarkable brine chemistry. Olaroz, Cauchari, and Hombre Muerto contain the highest Li + grades in brines of all of the Argentine Puna, and represent the most interesting perspectives of the Argentine plateau and in a regional context.

Argentinian Puna, showing the main salars in this region. Table 2. Comparative attributes of the Uyuni, Atacama, and the Argentine Puna's salars. The mean Li + a concentration and the Li:Mg a ratio in Uyuni corresponds to the mean values of 38 samples (from sample 100 to sample 133 and samples 146, 147 and 147A) reported by [START_REF] Ericksen | Geology and resources of salars in the central Andes: U.S. Geologial Survey[END_REF]. The mean Li + b concentration and Li:Mg b ratio corresponds to mean values of the 164 samples (from sample UA to sample RZ) reported by [START_REF] Risacher | Quaternary geochemical evolution of the Salar of Uyuni and Coipasa[END_REF]. The mean Li + c concentration and Li:Mg c ratio of Hombre Muerto correspond to the mean value of the 9 samples (samples 4, 5, 6, and 9 to 14) reported by Godfrey et al. (2013). The mean Li d concentration in Hombre Muerto corresponds to the mean Li + concentration of 100 m-deep brines reported by Garret (2004) and [START_REF] Kesler | Global lithium resources: relative importance of pegmatites, brine and other deposits[END_REF]. The mean Li + e concentration corresponds to the value reported by Ovejero [START_REF] Toledo | Evapofacies halítica en el Salar del Rincón, Departamento Los Andes[END_REF] for brines at 10-to 50-m depth in the Salar del Rincón. Data for Guayatayoc correspond to values reported by López Steinmetz (2017), data for Jama, Olaroz, Cauchari, and Salinas Grandes are from López [START_REF] López Steinmetz | Northern Puna plateau-scale survey of Li brine-type deposits in the Andes of NW Argentina[END_REF]. Remaining data is original from this contribution (Table 1).

  Figure B is a subset of Figure A. 1:Guayatayoc; 2: Salinas Grandes; 3: Olaroz; 4: Cauchari; 5: Jama; 6: Pastos Grandes; 7: Pozuelos; 8: Centenario; 9: Ratones; 10: Diablillos; 11: Hombre Muerto; 12: Incahuasi; 13: Antofalla-Botijuelas; 14: Río Grande; 15: Arizaro; 16: Pocitos-Quirón; 17: Rincón.

Fig. 2 .

 2 Fig. 2. (A) Schematic geological map of Southern Puna showing the location of eleven

Fig. 3 .

 3 Fig. 3. Schematic maps displaing the location of shallow brine sampling sites (colored crosses)

Fig. 4 .

 4 Fig. 4. Ternary diagrams displaying relative ionic concentrations (mg L -1 ) measured in brines

Fig. 5 .

 5 Fig. 5. Scatter diagrams plotting the concentrations of Na + versus Cl -(A), Ca 2+ versus CO3 2-

Fig. 6 .

 6 Fig. 6. Scatter diagrams showing the variation of the Li:K (A and B), the Li:B (C and D), and

Fig. 8 .

 8 Fig. 8. Diagram showing the salinity grades (A) and the TDS normalized concentrations of Li +

Acknowledgments

The authors wish to thank EG editor L. Meinert for his help throughout the reviewing process and two anonymous reviewers for their constructive comments which led to significant improvements of this manuscript. This study was founded by grant 2016-2017 and 2018-2019 of the SECTER -UNJu. Special thanks to Capt. Christian Meuric for large field logistical support. There are no conflicts of interest to declare.

Godfrey, L.V.; Chan, L.-H.; Alonso, R.N.; Lowenstein, T.K.; McDonough, W.F.; Houston, J.; Li, J.; Bobst, A.; Jordan, T.E., 2013, The role of climate in the accumulation of lithiumrich brine in the Central Andes: Applied Geochemistry, v. 38, p. 92-102. Grosse, P.;[START_REF] Montero López | Late Miocene ignimbrites at the southern Puna-northern Sierras Pampeanas border (∼27° S): stratigraphic correlation[END_REF]Petrinovic, I.A., 2017, Volcanes compuestos Chile and Bolivia, respectively (data from [START_REF] Ericksen | Geology and resources of salars in the central Andes: U.S. Geologial Survey[END_REF][START_REF] Risacher | Quaternary geochemical evolution of the Salar of Uyuni and Coipasa[END_REF].

Figure Captions

See Table 2 for data values. Note that regardless surfaces and Li:Mg ratios (A), only salars having the highest Li grades are currently operated (B).

Table Captions


José Kpeou

Didier BÉZIAT b Stefano Salvi

Guillaume Estrade

Gaétan Moloto-A- Kenguemba

Pierre Debat

Gold mineralization related to Proterozoic cover in the Congo Craton (Central African Republic): a consequence of Panafrican events

Keywords: Quartz veins, Late Panafrican, orogenic gold mineralization, fluid inclusions, Moboma, Central African Republic

Despite its high endowments, the gold mining potential of Central African greenstone belts is seemingly underrated when compared to equivalent belts in neighbor West Africa. This is probably because, over the past half century, only minor exploration efforts were ever made in this region. In the southwest of the Central African Republic, near the locality of Moboma, gold-bearing quartz veins are hosted in greenschist facies Paleoproterozoic formations that are intruded by numerous dolerite dykes. These rocks occur in a strongly deformed terrane that marks the front of the Panafrican Oubanguides nappe, developed during an E-W regional shortening. Presence of multiple banding indicates repeated reactivation of the quartz veins and circulation of H 2 O-CO 2 -NaCl fluids, similar to those characterizing typical orogenic gold-bearing settings. Fluid inclusion petrography and microthermometry permitted to distinguish two different fluids: one, aqueous-carbonic, circulated at relatively high temperature (Th = 250-270 °C) and was responsible for the main stage of Au deposition; a second fluid of low-salinity trapped in microcracks and in a late quartz generation, interpreted as meteoric, precipitated silver-poor native gold. At a later stage, supergene alteration caused the formation of discrete gold nuggets in the upper levels of the mineralization. The competent nature of the dolerite dykes and quartzite intersected by these quartz veins contributed to focus rock fracturing, localizing fluid circulation and the mineralization. The alteration assemblage developed in the veins is equivalent to that found in the dolerite dykes, which was dated at 571 Ma, thus pointing to a Panafrican age for the mineralization at Moboma.

Introduction and Exploration History

Compared to the neighboring West African Craton, the gold mining potential of Central African greenstone belts is highly underestimated, due to the much lower exploration efforts that have been undertaken in this part of the continent in the past. Nevertheless, gold remains the second mineral resource of the Central African Republic (CAR) and, with a total endowment that exceeded 12 t between 1929 and 1963, it has occupied a privileged place in the history of CAR subsoil development. This notwithstanding, gold production has been declining from 1952 in this country and, by 1980, it had reduced to practically nil [START_REF] Biandja | Approche métallogénique du "Greenstone Belt" de Bogoin (RCA)[END_REF]World Bank, 2008), with only a timid climb to this day, official gold production from the Central African Republic being estimated at 60 kg in 2016 (https://www.ceicdata.com/ en/indicator/central-african-republic/gold-production).

In a synthesis of the geology and metallogeny of the various countries constituting Central Africa, Milesi et al. (2006) made an inventory of their mineral resources, distinguishing between mineralizations in the Archean craton, in Proterozoic and Panafrican belts, and in Phanerozoic basins. In the CAR, they only report alluvial and eluvial placer mineralizations in sediments of estimated Paleozoic to Mesozoic ages, with the district of Roandji providing gold, while the Lobaye and Mambéré basins being source of diamond. However, the vast majority of primary gold deposits known to date are associated with Archean to Paleoproterozoic formations [START_REF] Mestraud | Géologie et ressources minérales de la République centrafricaine : état des connaissances à la fin 1963[END_REF]. They occur either in the form of stockwork of quartz-sulfide veins (pyrite and/or arsenopyrite and gold) intersecting granites (Bouar-Baboua and Irdéré deposits) or their micaschist host (Ouham deposits); ferruginous quartzite (Bogoin deposit); and gold-bearing pyritic horizons scattered in shale. All of these different mineralization styles can be found in the same site, as is the case at the Roandji deposit.

However, there exists yet another population of gold-bearing quartz veins, those of Moboma, which are located in low-grade metamorphic formations, called "upper group" by [START_REF] Mestraud | Géologie et ressources minérales de la République centrafricaine : état des connaissances à la fin 1963[END_REF]. These veins occur in an artisanal gold mining area located in the Lobaye region (Fig. 1), about 125 km SSW of the capital of the CAR, Bangui. Discovered in 1938, the deposit was mined by the Société Minière de la Moboma (SMM) until 1950, producing a total of 535 kg of gold [START_REF] Pianet | Rapport d'e pertise sur le gisement de Moboma[END_REF]. Since then, the deposit is exploited only by the villagers and no new geological data have been published on this region after the departure of the SMM. Because the only work done in the area has been limited to the surface, only exceptionally reaching depths past the hematite-goethite oxidation zone [START_REF] Barbeau | Rapport préliminaire sur les gîtes de la Moboma et de la Sosso[END_REF][START_REF] Delafosse | Etude préliminaire du gisement de la Moboma[END_REF], this paper provides the first comprehensive study of the primary mineralization. In it, we provide new structural and mineralogical evidence on the gold mineralization, discuss its timing of emplacement, and suggest a model for its formation.

Geological Setting

Regional geological framework

The study area (Fig. 1) is located in the southern part of the CAR, within the Central African Orogenic Belt, which covers the northern part of the Congo craton and comprises its metasedimentary Proterozoic cover (Affaton et al., 1991;[START_REF] Alvarez | Evidence for a Neoproterozoic carbonate ramp on the northern edge of the Central African craton: relations with late Proterozoic intracratonic troughs[END_REF][START_REF] Bessoles | Géologie de l'Afrique: la chaîne panafricaine: "zone mobile d'Afrique centrale (partie sud) et zone mobile soudanaise[END_REF][START_REF] Feybesse | The West Central African belt: a model of 2.5-2.0Ga accretion and two-phase orogenic evolution[END_REF][START_REF] Lavreau | A Fluid Inclusion and Stable Isotope Study at the Loulo Mining District, Mali, West Africa: Implications for Multifluid Sources in the Generation of Orogenic Gold Deposits[END_REF][START_REF] Moloto-A-Kenguemba | Evolu on géotectonique paléoprotérozo que à néoprotérozo que de la couverture du craton archéen du Congo au con ns du Congo, du Cameroun et de Centrafrique[END_REF][START_REF] Ngako | Les déformations continentales panafricaines en Afrique Centrale-Résultat d'un poinçonnement de type himalayen[END_REF][START_REF] Toteu | Geodynamic evolution of the Pan-African belt in central Africa with special reference to Cameroon[END_REF] and the Panafrican metamorphic Oubanguides nappe. The latter, which stretches for nearly 1000 km across central Africa, was thrusted southwards during the Panafrican orogeny, around 620 Ma [START_REF] Nédélec | Evolution structurale et métamorphique des schistes de Mbalmayo (Cameroun). Implications pour la structure de la zone mobile panafricaine d'Afrique Centrale, au contact du craton du Congo[END_REF][START_REF] Nzenti | Origin and evolution of the late precambrian high-grade Yaounde Gneisses (Cameroon)[END_REF][START_REF] Ouabego Kourtene | Contribution à l'étude de la chaine panafricaine des Oubanguides en République Centrafricaine[END_REF][START_REF] Pin | U-Pb zircon evidence for a pan-african granulite facies metamorphism in the Central African Republic. a new interpretation of the highgrade series of the northern border of the congo craton[END_REF][START_REF] Toteu | U-Pb and Sm-Nd edvidence for Eburnian and Pan-African high-grade metamorphism in cratonic rocks of southern Cameroon[END_REF]. Two large basins of Cretaceous age (Carnot and Congo basins), known to be diamond bearing, overlie all Precambrian formations.

The cover formations, little or unaffected by metamorphism, consist of Proterozoic sediments, specifically, i) arkose formations including the Mbaïki-Bangui-Boali (MBB) series, which host of the mineralized zone and are analogous to the Pama Boda arkose series to the North and to the Nola, Sembé and Lower Dja series, in the southwestern extension of MBB [START_REF] Bessoles | Géologie de l'Afrique: la chaîne panafricaine: "zone mobile d'Afrique centrale (partie sud) et zone mobile soudanaise[END_REF]; ii) metasedimentary formations of the Bangui basin, comprising the Bimbo and Fatima series [START_REF] Poidevin | Les ceintures de roches vertes de la République Centrafricaine (Bandas, Boufoyo, Bogoin, Mbomou). Contribution à la connaissance du précambrien du nord du craton du Congo[END_REF]. Carbonate formations positioned above the black Bimbo sandstone have been dated at 575 Ma ( 87 Sr/ 86 Sr isotopic characteristics of the Bangui limestones, [START_REF] Poidevin | Stratigraphie isotopique du strontium et datation des formations carbonatées et glaciogéniques néoprotérozoiques du Nord et de l' uest du craton du Congo[END_REF]. These Proterozoic formations are discordantly overlain by the Panafrican Oubanguides nappe and by the Mesozoic Carnot sandstone series. [START_REF] Poidevin | Les ceintures de roches vertes de la République Centrafricaine (Bandas, Boufoyo, Bogoin, Mbomou). Contribution à la connaissance du précambrien du nord du craton du Congo[END_REF] suggests the existence of a prolongation of the Oubanguides nappe under the Carnot formation, overlapping a western Archean panel and the western part of the Proterozoic units (cross-section in Figure 1). [START_REF] Poidevin | Les ceintures de roches vertes de la République Centrafricaine (Bandas, Boufoyo, Bogoin, Mbomou). Contribution à la connaissance du précambrien du nord du craton du Congo[END_REF] and [START_REF] Cornacchia | Les séries précambriennes d'origine sédimentaire et volcano-sédimentaire de la République centrafricaine[END_REF] propose that the boundary between the western and eastern Archean blocks could correspond to a Paleoproterozoic suture zone, marking a collision chain overthrusted on the Archean domain in the center of the CAR.

Local geology

The Moboma deposit (latitude 3° 42' N and longitude 17° 51' E) is located 80 km southwest of CAR's capital Bangui (Fig. 2A). Three rock formations are present in the deposit: a quartzo-pelitic complex, dolerite dykes, and quartz veins (Fig. 2B). The quartzopelitic complex is overthrusted from the north-west by the units of the Panafrican Oubanguides nappe (distant about 50 km) and is overlain to the east by the Neoproterozoic series of the Bangui basin (whose border is approximately 20 km southeast of Moboma). The pelite-quartzite transition is gradual and follows the original sedimentary bedding. These metasedimentary formations are in continuity with the Nola series, which are considered by [START_REF] Lescuyer | Africa GIS and SIG Afrique network-Geological and metallogenic information system, tools for sustainable development[END_REF] Several dolerite dykes are emplaced within these metasedimentary formations. They dip vertically and follow the N-S-trending regional schistosity (Figs. 2B,3A). Despite the extremely poor outcrop conditions, it could be estimated that these dykes measure a few tens of meters in thickness and extend for about a kilometer along strike. Similar doleritic dykes have been studied by Vicat et al. (1997) and Moloto-A- [START_REF] Moloto-A-Kenguemba | A late Neoproterozoic paleomagnetic pole for the Congo craton: Tectonic setting, paleomagnetism and geochronology of the Nola dike swarm (Central African Republic)[END_REF] in the neighboring region of Nola (Fig. 1), and by [START_REF] Poidevin | Etude géologique des dolérites de la basse vallée de la Lobaye (République Centrafricaine)[END_REF] in the region of Lobaye. The petrographic characteristics described by these authors are identical to those of the Moboma dolerites, i.e., intergranular to sub-ophitic texture, characterized by a plagioclaseaugite-ilmenite association. Based on their chemical composition, the Nola dolerites are related to an olivine tholeiite series of continental tholeiite chemical affinity. They were assigned an age of 571 ± 6 Ma ( 40 Ar / 39 Ar on amphibole, Moloto-A- [START_REF] Moloto-A-Kenguemba | A late Neoproterozoic paleomagnetic pole for the Congo craton: Tectonic setting, paleomagnetism and geochronology of the Nola dike swarm (Central African Republic)[END_REF].

Numerous quartz veins outcrop in the Moboma deposit. They intersect all formations but are particularly abundant within the dolerites where they form a dense network (Fig 2B &C).

More details on these veins are provided below.

Field evidence indicates that the tectonic history of the Moboma region is dominated by two successive major deformation phases (D 1 and D 2 ), which are accompanied by minor deformation phases causing only small local disturbances. The D 1 phase is visible to the west of the Moboma prospect and is manifested by a schistosity marked by secondary sericite, chlorite and albite, aligned parallel to the stratification. This S 0 S 1 schistosity trends N-S and dips 40° W, and was observed by [START_REF] Mestraud | Géologie et ressources minérales de la République centrafricaine : état des connaissances à la fin 1963[END_REF] at Moboma and by Moloto-A- [START_REF] Moloto-A-Kenguemba | A late Neoproterozoic paleomagnetic pole for the Congo craton: Tectonic setting, paleomagnetism and geochronology of the Nola dike swarm (Central African Republic)[END_REF] in the Nola formations; the latter authors associate these structures with the formation of the Oubanguides nappe. Phase D 2 , characteristic of the Moboma prospect, is evidenced by a S 2 foliation, developed in tight N-S-to N20-trending vertical planes that are well shown in the metasediments but are more discrete in the dolerites, overall defining a zone of strong deformation. The S 2 foliation is underlined by a greenschist-facies assemblage consisting mostly of chlorite-sericite in metasediments (Fig. 3C), and of chlorite-epidote-actinolite-albite in dolerite. It is superimposed on the S 0 S 1 schistosity, in most cases obliterating it. Other minor deformation phases consist of local drag-folds or micro-shears (Fig. 3D), which do not affect the larger structures.

Material and Methods

Petrographic studies were carried out on about twenty representative outcrop samples from different rock types (quartz veins, dolerite and metasedimentary rocks) of the Moboma deposit. A number of samples were also recovered from a depth of about 10 meters from artisanal mining pits. These samples are generally less attained by supergene alteration. Mineralogy and textural relationships were investigated using optical microscopy and back-scattered electron (BSE) imaging using a JEOL JSM 6360LV scanning electron microscope (SEM) equipped with a silicon drift detector analysis system, at the Geosciences Environment Toulouse (GET) laboratory at the University of Toulouse. The instrument was also used to obtain energy dispersive X-ray phase maps. Element concentrations in sulphides and gold grains were determined using a Cameca-SX five electron probe microanalyses (EPMA) at the Centre Raimond Castaing (University Paul Sabatier -Toulouse III). Operating conditions were: an accelerating voltage of 25 kV and a beam current of 20 nA. Calibration standards used were chalcopyrite for S, Fe and Cu, and pure metals for Au, Ag, Te and Bi.

Emission lines used were (1) Kα for S, Fe, Cu; (2) Lα for Ag, Au and Te; and (3) Mα for Bi.

Primary and secondary fluid inclusions assemblages were identified in 30-μm-thick polished thin sections and doubly polished slices of 150 μm thickness, using the criteria of [START_REF] Roedder | Fluid inclusions[END_REF] and [START_REF] Goldstein | Systematics of fluid inclusions in diagenetic minerals[END_REF]. Microthermometric measurements were performed at the University of Toulouse, following the procedures outlined by [START_REF] Roedder | Fluid inclusions[END_REF] and [START_REF] Shepherd | A practical guide to Fluid Inclusion Studies[END_REF], using a Linkam THMGS 600 heating-freezing stage mounted on a BX-51 Olympus microscope. The stage was calibrated using synthetic pure H 2 O inclusions (0° and 374.1°C) supplied by Syn Flinc and with natural pure CO 2 inclusions (-56.6°C) from Camperio (Ticino, Switzerland). Measurements below 0°C are accurate to ±0.1°C, whereas at the highest temperature measured (~400°C), they are accurate to ±1°C. Cryogenic experiments were carried out before heating to reduce the risk of decrepitating the inclusions. Salinity (expressed as wt.% eq. NaCl), bulk composition, and density data were calculated using the software package Fluids of [START_REF] Bakker | Package FLUIDS 1. Computer programs for analysis of fluid inclusion data and for modelling bulk fluid properties[END_REF].

Host-Rock Petrography

Metasediments

The quartzo-pelitic complex is formed by alternating layers of decimetric to metric thickness, composed of very fine-grained green and red metapelites and metasandstones to quartzites, the latter being very weakly recrystallized. The quartz, sericite, chlorite, albite paragenesis widely observed in the metapelitic rocks indicates that metamorphism did not exceed the greenschist facies in our study area.

Dolerite

Dolerite outcropping in the Moboma area forms a dark, massive rock with a doleritic to sub-ophitic structure. In the vicinity of quartz veins, this rock is highly altered and contains only relict crystals of the primary magmatic paragenesis. Plagioclase has secondary albite composition and clinopyroxene is extensively replaced by actinolite; apatite and ilmenite are also present. In order of decreasing abundance, the alteration paragenesis consists of carbonate, chlorite, albite, epidote and quartz. Sulfides are also common (mainly pyrite) as well as iron oxides and -hydroxides after Fe-Ti oxides (Fig. 3B). The above relationships indicate an assemblage consisting of actinote, albite, epidote and chlorite, consistent with greenschist facies conditions, similarly to the metapelites. The absence of hornblende-type amphibole in the Moboma dolerite excludes amphibolite facies conditions as suggested by [START_REF] Moloto-A-Kenguemba | A late Neoproterozoic paleomagnetic pole for the Congo craton: Tectonic setting, paleomagnetism and geochronology of the Nola dike swarm (Central African Republic)[END_REF] for the neighboring Nola region.

Quartz Veins

The majority of the quartz veins display a N40 to N60 orientation and sub-vertical dips; they vary in thickness from a few to about 50 cm (Fig. 3E,4A) and extend for up to a hundred meters along strike. A subordinate NNW-SSE trending fracture system also exists but only rarely it is host to quartz veins. Contacts between all veins and the doleritic and metasedimentary rocks are sharp. All veins are composed mostly of quartz (95%) and contain gold mineralization. Gold forms free native grains occurring with quartz within the veins, in their selvages, and in the immediately surrounding rocks. The internal structure of the quartz veins is characterized by a strike-parallel banding (Fig. 4A andB) marked by variations in quartz crystal size. The middle part of the veins consists of a band of large crystals (euhedral to subhedral crystals of mm to cm size) while on the vein edges, in contact with the host, commonly occurs a band of varied thickness, made up of small crystals of only a few tens of microns in size.

Three textural types of quartz are recognized and occur in most veins: a dark quartz (Qd), a clear quartz (Qcl) and fragmented or crushed quartz (Qcr). The former two quartz types consist of large crystals that, together, make up the majority of the vein. The dark color in Qd is due to the presence of abundant fluid inclusions. This variety is found preferentially toward the exterior parts of veins, just before the fine-grained Qcr quartz that mark the vein edges (Fig. 4E). Clear quartz is preferentially located in the central part of veins (Fig. 4E) and displays three main habits: i) isolated subhedral crystals or several individuals grouped in bands that parallel the vein walls (Fig. 4C); ii) overgrowths on dark quartz, in crystallographic continuity (Fig 4E ); iii) miarolitic quartz with large crystals filling lenticular geodes of up to several tens of cm in size (Fig. 4B). Locally, this quartz also forms small veinlets that cut all other features within the veins (Fig. 4C &D). In addition, most veins contain millimetric to centimeter-thick, more or less continuous borders, formed of tiny dark and smaller clear quartz crystals (a few tens of microns) recrystallized along grain and sub-grain boundaries (what is named 'crushed quartz', Qcr, Fig. 4C, D &E). The contact between these fine-grained borders with bands made of larger Qd and Qcl crystals is generally irregular (Fig 4D &E), although sharp contacts have been observed, particularly with clear quartz (Fig 4C); these contacts may be marked by alignments of pyrite grains (Fig. 4D) or by iron oxy-hydroxides, in which case they appear as reddish lines (Fig. 3F).

The principal mineral phases occurring in the quartz veins are sulfides (pyrite, chalcopyrite), carbonates (ankerite), white mica, iron oxides and oxy-hydroxides (hematite, goethite) and, less commonly, galena, arsenopyrite, barite, sphalerite and gersdorffite.

These minerals are associated with the various generations of quartz described previously and with supergene alteration observed locally on outcrop at the surface.

Dark quartz (Qd) assemblage

Dark quartz (Qd) is generally associated with scattered white mica flakes and an assemblage of metallic minerals, such as pyrite, chalcopyrite, galena, sphalerite, gersdorffite, and native gold (Fig. 5A &B). Pyrite, by far the most abundant sulphide, forms euhedral to subhedral crystals (grain size of 30 to 75 μm) locally arranged in alignments several crystals long (Fig. 4E), following the boundaries of the bands (Fig. 4D). This sulphide is often corroded by iron oxy-hydroxides, contains small crystals of chalcopyrite, sphalerite, and gersdorffite; most commonly galena overgrows pyrite (Fig. 5A). Gold occurs as visible grains of several tens of μm across, which are included within Qd crystals (Fig. 5B) or are interstitial between Qd grains. Iron oxy-hydroxides occur either as crack filling or at grain boundaries, locally overgrowing gold grains (Fig. 5B).

Border quartz (Qcr) assemblage

The fine-grained bands that mark the borders of most veins, resulting from local fragmentation (crushing) and recrystallization of quartz (Qcr) and other vein components, consist of the same mineral assemblage that accompanies dark quartz, with particularly abundant interstitial ankerite and white mica. Gold is also present as interstitial grains (Fig. 5C), and has a fineness identical to that of gold grains in dark quartz.

Clear quartz (Qcl) assemblage

Clear quartz (Qcl) occurs in a variety of habits. It can form euhedral crystals, overgrowth on dark quartz, and fill geodes. Similarly to the above quartz types, it may also contain visible gold grains (Fig. 5D). Gold in clear quartz, however, differs from the other two quartz types as its silver grades are distinctly lower (less than 2% Ag, Table 1).

Supergene alteration

In addition to the assemblages that accompany the different types of quartz described above, all veins near the surface show an important development of a late alteration consisting of iron oxides and oxy-hydroxides. These occur as: i) overgrowths on quartz near vein walls; ii) filling fractures in mineralized quartz veins (Fig. 6A) and iii) in zones of dissolution within the veins (Fig. 6B); iv) pseudomorphs after primary pyrite and ankerite (Fig. 6C &D); v) thin aureoles surrounding quartz crystals (Fig. 6E). Locally, barite accompanies these minerals (Fig. 6C). This assemblage clearly indicates a late supergene weathering and is commonly accompanied by alteration of mica into kaolinite plus hematite (Fig. 6F).

Gold occurrence and fineness

Textural and chemical data suggest the presence of two generations of gold at Moboma. In dark and crushed quartz, gold occurs as free-grains of several tens of μm across, which are included within Qd crystals (Fig. 5B) or are interstitial between Qcr grains (Fig. 5C).

In both cases, gold contains ~5% Ag (Table 1) and its fineness, calculated from the relationship 1,000Au / (Au + Ag) (wt %) is 945 ± 10, a value that is characteristic of orogenic deposits worldwide (fineness > 900; e.g., [START_REF] Morrison | Geological and geochemical controls on the silver content (fineness) of gold in gold-silver deposits[END_REF]Velásquez et al., 2014). Gold in clear quartz differs from gold found in the other two quartz types by the size of the grains which, in the former case, can reach several mm in diameter. The contents of silver are also different, as they are systematically lower (less than 2% Ag, Table 1) in clear-quartz gold. Such a low Ag content is commonly reported for gold issued from remobilization of primary, silver-rich gold.

Fluid Inclusions

Although fluid inclusions are abundant in all vein quartz at Moboma, the great majority are very small (<2 µm), and many of the larger inclusions show evidence of decrepitation. Nevertheless, pristine primary inclusions could be identified, either forming isolated groups or as assemblages defining growth zones in the subhedral quartz crystals.

We therefore carried out a reconnaissance fluid-inclusion study to determine the salient characteristics of the fluids.

Based on their appearance at room temperature and on microthermometric data, two types of fluid inclusions were defined: aqueous-carbonic inclusions (Type 1), commonly consisting of an aqueous liquid plus liquid and vapor carbonic phases, and aqueous inclusions (Type 2). Both types occur in dark quartz (Qd) and in quartz along vein borders (Qcr), whereas clear quartz (Qcl) only contains aqueous fluid inclusions (Type 2). Results of microthermometry are listed in Table 2. Salinities were calculated based on the temperature of clathrate dissociation and of final ice melting, respectively, for Type-1 and Type-2 fluid inclusions [START_REF] Bodnar | Interpretation of Data from Aqueous-Electrolyte Fluid Inclusions, in: Fluid Inclusions: Analysis and Interpretation[END_REF][START_REF] Diamond | Review of the systematics of CO2-H2O fluid inclusions. Lithos, Fluid Inclusions: Phase Relationships -Methods -Applications[END_REF].

Fluid inclusions in dark quartz (Qd)

In dark quartz, Type-1 fluid inclusions occur along growth zones, are of small size (<2 to 10 μm) and show somewhat variable aqueous liquid/carbonic ratios, although the volume proportions of the carbonic phase remain lower than ~50% (Fig. 7A). The temperatures of melting of CO 2 ice, Tm (CO 2 ), are comprised between -57.9 °C and -57.2 °C, which is lower than the melting temperature of pure carbon dioxide (-56.6 °C), suggesting the presence of small amounts of other gases, likely methane, dissolved in the carbonic phase.

Homogenization of the carbonic phase, Th(CO 2 ), occurred to the liquid phase, at temperatures ranging from 16.1 °C to 26.1 °C. Clathrate dissolved in a small range from 8 °C to 8.5 °C, corresponding to salinities from 3.0 to 3.9 wt.% eq. NaCl. Total homogenization temperatures show a unimodal distribution with a peak around 260-270 °C and a small dispersion toward lower temperatures (Table 2). Fluid inclusions of Type 2 in dark quartz (Fig. 7B) are arranged in parallel planes corresponding to microfractures, and their aspect and microthermometric properties are the same as those of Type-2 inclusions occurring in clear quartz (Qcl), described below.

Fluid inclusions in border quartz (Qcr)

In some of the larger grains of crushed quartz occurring in the border zones, we could observe the presence of Type-1 fluid inclusions. They generally measure only a few microns or less and contain an aqueous liquid plus liquid and vapor CO 2 phases at room temperature (Fig. 7C). Locally, two-phase aqueous fluids inclusions (Type 2) can be observed, generally rich in vapor, aligned along microfractures contained within a quartz grain. However, only the microthermometric characteristics of the three-phase primary fluid inclusions (Type 1) could be determined, the aqueous inclusions being too small to observe phase changes (<< 5 μm) (Table 2). In Type-1 fluid inclusions, the Tm(CO 2 ) ranged between -58.1 and -57.2 °C, comparable to values obtained from Qd quartz. The dissolution of clathrate took place between 5.2 and 7.9 °C, corresponding to salinities between 4.1 and 8.8 wt.% eq. NaCl, somewhat higher than those obtained for Type-1 inclusions in Qd (3.5 wt.% eq. NaCl, in average). The homogenization of the carbonic phase Th(CO 2 ) occurred to the liquid phase within a temperature range between 11.5 °C and 25.9 °C. The total Th distribution is unimodal, with a peak around 260-270 °C, which is identical to the values obtained for Qd quartz.

Fluid inclusions in clear quartz (Qcl)

Only Type-2 fluid inclusions occur in clear quartz. They have consistent L/V ratio, with the majority showing vapor bubbles occupying less than 50% of the inclusion volume, although partially decrepitated or necked down occurrences can be observed. They measure up to 10 µm (Fig. 7D) and are mostly found in clear quartz that overgrows on dark quartz. Tm (Ice) ranged from -2.4 to -0.7 °C, corresponding to salinities between 1.2 to 4 wt.% eq. NaCl.

Homogenization temperatures varied from 180 to 275 °C.

Discussion

Timing of gold mineralization

All known gold deposits occurring in Archean greenstone belts in the Central African Republic (e.g., Bogoin, Banda, Boufoyo) are regarded as pre-Panafrican. They consist of quartz veins developed within shear-zones [START_REF] Biandja | Approche métallogénique du "Greenstone Belt" de Bogoin (RCA)[END_REF], much as are most orogenic deposits described in the Paleoproterozoic of West Africa [START_REF] Goldfarb | West Africa: The World's Premier Paleoproterozoic Gold Province[END_REF]. Although there are no specific studies done on the dolerites at Moboma, they are in clear continuity with the dolerite dyke swarm occurring in the neighboring Nola region (Fig. 1), are emplaced in analogous units (the MBB and the Nola series) and are petrographically identical. It is thus very likely that the Moboma dolerite belong to the Nola dyke swarm, which is known to also extend to the Lower Dja and Sembé areas, respectively in nearby Cameroon and Congo (e.g., Moloto-A-Kenguemba et al., 2008;[START_REF] Van Den Hende | Notice explicative de la carte géologique de reconnaissance du Cameroun au l/500 000[END_REF]Vicat et al., 1997). Thus, they could be considered as the northeastern extension of the "doleritic complex" that intrudes the northern edge of the Congo craton (Vicat et al., 1997).

The Nola dolerites have been dated at 571 ± 6 Ma, although this age is considered by the authors as representing an hydrothermal overprint that took place during retrograde metamorphism to greenschist facies (Moloto-A- [START_REF] Moloto-A-Kenguemba | A late Neoproterozoic paleomagnetic pole for the Congo craton: Tectonic setting, paleomagnetism and geochronology of the Nola dike swarm (Central African Republic)[END_REF]. At Moboma, the mineralized quartz veins crosscut the dolerite dykes and are affected by the same metamorphic assemblage, in addition to the ankerite-pyrite alteration related to the emplacement of the quartz veins. It is thus likely that the quartz veins were emplaced either contemporaneously, or slightly later, then the metamorphism that affected these dykes, suggesting that gold mineralization at Moboma may also be related to the same Panafrican event that was dated at Nola at 571 Ma.

Structural control

The principal structural elements in the Paleoproterozoic formations of the Moboma region are the N20-trending 40°-NW-dipping schistosity (S 0 S 1 ) visible outside of the prospect, the N-S vertical foliation (S 2 ) and the N40-to N60-trending vertical quartz veins characteristic of the prospect. A sequence of events that is coherent with these structures would commence with the emplacement of the Oubanguides nappe, characterized by the S 0 S 1 schistosity, recognized about 50 km NW of Moboma. This took place during the Panafrican orogeny (e.g., [START_REF] Rolin | Présence d'un chevauchement ductile majeur d'âge panafricain dans la partie centrale de la République Centrafricaine: résultats préliminaires[END_REF], as indicated by the 620 Ma U-Pb age on zircon from a metamorphosed metabasite near Yaoundé [START_REF] Toteu | U-Pb and Sm-Nd edvidence for Eburnian and Pan-African high-grade metamorphism in cratonic rocks of southern Cameroon[END_REF]. Followed the development of the S 2 foliation, during high-strain deformation that caused the NS-trending, vertical, transcurrent dextral shear zone under greenschist facies conditions. This led to the opening of tension gaps that were later filled by quartz and the mineralization. This mode of formation evokes that described for the Ashanti area [START_REF] Allibone | Structural Controls on Gold Mineralization at the Ashanti Deposit, Obuasi, Ghana[END_REF], with its thick (hundreds of meters) zone of strong deformation. The geological formations present at Moboma are heterogeneous (e.g., metasedimentary series enclosing dolerite dykes) and responded differently to deformation. For instance, the schistosity developed a tight, well developed foliation in the metasediments while forms widely spaced planes in the dolerites. It is thus likely that the emplacement of the quartz veins was controlled to a great extent by the difference in competence among the host rocks, thus favoring the more competent dolerite and quartzite (Fig. 8A). The preferential localization of gold-bearing quartz veins in the dolerites had already been emphasized in the older literature (e.g., [START_REF] Barbeau | Rapport préliminaire sur les gîtes de la Moboma et de la Sosso[END_REF][START_REF] Delafosse | Etude préliminaire du gisement de la Moboma[END_REF][START_REF] Junner | Rapport sur la Socièté Minière de la Moboma[END_REF] and more recently by [START_REF] Moloto-A-Kenguemba | Evolu on géotectonique paléoprotérozo que à néoprotérozo que de la couverture du craton archéen du Congo au con ns du Congo, du Cameroun et de Centrafrique[END_REF]. This metabasite-quartz vein association is reminiscent of those described in several gold deposits, such as those of Kalgoorlie in Australia [START_REF] Phillips | Factors in the formation of the giant Kalgoorlie gold deposit[END_REF], Pampe in Ghana [START_REF] Salvi | The Pampe gold deposit (Ghana): Constraints on sulfide evolution during gold mineralization[END_REF] or Sortekap in Greenland [START_REF] Holwell | Orogenic gold mineralisation hosted by Archaean basement rocks at Sortekap, Kangerlussuaq area, East Greenland[END_REF].

Nature and origin of the ore fluids

The study of fluid inclusions, albeit preliminary, does reveal the existence of a H 2 O-CO 2 -NaCl fluid of low salinity (~4 wt.% eq. NaCl) (Type 1) related to the earliest quartz generation, which likely represents the primary mineralizing fluid in the deposit. Values of Tm (CO 2 ) slightly lower than those of the melting point of carbon dioxide (around -58 °C) indicate that this fluid could also contain traces of other constituents, such as CH 4 , N 2 or H 2 S. We do not have independent control on pressure to determine the actual temperatures of fluid trapping, however, given the relatively low intensity of metamorphism observed in the metasediments and dolerite, we estimate a correction in the order of about 50 °C (i.e., ca. 320 °C). This type of fluid is typical of the fluids associated with orogenic gold deposits worldwide [START_REF] Goldfarb | Orogenic gold: Common or evolving fluid and metal sources through time[END_REF], and more particularly the Proterozoic West African Craton gold deposits (e.g., Béziat et al., 2008;[START_REF] Goldfarb | West Africa: The World's Premier Paleoproterozoic Gold Province[END_REF][START_REF] Lavreau | A Fluid Inclusion and Stable Isotope Study at the Loulo Mining District, Mali, West Africa: Implications for Multifluid Sources in the Generation of Orogenic Gold Deposits[END_REF], and is generally considered to derive from greenschist facies metamorphic devolatilization reactions taking place at the regional scale (e.g., [START_REF] Gaboury | Parameters for the formation of orogenic gold deposits[END_REF]. Although metamorphism is of very low grade in the Moboma rocks hosting the deposit, conditions were higher in the underlying Archean metasediments, making them better candidates for producing the fluid and sourcing gold (e.g., [START_REF] Tomkins | On the source of orogenic gold[END_REF]. The CO 2 -free aqueous fluid inclusions (Type 2) found in some of the quartz generations (mostly Qcl) are also commonly observed in orogenic deposits and have been considered to result from post-trapping modifications of primary inclusions (e.g., Velásquez et al., 2018;Wille and Klemd, 2004). However, because their occurrence in the Moboma veins is restricted to the Qcl that overgrowth older quartz generations, we would rather suggest that they represent trapping of a different fluid that circulated in the rocks after metamorphism. Given the low homogenization temperatures and salinity, absence of CO 2 , and limited occurrence to the quartz overgrowths, we suggest that these fluid inclusions trapped a meteoric fluid.

Mineralization model

The textural relationships between the various quartz habits in the different bands observed in the veins suggest a multistage formation mechanism and that the vein systems formed by several filling episodes [START_REF] Robert | Archean gold-bearing quartz veins at the Sigma Mine, Abitibi greenstone belt, Quebec; Part II, Vein paragenesis and hydrothermal alteration[END_REF], related to the structural evolution of the region. At the early stages of opening (stage 1), vein filling consisted essentially of equidimensional quartz crystals growing in contact with the vein walls and perpendicular to them (Qd). They were accompanied by ankerite, muscovite, and a metal paragenesis consisting essentially of pyrite, as well as few scattered grains of native gold containing about 5% Ag. Subsequent reiteration of vein openings caused the veins to extend laterally and their thickness to increase (stage 2). This episodic reopening phenomenon, generally explained by an oscillation of stress within the rocks [START_REF] Ramsay | The crack-seal mechanism of rock deformation[END_REF], resulted in coupled brecciation and recrystallization of previously deposited quartz, producing a mesh of very small interstitial grains (Qcr). These phenomena are localized preferentially in the contact zone between vein and wall rock, as well as in narrow, elongated micro-shear zones, internal to the vein. The formation of ribbons and miarolitic cavities composed of clear quartz (Qcl) likely took place after the end of this process, and suggests reopening of these veins, preferentially in their central parts (Fig. 8B, stage 3). Undeformed, clear quartz also precipitated as overgrowths on the dark quartz crystals. The occurrence of Ag-poor gold associated with Qcl may indicate that primary gold was put in solution during this process and reprecipitated, probably after only little transport. Remobilization commonly produces gold with higher purity compared to that of primary gold (Table 1) (e.g., berth r et al., 1997b). The meteoric fluid represented by the Type-2 fluid inclusion population, characteristic of clear quartz, could represent the fluid responsible for the remobilization.

At a later stage, supergene alteration affected the mineralization, as evidenced by the occurrence of a late paragenesis of oxides (hematite), iron oxy-hydroxides (goethite), accessory barite and clays minerals, more abundant in surface samples than in those from the deepest pits. These secondary processed may have played an important role in the enrichments observed in some of the veins at Moboma and could be at the origin of the large nuggets discovered during the eluvial exploitation [START_REF] Mestraud | Géologie et ressources minérales de la République centrafricaine : état des connaissances à la fin 1963[END_REF].

Conclusions

Structural and mineralogical studies of quartz veins show that gold mineralization of Moboma shares a number of characteristics with orogenic Au deposits worldwide (mineral paragenesis with pyrite + ankerite, nature of H 2 O-CO 2 -NaCl fluids, formation of veins by multiple open-space filling). These auriferous quartz veins were probably set up in a regional transcurrent shear zone, posterior to formation of the regional schistosity, itself a

Figure 8

Schematic representations of A) the geometry in space of the quartz vein system and B) quartz vein evolution and mode of formation of the gold mineralization.

  and Moloto-A-Kenguemba et al. (2008) as being of Paleoproterozoic age.

  consequence of Panafrican thrusting. Field evidence, together with published data, suggest that mineralization was contemporaneous with the hydrothermal alteration of dolerite dykes -dated at 571 Ma -that are crosscut by the gold-bearing veins. Evidence for vein emplacement during late-Panafrican deformation makes Moboma stand out as an unusual deposit in Central African Republic. It also highlights the exploration potential of the edge of the Panafrican nappe. stratigraphic series are: D: Dja, HS: Haute Sangha, MBB: M'Baïki-Bangui-Boali, N: Nola, PB: Pama-Boda, S: Sembé.

Figure 2

 2 Figure 2

Figure 3

 3 Figure 3

Figure 4 A

 4 Figure 4

Figure 5

 5 Figure 5

Figure 6 A

 6 Figure 6

Figure 7

 7 Figure 7

Acknowledgments

Financial support for this study was provided by the French CNRS, the University of Toulouse and the Campus France agency. We wish to thank the BRGM's documentation service for access to its database and a large number of documents relating to the former work done in the Central African Republic. We thank German Velásquez and Youssef Driouch for their critical review of the manuscript and Pierre Chevalier for insightful comments.

Conflicts of Interest:

The authors declare no conflict of interest.

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Tectonothermal Evolution and Relationships with the Volta Basin, in: Dallmeyer, R.D., Lécorché, J.P. (Eds.), The West African Orogens and Circum-Atlantic Correlatives, Berlin,Heidelberg, https://doi.org/10.1007/978-3-642-84153-8_6 Velásquez, G., Béziat, D., Salvi, S., Siebenaller, L., Borisova, A.Y., Pokrovski, G.S., Parseval, P.D., 2014. Formation

Figure captions