J Gauriau
email: gauriau@usc.edu

M Harlaux

A.-S André-Mayer

A Eglinger

A Richard

A Fontaine

M G Lefebvre

D Béziat

J Villeneuve

D Lemarchand

Chemical and boron isotope composition of tourmaline from the Kiaka orogenic gold deposit (Burkina Faso, West African Craton) as a proxy for ore-forming processes

Keywords: Tourmaline, Boron isotopes, Orogenic gold deposit, West African Craton, Burkina Faso

The Kiaka orogenic gold deposit (Burkina Faso), located in the Paleoproterozoic domain of the West African Craton, is characterized by a two-stage gold mineralization hosted in volcanosedimentary metamorphic rocks that was formed during the Eoeburnean (2.20-2.13 Ga) and Eburnean (2.13-2.05 Ga) orogenic cycles. These two stages include an early disseminated lowgrade gold mineralization and a late vein-hosted high-grade gold mineralization. A paragenetic study indicates that the first gold stage was coeval with the deposition of hydrothermal tourmaline. The aim of this paper is twofold: (i) to determine the processes responsible for the economic disseminated gold mineralization, and (ii) to identify the source of the mineralizing fluids. To this end, we combine in situ analyses obtained from electron probe microanalysis and secondary ion mass spectrometry measurements. A dravite composition characterizes the hydrothermal tourmaline hosted in metamafic volcanic rocks and metagreywackes, and δ 11 B values range within the two intervals [-25.1‰; -22.0‰] and [-19.6‰; -15.1‰]. Our results suggest that tourmaline formed from a high-temperature (ca. 400°C), reduced, and low-salinity hydrothermal fluid that interacted with the local host rocks. From the modeling of tourmalinefluid boron isotope fractionation, we infer a metamorphic fluid origin derived from devolatilization of deeply-buried muscovite schists during the regional prograde to peak metamorphism prior ca. 2.13 Ga. This metamorphic fluid-rock interaction model may possibly extend to other orogenic gold deposits in the West African Craton.

INTRODUCTION

Tourmaline is the most important borosilicate mineral on Earth and represents a reliable monitor of the physico-chemical conditions of the environment in which it crystallized due to three main properties: (i) it is a stable mineral over a large range of pressure and temperature conditions, (ii) it has a complex crystal chemistry incorporating a large range of chemical elements, and (iii) it shows negligible intracrystalline diffusion, making it an excellent petrogenetic indicator for determining fluid sources and processes [START_REF] Dutrow | Tourmaline: a geologic DVD[END_REF][START_REF] Slack | Tourmaline as a Recorder of Ore-Forming Processes[END_REF][START_REF] Van Hinsberg | Tourmaline: an ideal indicator of its host environment[END_REF]. Tourmaline is a common mineral in orogenic gold deposits worldwide, often found disseminated in gold-bearing quartz veins or as metasomatic replacements along the host rock vein selvages. Deposition of tourmaline and gold is generally interpreted to be synchronous [START_REF] King | Strontium isotope compositions of tourmaline from lode gold deposits of the Archean Abitibi Greenstone belt (Ontario-Quebec, Canada): Implications for source reservoirs[END_REF][START_REF] Mueller | Strontium isotope systematics of hydrothermal minerals from epigenetic Archean gold deposits in the Yilgarn Block, Western Australia[END_REF][START_REF] Kotzer | An empirical oxygen-and hydrogen-isotope geothermometer for quartz-tourmaline and tourmaline-water[END_REF][START_REF] Anglin | Sm-Nd dating of scheelite and tourmaline; implications for the genesis of Archean gold deposits, Val d'Or, Canada[END_REF][START_REF] Frei | Mono-sample Pb-Pb dating of pyrrhotite and tourmaline: Proterozoic vs. Archean intracratonic gold mineralization in Zimbabwe[END_REF][START_REF] Slack | Tourmaline associations with hydrothermal ore deposits[END_REF][START_REF] Rowins | A reinterpretation of the role of granitoids in the genesis of Neoproterozoic gold mineralization in the Telfer Dome, Western Australia[END_REF][START_REF] Jiang | Stable and radiogenic isotope studies of tourmaline: an overview[END_REF][START_REF] Béziat | The Guibaré and Fété Kolé gold-bearing tourmaline-quartz veins in the Birimian greenstone belt of Burkina Faso[END_REF], suggesting that tourmaline can be used as a proxy for tracing the oreforming processes.

Fluid and metal sources in orogenic golds deposits, including Au and B, are still debated, therefore a large diversity of genetic models have been proposed. This diversity may either reflect an intrinsic variability of orogenic gold deposits or may show divergence between interpretations. The current paradigm for the formation of orogenic gold deposits involves a metamorphic fluid derived from devolatilization of thick metasedimentary sequences during prograde to retrograde metamorphic evolution of greenstone belts (e.g. [START_REF] Wyman | Alkaline magmatism, major structures, and gold deposits: implications for greenstone belt gold metallogeny[END_REF][START_REF] Goldfarb | Distribution, character and genesis of gold deposits in metamorphic terranes[END_REF]Phillips and Powell 2010;[START_REF] Goldfarb | Orogenic gold: Common or evolving fluid and metal sources through time[END_REF][START_REF] Fusswinkel | Fluid evolution of the Neoarchean Pampalo orogenic gold deposit (E Finland): Constraints from LA-ICPMS fluid inclusion microanalysis[END_REF][START_REF] Groves | A holistic model for the origin of orogenic gold deposits and its implications for exploration[END_REF]. Alternatively, a magmatic origin for fluids and metals related to reduced intrusions has been proposed [START_REF] Burrows | Generation of magmatic H2O-CO2 fluid enriched in Mo, Au and W within an Archean sodic granodiorite stock, Mink Lake, North Western Ontario[END_REF][START_REF] Wang | An overview of the relationship between granitoid intrusions and gold mineralization in the Archaean Murchison Province, Western Australia[END_REF][START_REF] Tomkins | On the source of orogenic gold[END_REF][START_REF] Xue | No mass-independent sulfur isotope fractionation in auriferous fluids supports a magmatic origin for Archean gold deposits[END_REF], as well as mixed magmatic-metamorphic fluid origins [START_REF] Rogers | Two stages of gold mineralization at Hutti mine[END_REF][START_REF] Molnár | Boron, sulphur and copper isotope systematics in the orogenic gold deposits of the Archaean Hattu schist belt, eastern Finland[END_REF]. The source of boron in orogenic gold environments may be derived from (i) metamorphic fluids produced during devolatilization reactions of metasedimentary sequences that occur above 650°C [START_REF] Spear | P-T paths from anatectic pelites[END_REF][START_REF] Van Hinsberg | Tourmaline: an ideal indicator of its host environment[END_REF], and/or (ii) magmatic fluids released from boron-rich reduced magmas [START_REF] Trumbull | Boron isotopes in the continental crust: granites, pegmatites, felsic volcanic rocks, and related ore deposits[END_REF]. In previous studies, tourmaline has been used as a proxy for (i) to identify trace elements that discriminate tourmaline from gold-bearing veins from orogenic gold deposits [START_REF] Sciuba | Chemical composition of tourmaline in orogenic gold deposits[END_REF]), (ii) tracing the origin of hydrothermal fluids and metals (distal vs.

proximal; metamorphic vs. magmatic) and for (iii) determining the ore-forming processes yielding to deposition of gold, such as in the Yilgarn craton [START_REF] Jiang | Chemical and boron isotopic compositions of tourmaline from the Archean Big Bell and Mount Gibson gold deposits, Murchison Province, Yilgarn Craton, Western Australia[END_REF], the Archean Abitibi belt [START_REF] King | Strontium isotope compositions of tourmaline from lode gold deposits of the Archean Abitibi Greenstone belt (Ontario-Quebec, Canada): Implications for source reservoirs[END_REF][START_REF] Beaudoin | The boron isotope composition of tourmaline from the Val-d'Or orogenic gold deposits[END_REF], the Archean Barberton greenstone belt [START_REF] Byerly | Tourmaline mineralization in the Barberton greenstone belt, South Africa: early Archean metasomatism by evaporite-derived boron[END_REF], the Archean Hattu schist belt [START_REF] Molnár | Boron, sulphur and copper isotope systematics in the orogenic gold deposits of the Archaean Hattu schist belt, eastern Finland[END_REF], the Hutti-Maski Greenstone Belt [START_REF] Hazarika | Diverse Tourmaline Compositions from Orogenic Gold Deposits in the Hutti-Maski Greenstone Belt, India: Implications for Sources of Ore-Forming Fluids[END_REF], and the Paleoproterozoic West African Craton [START_REF] Labou | Petrological and geochemical study of Birimian ultramafic rocks within the West African Craton: Insights from Mako (Senegal) and Loraboué (Burkina Faso) lherzolite/harzburgite/wehrlite associations[END_REF]; this study).

The Kiaka orogenic gold deposit (Burkina Faso) is hosted in the Manga Fada N'Gourma Paleoproterozoic greenstone belt, located on the southeast domain of the West African Craton.

The regional host rocks consist of metavolcanic and metasedimentary units composed of metapelites, greywackes, and schists. A dioritic pluton dated at 2140 ± 7 Ma [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]) intruded the schists contemporaneously with a regional shearing event. The Kiaka deposit contains a polyphase gold mineralization formed between ca. 2150 Ma and 2080 Ma that is interpreted to be coeval with the deposition of hydrothermal tourmaline [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF][START_REF] Mignot | Re-Os Geochronological Evidence for Multiple Paleoproterozoic Gold Events at the Scale of the West African Craton[END_REF]. This paper provides new data on the chemical and boron isotopic composition of hydrothermal tourmaline, as well as whole-rock boron isotope composition of the host rocks. The objective is to trace the ore-forming processes and the origin of mineralizing fluids in the Kiaka gold deposit, focusing specifically on hydrothermal tourmaline associated to the ca. 2150 Ma disseminated gold mineralization. The origin of the mineralizing fluids is discussed relative to the two potential sources: the dioritic intrusion (magmatic origin) and the metasedimentary rocks (metamorphic origin).

GEOLOGICAL CONTEXT The Paleoproterozoic Baoulé-Mossi domain

The Paleoproterozoic southern part of the West African Craton (WAC) is composed of lowgrade volcanic and volcano-sedimentary Birimian rocks (ca. 2.20-2.14 Ga) and siliciclastic sediments (ca. 2.13-2.09 Ga) that were intruded by voluminous granites [START_REF] Grenholm | Paleoproterozoic volcano-sedimentary series in the ca. 2.27-1.96 Ga Birimian Orogen of the southeastern West African Craton[END_REF].

The oldest Birimian sequence consists of tholeiitic basalt, gabbro, pyroxenite, and minor calcalkaline ultramafic-mafic to differentiated suites [START_REF] Baratoux | Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny (2.2-2.0 Ga), western Burkina Faso[END_REF]. The geodynamic context of formation of the Birimian rocks remains unclear and several interpretations have been proposed, including a plume-related oceanic plateau [START_REF] Abouchami | A major 2.1 Ga event of mafic magmatism in west Africa: An Early stage of crustal accretion[END_REF][START_REF] Boher | Crustal growth in West Africa at 2.1 Ga[END_REF]Augustin and Gaboury 2017) and an oceanic context evolving from a MORB-like domain to an increasingly mature volcanic arc domain [START_REF] Pouclet | Geodynamic setting of the Birimian volcanism in central Ivory Coast (western Africa) and its place in the Palaeoproterozoic evolution of the Man Shield[END_REF][START_REF] Baratoux | Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny (2.2-2.0 Ga), western Burkina Faso[END_REF][START_REF] Labou | Petrological and geochemical study of Birimian ultramafic rocks within the West African Craton: Insights from Mako (Senegal) and Loraboué (Burkina Faso) lherzolite/harzburgite/wehrlite associations[END_REF]. This period characterizes the Eoeburnean cycle, defined by an early magmatic accretion stage followed by an arc construction stage in a convergent setting [START_REF] Baratoux | Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny (2.2-2.0 Ga), western Burkina Faso[END_REF][START_REF] Mcfarlane | Episodic collisional orogenesis and lower crust exhumation during the Palaeoproterozoic Eburnean Orogeny: Evidence from the Sefwi Greenstone Belt, West African Craton[END_REF]. The Eburnean orogenic cycle started with the filling of major sedimentary basins by flysch sequences derived from the erosion of the Birimian crust and their inversion between ca. 2.13 Ga and 2.10 Ga [START_REF] Davis | Craton-scale lithostratigraphic correlation as an insight for the geodynamic evolution of the southern West African Craton[END_REF][START_REF] Grenholm | Paleoproterozoic volcano-sedimentary series in the ca. 2.27-1.96 Ga Birimian Orogen of the southeastern West African Craton[END_REF]. Different depositional settings have been proposed such as syn-orogenic foreland basin sedimentation [START_REF] Feybesse | The paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF][START_REF] Block | Paleoproterozoic juvenile crust formation and stabilisation in the south-eastern West African Craton (Ghana); New insights from U-Pb-Hf zircon data and geochemistry[END_REF] or back-arc and fore-arc basin sedimentation controlled by extensional detachments [START_REF] Perrouty | Revised Eburnean geodynamic evolution of the gold-rich southern Ashanti Belt, Ghana, with new field and geophysical evidence of pre-Tarkwaian deformations[END_REF][START_REF] Grenholm | Paleoproterozoic volcano-sedimentary series in the ca. 2.27-1.96 Ga Birimian Orogen of the southeastern West African Craton[END_REF]. The paroxysm of the Eburnean cycle is characterized by tectonic accretion and crustal reworking between ca. 2.09 and 2.05 Ga (e.g. Masurel et al. submitted). Large transpressional shear zones cross-cut the Baoulé-Mossi domain contemporaneously with the emplacement of high-K calc-alkaline to shoshonitic granitic plutons [START_REF] Feybesse | The Archaean/Proterozoic contact zone in West Africa: a mountain belt of décollement thrusting and folding on a continental margin related to 2.1 Ga convergence of Archaean cratons?[END_REF][START_REF] Egal | Late Eburnean granitization and tectonics along the western and northwestern margin of the Archean Kénéma-Man domain (Guinea, West African Craton)[END_REF][START_REF] Parra-Avila | The Eburnean magmatic evolution across the Baoulé-Mossi domain: Geodynamic implications for the West African Craton[END_REF] in syn-to post-collisional settings [START_REF] Eglinger | New insights into the crustal growth of the Paleoproterozoic margin of the Archean Kéména-Man domain, West African craton (Guinea): Implications for gold mineral system[END_REF][START_REF] Masurel | Sadiola Hill: A World-Class Carbonate-Hosted Gold Deposit in Mali, West Africa[END_REF][START_REF] Thébaud | The Paleoproterozoic (Rhyacian) gold deposits of West Africa[END_REF].

Paleoproterozoic gold metallogeny of the Baoulé-Mossi domain

West Africa is currently the largest Paleoproterozoic gold-producing region in the world with an annual gold production of ca. 250 t [START_REF] Béziat | Gold metallogeny in the birimian craton of Burkina Faso (West Africa)[END_REF][START_REF] Markwitz | Compilation of West African mineral deposits: spatial distribution and mineral endowment[END_REF][START_REF] Robertson | West African goldfields[END_REF][START_REF] Goldfarb | West Africa: The World's Premier Paleoproterozoic Gold Province[END_REF]. The majority of gold deposits are closely located to major first-order tectonic structures such as the Markoye shear zone (MSZ, Fig. 1A) in Burkina Faso [START_REF] Tshibubudze | Structural setting of gold deposits in the Oudalan-Gorouol volcano-sedimentary belt east of the Markoye Shear Zone, West African Craton[END_REF], the Ashanti shear zone (AS, Fig. 1A) in Ghana [START_REF] Perrouty | Revised Eburnean geodynamic evolution of the gold-rich southern Ashanti Belt, Ghana, with new field and geophysical evidence of pre-Tarkwaian deformations[END_REF]) and the Senegal-Mali shear zone (Ledru et al. 1991, Lawrence et al. 2013) that affected greenstone belts. Gold deposits in the southern WAC formed during the long-lived period of a magmatic accretion stage, defined as the Eoeburnean cycle, and broadly during the reworking and tectonic accretion stages of the Eburnean orogenic cycle (Masurel et al. submitted). Few absolute, and some relative ages constrain gold mineralization between ca. 2.18 and 2.06 Ga, and a few are linked to polyphase mineralization processes (such as the Obuasi Au deposit; [START_REF] Fougerouse | Evidence for two stages of mineralization in West Africa's largest gold deposit: Obuasi, Ghana[END_REF][START_REF] Mignot | Re-Os Geochronological Evidence for Multiple Paleoproterozoic Gold Events at the Scale of the West African Craton[END_REF]Augustin et al. 2017). The oldest orogenic gold deposits are mostly known in the eastern part of the Baoulé-Mossi domain, formed during the Eoeburnean deformation between ca. 2.14 and 2.13 Ga (Lawrence et al. 2017;[START_REF] Mignot | Re-Os Geochronological Evidence for Multiple Paleoproterozoic Gold Events at the Scale of the West African Craton[END_REF]. They include the ~5.8 Moz Kiaka gold deposit, hosted by volcano-sedimentary rocks which were deformed and metamorphosed under amphibolite facies conditions [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]. Early Eburnean gold deposits (ca. 2.11-2.09 Ga) correspond to different types of gold deposits including orogenic (Obuasi deposit; [START_REF] Fougerouse | Evidence for two stages of mineralization in West Africa's largest gold deposit: Obuasi, Ghana[END_REF], conglomerate-hosted paleo-placer (Tarkwa deposit; [START_REF] Perrouty | Revised Eburnean geodynamic evolution of the gold-rich southern Ashanti Belt, Ghana, with new field and geophysical evidence of pre-Tarkwaian deformations[END_REF], granitoid-hosted (Chirano deposit; Allibone et al. 2004), intrusion-related (Morila deposit;McFarlane et al. 2011), and skarn (Tongon deposit;Lawrence et al. 2017) gold deposits. Some gold deposits formed during the late Eburnean tectono-thermal activity between ca. 2.09 and 2.05 Ga following docking of the Archean and Paleoproterozoic domains within the southern WAC, synchronous with widespread high-K and shoshonitic granites [START_REF] Eglinger | New insights into the crustal growth of the Paleoproterozoic margin of the Archean Kéména-Man domain, West African craton (Guinea): Implications for gold mineral system[END_REF][START_REF] Masurel | Sadiola Hill: A World-Class Carbonate-Hosted Gold Deposit in Mali, West Africa[END_REF].

Economic gold mineralization associated with this transcurrent tectonics and crust-derived magmas are widespread in the western portion of the Baoulé-Mossi domain [START_REF] Thébaud | The Paleoproterozoic (Rhyacian) gold deposits of West Africa[END_REF]Masurel et al. submitted).

Local geology and structural framework of the Kiaka deposit

The Kiaka orogenic gold deposit is located within an elongated volcano-sedimentary sequence known as the Manga Fada N'Gourma greenstone belt, which extends along 250 km from northeast to southwest (Hottin and Ouégrago 1976;[START_REF] Naba | Structure and emplacement model for late-orogenic Paleoproterozoic granitoids: the Tenkodogo-Yamba elongate pluton (Eastern Burkina Faso)[END_REF][START_REF] Ganne | Modern-style plate subduction preserved in the Palaeoproterozoic West African craton[END_REF]). The deposit is located at the intersection between two major structures, the Markoye Shear Zone (MSZ) and a D2-related NE-trending shear zone (Fig. 1A-B; [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]. The volcanosedimentary sequence hosting the Kiaka deposit forms a NE-striking corridor, strongly dipping to the northwest and commonly subparallel to the regional foliation (Fig. 1B). Host rocks consist of conglomerate, metagreywacke, and metapelite, which are intercalated with garnetorthopyroxene schists and mafic rocks [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]. Several magmatic rocks have been described in the Kiaka area, such as a biotite-amphibole diorite plug dated at 2140 ± 7 Ma [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]) and an unfoliated biotite granite, dated between 2120 and 2100 Ma, intruding the tonalite-trondhjemite-granodiorite (TTG) basement [START_REF] Castaing | Notice explicative de la carte géologique et minière du Burkina Faso à 1/1 000 000[END_REF][START_REF] Vegas | Structure and emplacement of granite plutons in the Paleoproterozoic crust of Eastern Burkina Faso: rheological implications[END_REF][START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]. Lithogeochemical data suggest similarities between the biotite-amphibole diorite and the Tenkodogo-Yamba batholith that is part of the TTG suite [START_REF] Naba | Structure and emplacement model for late-orogenic Paleoproterozoic granitoids: the Tenkodogo-Yamba elongate pluton (Eastern Burkina Faso)[END_REF]).

Deposit-scale study suggests the existence of at least three main deformation phases reported as D1Ki, D2Ki, and D3-4Ki [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]. The two first phases (D1Ki and D2Ki) occurred under ductile conditions (P=550-650°C, T=6-8 kbars; [START_REF] Ganne | Modern-style plate subduction preserved in the Palaeoproterozoic West African craton[END_REF][START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF], whereas the others (D3Ki-D4Ki) took place at the ductile-brittle transition (P<4 kbars, T=200-450°C; [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF][START_REF] Ganne | Modern-style plate subduction preserved in the Palaeoproterozoic West African craton[END_REF][START_REF] Block | Petrological and geochronological constraints on lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton[END_REF]. The D1Ki phase corresponds to a relic structural event marked by P1Ki folds, resulting from EW to WNW-ESE compression, which are preserved in low-strain domains between D2Ki shear zones [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]. The S2Ki regional foliation, parallel to the isoclinal P1Ki folding, records a biotite-sillimanitegarnet-amphibole assemblage, suggesting lower to middle amphibolite facies conditions (Fig. 3A). At the regional scale, the NE-trending D2-related shear zones and the NS-trending MSZ are the main structures controlling the emplacement of the gold mineralization at Kiaka (Fontaine et al. 2017). These structures record dextral-reverse D1 motion and sinistral-reverse D2 reactivation [START_REF] Tshibubudze | The Markoye Shear Zone in NE Burkina Faso[END_REF][START_REF] Tshibubudze | Structural setting of gold deposits in the Oudalan-Gorouol volcano-sedimentary belt east of the Markoye Shear Zone, West African Craton[END_REF]. Mineralized envelopes are spatially associated with the NE-trending shear zones and some of them extend along the axial plane of P2Ki isoclinal folds [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]. The sinistral-reverse D2 reactivation is coeval with the emplacement of the Kiaka diorite dated at 2140 ± 7 Ma. The D3-4Ki phases correspond to quartz-carbonate veins that formed during reactivation along D2-related shear zones and retrogression at greenschist facies conditions [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF], likely during or after the ductile-brittle transition.

Tourmaline paragenesis and space-time relationships with gold mineralization

A relative chronology between tectono-metamorphic and hydrothermal events has been proposed by [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF], based on crosscutting relationships, hydrothermal assemblages, and ore mineralogy, identifying two successive stages of gold mineralization and associated hydrothermal alteration in the Kiaka deposit (Fig. 2):

(i) The first main stage corresponds to the early sulfide-rich disseminated gold mineralization related to the D2Ki phase, coeval with emplacement of the biotite-amphibole diorite intrusion (Fig. 2). This mineralization is volumetrically the most important (>80% of the orebodies) and constitutes most of the gold resources with average grades of 1-3 g/t. The ore assemblage consists of 50µm-1mm large anhedral pyrrhotite (60-80%) that is commonly associated with sub-millimetric (200-300 µm) pyrite (10-20%), chalcopyrite (5-10%), arsenopyrite (5-10%), and rare löllingite disseminated in sericitized metagreywacke and partly in metamafic volcanic rocks, containing a biotite-titanite-clinozoisite assemblage. Gold probably occurs as nano-inclusions hosted in, or within, the crystal lattice of pyrite, arsenopyrite, and löllingite. This stage was dated at 2157 ± 24 Ma (Re-Os on pyrrhotite; Le [START_REF] Mignot | Re-Os Geochronological Evidence for Multiple Paleoproterozoic Gold Events at the Scale of the West African Craton[END_REF][START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]).

(ii) The second stage is related to formation of a high-grade gold mineralization at 50-60 g/t Au hosted in hydrothermal veins and breccias that formed during the D3Ki to D4Ki phases (Fig. 2). The ore assemblage is composed of pyrrhotite, chalcopyrite, arsenopyrite, electrum, native gold (2-20 µm), and tellurobismuthite, intergrown with clinozoisite, quartz, calcite, and chlorite.

According to [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF] and new petrographic observations, two generations of tourmaline are observed in the Kiaka deposit. A first generation of tourmaline (Tur1) is locally found as rounded detrital grains (<10 µm) in quartz-biotite metagreywacke, representing <5% in volume of the host rock. Tur1 is also found within garnet-biotite-chlorite-muscovite layers hosted in muscovite schists (Fig. 3A-B), which are affected by the S2Ki foliation and associated folds, suggesting an early formation prior, or during, the D1Ki stage. A second generation of tourmaline (Tur2) is found in the metagreywacke and metamafic volcanic rock (referred subsequently as metamafic rocks) located close to or within the mineralized zones. This tourmaline is found as large (typically 50-500 µm) zoned porphyroblasts containing sulfide and silicate inclusions. It is locally oriented along the S2Ki foliation. In the metagreywacke, Tur2 is intergrown with an assemblage of biotite, clinozoisite, quartz, K-feldspar and disseminated sulfides, including pyrrhotite, chalcopyrite, pyrite, and arsenopyrite (Fig. 3C-D). In the metamafic rock, Tur2 is intergrown with chlorite (Chl 1, pre-to syn-D1Ki), biotite, titanite, clinozoisite, and pyrrhotite, which are characteristics of the D2Ki hydrothermal assemblage.

Chlorite (Chl 1) inclusions oriented along an early foliation (syn-D1Ki) within the poikiloblastic Tur2 porphyroblasts (Fig. 5), as well as a chlorite (Chl 2)-biotite assemblage (Fig. 3F), related to the D3Ki stage and overgrown by Tur2, constraint the formation of Tur2 within syn-to late-D2Ki event. The assemblage of Chl 1 and biotite is interpreted as a relic from stage D1Ki that was overprinted by ore stage fluids during the D2Ki event in replacement zones. In contrast, the assemblage of titanite, clinozoisite, and pyrrhotite is considered to be earlier than D3Ki, because it is cut by chlorite (Chl 3)-calcite veins (Fig. 3E-F). Therefore, the second generation of tourmaline (Tur2) is interpreted to be of hydrothermal origin and to have formed during the D2Ki event, coevally with formation of the Kiaka disseminated gold mineralization. In this paper, a specific focus is given on Tur2 hosted both in metagreywacke and metamafic rocks, in order to determine the fluid sources and processes related to the formation of the first gold event, i.e. the early sulfide-rich disseminated gold mineralization related to the D2Ki event.

Early detrital Tur1 was not investigated because it formed before the gold mineralization and thus cannot be used as a tracer of ore-forming processes and mineralizing fluids.

SAMPLE DESCRIPTION AND ANALYTICAL METHODS

Sample description

Representative samples of the four main lithologies of the Kiaka deposit were collected from drill cores (spacing >400 m) intersecting the main orebody and alteration zones. Investigation of these lithologies was carried out in order to document the hydrothermal alteration by the gold-bearing fluids and to identify the presence of hydrothermal tourmaline. These lithologies include:

(i)

Muscovite schists: they are mostly located in the southern part of the deposit in the footwall of the D2-related shear zone (Fig. 1). They represent 50 to 100 m thick lenses intercalated between metagreywacke and metavolcanic rocks. The muscovite schists contain folded tourmaline (Tur1)-garnet-biotite-chlorite-muscovite layers (Fig. 3A-B) and rare pyrrhotite grains. The muscovite schist sample (drill KDH75, depth 64.9 m) is located at about 1-2 m from the mineralized zones and is weakly hydrothermally altered.

(ii) Diorite: it consists of sub-vertical intrusive plugs within the metasedimentary sequence in the northern part of the deposit (Fig. 1C). It is interpreted to be contemporaneous with the D2Ki event generating sinistral displacement along mineralized, NE-trending D2 shear zones. The diorite is dominantly composed of plagioclase, amphibole, chlorite, biotite, and quartz. No tourmaline was found in the diorite. Sample KDH337 (depth 44.1m) is located at about 50 m from the mineralized zones and is weakly hydrothermally altered, containing rare grains of pyrrhotite and chalcopyrite associated with titanite.

(iii) Metagreywacke: it is the dominant lithology at Kiaka and forms the southern footwall of a D2-related shear zone hosting the main mineralized zone. Composed of quartz, feldspar, and sillimanite with a granoblastic texture, metagreywacke locally contain disseminated tourmaline grains (Tur2), about 50 µm in size (Fig. 3C-D). Sample KDH29 (depth 311.8 m; Fig. 3C-D; Fig. 4) is from a phlogopite-rich alteration zone close to the orebody.

(iv) Metamafic rock: it is intercalated within the metasedimentary sequence forming >50 m thick sequences dipping towards the northwest (Fig. 1). Sample KDH291 (depth 56.2 m; Fig. 3E-F) comes from an alteration zone in proximity to the mineralized envelope. Within this zone, the metamafic rock contains porphyroblasts of hydrothermal tourmaline (Tur2) reaching few millimeters in size, embedded within a chloritized, fine-grained matrix, that locally contains pyrrhotite, titanite and clinozoisite (Fig. 3E).

Scanning electron microscopy

Mineralogical observations of tourmaline were carried out using transmitted-light microscopy and scanning electron microscopy (SEM) at the GeoRessources laboratory (Nancy, France).

Tourmaline grains were investigated using a Hitachi S-4800 and a JEOL J7600F scanning electron microscopes, both equipped with SDD-type energy-dispersive X-ray spectrometers (EDS). Backscattered electron (BSE) images were acquired on carbon-coated polished thin sections with an acceleration voltage of 15 kV, adjusting the image contrast to reveal internal zoning within the tourmaline grains. SEM was used to characterize the internal textures of tourmaline and to select representative grains for the in situ chemical and isotopic analyses.

Electron probe microanalysis

Major and minor element composition of tourmaline was determined using a Cameca SX100 2). X-ray elemental maps were acquired with a 15 kV accelerating voltage, a 100 nA beam current, and a 1 µm 2 electron beam (Fig. 6).

The elements Na, Si, Mg, Al, Ca, Ti, and Fe were measured with dwell times of 300 msec each and an acquisition step of 10 µm, for a total map size of 2900 x 3890 µm.

Secondary ion mass spectrometry

Boron isotopic composition of tourmaline was measured by secondary ion mass spectrometry (SIMS) using a Cameca IMS 1280-HR instrument at the CRPG-CNRS (Vandoeuvre-lès-Nancy, France). Analyses were performed on gold-coated polished thin sections that were previously investigated by SEM and EMPA, with a 20 nA beam of primary ions O -accelerated at 13 kV with an ellipsoid ablation spot of 20 µm long axis. The secondary ions 10 B + and 11 B + were accelerated at 10 kV and were measured in mono-collection with the axial Faraday cup with a mass resolution power M/ΔM = 2000. Each calculated isotopic ratio 11 B/ 10 B corresponds to 30 cycles of measurement with counting times of 8 sec per cycle on mass 10 and of 4 sec per cycle on mass 11. For each spot, a pre-sputtering of 60 sec was applied immediately before each analysis for surface cleaning. Instrumental mass fractionation (IMF) was determined and corrected using two tourmaline reference materials corresponding to dravite (Harvard #108796)

and schorl (Harvard #112566) described by [START_REF] Dyar | Reference minerals for the microanalysis of light elements[END_REF] and [START_REF] Leeman | Boron isotopic analysis of proposed borosilicate mineral reference samples[END_REF].

Each standard was measured several times during the analytical SIMS session following a standard-sample bracketing procedure. The summary of the B isotope analyses on the tourmaline reference materials measured by SIMS is reported in ESM Table 1. The matrix effect was corrected using the dravite Harvard #108796 reference material because the tourmaline samples analyzed in this work have dravitic composition. After correction of IMF, the measured 11 B/ 10 B ratios were reported in δ 11 B notation (in ‰) relative to the NIST SRM 951, whose 11 B/ 10 B ratio is 4.04362 [START_REF] Catanzaro | Boric acid: isotopic and assay standard reference materials[END_REF]. The external error (2σ) is about 0.13‰ over 28 analyses for the dravite Harvard #108796 and 0.26‰ over 14 analyses for the schorl Harvard #112566. The external error used is that of the dravite Harvard #108796. The total error is the quadratic sum of the internal and external errors and range between 0.2 and 1.2‰ (2σ) on the tourmaline samples with a median value of 0.3‰ (2σ).

Multi-collectorinductively coupled plasmamass spectrometry

Whole-rock boron content and isotopic compositions of the muscovite schist and diorite were measured using a Thermo Scientific Neptune multi-collectorinductively coupled plasmamass spectrometer (MC-ICP-MS) at the LhyGeS (Université de Strasbourg-EOST, France).

The B extraction procedure was adapted from [START_REF] Lemarchand | Boron isotopes in different grain size fractions: Exploring past and present water-rock interactions from two soil profiles (Strengbach, Vosges Mountains). Geochim Cosmochim Ac 98:78-93 London D[END_REF] and is summarized as follows: 10 mg of powdered sample were mixed with 250 mg of analytical grade K2CO3 in a 12 mL PtAu 05 crucible and placed in a muffle furnace at 950°C for 45 min. After cooling, the fusion residue was dissolved in 35 mL of bidistilled HCl 0.2N. Boron was then purified from a 0.5 mL aliquot by a two-step chemical procedure using the cation exchange resin Biorade AG50WX12 and the B-specific resin Amberlite IRA743. The recovered solution was simultaneously analyzed for both B concentration and isotope ratios using the standard-sample-standard bracketing method. The operational settings for B analyses by MC-ICP-MS are described in details by [START_REF] Roux | A Rapid Method for Determining Boron Concentration (ID-ICP-MS) and δ 11 B (MC-ICP-MS) in Vegetation Samples after Microwave Digestion and Cation Exchange Chemical Purification[END_REF]. Accuracy has been verified by repeated analyses of solution and rock standards (ERM AE120 = -20.1‰; ERM AE121 = +20.0‰; NRC NASS5 = +39.8‰; GSJ JB2 = +7.2‰). Total B contamination is 15 ng and the analytical uncertainties are 5% and 0.4‰ (2σ) for B concentration and isotope ratios, respectively.

RESULTS

Petrography and textural features of tourmaline

Hydrothermal tourmaline (Tur2) hosted in metagreywacke (sample KDH29-311.8 m; 1.08 g/t Au) ranges in size from 20 to 400 µm with euhedral to sub-euhedral shapes (Fig. 3C-D; Fig. 4).

No foliation is clearly preserved in this sample. A metamorphic recrystallization can be observed whereas Tur2 and biotite seem to represent a mineral equilibrium assemblage and to be part of the same hydrothermal D2Ki event. In addition, pyrrhotite and pyrite are characteristic of the assemblage of the low-grade disseminated ore in the Kiaka deposit. The tourmaline grains in the metagreywacke sample occasionally host micro-inclusions of apatite, quartz and pyrrhotite oriented along intra-grain fractures. At microscopic scale, tourmaline has light green to brownish colors in polarized light and is greenish in crossed polarized light. The grains appear homogeneous without core-rim zoning on BSE images (Fig. 4). Nine representative tourmaline grains were selected for in situ chemical and isotopic analyses.

Hydrothermal tourmaline (Tur2) hosted in metamafic rock (sample KDH291-56.2 m; 0.05 g/t Au) is characterized by euhedral to sub-euhedral habit and is commonly millimetric in size (Fig. 3E). Under the microscope, poikiloblastic tourmaline porphyroblasts have light yellow to beige colors in polarized light and are yellow-brownish in crossed polarized light. Three representative millimetre-size tourmaline grains intergrown with the biotite-titanite-clinozoisite D2Ki assemblage were selected for the in situ chemical and isotopic analyses (Fig. 5). Those three grains show different crystal orientation relative to the thin section plane, and their internal zoning and compositional variations were investigated in detail. The first tourmaline grain (Grain 1) shows an oblique section relative to the c-axis of about 3 mm wide, the second (Grain 2) shows a transversal section relative to the c-axis of about 9 mm in length, and the third (Grain 3) shows an oblique section relative to the c-axis (Fig. 5). The poikiloblastic tourmaline porphyroblasts host numerous sulfide inclusions (<100 µm up to 0.1 mm in size), including pyrrhotite, chalcopyrite and pyrite, as well as accessory clinozoisite, biotite, amphibole, and chlorite (Chl 2) (Fig. 2). At the grain scale, tourmaline hosted in metamafic rock shows a corerim zoning on BSE images (Fig. 5C). The core represents the main part of the grains and shows a patchy zoning with dark to medium grey contrasts on BSE images. The rim forms a 100-300 µm thick overgrowth in optical continuity with the core and shows brighter contrasts on BSE images. No dissolution textural evidence between core and rim was observed. X-ray elemental maps show intra-crystalline chemical zoning for Al, Fe, Mg, Ca, and Ti (Fig. 6). The rim is marked by higher Fe contents relative to the core, which is Mg-rich. The intra-grain variation in Ca, Ti, and Al, corresponds to the patchy zoning within the core related to the primary crystal growth of tourmaline.

Chemical composition of tourmaline

Chemical compositions and structural formulae of representative tourmaline grains (Tur2) from the Kiaka deposit are reported in the ESM Tourmaline hosted in the metamafic rock (sample KDH291-56.2 m) belongs to the alkali-group and plots within the compositional field of Al-poor and Fe 3+ -rich metasedimentary rocks in the Al-Fe-Mg diagram (Fig. 7F). Tourmaline has a dravite composition with Mg/(Fe+Mg) = 0.67-0.77 and Xvacancy/(Xvacancy+Na) = 0.11-0.29 (Fig. 7A). It has moderate contents of Ca (0.10-0.64 apfu), Na (0.50-0.70 apfu), and Al (5.55-6.27 apfu). No correlation is found between the Ca and Ti contents (0.04-0.19 apfu) and the color zoning of tourmaline in polarized light. The core and rim show minor compositional variations that are principally marked by variations of the Fe/Mg ratio, as also seen on X-ray elemental maps (Fig. 6). The chemical variations for tourmaline hosted in the metamafic rock are mostly due to the Mg+1Fe-1 and □Al+1(NaMg)-1 exchange vectors, with possible contribution of the (AlO)+1(MgOH)-1 deprotonation vector (Fig. 7).

Tourmaline hosted in the metagreywacke (sample KDH29-311.8 m) has a distinct chemical composition compared to the metamafic rock-hosted tourmaline. Tourmaline belongs to the alkali-group with a dravite composition and plots into the compositional field of Al-rich metasedimentary rocks in the Al-Fe-Mg diagram (Fig. 7F). It differs mainly from tourmaline hosted in the metamafic rock by lower contents of Ca (0.16-0.22 apfu), Mg (1.75-1.89 apfu),

and Na (0.45-0.54 apfu), and by higher contents of X-vacancy (0.25-0.35 apfu), and Al (6.37-6.72 apfu). Tourmaline has dravite composition with Mg/(Fe+Mg) = 0.68-0.73 and Xvacancy/(Xvacancy+Na) = 0.32-0.44. Variations of tourmaline compositions are mostly explained by the Mg+1Fe-1 and □Al+1(NaMg)-1 exchange vectors joining the dravite to foitite endmembers (Fig. 7B-C).

Boron isotope composition of the host rocks and tourmaline

Whole-rock boron isotope analyses were carried out on representative samples of unaltered diorite (sample KDH337-44.1) and muscovite schist (sample KDH75-64.9) collected outside the mineralized envelopes. Boron concentration and isotope analyses yielded reproducible values for each sample (Table 1). The diorite has a B content of 5.6 ppm with an average δ 11 B value of -13.9‰ (n = 2), whereas the muscovite schist contains 71 ppm B and has an average δ 11 B value of -29.2‰ (n = 2). Because the samples are devoid of hydrothermal alteration and the isotopic data are reproducible, the whole-rock boron isotope compositions are considered to be representative of each rock precursor.

In situ boron isotope compositions of tourmaline hosted in the metagreywacke (sample KDH29-311.8 m) and the metamafic rock (sample KDH291-56.2 m) are shown in Fig. 8 and Table 2.

DISCUSSION

Chemical composition of tourmaline as a monitor of fluid-rock interactions

Tourmaline grains hosted in the metamafic rock and the metagreywacke belong to the dravite series, but they differ by their chemical and boron isotopic compositions. Tourmaline hosted in the metamafic rock is Al-poor (5.75-6.27 apfu), Fe-rich (0.63-1.05 apfu) and Mg-rich (2.00-2.30 apfu), whereas tourmaline hosted in metagreywacke is Al-rich (6.37-6.72 apfu), Fe-poor (0.65-0.83 apfu) and . This is consistent with the whole-rock contents of Al2O3, Fe2O3 and MgO in metamafic rock and metagreywacke, respectively (Table 1; [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF], which suggests a host rock control on the chemical composition of tourmaline (except for boron), as proposed by other studies (e.g. [START_REF] Van Hinsberg | Tourmaline: an ideal indicator of its host environment[END_REF][START_REF] Hazarika | Diverse Tourmaline Compositions from Orogenic Gold Deposits in the Hutti-Maski Greenstone Belt, India: Implications for Sources of Ore-Forming Fluids[END_REF]2016;[START_REF] Kalliomäki | Major and trace element geochemistry of tourmalines from Archean orogenic gold deposits: Proxies for the origin of gold mineralizing fluids?[END_REF][START_REF] Harlaux | Origin of the atypical Puy-les-Vignes W breccia pipe (Massif Central, France) constrained by trace element and boron isotopic composition of tourmaline[END_REF]). In addition, metamafic rock-hosted tourmaline has lower X-vacancy contents (0.08-0.24 apfu) and higher Na+Ca contents (0.76-0.92 apfu), whereas metagreywacke-hosted tourmaline has higher X-vacancy contents (0.25-0.35 apfu) and lower Na+Ca contents (0.65-0.75 apfu). This is also consistent with the higher whole-rock contents of CaO, Na2O and K2O in the metamafic rock compared to metagreywacke (Table 1; [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]). In the X-vacancy vs. Al total and Fe vs. Al diagrams (Fig. 7C,D), metamafic rock-hosted tourmaline shows large Al variations (5.75-6.27 apfu) reflecting the (AlO)+1(MgOH)-1 deprotonation exchange vector, whereas metagreywacke tourmaline has higher Al and X-vacancy contents projecting along the □Al+1(NaMg)-1 exchange vector.

These variations may reflect variable degrees of interaction between the hydrothermal fluid and the host rocks. For comparison, syn-ore hydrothermal tourmaline from orogenic gold deposits in the Loulo Gold District in Mali [START_REF] Labou | Petrological and geochemical study of Birimian ultramafic rocks within the West African Craton: Insights from Mako (Senegal) and Loraboué (Burkina Faso) lherzolite/harzburgite/wehrlite associations[END_REF] shows large variations of the Mg/(Fe+Mg) ratio (0.27-0.96) and have lower Ca contents (0.01-0.09 apfu) and higher Na contents (0.62-0.89) compared to hydrothermal tourmaline from Kiaka (Fig. 7). The composition of the X-site in tourmaline is dominantly controlled by the Na and Ca concentrations in the coexisting aqueous fluid, being therefore a function of the fluid salinity (von [START_REF] Goerne | Synthesis of tourmaline solid solutions in the system Na2O-MgO-Al2O3-SiO2-B2O3-H2O-HCl and the distribution of Na between tourmaline and fluid at 300 to 700°C and 200 MPa[END_REF][START_REF] Goerne | Experimental determination of Na-Ca distribution between tourmaline and fluid in the system CaO-Na2O-MgO-Al2O3-SiO2-B2O3-H2O[END_REF]Dutrow andHenry 2016, 2018). The relative high contents of Na (0.45-0.70 apfu, average = 0.61 apfu) and Ca (0.10-0.37 apfu, average = 0.24 apfu) and the relative low contents of X-vacancy (0.08-0.35 apfu, average = 0.15 apfu) in hydrothermal tourmaline (Tur2) suggest that the fluid was of low to moderate salinity. The tourmaline-bearing ore assemblage is interpreted to have formed during the D2Ki stage at temperatures of 371-413°C based on chlorite (Chl 2) thermometry [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]. Additionally, arsenopyrite in contact with pyrrhotite and löllingite in quartz-biotite metagreywacke contains 35.4-36.5 atm% of As, yielding temperatures of 360-475ºC [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]. Assuming a median temperature of 400°C and an average Na = 0.6 apfu in tourmaline, a salinity of ca. 3 wt.% NaCl equiv can be estimated for the fluid in equilibrium with the assemblage tourmaline + quartz + chlorite (von [START_REF] Goerne | Synthesis of tourmaline solid solutions in the system Na2O-MgO-Al2O3-SiO2-B2O3-H2O-HCl and the distribution of Na between tourmaline and fluid at 300 to 700°C and 200 MPa[END_REF]. Tourmaline precipitation from hydrothermal fluids requires acidic to near-neutral conditions (pH <6) that favor the formation of stable trigonal complexes [B(OH)3] in the fluid [START_REF] Morgan | Experimental reactions of amphibolite with boron-bearing aqueous fluids at 200 Mpa: implications for tourmaline stability and partial melting in mafic rocks[END_REF][START_REF] Henry | Metamorphic tourmaline and its petrologic applications[END_REF]. The development of pervasive sericitization associated with disseminated tourmaline and clinozoisite in the metagreywacke [START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]) suggests that the fluid was moderately acidic. The composition of tourmaline from Kiaka is dominantly controlled by the Fe 2+ +1Mg 2+

-1 and □Al+1(NaMg)-1 exchange vectors with no contribution of the Fe 3+ +1Al 3+

-1 homovalent substitution (Fig. 7), which suggests low Fe 3+ /Fe 2+ ratios and reduced fluid conditions. These arguments suggest that hydrothermal tourmaline (Tur2) likely formed from moderately high temperature (ca. 400°C), moderately acidic, reduced, and low-salinity hydrothermal fluids and that its chemical composition (except for B, Na and Ca) was buffered by the host rock during fluid-rock interactions.

Origin of fluids and boron for formation of the Kiaka gold deposit

The source of boron and mineralizing fluids at Kiaka can be discussed in terms of proximal vs.

distal, as well as magmatic vs metamorphic origin. In the metagreywacke and the metamafic rock, the boron budget is largely dominated by hydrothermal tourmaline (Tur2), which is the main boron-bearing mineral in those rocks. Considering that basalts have typically very low boron contents (Table 1; <0.1 to 5 ppm; [START_REF] Leeman | Geochemistry of boron and its implications for crustal and mantle processes[END_REF][START_REF] Marschall | Boron isotopes in the ocean floor realm and the mantle[END_REF], it is unlikely that the boron required to form Tur2 in the metamafic rock was derived from the basaltic precursor. The metagreywacke contains two generations of tourmaline: one generation is detrital (Tur1), being part of the mineral assemblage of the protolith, whereas the other one is hydrothermal (Tur2), being formed during the D2Ki event. As detrital Tur1 remained stable in the metagreywacke during the prograde and retrograde metamorphism, it seems unlikely that Tur1 was a viable source of boron for formation of Tur2, even if we cannot definitively exclude the potential destabilization of Tur1 in deep-seated metagreywacke lithologies.

We may also exclude the diorite as a proximal boron source because of its low boron content (B = 5.6 ppm, Table 1) and its absence of significant hydrothermal alteration, arguing against the release of boron-rich magmatic fluids or post-magmatic hydrothermal alteration. Another possible candidate could be the younger unfoliated biotite granites that have intruded the TTG basement between ca. 2120 Ma and 2100 Ma [START_REF] Vegas | Structure and emplacement of granite plutons in the Paleoproterozoic crust of Eastern Burkina Faso: rheological implications[END_REF][START_REF] Baratoux | Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny (2.2-2.0 Ga), western Burkina Faso[END_REF]). These granitic rocks emplaced through transcurrent shearing along the Manga Fada N'Gourma greenstone belt and the related amphibolite-facies metamorphism attributed to contact metamorphism [START_REF] Naba | Structure and emplacement model for late-orogenic Paleoproterozoic granitoids: the Tenkodogo-Yamba elongate pluton (Eastern Burkina Faso)[END_REF][START_REF] Vegas | Structure and emplacement of granite plutons in the Paleoproterozoic crust of Eastern Burkina Faso: rheological implications[END_REF] has been dated at 2105 ± 30 Ma (Pb-Pb on zircon; [START_REF] Castaing | Notice explicative de la carte géologique et minière du Burkina Faso à 1/1 000 000[END_REF]. However, none of these granitic rocks has been described in the district of Kiaka and no granite intercept was found in the investigated drill cores.

A proximal origin of boron-rich fluids may therefore be discarded. The boron isotope compositions of hydrothermal tourmaline (Tur2) from Kiaka (δ 11 B = -25.1‰ to -15.1‰; Fig. 8) fall outside the typical δ 11 B range for tourmaline hosted in granitic rocks (-15‰ to +5‰; [START_REF] Marschall | Tourmaline Isotopes: No Element Left Behind[END_REF][START_REF] Trumbull | Boron isotopes in the continental crust: granites, pegmatites, felsic volcanic rocks, and related ore deposits[END_REF], although they partly overlap with the δ 11 B range of tourmaline from granite-related veins (-22‰ to -3‰; Fig. 8).

A magmatic-hydrothermal origin of the fluid can be excluded, because (i) no contemporaneous granitic intrusion is found nearby the Kiaka deposit, (ii) age constraints exclude the diorite plug as a magmatic source of boron, and, to a lesser extent, (iii) the mineralization is hosted in thick metasedimentary-metavolcanic units. 1), which is commonly associated with micas [START_REF] Leeman | Geochemistry of boron and its implications for crustal and mantle processes[END_REF][START_REF] Nakano | Boron isotope geochemistry of metasedimentary rocks and tourmalines in a subduction zone metamorphic suite[END_REF][START_REF] Trumbull | Boron isotopes in the continental crust: granites, pegmatites, felsic volcanic rocks, and related ore deposits[END_REF].

Muscovite breakdown reactions in metapelitic rocks at P ≤ 4 kbar and T ≤ 650ºC [START_REF] Thompson | Dehydration melting of pelitic rocks and the generation of H2Oundersaturated granitic liquids[END_REF][START_REF] Vielzeuf | Experimental determination of the fluid-absent melting relations in the pelitic system[END_REF][START_REF] Spear | P-T paths from anatectic pelites[END_REF]) can produce a low-salinity, reduced, and boron-rich metamorphic fluid [START_REF] Moran | Boron depletion during progressive metamorphism: implications for subduction processes[END_REF][START_REF] Henry | Metamorphic tourmaline and its petrologic applications[END_REF][START_REF] Nakano | Boron isotope geochemistry of metasedimentary rocks and tourmalines in a subduction zone metamorphic suite[END_REF][START_REF] Kawakami | Boron in metapelites controlled by the breakdown of tourmaline and retrograde formation of borosilicates in the Yanai area, Ryoke metamorphic belt, SW Japan[END_REF]. In order to evaluate whether hydrothermal tourmaline could have formed from boron-rich fluids derived from devolatilization of muscovite schists at depth, we calculated the tourmaline-fluid B isotope fractionation according to the Rayleigh fractionation model of [START_REF] Marschall | Experimental boron isotope fractionation between tourmaline and fluid: confirmation from in situ analyses by secondary ion mass spectrometry and from Rayleigh fractionation modelling[END_REF] and using the fractionation factors of [START_REF] Meyer | Boron-isotope fractionation between tourmaline and fluid: an experimental re-investigation[END_REF]. Considering a temperature of 650ºC for the muscovite breakdown reaction, an average δ 11 B = -29.2‰ for the muscovite schist (Table 1), and a muscovite-fluid boron isotopic fractionation of -7.7‰ [START_REF] Wunder | The geochemical cycle of boron: constraints from boron isotope partitioning experiments between mica and fluid[END_REF] propose that the source of the boron-rich fluids was derived from muscovite schists similar to those exposed at Kiaka but located at a deeper level in the metamorphic nappe. The present results indicate that neither proximal country rocks of the Kiaka deposit nor ascending granitic magmas could be viable sources of fluids and boron. However, the isotopic data are consistent with fluids and boron being issued from devolatilization of deep-seated muscovite schists during the D2Ki event, which relates to the prograde-to-peak amphibolite facies regional metamorphism. This interpretation is supported by mineral-equilibria modelling suggesting that exhumation of metasediments can produce important amounts of metamorphic fluids at temperatures near 500ºC [START_REF] Vry | The role of exhumation in metamorphic dehydration and fluid production[END_REF].

CONCLUSIONS

In the Kiaka orogenic gold deposit, hydrothermal tourmaline (Tur2) is commonly associated with the proximal alteration assemblage in the host rocks. Mineralogical observations suggest that deposition of Tur2 was synchronous with the early sulfide-rich disseminated gold mineralization coeval with the D2Ki phase, corresponding to the regional prograde-to-peak metamorphism. Hydrothermal tourmaline hosted in metamafic rock and metagreywacke has a dravite composition, but shows variable major element contents, mainly in Al, Mg, and Fe, thus suggesting a host rock buffering. Boron isotope compositions of tourmaline range between -25.1‰ and -15.1‰, with differences between tourmaline hosted in the metamafic rock (δ 11 B = -19.6‰ to -15.1‰) and tourmaline hosted in the metagreywacke (δ 11 B = -25.1‰ to -22.0‰).

Modeling of tourmaline-fluid boron isotope fractionation argue for a metamorphic origin of the mineralizing fluids that were produced during devolatilization of deep-seated muscovite schists.

We propose that hydrothermal tourmaline (Tur2) and the synchronous disseminated gold mineralization resulted from moderately high-temperature (ca. 400ºC) interactions between a boron-rich hydrothermal fluid and the metamorphic host rocks at recorded amphibolite facies conditions.

Figure captions

Figure 1. A. Location and geological setting of the Kiaka orogenic gold deposit in the West African Craton (modified after [START_REF] Castaing | Notice explicative de la carte géologique et minière du Burkina Faso à 1/1 000 000[END_REF][START_REF] Naba | Structure and emplacement model for late-orogenic Paleoproterozoic granitoids: the Tenkodogo-Yamba elongate pluton (Eastern Burkina Faso)[END_REF]Milési et al. 2004;[START_REF] Vegas | Structure and emplacement of granite plutons in the Paleoproterozoic crust of Eastern Burkina Faso: rheological implications[END_REF][START_REF] Ganne | Modern-style plate subduction preserved in the Palaeoproterozoic West African craton[END_REF]. The Kénéma Man domain (Archean nucleus) and the Baoulé-

Kénéma Man domain

  The full range of δ 11 B values of tourmaline extends from -25.12‰ to -15.10‰ (n = 49) and overlaps the lower δ 11 B range of tourmaline hosted in orogenic gold deposits worldwide (δ 11 B = -24‰ to +19‰; Fig. 8). Tourmaline hosted in the metamafic rock has heavier boron isotopic composition (δ 11 B = -19.6‰ to -15.1‰; mean = -17.8‰; mode ~ -19.5‰; n = 31) compared to tourmaline hosted in the metagreywacke (δ 11 B = -25.1‰ to -22.0‰; mean = -24.2‰; mode ~ -24.0‰; n = 18). Isotopic analyses of single tourmaline grains reveal δ 11 B variations up to 4.7‰ from core to rim, but show no systematic δ 11 B variation related to sector and patchy zoning. No correlation was found between the chemical and boron isotopic composition of tourmaline.

  A meta-evaporitic fluid origin (δ 11 B = +5‰ to > +30‰) can also be ruled out. In contrast, the B isotope values of Tur2 are compatible with those of tourmaline hosted in metasedimentary rocks (δ 11 B = -26‰ to +1‰; Nakano and Nakamura 2001; Bebout and Nakamura 2003; Marschall and Jiang 2011; Berryman et al. 2017; Fig. 8). Despite the general diachronism observed from east to west in the southern WAC (Parra-Avila et al. 2017), comparison with the Loulo Mining District in Mali is of interest. Lambert-Smith et al. (2016) have used geochemistry and boron isotopy of tourmaline to discriminate fluid source regions. The  11 B values in syn-ore tourmaline indicate a dominant fluid contribution from clastic metasediments and carbonate rocks, suggesting metamorphic devolatilization in the host terrane as the most likely process that produced the ore-forming fluids. It seems likely for the Kiaka deposit that hydrothermal tourmaline crystallized from a boron-rich metamorphic fluid, of distal origin, derived from deep-seated metasediments. Boron isotope compositions of tourmaline hosted in the metamafic rock (δ 11 B = -19.6‰ to -15.1‰) and the metagreywacke (δ 11 B = -25.1‰ to -22.0‰) show a significant difference of δ 11 B values (up to 10‰), which suggests variable degrees of interactions between the boronrich hydrothermal fluid and the host rocks, as also supported by the chemical compositions of tourmaline. The muscovite schists may represent a potential source for the boron-rich fluids because they have a relatively high B content (71 ppm, Table

  , we estimated a δ 11 B = -21.5‰ for the expelled metamorphic fluid. At 650ºC, the minimum boron content of this fluid would be about 1.0 to 2.5 wt.% B2O3 (London 2011). The evolution of the B isotopic composition of this fluid during its ascent through metamorphic rocks at amphibolite facies conditions was calculated using the fractionation for a stepwise decreasing temperature from 650 to 350ºC following the model of[START_REF] Büttner | Fluid sources in the Twangiza-Namoya Gold Belt (Democratic Republic of Congo): Evidence from tourmaline and fluid compositions, and from boron and Rb-Sr isotope systematics[END_REF]. Considering an initial fluid composition of δ 11 B = -21.5‰ at 650ºC, progressive fractionation by removing 60 to 90% of boron from the fluid (F = 0.1-0.4) results in a shift towards higher δ 11 B values of -19.6‰ to -14.9‰ (Fig.9).Tourmaline precipitating from this fluid would acquire δ 11 B compositions comprised between -20‰ and -18‰. At 400ºC, a representative temperature for Tur2 formation at Kiaka, the fluid would have a δ 11 B = -16.3‰ and the corresponding tourmaline would acquire a δ 11 B = -19.1‰ (Fig.7). These δ 11 B values fall into the range of δ 11 B = -25.1‰ to -15.1‰ (average = -20.1‰) measured for hydrothermal tourmaline from Kiaka. The differences of δ 11 B values between tourmaline hosted in the metagreywacke (average = -24.2‰) and the metamafic rock (average = -17.8‰) are interpreted to reflect variable degrees of interactions, at constant temperature (ca. 400ºC), between the host rocks of initially distinct isotopic compositions and the boronrich metamorphic fluid.In summary, hydrothermal tourmaline (Tur2) from Kiaka formed through moderately hightemperature fluid-rock interactions between boron-rich metamorphic fluids and the country rocks, resulting in the local uptake of Al, Fe, and Mg required for tourmaline formation. We

  Mossi domain (Paleoproterozoic juvenile crust) form two domains separated by the Sassandra Fault (SF). Volcano-sedimentary belts include Banfora (BA), Houndé (HO), Boromo (BO), Lawra (LW), Goren (GO), Bui (BU), Ashanti (AS) and Sefwi (SW), Haute Comoé basin (HC), and Manga Fada-N'Gourma belt (MFG). These belts are separated into dark green for mafic volcanics rocks and light green for volcano-sedimentary rocks and intermediate to felsic igneous rocks[START_REF] Baratoux | Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny (2.2-2.0 Ga), western Burkina Faso[END_REF]. A spatial association is found between gold deposits of the Baoulé-Mossi domain and major tectonic structures such as the Grenville-Ferkessedougou-Bobo-Dialousso (GFBF), Ouango-Fitini (OF) and Markoye (MSZ; this study) shear zones. For interpretation of the colors, the reader is referred to the web version of this article. B. Geology of the Manga Fada-N'Gourma greenstone belt with MSZ (thick black lines) and location of the Kiaka gold deposit[START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]. C. Local geology of the Kiaka deposit showing the location of the drillholes and cross-sections. Mineralized envelopes are projected at surface from drillhole data (modified from Fontaine et al. 2017). D. NW-striking section 6100 showing the location of the diorite plug.

Figure 2 .

 2 Figure 2. Paragenetic sequence of the Kiaka gold deposit and relationships with the structural

Figure 3 .

 3 Figure 3. Mineralogical and textural features of tourmaline in the alteration zones of the Kiaka

Figure 4 .

 4 Figure 4. Petrography of hydrothermal tourmaline (Tur2) hosted in metagreywacke (sample

Figure 5 .

 5 Figure 5. Petrography of metamafic rock-hosted hydrothermal tourmaline (Tur2)

Figure 6 .

 6 Figure 6. X-ray elemental maps of a poikilitic tourmaline porphyroblast (Tur2, Grain 2) hosted

Figure 9 .

 9 Figure 9. Boron isotope fractionation curves between tourmaline and fluid for a stepwise

  Fig.1C

  Fig. 4B

  Fig.3B

  Fe on LIF), and vanadinite (Cl on LPET). Detection limits are approximately 330 ppm for Si, 300 ppm for Al, 1000 ppm for Fe, 265 ppm for Mg, 250 ppm for Ti, 365 ppm for Mn, 515 ppm for Na, 370 ppm for Ca, 200 ppm for K, and 140 ppm for Cl. Structural formulae Fe 2+ , Mg 2+ , Mn 2+ , Al 3+ , Li + , Fe 3+ , or Cr 3+ ; Z = Al 3+ , Fe 3+ , Ti 4+ , Mg 2+ , or Cr 3+ ; T = Si 4+ , Al 3+ , or B 3+ ; V = OH -, O 2-; and W = OH -, F -, or O 2-[START_REF] Henry | Nomenclature of the tourmaline-supergroup minerals[END_REF]. Chemical compositions

	of tourmaline were calculated using the WinTcac software (Yavuz et al. 2014) normalizing to
	15 cations in Y+Z+T sites and assuming stoichiometric 3 atoms for B and 4 atoms for OH+F,
	based on the general formula XY3Z6(T6O18)(BO3)3V3W, where X = Na + , Ca 2+ , K + , or vacancy
	site; Y =
	performed with the following operating conditions: beam current of 20 nA, acceleration voltage
	of 15 kV, and beam diameter of 1 µm. Counting times on element peaks and backgrounds were
	16 sec and 8 sec, respectively, for Si, Al, K, Ca, Fe, Mg, Mn, Ti, and 30 sec and 15 sec,
	respectively, for Na, Cr, and Cl. The following standards and monochromators were used for

electron microprobe analyzer (EMPA) equipped with five wavelength-dispersive X-ray spectrometers (WDS) at the GeoRessources laboratory (Nancy, France). Analyses were the EMPA measurements: albite (Si, Na on TAP), olivine (Mg on TAP), corundum (Al on TAP), orthoclase (K on LPET), andradite (Ca on PET), ilmenite (Ti on LPET, Mn on LIF), hematite (of tourmaline are reported in weight per cent (wt.%) oxides and the structural formulae are expressed in atoms per formula unit (apfu) (ESM Table

Table 2

 2

	. Overall, tourmaline compositions show

1.70 wt.%), CaO (0.58-2.10 wt.%), MnO (<0.1 wt.%), and TiO2 (0.19-1.51 wt.%). The measured K2O and Cl values are very low (up to 0.14 wt.%), close to or below the EMPA limits of detection.

- Gounkoto

	A 15°N	Late Tarkwaian basins Post-proterozoic domain Proterozoic domains	shear zones Major suture 1 st or 2 nd order
			Archean domain	Gold deposits
	Sabodala	1 2	irimian greenstone belts 1: mafic to intermediate volcanic rocks -2: intermediate to acid volcaniclastic and volcanosedimentary rocks	A &	GO	MSZ
			Birimian plutonic belts	Kiaka deposit		B
		Tabakoto					MFG
					HO	BO
							ig. 1B
			SF	BA	OF		LW	BU	P an A fri ca n or og en fro nt
			GFBF	HC		SW	N
							AS
							0°B
							250 km	0°F	5°1
						6 1 0 0 N	KDH29	740 500 mE
							Metamafic rock
							Conglomerate
	1 289 500 mN	500 m					KDH337	Qtz-Bt metagreywacke Fsp-phyric metagreywacke
							KDH291	Grt-opx schist
	N						Aluminosilicate metapelite
							Shear zone	Diorite
							(> 0.3 g/t Au) Ore zones	KDH75	Drill Holes
	1 289 000 mN
				KDH75
	1 288 500mN	738 500 mE	739 000 mE

Acknowledgments

We are grateful to AMIRA International and industry sponsors, including AusAid and the ARC Linkage Project LP110100667, for their support on the WAXI project (P934A). This study is part of the WAXI 2 program (http://waxi2.org/). We also thank Drs. James Lambert-Smith and Crystal LaFlamme for their constructive and thorough reviews that greatly helped improving the quality of the original manuscript. Finally, we thank the Editor-in-Chief Dr. Georges Beaudoin and the Guest Editor Dr. Nicolas Thébaud for their comments and the editorial handling.

light (TL) and backscattered electrons (BSE) (same as Figure 5C). Relative intensities of the elements Fe, Mg, Al, Ca, Na, and Ti (C, D, E, F, and G) are shown with count/sec. scales. [START_REF] Henry | Tourmaline as a petrogenetic indicator mineral: an example from the staurolite-grade metapelite of NW Maine[END_REF]: 1-Li-rich granitoids, pegmatites and aplites; 2-Li-poor granitoids, pegmatites and aplites; 3-Fe 3+ -rich quartz-tourmaline rocks (hydrothermally altered granitoids); 4-Al-rich metapelites and metapsammites; 5-Al-poor metapelites and metapsammites; 6-Fe 3+ -rich quartz-tourmaline rocks, calc-silicate rocks and metapelites; 7-Low-Ca meta-ultramafic rocks and Cr-V-rich metasediments; 8-Metacarbonates and metapyroxenites.


domain: math info

text:
[2] M. Garland and P. S. Heckbert. Surface simplification using quadric error metrics. In Proc. SIGGRAPH '97, pages 209-216, 1997.

Physics driven geometrical model reduction

  ESTEW -DOREA -2020 -Réf: DOR/PRE/2020/007 Outline • Previous results on new view factor computation methods • Physics driven geometrical model reduction • Problem statement • Reduction process coupled to an oracle for the physical component (here thermal) • Limits • Conclusion & future works ESTEW -DOREA -2020 -Réf: DOR/PRE/2020/007 Progressive geometric view factors • Polygon-based quadrature • Adaptive splitting • Predictions More details here: V. Vadez, F. Brunetti, P. Alliez. Progressive Geometric View Factors for Radiative Thermal Simulation. International Conference on Environmental Systems (ICES2020) Page 4 ESTEW -DOREA -2020 -Réf: DOR/PRE/2020/007 Physics driven geometrical model reduction • Problem statement: • Radiative thermal computations are time consuming • Goal: Geometric reduction consulting physics unaware but physics driven thanks to an oracle User inputs: Geometric file & decimation step Output: Distortion • Global generic architecture: • Preparation: reference calculation case request and clustering • Reduction: decimation iterations for each cluster • Reassembling: reduced geometry reconstitution and calculations request • Distortion: stop criteria according to user input and calculations results (max distortion) -DOREA -2020 -Réf: DOR/PRE/2020/007 Physics driven geometrical model reduction • Current prototype thermal limitations • View surfaces face to face only (no solar or albedo fluxes computed) • Node-to-node radiative couplings • Thermal equation solved: steady state • No conductive part in considered model (so only radiative couplings influence final temperatures aka radiative component totally drives temperatures) • Same thermo-optical properties everywhere Note: if solar and albedo computations are performed, if conductive couplings are added, if thermo-optical properties are different for each node -> final reduced output would be different BUT reduction algorithm does not change Physics driven geometrical model reduction • State of the art on geometric model reduction • Edge-collapse or halfedge-collapse • Cost function & placement function • Lindstrom-Turk [1]: optimizes shape, volume & boundaries • Garland-Heckbert [2]: encodes approximate distance to the original mesh by using quadric matrices that it assigns to each vertex • Volume preserving memory-aware • Normal preserving [1] Peter Lindstrom and Greg Turk. Fast and memory efficient polygonal simplification. In IEEE Visualization, pages 279-286, 1998.

•

  number of faces over iteration number Page 17 ESTEW -DOREA -2020 -Réf: DOR/PRE/2020/007 Physics driven geometrical model reduction • Conclusion • Volume and boundaries preserved thanks to Lindstrom-Turk edge collapse • Results are acceptable until geometry is too much approximated • Fast computation times (around 2 minutes for reduction and thermal calculus for this model). And faster at each iteration. The prototype shows that the algorithm can be improved. Even if the geometry is drastically corrupted, the temperatures are still acceptable thanks to the chosen reduction metric preserving the areas. Page 18 ESTEW -DOREA -2020 -Réf: DOR/PRE/2020/007 Physics driven geometrical model reduction • Future works on geometric model reduction • Progressive reduction (with adaptive ratio for each reduction step) • Reduce large clusters first to avoid small clusters to be too much approximated • Future works for thermal simulation • Test on a complete radiative model by adding maskings & shadows for incident fluxes (albedo, solar & Earth) • Test on a real model, with ESA permission Sentinel-3 and BepiColombo are good candidates • Examine standard exchange formats between our reduction outputs and STEP-TAS & STEP AP203 Sensitivity analysis on thermo-optical properties, use of machine learning to acquire thermal expertise about what drives the reduction process

Physics driven geometrical model reduction