Quentin Masurel
email: qmasurel@hotmail.com

Aurélien Eglinger

Nicolas Thébaud

Andrew Allibone

Anne-Sylvie André-Mayer

Helen Mcfarlane

John Miller

Mark Jessell

Laurent Aillères

Olivier Vanderhaeghe

Stefano Salvi

Lenka Baratoux

Stephane Perrouty

Graham Begg

Denis Fougerouse

Patrick Hayman

Ousmane Wane

Asinne Tshibubudze

Luis Parra-Avila

Alain Kouamélan

Prince Ofori Amponsah

Gold metallogeny of the Paleoproterozoic part of the southern West African Craton: capturing distinct pulses within an orogenic cycle

Keywords: West African Craton, Paleoproterozoic, gold, Eburnean Orogeny

M D m , w ' premier Paleoproterozoic gold province (~10,000 metric ton gold endowment). Structural, metamorphic, and geochronological data suggest gold mineralisation occurred during three 2 episodes that span much of the Eoeburnean and Eburnean orogenic cycles. Eoeburnean orogenic and rare skarn-hosted gold deposits formed between ca. 2200 and 2135 Ma during repeated episodes of volcanism, plutonism, and shortening, which thickened the Paleoproterozoic crust. Early Eburnean orogenic and placer gold deposits formed between ca. 2110 and 2095 Ma during inversion, metamorphism, and subsequent oblique shortening of intra-orogenic basins filled after ca. 2135 Ma. This episode of mineral m w -M D m w m -Man Domain at ca. 2095 Ma. Late Eburnean orogenic and less common intrusion-related gold deposits formed between ca. 2095 and 2060 Ma during strike-slip to oblique-slip tecto , --m w w -M D m wm -Mossi Domain, whereas the Early Eburnean and Late Eburnean deposits include ca. 50-70% and 20-40%, respectively. Here, we highlight the favourable confluence of accretioncollision tectonics, involving juvenile crust formation as well as protracted magmatic, metamorphic, and deformation histories that resulted in di m -Mossi Domain.

Introduction

Tectonic and associated geodynamic processes ultimately control the character, timing, and distribution of ore-forming mineral systems [START_REF] Robert | Gold metallogeny of the Superior and Yilgarn cratons[END_REF][START_REF] Kerrich | Metallogenic provinces in an evolving geodynamic framework[END_REF][START_REF] Bierlein | Lithospheric controls on the formation of provinces hosting giant orogenic gold deposits[END_REF][START_REF] Mccuaig | The mineral system concept: the key to exploration targeting[END_REF]. Developing an understanding of -m mm m m m -Mossi Domain of the southern part of the West African Craton (sWAC) have studied the age and geological setting of gold mineralisation within specific deposits and camps (Wassa, [START_REF] Perrouty | Geological setting of the Wassa gold deposit, SW Ghana[END_REF]Siguiri, Lebrun et al. 2017;Obuasi, Blenkinsop et al. 1994;Allibone et al. 2002a;[START_REF] Fougerouse | Evidence for two stages of m ' deposit: Obuasi, Ghana[END_REF]Oliver et al. 2020;Pampe, Salvi et al. 2016a;Sadiola, Masurel et al. 2017a;Kiaka, Fontaine et al. 2017;Mana, Augustin et al. 2017;Loulo, Lawrence et al. 2013a;Allibone et al. 2020). These studies indicate that gold deposits formed di m In this review, we integrate published data acquired from deposit through craton scales to propose a revi --Mossi Domain. We then use this interpretation to place the various gold deposits in a craton-wide temporal and spatial context. Lastly, we com-pare the geological and metallogenic history of the Pal -Mossi Domain) with that of the Neoarchean Superior (Canada) and Yilgarn (Western Australia) cratons.

-Mossi Domain

The southern portion of the West African Craton (Fig. 1) comprises the Pal -M D m m -Man Domain (Bonhomme 1962;[START_REF] Boher | Crustal growth in West Africa at 2.1 Ga[END_REF][START_REF] Feybesse | The paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF]). This paper focuses -Mossi Domain, which in-cludes elongate belts of volcanic, volcano-sediment , m m m w --Mossi Domain provides the geological con-text of gold mineralisation during Paleoproterozoic orogenic activity. Geochronological data discussed in this review is provided in Electronic Supplementary Data (ESM 1).

Juvenile mafic volcanism (ca. 2350-2200 Ma)

-M D m m m -Boromo belt, [START_REF] Ganne | Modern-style plate subduction preserved in the Palaeoproterozoic West African craton[END_REF]Nangodi belt, Sylvester and Attoh 1992;Sefwi belt, Hirdes et al. 1993; Ashanti belt, [START_REF] Dampare | Geochemistry of Paleoproterozoic metavolcanic rocks from the southern Ashanti volcanic belt, Ghana: petrogenetic and tectonic setting implications[END_REF]Mako belt, Dioh 1986;Diallo 1994;[START_REF] Dia | Isotopic constraints on the age and formation of Paleoproterozoic volcanic arc complex in the Kédougou inlier, eastern Senegal, West Africa[END_REF][START_REF] Abouchami | A major 2.1 Ga event of mafic magmatism in West Africa: an early stage of crustal accretion[END_REF]Labou et al. 2020) 2016; [START_REF] Petersson | Zircon U-Pb-Hf evidence for subduction related crustal growth and reworking of Archaean crust within the Palaeoproterozoic Birimian terrane, West African Craton, SE Ghana[END_REF]) and Sm-Nd whole rock analyses [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][START_REF] Doumbia | Petrogenesis of juvenile-type Birimian (Paleoproterozoic) granitoids in Central Côte-d'Ivoire, West Africa: geochemistry and geochronology[END_REF] The minimum age of tholeiitic volcanism is indirectly constrained by the transition to intermediate-felsic volcanism and the widespread onset of diorite-granodiorite-tonalitetrondhjemite plutonism at ca. 2200 Ma (De [START_REF] Kock | Geochronology of the Birim Supergroup of the West African craton in the Wa-Bolé region of west-central Ghana: implications for the stratigraphic framework[END_REF][START_REF] Parra-Avila | The geochronological evolution of the Paleoproterozoic Baoulé-Mossi domain of the southern West African Craton[END_REF]. The geodynamic context of the juvenile tholeiitic oceanic crust remains uncertain, al-though 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), island arc [START_REF] Sylvester | Lithostratigraphy and composition of 2.1 Ga greenstone belts of the West African Craton and their bearing on crustal evolution and the Archean-Proterozoic boundary[END_REF][START_REF] Ganne | Modern-style plate subduction preserved in the Palaeoproterozoic West African craton[END_REF]Labou et al. 2020), back-arc basin [START_REF] Vidal | The Paleoproterozoic (Birimian) of Haute-Comoé in the West African Craton, Ivory Coast: a transtensional back-arc basin[END_REF], and mid-ocean ridge [START_REF] Pouclet | Geodynamic setting of the Birimian volcanism in central Ivory Coast (western Africa) and its place in the Paleoproterozoic evolution of the Man Shield[END_REF]Labou et al. 2020) settings have been proposed.

Arc constructional stage, Eoeburnean shortening, and crustal thickening (ca. 2200-2135

Ma)

Lithostratigraphic studies of various volcano-sedimentary belts in the Baoulé-Mossi domain indicate calc-alkaline supracrustal rock successions overlie the basal tholeiitic flows, locally with unconformable contacts [START_REF] Davis | Craton-scale lithostratigraphic correlation as an insight for the geodynamic evolution of the southern West African Craton[END_REF]. For example, the supracrustal rock succession in the Oumé-' re comprises basal mafic to intermediate lavas and tuffs, overlain by felsic lavas, lapilli tuffs, and ignimbrites intercalated with bimodal volcaniclastic, and minor epiclastic rocks, all interpreted to have been deposited in an evolving volcanic arc (Mortimer 1990(Mortimer , 1992a(Mortimer , 1992b(Mortimer , 2016)). The transition from mafic to intermediate volcanism in the Oumé-Féttekro belt occurred between ~2185 and 2160 Ma (Kouamélan 1996), which is consistent with ages reported for similar rock sequences elsewhere in the sWAC (e.g. northwestern Ghana, De [START_REF] Kock | Geological Map Explanation, Map Sheet 0803B[END_REF][START_REF] Block | Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: interplay of coeval contractional deformation and extensional gravitational collapse[END_REF]southwestern Ghana, Agyei-Duodu et al. 2009;Hirdes et al. 2007;[START_REF] Mcfarlane | The geodynamic and tectonic evolution of the Paleoproterozoic Sefwi greenstone belt, West African craton[END_REF]eastern Senegal, Dia et al. 1997;Hirdes and Davis 2002;[START_REF] Dioh | Diversity of the Paleoproterozoic granitoids of the Kédougou inlier (eastern Senegal): Petrographical and geochemical constraints[END_REF]Gueye et al. 2007).

Limited geochronology suggests the onset of diorite-granodiorite-tonalite-trondhjemite plutonism occurred as early as ~2250 Ma but became widespread across the sWAC after ~2200 Ma, with peak in magma flux between ~2160 and 2150 Ma (Agyei- [START_REF] Duodu | [END_REF][START_REF] Kock | Geochronology of the Birim Supergroup of the West African craton in the Wa-Bolé region of west-central Ghana: implications for the stratigraphic framework[END_REF][START_REF] Tshibubudze | The relative and absolute chronology of strato-tectonic events in the Gorom-Gorom granitoid terrane and Oudalan-Gorouol belt, northeast Burkina Faso[END_REF][START_REF] Petersson | Zircon U-Pb-Hf evidence for subduction related crustal growth and reworking of Archaean crust within the Palaeoproterozoic Birimian terrane, West African Craton, SE Ghana[END_REF][START_REF] Parra-Avila | The geochronological evolution of the Paleoproterozoic Baoulé-Mossi domain of the southern West African Craton[END_REF][START_REF] Mcfarlane | The geodynamic and tectonic evolution of the Paleoproterozoic Sefwi greenstone belt, West African craton[END_REF], 2019). However, the age and chemistry of extensive granitoid terranes between the greenstone belts and sedimentary basins remain largely unknown. These calc-alkaline plutonic rocks are not TTGs as per the definition of Moyen and Martin ( 2012), but were generated by partial melting of a mafic source [START_REF] Ganne | Modern-style plate subduction preserved in the Palaeoproterozoic West African craton[END_REF]. They intrude both the large granitoid domains and adjacent belts of volcano-sedimentary rocks throughout the Baoulé-Mossi domain.

The onset of deformation in the Baoulé-Mossi domain followed the switch from basic volcanism, regional extension and/or oceanic rifting of uncertain origin to a demonstrably active convergent margin setting with associated intermediate to felsic volcanism. Structural evidence associated with this early deformation, such as layer-parallel foliation and thrusts, was recognized across the Goren (Hein 2010), Bouroum-Yalago [START_REF] Vanin | Taparko-Bouroum Project, Burkina Faso[END_REF]), Bélahouro (McCuaig et al. 2016), Ouadalan-Gorouol (Tshibubuze et al. 2015), and Fada N' m Naba 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] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]) belts in Burkina Faso, and the Bolé-Bulenga Domain in NW Ghana [START_REF] Block | Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: interplay of coeval contractional deformation and extensional gravitational collapse[END_REF]. Although variability in the strain regime has been documented across distinct volcano-sedimentary belts and granitoid domains, this early deformation is correlated with crustal thickening (clockwise P-T-t paths), high-grade metamorphism (amphibolite-to granulite-facies), and subsequent partial melting of the crust [START_REF] Ganne | Thermo-mechnical modeling of lower crust exhumationconstraints from the metamorphic record of the Paleoproterozoic Eburnean orogeny, 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]. [START_REF] Ganne | Modern-style plate subduction preserved in the Palaeoproterozoic West African craton[END_REF][START_REF] Block | Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: interplay of coeval contractional deformation and extensional gravitational collapse[END_REF]. This collision was broadly coeval with the short-lived period of reworking of Archean crust between ca. 2140-2130 Ma documented in the Kibi belt and Suhum basin in southeastern Ghana (Leube et al., 1990;Taylor et al., 1992;[START_REF] Petersson | Zircon U-Pb-Hf evidence for subduction related crustal growth and reworking of Archaean crust within the Palaeoproterozoic Birimian terrane, West African Craton, SE Ghana[END_REF][START_REF] Petersson | Extensive reworking of Archaean crust within the Birimian terrane in Ghana as revealed by combined zircon U-Pb and Lu-Hf isotopes[END_REF]. Some authors associate this phase of early convergence with N-S-directed bulk crustal shortening based on the rarely observed orientations of ENE-striking fabrics and structures in the field [START_REF] Tshibubudze | The relative and absolute chronology of strato-tectonic events in the Gorom-Gorom granitoid terrane and Oudalan-Gorouol belt, northeast Burkina Faso[END_REF][START_REF] Block | Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: interplay of coeval contractional deformation and extensional gravitational collapse[END_REF]). These same authors interpreted the later switch to WNW-ESE to NW-SE-directed bulk crustal shortening at ~2120 Ma to reflect a change in boundary conditions applied to the lithosphere, such as a change in the vergence of subduction and/or direction of plate convergence at the onset of the " " orogeny. Alternatively, the Paleoproterozoic tectonic evolution of the sWAC may reflect incremental/progressive NW-SE-directed bulk crustal shortening [START_REF] Ledru | Transcurrent tectonics and polycyclic evolution in the Lower proterozoic of Senegal-Mali[END_REF][START_REF] Leake | The petrogenesis and structural evolution of the early Proterozoic Fettekro greenstone belt, Dabakala region[END_REF][START_REF] Eisenlohr | The structural development of the early Proterozoic Birimian and Tarkwaian rocks of southwest Ghana, West Africa[END_REF][START_REF] Feybesse | The paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF]Grenhölm et al. 2019), with these apparent structural components reflecting (i) heterogeneous strain distribution and/or (ii) the domainal development of structural fabrics and/or (iii) the rotation of early fabrics and structures during overprinting deformation.

Deep water -Mossi Domain (ca. 2135-2105 Ma)

Thick sedimentary rock series dominated by flysch sequences were deposited in major basins, some of which were > 200 km across, between ~2135 and 2105 Ma. The deposition of voluminous flysch sequences (e.g. Kumasi in Ghana, Sunyani-Como ' , Bagoé in Mali, Siguiri in Guinea, Dialé-Daléma in Senegal, Fig. 2a), derived from the erosion of continental crust constrains the minimum age of uplift following the aforementioned period(s) of metamorphism and deformation in the Baoulé-Mossi domain.

Major shear zones that commonly bound these basins probably initiated as extensional faults during this period. These fundamental structures persist throughout the following history of the craton outlined below, being reactivated several times, including during gold mineralization. Available geochronology data suggest diachronous development of these sedimentary basins, with an apparent younging in maximum depositional ages from SE to NW across the Baoulé-Mossi domain (Fig. 2a). This apparent diachroneity appears further supported by the general migration of a magmatic and tectono-metamorphic fronts across the sWAC from southeast to northwest with a ~5 to 15 myr " ", Hirdes et al. (1996Hirdes et al. ( , 2002) ) and recently by [START_REF] Parra-Avila | The geochronological evolution of the Paleoproterozoic Baoulé-Mossi domain of the southern West African Craton[END_REF]. The following description of the basin lithologies and their ages is largely a synthesis of data presented in [START_REF] Davis | Craton-scale lithostratigraphic correlation as an insight for the geodynamic evolution of the southern West African Craton[END_REF] and Grenhölm et al. (2019), which include comprehensive reviews of the ages of detrital zircon grains from clastic and volcano-sedimentary series in the Baoulé-Mossi domain.

Flysch-type sedimentary sequences that include sandstones, siltstones, and argillites with minor volcaniclastic rocks, chemical sedimentary rocks and volcanic rocks, were deposited in large scale basins across the Baoulé-Mossi domain (Milési et al. 1989;[START_REF] Feybesse | The paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF],

and together with the older volcano-sedimentary belts define the first-order architecture of the craton. These thick deep-water rock sequences have a similar overall stratigraphy at the scale of the sWAC. In NW Ghana, the Maluwe series was deposited between ~2135 and 2125 Ma (De [START_REF] Kock | Geological Map Explanation, Map Sheet 0803B[END_REF][START_REF] Kock | Geochronology of the Birim Supergroup of the West African craton in the Wa-Bolé region of west-central Ghana: implications for the stratigraphic framework[END_REF]. In SW Ghana, the Kumasi and Sunyani-Comoé series were deposited between ~2135 and 2110 Ma [START_REF] Davis | U-Pb age constraints on deposition and provenance of Birimian and gold-bearing Tarkwaian sediments in Ghana, West Africa[END_REF]Oberthür et al. 1998;Hirdes et al. 2007;[START_REF] Vidal | Evolution of a "w[END_REF][START_REF] Duodu | [END_REF][START_REF] Adadey | Geological map explanation -map sheet 0503 B at[END_REF]Grenhölm et al. 2019).

In contrast, the Bagoé series in Mali (Liégois et al. 1991;[START_REF] Wane | The onset of Eburnean collision within the Kénéma-Man craton evidenced by plutonic and volcanosedimentary rock record of the Massigui region, southern Mali[END_REF], the Siguiri series in Guinea [START_REF] Lebrun | Geochronology and lithostratigraphy of the Siguiri district: implications for mineralization in the Siguiri basin (Guinea, West Africa)[END_REF], and Kofi-Dialé-Daléma series in the Kédougou-Kénieba inlier (Hein et al. 2015;[START_REF] Js | Paleotectonic setting of the south-eastern Kédougou-Kénieba inlier, West Africa: new insights from igneous trace element geochemistry and U-Pb zircon ages[END_REF]Masurel et al. 2017c;[START_REF] Allibone | Paleoproterozoic golf deposits of the Loulo district, western Mali[END_REF] were deposited between ~2110 and 2095, ~2115 and 2095 Ma, and ~2120 and 2095 Ma, respectively. The restricted presence of impure carbonate rocks in the Siguiri, Kofi, and Dialé-Daléma series (Milési et al. 1989;[START_REF] Ledru | Transcurrent tectonics and polycyclic evolution in the Lower proterozoic of Senegal-Mali[END_REF][START_REF] Lebrun | Geochronology and lithostratigraphy of the Siguiri district: implications for mineralization in the Siguiri basin (Guinea, West Africa)[END_REF][START_REF] Js | Paleotectonic setting of the south-eastern Kédougou-Kénieba inlier, West Africa: new insights from igneous trace element geochemistry and U-Pb zircon ages[END_REF]) together with the similar maximum and minimum depositional ages suggest that these carbonate formations belong to the same regional-scale sedimentary basin and may reflect a hinterland-foreland setting relationship between eastern and western sWAC as first proposed by Grenhölm et al. (2019).

N w m

-Mossi Domain. Some authors favour a syn-orogenic foreland basin setting [START_REF] Feybesse | The paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF][START_REF] Block | Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: interplay of coeval contractional deformation and extensional gravitational collapse[END_REF] Multistage shortening and deformation of these sedi-mentary series between ca. 2110 and 2095 Ma resulted in the development of 1--10-km-scale isoclinal folds, associ-ated axialplanar cleavage, major intra-basin reverse shear zones, and reactivation of the basin margin faults as major reverse shear zones [START_REF] Hirdes | Reassessment of proterozoic granitoid ages in Ghana on the basis of U/Pb zircon and monazite dating[END_REF]Allibone et al. 2002aAllibone et al. , 2002b;;Oliver et al. 2020). Limited mapping indi-cates that the deep water series now comprise kilometrescale isoclinally folded slices imbricated along intra-basin faults that likely evolved from major isoclinal folds as shortening progressed. No intact, complete stratigraphic section has been recognised to date, precluding constraints on the original thickness of any deep water series. These folds and the major intra-basin and basin margin fa 2105 and 2085 Ma (Allibone et al. 2002a(Allibone et al. , 2002b[START_REF] Allibone | Paleoproterozoic gold deposits hosted by albite-and carbonate-altered tonalite in the Chirano district, Ghana, West Africa[END_REF][START_REF] Feybesse | The paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF]Oliver et al. 2020). ' , m N'Z -Brobo shear zone occurred after ca. 2100 Ma (Mortimer 1990;[START_REF] Leake | The petrogenesis and structural evolution of the early Proterozoic Fettekro greenstone belt, Dabakala region[END_REF]. Relatively minor, but economically significant, N-to NNE-striking primary strike-slip faults that refract across the earlier structural grain also developed during this period (Allibone et al. 2002a). series sedimentary rocks [START_REF] Schwartz | The Falémé iron district, Senegal[END_REF]Delor et al. 2010;Lawrence et al. 2013aLawrence et al. , 2013b;;Masurel et al. 2017c;Allibone et al. 2020). Stable isotope (B, C, O) data, however, imply that hydrothermal minerals within these gold deposits were largely derived from metamorphosed calcareous, Na-B-rich evaporitic, and siliciclastic sedimentary rocks within the Kofi series rather than from the skarns or associated granites (Lambert-Smith et al. Orogenic gold deposits that formed during this period may include the later mineralising event at the Obuasi deposit [START_REF] Fougerouse | Evidence for two stages of m ' deposit: Obuasi, Ghana[END_REF]Oliver et al. 2020), gold mineralisation at the major Ahafo deposit in western-most Ghana (Baah-Danso 2011; Masurel et al. 2021), the Siguiri deposit in Guinea [START_REF] Lebrun | Structural controls on an orogenic gold system: the world-class Siguiri gold district, Siguiri basin, Guinea, West Africa[END_REF], and both mineralising events at the Kalana deposit in southern Mali (Salvi et al. 2016c).

Amphibolite

The second mineralising event at the Obuasi deposit in-cludes some particularly high-grade parts of the resource (Fig. 8a) that formed during late transpressional reactivation of segments of the Obuasi and Main Reef shear zones, both older contractional structures active during earlier basin inversion. New mineralised faults also formed at this time, including the Ashanti Fissure at the northern end of the deposit and several minor faults in various prospects south of the deposit (Allibone et al. 2002a;[START_REF] Fougerouse | Evidence for two stages of m ' deposit: Obuasi, Ghana[END_REF]Oliver et al. 2020). It remains unclear whether this gold was remobilised from rocks mineralised during the Early Eburnean phase, or introduced during late Eburnean transpressional deformation [START_REF] Fougerouse | Gold remobilisation and formation of highgrade ore shoots driven by dissolution-reprecipitation replacement and Ni substitution into auriferous arsenopyrite[END_REF]. Some mineralised lodes in other deposits along the south-east margin of the Kumasi basin, such as the ~17 Moz Bogoso-Prestea gold system (Fig. 8b), are developed in similar structural settings to the later mineralised rocks at Obuasi (Allibone et al. 2002b). Gold Giant paleoplacer gold deposits are generally considered to form in retro-arc foreland basins, inboard of the accretionary system where detrital gold is transported, deposited, and reworked (Solomon and Groves 1994). Gold-bearing con-glomerates were deposited within the Tarkwa basin as it was deformed within an actively accreting margin (Figs. 10b andM . 1991; [START_REF] Davis | U-Pb age constraints on deposition and provenance of Birimian and gold-bearing Tarkwaian sediments in Ghana, West Africa[END_REF][START_REF] Pigois | Age constraints ont Tarkwaian paleoplacer and lode-gold formation in the Tarkwa-Damang district, SW Ghana[END_REF][START_REF] White | The nature and origin of gold minelization at Damang mine, Ghana[END_REF], Le Mignot et al. 2017b). Eoeburnean gold deposits such as those in the Ashanti belt in southwest -Mossi Domain at ca. 2110 Ma [START_REF] Ledru | Markers of the last stages of the Paleoproterozoic collision: evidence for a 2 Ga continent involving circum-South Atlantic provinces[END_REF][START_REF] Feybesse | The paleoproterozoic Ghanaian province: Geodynamic model and ore controls, including regional stress modeling[END_REF].

The relative proportions of orogenic gold deposits formed during the early and late Eburnean phases is uncertain because of the following: (1) the age of some major deposits remains mechanism that promoted permeability and fluid flow, thus driving the forma-tion of larger gold deposits [START_REF] Ledru | Transcurrent tectonics and polycyclic evolution in the Lower proterozoic of Senegal-Mali[END_REF](Ledru et al. M . 1992;;[START_REF] Lebrun | Structural controls on an orogenic gold system: the world-class Siguiri gold district, Siguiri basin, Guinea, West Africa[END_REF].

Comparison with the Neoarchean Yilgarn and Superior Cratons

Some aspects of , m -, wm -Mossi Domain close-ly resemble those of the Neoarchean Superior (Canada) and Yilgarn (Australia) cratons [START_REF] Robert | Gold metallogeny of the Superior and Yilgarn cratons[END_REF]. Orogenic cy-cle(s) in each region included a change from early mafic vol-canism to later siliciclastic and volcanoclastic sedimentation above unconformities, followed by bulk incremental deforma-tion, peak regional metamorphism, and granitic plutonism. Key geological features of the three most richly endowed gold subprovinces in these cratons: Ashanti, Abitibi, and Kalgoorlie, and compared in Fig. 12.

Waning volcanic activity in these subprovinces corresponded with an increase in clastic -Mossi Domain such as local versus regional deformation and progressive versus multiphase deformation, the three subprovinces expe-rienced a similar history of deformation to that outlined by [START_REF] Robert | Gold metallogeny of the Superior and Yilgarn cratons[END_REF]. Diachronous deformation has been demonstrated in all cratons, although this is not necessarily apparent at the belt scale [START_REF] Robert | Gold metallogeny of the Superior and Yilgarn cratons[END_REF][START_REF] Czarnota | Geodynamics of the Eastern Yilgarn Craton[END_REF][START_REF] Parra-Avila | The geochronological evolution of the Paleoproterozoic Baoulé-Mossi domain of the southern West African Craton[END_REF]. The diversity of gold mineralisation styles, ore and alteration assemblages, and overprinting relationships indicate sever w m , w w

. 2018). The diachronous nature of gold mineralisation in these cratons reflects the timing of terrane accretion in different supracrustal belts [START_REF] Robert | Gold metallogeny of the Superior and Yilgarn cratons[END_REF][START_REF] Vielreicher | The giant Kalgoorlie goldfield revisited[END_REF].

Despite these many similarities, there are also a number of important differences between these Paleoproterozoic and Neoarchean subprovinces, which reflect contrasting aspects of their geodynamic evolution and which have important implica-tions for exploration targeting (Fig. 12). Firstly, diachronous gold mineralisation in the Abitibi and Kalgoorlie subprovinces

  through space and time has improved our ability to explain the heterogeneous distribution of gold mineralisation in order to target prospec-tive areas more effectively (Groves and Bierlein 2007).

  in NW Ghana[START_REF] Siegfried | Geological Map Explanation, Map Sheet[END_REF]; (ii) the 2295 ± 19 to 2285 ± 11 Ma xenocrystic zircons from the 2253 ± 15 Ma Kel Enguef migmatitic orthogneiss in the Ouadalan-Gorouol belt in north-ern Burkina Faso (Tshibubudze et al. 2013); (iii) the 2312 ± 17 Ma inherited zircon cores from the 2154 ± 1 Ma Dabakala tonalite (Gasquet et al. ages obtained from Lu-Hf analyses on magmatic zircons (Parra-Avila et al.

  . The onset of basin formation in t -Mossi Domain broadly coincided with the cessation of the Eoeburnean orog-eny at ca. 2140 Ma. However, this temporal relationship does not support or preclude any of the alternative tectonic settings for basin formation. Coarser grained clastic rock sequences than those present in the aforementioned large-scale basins were locally deposited in northwest Ghana between ca. 2130 and 2120 M m et al. 2019). These rocks, referred to as the Bui (Kiessling et al. 1997) and Chako (Block et al. 2016) series, largely consist of sandstones and conglomerates, and have been interpretedas having been deposited predominantly in shallow water ries was deposited on older Eoeburnean-aged rocks, whereas Zitsmann et al.(1997) indicated that the Bui series overlies the Maluwe deep water series with a conformable to paraconformable contact (Fig.2a). The Bui and Chako series are interpreted as molasse, which was deposited in local depocentres following the collision between Eoeburnean-aged terranes of northwest Ghana and southwest Burkina Faso and the block of Archean material now confined to the lower crust of southeast Ghana and Togo at ca. 2135 Ma (De[START_REF] Kock | Geochronology of the Birim Supergroup of the West African craton in the Wa-Bolé region of west-central Ghana: implications for the stratigraphic framework[END_REF][START_REF] Block | Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: interplay of coeval contractional deformation and extensional gravitational collapse[END_REF].Deformation and metamorphism of the deep water sedimentary series during the onset under lower to upper greenschist-facies conditions between ca.2110 and 2095 Ma, during the early stages of the Eburnean orogeny[START_REF] Perrouty | Revised Eburnean geodynamic evolution of the gold-rich southern Ashanti belt, Ghana, with new field and geophysical evidence of pre-Tarkwian deformations[END_REF][START_REF] Lebrun | Geochronology and lithostratigraphy of the Siguiri district: implications for mineralization in the Siguiri basin (Guinea, West Africa)[END_REF] Oliver et al. 2020; Allibone et al. 2020). Deformed rocks of the Kumasi series were intruded by calc-alkaline biotite-gran-ites between ca. 2110 and 2105 Ma, and these intrusive rocks were affected by Eburnean deformation and metamorphism indicating both occurred after ca. 2110-2105 Ma[START_REF] Hirdes | Reassessment of proterozoic granitoid ages in Ghana on the basis of U/Pb zircon and monazite dating[END_REF][START_REF] Hirdes | Explanatory notes for the geological map of southwest Ghana 1:100000: sheets Wiawso (0603D), Asafo (0603C), Kukuom (0603B), Goaso (0603A), Sunyani[END_REF][START_REF] Davis | U-Pb age constraints on deposition and provenance of Birimian and gold-bearing Tarkwaian sediments in Ghana, West Africa[END_REF] calc-alkaline diorite and quartzfeldspar-porphyry at ca. 2105 Ma (Hein et al. 2015; Masurel et al.

  , obvious in geological maps of the craton (Fig.1). Syn-metamorphic quartz-car-bonate veining occurred throughout the history of basin inversion[START_REF] Fougerouse | Evidence for two stages of m ' deposit: Obuasi, Ghana[END_REF] Oliver et al. 2020 Oliver et al. ). al. 2020)). Field structural relationships imply docking of the Archean and Paleoproterozoic domains was associated with region-wide dextral reactivation of major shear zones such as the Sassandra fault, Banifing shear zone, between ca. 2110 and 2095 Ma (Fig.2a). Sedimentary rocks of these series largely comprise con-glomerates and cross-stratified to planar-bedded sand-stones, with subordinate siltstones and rhyolitic volcanic rocks also occurring locally in some series. Conglomerates are typically polym , -Avila et al. 2015; Lebrun et al. 2016)Adadey et al. 2009; Perrouty et al. 2012). Deposition of the Tarkwa series was coeval with intrusion of ca. 2100 Ma gabbro sills into the clastic sedimentary rocks (Adadey et al. 2009; Perrouty et al. 2012; White et al. 2015). These shallow water to subaerial series have been interpreted as molasse deposited in short-lived basins be-cause their deposition ages overlap with the crystallisation ages of crosscutting syn-kinematic granites (Tunks et al. 2004; Goldfarb et al. 2017). Their internal structural char-acter implies they were deposited during increment N --w m -M . 1998; Feybesse et al. 2006; Baratoux et al. 2011; Masurel et al. 2017c; Wane et al. 2018). Domain to deformed and metamorphosed Paleoproterozoic sedimentary rocks of the Siguri Series (Lebrun et al. 2016; Eglinger et al. 2017). These relationships indicate docking of the Archean and Paleoproterozoic rocks occurred at ca. 2095 ± 5-10 Ma. Docking of the two domains appears to have coincided with dextral reactivation of t , , w -Mossi Domain (Fig. 1). Associated deformation may also have resulted in the oroclinal bending of the Siguri Series and Seikorole shear zone, such w -w m m -Man Domain, whereas the general structural grain of rocks to the north and east typically strikes north to NNE (Fig. 1). The direction of converg w from either the southwest or west, in-denting the southwest margin of the Baoul -Mossi Domain. Late Eburnean strike-slip to oblique-slip tectonics and crustal reworking following arccontinent collision (ca. 2095-2060 Ma) et al. 2002a, 2002b; McFarlane et al. 2019b; Oliver et al. 2020). Strain during this period was largely localised along major shear zones that form geological domain boundaries with little mod-ification to the pre-existing struct m et al. 2017b, 2017c; Allibone et al. 2020). In SW Ghana, segments of the Ashanti-Konongo-Prestea, Bibiani, and Kenyase-Yamfo shear zones were reactivated as sinistral-reverse shear zones between ca.

  during this period. Granulite-facies mineral assemblages developed within ductile shear zones along the northern m et al. 2017, Fig.2b). These magmas formed (i) by extraction of juvenile melt from a depleted mantle source that had b . This late episode of volcanism is referred to as the Bandamian Cycle(Hirdes et al. 199 M , al. 2020). Eruption of juvenile mafic-ultramafic volcanic rocks through crust thickened during the earlier docking of the Archean and Paleoproterozoic domains is inferred to reflect upwelling of the asthenosphere and associated extension caused by litho-spheric delamination or slab breakoff[START_REF] Eglinger | New insights into the crustal growth of the Paleoproterozoic margin of the Archean Kénéma-Man domain, West African craton (Guinea): Implications for gold mineral system[END_REF] such rocks are absent from northern and eastern parts of the domain. These rocks intrude sedimentary 2079 ± 2 Ma,Hirdes and Davis 2002) in southern Mali and Senegal respec-tively. Together, these high-K calc-alkaline and peraluminous granites form a in Guinea and throughout the Paleoproterozoic rocks of central-southern Mali.Peraluminous two-mica leucogranites formed by either partial melting of metasedimentary source rocks[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], and/or older calc-alkaline granites[START_REF] Tapsoba | Chemical and Sr-Nd isotopic compositions and zircon U-Pb ages of the Birimian granitoids from NE Burkina Faso, West African Craton: Implications on the geodynamic setting and crustal evolution[END_REF]Domain. Extension and unroofing of deep crustal rocks in the Sefwi belt at ca. 2075 Ma (McFarlane et al. 2019a) may also have been another expression of this asthenospheric upwelling and associated lithospheric delamination. in this section are divided into Eoeburnean, Early Eburnean, and Late Eburnean groups, which formed between ca. 2200 and 2135 Ma, ca. with those discussed by Groves et al. (1998) and Goldfarb et al. (2001, 2005), Sillitoe and Thompson (1998)m and Lang and Baker (2001), respectively. Eoeburnean gold deposits (ca. 2200-2135 Ma) of gold are hosted by amphibolite-facies biotite-kyanite-garnetamphibole-bearing schists within the Tenkodogo-N' m D mBurkina Faso[START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF] Gauriau et al. 2020). The early disseminated gold event atKiaka occurred during sinistral-reverse displacement along the NNE-striking Markoye shear zone at 2157 ± 24 Ma[START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF], broadly coeval with region-wide lower amphibolite-facies metamorphism and intrusion of a syn-kinematic diorite at 2140 ± 7 Ma are from[START_REF] Markwitz | Compilation of West African mineral deposits: Spatial distribution and mineral endowment[END_REF]. The geology legend is identical to that used in Fig.1(Fontaine et al. 2017). An early orogenic gold event within the ~8 Moz Mana gold district in western Burkina Faso oc-curred contemporaneously with intrusion of the syn-kinematic 2172 ± 6 Ma Wona-Kona and 2176 ± 8 Ma Siou plutons during E-W-directed Eoeburnean shortening and regional up-per greenschist-facies metamorphism (Augustin et al. 2017). The ~0.75 Moz Julie gold deposit in NW Ghana is focussed along E-W-striking structures that developed between ca. 2145 and 2130 Ma (Amponsah et al. 2016; Salvi et al. 2016b), broadly coeval with regional amphibolite-facies metamorphism at 2137 ± 8 Ma (Block et al. 2016). Early orogenic gold mineralisation at the ~10 Moz Wassa deposit in SW Ghana (Perrouty et al. 2015) occurred after emplace-ment of the host porphyry at 2191 ± 6 Ma but before emplacement of the crosscutting Benso granodiorite at 2157 ± 5 Ma (Parra-Avila et al. 2015). Pyrite grains aligned and stretched within the early fabric at Wassa host gold particles (Perrouty et al. 2016) crystallised at 2164 ± 14 Ma (Le Mignot et al. 2017b). The geometry of the Wassa deposit is controlled by a kilometre-scale NE-striking upright synform (Fig. 4a, b), which affected rocks of both the Sefwi volcanic series and the Tarkwa molasses series. The S1 foliation and co-planar gold-bearing quartz veins, which host most of the gold re-source, are both folded by the Wassa synfom (Perrouty et al. 2016).Gold skarns also formed locally during the latter part of the Eoeburnean period. The ~3.8Moz Tongon gold deposit (Fig.3) is confined to a deposit formed between 2139 ± 21 Ma and 2128 ± 21Ma[START_REF] Lawrence | The , ' m mineralization[END_REF], before emplacement of the post-mineralisation Tongon granodiorite at 2139 ± 21 Ma[START_REF] Lawrence | The , ' m mineralization[END_REF]. Despite these imprecise dates, crosscutting relationships have been used to suggest that the ore-forming process occurred during the culmination of Eoeburnean deformation contemporaneously with peak meta-morphism in northwest Ghana and western Burkina Faso at ca. 2140 Ma[START_REF] Lawrence | The , ' m mineralization[END_REF].Early Eburnean gold deposits (ca. 2110-2095 Ma)Both orogenic and paleoplacer deposits formed during the Early Eburn ,[START_REF] Davis | U-Pb age constraints on deposition and provenance of Birimian and gold-bearing Tarkwaian sediments in Ghana, West Africa[END_REF][START_REF] Pigois | Age constraints ont Tarkwaian paleoplacer and lode-gold formation in the Tarkwa-Damang district, SW Ghana[END_REF][START_REF] Tunks | Vein mineralization at the Damang gold mine, Ghana: controls on gold mineralization[END_REF]).Gold-bearing quartz pebble conglomerates from the Banket series at Tarkwa (Main and Western Reefs, Fig.5a) and lower grade reefs at Damang (Fig.5b) were deposited in a fluvial/lacustrine setting in a larger foreland basin developed on Eoeburnean Ashanti belt basement[START_REF] Davis | U-Pb age constraints on deposition and provenance of Birimian and gold-bearing Tarkwaian sediments in Ghana, West Africa[END_REF][START_REF] White | The nature and origin of gold minelization at Damang mine, Ghana[END_REF]). The ages of detrital zircons in the have suggested that both orogenic gold deposits which formed during the early stages of inversion of the Kumasi series as well as older Eoeburnean gold deposits such as Wassa could have contributed gold to the Tarkwa paleoplacer deposits. This interpretation remains controversial as δ13CCO2 values of high-density CO2-rich, water-poor inclu-sions hosted in quartz pebbles from the paleoplacer deposits at Tarkwa differ considerably from the δ13CCO2 values of similar high-density CO2-rich inclusions in vein quartz from the m m . 2015). At Obuasi and other similar deposits haloes of auriferous arsenopyrite up to 40 m wide developed around mineralised reverse shear zones and along isoclinal fold hinges (Fig. 6a, bage constraints indicate that regional greenschist-facies metamor-phism of the Kumasi series was coeval with gold mineralisation at ca. 2095 Ma (Fougerouse et al. 2017; Oliver et al. 2020). Ongoing shortening of the Kumasi series, or subsequent transpression in the final stages of basin inversion, led to the formation of other gold deposits elsewhere along both margins of the Kumasi basin (~17 Moz Prestea-Bogoso gold deposit, Appiah 1991; Allibone et al. 2002b), and shear zones within the Kumasi basin (~0.5 Moz Pampe gold deposit, Salvi et al. 2016a). Depositscale structural rela-tionships at the ~7.1 Moz Edikan (Tourigny et al. 2018), the ~8.4 Moz Akyem (Newmont annual report 2017), the ~4.9 Moz Chirano (Allibone et al. 2004), and the ~6.8 Moz Obotan-Esasse (Asanko Mining annual report 2018) gold de-posits (Fig. 3) suggest a similar timing of mineralisation to that at Obuasi. Several orogenic gold deposits in Burkina Faso and N (Augustin et al. 2017). Fabric-mineral-vein crosscutting relationships and the age of the host rock sequence at the ~1.9 Moz Inata deposit in the Bouroum belt of northern Burkina Faso indicate that gold deposition occurred after ca. 2120 Ma (McCuaig et al. 2016), whereas Re-Os dating of pyrite implies gold mineralisation occurred at 2090 ± 29 Ma (Fougerouse 2011). The ~6.5 Moz Essakane gold deposit in northern Burkina Faso formed during sinistral transpressional reactiva-tion of the Markoye shear zone during the onset of the Eburnean orogeny (Tshibubudze and Hein 2016). The second mineralising event at Kiaka occurred during retrogression from lower amphibolitefacies to upper greenschist-facies conditions coeval with the reactivation of the Markoye shear between ca. 2125 and 2090 Ma, whose timing is bracketed by the emplacement of pre-and post-mineralisation granites[START_REF] Castaing | Carte géologique et minière au 1:1000 000 du Burkina Faso[END_REF][START_REF] Fontaine | Geology of the world-class Kiaka polyphase gold deposit, West African Craton, Burkina Faso[END_REF]. The age of the orogenic gold overprint on the Gaoua Eoeburnean porphyry-Cu system is not known but may have occurred between ca. 2100 and 2090 Ma, during the waning stages of the Eburnean tectono-thermal event in Burkina Faso (LeMignot et al. 2017a). The ~0.1 Moz Bepkong gold deposit in the Wa-Lawra belt of NW Ghana is associated with structures that developed during sinistral displacement along the first-order Jirapa shear zone at ca. 2100 Ma implying it also formed during the Early Eburnean period[START_REF] Amponsah | Geology and geochemistry of the shear-hosted Julie gold deposit, NW Ghana[END_REF][START_REF] Block | Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: interplay of coeval contractional deformation and extensional gravitational collapse[END_REF].Orogenic gold mineralisation, comparable w -Mossi Domain, is much rarer in the western part. Part of the gold resource at the ~13.5 Moz Siguiri deposit in Guinea may have developed during the Early Eburnean period,[START_REF] Lebrun | Structural controls on an orogenic gold system: the world-class Siguiri gold district, Siguiri basin, Guinea, West Africa[END_REF]. The maximum age for gold mineralisation in the Siguiri area is provided by the youngest detrital zircon age population from the Fatoya sedimentary host rocks, which is 2113 ± 5 Ma[START_REF] Lebrun | Geochronology and lithostratigraphy of the Siguiri district: implications for mineralization in the Siguiri basin (Guinea, West Africa)[END_REF].Other less common styles of gold mineralisation developed during Early Eburnean time.These include the early formed intrusion-related Au-(Sb, Bi, Te) mineralisation at the ~7.5Moz Morila gold deposit within the amphibolite-facies aure-ole of a tonalite-granodioriteleucogranite intrusive complex emplaced between 2098 ± 4 Ma and 2091 ± 4 Ma[START_REF] Mcfarlane | Geology and Intrusion-Related Affinity of the Morila Gold Mine, Southeast Mali[END_REF]. The association between early mineralisation at Morila, Te-Bi minerals, and leucogranite veinlets imply mineralisation was directly associated with granite magmatism. many gold deposits that formed during this period are comparable with ~12 Moz Sadiola district occurred between ca. 2090 and 2060 Ma coe-val with emplacement of nearby high-K granites, but after basin inversion and metamorphism of the host Kofi series(Masurel et al. 2017a

  2016, 2020). Only the gold prospects within the imme-diate vicinity of the dyke swarm alo m m the direct input of magmatic fluid during mineralisation (Lambert-Smith et al. 2020). ' , the ~2.5 Moz Bonikro gold de-posit (Ouatarra 2015) includes early intrusion-related gold mineralisation (Fig. 7b) formed at 2086 ± 4 Ma (Masurel et al. 2019) and later orogenic gold mineralisation formed at 2074 ± 16 Ma (Re-Os on molybdenite, Masurel et al. 2019). At Morila, the Early Eburnean intrusion-related gold (Bi, Sb, Te) mineralisation was overprinted by orogenic gold (As) mineralisation at 2074 ± 14 Ma (McFarlane et al. 2011). At both Morila and Bonikro, the later orogenic gold event ac-counts for the bulk of the gold resource. Orogenic gold mineralisation that occurred during the later part of the Eburnean orogeny (ca. 2095-2060 Ma) formed along retrograde P-T-t paths, and was associated with either local transpression, wrench, or transtensional deformation (Goldfarb et al. 2017), and likely cannot be correlated across the whole region.

  mineralisation in the ~10 Moz Ahafo district formed during transpressional sinistral-reverse reactivation of the Kenyase-Yamfo fault system(Baah-Danso 2011al. 2007;[START_REF] Jessell | Crustal-scale transcurrent shearing in the Paleoproterozoic Sefwi-Sunyani-Comoé region, West Africa[END_REF]. Gold mineralisation in the ~13.5 Moz Siguiri district in Guinea occurred during transpression and transient transtension after region-wide compressional de-formation at ~2095 Ma[START_REF] Lebrun | Structural controls on an orogenic gold system: the world-class Siguiri gold district, Siguiri basin, Guinea, West Africa[END_REF]. At the ~2 Moz Kalana deposit further east of the Siguiri basin, gently dip-ping auriferous quartz veins localised along thrust faults and near vertical auriferous quartz veins in extensional fractures crosscut the regional penetrative schistosity and were interpreted , but have also been described at the Obotan deposit in the Kumasi basin and the Damang deposit in the Ashanti belt[START_REF] Tunks | Vein mineralization at the Damang gold mine, Ghana: controls on gold mineralization[END_REF]). The orogenic gold event that overprinted paleoplacer gold at Damang is hosted within a contractional subhorizontal fault-fracture mesh and associated quartz vein arrays[START_REF] Tunks | Vein mineralization at the Damang gold mine, Ghana: controls on gold mineralization[END_REF][START_REF] White | The nature and origin of gold minelization at Damang mine, Ghana[END_REF] White et al. 2015, Fig. 9a, b), whose associated alteration haloes include hydrothermal xenotime dated at 2063 ± 9 Ma[START_REF] Pigois | Age constraints ont Tarkwaian paleoplacer and lode-gold formation in the Tarkwa-Damang district, SW Ghana[END_REF]. Late gold mineralisation at Wassa, dated at 2055 ± 18 Ma (Re-Os on pyrite, LeMignot et al. 2017b), is hosted by quartz veins that crosscut the earlier foliation which hosts auriferous pyrite[START_REF] Perrouty | The Wassa deposit: a poly-deformed orogenic gold system in southwest Ghanaimplications for regional exploration[END_REF]. -tivity had ceased by this time, whereas the younger ages ob-tained from hydrothermal minerals at the Damang and Wassa deposits indicate hydrothermal activity continued for a further 10 to 20 myr in these locations. Such differences in timing reflect the progressive waning of orogenic as Orisian (ca. 2000 Ma) deformation and plutonism in parts of northeast South America. hosts sev-eral minor porphyry-Cu and VMS base metal occurrences as well as the much more widespread gold deposits described above (Fig.3). The distribution and age of these porphyry-Cu and VMS occurrences provide additional clues as to the tectonic context of the formation of nearby gold deposits of broadly the same age. In Burkina Faso, porphyry-Cu mineralisation at Gaoua was b -diorite and 2172 ± 9 Ma post-mineralisation Gongondy gabbro[START_REF] Brownscombe | The Tinga anomaly: A new style of gold mineralization in Ghana?[END_REF]. The ages and spatial distribution of the Gaoua porphyry-Cu and Perkoa-style VMS occurrences suggest western Burkina Faso was located in an arc/back-arc setting[START_REF] Seedorff | Porphyry deposits: characteristics and origin of hypogene features[END_REF][START_REF] Franklin | Volcanogenic massive sulfide deposits[END_REF] throughout much of Eoeburnean time between ca. 2180 and 2140 Ma (Figs. 10a and 11). Eoeburnean orogenic gold mineralisations such as Kiaka (I), Julie, and Wassa (I) are interpreted to have formed during the inver -; (2) the higher metamorphic grade and lack of preservation of prospective upper to mid-crustal rocks in parts of northwest Ghana and eastern Burkina Faso; and (3) reworking of dehydrated, un-prospective felsic Archean material by calc-alkali --m m -Mossi Domain at ca. 2140 Ma (De Kock et al. 2011; Petersson et al. 2016). This Archean ba m , w m m -Man nucleus, is inferred to occur in southeast Ghana and Togo based on xenocrystic zircon cores (De Kock et al. 2011), Lu-Hf data[START_REF] Petersson | Zircon U-Pb-Hf evidence for subduction related crustal growth and reworking of Archaean crust within the Palaeoproterozoic Birimian terrane, West African Craton, SE Ghana[END_REF][START_REF] Petersson | Extensive reworking of Archaean crust within the Birimian terrane in Ghana as revealed by combined zircon U-Pb and Lu-Hf isotopes[END_REF], and the presence of Archean detrital zircons in

  the sources of gold in the Tarkwa paleoplacer deposit(Kl m . (2015). However, given the protracted nature of metamorphism, gold mineralisation, and associated deformation along the eastern margin of the Kumasi basin[START_REF] Eisenlohr | The structural development of the early Proterozoic Birimian and Tarkwaian rocks of southwest Ghana, West Africa[END_REF]

  unknown (Syama); (2) the relative proportions of gold precip-itated during different events in multistage deposits is poorly defined (Obuasi); and (3) the amount of gold erodedMignot et al. 2017b; Goldfarb et al. 2017 and references therein). Many deposits either formed (Loulo, Sadiola) or were upgraded (Obuasi) during late Eburnean tectono-thermal activity be-tween ca. 2095 and 2060 Ma, soon after the change from orogen-perpendicular shortening to prolonged, diachronous, multistage transpressional to transtentional tectonics at ca. resulted in major structures being unfavourably oriented for dip-slip displacement. Stress field switches such as this have been proposed as a

  Alternatively, late Eburnean granitic plutonism and orogenic gold mineralisation could also reflect the strike-slip reactivation of trans-lithospheric faults and lithostratigraphic boundaries during lateral escape of the crust, which induced decompression melting and to span much of Eburnean orogeny from ca. 2098 Ma (Morila) through to ca. 2074 Ma (Bonikro). Intrusion-related gold deposits worldwide typically formed in back-arc settings or on deformed accretionary margins(Sillitoe and Thompson 1998; Lang and Baker 2001, Fig. 10c). . In French Guiana, an early gold event was associated with a period of crustal growth and mag-matic accretion, following the production of an older basaltic crust. A second orogenic gold event coeval with deposition of gold paleoplacers occurred during a period of collision and crustal reworking. Thi m -Mossi Domain.Although two or more pulses of Paleoproterozoic gold mineralisation appear to have occurred within the Guiana Shield, limited data suggests many of the discovered gold deposits formed during sinistral shearing along major crustal discontinuities, Hydrothermal titanite at the Karouni orogenic gold deposit in Guyana crystallised at 2084 ±14 Ma[START_REF] Tedeschi | The Karouni gold deposit, Guyana, South America: Part I. Stratigraphic setting and structural controls on mineralization[END_REF]. These dates correlate closely with the major late Eburnean period of orogenic gold mineralisation in West Africa. Younger gold mineralisation occurred after ca. 2030 Ma in the Guiana Shield (Tedeschi et al. 2018), whereas no gold min m -Mossi to date. The geolog-ical context of this younger mineralisation within the Guiana Shield remains unclear.

  . The Porcupine Group consists of turbiditic rocks several kilometres thick, which were deposited in a deep water setting. The younger fault-bounded Timiskaming Group consists, and Superior Craton (Pontiac, Quetico, English River, Frieman et al. 2017) have no equivalents in the 2008; Squires et al. 2010; Czarnota et al. 2010; Vielreicher et al. 2016) cratons with the Paleoproterozoic terranes of the Baoul -Mossi Domain (this study and references therein) Neoarchean Yilgarn Craton. Sedimentary rocks comparable with the deep water series in the Ashanti and Abitibi subprovinces may be represented in the Kalgoorlie subprovince by the Merougil series (Squires et al. 2010). Late orogenic coarse-clastic rock successions in the Kalgoorlie subprovince, such as Kurrawang series (Krapez et al. 2000), are more comparable in sedimentological character and scale with molasse, than deep water rock sedimentary sequences. Plutonism accompanied in-termediate to felsic volcanism and outlasted clastic sedimentation in all three subprovinces. In addition, there is a common evolu-tion from early mafic to intermediate plutons and/or TTGs, to high-K granites and/or two-mica-granites throughout the orogen-ic cycle (in the Yilgarn Craton, Zibra et al. 2020; in the West African Craton, McFarlane et al. 2019a). D m m w

  layered mafic-ultramafic complex, which intruded Archean crust. Maficultramafic layers within the complex consist of lower MgO websterite, pyroxenite, peridotite, and gabbro-norite and were genetically related to the Ni-Cu-(PGE) mineralisation that occurred between 2091 ± 18 and 2080 ± 13 Ma (Gouedji et al. 2014). Thirdly, the tectonostratigraphic setting of the most richly endowed parts of the three cratons differs. Major gold districts in the Superior and Yilgarn cratons such as Sunrise Dam in the Kalgoorlie subprovince and Val-' , w wm -Mossi Domain occurs either within early Eburnean large-scale sedimentary basins or along fault sys-tems that coincide with their margins (Goldfarb et al. 2017; Oliver et al. . Such late volcanism is absent in both the Neoarchean Yilgarn and Superior cratons, and this may reflect the intrinsic buoyancy of Archean lithospheric roots due to their high Mg and low Fe contents, which inhibited lithospheric de-lamination thus preventing the generation of widespread post-orogenic melts (Begg et al. 2009). Conclusions m -Mossi Domain formed during three pulses within a >100-myr-long period of crustal growth and differentiation. The spatial and temporal distribution of known gold deposits is consistent with accretion-collision tectonics, characterised by a complex interplay between deformation, crustal thickening, burial, metamorphism, partial melting, and exhumation. The most prolific period for ore deposition correlated with the Eburnean orogeny. Changes in the geodynamic setting, which affected the kinematics of Eburnean deformation and the char-acter of associated plutonism, coincided with increases in the flux of gold mineralisa -Mossi Domain between ca. 2110 and 2060 Ma. This review raises at least one fundamental question. Was the total gold budget acquired right from the earliest geodynamic stages through accretion of juvenile crust and then Domain, about which little is known and which remain largely under-explored, offer prospective targets for economically significant Archean and/or Paleoproterozoic gold ores (New Liberty, Ndablama, and Kana Hills deposits in Liberia; Baomahun and Kalmaro deposits in Sierra Leone, Foster and Piper 1993; Markwitz et al. 2016). Augustin J, Gaboury D (2017) Paleoproterozoic plume-related basaltic rocks in the Mana district in western Burkina Faso, West Africa: Implications for exploration and the source of gold in orogenic gold deposits. J Afr Earth Sci 129:17-30 Augustin J, Gaboury D, Crevier M (2017) Structural and gold mineralizing evolution of the world-class orogenic Mana district, Burkina Faso: multiple mineralizing events over 150 Ma. Ore Geol Rev 91:981-1012 Ayer J, Amelin Y, Corfu F, Kamo S, Ketchum J, Kwok K, Trowell N (2002). Evolution of the southern Abitibi greenstone belt based on U-Pb geochronology: autochtonous volcanic construction followed by plutonism, regional deformation and sedimentation. Precambr Res 115:63-95 Baah-Danso E (2011) The structural evolution of the Subika deposit, Ahafo, Sefwi belt, Ghana. MSc. thesis, The University of Western Australia, 82p Ballo I, Hein KA, Guindo B, Sanogo L, Ouologuem Y, Daou G, Traore A (2016) The Syama and Tabakoroni goldfields, Mali. Ore Geol Rev 78:578-585 Baratoux L, Metelka V, Naba S, Jessell MW, Grégoire M, Ganne J (2011) Juvenile Paleoproterozoic crust evolution during the Eburnean orogeny (ca. 2.2-2.0 Ga), western Burkina Faso. Precambr Res 191:18CJ, Deen T, Bowden P (2009) The lithospheric architecture of Africa: Seismic tomography, mantle petrology, and tectonic evolution: Geosphere 5:25-50 Bierlein FP, Groves DI, Cawood PA (2009) Metallogeny of accretionary orogens -The connection between lithospheric processes and metal endowment. Ore Geol Rev 36province. Econ Geol 112:123-143 Goldfarb RJ, Baker T, Dubé B, Groves DI, Hart CJR, Gosselin P (2005) Distribution, character, and genesis of gold deposits in metamorphic terranes. Econ Geol 100th Anniversary Volume:407-450 Goldfarb R, Groves D, Gardoll S (2001) Orogenic gold and geologic time: a global synthesis. Ore Geol Rev 18:1-75 Grenhölm M, Jessell M, Thébaud N (2019) A geodynamic model for the Paleoproterozoic (ca. 2.27-1.96 Ga) Birimian orogen of the southern West African Craton -Insights into an evolving accretionary-collisional orogenic system. Earth Sci Rev 192:138-193 Groves DI, Bierlein FP (2007) Geodynamic settings of mineral deposit systems. J Geol Soc London 164:19-30 Groves DI, Goldfarb RJ, Gebre-Mariam M, Hagemann SG, Robert F (1998) Orogenic gold deposits: A proposed classification in the context of their crustal distribution and relationship to other gold deposit types. Ore Geol Rev 13:7-27 Gueye M, Siegesmund S, Wemmer K, Pawlig S, Drobe M, Nolte N, Layer P (2007) New evidence for an early Birimian evolution in the West African Craton: an example from the Kédougou-Kénieba inlier, southeast Senegal. S Afr J Geol 110:511

  overlap with this range of xenocrystic zircon ages. Detrital

	zircon grains in younger sedimentary rocks gradually inc
	M , w	m	-	M	m	. 2019).

  The style of this early deformation reflects the dominance of horizontal bulk crustal shortening over more local accommodation mechanisms such as ascending magmas into the crust, and/or doming of partially molten crust[START_REF] Ganne | Modern-style plate subduction preserved in the Palaeoproterozoic West African craton[END_REF][START_REF] Block | Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: interplay of coeval contractional deformation and extensional gravitational collapse[END_REF]. The maximum age for the onset of deformation remains poorly constrained but occurred sometime after ~2200 Ma. The minimum age for crustal thickening and partial melting was interpreted from leucosome-

hosted monazite grains dated at 2137 ± 8 Ma in NW Ghana

[START_REF] Block | Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: interplay of coeval contractional deformation and extensional gravitational collapse[END_REF]

. Peak metamorphism in the Mana region in western Burkina Faso has been similarly dated at 2135 ± 11 Ma and correlated with metamorphic zircon overgrowths on magmatic zircons from the 2176 ± 8 Ma Siou granodiorite (U-Pb on zircon,

Augustin et al. 2017

). These results are also consistent with HP-LT blueschist-facies series metamorphism (P>10 kbar, T<450°C) of supracrustal rocks in the Tenkodogo N' urma domain in eastern Burkina Faso

[START_REF] Ganne | Modern-style plate subduction preserved in the Palaeoproterozoic West African craton[END_REF]

). These early tectono-metamorphic events have been historically referred to as " " (Hein et al. 2010 in Burkina Faso) " " (De Kock et al. 2011 in northwest Ghana). The Eoeburnean orogeny culminated with the collision between independent crustal blocks in modern-day northwest Ghana and southwest Burkina Faso at ca. 2135 Ma (

Acknowledgements

AMIRA International, the industry sponsors, and sponsors in kind are gratefully acknowledged for their support of the WAXI-3 project (P934B). This study was also partly supported by the Hammond and Nisbet fellowship at the University of Western Australia.

This study is one of the key outcomes of collaborative research and we wish to recognize our WAXI-3 research colleagues from partner institutions worldwide. The authors also wish to acknowledge the excellent work undertaken by the Bureau de Recherches Géologiques et Minières (BRGM) more than 25 years ago (Milési et al. 1991(Milési et al. , 1992)), which provided a solid foundation for this study. Finally, the authors would like to thank Jochen Kolb, Richard Goldfarb, and Thomas Blenkinsop for their constructive feedback, which certainly helped improve the quality of the manuscript.


Nicola Ciarfaglia

Jany Dandurand

Antonio Laezza

Antonietta Pepe

Louise Lods

Antoine Lonjon
email: antoine.lonjon@univtlse3.fr

Brigida Bochicchio

A Lods

J Lonjon

A Dandurand

B. Pepe

Thermal and dynamic mechanical behavior of poly(lactic acid) (PLA)-based electrospun scaffolds for tissue engineering

Keywords: biomaterials, degradation, ditîerential scanning calorimetry, fibers, mechanical properties

Electrospun scaffolds can find numerous applications, including biomedical; for example, tissue engineering. Poly-L-lactic acid is considered suitable for these applications, but its low-thermal stability and its poor mechanical prop erties limit this polymer use. The aim of this work is to obtain a modulation of the final scaffolds characteristics such as fibers dimension, wettability, elastic ity, and resistance to rupture through the choice of the polymers to be electrospun. Different electrospun scaffolds containing gelatin, Poly-DL-lactic acid, different percentage s of cellulose nanocrystals and an elastin peptide have been produced. Thermal stability, physical structure, and its mechanical behavior have been studied. Results suggest that the electrospun scaffolds show better thermal and mechanical properties than bulk materials; that is, the scaffolds with the best hydrophilic and thermomechanical properties are the samples containing 3% (wt/wt) of CNCs and elastin peptide.

INTRODUCTION

Electrospinning is widely used to prod uce materials made of fi bers with regular diameters ranging from 2 nm to several micrometers from solutions of polymers under a strong electric field in the order of thousands of Volts. The added value of electrospinning consists in the possi bility to easily tune fi bers diameter and porosity, that is, the ratio between the volume of voids called pores and the total volume of the material considered, together with the surface-to-volume ratio as well.1 These findings are considered appealing characteristics in the perspective of the use of electrospun fibers as potential tools in biomedi cal devices and innovative materials as well. For example, they are known as biomedical devices, 2 used in drug delivery and tissue engineering,3 and as fùters, protective textile, catalysts and storage cells for hydrogen fuel cells. 4 Furthermore, in the Jast decades electrospun fibers were widely investigated because their production requires minimum consumption of solvents. 5 In the last few years, many electrospun scaffolds have been successfully produced from different polymers, such as polyamide 11 for kidney cells culturing, 6 poly-(vinyli dene fluoride), in order to enhance the piezoelectric phase,7 polyurethane for bone tissue regeneration, 8 poly (-caprolactone) for potential dental applications, 9 and in blends with polyethylene glycol as a drug delivery system, 10 and many others. Furthermore, it is even possible to electrospun blends of polymers, as done for example by Oliveira Lobo et al.11 Poly-L-lactic acid (PLA) is the best candidate of poly mers used for the production of electrospun scaffolds. Actually, it is considered ideal for biomedical applica tions and tissue engineering. 13 -15 Nevertheless, it is weak for the low-thermal stability and the poor mechanical properties, which make difficult its use in biomedical applications. The choice of proper polymeric blend could affect the mechanical properties of the scaffold For example, silicon dioxide nanoparticles, 16 or cellulose nanocrystals (CNCs) have been used as nanofillers for their ability to confer strength to the material.17

CNCs are the product of hydrolysis of cellulose fibers composed of amorphous and crystalline regions. 1 8 Several papers concem the use of CNCs and nano whiskers as reinforcing agents of polymeric matrices composed of polyvinyl acetate, polyethylene oxide. 1 9-22 Finally, CNCs was added to PLA in electrospun scaffolds, for applications in tissue engineering. 23 The aim of this work is to elaborate scaffold nature inspired and biodegradable polymers. The working hypothesis was the modulation the properties such as fibers dimension, wettability, elasticity, and mechanical properties.

Other components that can improve mechanical prop erties of electrospun fibrous structures are natural pro teins, including collagen, gelatin (GE), and elastin.24 Taking advantage of these studies, herein different bio inspired electrospun scaffolds were produced and charac terized through ther momech anical techniques. Six kinds of different polymeric matrices were electrospun starting from polymeric blends of different hydrophobicity degree. All of them were solubilized in 1,1,1,3,3,3 Hexafluoro-2-Propanol (HFP). The polymers were chosen as a func tion of biodegradable and biocompatible properties. First of all, GE was appealing for the presence of bioactive motifs and for its low immunogenicity. 25 The scaffolds produced in this work are reported, in order to facilitate reading, with names that give informa tion about their composition. Letters G, P, and N mean "Gelatin GE", "Polymer PDLLA," and "Nanocellulose CNCs" respectively, while the numbers represent CNCs percentage in the polymeric blends. Finally, prefix El and suffix k meanings "elastin peptide" and "cross-linked," respectively.

The morphology analysis of the samples was carried out by scanning electron microscopy (SEM) and from SEM images average fibers diameters were calculated Thermal properties and hydrophilic degree of the electrospun scaffolds were assessed by thermogravimetric analysis (TGA) while the physico-chemical structure evo lutions of the electrospun scaffolds (also after conducting the swelling test on them) were studied by differential scanning calorimetry (DSC) technique. Finally, Dynamic mechanical analysis (DMA) was used in order to investi gate mechanical properties of the samples. In El8NGP mixture (EX15:CNCs:GE:PDLLA ratio 1:7.5:22.5:67.S, 13.1% wt/vol) elastin peptide and GE were added at the same time.

EXPERIMENTAL

Materials

The polymeric mixtures were loaded into a 5 ml plastic syringe with a 18 G stainless steel and then electrospun at 19 kV (gamma high-voltage generator), with a flow of the volumetric pump of 1.6 ml/h. The target was a copper plate covered with an aluminum sheet with a distance from the needle of 19 cm Example of GP, 8NGP, and El8NGP scaffolds composition is listed in Table 1.

Cross linking

Electrospun scaffolds were cross-linked using a 45.0 mM EDC/NHS (1:1) 85.5% ethanolic solution. Scaffolds were left at room temperature under stirring on an orbital shaker (PSU-lOi, Biosan, Riga, Latvia) for 24 h at 60 rpm, then washed with deionized H 2 0 (20 ml x 3 times) to remove the residual cros slinker and finally dried in 95.0% ethanol (1 h at 60 rpm).

3

METHODS

Scanning electron microscopy

The morphological analysis was carried out using a Jeol (Japan) JSM7800F Prime scanning electron microscope and SEM images were acquired with a voltage of 3 kV, after platinum metallization. The images were analyzed using ImageJ software (Java open source) supplied with DiameterJ plug-in.

Thermogravimetric analysis

Thermal stability of the electrospun scaffolds was investi gated by TGA using a Q50 device (TA Instruments, New Castle). Samples with a mass between 5 and 7 mg were eut from the board of the scaffolds and placed into the instrument under a synthetic air flow of 10 ml/min (oxi dizing atmosphere). They were heated from room tem perature to 700 °C at 10 °C/min. Three samples were analyzed for each electrospun scaffold.

Differential scanning calorimetry

The physical structure was analyzed using DSC7 (Perkin Elmer) at a range of 30-200 °C at 20 °C/min under nitro gen flow. Samples with a mass ranging 4-7 mg were eut from the board of the scaffolds, and placed in closed alu minum pans. After the first scan, the samples were immediately cooled at the same rate, and the second scan was recorded. Glass transition temperature T g and heat capacity jump �c;, were determined by the tangent method while the melting temperature T m was taken at the maximum of the peak. DSC measurement was conducted also after the swelling test on the electrospun scaffolds at low temperature with Diamond DSC (Perkin Elmer), under helium atmoephere. In this case, samples were placed in pans and hydrated with 30 µl of ultrapure water. After the experiments we wait 60 min at 25 °C in order to reproduce the swelling test. Two cooling scans were carried out, between 25 and -12ü°C, and 80 and -120 °C at a rate of lü°C/min and two heating scans from -120 to 80°C and from -120 to 85°C at a rate of 10 °C/min, with an identical empty reference pan.

Dynamic mechanical analysis

DMA was conducted using an ARES G2 strain-controlled rheometer (TA Instruments). Samples dimensions were

RESULTS AND DISCUSSION

Morphology observation

Scanning electron microscopy images were used in order to analyze electrospun fibers morphology before and after cross-linking reaction. In presented: herein, there is again a different morphology, similar to that of the G Pk sample, after cross-linking reaction, with a curvilinear trend of the fibers.

The other samples containing CNCs, that is, lNGP, 3NGP, and SNGP scaffolds, with their cross-linked fees, presented morphology very similar to 8NGP matrix (data not shown). This similarity in SEM images suggested that CNCs was well dispersed in the polymeric blends for all these concentrations, as confirmed by Yang et al., 32 who produced matrices composed of PLA and cellulose nanofibrils (CNFs) as nanofiller with concentrations of 3%, 5%, 7%, and 10% via electrospinning. CNFs have also been used in order to reinforce scaffolds made up of PLA and poly(butylene succinate) (PBS), 33 and other fibrous particules like carbon nanotubes (CNTs) and GE to blends of PLA have led to well-formed and uniform non woven scaffold, thanks to their incorporation in the poly meric suspen sion. 34 These fibrillar beadless and interconnected porous microstructu res were also found in El8NGP samples (Figure l(e)), with curled fibers after cross-linking (Figure l(f)). Rnjak-Kovacina et al., 35 and Jiménez Vàzquez et al. 36 have developed electrospun scaffolds with fibers structure similar to ours samples and com posed of h uman tropoelastin and collagen and bovine elastin and collagen derived from chicken skin, respectively.

From SEM analysis it can be concluded that ail the samples showed a similar morphology and an analogous behavior after cross-linking reaction. The mean diameter value measured on 1000 fibers from three different images is around 1116 ± 290 nm.

Thermal stability

In order to analyze the thermal stability of the samples, TGA was carried out under oxidizing atmosphere (syn thetic airflow). For each electrospun scaffolds, three anal ysis were done.

In Figure 2 a comparison between TGA thermograms of respectively GP, 8NGP, and El8NGP scaffolds is presented.

This graph shows the normalized weight and the derivative thermogravimetric (DTG) curves in fonction of the temperature. On GP scaffold (Figure 2), three main thermal phenomena were noted. The first one was observed at a maximum of intensity at about 63 °C due to the loss of the water present in the matrix. In fact, despite PDLLA is essentially a hydrophobie polymer, the pres ence of the hydrophilic regions of GE (which is partially hydrolyzed collagen) increased the ability of the electrospun scaffold to bind water. In literature this "'° desorption phenomenon was observed when GE was electrospun. 37 The main degradation started at a temper ature around 250 °C and reached a maximum of intensity at 327.1 ± 0.3 °C, (Table 2) with the loss of about 80% of the scaffold mass. Subsequently, a high -temperature degradation phe nomenon (deriving from the oxidation of the first degra dation residue) with a lower slope of the normalized weight curve, was present and led to the total decomposition of the sample, at a temperature around 560 °C. The same trend for 8NGP and El8NGP scaffolds can be noted, with an increase of the degradation tempera ture with the presence of elastin peptide (335.8 ± 0.8 °C).

CNCs have been used as nanofiller even in the case of electrospun scaffolds made up of GE. 38 We have tested various contains of CNCs in our scaf folds. The scaffolds named lNGP, 3NGP, and SNGP, which contain a percentage of CNCs of 1%, 3%, and 5% (wt/wt) respectively, presented thermograms very similar (data not shown). Table 2 shows the values of thermal degradation. For the first two typologies of mats, an increase of degradation temperature, which reached the maximum value of 338 ± 2°C for the 3NGP scaffold was observed. This increased temperature with the presence of CNCs is in agreement with the works of Shi et al., 39 and Zhou et al., 40 in electrospun blends of PLA with dif ferent amounts of CNCs. When the amount of added CNCs became higher (scaffolds SNGP and 8NGP) a decrease of the degradation temperature was observed.

Therefore, the presence of the CNCs, up to 3% (wt/wt) seems to improve the thermal stability of the matrices, probably due to char formation from CNCs before the thermal decomposition of GP scaffold, inhibiting the thermal conductivity and then leading to a decrease of the degradation temperature of these scaffolds. 41 In lit erature a decrease of mechanical properties is described after the percolation threshold. 42 Also the peptide effect seems to improve the thermal stability of the samples. As a matter offact, if we compare 8NGP and El8NGP, we note an increase of 5 °C for degra dation temperature. The slight increase could be explaining by a better crosslinking induced by the pres ence of peptide. A higher crosslinking provides more cohesion between polymer chains. This cohesion increase needs more energy to obtain the thermal degradation.

Figure 3 shows the TGA thermograms before and after cross-linking reaction for 8NGP scaffold.

The main difference between these two samples was the water content in the matrix. The scaffold cross-linked did not show loss of water in this case.

Table 2 highlights how the quantity of water present in the electrospun scaffolds increases with the addition of CNCs and elastin peptide: 3NGP and El8NGP scaffolds showed the maximum percentage of water, 5.1 % and 4.6% respectively. This behavior was probably due to the presence of the hydroxyl groups of CNCs and to the hydrophilic regions of the peptide, which can better form hydrogen bonds with water, leading to a higher hydro philicity of the scaffolds. The finding that all the cross linked samples showed a smaller presence of water can be explained by the fact that cross-linking induces a decrease of hydrophilic sites and of mobility.

• • ••• •• • . Physlcal aglng • • ••• • • ••• •••• • ••• • • ••••• •••• ••• •• • •••••••••••• ••• • •••• • ••• . ..•.

Physical structure

Figure 4 presents the first heating scans of non-cross linked electrospun scaffolds (GP, 8NGP, and El8NGP).

The thermal parameters of ail samples are reported in Table 3.

The scaffolds showed a glass transition temperature, near 56 °C associated with amorphous phase and a peak is superimposed on the heat capacity step attributed to the physical aging. We can note only one T g , consistent with a good miscibility of the two polymers. In GE/PLA blends Alippilakkotte noted that the miscibility was 3).

• • ••• • .... .. • • •••••••• ••••
The presence of interstitial water was confirmed by the broad complex endothermic peak located in a tem perature range between 70 and 150°C. At high tempera ture, a melting peak was observed at aro und 149 °C indicative of crystalline phase. nanocomposite fibers via melt-spinning. It interesting to note that add elastin peptide drive to an analogous T m .

In the second heating scans the endothermic event at the glass transition vanishes and the data, listed in Table 3 shown a good stab ility of glass transitio n temper ature with the low level of nanocellulose, and a decrease of 5 °C of T g in scaffolds with 8% of CNCs or elastin pep tide. The melting temperature decrease stro ngly when it can be measured.

In Figure 5 we compared the 8NGP scaffold first heating scan before and after cross-linking reaction .

We can observe an increase of both thermal transi tions, glass, and melting. The values reported in Table 3 confirm this increase of thermal properties for all the samples. But it seems to be that the crystalline phase (Tm values) was less affected than amorphous phase (T g values) by the cross-linking.

In the second heating scans, the glass transition tem perature of ail the cross-linked scaffolds, decrease until 3NGPk when at higher concentrations T g increases again: this finding suggests that a percolation threshold phe nomenon is present at 3% CNCs (wt/wt). This phenome non was observed for example when CNTs were added in polymeric matrices. 42

• 46 Finally, T m value decreases at about 100 °c in the second heating scan.

This st udy shows that the different products used for the scaffold formulation have a good miscibility. In the first heating ramp, we observed the sample fingerprint linked to process and composition. Physical aging phe nomenon observed superimposed at the glass transition is due to the electrospinning process, and can make diffi cult T g measurement with good accuracy, this endother mic fingerprint vanishes which thermal cycling. An addition of CNCs to PDLLA and GE with various ratio formed a hydrogen bonding network, restricting the seg mental mobility retarding the cooperative motion of the polymer chains, and leading to the increase of glass tran sition temperature. A similar T g behavior was observed by Cai et al., 47 which produced electrospun scaffolds composed of poly-DL-lactic acid with a tiller different from CNCs, that is, nanodiamonds. With elastin peptide the thermal response made unchanged A broad endothermic peak superimposed with several transitions (70 and 150°C) was attributed to the vaporiza tion of bound water.

Cross-linking treatrnent induces an enhancement of thermal parameters, T g (excepted 3%) and T m . This result could be due to the effect of the cross-linking reaction of GE. In literature we found analogous results: with genipin cross-link GE electrospun scaffolds, 48 or the use of vinyltrimethoxysilane to form cross-linked bonds on PLA electrospun scaffolds. 49 In presence of elastin peptide not any specific behavior was observed.

A second heating scan modify significantly the ther mal behavior of amorphous and ciystalline phase of scaf folds, plasticization effect on amorphous phase and smaller ciystals demonstrating the non-reversible effect of temperature on the electrospinning scaffold.

Adding elastin peptide at the formulation does not modify the thermal properties.

In Table 3 we reported also the calculated electrospun scaffolds ciystallinity values, calculated from DSC, using the enthalpy of melting on first heating scan and the the oretical LiH m for a 100% ciystalline PLA, 93J/g. 50 • 51 GP scaffold presented a crystallinity of 19.6%, for non-cross linke d samples, there was an increase of ciystallinity with CNCs addition until SNGP scaffold (29.5%), while, at 8% (wt/wt) concentration and with peptide addition ciystal linity decreased, assuming values of 13.8% and 16.3%. This datum suggests that a low amount of CNCs could act as nucleating agent for improving the ciystallization of PLA molecular chains, 39 and suggest that the increase of ciystallinity could contribute to the enhance of degra dation temperature, 52 as found in TGA studies. On the 4.

The first heating run, bring to the fore the structural water melting T m . For GPk scaffold it assumed a value of 10.3 °C this value decreased with the addition of CNCs (samples lNGPk, 3NGPk, and SNGPk), but became higher in 8NGPk scaffold and peptide addition at 9.2 °C. The same trend has been found for the second heating scan. Using ImageJ software (Java open source) supplied with the DiameterJ plug-in, average fibers diameter were calculated from SEM images of the electrospun scaffolds and the DSC swelling test values, and listed with their SD, in Table 4. It is interesting to note that it seems to be a correlation between the structural water melting tem perature and the scaffolds average fibers diameter: as a matter of fact the bigger is the average fibers diameter, the smaller is T m . This result suggests that during the cooling scan water molecules could rearrange themselves better, leading to a higher ciystallinity and to a higher T m of the water, if the fi bers diameter was smaller.

Dynamic mechanical behavior

Mechanical properties of the scaffolds were analyzed by DMA In Figure 6 storage modulus E' and loss modulus E" are reported as a function of the temperature for GP, 8NGP, 8NGPk, and El8NGP electrospun scaffolds. In Figure 6, the viscoelastic step associated with the anelastic manifestation of glass transition call a mode, is visible on storage modulus. The temperature of a relaxa tion is consistent with previous calorimetric results. For 8% CNCs addition the glassy plateau decreases strongly, and the a relaxation is increased Add elastin peptide at this ratio (El8NGP) enhanced the modulus in the whole temperature range, and shifted a relaxation to a lower "iv" The influence of CNCs added at GP is studied and reported in Figure 7.

We observed that a relaxation increases with increas ing the nanocellulose ratio, and the glassy modulus increases until 3% of CNCs and decreases strongly for higher ratio. This datum is in agreement with the work of Pirani et al., 53 which presented biodegradable nanocomposites of CNCs and electrospun PLA and found the maximum value of the storage modulus at a CNCs loading level of 3% (by the weight). An increase of E' has Ta values extracted from loss modulus E" maximum are reported in Figure 8.

We can observe a continuous increase of Ta with increasing CNCs ratio. A decrease of this parameter appears adding elastin peptide, and finally the cross-linking treat ment enhances Ta to lü°C. This behavior was consistent with calorimetric parameters and associated with the mac romolecules mobility loss, and with the increase of the stiff ness due to the reticulation. Glassy modulus observed around 150 MPa at low level of CNCs shown an important decrease (around 100 MPa) after 3% this value corresponds at a percolation threshold, and can give rise to a modulation of the mechanical properties. Uniaxial tensile testing of the scaffold in the dry state, previously described (Ciarfaglia et al.), showed that addition of 8% of CNC, both in the pres ence and absence of elastin peptide reduced significantly Young's moduli, thus increasing elasticity, while in the hydrated state no differences were observed. Interestingly, 8NGPk and E18NGPk scaffolds showed higher elongation at break inferring a role as plasticizer in the dry state. 55 Our scaffolds are suitable to be useful in skin tissue engineering; natural skin possesses tensile modulus between 15 and 150 MPa, 17 • 56 • 57 and tunable adding CNCs or peptide with or without cross-linking. Our values are measured in dry state. Lower values are expected in physi ological environment due to the plasticization effect.

CONCLUSIONS

In this work, thermomechanical properties of different electrospun scaffolds, composed of GE, Poly-DL-Lactic

Acid, various ratio of CNCs and an Elastin peptide were analyzed. All the components of the polymeric blends showed a good miscibility. CNCs and elastin peptide were well dispersed in the solvent: their addition to the mixtures changed the concentration blends but did not affect their properties and the electrospinning process. SEM images showed a well-defined three-dimensional fibrillar microstructures and interconnected porous microstructures, with a more curvilinear trend after cross-linking re action. TGA presented an increase of deg radation temperature and hydrophilic properties with addition of CNCs up to 3% (wt/wt) and elastin peptide . By using DSC it was found an increase of the T g with CNCs addition for the scaffolds series. Furthermore, a correlation between water melting temperature and fibers diameter average, after the swelling test, bas been found. Finally, DMA showed an important increase of the Ta after the cross-linking and brought to the fore a concentration threshold on storage modulus values. These results suggest that the electrospun scaffolds show better thermal and mechanical properties than bulk materials and the scaffolds 3NGP and El8NGP have the more interesting parameters for appli cation. Therefore, these polymeric blends are useful for the preparation of electrospun scaffolds biocompatible and suitable for applications in the field of tissue engineering. Further studies will be conducted to check the biocompatibility of these matrices.

ORCID

Antoine Lonjon Cl:) https://orcid.org/0000-0002-4346-6543

  and by Zhang et al., 12 which produced matrices of biolog ical interest.

  Poly-lactic acid was herein used in the racemic form (PDLLA). As a mat ter of fact, PDLLA [Poly(D,L-lactic)acid] with a lower crystalline level undergoes faster degradation than semi crystalline PLA 26 That characteristic let it be preferred in the field of the tissue engineering. 27 Elastin is a highly hydrophobie and cross-linked protein assembled from a soluble precursor protein called tropoelastin. It is the pro tein responsible for the elasticity of vertebrates' tissue such as large blood vessels, lung, and skin. It was ascertained that the conformational features related to the single exon coded polypeptides are the same as those suggested when they are insert in the entire protein, because each exon encodes an independent and self-contained structure. Therefore, it is possible to use the exon-by-exon analysis to study the characteristics of a single domain. In the works of Tamburro et al., a coherent recomposition of the elastin domains, with the intention of giving an acceptable solu tion of the elastin structure-function problem was pro posed. 2 8 , 2 9 Finally, a peptide encoded by exon 15 (EX15) of Human Tropoelastin and CNCs at 1, 3, 5, and 8% (com pared to the weight of the other components) were added to the polymeric blends in order to irnprove biocompatibil ity and strength, respectively. Due to GE solubility in water and in biological fluids, a cross-linking reaction was carried out on the electrospun scaffolds.

  Figure l(a), SEM image of GP scaffold is shown: it presented a well-defined three-dimensional fibrillar microstructures and interconnected porous microstructures. A linear and random-oriented structure of the fibers was noted and the absence of sol vent and beads suggested a good solubilization of the polymers in the blend. After the cross-linking reaction with EDC, the scaffold showed a change in the morphol ogy (Figure l(b)), with a more curvilinear trend of the fibers. These characteristics were also obtained in the work of Piccirillo et al., which prepared several electrospun scaffolds with different GE/PLA ratios.30 GE can lead to the production of bead-free fi bers and smooth morphology of the scaffolds even when it is electrospun in blend with polycaprolactone.31 Also for 8NGP scaffold (Figure l(c)) as for GP one, a nonwoven matrix with fibrillar and interconnected porous microstructures presenting randomly oriented fibers was observed. CNCs were finely dispersed in the polymeric mixture because the presence of beads was not noted. In Figure l(d) SEM image of 8NGPk scaffold is

FIGURE 1

 1 FIGURE 1 Scanning electron microecopy images of (a b) GP, (c d) 8NGP, and (e f) El8NGP scaffolds before and after cross linking reaction

FIG URE 2

 2 FIG URE 2 Comparison of thermal behavior by TGA thermograms of the three typologies of the electrospun scaffolds. Loss mass and derivated Joss mass are reported (10 °C/min). Square, circle, and up triangle, represents GP, 8NGP, and El8NGP samples respectively. Loss mass is reported as filled symbols, while derivated Joss mass is reported as empty symbols. TGA, thermogravimetric analysis

FIGURE 3

 3 FIGURE 3 Comparison between the TGA thermograms of 8NGP electrospun scaffolds before (square) and after (circle) cross linking reaction. Loes mass is reported as filled symbols, while derivated Joss mass is reported as empty symbols. TGA, thermogravimetric analysis

FIG URE 4

 4 FIG URE 4 First heating thermograms of non cross linked electrospun scaffolds performed at 20"C/min. Square, circle, and up triangle represents GP, 8NGP, and El8NGP sarnples respectively

FIGURE 5

 5 FIGURE 5 First heating DSC thermograms of 8NGP electrospun scaffolds before (square) and after (circle) cross linking reaction. DSC, differential scanning calorimetry

T m H 2

 2 -linking reaction led to a decrease of ciys tallinity except for 8NGPk scaffold.DSC was also used in order to analyze the thermal prop erties of the electrospun scaffolds after conducting the swell ing test. The swelling test was done only on the cross-linked scaffolds (GE is water soluble without the cross-linking reac tion). Results obtained are presented in Table

12 'FIG URE 6

 126 FIG URE 6 DMA thermograms of GP (square), 8NGP (circle), El8NGP (up triangle), and 8NGPk (down triangle) electroepu n scaffolds. Storage moduli are reported in fi.lied symbols while loss moduli are reported in empty symbols. DMA, dynamic mechanical analysis

FIG URE 7

 7 FIG URE 7 Effect of peroentage of oelluloee nanocrystals on the DMA behavior ofscaffulds: DMA thermograms ofGP (square), lNGP (circle), 3NGP (triangle), and SNGP (exagon) electroepun scaffolds. Storage moduli are reported in filled symbols while loes moduli are reported in empty symbols. DMA, dynamic mechanical analysis

FIGURE 8

 8 FIGURE 8 Histogram of Ta values obtained for the scaffolds studied in this work

GE (type B powder, 225 Bloom strength, p.I. 3.8-5.5 at 25 °C) from bovine skin, was bought from Sigma Aldrich (St. Louis, MO); Poly-DL-Lactic Acid (PDLLA EasyFil PLA-PolyLactic Acid, transparent pellets, molecular weight 126,000 g/mol, density 1240 Kg/m 3 ) was pur chased from Form Futura (Netherlands), cellulose nanocrystals (CNCs, spray dried powder, particle size 1-50 µm, pH in water 6-7, 88% crystalline fraction) was obtained from CelluForce (Montreal, Quebec, Canada) while elastin peptide was synthetized as previously reported 2 9 For electrospinning, all the components were dissolved in 1,1,1,3,3,3 Hexafluoro-2-Propanol (HFP) (Iris Biotech GMBH, Marktredwitz, Germany). For cross linking, N-hydroxysuccinimide (NHS) was obtained from Sigma Aldrich, N-(3-dirnethylaminopropyl)-N'-ethyl carbodiimide hydrochloride (EDC-HCI) was purchased from Novabiochem (Darmstadt, Germany) and ethanol was bought from Carlo Erba Reagents (Cornaredo, MI). 2.2 2.2.1 Fabrication of PLA-based scaffolds

	Electrospinning
	GP mixture (GE:PDLLA ratio 1:3, 12.0% wt/vol) was pre•
	pared by dissolving GE in HFP at 37°C for 2 h and then
	adding PDLLA. The mixture was kept at 37°C under
	magnetic stirring for 3 h. All the NGP mixtures (lNGP,
	3NGP, SNGP, and 8NGP) were obtained by dispersing
	CNCs in HFP at room temperature and by keeping the
	dispersions under magnetic stirring for 24 h. The blends
	underwent some sonication cycles. Later, GE was dis•
	solved in the polymeric mixtures and kept under stirring
	for 2 h at 37 °C. Finally, PDLLA was added and stirred for
	3 h at 37 °C.

TABLE 1

 1 Polymeric blends compositions ofGP, 8NGP, and El8NGP samples of 35 x 12.5 mm 2 with a thickness of 0.3 mm for the non-cros.s-linked scaffolds and of O. 7 mm for th ose cros.s linked. The scaffolds were analyzed in tensile geometry mode and the temperature range of the heating scans was performed between O and 80 °C at a heating rate of 3 °C/min. The strain and the frequency were fixed at 0. 03% and 1 Hz respectively within the material linearity range.

		Gelatin (GE)	Poly DL lactic acid	Cellulose nanocrystals	Elastin peptide	Solvent (HFP)
	Sample name	mass(g)

(PDLLA) mas (g) (CNCs) mass (g) (EXIS) mass (g) volume (ml) GP 0.09 0.27 / / 3.0 8NGP 0.09 0.27 0.03 / 3.0 El8NGP 0.09 0.27 0.03 0.0039 3.0 E�(T) (elongational storage modulus) and E:(T) (elongation Joss modulus) were determined.

TABLE 2 Degradation temperature, calculated using derivated Joss mass curves and percentage of water lost, calculated on the TGA thermograrns De gradation Scaffold temperature ( °C)

 2

	Lost water(%)

T m ( °C) �CP (J g-1 C-1) 2nd 1st 2nd 1st

  Mukherjee et al. obtained T m values ranging from 102 to 104 °C from the analysis of pigskin GE.44 With the addition of CNCs, the melting temperature was measured between 149 and 155 °C. A similar T m was registered by Muijca-Garcia et al.,45 in PLA-CNCs

					2nd	Xe(%)
	51.7	148.9	I	0.6	0.3	19.6
	50.7	149.0	I	0.8	0.2	15.9
	52.4	155.1	144.7	0.6	0.2	29.1
	53.7	156.3	146.7	0.5	0.3	18.9
	53.8	155.3	149.1	0.6	0.2	24.2
	44.6	154.7	I	0.7	0.2	19.0
	51.5	155.0	144.1	0.6	0.3	29.5
	50.7	156.9	144.3	0.7	0.3	17.5
	53.2	149.0	I	0.8	0.3	13.8
	52.4	156.2	145.7	0.7	0.2	20.0
	54.8	154.9	145.1	0.5	0.3	16.3
	52.3	156.1	146.3	0.7	0.3	15.6

TABLE 4 Transition temperatures and melting enthalpi es of water present Scaffold 1st 2nd 1st 2nd Average fibers diameter (µm) in the electrospun scaffolds after

 4

	GPk	10.3	11.3	244.9	241.8	0.9 ±0.2	swelling test found by using ose
	lNGPk	7.3	6.6	267.9	231.6	1.6 ± 0.9
	3NGPk	8.3	8.0	182.9	173.8	1.7 ± 0.4
	5NGPk	5.8	7.4	264.2	260.3	2.2 ± 0.3
	8NGPk	10.8	10.3	209.5	205.4	0.8 ± 0.3
	El8NGPk	9.2	8.8	178.3	176.9	1.2 ± 0.3