# Gold concentration during polyphase deformation: Insights from Boulanger Project, French Guiana

Brice Lacroix, Dennis Lahondes, Pierre-Jean Hainque, Etienne Le Goff, Dominique Fournier, Alix Hauteville, Blandine Gourcerol, Aurélien Eglinger, Anne-Sylvie André-Mayer

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## To cite this version:

Brice Lacroix, Dennis Lahondes, Pierre-Jean Hainque, Etienne Le Goff, Dominique Fournier, et al. Gold concentration during polyphase deformation: Insights from Boulanger Project, French Guiana. Journal of South American Earth Sciences, 2024, 146, pp.105074. 10.1016/j.jsames.2024.105074. hal-04679874

HAL Id: hal-04679874

https://brgm.hal.science/hal-04679874v1

Submitted on 28 Aug 2024

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# Gold concentration during polyphase deformation: Insights from Boulanger Project, French Guiana

Brice Lacroix<sup>a,b,c,*</sup>, Dennis Lahondes<sup>b,d</sup>, Pierre-Jean Hainque<sup>b,c</sup>, Etienne Le Goff<sup>b</sup>, Dominique Fournier<sup>b,d</sup>, Alix Hauteville<sup>e</sup>, Blandine Gourcerol<sup>f</sup>, Aurélien Eglinger<sup>e,g</sup>, Anne-Sylvie Andre-Mayer<sup>e</sup>

## ABSTRACT

The Boulanger gold deposit is hosted within the Rhyacian greenstone belts from the Guiana shield located within the North Guiana Through, French Guiana. The Boulanger deposit is characterized by a complex polyphase deformation and mineralization history involving a coaxial shortening phase, D₁, followed by a trans-current deformation event, D₂. The main ore bodies consist of a series of quartz-tourmaline-pyrite-carbonate extension and shear veins in response to NNE-SSW shortening event (D₁), and adjacent altered host-rocks. The proximal alteration bordering the vein system is characterized by pyrite-tourmaline (main ore assemblage). This shortening event is followed by transcurrent deformation, D₂. Locally, D₂ is expressed by the formation of N020-030 narrow zones (F₂ axial trace) along which D₁ structures are reoriented. The structural analysis performed on both S₁/₀ foliation and on geophysical data interpretation highlights the presence of Type-1 interference patterns. These zones are associated with significant gold enrichment and the presence of a new ore assemblage dominated by pyrrhotite-pyrite. Quantitative structural analysis approach, based on measured foliations (S₁/₀) and gold grade demonstrates that D₂-related folding is associated with important gold endowment. These findings suggest that the characterization of the exact fold interference pattern is fundamental for the targeting of favorable enriched zones at the deposit scale.

## 1. Introduction

Greenstone belts are areas of great interest as they host numerous mineral deposits including volcanic massive sulphide (VMS), Ni-Cu platinum-group element (PGE) sulphide deposits, and more particularly, orogenic gold deposits, which compose an important source for global gold production (Groves et al., 1998; Poulsen, 2000; Goldfarb et al., 2001). Orogenic gold systems formed in deformed terranes at different geological times (from Paleozoarchean to early Cenozoic; Neu-mayr et al., 1998; Craw and Koons, 1989) and at various crustal levels (from low-grade greenschist to high-grade granulite metamorphic con-ditions) (e.g., Poulsen, 2000; Goldbarb et al., 2005; Dubbé and Gosselin, 2007). Several decades of active research have almost univocally demonstrated that orogenic gold deposits formed from aqueous-carbonic fluid released during metamorphism through devo-latilization reactions in the middle to lower crust (e.g., Cox et al., 1991;

Philipps and Powell, 2010; Goldfarb and Groves, 2015). The transfer pathways of mineralizing fluids as well as the trapping mechanisms of gold mineralization are strongly controlled by crustal structures such as faults and/or shear zones (increasing porosity and permeability) (e.g., Voissey et al., 2020) as well as the deformation conditions during late orogenic stages (e.g., Combes et al., 2022). While pre-existing structures predating gold input may be of important control for the early stage of ore deposition (e.g., Chauvet et al., 2019), later deformation phases postdating gold input are significant for remobilization/reconcentration processes (also referred as gold endowment) (e.g., Tomkins and Mav-rogenes, 2002). Indeed, as reported by several studies, orogenic gold mineralization is not generally associated with a single tectonic event, but rather benefits from polyphase deformation (e.g., gold grade endowment) (Thébaud et al., 2018; Masurel et al., 2022). This has been especially well documented in orogenic gold deposits from Alaska (Juneau terranes; Goldfarb et al., 2008) where grade endowment is

<footnote><sup>a</sup> Department of Geology, Kansas State University, USA</footnote>

<footnote><sup>b</sup> GexplOre, Nancy, France</footnote>

<footnote><sup>c</sup> UMR, 6249, Chrono-Environnement, Université de Franche-Comté, Besançon, France</footnote>

<footnote><sup>d</sup> Reunion Gold Corporation, Longueil, (Quebec), Canada</footnote>

<footnote><sup>e</sup> GeoRessources, Université de Lorraine, Nancy, France</footnote>

<footnote><sup>f</sup> BRGM, 45060, Orléans, France</footnote>

<footnote><sup>g</sup> InnovExplo inc., Longueil, (Quebec), Canada</footnote>

<footnote>* Corresponding author. Department of Geology, Kansas State University, USA.
E-mail address: blacroix@ksu.edu (B. Lacroix).</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** A. Geology of the Guiana Shield (modified from Kroonenberg et al., 2016; Tedeschi et al., 2020). Yellow stars: location of Boulanger gold-deposit. B. Geological map of French Guiana with location of the main Au deposits (Modified from Delor et al., 2003; Combes et al., 2022).</figcaption></figure>
linked to post-subduction transpressive deformation, (e.g., during strike
slip reactivation of reverse crustal structures) (Goldfarb et al., 1993;
Goldfarb and Groves, 2015). Such polyphase deformation evolution
generally leads to a complex tectonic history involving several stages of
folding, overprint by late brittle deformation. Therefore, accurately
determining the deformation evolution at deposit scale is crucial to
identify new deposits and/or mineralized extensions. To achieve these
goals, it is crucial to understand the evolution of deformation events at a
deposit scale combined with the associated vein paragenesis (Perret
et al., 2020; Combes et al., 2022).

In this contribution, we present new structural and geophysical
datasets from one gold deposit located in the Rhyacian greenstone belts
from French Guiana: the Boulanger Project (Fig. 1). We particularly
emphasize the important role of polyphase deformation in the gold
remobilization processes.

## 2. geological Background

The Guiana Shield corresponds to the northern portion of the
Amazonian Craton (Delor et al., 2003). With a total area of 900,000 km²,
it covers eastern Venezuela, Guyana, Suriname, French Guiana, the
northern end of Brazil and easternmost Colombia (e.g., Delor et al.,
2003; Daoust et al., 2011; Tedeschi et al., 2020) (Fig. 1A). Although
remnants of the Archean basement are preserved in the Amapá Block in
northern Brazil and in the Imataca Complex in eastern Venezuela
(Avelar et al., 2003; Tassinari et al., 2004; Barreto et al., 2013; Tedeschi
et al., 2020), the Guiana Shield is mainly composed of Paleoproterozoic
rocks accreted and affected by tectonics during the transamazonian
orogeny at 2.2–2.0 Ga (Vanderhaeghe et al., 1998; Milesi et al., 2003;
Delor et al., 2003a, 2003b). The oldest Paleoproterozoic terranes are
located along the northeast margin of the Guiana Shield which corre-
sponds to the trans-Amazonian Province (Vanderhaeghe et al., 1998;
Santos et al., 2000; Tedeschi et al., 2020). In French Guiana, the
trans-Amazonian Province is composed by large Rhyacian (2.20–2.05
Ga) granite-greenstone belts (northern and southern belts) which
include Rhyacian volcano-sedimentary sequences, intrusive granitoids
and TTG (tonalite-trondhjemite-granodiorite), and the Upper Detrital

Unit that formed along the North Guiana Trough (NGT) contemporaneously to potassic granite suite (2.1–2.08 Ga) (Vanderhaeghe et al., 1998; Santos et al., 2000; Tedeschi et al. 2018, 2020; Guiraud et al., 2020). The greenstone belts are known to host multiple gold deposits (e. g., Milesi et al., 1995; Milesi et al., 2003; Guiraud et al., 2020; Combes et al., 2021).

The geodynamic evolution of the northern and southern greenstone
belts of French Guiana cannot be fully elucidated due to the strong
overprinting by later events (mostly D₂, from Delor et al., 2003). The
evolution proposed here is largely based on structural constraints made
through volcano-sedimentary greenstone belts and regionally associated
plutonic rocks (see review of Delor et al., 2003 for further details). A
two-step tectonic evolution has been proposed for northern French
Guiana involving two major tectono-metamorphic events, D₁ and D₂,
both composing the Transamazonian orogeny (Ledru et al., 1991; Van-
derhaeghe et al., 1998; Delor et al., 2003). These events affected the
greenstone belts and the granite-gneiss complexes (Lasserre et al., 1989;
Ledru et al., 1991), whereas only the major D₂ deformation has been
recognized in the Upper Detrital Unit (UDU) in northern French Guiana.

The Main Transamazonian orogeny, D₁ (ca. 2.18–2.13 Ga), is asso-
ciated with magmatic accretion and crustal recycling (Vanderhaeghe
et al., 1998; Delor et al., 2003). This event follows the formation of
tholeithic juvenile oceanic crust exposed within the “Ile de Cayenne”
complex, by the formation of calk-alkaline greenstone-TTG magmatic
arcs and associated TTG (ca. 2.18–2.16 Ga) (also known as “Complexe
Guyanais”). Absolute time correlations are difficult to make in the
volcano-sedimentary sequences, and more accurate constraints have
been established based on plutonic rock relationships. Thus, the earliest
datable deformation structures are round, imbricated magmatic fabrics
associated with central French Guiana calc-alkalic and TTG batholiths
(ca. 2.15 to 2.13 Ga) and down-dip mineral lineations indicative of
normal movement in thermal aureoles around ca. 2.13 Ga tonalites in
the northern basin (Delor et al., 2001; 2003 b). These details point to the
close relationship between all D₁ events (thrusting and
magmatism-related deformation) and calc-alkalic and TTG plutonism.

The second deformation events, D₂, corresponds to a transcurrent
tectonic event which produced E-W to NW-SE sinistral transtensional

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Geological map of Boulanger district interpreted from airborne magnetic survey (Resources Reunion internal report) and synthetic stratigraphic column of Crique Filon area. The white rectangle corresponds to the extent of Fig. 4.</figcaption></figure>
Massive andesite (VAND)

<figure><img src="image_6.png" /></figure>

Tuffaceous andesite (TAND)

<figure><img src="image_7.png" /></figure>

**Fig. 3.** Macroscopic observations of massive (VAND) and Tuffaceous (TAND) Andesite made along drill cores. A. Amygdules filled with quartz and epidote. B. Example of quartz – epidote extension vein crosscutting S₁/₀ foliation. Note the epidote alteration. C. Classic late carbonate veinlets crosscutting massive andesite/ basalt. D. presence of magnetite (black layers) highlighting S₁/₀. Core diameter is 4.76 cm (NQ drill size). E. Example of folds affecting S₁/₀. F. Fully chloritized Lapilli fragments. Note that the fragments are stretched parallel to the foliation. G. Euhedral mm-size pyrite crystals. D. Tourmaline parallel to the foliation. H. Example of D₁ quartz-tourmaline extension veins crosscutting the foliation. I. Example of quartz-carbonate veins transposed to the main foliation. Core diameter is 4.76 cm (NQ drill size).
<figure><img src="image_8.png" /><figcaption>Fig. 4. Detailed geological map of Crique Filon prospect. The stereodiagrams show the distribution of the pole of S₁/₀ for each drill core (Drill location on the map).</figcaption></figure>

basins, mapped across French Guiana (Lasserre et al., 1989; Ledru et al., 1991; Egal et al., 1992; Milesi et al., 1995; Vanderhaeghe et al., 1998; Delor et al., 2003, 2003b) (Fig. 1A). In northern French Guiana, the D₂ event is contemporaneous to the opening of a series of “pull-apart”-type basins aligned approximately E-W and filled by conglomerate and clastic sediments which compose the Upper Detrital Unit (Ledru et al., 1991; Egal et al., 1992; 1994, 1995) (Fig. 1A).

According to Milesi et al. (1995) and Delor et al. (2003), the D₂ event is the major event responsible for gold-mineralization. The age of this deformation event is bracketed by younger granite and leucogranite with ages varying from ca. 2.1 to 2.0 Ga (Vanderhaeghe et al., 1998; Milesi et al., 2003). This granitic suite is widely documented in southern and western French Guiana (Milesi et al., 1995; Delor et al., 2003) and was emplaced during D₂.

## 2.1. General greenstone belt stratigraphy

The Rhyacian greenstone belts are known by different names across the Guiana Shield (Tedeschi et al., 2020). These names include the Barama-Mazaruni Supergroup in Guyana, Pastora Group in Venezuela, Marowijne Supergroup in Suriname, and the Paramaca Series in French Guiana. In French Guiana, the stratigraphy of the Paramaca Greenstone Belts has been described by Guiraud et al. (2020) and consists of.

1. A lower sequence of basalts and gabbros, interlayered with kometitic flows (Delor et al., 2003). Locally, these mafic units are metamorphosed to amphibolites;

2. Calc-alkaline intrusive magmas and related andesitic to rhyolitic flows, and intermediate composition tuffs. The tuffaceous series is interlayered with clastic sediments and cherts;

3. A turbiditic sedimentary sequence (known as the “Armina Formation”) composed of intercalated pelite, greywackes, fine sandstones and rare conglomerates interbedded with tuffs.

## 2.2. Mineralization

The Rhyacian greenstone belts of the Guiana Shield are preferential hosts for primary gold mineralization (e.g., Milesi et al., 2003; Delor et al., 2003; Tedeschi et al., 2020; Guiraud et al., 2020; Combes et al., 2022) (Fig. 1B). Three types of primary gold deposits have been recognized in French Guiana: 1) stratiform (VMS-type) (e.g., Dorlin, Montagne d'Or), 2) orogenic (Loulouie, Yaou, Combes et al., 2022), and 3) paleo-placer as gold-bearing conglomerates (Milesi et al., 2003). Numerous orogenic gold deposits (including the Boulanger deposit) are distributed along the North Guiana Through (NGT), an ESE-WNW trending crustal-scale shear zone located between French Guiana and Suriname within the northeastern part of the Guiana Shield (Fig. 1B).

## 3. Deposit geology

The Boulanger deposit consists of a series of showings including Doyle, Crique Filon and Crique Loulou belonging to the same N120-130 mineralizing trend (Fig. 2). The description below essentially concerns Crique Filon and Doyle occurrences (Fig. 2).
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Field and drill core evidence of S₁/₀ folding and axial plane foliation. Note that bedding and S₁/₀ are commonly sub-parallel to the bedding. Stereodiagrams. Left: structural data from A. Right: structural data from B.</figcaption></figure>

## 3.1. Lithostratigraphy

The stratigraphy at Boulanger belongs to the Paramaca Series, a Paleoproterozoic volcano-sedimentary series dominated by mafic to intermediate compositions (Vanderhaeghe et al., 1998; Delor et al., 2003). Locally, the mineralization (defined by massive quartz-tourmaline ± pyrite veins; see sections 3.2.2) is hosted by massive andesite (VAND) and/or Andesitic tuffs (TAND) (Fig. 3). The descriptions proposed below are based on macroscopic observations.

### 3.1.1. Massive andesite (VAND)

VAND correspond to aphanitic to microcrystalline massive greenish volcanic rock which could locally present evidence of amygdales (Fig. 3A). VAND are crosscut by abundant mm—to cm-thick quartz-calcite and quartz-carbonate-chlorite veins (Fig. 3B and C). Quartz-rich veins are generally associated with an epidote alteration halo (Fig. 3C). The amygdales are generally filled by quartz-pyrite assemblage with also common epidote alteration (Fig. 3A). Sulfide and oxide minerals are present and correspond to disseminated magnetite, pyrite and locally pyrrhotite (Fig. 3D).

### 3.1.2. Andesitic tuffs (TAND)

Andesitic tuffs (TAND) are heterogeneous in terms of texture, grain sizes and mineral contents. Our current petrographic observations are based on available drill cores, and therefore do not allow to fully assert the lateral facies variation of tuffaceous series in great details. Andesitic tuffs usually show millimetric to centimetric laminations, with variable amounts of tourmaline, sericite, chlorite, quartz and carbonate (Fig. 3E, F, G). A foliation parallel to the bedding, referred as S₁/₀, is present (Figure E, G). Andesite tuffs are commonly associated with the presence of euhedral pyrite (Py I), developed within the microcrystalline matrix, and tourmaline along the foliation (Fig. 3G–I). Pyrite crystals (Py I)

commonly show the presence of pressure shadow parallel to S₁/₀ suggesting they formed prior or during S₁/₀ foliation. S₁/₀ foliation features a L1 pyrite-bearing mineral lineation that plunges 65° toward the N085 (Fig. 3 H). Locally, TAND contain mm-to cm-sized mafic chloritized lapilli fragments (Fig. 3F). Lapilli fragments are stretched parallel to S₁/₀ foliation and record strain varying from 1 to up to 7. Lapilli fragments can be affected by boudinage with interboudin domains filled by secondary pyrite (Py II) (Fig. 3F), suggesting that pyrite is also synkine-matic. Whereas pyrite is the dominant sulfide minerals, disseminated pyrrhotite parallel to sub-parallel to the foliation has been locally described (Fig. 3J). Numerous quartz-carbonate-tourmaline veins have been observed within TAND (Fig. 3E–I, J). They are either transposed to the main foliation, featuring pinch-and-swell texture (Fig. 3E–J), or crosscut the foliation (Fig. 3I).

## 3.2. field and structural observations

Detailed structural analyses have been conducted on both outcrops and along oriented drill cores from the 2020 drilling campaigns done by Reunion Gold Corporation (Crique Filon target: CFD20-21, CFD20-22, CFD20-24, CFD20-25, CFD20-26, CFD20-27, CFD20-30, CFD20-31, CFD20-32, CFD20-33; Fig. 4). In total, three deformation stages have been defined.

### 3.2.1. coaxial folding and quartz-tourmaline vein emplacement

#### 3.2.1.1. S₁/₀ foliation.
Although intensively weathered, evidence of bedding (S₀) affected by tight to isoclinal folds has been observed within tuffaceous rocks at surface and along drill cores (Fig. 5). The overall stratigraphic series is affected by a pervasive regional axial planar foliation, referred as S₁/₀, parallel to sub-parallel to the bedding (Fig. 3D, E, G and Fig. 6A). At regional scale, S₁/₀ orientation is homogeneous,
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Stereodiagrams (equal area, lower hemisphere) showing the different structures measured along drill cores. A. Pole of S1/0. B. Measured fold axes. C. Measured mineral lineation (tourmaline, pyrite, sericite). D. F2 fold axes determined by structural analysis.</figcaption></figure>

mainly oriented N120-N140 (Fig. 6A), consistent with regional tectonic grain (Delor et al., 2003; Vanderhaeghe et al., 1998). The distribution of foliation poles support that S₁/₀ is affected by tight folds oriented N120-140 (Figs. 5 and 6A). S₁/₀ foliation features tourmaline, pyrite or sericite stretching lineation (l₁), which is plunging of about 65° to N088 (Fig. 3H; Fig. 6C). Occasionally, some crosscutting relationship between S₁/₀ (axial plane foliation) and the bedding are visible along fold hinges (Fig. 5). Most of S₁/₀ foliation is pseudo-parallel to the fold axial planes, suggesting both structures are contemporaneous and likely formed in response to a NE-SW compressive event (D₁). Although measured fold axes generally follow the regional N130 trend, their plunging directions vary from SE to NW (Fig. 6B), suggesting the presence of polyphase deformation.

### 3.2.1.2. Quartz tourmaline veins (VQTL).
Quartz tourmaline vein (VQTL) system which composes to the main exploration target, is defined by a set of extension and shear veins (Fig. 7A and B; Fig. 8).

Shear veins consist of massive (thickness varying from 20 cm to > 12 m) tabular bodies oriented N120 and dipping 75° toward the south (Fig. 7A and B, Fig. 8A) and developed along shear zones. Shear veins are laminated and generally contain inclusion trails, rhombus shape quartz, suggesting the opening by oblique crack-seal texture (Labaume et al., 1991; Lacroix et al., 2020; Bons et al., 2012). Locally, shear veins are crosscut by dilatational jogs where brecciated Tourmaline-rich material can be observed (Fig. 8B). Proximal alteration of massive shear veins consists to meter-wide intense tourmaline alteration (almost 100%

tourmaline), whereas the more distal altered host rock consists to tourmaline-rich mafic tuffs, is associated with the mm-to cm-wide tourmaline ± calcite replacement zones parallel to S₁/₀. These shear veins can reach several hundred meters of lateral extension (Fig. 4). The N120-140 orientation of quartz-tourmaline veins also agrees with the trend defined by mapping based on the distribution of quartz boulders and silicified zones (Fig. 4).

Extension veins consist of sub-horizontal cm-to dm-thick planar bodies striking at about N320 and dipping about 10° toward NE (Fig. 7C). Extension veins are commonly filled with interdigitated quartz and tourmaline minerals perpendicular to the contact with host rock supporting a Mode I opening and mineral crystallization from the host-rock toward vein center (Fig. 8C) (Bons et al., 2012). Pyrite and chalcopyrite are also present, generally along the outer part in contact with the host rock (Fig. 3I). Quartz - tourmaline extension veins are associated with a sericite-tourmaline alteration of the proximal host rock (Fig. 3H).

Along drill cores, extension and shear veins do not show clear cross-cutting relationship. However, several observations suggest that they are rather connected. Kinematic indicators of shear veins, such as slickenslide, striation, and the angular relationship with contemporaneous extension veins and foliation suggest that they formed along reverse shear/faults in response of a NE-SW compressive stress (D₁) (Fig. 7D).

The geometry of both extension and shear veins gives an opportunity to determine the stress field orientation during deformation (e.g.,
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Example of D₁ quartz-tourmaline vein system observed within Boulanger prospect. **A** and **B**. Field picture and interpretation showing the geometry of D₁ mineralized system. Note the presence of sub-horizontal (flat; blue) veins connected to steeply southwest dipping massive shear vein (red). **C.** stereographic projection of both shear (red) and extension (blue) veins. **D.** Block diagram showing the geometry of the D₁-mineralized system.</figcaption></figure>

Sibson, 2020). Extension veins form perpendicular to the minimum principal stress ($\sigma_3$) and along the $\sigma_1-\sigma_2$ plane (e.g., Bons et al., 2012; Van Noten et al., 2011; Lacroix et al., 2013). In contrast, shear veins form oblique to both the maximum and minimum principal stresses, $\sigma_1-\sigma_3$ (Fagereng et al., 2010; Bons et al., 2012; Sibson, 2020) with $\sigma_2$ given by the intersection between EV and SV. Using this spatial relationship, the paleo-stress field is characterized by a sub-vertical minimum principal stress, and sub-horizontal maximum principal stress, $\sigma_1$, oriented N026 (Fig. 7C).

### 3.2.2. Evidence of a second deformation event

Within Crique Filon area, both $S_{1/0}$ foliation measured in the field and along cores are locally re-oriented along a NS trend (Fig. 4). A similar statement can also be made by quartz-tourmaline massive veins intersected by drill cores (red polygons on Fig. 4; note that they correspond to surface projection of the mineralized system intersected at 100 m depth). Such structural reorientation suggests the presence of another deformation event.

In order to better constrain this second event, we used $S_{1/0}$ foliation dataset from surface and from ten oriented DDHs from the 2020 drilling campaigns within the Crique Filon target (CFD20-21, CFD20-22, CFD20-24, CFD20-25, CFD20-26, CFD20-27, CFD20-30, CFD20-31, CFD20-32, CFD20-33) to determine the axial surface orientation of the shear event $D_2$ ($F_2$) (Fig. 9). The distribution of $S_{1/0}$ foliation defines a series of several distribution planes (π-planes) (Fig. 9). All the calculated fold axes share the same N030-040 axial surface (Fig. 6D). Note that the distribution of poles of foliation agree with type-1 fold interference pattern (e.g., Ramsay, 1967) (Fig. 9). In such polyphase folding context, $F_1$ fold limbs ($S_{1/0}$) are affected by steeply plunging $F_2$ folds with the fold plunge and plunging direction both controlled by the pre-$D_2$ dip angle of $S_{1/0}$ foliation.

$D_2$ deformation is not pervasive and only observed along narrow domains oriented N040 that seem spatially correlated to the drainage system such as Crique Filon.

### 3.2.3. $D_3$-late brittle deformation

Late deformation event observed within the studied area ($D_3$) is marked by the development of conjugated brittle fault surfaces oriented N085/55 and N260/40, respectively (Fig. 10). The fault surface can feature striations plunging at about 20° toward N110, in agreement with an important strike-slip component (Fig. 10). Because these late structures are not associated with gold mineralization and do not play a significant role on the mineralization shifting, they will not be discussed further.

## 4. mineralization

The Au mineralization at the Crique Filon prospect is complex and records a polyphase or progressive deformation history. In order to better understand the relationship between deformation and mineralization events, we discuss the mineralization as $D_1$-and $D_2$-related mineralization. Note that this description is based on macroscopic observations only.

### 4.1. $D_1$ mineralization

Volumetrically, the most significant mineralized structures consist of extension and shear vein system predominantly composed of quartz and tourmaline (Tur I) with minor amount of carbonate and pyrite (Py II) ± chalcopyrite (Fig. 11A; Fig. 12), as well as the adjacent altered host rock (Fig. 8). Along drill cores, the quartz-tourmaline vein thickness varies from 1 cm to up to 11 m (Fig. 8A), with gold grade generally below 2 g/
<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Example of quartz–tourmaline vein system intersected along drill core. A. Massive (>1 m thick) shear vein (Drill hole CF20-25, 100.8–111.53 m depth interval). B. Detail of brecciated VQTL filled by tourmaline-reach matrix (Drill hole CF20-21 at 162 m depth). C. Quartz-tourmaline extension vein. Core diameter is 4.76 cm (NQ drill size).</figcaption></figure>

ton. The mineralized envelope is defined by a hydrothermal alteration of the direct host rocks marked by the assemblage tourmaline – sericite – pyrite (Fig. 3I and 11A). The intensity of tourmaline alteration could vary from partial, with the development of tourmaline-rich foliation with tuffaceous rocks adjacent small veins, to pervasive forming massive tourmaline bodies within the direct host rock adjacent to massive shear veins.

## 4.2. D₂ mineralization

Macroscopic observations made along the highest-grade drill in-tersections systematically show the presence of pyrrhotite-pyrite ore assemblage (Fig. 12), along with the presence of visible gold along late fractures affecting D₁ quartz-tourmaline veins (Fig. 11 B, C, D). Within these depth-intervals, pyrrhotite is either disseminated along the folia-tion, forming mm-scale grains parallel to S₁₀ foliation (Fig. 11C and D), or more massive and abundant along S₁₀ fold hinges, suggesting space opening by foliation delamination (Fig. 11 C). Because this mineraliza-tion stage is not pervasive, e.g., associated with the pervasive fluid flow through the porous media, no important alteration front is documented. These new observations suggest that the highest gold grades are not macroscopically associated with the same ore assemblage and structures as for D₁-mineralization (Fig. 8). We suggest that this new ore assem-blage corresponds to a second mineralization/deformation stage (Fig. 12). Indeed, highest gold grades are almost systematically associ-ated to the presence of pyrrhotite and D₁-quartz veins fractures (Fig. 11B), and/or small bodies formed by the delamination of S₁/₀ foliation (Fig. 11C and D) in response to structural reorientation. Some evidence of visible gold along fractures developed within quartz veins have also been reported along DDH CFD20-25, which once again, sup-ports a late mineralization event. Interestingly, the zone affected by D₂

<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Conceptual block diagram showing the re-orientation of D₁ structures and mineralized system along N020-030 D₂ corridor. The stereodiagram (equal area, lower hemisphere) shows the distribution of the poles of S₁₀ foliation measured along drill core from Crique Filon area.</figcaption></figure>

reorientation (e.g., Crique Filon; Fig. 4) and exhibit the best Au-grade (3 m at 22.3 g/t). These zones are characterized by higher Au-grade, and the presence of pyrite and cm-thick bodies of pyrrhotite (Fig. 11C and D). Note that these latter features are not present (or at least at macro-scopic scale) within D₁ mineralization.

# 5. Geophysical interpretation

Due to the deeply weathered profile, which is typical from humid and tropical region, surface geological information is sparse and almost exclusively relies on drill cores (e.g., Combes et al., 2022). To increase knowledge on the geology and structures at property scale, the fabrics documented on the field and along drillcores at micro- and macro-scales are integrated to the regional Airborne and Ground geophysical data in order to propose the tectonic framework of the Boulanger district. Regional-scale interpretation of the French Geological Survey airborne radiometric and magnetic have been already proposed by Delor et al. (2003). However, the regional-scale of investigation did not allow to resolve geophysical heterogeneity at the Boulanger deposit-scale.

## 5.1. Lithological distribution

At deposit scale (e.g., Crique Filon site), the distribution of the main stratigraphic units is well defined from surface geological mapping, trenching and drill core data (Fig. 4). The induced polarization map presented in Fig. 13B highlights zones with differences in the subsurface
<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Example of D3 brittle structures observed on surface. Note that most of the measured striations indicate a strike-slip movement (stereodiagrams are equal area, lower hemisphere).</figcaption></figure>

chargeability. Induced polarization (IP) is largely used for mapping ore bodies and host rocks that are associated with disseminated sulfides and massive sulfide bodies (Sumner, 2012; Gurin et al., 2018). Based on our drill core description and mapping, the higher chargeability strati-graphic units may be spatially correlated with the andesitic tuffs (TAND), as they contain important euhedral primary pyrite (Fig. 3G). In contrast, the low chargeability areas correspond to the mafic volcanic rocks (VAND). Note that both low- and high-chargeability units define folded patterns oriented N120-130 parallel to D₁ structures (e.g., S₁/₀, F₁ fold axes). Interestingly, the main known quartz-tourmaline minerali-zation intersected along diamond drill holes and documented at surface (Fig. 4) are located along the sharp contacts between the two contrasted chargeability surfaces (Fig. 13B). These observations suggest that the contact between andesite and tuffaceous rocks localizes the minerali-zation, supporting our observations along drill cores.

Similar observations can be made from the Total Magnetic Intensity grid (TMI) (Fig. 13A). The high magnetic zones are associated with mafic to intermediate massive volcanic rocks (VAND) from the Para-maca series. Indeed, this is supported by the presence of disseminated magnetite (Adagunodo et al., 2015) within massive andesite (Fig. 3D), magnetite being known as one of the few minerals responsible for sig-nificant magnetic anomalies (Adagunodo et al., 2015). In contrast, the low magnetic area highlights the tuffaceous andesite. TMI data also highlight the contact between the Paramaca volcaniclastic series and the granodiorite to the south, as well as the presence of a strong magnetic NW-SE lineament which corresponds to a regional dolerite (Delor et al.,

2003) (Fig. 13A).

## 5.2. Identified structures

Both airborne Magnetic and ground Induced polarization data are used to delineate the main structures at property scale. Both the IP and magnetic maps highlight clear evidence of type-1 fold interference patterns (e.g., Ramsay, 1967) (Fig. 13A and B). The presence of such fold geometry is also supported by the distribution of S₁/₀ (Fig. 9) and fold axes (Fig. 6B) both measured on surface and along drill cores. Such fold interference pattern is likely to be produced by polyphase folding involving the superposition of F₁ fold axial surface oriented N120, and N020 trending upright F₂ fold axial surface.

The observations from the Total Magnetic Intensity grid (TMI) (Fig. 13A) emphasize (1) the contact between the Paramaca series and the granodiorite, (2) the presence of a N130 striking dolerite dike, (3) the lithological magnetic variation among the Paramaca series, and (4) the type-1 fold geometry affecting the Paramaca series (Fig. 13A). The TMI map also shows the presence of a low magnetic lineaments oriented N020-030. Interestingly, this lineament is spatially correlated to the Crique Yaoni stream (oriented N030) which composed the main target for current alluvial gold production (Jebrak et al., 2021).

The main results of geophysical data interpretation include: (1) the presence of N110-120 fold structures which allow the replication of the Paramaca volcanoclastic series, (2) the presence of Type-1 fold inter-ference patterns and their related N020 F2 axial surface (Fig. 13).
<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Example of D₂ mineralization overprinting D₁ structures. A. D₁ mineralization associated with quartz, pyrite and carbonate, intersected by drill hole along the structural re-orientation of Crique Filon (DDH # CFD20-25, Fig. 4). Note that the mineralized structure is affected by pinch-and-swell suggesting structural overprint. B. Presence of visible gold along late fracture affecting massive D₁ VQTL (DDH # CFD20-24, Fig. 9). C and D. Example of polyphase folding developed within S₁/₀ (DDH # CFD20-24, Fig. 9). Note the presence of pyrrhotite along the fold hinge. Scale bars: ~2.5 cm.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 12.** Paragenetic sequence of the Boulanger Gold deposit with emphasis on deformation events.</figcaption></figure>

# 6. Discussions

## 6.1. From deposit scale to regional-scale structural evolution of the Boulanger district

The structural evolution proposed below is based on an integration of regional and deposit-scale field data including airborne magnetic,

Induced Polarization and structural analyses completed by Reunion Gold Corp.

Our dataset clearly illustrates that the studied area is affected by polyphase deformation invoking at least two stages of deformation/folding, D₁ and D₂. This is especially well documented by the local reorientation of S₁/₀ at Crique Filon (e.g., Fig. 4) and the type-1 fold superposition pattern highlighted by geophysical data (Fig. 13).
<figure><img src="image_17.png" /><figcaption>Fig. 13. A. Total Magnetic Intensity grid (TMI) map. B. Chargeability map derived from ground Induced Polarization survey.</figcaption></figure>

Historically, fold interference patterns as portrayed by Ramsay (1967) would have been described as the consequence of two successive deformation phases (Ramsay, 1967; Fossen et al., 2019). However, as discussed by Fossen et al. (2019), we suggest that this fold interference pattern developed during progressive deformation involving two sub-phases, D₁ and D₂, described below.

### 6.1.1. D₁: pure compressional event

Deformation event D₁ is principally associated with the development of tight to isoclinal folds and a crenulation foliation S₁/₀ within the volcanoclastic Paramaca Series (e.g., Fig. 5). The structural analysis made on S₁/₀ suggests that the fold axes and axial surfaces are both oriented N130-150, consistent with the regional structural grains highlighted by both airborne and ground geophysical surveys (see section 5). S₁/₀ is homogeneous, with an average of N135/80 (Fig. 6A), pseudo-parallel to the fold axial surfaces suggesting that both structures formed contemporaneously in response to a NE-SW shortening (Fig. 14A). This NE-SW shortening event also produced important bedding-parallel shear along fold limbs, and at contact between the two rheologically contrasted tuffaceous and massive andesites, which allow the emplacement of quartz-tourmaline-pyrite-carbonate shear and extension mineralized vein system (Figs. 7 and 8). Stress inversion performed on the mineralized system also suggests a NNE-SSW maximum horizontal stress (Fig. 7C), in line with both regional NW-trending fold and foliation. These findings agree with the geodynamical model of the Guiana Shield proposed by Delor et al. (2003)

which defines a first shortening event (D1) oriented NS.

### 6.1.2. D₂: transcurrent deformation

Numerous evidence of transcurrent deformation are present at property scale. Whereas the exact kinematic of transcurrent deformation remains questionable, dextral evidence such as asymmetric shear folds affecting early veins are predominant. In addition, D₂ transcurrent event is marked by the systematic change in plunge and plunge direction of the F₁ fold axes (Type-1 fold superposition; Fig. 6B, 9 and 13). Indeed, measured fold axes are either plunging toward N130 or N300 (Fig. 6B), suggesting the F₁ fold system developed within volcanoclastic rocks is affected by a second stage of folding producing Type-1 interference pattern (e.g., Ramsay, 1967). We interpret that this polyphase folding results from a dextral transcurrent deformation event, D₂, in line with the geodynamical proposed by Delor et al. (2003).

In the case of Boulanger, we interpret that the property scale Type-1 interference pattern (e.g., Ramsay, 1967) results from the transition from compressional to transcurrent deformation (Fig. 14B). Indeed, complex fold interference patterns can form along shear zones where strain is partitioned between pure shear (D₁) and simple shear dominated domains (Clegg and Holdsworth, 2005; Fossen et al., 2019). The style of interference pattern recognized at Boulanger deposit, slightly differs from the type-2 fold geometry documented by Vanderhaeghe et al. (1998) in the Upper Detrital Unit exposed at Montagne Tortue located at 25 km southwest from the studied area. Similar transition to a transcurrent regime has been proposed elsewhere in the Guiana shield
<figure><img src="image_18.png" /><figcaption>Fig. 14. Conceptual tectonic evolution model of southern Boulanger.</figcaption></figure>

(French Guiana, Suriname and Guyana) by Vanderhaeghe et al. (1998), Delor et al., (2003) and Tedeschi et al. (2018).

A geodynamic evolution of the Guiana shield has been proposed by Vanderhaegue et al. (1998) and further discussed by Delor et al. (2003). At regional scale, D₁ is responsible for the main structural fabric observed along the entire North Guiana Trough (Delor et al., 2003; Vanderhaeghe et al., 1998; Daoust et al., 2011) located in French Guiana and Suriname (referred as D₁ in Delor et al., 2001). Although, the timing of D₁ deformation event and related mineralization is not constrained yet, Vanderhaeghe et al. (1998) have performed radiochronology on small syntectonic granitic plutons from Maripa area, located at 35 km SW from Boulanger. Based on their results, they propose convergence and oblique-convergence between 2093 Ma and 2083 Ma.

## 6.2. GOLD concentration during transient deformation

As described above, it is clear that Au mineralization is associated with the development of quartz-tourmaline extension and shear vein system during D₁ compressive event (ground preparation). However, our detailed observations along drill cores suggest the development of a late pyrrhotite-pyrite assemblage along small scale F₂ fold hinges (affecting S₁/₀) as well as the deposition of visible gold along fractures affecting massive VQTL (Fig. 11B), suggesting that this mineralization postdates the quartz-tourmaline vein system (shear and extension veins). We infer that this second ore stage is contemporaneous to D₂ folding event during transcurrent deformation event (see section above).

To illustrate the gold endowment related to polyphase folding, we have produced an Au grade discretization of S₁/₀ foliation as function of Au grade (Fig. 15). For this, each drill interval on which foliation has been measured, has been plotted as a function to the corresponding Au results (Au assay every meter). For low grade (e.g., Au g/t varying from 0.05 to 0.1 g/t), S₁/₀ foliation depicts D₁ fold trend (Fig. 15). For gold

grade varying between 0.1 and 0.5 g/t, the foliation seems to be influenced by both D₁ and D₂ (Fig. 15). Finally, S₁/₀ foliation from the highest gold grade intervals (>0.5 g/t) clearly display F₂ fold interference (Fig. 15). The different calculated fold axes (given by the different π-planes) are aligned along a N030 trend, which is consistent with 1) F₂ axial plane estimated from S₁/₀ foliation (see section 4.3), and 2) the low magnetic N020 trend highlighted by airborne geophysical data (Fig. 13A). The results from this structural analysis demonstrate that the D₁ events is associated with low-to medium-Au grades (up to 0.5 g/t Au) (Fig. 15).

The Boulanger deposit show many similitudes with the Clearwater deposits, located in the Eastmain greenstone belt from James Bay area (Canada) (Lacroix et al., under review). Similarly, the mineralization at Clearwater is polyphase and associated with a two-fold mineralization process. The first stage of mineralization is associated with the development of a quartz-tourmaline vein system similar to Boulanger in response to a North-South shortening event (D₁) at the transition between greenschist and Amphibolite metamorphic conditions (Lacroix et al., under review). This shortening event is, then, followed by the transcurrent reactivation of crustal structures, locally producing a crenulation foliation S₂, the formation of type-2 fold interference pattern, as well as the delamination of the initial S₁/₀ foliation which allows the opening of spaces filled by pyrite-pyrrhotite assemblage (similar to Boulanger deposit).

Similar gold endowment involving late transcurrent deformation have been documented in different Precambrian hosted-greenstone (De Ronde and de Wit, 1994; Jones and Kisters, 2022) and Mesozoic-Cenozoic Au-deposits (e.g., Goldfarb et al., 1993; Goldfarb et al., 2008; Goldfarb and Groves, 2015; Lacroix et al., 2020), suggesting that the transition from collisional compressive regime to transcurrent deformation is an important mechanism in the formation and maturity of orogenic gold systems (Goldfarb et al., 2008). This is particularly
<figure><img src="image_19.png" /><figcaption>Fig. 15. Stereodiagrams showing the distribution of the poles of S₁/O foliation on drill cores from the 2020 drilling campaign for specific gold grade (in ppm).</figcaption></figure>

well documented for several subduction-related orogenic gold deposits from the Alaskan Cordillera, in which the main gold stage occurred during transpressive reactivation of compressive crustal scale faults (Goldfarb et al., 1993, 2008; Goldfarb and Groves, 2015). More recently, Lacroix et al. (2020) reported similar observations within the Los Burros deposits from Central California. These similitudes between different gold deposits worldwide support the hypothesis proposed by Goldfarb et al. (2008) in which the transition from collisional compressive regime to transcurrent deformation is an important mechanism in the formation and maturation of orogenic gold systems. Whereas the role of the transition between pure compression to transcurrent deformation in gold deposition is now undoubtedly documented, the exact physico-chemical mechanisms leading to gold deposition and/or reconcentration remain

questionable.

The role of fold interference patterns, and refolding, within the development of ore deposit, and ore-shoot has been already proposed within the Barberton greenstone belt of South Africa (Witt and Ronde, 1994; Jones and Kisters, 2022) in which 85% of the presently and historically mined reefs occur in the hinge region of refolded regional syncline (Jones and Kisters, 2022). Whereas the exact processes leading to ore maturation are not understood yet, these authors acknowledge the control of fluid flow and mineralization during the refolding event. In the case of Boulanger deposit, we infer that D₂ structural overprint linked to the kinematic transition is spatially correlated with the highest gold grade as well as the presence of pyrrhotite.
### 6.3. Conceptual mechanical model

As discussed above, the transition from pure shortening to transcurrent regime has been documented in several greenstone belts worldwide, including the Guiana Shield, Abitibi Greenstone belts, West and South Africa (e.g., De Ronde and de Wit, 1994; Milesi et al., 1995; Vanderhaeghe et al., 1998; Delor et al., 2003; Hein et al., 2015). Whereas the exact mechanisms conducting to such a transition are still debated (e.g., Gapais et al., 2021), we propose that the passage from pure compressive to transcurrent regimes is a direct response of crustal thickening. Under normal Andersonian conditions ($σ$ vertical = $σ_1$, $σ_2$ or $σ_3$), the development of both sub-horizontal folds (upright fold with horizontal axis) and the quartz-tourmaline vein system as observed at Boulanger deposit require specific stress conditions in which the minimum principal stress, $σ_3$, is vertical (e.g., Anderson, 1951; Sibson, 1985). Assuming that the vertical stress is exclusively controlled by the thickness of the overburden rock sequence, $σ_3$ would significantly increase during $D_1$ crustal thickening event by the development of tight folds and penetrative $S_{1/0}$. Under such conditions, $σ_3$ continues to increase until it exceeds the intermediate principal stress, $σ_2$. At that stage, the new stress field orientation conditions, in which $σ_3$ becomes horizontal and $σ_2$ vertical, initiate the $D_2$ transcurrent regime. In a logical manner, such transition should be accompanied by prograde metamorphism (increasing temperature). As described above, the maturation of gold grade during $D_2$ event is untimely associated with the precipitation of pyrrhotite along space formed by foliation delamination. Whereas the exact composition of pyrrhotite and pyrite have not been investigated in this study, it is relatively well admitted that the pyrite-pyrrhotite transformation occurs in prograde metamorphic conditions (500–550 °C temperature range) (e.g., Zabin and Kremenetskiy, 1993; Groves et al., 2020; Gaboury, 2019). In the case of Boulanger deposit, it is possible that pyrrhotite formed from pyrite present within tuffaceous rocks during crustal thickening event, $D_2$, which supports the proposed crustal thickening model.

### 6.4. Implications for mineral exploration

We propose that resolving the geometry of fold interference pattern is fundamental in understanding the distribution of the gold grade at deposit-scale. Indeed, our structural analysis coupled with Au-grade highlights the presence of grade control along the $F_2$ axial plane formed along refolded $D_1$ structures such as $S_{1/0}$ foliation and quartz-tourmaline veins. The presence of fold superposition developed within the Rhyacian Paramaca greenstone belt in the location studied here offers new prospective for gold exploration within the Guiana Shield. As suggested by the present study, the gold-grade significantly increases during $D_2$ fold superposition, with the development of an ore shoot along $D_2$ axial planes oriented N020. Therefore, we propose that the intersection of $D_1$ and $D_2$ mineralized structures may represent favorable zones to investigate. In this context, refining the polyphase fold geometry (e.g., interference pattern) is fundamental. Accordingly, the recognition of the exact fold geometry appears critical for exploration targeting.

## 7. Conclusions

We report new structural and petrological dataset of the Boulanger gold deposit, hosted within the Rhyacian greenstone belts from the Guiana shield located within the North Guiana Through, French Guiana. The Boulanger deposit is characterized by a complex polyphase deformation and mineralization history involving a coaxial shortening phase, $D_1$, followed by a transcurrent deformation event, $D_2$. The main ore bodies consist of a typical orogenic gold deposit associated with the emplacement of quartz-tourmaline-pyrite-carbonate extension and shear veins in response to NNE-SSW compressive event ($D_1$). This shortening event is followed by transcurrent deformation, $D_2$. Locally,

$D_2$ is expressed by the formation of N020-030 narrow zones ($F_2$ axial trace) along which $D_1$ structures are reoriented. The structural analysis performed on both $S_{1/0}$ foliation and on geophysical data interpretation highlights the presence of Type-1 interference patterns. These zones are associated with significant gold enrichment and the apparition of a new ore assemblage dominated by pyrrhotite-pyrite. Quantitative structural analysis approach, based on measured foliations ($S_{1/0}$) and gold grade demonstrates that $D_2$-related folding is associated with important gold endowment. These findings suggest that the characterization of the exact fold interference pattern is fundamental for the targeting of favorable enriched zones at the deposit scale.

### CRediT authorship contribution statement

**Brice Lacroix:** Writing – review & editing, Writing – original draft, Validation, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Dennis Lahondes:** Writing – review & editing, Resources, Project administration, Investigation. **Pierre-Jean Hainque:** Writing – review & editing, Validation. **Etienne Le Goff:** Supervision, Resources, Project administration. **Dominique Fournier:** Writing – original draft, Project administration, Investigation, Funding acquisition, Formal analysis. **Alix Hauteville:** Visualization, Investigation. **Blandine Gourcerol:** Conceptualization. **Aurélien Eglinger:** Writing – review & editing. **Anne-Sylvie Andre-Mayer:** Writing – review & editing.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Brice Lacroix reports financial support was provided by Kansas State University. Brice Lacroix reports travel was provided by Reunion Gold Corporation.

### Data availability

Data will be made available on request.

### Acknowledgments

Financial and logistical support was also provided by Reunion Gold Corporation (Québec, Canada) and Kansas State University (Manhattan KS, USA). This study benefited greatly from additional geological mapping, core logging and structural data made available to us by Reunion Gold Corporation. We are grateful to the exploration geologist team at Boulanger, and Justin Van der torn for suggestions to improve the English. We thank Journal of South American Earth Sciences editor M. da Costa Pereira Lavalle Heilbron, G. Pires and an anonymous reviewer for their constructive comments that helped to improve this manuscript.

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