The analysis of aeromagnetic data aims to identify structural lineaments able to tie pegmatite rocks in Bougouni region. Magnetic data analysis concerns the realization anomaly map and the application of mathematical transformations (e.g., reduction to the equator, upward continuation, analytical signal... etc.). These transformations allow to enhance the appearance of geological structures. Used aeromagnetic data were collected between February and December 2001 by Kevron Pty Ltd on behalf of the Malian government. The first processing was carried out by Fugro Airborne Surveys Pty Ltd [5] . The characteristics of the magnetic data survey are recorded in Table 1 .

Fieldwork expeditions were conducted in 2018 and 2019. A literature review was conducted prior to fieldwork. As a result, we identified a number of outcrops collected during previous campaigns and whose reports are available at the National Direction of Geology and Mines of Mali (DNGM). We were also able to study new outcrops not reported in the literature.

A local worker from Zantoulé (Kola community) was recruited to facilitate interaction with the people of his community. He also provided security and logistical support. During these campaigns, 200 outcropping levees were identified in 25 targets ( Figure 2 ). All 200 outcropping pegmatite dykes were described macroscopically.

The total magnetic intensity (TMI) represents the magnetic response of the underlying geological formations [2] [28] . It is an asymmetric anomaly with respect to the causative sources [29] . The TMI constitutes the response of the dominant magnetic bodies in the studied area. [3] and [4] qualify the TMI as allowing to define the variations in magnetic susceptibility contrast related to geological rocks rich in ferromagnesian elements and those poor.

The magnetic anomaly map of the Bougouni area shows numerous circular anomalies as well as magnetic lineaments. These anomalies are more dominant in the eastern part of the study area. They are marked by a magnetic contrast combining low and high values. This indicates the presence of circular bodies similar to massive geological formations such as crystalline rocks (magmatic and/or metamorphic). These geological massifs are smaller in size to the west and south of the Bougouni region ( Figure 3 ). The observed magnetic lineaments are characterized by low magnetic signals. These lineaments are E-W; NW-SE; NNE-SSW to NE-SW directions. Many E-W directions are cross cut by NNE-SSW lineaments ( Figure 3 ).

The asymmetry of the anomaly constitutes an oblique magnetization [30] which complicates magnetic data interpretation. The reduction filters (pole or equator) can be removed this. In low-latitude regions such as Bougouni (<15˚ inclination), reduction to the equator (RTE) is more effective [2] . The RTE permits to bring the observed anomalies back to the vertical of the causative sources.

The application of the RTE in the Bougouni area is allowed to smooth the magnetic signal whose positive pole goes from 82.93 nT ( Figure 3 ) to 81.2 nT ( Figure 4 ). In this map, they are noted circular anomalies and lineaments. These lineaments have the same orientation in the TMI map. Also, NE-SW direction has a low magnetic signal compared to the others. The NE-SW are felt in places by the discontinuities of the EW and NW-SE lineaments ( Figure 4 ).

The upward continuation (UC) is a mathematical transformation that has been widely discussed by [31] . It is the displacement of the measurement plane from a position P to a position P’ (above) in order to assess the geometry and amplitude of the causative sources [2] [31] [32] . This transformation accentuates the effect of deep sources and attenuates that of superficial sources. The UC allow to classify sources according to their rooting.

The application of this transformation shows a progressive attenuation of the anomalies as a function of the UC distance ( Figure 5 ). The signal amplitude of

sources decreases with the increasing of UC distance. This allows to appreciate the degree of rooting of the anomalies observed in the Bougouni area. The lineaments are more attenuated towards 1200 m ( Figure 5c ). of extended distance while many circular anomalies still persist beyond 2500 m ( Figure 5d ). Thus, those anomalies can be qualified as deep sources.

The analytical signal is a transformation that enhances the bodies related to the disturbing sources of the observed anomalies [33] . It accentuates the shape of the anomalies above these disturbing sources [33] . It is also more effective to delimit the contour of anomalies related to geological bodies [32] [33] . This transformation is more appropriate in the determination of lineaments within crystalline terrain due to their non-negligible remanent and induced magnetizations [33] .

The analytical signal map of Bougouni area more clearly defines the outline of observed circular anomalies and lineaments. The circular anomalies are expressed in positive magnetic signals ( Figure 6 ) and are in places crossed by magnetic lineaments. The discontinuities (NW-SE) of the East-West lineaments are more accentuated by this transformation.

The derivative tilt transformation allows to enhance the amplitude of very short wavelengths related to a geological body [33] . [34] affirm that this transformation is able to represent all anomalies in a similar way even when they have very low signal amplitudes. The derivation processing consists of having a largely refined pushing in order to offer a multitude of lineaments oriented in all possible directions.

The map of the inclined derivative of the Bougouni area offers ( Figure 7 ) many magnetic lineaments including those identified by the previous transformations. Many circular anomalies have their surface dominated by small lineaments. These lineaments are of very low magnetic intensity especially since they are only identified by this transformation of the inclined derivative ( Figure 7 ). They are similar to the zones of discontinuities observed on surface of outcropping rocks.

The metasedimentary rocks of the Bougouni area are located in Bougouni-Kékoro Basin (BKB). This basin occups mainly the central part of the Bougouni Pegmatite Province (BPP). Metasedimentary rocks are intruded by several granitoidic plutons. They are divided into: i) coarse-grained metasediments and ii) fine-grained metasediments ( Figure 1B & Figure 2 ). Subvertical to vertical stratification affects these rocks. Coarse-grained metasediments are most exposed and located many places around the Baoulé River. They are less weathering. The stratification of this formation is testified by the presence of metagreywackes and schists alternance. The stratification S0 and S1 (first phase of deformation) are locally sub-parallel to each other ( Figure 8A ). Concerning fine-grained metasediments, they dominate the BKB margins ( Figure 1B & Figure 2 ). These formations are observed in a pit of gold artisanal mining site (AMS) in Tiéouléna village ( Figure 2 ). They are highly altered.

The granitoids of the Bougouni area are composed of: i) the Massigui plutonic complex in the south-eastern part, and ii) the Bougouni plutonic complex in the northwestern part ( Figure 1B ). The petrographic composition of the Bougouni plutonic complex is various. They consist of: i) granodiorites, ii) tonalites, iii) biotite-bearing monzogranites and iv) two mica-bearing monzogranites ( Figure 2 ). They are oriented NE-SW within the metasedimentary rocks of the Bougouni-Kékoro and Yanfolila basins ( Figure 1B ).

The granodiorites, as subcircular bodies, spread on tens of m2 to km2, those granodiorites commonly occur as flat slabs ( Figure 8F ). In some places, they occur as small metric “whale backs” with slight positive relief ( Figure 8E ). Tonalites outcrop in the west-central part of the study region ( Figure 2 ) on about half a kilometer long. A network of quartz veins (1 to 2 cm thick) cuts the entire outcrop. Biotite monzogranites occur in many places, as batholiths and small circular plutons ( Figure 2 ). They are often occurred as flat slabs and as whalebacks ( Figure 8G ). Two-mica monzogranites are the most abundant facies in the region (up more than 50% of the plutonic rocks). They are located in the northwest and in the southeast of the Bougouni area ( Figure 2 ). These rocks occur as scattered decametric to hectometric slabs. They often occur as small plutons of a few meters to decameters in height. These facies dominate a large hill in Dogo and Tyinra in the north.

In addition to granitoids, there are some mafic rocks represented by quartz-gabbroic diorite and dolerite ( Figure 2 ). On the field, they appear as scattered “balls” ( Figure 8H ). The dolerites are arranged in swarms of dykes. These last have been attached to post paleoproterozoic age. These facies appear as lenticular bands and cluster in areas of weakness, such as contact and fault zones ( Figure 2 ).

In the field, Li-poor dykes are centimeter to decameter thick and a length varies from 5 to 500 meters. These dykes most outcrop in Diamana sector, Sido, Zantogola, Dialakoro, Bougoula and Madina sectors ( Figure 2 ). These dykes contain the garnet mineral. This garnet occurs as circular crystal. It is light pink in the aplitic facies ( Figure 9C ) and reddish to brownish in pegmatitic one. Its size is variable from 0.1 to 1 cm ( Figure 9B ).

The pegmatitic and aplitic facies of the Li-poor dykes are distinctive dykes. Especially at Diamana ( Figure 9A & Figure 10 ), aplites and pegmatites cut mutually. The aplites are generally organized in anastomosing veins ( Figure 8F ). The Li-poor dykes facies mainly host of feldspar and quartz with muscovite and some garnet with rare biotite ( Figure 9B , Figure 9C & Table 2 ). The aplite facies are isogranular with a fine size (<0.1 cm) ( Figure 9C ). The pegmatites are heterogranular and characterized by medium to coarse crystals with size varies from 0.5 to 3 cm ( Figure 9B ). In outcrops, the difference between Li-poor dykes and ones of Li-rich is tied to this presence of garnet in the Li-poor rocks ( Figure 9B ).

The Li-poor dykes of Diamana sector are located 45 km to the northeast of Bougouni ( Figure 2 ). The outcrops are located between 6 and 7 km around the village ( Figure 10 ). Those dykes are host in the granodiorites. In 2018, somes of those dykes have been confirmed in 1 - 2 m deep trenchs done by National Geological Direction of Mali (DNGM). Within the pegmatites are enclaves of granodiorites.

The locality of Boundio is located 30 km in the municipality of Zantièbougou in at the eastern of Bougouni ( Figure 2 ). The dykes of this sector outcrop near to granodiorite massif. The pegmatite dyke is around of 50 cm thick and oriented N65˚. Another dyke oriented N70˚ with 30 cm thick has been identified. This is divided into three ramifications to the west of first.

The target of Madina Kourlamini is located in 25 km to the southwest of Bougouni ( Figure 2 ). It is an outcrop of granodiorite cut by pegmatites dykes oriented from N75 to N95. An N15 pegmatite dyke enriched in biotite has been identified within this target. Diorite and metasedimentary enclaves are also noticed in this sector.

The Sido target is located 34 km to the northwest of Bougouni ( Figure 2 ). It is a 50 - 60 m diameter circular outcrop of biotite-monzogranite which is crossed by pegmatite veins and dykes. The contact with the host rock is unclear.

The Bougoula target is situated to 65 km west of Bougouni ( Figure 2 ) and is composed of granodiorites. These granodiorites are spread over three small hills about 5 hectares area. They contain feldspar phenocrysts (2 to 5 cm) and are cut by outcrops of pegmatites and aplites. Some places, pegmatites and aplites present as veins. These dykes and veins are trending N30-50˚ and are between 5 and 50 cm thick. In same area, metasedimentary enclaves are noted within the granodiorite facies.

Dialakoro target is located on the right bank of the Baoulé River 25 km south of Bougouni ( Figure 2 ). Here, pegmatite dykes’ outcrop in a valley within the metasediments and have N170˚ orientation. They have 50 cm thick and are length to 200 m about. This dyke seems continued on the south in Kologo village where its outcrops in small body.

In Denkelena sector, pegmatite target is located at 40 km west of Bougouni. This target is near to Bougoula one ( Figure 2 ) and composed of granodioritic facies. These facies are distributed in three outcrop areas covering 2.5 km². In this sector, pegmatite veins and dykes cut an outcrop of granodiorite and have a thickness between 5 to 20 cm.

The main targets of the Li-rich dykes’ outcrop in: N’gouanala, Sogola, Goulamina, Kola, Sinsinkourou, Massala, Tinguéléni and Massafala ( Figure 10 ). The pegmatite facies of Li-rich are mostly in lonely dykes. However, they are sometimes accompanied by aplite within the same dykes with a magmatic bedding structure ( Figure 9E ). This magmatic bedding suggests that dykes and their fluids use the same structural network. The metasedimentary rocks host mainly those Li-rich dykes ( Figure 2 ). These dykes are lengths up to 560 m. Simalar to the Li-poor facies, these facies are composed of feldspar and quartz with occasional muscovite minerals. It is also noted an abundance of spodumene with rare biotite ( Figures 9E-9H , Table 2 ). The aplite facies are heterogranular and some places isogranular and fine-grained (<0.1 cm) ( Figure 9D ). The pegmatites are also heterogranular, but characterized to crystals with medium to large grains ( Figures 9E-9H ). The abundance of spodumene is highly in these pegmatites (35% to 45% vol.). This mineral has a stick form ( Figures 9F-9H ) with a length between 2 and 20 cm long. Its mean size varies from 0.5 to 2 cm. In the field, spodumene is easily recognized by its prismatic and colorless appear. It is 90˚ double cleavage approximately ( Figure 9F , Figure 9G ). Fresh spodumene crystals are whitish and slightly translucent ( Figure 9G & Figure 9H ), whereas one’s weathering, they become milky white, greenish, and brownish. Their differential alteration gives a brownish color ( Figure 9F ). Li-rich aplites also contain spodumene. However, they have small sizes (<0.2 cm) making them difficult to identify. It is important to note that Li-rich aplites contain millimetric crystals of tourmaline which make easy their identification. This can also distinguish this aplite to ones of Li-poor.

The Massafala target represents the fifth of Diamana sector and is located to the north of this village ( Figure 7 & Figure 10 ). It noted two granodiorite outcrops with different sizes. The smaller outcrop is crossed by some veins of pegmatite.

These veins are of variable thickness and orientation. Also, it is noted the diorite enclaves within those granodiorites. These enclaves and pegmatites are not observed in the main outcrop. At the same place, a vein of pegmatite oriented N155 has been identified. This vein is 15 cm thick and 10 m long.

For the Goulamina target, it is located 68 km WSW of Bougouni town ( Figure 2 ). This target is composed of an along relief outcrop (250m/40m) oriented N160 ( Figure 10 ). Many pegmatites outcrop in this area. It is also noticed an alternation of pegmatitic and aplitic facies ( Figure 9E ). The pegmatitic facies are more dominant over the aplitic ones. The development of thick lateritic cover can be observed which often masks the contact between host rock and dyke. This target corresponds to “Leo Lithium” permit (now Ganfeng). Nowadays, this target is in exploitation phase.

The Sinsinkourou target is the magmatic outcrops located 12 km to the southeast of Bougouni ( Figure 2 ). It is composed of a large batholith and the scattered diorite-granodiorite slabs which are crossed by pegmatite and aplite dykes. It is noticed metasedimentary enclaves in these outcrops. Also, a main pegmatite dyke is noted. It last, is oriented between N70 and N80, and has 2.5 - 6.7 m thick. This main pegmatite outcrops in 500 m + 40 m length running through the central of laterite hill ( Figure 10 ). In the field, the enclaves of granodiorite (30 cm in diameter) are noted within this dyke. The crystals are coarser in the core and relatively finer at the walls of this dyke. The main dyke is connected:

The Sogola target is located 23 km to the southwest of Bougouni ( Figure 2 ) around 1 to 1.5 km of Sogola village. This sector corresponds with Kodal Minerals exploration permit. The target contains four outcrops area of pegmatite. Generally, metasedimentary rocks are intruded by these pegmatite dykes. There are locally metasedimentary enclaves in these pegmatite:

The N’gouanala target is located at 25 km in the south of Bougouni ( Figure 2 ) no far from Dialakoro target (~4 km). The N’gounala target outcrops in Kodal Minerals permit. It is organized as a patchwork of pegmatite dykes intruded into the metasediments. The main body (pegmatite) is a hill-shaped dyke (200 m × 50 m). In west, this main dyke is divided into two ramifications (N160 and N110). In addition, there are many other pegmatite dykes trending from N105 to N130 with a thickness up to 4 m ( Figure 10 ).

The Kola target is also in the south at 7 km of Bougouni ( Figure 2 ). This target corresponds to an exploration permit held by Kodal Minerals. It presents many similarities with the N’gouanala target and outcrops are organized as small hills again. They consist of pegmatite dykes included in metasedimentary rocks. The contact between pegmatite dykes and host rock is sharp and very micaceous. The outcrop is composed of four pegmatite dykes ( Figure 10 ) while three (3) are subparallel with N85 and N110 trending. The thickness varies from 1.2 to 10 m. The outcrop length ranges from 50 to 560 m. The central dyke is cut by the fourth dyke, trending N60 - 70 with 1.5 m thick and 200 m length.

The Tinguéléni target is located about 70 km to the west of Bougouni ( Figure 2 ). The outcrop contains a massif of biotite- monzogranite with 200 m along and 100 m wide. It is approximately flat slabs shaped. It is noticed shows biotite enrichment on its edges, in contact with pegmatite and aplite veins and dykes that cross it. In the outcrop area, it is noted a series of parallel pegmatite dykes while two are N10 - trending. They are about 50 cm thick ( Figure 10 ). The crystals are generally oriented perpendicular to walls of pegmatite dykes. Without spodumene, a pegmatite dyke and an aplite’s vein are both associated with main pegmatite bodies. The first is oriented N65. For the last, it is N25 orientation and contains crystals of tourmaline and garnet, especially in its core.

The Massala target is located about ten kilometers to the east of Bougouni ( Figure 2 ). The target consists of a metasedimentary and quartz-granodiorite complex. It is cut by pegmatite veins and dykes ( Figure 10 ). Pegmatite dykes are often fragmented and tilted. Mostly of in situ pegmatite dykes, are intruded in quartz-granodiorite. Pegmatites of Massala are enriched in spodumene and depleted in muscovite from north to south.

The Zantogola outcrop is located to the northeast of Bougouni ( Figure 2 ). Pegmatite dykes are at two outcrops (Koleba and Faradièkourou). Koleba Outcrop is a Li-poor pegmatite dyke trending N40 with 200 m long and 4 m thick. No far from this dyke, it notes dolerite outcrops. For Faradièkourou outcrop, it appears in massive with a dimension 400 × 100 m. It is cut by veins and pegmatite dykes Li-rich. These dykes have different orientations.

The target of Foulaboula is located 16 km in south-west between Bougouni and Sogola ( Figure 2 ). In field, it is noticed two outcrops highly altered pegmatites. The first outcrop corresponds to shred of pegmatite appearing in the bed of the Baoulé River and is intrusive within metasediments. The second corresponds to pegmatite dyke with N90 trend. It is 5 m thick and 50 m long. This dyke outcrops at the edge of a valley not far from Baoulé River.

Located at 45 km in west of Bougouni not far from Bougoula ( Figure 3 ), Sibirila target is a small outcrop of spodumene pegmatite. It is oriented N60, with a thickness of 3 m and length of 50 m. It is intrusive within a biotite-monzogranite.