Figure 1. Geological map showing the Upper Zaire granitoid massif, the associated Kibalian greenstone belts, and cratonic blocks.

The study area is located in the Lassa region of Wamba Territory, Haut-Uélé Province, northeastern Democratic Republic of the Congo, approximately 130 km south of Isiro (Figure 1). Geographically, the area lies between longitudes 27˚32'00" E and 27˚32'30" E and latitudes 1˚55'00" N and 1˚57'30" N (Figure 2). It belongs to the Kibalian greenstone belt, which is enclosed within the Upper Zaire granitoid massif and forms part of the northeastern Congo Craton ([13]; [16]).

The region has a humid tropical climate, abundant rainfall throughout the year, and dense equatorial rainforest. This vegetation substantially limits bedrock exposure, such that geological observations are mainly possible along river channels, erosion surfaces, road cuts, and artisanal-mining excavations.

The study area is accessible through a network of secondary roads connecting Isiro with Wamba Territory and surrounding villages. Artisanal-mining activities have locally improved rock exposure, facilitating lithological mapping and sampling.

Figure 2. Location map of the study area in Haut-Uélé Province.

The Lassa area belongs to the Paleoproterozoic Kibalian greenstone belt, one of the principal greenstone belts of the northeastern Congo Craton. The Kibalian belt consists of metavolcanic and metasedimentary sequences intruded by granitoid bodies during the Eburnean orogenic event. These units record a complex tectono-metamorphic evolution involving crustal accretion, deformation, metamorphism, and hydrothermal activity ([3]). This framework is consistent with the regional geological map of Zaire ([8]) and the lithostratigraphic evolution of northern DRC ([24]).

In the study area, the lithological assemblage includes granitoids, doleritic intrusions, metavolcanic rocks, banded iron formations, quartz veins, and locally silicified rocks. Quartz veins occur as fracture fillings and are commonly associated with iron-oxide alteration. Granitoids constitute the dominant basement rocks, whereas BIF and metavolcanic units occur as discontinuous bands within the greenstone sequence.

Structurally, the area is affected by brittle and ductile deformation expressed by faults, fractures, foliations, and quartz veins trending predominantly NE-SW and NW-SE. These structures acted as preferential pathways for hydrothermal fluids responsible for silicification, hematitization, chloritization, and iron enrichment. Such hydrothermal-alteration features are commonly associated with mineralized systems throughout the Kibalian greenstone belt.

Kibalian rocks are oriented mainly NW-SE to NNW-SSE and N-S, with subordinate NE-SW and E-W trends in the granitic basement, the orthogneissic complex, and the Upper Kibalian, particularly within the itabirite beds. These Upper Kibalian formations have undergone intensive folding ([14]).

These rocks can be grouped into two main sets: an elongated assemblage of metamorphic rocks surrounded by various granitic rocks, including a western Upper Kibalian metasedimentary facies (Ks) and an eastern Lower Kibalian metavolcanic facies (Ki). The two sets are separated by a major unconformity; consequently, the distinction between the Lower and Upper Kibalian is justified, at least locally ([13]). The regional geological framework of the study area is illustrated in Figure 3.

The tectonic control on relief is particularly evident along the upper Bomokandi River near Rungu and in the Moto mining region, where itabirite bands appear to provide useful stratigraphic markers.

In the eastern Moto area (Makoro region), the itabirites form N-S-oriented bands. Similar trends occur in the western area between Telekudu-Moku in the south and Motobi-VKV in the north, and farther west toward Tora. In the central sector around Doko-Durba, however, the dominant trends are approximately E-W ([17]). These characteristics are comparable to those reported from the Kilo-Moto mining district ([15]).

A broad basin-like structure is also expressed by the itabirites. Two tectonic domains can be distinguished: the Dungu domain, which has a general ENE trend, and the Niangara domain, which has a NW trend.

Figure 3. Geological and structural sketch of the central part of the Kibalian belt at Moto (belt boundary after [16]). Ki = Lower Kibalian (schistose and massive facies); Ks = Upper Kibalian (quartzite and itabirite levels); + = granitoid massif. The West Nile Gneissic Complex occurs to the north ([13]).

Field investigations were conducted at thirteen observation and sampling stations to characterize lithological units and identify representative sampling sites. Geological mapping included observations of lithology, structural features, alteration patterns, and quartz-vein distribution. Representative rock samples were collected from fresh outcrops and artisanal-mining excavations to minimize the effects of weathering. Three representative specimens (BD01, BD02, and BD06) were selected for petrographic analysis, whereas three samples (BD05, BD06, and BD01b) were selected for whole-rock geochemical and metallographic analyses.

Petrographic analyses were conducted on polished thin sections using a polarizing optical microscope to determine mineralogical composition, textures, and alteration features.

Whole-rock geochemical analyses were performed by X-ray fluorescence at CREN-K (Centre de Recherche en Énergie et Sciences Nucléaires de Kinshasa), DRC. Major elements were determined and reported as oxide weight percentages (wt.%), whereas trace elements were expressed in parts per million (ppm). The results were processed using conventional geochemical classification diagrams and elemental ratios to characterize the lithologies and evaluate hydrothermal-alteration processes.

The field survey enabled the preparation of a location map for the observation and sampling stations, identified by the prefix BD (Figure 4). The geographical coordinates of the principal observation and sampling stations are summarized in Table 1.

Figure 4. Map showing the locations of observation and sampling stations.

Field investigations showed that the study area is dominated by granitoids, banded iron formations, doleritic dykes, quartz veins, and locally silicified rocks. Outcrops are generally scarce because of dense equatorial vegetation and a thick lateritic weathering profile. Most geological observations were therefore made along river channels, in artisanal-mining pits, and at road cuts.

The granitoids are generally coarse-grained and locally affected by fracturing and hydrothermal alteration. Quartz veins occur as isolated veins or vein networks cutting across the host rocks. Their thickness ranges from a few centimetres to several metres, and they commonly display massive, milky-white, or translucent textures. Iron-oxide staining is frequently observed along fractures and vein margins.

The BIFs occur as discontinuous bands composed predominantly of alternating silica-rich and iron-rich layers. At several localities, these rocks exhibit intense silicification and hematitization, indicating substantial hydrothermal overprinting. Doleritic dykes cross-cut both the granitoids and the metavolcanic sequence, confirming that they represent younger intrusive events.

Hydrothermal alteration is widespread throughout the study area and is mainly expressed as silicification, iron-oxide enrichment, chloritization, and local sericitization. These alteration features are particularly developed around quartz veins and fracture zones, suggesting that structural discontinuities controlled fluid circulation during hydrothermal events.

Table 1. Coordinates of the principal observation and sampling stations in the Lassa region.

Three petrographic facies were identified and are described below.

Dolerite Facies

The dolerite facies is represented by sample BD01, collected from a massive outcrop intersected by a quartz vein approximately 50 cm thick, striking N235˚ and dipping 78˚ NW (Figure 5).

Figure 5.Outcrop view of the dolerite and quartz vein.

Macroscopically, the rock is greyish green, medium-grained, and massive. It consists predominantly of feldspar and quartz associated with dark ferromagnesian minerals. Disseminated iron sulphides are locally present, and the dolerite is cross-cut by a conspicuous milky quartz vein, indicating a later brittle-deformation event.

Under the polarizing microscope, the rock displays a typical doleritic (subophitic) texture characterized by elongated plagioclase laths intergrown with pyroxene. Pyroxene occurs as brown crystals in plane-polarized light and green crystals under crossed polars. Plagioclase is colourless in plane-polarized light and displays characteristic grey interference colours under crossed polars. Minor opaque minerals are also present. The observed mineral assemblage and characteristic doleritic texture confirm the classification of sample BD01 as dolerite (Figure 6).

Figure 6. Microscopic observation of sample BD01 (dolerite).

Granite Facies

The granite facies is represented by sample BD02, collected from a massive granitoid outcrop exposed in the study area (Figure 7). Macroscopically, the rock is grey, medium-grained, and exhibits a typical granular texture. Its principal mineral constituents are quartz, orthoclase, muscovite, and biotite, which are uniformly distributed throughout the rock.

Figure 7. Outcrop view of the granite.

Microscopic examination reveals a holocrystalline granular texture composed of medium-sized interlocking crystals. Quartz occurs as anhedral, colourless grains in plane-polarized light and displays first-order grey interference colours under crossed polars. Orthoclase is abundant and is readily identified by its characteristic Carlsbad twinning, whereas albite exhibits well-developed polysynthetic twinning. Muscovite occurs as colourless flakes with moderate birefringence, while biotite occurs as brown pleochroic crystals. Minor opaque Fe-Ti oxides and disseminated iron sulphides are locally observed. This mineralogical assemblage and texture are consistent with the identification of sample BD02 as granite (Figure 8).

Figure 8. Microscopic observation of sample BD02 (granite).

Banded Iron Formation (BIF) Facies

The banded iron formation (BIF) facies is represented by sample BD06, collected from a foliated outcrop showing alternating iron-rich and silica-rich bands (Figure 9). Macroscopically, the rock is greyish brown to dark grey and displays a well-developed foliation striking N273˚ and dipping 32˚. The principal minerals identified in hand specimen are hematite, magnetite, and quartz, which define the characteristic banded appearance of the rock.

Figure 9.Macroscopic observation of BIF.

Microscopic examination shows a well-developed banded texture consisting of alternating quartz-rich and iron-rich layers. The siliceous bands are composed predominantly of anhedral quartz grains that are colourless in plane-polarized light and display first-order grey interference colours under crossed polars. The ferruginous bands contain abundant opaque iron oxides, mainly hematite and magnetite, disseminated within the siliceous matrix. This regular alternation of silica-rich and iron-rich layers is characteristic of banded iron formations and confirms the identification of sample BD06 as BIF (Figure 10).

Figure 10. Microscopic observation of sample BD06 (BIF).

3.3.1. Geochemical Analysis

Three representative samples (BD05, BD06, and BD01b) were analysed for major oxides and trace elements by energy-dispersive X-ray fluorescence (ED-XRF) at the Regional Nuclear Research Center of Kinshasa (CREN-K), Democratic Republic of the Congo.

Approximately 5 g of powdered sample was mixed with 1 g of Fluxana binder, homogenized, and pressed into pellets using a hydraulic press before analysis. Major oxides are reported in wt.%, whereas trace elements are expressed in ppm.

The analyses were performed using an XEPOS III ED-XRF spectrometer following the manufacturer’s analytical protocol. Instrument calibration was achieved using certified reference materials (ISE2232, ISE1042, IPE1314, IPE2444, and PT2304 SOIL). Analytical precision and accuracy were evaluated using the laboratory quality-control procedure, and the results are reported with 95% confidence intervals.

The analytical results for major oxides and trace elements are presented in Table 2.

Table 2.Major-oxide (wt.%) and trace element (ppm) compositions of the investigated samples.

3.3.2. Metallogenic Analysis

Polished sections were prepared from the mineralized samples BD05, BD06, and BD01b to characterize opaque mineral assemblages under reflected-light microscopy. These samples were selected because they are representative of the main lithologies and contain visible metallic minerals observed in the field.

Sample BD06 (BIF). Macroscopically, the rock is characterized by alternating millimetre-scale silica-rich and iron-rich bands. Reflected-light microscopy reveals abundant hematite occurring as irregular patches disseminated within a quartz-rich gangue. The hematite grains are distributed throughout the rock, confirming the ferruginous nature of the BIF (Figure 11 and Figure 12).

Figure 11. BIF sample BD06.

Figure 12. BIF sample BD06 under reflected light: (A) Reflected light; (B) Reflected light.

Sample BD05 (BIF). In hand specimen, the rock has a massive grey-black appearance with iron-rich bands alternating with silica-rich layers. Microscopic observations indicate that the gangue consists predominantly of quartz, within which hematite occurs as fine disseminated grains and thin laminae (Figure 13 and Figure 14).

Figure 13.BIF sample BD05.

Figure 14.BIF sample BD05 under reflected light.

Sample BD01b (quartz vein). The sample consists of a milky quartz vein containing disseminated bright-yellow sulphide grains identified as pyrite. Under reflected light, pyrite occurs as euhedral to subhedral crystals embedded in a quartz matrix, indicating sulphide mineralization associated with the quartz vein (Figure 15 and Figure 16).

Figure 15. Quartz vein, sample BD01b.

Figure 16.Quartz vein, sample BD01b, under reflected light.

The petrographic investigation identified three principal lithological facies in the Lassa area: dolerite, granite, and banded iron formation. These lithologies reflect distinct geological processes that contributed to the evolution of the Kibalian greenstone belt. Their coexistence represents an association of intrusive igneous rocks and chemical sedimentary formations, a characteristic feature of Precambrian greenstone terranes ([2]; [13]).

The dolerite is characterized by a typical doleritic texture composed of plagioclase, pyroxene, and olivine, indicating crystallization from a mafic magma at shallow to intermediate crustal levels. Quartz veins cutting the dolerite demonstrate that brittle fracturing and subsequent hydrothermal-fluid circulation occurred after magma emplacement. Such structural relationships are commonly reported in the Kibalian terrane and other Archean greenstone belts ([14]; [17]).

The granite displays a holocrystalline granular texture dominated by quartz and feldspars associated with muscovite and biotite. This mineral assemblage is consistent with the slow crystallization of felsic magma at depth and reflects the widespread granitoid intrusions that characterize the Kibalian basement ([2]; [13]).

The BIF is distinguished by alternating silica-rich and iron-rich bands composed mainly of quartz, hematite, and magnetite. Such banding is typical of Precambrian chemical sedimentary rocks and reflects successive precipitation of silica and iron under fluctuating physicochemical conditions in ancient marine environments ([12]; [18]).

Overall, the petrographic characteristics confirm that the Lassa area preserves the typical lithological association of the Kibalian greenstone belt, where granitoids, mafic intrusions, and banded iron formations provide a favourable geological framework for subsequent hydrothermal activity and associated mineralization. These mineral assemblages are consistent with those described in other Precambrian terranes ([19]).

The geochemical compositions of the investigated samples reveal marked variations that reflect contrasting lithologies and the influence of hydrothermal processes. Major-oxide and trace-element distributions are consistent with the petrographic observations and provide insight into the geological evolution of the Lassa area. Major-element geochemistry is widely used to evaluate rock composition, alteration processes, and crustal evolution ([23]).

The BIF sample BD05 is characterized by very low SiO₂ (7.20 wt.%), high Al₂O₃ (38.60 wt.%), Fe₂O₃ (15.80 wt.%), and relatively high loss on ignition (LOI = 11.70 wt.%). These features indicate intense weathering and secondary alteration that probably modified the primary composition of the iron formation. The elevated LOI further suggests hydration and supergene-alteration processes ([22]; [18]). The unusually high Al₂O₃ content of BD05 may reflect intense weathering, clay-mineral enrichment, or localized hydrothermal alteration. Consequently, this composition is interpreted as representing an altered BIF rather than its original primary geochemical signature.

Compared with BD05, sample BD06 contains higher SiO₂ (45.50 wt.%) and Fe₂O₃ (22.30 wt.%), together with lower Al₂O₃ (12.50 wt.%). This composition is consistent with a silica-rich banded iron formation in which iron oxides and quartz are the dominant mineral phases. The relatively elevated MnO content (0.15 wt.%) may reflect interaction with Mn-bearing hydrothermal fluids and is consistent with hydrothermal alteration associated with iron enrichment ([12]; [11]).

The quartz-vein sample BD01b is distinguished by its high SiO₂ content (71.80 wt.%), confirming its predominantly siliceous composition. Its relatively low Fe₂O₃ content (6.10 wt.%) is consistent with petrographic observations of a quartz-rich vein containing only minor sulphide minerals. Nevertheless, enrichment in Cr (243.70 ppm), Cu (193.60 ppm), and Zn (135.30 ppm), together with the occurrence of disseminated pyrite observed under reflected light, indicates that hydrothermal fluids capable of transporting metallic elements affected the quartz vein ([10]; [21]).

Overall, the geochemical data indicate that the BIFs record iron enrichment accompanied by variable silicification. Silica enrichment in banded iron formations is commonly associated with hydrothermal remobilization and silica precipitation during post-depositional alteration ([12]), whereas the quartz vein records intense silicification associated with hydrothermal sulphide mineralization. These characteristics support the interpretation that the Lassa area provides a favourable geological setting for further exploration of hydrothermal mineral systems ([9]; [10]). Comparable silica-rich hydrothermal alteration has also been reported from Proterozoic successions of Central Africa ([20]).

The petrographic, geochemical, and mineragraphic data indicate that the Lassa area has geological characteristics favourable for hydrothermal mineralization. Quartz veins cutting the host rocks, together with disseminated pyrite observed in sample BD01b, suggest that hydrothermal fluids circulated through fracture systems after emplacement of the host lithologies. Such structural and mineralogical associations are typical of hydrothermal systems developed within Precambrian greenstone belts ([10]; [9]). Similar structural controls have been described for the Kilo-Moto gold district. The relationship between regional structures and hydrothermal mineralization has long been recognized in the Kilo area ([25]).

The BIF samples contain abundant hematite and magnetite associated with quartz-rich layers. These iron-rich formations may have acted as chemically reactive host rocks during hydrothermal-fluid circulation, promoting sulphide precipitation where suitable structural and physicochemical conditions existed. Similar relationships between BIFs and hydrothermal mineralization have been documented in several Archean greenstone belts worldwide ([12]; [21]).

The quartz-vein sample is characterized by a silica-rich composition and disseminated pyrite associated with Cr, Cu, and Zn enrichment. Although these features indicate hydrothermal activity, they do not by themselves demonstrate the presence of an economic gold deposit because no direct gold analyses were conducted in this study. Further investigations, including Au assays, sulphide geochemistry, fluid-inclusion studies, and stable-isotope analyses, are required to evaluate the origin and economic significance of the mineralization ([10]; [9]).

Overall, the integration of petrographic, geochemical, and mineragraphic evidence suggests that the Lassa area is a promising target for future mineral exploration. Nevertheless, the present study should be regarded as a preliminary geological assessment, and further multidisciplinary investigations are required to confirm the extent and economic potential of the hydrothermal mineralization.