Tectonic Inversion in Sediment-Hosted Copper Deposits: The Luangu Area, West Congo Basin, Republic of the Congo

Abstract

Complex Neoproterozoic tectonic processes greatly affected the West Congo Basin, resulting in a series of dispersed copper deposits in the Niari Sub-basin, Republic of the Congo. Structural observation and analysis can help in understanding both the transportation pathways for copper accumulation and the detailed tectonic evolution processes. This study examines cases from four copper mine sites in the Luangu region of the Niari Basin, using a set of codes that consider the three regional tectonic regimes (extension, extrusion, and contraction) and three deformation criteria (maximum effective moment criterion, tensile fracture criterion, and the Coulomb criterion). By combining these two aspects, nine new codes are introduced: the extension maximum effective moment criterion (EM), extension tensile fracture criterion (ET), extension Coulomb criterion (EC), strike-slip maximum effective moment criterion (SM), strike-slip tensile fracture criterion (ST), strike-slip Coulomb criterion (SC), compression maximum effective moment criterion (CM), compression tensile fracture criterion (CT), and compression Coulomb criterion (CC). By analyzing and applying these codes to the selected sites, we show that the new codes can present a geometric coordination catering to an exhumation-related inversion process from extension, strike-slipping, to contraction. The existence of SM- and CM-related structures that occurred during regional extrusional and contractional events may indicate a deeper level of exhumation for layers related to copper deposits in the field sites. A new tectonic evolution model is presented, considering the hypothesis of vertical principal stress changes while the two horizontal principal stresses remain relatively constant during copper mineralization affected by the Western Congo Orogen. The application of the nine codes facilitates the determination of interrelations between different tectonic regimes.

1. Introduction

Previous studies have revealed sediment-hosted stratiform copper deposits (SSC), supergene-enriched copper deposits, and vein-type copper deposits within the Central African Copper Belt [1,2], with total estimated resources of more than 150 million tons of copper and 8 million tons of cobalt [3]. The formation mechanisms of sediment-hosted deposits are still debated; however, it is likely that structural factors play a significant role.

Deformation is localized within sediment-hosted deposits in the Central African Copper Belt [2], but the main phase of stratiform mineralization occurred during burial after the Lufilian Orogeny [1], following the diagenetic phase and the formation of siderite. This may be related to Pan-African orogenic superimposition. In tradition, researchers believed that sediment-hosted deposits did not exhibit early structural movements, but are controlled by extensional tectonic systems [3]. Recent studies reveal the mineralization of the sediment-hosted copper deposits may also be related to tensile fractures which act as pathways for fluid flow and metal transport in orogenic-related fold-thrust tectonics [4,5,6]. Tensile fractures are governed by the tensile criterion and have no direct relationship with the tectonic regime, as they can occur in multiple settings. Extension refers to a tectonic regime defined by extensional structures, especially normal faults. However, tensile fractures with vertical occurrences can locally belong to extensional structures. Sometimes, vertical tensile fractures appear as a set of en echelon fractures, indicating a general strike-slip movement. Horizontal tensile fractures result from contraction. Therefore, mapping outcrop structures is a key to determining copper mineralization mechanisms when gravity and aeromagnetic surveys do not support the presence of such deep-seated bodies.

Fractures in sedimentary rocks include tension fractures (brittle), shear fractures (brittle), and ductile (local plasticity) shear zones, which are respectively controlled by the tensile criterion (T, the tensile strength of the rock, e.g., [7,8]), the Coulomb criterion (C, e.g., [7,8]), and the maximum effective moment criterion (M, e.g., [9]). Different criteria can be combined in Mohr space as shown in Figure 1. The conjugate angle 2α of these three can be used to differentiate the conditions easily in outcrops, being approximately 0° (T), 60° (C), and 110° (M), respectively [8]. Similarly, there are three regional stress regime types—extension, strike-slip, and contraction [10]—coexisting within the same ore deposit fields (

Table 1. Lithostratigraphic units in the Luangu area and its surrounding region corresponding to Figure 2. The green dashed line in the table represents the studied copper unit.

Craton Texture	Unit	Rock Assemblage (and Thickness)	Time	Tectonic Setting	References
	Mpioka Subgroup	Alternating sandstone-clayey sediments	~540 Ma, ~1000 m	Late orogenic related	[11]
	Schisto-Calcaire Subgroup	I–V carbonate-dominated subgroups	~630–580 Ma, ~1100 m	Related to the rifting of the basin.	[11]
	Upper diamictite formation	Diamictite	~640–635 Ma; ~150 m	Related To the Macaúbas Rift II: Cryogeonian	[12,13,14]
	Haut Shiloango Subgroup	III clast-supported conglomerates and breccias	~678–640 Ma; ~1050 m
		II nodular wackestone
		I alternating limestones and claystones
	Lower diamictite formation	Diamictite with basalt sheets	~694–678 Ma; ~400 m
	Sansikwa Subgroup	Continental rift basin fill intruded with dolerite sills	Earlier than 694 Ma ~1650 m
Nonconformity
Basement (Before Early Neoproterozic)	Lufu granite		917 ± 14 Ma	Related To the Macaúbas Rift I: Tonian	[13]
	Gangila Meta basalt		920 ± 8 Ma
	Inga metarhyolite		924 ± 25 Ma
	Yelala metaconglomerate		~930 Ma
	Kimezian Basement		~2.1 Ga		[15,16]

). Each stress regime exhibits typical structures that can be mapped using the three different faulting criteria mentioned above.

This paper presents a case study on tectonic regime changes in the Luangu area of the Niari Sub-basin in the Neoproterozoic West Congo Basin, Republic of the Congo (Figure 2). Regionally, copper deposits are distributed along sinistral strike-slip shear zones that terminate at the thrust front of the fold-and-thrust belt of the West Congo Orogen (Figure 2C; [17]). However, they are also sediment-hosted and relevant to regional extension processes (

Table 2. Copper mine locations and deformation features.

Mines	GPS	Ductile Shearing	Extensile Fractures	Shear Fractures
L1, Northwest Luangu	+13°55′50″–04°19′30″	Layer-parallel sliding; stretch lineations	Veinlet stockworks,	Silication alteration in wall rocks, iron stained
L2, Central West Luangu	+13°55′50″–04°20′15″	Layer-parallel sliding; folding	Fissures with malachite veins	Silication alteration in wall rocks, iron stained
L3, Southwest Luangu	+13°56′00″–04°21′15″	Layer-parallel sliding; folding	Disseminated	Silication alteration in wall rocks
LE, Northeast Luangu	+13°59′00″–04°19′25″	Layer-parallel sliding; folding	Stockworks	Fault zone with silication alteration

; e.g., [12,13]), which presents a challenge in clarifying the various ore-controlling models affected by different regimes. By selecting a north–south rectangular area across the Luangu region, we provide observational results from the field to regional scales by combining deformation criteria theory with structural analysis. Structural features in different stress fields at the outcrop scale delineate ore-controlling systems for mineralization. A copper formation model controlled by tectonic regimes is discussed.

Figure 2. (A,B) Tectonic locations of the Congo Craton and the West Congo Basin (after [11]). (C) Tectonic framework in the area southeast of the West Congo Basin after [18,19,20,21]. Orange color dashed lines with arrows in the I-I’ profile show some possible movement directions of copper (Cu). The orange region in the profile indicates the current study layer which had been deformed by extensional, strike-slipping, and thrusting and folding structures.

Figure 2. (A,B) Tectonic locations of the Congo Craton and the West Congo Basin (after [11]). (C) Tectonic framework in the area southeast of the West Congo Basin after [18,19,20,21]. Orange color dashed lines with arrows in the I-I’ profile show some possible movement directions of copper (Cu). The orange region in the profile indicates the current study layer which had been deformed by extensional, strike-slipping, and thrusting and folding structures.

2. Geological Setting

The study area is located in the Niari sub-basin, a southeast part of the West Congo Basin rifted upon the southwest boundary of the Proterozoic Congo Craton (Figure 2A,B; [11,22,23,24,25]). The Paleoproterozoic basement is distributed in the west of the study area and is locally intruded by younger granites dated at 1.0 Ga (Figure 2; [26]). This basement is capped by 5–6 km of thick basalts and rhyolites, dated between 912 ± 7 Ma and 920 ± 8 Ma (U-Pb zircon dates; [18]). The Neoproterozoic cover rocks are carbonate platforms and foreland metasediments of the West Congolian Group [27,28]. With a nonconformable relationship to the basement, the West Congolian Group includes, upwards, a continental rift basin fill (Sansikwa Subgroup) intruded with dolerite sills, as well as marine carbonate successions with interbeds of glacial deposits and basalt sheets (Table 1; [11,12,13,14]).

The Luangu deposit is hosted by the West Congo Group within a cyclical transgressive–regressive zone, situated between the underlying basement complex rocks and the overlying terrigenous red and tan siltstones and mudstones (Table 1). The timing of mineralization is not well constrained. Mineralization either occurred between 1000 and 700 Ma during the diagenetic period of the covering sedimentary system [29,30], or coincided with the Pan-African orogeny in the Neoproterozoic Ediacaran period or slightly later (635–541 Ma for the Schisto-Calcaire/Lukala Group [11]; Mpioka Group, about 540 Ma [11]; see Table 1). Regionally, there is an approximately east–west-trending copper mineralization zone in the Luangu region (see Figure 3). Various cupriferous deposit locations within the Luangu region have been verified to be situated in the upper section of the West Congolian Group, between the carbonate Schisto-Calcaire Subgroup and the terrigenous Mpioka Subgroup (as indicated by the green dashed line in Table 1). The cupriferous units have also been significantly deformed by layer-parallel detachments, strike-slip faults, and reverse faults (see Figure 3). A series of north–south-trending fold-and-thrust structures, along with some northeast–southwest- and northwest–southeast-trending conjugate strike-slip structures, can also be mapped to superpose upon the mineralization zone into north–south-trending stripes (see Figure 3). In general, three stages of evolution after the formation of the basement can be identified:

Rift Basin Period (about 0.93 Ga to about 0.54 Ga [11,12,31,32]): Characterized by extensional detachment, new crustal growth, and the formation of the Luangu copper mineralization zone.

Pan-African Orogeny (Neoproterozoic to Early Paleozoic): This was a notable period of tectonic, magmatic, and metamorphic activity that contributed to the formation of Gondwanaland [32,33,34,35]. Regional strike-slip faults and related reverse faulting and folding structures serve as the main ore-controlling structures of the region and may overlie some mineral bodies.

Gondwana Continental Rifting Period (Mesozoic): This period was marked by rifting related to the formation of the Atlantic Ocean [36,37,38,39].

New structural features from outcrops of some copper mines—L1, L2, L3, and LE—are summarized and listed in Table 2. Due to alteration, there is significant variation in the lithology of sedimentary wall rocks, necessitating the use of original sedimentary structures for identification. Further studies on ore-controlling structures in oxide ores could be beneficial for better understanding the mineralization process.

3. New Structural Codes Based on Both Structural Regimes and Faulting Criteria

Nine new structural codes are defined to help perform structural analysis in the Luangu region in this paper. In this section, the codes are explained, including how the nine codes are formulated and defined.

There are three types of regional deformation regimes including extension, strike-slipping, and contraction [10]. Here, we will provide a new but more general interpretation of geometric relationships during the superposition of different tectonic regimes in an inversion process. When observing stress field patterns, if the directions of the maximum and minimum horizontal compressions (σH, σh) across the aforementioned regimes are constant, and only the magnitude of the vertical principal stress changes with time or space, this suggests the presence of an ore-forming system (see Figure 4). It is important to study vertical or regional variations in the stress field based on signature structures characteristic of the tectonic stress field. The inversion system logically encompasses tectonic processes from early to later stages, such as rifting, strike-slip faulting, reverse faulting, and folding (e.g., [7,40,41]), similar to the evolution of the West Congo Basin.

Figure 4 illustrates the paleostress field sequence during basin inversion processes, highlighting strain relationships. From the deformation perspective, an ore-forming system that combines the extension regime and the strike-slip regime should align with the maximum principal strain axis; the strike-slip regime and the contraction regime (including fold-thrust and reverse faults) should align with the minimum principal strain axis. Additionally, the medium principal strain axis (y) in the contraction regime should be parallel to the maximum principal strain axis (x) in the extension regime. Extensional structures focus primarily on investigating interlayer sliding, combinations of high and low angles, and attitude relationships, among others. Extrusion structures including X-shaped shear fracture combinations are typically observed on the original bedding plane (S0). Contraction structures including reverse faults and related folds are caused by the stress field when both the axes of the minimum principal stress and the maximum principal stress are horizontal. It is crucial to analyze the relationships between structures formed during the three different regimes above.

There are also three types of structural criteria available for mapping in outcrops: the tensile fracture criterion, the Coulomb criterion, and the maximum effective moment criterion [9]. These three criteria can form a unified deformation criterion to explain natural shear planes and cracks (Figure 1; [8]).

If we consider three stress regimes, which are extension, strike-slipping, and compression, and the three deformation criteria, there will be nine types, including extension maximum effective moment criterion (EM); extension tensile fracture criterion (ET); extension Coulomb criterion (EC); strike-slip maximum effective moment criterion (SSM); strike-slip tensile fracture criterion (SST); strike-slip Coulomb criterion (SSC); compression maximum effective moment criterion (CM); compression fracture criterion (CT); and compression Coulomb criterion (CC) (

Table 3. Classification of nine codes for different stress regime structures in copper deposits.

Stress Regime	Related Structures in Outcrops	Codes and 2αf		Related Reports
Extensional	Detachment or layer-parallel sliding plane; normal faults, extensional veins/fractures, vein structures/tensile fractures	EC	<90°	[42,43]
		ET	~0°
		EM	~110°
Strike-Slip or Extrusion	Strike-slip faults, releasing/restraining bends, dilational jogs, vein structures/tensile fractures	SC	<90°	[44]
		ST	~0°
		SM	~110°
Compressional	Thrust or reverse faults, folds, duplexes, imbricate thrust systems; sulfide-filled foliation boudinage structures, vein structures/tensile fractures	CC	<90°	[5,6]
		CT	~0°
		CM	~110°

Note: Codes definition and explanation are the same as in Figure 4. 2α_f is the conjugate angle between shear planes containing σ₁.

). Therefore, considering both the tectonic regime and geometric factors, nine common classifications have been identified from outcrop studies.

4. Results

4.1. Primary and Weathering Structures in the Cupriferous Units

Field surveys, drill core analyses, and geophysical techniques on copper mineralization have been conducted, revealing a new cupriferous microbial dolomite unit in the western Luangu region (see Figure 5 and Figure 6). Thick dolomite with stromatolites occurred in a warm, shallow water environment within a Neoproterozoic rifting basin [11]. On the way from L2 to L3, particularly near the outcrop close to L3, one can observe an exposed outcrop of inclined bedding surfaces running north–south (see Figure 5 and Figure 6). From a distance, it exhibits directional distribution characteristics. The diameter of the layered stones is generally 10–30 cm. The curved layers display alternating bright and dark patterns (see Figure 6). The ore rock types here have been mostly proven to be weathering-related (Figure 7). For example, chessboard-like structures are commonly observed where only silicified veins remain, while other parts of the primary sandstone rocks have weathered away. Larger ore bodies are usually confined to fault planes. In oxide ore zones, weathered ores are not conducive to diagenetic analysis; however, in many cases, the siliceous framework remains intact, with neogenic veinlets, malachite, azurite, etc., attached to the framework (see Figure 3; Table 2).

4.2. Results and Analyses on Ore-Controlling Structures

Outcrop structures from four mine sites in the Luangu copper deposits are carefully studied and depicted in this section.

Table 4. Coded results and geometric relationships between different regimes.

Mine Site	Coded Results	Contraction and Extrusion Share the Z Axis or Not?	Extension and Extrusion Share the X Axis or Not?	Is Y Axis in Contraction Parallel the X Axis in Extension?
L1, Northwest Luangu	EM; SC; CT, CC	Deduced YES	Deduced YES	YES by CC and EM from Figure 8F
L2, Central West Luangu	EM, ET, EC; SM; CM	YES from Figure 9E	Deduced YES	Deduced YES
L3, Southwest Luangu	EM; ST, SC; CM	YES from Figure 10C,E	YES from Figure 10B,C	YES from Figure 10B,E
LE, Northeast Luangu	EM; SM, SC; CM, CT, CC	Deduced YES	Deduced YES	YES from Figure 11A,B

contains the results of observations of the new codes and the relations between the different tectonic regimes experienced.

4.2.1. L1

L1 is currently a mining pit but does not have a well-exposed outcrop profile (Figure 3 and Figure 5; Table 2; Figure 8A). Apart from the oxidized copper ore, primary ore minerals without oxidation cannot be found here (Figure 6A,B).

The euhedral quartz veins can grow in extensile cracks near the Earth’s surface environment (possibly coded as CT in contraction), along with oxidized copper minerals attached to their tips (Figure 8B–D). The attitude can be measured in an excavation outcrop (Figure 8E). Layer-parallel extension caused stretching lineations with a mean orientation of 10°–178° (L_M in Figure 8F). L_M is from the low-angle detachment coded as EM. Bedding layers might have also experienced conjugate strike-slip (SC in Figure 8F) and folding processes (CC in Figure 8F). According to Figure 4, the set of structures in L1 is suitable for the extension inversion process caused by a decrease in stress solely in the vertical direction over time. The earlier deformation is characterized by layer-parallel extension controlled by the maximum effective moment criterion. The later, but shallower, deformation involves strike-slip and contraction, both controlled by the Coulomb criterion.

Figure 8. Field outcrop and vein-type ore from L1: (A) Field quarry. (B) Ore, approximately 20 × 30 × 50 cm in size, featuring developed jointing with quartz crystal clusters forming on the joint surfaces. (C,D) depict the tips of the quartz crystal clusters turning dark in color, potentially indicating copper mineralization. (E) is the outcrop used for orientation measurement. (F) shows an equal-area stereographic projection plotted on the lower hemisphere to display layer surfaces and stretch lineations. The plotted blue dashed circles represent bedding layers that have been transformed by layer-parallel sliding, with stretch lineations on their surfaces.

Figure 8. Field outcrop and vein-type ore from L1: (A) Field quarry. (B) Ore, approximately 20 × 30 × 50 cm in size, featuring developed jointing with quartz crystal clusters forming on the joint surfaces. (C,D) depict the tips of the quartz crystal clusters turning dark in color, potentially indicating copper mineralization. (E) is the outcrop used for orientation measurement. (F) shows an equal-area stereographic projection plotted on the lower hemisphere to display layer surfaces and stretch lineations. The plotted blue dashed circles represent bedding layers that have been transformed by layer-parallel sliding, with stretch lineations on their surfaces.

4.2.2. L2

The copper ore body in the site L2 is NW–SE-trending, controlled by regional strike-slip structures (Figure 5; Table 4; Figure 6C and Figure 9A). In the field, it appears as steep-standing blocks, specifically lenses of ore bodies within the strike-slip fault systems (Figure 9B).

The cupriferous shear zone, controlled by EM, ET, and EC, is marked in Figure 9C,D, indicating that it may have formed on extensional detachment fault surfaces on top of the SC-III unit (Figure 3 and Figure 5).

Figure 9. Deformation structures from L2. (A) The NW–SE-trending and vertical ore body, approximately 5 m wide in the field. (B) Strike-slip-related slickenside lineations coded SM. (C) The cupriferous shear zone is parallel to the bedding layers marked EM, with high-angle shear fractures marked EC. (D) In addition to the EM-EC structures, extensional cracks filled with cupriferous veins are observed and marked as ET. (E) An excellent complex folding and reverse faulting outcrop, outlined and compared as different regimes with defined codes EM, EC, SM, and CM. (F) A stereographic projection plotted on the lower hemisphere with data measured on the bedding layer at the top of the (D) profile.

Figure 9. Deformation structures from L2. (A) The NW–SE-trending and vertical ore body, approximately 5 m wide in the field. (B) Strike-slip-related slickenside lineations coded SM. (C) The cupriferous shear zone is parallel to the bedding layers marked EM, with high-angle shear fractures marked EC. (D) In addition to the EM-EC structures, extensional cracks filled with cupriferous veins are observed and marked as ET. (E) An excellent complex folding and reverse faulting outcrop, outlined and compared as different regimes with defined codes EM, EC, SM, and CM. (F) A stereographic projection plotted on the lower hemisphere with data measured on the bedding layer at the top of the (D) profile.

The latest contraction structures can also be observed in well-exposed outcrops in the field from mine L2 (Figure 9D,E). The fold-thrust structures align with the projection results. Statistical analysis of 25 bedding planes results in a beta axis of or y-axis orientation of 2–323, representing the medium strain orientation during the final contraction process.

Analysis indicates that extensional structures have been modified by both strike-slip and thrust regimes. From early to late, there is layer-parallel shearing (EM) with high-angle normal faulting (EC), strike-slip faulting (SM), and high-angle reverse faulting (CM).

4.2.3. L3

The site L3 is located in the southwestern part of the Luangu study area, some several meters under the top surface of the Schisto-Calcaire carbonate rock unit and the below surface of the Mpioka red sandstone unit, where malachite mineralization is observed near the surface but inside the Schisto-Calcaire unit (Figure 10A). In the field, strain ellipsoid markers and iron mineralization are visible.

The large extensional lineations in the outcrop also indicate that the maximum strain axis is 165° (see Figure 10B). A closer observation also shows small strike-slip structures. The en echelon cracks on the surface containing large lineations can be coded as ST (Figure 10C). The ST cracks opened at the larger shear plane are marked as SC. Sinistral and dextral kinematic senses can be determined by a set of SC-ST geometric systems (Figure 10C). An arcuate anticline with sub-kink folds (CM) and sub-reverse blind faults (CM) has been identified, with a β range of 12°–162° (see Figure 10D–F). It is inferred that this fold has a medium strain axis orientation of about 162°. Therefore, the medium strain axis in the contraction system is parallel to the earlier extensional maximum strain axis, which can be explained by the method shown in Figure 4.

Figure 10. Field outcrop structures and elemental studies at L3. (A) Boundary between the Mpioka Subgroup and Schisto-Calcaire-III, showing red sandstone layers above the unconformity. (B) Depicts the fold hinge area and the initial stretching lineation bands (coded EM) when facing north. (C) Shows extensile cracks (ST), sinistral and dextral strike-slip shear fracture sets (SC). (D,E) Illustrates thrust-and-fold structures when facing south (CM). The dotted lines in (D) illustrate two reverse kink zones, and the dotted lines in (E) outline curved bedding surfaces. (F) Displays equal-area stereographic projections of plane elements with a beta axis of 12–162°.

Figure 10. Field outcrop structures and elemental studies at L3. (A) Boundary between the Mpioka Subgroup and Schisto-Calcaire-III, showing red sandstone layers above the unconformity. (B) Depicts the fold hinge area and the initial stretching lineation bands (coded EM) when facing north. (C) Shows extensile cracks (ST), sinistral and dextral strike-slip shear fracture sets (SC). (D,E) Illustrates thrust-and-fold structures when facing south (CM). The dotted lines in (D) illustrate two reverse kink zones, and the dotted lines in (E) outline curved bedding surfaces. (F) Displays equal-area stereographic projections of plane elements with a beta axis of 12–162°.

4.2.4. LE

The cupriferous shearing zone between the thick dolomite (Schisto-Calcaire III) and the red sandstone layers (Mpioka Subgroup) at two pits from the northeastern Luangu (LE) mine site was observed and measured (Figure 3; Figure 11). The natural erosion in the north pit is much deeper than that of the south mining pit and has caused the cupriferous shearing zone to be lost.

The cupriferous shearing zone with malachite and limonite can be observed in the south pit of the EL site. The extensional detachment surface is nearly horizontal with smooth undulations (Figure 11A). There are some smaller sheath folds that can be found in the shallower dip planes. The kinematic sense of the detachment zone (EM) is roughly with the top plate moving to the south, which is consistent with that in other sites of Luangu. Extrusion-related shear planes (SC) can also be found, cutting through the detachment surfaces.

The overall dip of the strata is gentle, but becomes steeper as it nears the faulting locations. Extrusion-related strike-slip faulting (SM) with cataclastics occurred in the north pit of the EL site, as well as contraction-related kink folding (CM), layer-parallel cracks and veins (CT), and reverse faulting (CC) (Figure 11B).

According to the geometric relations revealed by the stereographic projection in Figure 11, the maximum extensional principal strain axis is nearly parallel to the fold axis and the strike of the reverse faults, which reflects an inversion process.

5. Discussion

In the previous section of this paper, we analyzed complex outcrop structures from four mine sites using our new codes (Table 4; Figure 8, Figure 9, Figure 10 and Figure 11). These observations are dynamically related to both the tectonic regimes and possible deformation criteria. However, the results remain too isolated to provide a comprehensive understanding of the tectonic evolution processes. Our results and analysis from four mine sites in Luangu clearly indicate that three different regimes were experienced. Here, we present a further discussion on regional evolution by comparing various sites of the Luangu copper deposits.

5.1. The Significance of the New Codes Series Used for Structural Analysis

Previous structural analysis only differentiated three regimes qualitatively according to reverse, strike-slip, and normal faults, and could only roughly determine a major tectonic regime on site. However, through our nine new codes, it is possible to uncover more details about the transitions between different tectonic regimes. Moreover, a quantitative determination of the coordination relations can be made and deduced from these codes recorded at outcrops, allowing us to establish a regime change order.

It is easy to understand that a location point should be controlled by a single tectonic regime at any given time. However, extension at one point could change to strike-slip at another time and eventually to contraction. The final stage should depend on regional tectonic conditions. In other words, when the basin rift is relatively localized, if the regional tectonic condition is strike-slip, then the final stage will be strike-slip; if the regional tectonic condition is contraction, then the final stage will be contraction. Some previous studies believe that regional copper deposits are more obviously controlled by a set of ENE–WSW strike-slip faults (Figure 2C; [11,17]) in a regional extrusion regime. However, structures associated with contraction are also shown to be prevalent in our study. From all pits, upright veins and horizontal veins can be observed to form due to fracturing that occurred under tensile criteria during different regimes (see the lower right picture in Figure 11B).

By classifying structures related to different regimes and levels at the field scale, a changing process for different erosion levels of each tectonic regime can be easily deduced (

Table 5. Deformation history of each site using the new codes.

Mine Site	Results	Extension		Strike-Slip		Contraction		Preservation Assessment
		Ductile	Brittle	Ductile	Brittle	Ductile	Brittle
		EM	ET/EC	SM	ST/SC	CM	CT/CC
L1, Northwest Luangu	EM; SC; CT, CC	√	-	-	√	-	√	Good
L2, Central West Luangu	EM, ET, EC; SM; CM	√	√	√	-	√	-	Limited
L3, Southwest Luangu	EM; ST, SC; CM	√	-	-	√	√	-	Limited
LE, Northeast Luangu	EM; SM, SC; CM, CT, CC	√	-	√	√	√	√	Limited
Ore preservation assessment		Best	Best	Good	Better	Bad	Good	-

Note: The tick mark ‘√’ indicates that structures related to different regimes and levels at the field scale have been found at the corresponding mine site.

). The tendency of these changes can be more easily understood in Figure 12, which compares different evolutionary routes in physical style. EM is likely the most prominent during a deep ductile extension stage, which is clearly related to the formation of the Luangu copper deposit (Figure 13). In L2 and LE, the copper grade reaches up to 3%–4% within the dolomite layer detachment zone (Figure 9 and Figure 11). However, other types of structures may hinder preservation conditions due to higher erosion levels. For the L1 site, this may indicate good preservation of the copper deposit, as no SM and CM structures have been observed there. For the L2, L3, and LE sites, this suggests limited preservation conditions for the copper deposit, as CM structures have been observed there (Table 5).

5.2. Models for Regional Distribution and Evolution of the Luangu Deposits

From the geological background in the Section 2, three stages of tectonic development can be concluded: basement rift and West Congo Basin formation, Pan-African orogeny, and the assembly and rifting of Gondwanaland. Large sediment-hosted copper deposits often experienced superimposed tectonic systems [1,3]. Since the four study sites are primarily located at the same tectonic level within a relatively small region, it is plausible to determine the superposition order in time, beginning with extension, followed by strike-slip, and finally contraction. The regime changes correspond to an inversion process during exhumation. This study further demonstrates the geometric, kinematic, and dynamic relationships resulting from tectonic inversion.

• Stage A: Rift valley basin period

This time period is a transition from N–S contraction to N–S extension; that is, from the Lufilian orogeny to the rift valley, resulting in the sedimentation of the West Congo Group (Figure 14A). Previous studies have suggested the presence of a regional Neoproterozoic coastal environment during the extensional regime [11,46]. Our research has also revealed the presence of abundant stromatolitic structures between L1, L2, and L3, indicating a lacustrine depositional setting. Within these formations, sedimentary copper deposits are found, as illustrated in field photographs (Figure 8, Figure 9, Figure 10 and Figure 11).

• Stage B: Pan-African orogenic period

This is a period characterized by E–W contraction but occasionally by extension due to deep tectonic heat upwelling, resulting in the formation of copper deposits in the study area via hydrothermal processes along fracture zones (Figure 14B). The major superimposed tectonic systems transformed the boundary between the red bed sediments of the Mpioka unit and the underlying Schisto-Calcaire unit, resulting in isolated copper-bearing units in the study area, although they remain within a defined zone on the regional map (Figure 2C).

Systems of extensional, extrusive, and thrust structures coexist in field observations, indicating an evolving inversion process at each site. Field studies indicate that the extensional-related EM, ET, and EC systems are major mineralization factors. However, the strike-slip-related SM, ST, and SC systems, as well as the contraction-related CM, CT, and CC systems, may also have ore control significance.

• Stage C: Assembly and rifting of Gondwanaland

This is a period characterized by E–W extension and contraction processes, resulting in the exhumation of copper deposits in the study area (Figure 14B). The extension is related to Gondwana continental rifting and the formation of the Atlantic. The current predominant stress system is east–west contraction [47]. Although the copper mineralization is more intense near the oceanic spreading center [48], it is difficult to determine whether this significantly affects the formation of ore deposits within the continent.

6. Conclusions

(1) This study introduced and defined nine codes (EM, ET, EC, SM, ST, SC, CM, CT, CC) considering both tectonic regimes and deformation criteria for structural analysis. The significance of the new code series used for structural analysis makes the deformation history easier to understand for each mine site.

(2) Taking the Luangu deposits as an example, analysis results from four mine sites are provided using the new codes and discussed through comparisons with related physical diagrams. The key geometric relationships between different regimes, as observed and measured directly, prove that contraction and extrusion share the Z axis at L2 and L3, extension and extrusion share the X axis at L3, and the Y axis in contraction is parallel to the X axis in extension at L1, L3, and LE.

(3) A further basin inversion model for regional copper deposits in the West Congo Basin is established based on the previous tectonic settings. The E–W compression at the end of the Pan-African Orogen may represent the primary stress field for the formation of significant copper bodies.