Re-Evaluation of Kombat-Style Mineralization and Implications for Exploration in the Otavi Mountainland, Namibia

This study re-evaluates the characteristics of Cu-Pb-Ag and Fe-Mn ore mineralization of the Kombat Mine and Gross Otavi Mine based on field geology, fluid inclusions, petrology, mineralogy, and geochemistry. This is to determine the genetic relationship between Fe-Mn and Cu-Pb-Ag mineralization. The study has established that the Cu-Pb-Ag ore at the Kombat Mine can be classified as a variant of MVT-type deposit, whereas the Fe-Mn ore can be classified as a stratiform-syn-sedimentary deposit. The formation of the MVT-type deposit is associated with a hydrothermal fluid system with a mean temperature of 183˚C and mean salinity of 12.85 wt. % NaCl equivalent. The syn-sedimentary Fe-Mn ore, which is largely associated with calc-silicate lithologies, consists mainly of magnetite and hematite with minor pyrite, hausmannite and jacobsite, and was deposited by diagenetic and mixed diagenetic-hydrogenetic processes under changing oxic and anoxic conditions within the sedimentary basin. Acceptable geochemical exploration indicators of the existing mineralization include anomalous values above 0.5% Cu, 0.2% S; 0.05% Pb; 0.04% As; 0.01% Zn; V, W, Mo, and Ag are 0.002%. Mineralogical indicators include chalcopyrite, bornite, covellite and galena with minor chalcocite, sphalerite, and tennantite for the Cu-Pb MVT-type ores at Kombat Mine.


Introduction
The Kombat Mine is one of the most significant copper mines in Namibia. It originally commenced mining in 1962 and operated until 2008, producing 12.5 million tonnes of ore grading 2.6% Cu over this period [1] [2]. Six defined ore blocks have been delineated to date that are collectively termed Kombat Mine and are located on the northern limb of the canoe-shaped, doubly plunging Otavi Valley Synclinorium, in the contact zone between phyllite of the Kombat Formation and underlying dolomites of the upper Tsumeb Subgroup (Figure 1). This extends for over 4 km [3]. The mine is unique in the Otavi Mountainland (OML) in that the base metal hosting massive and semi-massive sulfides, fracture veins and galena-rich alteration breccias are spatially associated with replacement iron-manganese oxide/silicate mineralization that hosts several unique minerals first described at Kombat Mine [4].
The unusual mineral association of the Kombat-style mineralization warrants a re-evaluation of the genetic relationship between iron-manganese and copper-lead silver ores.
Kombat-style mineralization was first reported by Sir Francis Galton in 1851 [1]. From 1909 and 1911, the historic Gross Otavi and the Kombat Mines, respectively, were exploited by the OtaviMinen-und Eisenbahn-Gesellschaft (OMEG) until 1925 when groundwater ingress shut down both mines [4]. The operations included limited surface production and underground mining at both Kombat and Gross Otavi. Operations restarted in 1962 and worked continuously until 2008, when the underground Kombat Mine was shut down following a flooding incident and depressed global base metal prices [5].
The existing resources estimates were based on series of exploration activities conducted since the 1950s. Tsumeb Corporation Ltd (TCL) acquired the assets from OMEG and carried out soil geochemical and ground magnetic (induced polarization and seismic) surveys in the vicinity of the Kombat Mine between the 1960s to the 1990s; however, documentation and results are not available for all surveys [5]. Surface diamond drilling covering 1600 m of strike length from No. 1 shaft towards the Asis Far West shaft was initiated by TCL and on purchase in 1987 by Goldfields of South Africa until 1998 and transfer of ownership to Ongopolo Mining Limited. Ongopolo Mining Limited sunk an 800 m shaft at Asis Far West with a loan guarantee from the Namibian government in 2005. In 2006, Weatherly Mining Namibia Limited purchased the assets and drilled over 1200 holes including diamond coring (10 holes), reverse circulation (258 holes: 27,750 m) and percussion (16,500 m) yielded positive results of near-surface copper mineralization over the three kilometres west of the Asis Ostore body to the No. 1 Shaft at the Kombat Minein 2007 [5].
The existing resources at the Kombat Mine are still substantial, open pit and underground indicated mineral resource estimate of 7.35 million tonnes at a grade of 0.91% copper, 0.88% lead and 0.58 g/t silver, plus 31.76 million tonnes of inferred mineral resources at a grade of 2.21% copper, 1.33% lead and 4.42 g/t silver [6]. The Kombat Mine is currently under care-and-maintenance status by Trigon Metals, with a plan to set up an open-pit targeting the historically overlooked low grade shallow Cu-Pb (Ag) mineralization which is associated with Fe-Mn mineralization. Although different historic exploration techniques can be applied for the investigation of Kombat-style mineralization, all methods are based on the genesis of the deposit.
Historic activities at the Kombat Mine were based on the "Fracture Zone model" favoured by Tsumeb Corporation Ltd (TCL) geologists up to 1998 and Otavi Minen-und Eisenbahn Gesellschaft (OMEG) geologists from 1906 to 1941 [7]. Later a "Roof Pendants (Ore Lenses) Hanging on Rollover Structures model" [8] was mostly adopted by many authors. Hence, the genetic relationship between Fe-Mn ore and Cu-Pb (Ag) ore at the Kombat deposit remains uncertain [9].
The current study focuses on a re-evaluation of the genetic model of the Kombat polymetallic deposit, the establishment of suitable mineralogical and geochemical indicators, which can be used in exploration for additional Kombat-style polymetallic Fe-Mn and Cu-Pb (Ag) carbonate replacement deposits in the Otavi Mountainland. The study perused the development of an integrated exploration model for Kombat-style mineralization based on the association between Fe-Mn and Cu-Pb (Ag) ore at the Kombat Mine, in order to increase the mineral resource at the mine. In addition, other exploration companies might also benefit from the use of the model elsewhere in the OML or in other localities of similar geological setting. Open Journal of Geology larly the Kombat Central Pit, the Iron-Manganese Pit near Shaft 03, the 900 East Pits (three pits relatively close to each other) and at the OMEG pits around the Gross Otavi Mine (Figure 2). Some rock samples were collected from the Kombat Mine underground level 2 (approximately at 40 m depth), the Asis Far West shaft development dumps, and from surrounding mineralized hills. In addition, underground samples from level 5 up to level 16 (approximately from 150 m to 600 m depth) were obtained from the collections of the National Earth Science Museum, Windhoek.
Sample preparation was carried out at the Geological Survey of Namibia (GSN). Samples were split. The first half was preserved, while a portion of the second half was prepared into polished blocks and thin sections to determine the nature of the ore, accessory and gangue minerals, as well as mineral paragenesis and microstructures. Other portions were crushed and milled (<75 μm fraction) to provide homogeneous pulp for XRF whole-rock geochemical analysis to quantify the elements and for XRD analysis to provide quantitative mineralogy.
At Queens University, Kingston, Ontario, Canada, nine (9) gangue mineral samples, mainly quartz and calcite crystals selected for fluid inclusion study, were placed in a heating and freezing stage chamber (Linkham TH600 stage), which is connected to a microscope for observation of fluid bubbles (liquid).
While a fraction of the material of each of the thirty-two (32) pulp samples we reanalyzed by XRD to determine the mineralogical composition of ore and host rocks at the University of Cardiff, UK.

Location of the Study Area
The study area is in the Grootfontein District, Otjozondjupa Region of northern

Geology of the Kombat Mine, Otavi Mountainland (OML)
The Kombat Mine is situated on the northern limb of the Otavi Valley Syncline [8] [10]. The basement rocks of the OML comprise felsic (alkaline/calc-alkaline granites and granodiorites) and mafic rocks (anorthosites, gabbros and amphi- from 900 to 600 Ma, involving intracratonic rifting followed by spreading, continental collision and final basin closure at 520 Ma [15]. These rocks underwent various stages of deformation and metamorphism during the Damara Orogeny resulting in distinct tectonostratigraphic zones in Figure 3, which are based on metamorphic grade, degree and style of deformation and stratigraphy; within and around the OML these are the Northern Platform (NP), the Northern Margin Zone (NMZ) and the Northern Zone (NZ). The geological succession of the NMZ, in which the Kombat Mine-sites are situated consists of the basal Nosib Group followed by the Otavi Group carbonates and glacial deposits, and finally the Mulden Group clastic sediments. The 780 -740 Ma Nosib Group represents horst-graben deposits [16] laid down during the break-up of the Mesoproterozoic Rodinia Supercontinent. It is made up of arkose, quartzite, shale, phyllite and local conglomerate (Nabis Formation) and the predominantly volcanogenic Askevold Formation (epidote, agglomerate, chlorite schist and dolostone). The clastic Nosib sequence is followed by the Otavi Group carbonates (Figure 2), which are subdivided into the Ombombo, Abenab and Tsumeb Subgroups by two global glaciation events, namely the 720 Ma Chuos Formation and the 635 Ma Ghaub Formation. The two formations consist of characteristic glacial diamictites and are followed by cap carbonates represented by thick successions of carbonate platform deposits [12] [14] [17]. These carbonates were laid down during deglaciation and renewed sea level rise [10]. The syntectonic Mulden Group deposited between the first (D1) and second (D2) phases of Damaran deformation on the Northern Platform comprises the erosional debris of the developing orogen. It consists of the lower Tschudi (sandstone, shale, siltstone, conglomerate) and the upper Kombat Formation (phyllite, shale) deposited at approximately ~575 Ma [18].

Geological Observations
Field investigation and sampling was conducted in the open pits, particularly the Kombat Central Pit, the Iron-Manganese Pit near Shaft 03, the 900 East Pits (three pits relatively close to each other) and at the OMEG pits around Gross Otavi. Ore samples were collected from the Kombat Mine underground level 2, the Asis Far West shaft development dumps and from surrounding mineralized hills. The ore samples represented vein-type to breccias and disseminated mineralization, whereas those collected from underground appear to consist of massive sulfide ore minerals.

Iron-Manganese (Fe-Mn) Pit
Fe-Mn ore is exposed in the walls of a 50 m diameter pit shown in Figure 5

900-East Pits
The three open pits are located within 50 m of each other, ranging from 20 to 50 m in diameter. Figure 8 illustrates pit walls consist of well-exposed remnants of laminated, folded calc-silicate layers that host malachite as fracture plane coat-    fractured to brecciated grey dolostone with ferruginous chert of the upper Hüttenberg Fm [17]. The dolostone contains the paleo-cavities that appear interconnected by steep fault planes (F: 90˚/205˚; 65˚/178˚). These cavities are characterized by a reddish-brown ferruginous and matrix-supported breccia containing chalcocite, malachite and hematite ore associated with quartz and calcite gangue minerals (Figure 9(b)). Also, some black manganese dendritic structures as well as euhedral to subhedral quartz veins have been observed (Figure 9(c)) and sampled for fluid inclusion analysis. One kilometer further to the west, an adit cut into a hill of laminated, grey, ferruginous and cherty dolostone hosting chalcocite and malachite mineralization along the ramp roof proved unsafe for sampling. However, a well-exposed botryoidal shaped unit of brownish calc-silicate and whitish siliceous surfaces along the ramp wall ( Figure  9(d)) was sampled.

Mineralogy and Petrology of the Ores
The polished block samples were viewed under 20-micron magnification of a reflected light microscope to determine their contents of ore minerals and to obtain photomicrographs of these ores. The image in Figure 11 shows that brown bornite (Bn) is the dominant ore mineral, followed by yellow to orange chalco-  and pyrite (Py); hematite (Hm) and jacobsite (Jb) appear as exsolution lamellae within the syn-sedimentary chalcopyrite generation, which is associated with bioturbation ( Figure 12(c)). Hence, the overall paragenetic sequence of the Kombat deposit in Figure 13 can be summarized as follows: Syn-sedimentary (iron-manganese + copper) → Hydrothermal (primary copper ore minerals) → Secondary copper ore minerals → Alteration of secondary ore minerals (supergene enrichment).
Thin-section images in Figure 14 show of opaque, presumably ore minerals (Samples AN19200 and AN19203; Figure   14(l), Figure 14(m)) could be due to thermally induced annealing, which created polycrystalline grain aggregates and dissolution textures [19]. In addition,

Analytical Methods
The samples collected from the study area were analyzed by X-ray fluorescence (XRF), X-ray diffraction (XRD), and heating and freezing (Linkham TH600) stage.

X-Ray Fluorescence (Portable XRF)
A total of forty (40)

Heating and Freezing (Linkham TH600) Stage
A total of nine (9)

XRD Major, Minor and Trace Minerals of Kombat Mine Samples
The X-Ray Diffraction (XRD) results presented in the remainder of the minerals occurs in trace quantities.

Trace Elements of Kombat Mine Samples
XRF results are plotted on the discriminant ternary diagram for sedimentary rocks in Figure 15, which shows that the Kombat samplesfall within the hydrothermal, diagenetic and mixed (hydrothermal-diagenetic-hydrogenetic) fields.
On sample localities based ternary diagrams in Figure [23] show that most of the Kombat Mine samples plotted in the Cu-Pb SEDEX field and are clearly distinct from VMS deposits as illustrated on individual sample localities in Figure 18 [24]. In addition, various elements were plotted against copper on the binary diagrams in Figure 19 & Figure 20 to display the Pearson correlation coefficient, a measure of the strength of a linear association between a selected element and copper denoted by r [25]. Figure 19 shows a negative Pearson correlation  Sample  Number  AN19192  AN19193  AN19194  AN19195  AN19196  AN19197  AN19198  AN19199  AN19200  AN19201  AN19203  AN19206  AN19208  AN19209  AN19210  AN19211  AN19212  AN19213  AN19214  AN19217  AN19219  AN19220  AN19221  AN19222  AN19225  AN19228  8649  8650  8659  8672  8673  and W). Thus, elements with moderate to strong linear association r ≥ 0.4 [25] with copper were selected for coefficient of determination (R 2 ) evaluation and the results in Figure 21 shows that W, S, Mo, Ag, Pb, Ca and Mn have a significant correlation of R 2 ≈ 0.70 with Cu.

Fluid Inclusion Analysis of Kombat Mine Samples
Results were obtained from an average of five primary or second-generation of fluid inclusions per sample of calcite, quartz, sphalerite, wulfenite and cerussite.  [28] in Figure 22 shows that total homogenization occurred at temperatures ranging from 160˚C to 200˚C.

Discussion
Hydrothermal cavities, fracture fills and breccia-style systems such as those at Gross Otavi Mine and the Kombat Mine pits are tectonically controlled, epigenetic characteristics of MVT-type deposits [30]. The deposition sites are determined   The copper-lead ores consist of bornite (Bn) and chalcopyrite (Cp) which are both being replaced by twin lamellae textures of chalcocite (Cc) and exsolution of covellite (Cv) after bornite and chalcocite, while covellite veins have been terminated by pyrite (Py). These copper ore minerals are associated with hydrothermal dissolution and dolostone replacement features common in and about MVT deposits [33]. The hydrothermal fluid generation requires three components: 1) Brine source: possibly through evaporation of seawater in the closing Khomas Ocean and halide dissolution generated the basinal sediment brine. 2) Sulfur source: through a metamorphic breakdown of sulfate minerals and organic materials, 3) Metal source: metals may have been leached from the underlying sedimentary package through which the brines migrated [34]. During brine migration, the replacement textures observed in Figure 14 could indicate leaching of copper, zinc and tin from clays, micas and amphiboles, whereas lead originates most likely from sandstones [35].

Genetic of the Kombat Style Deposit
Thus, the primary hydrothermal sulfide deposits formed by metamorphic dehydration, which supplied the sulfur and brines through tectonic pumping related to the Damara Orogeny, while the upward migration of the hydrothermal brines 2) Penetrative fractures acted as passageways for ascending hydrothermal fluids rich in Cu, Pb and Ag; subsequent hydraulic fractures (veins) are filled with primary copper and lead minerals (Figure 6(b)). Silver most likely occurs as a trace element in both sulfide hosts.
3) Further deformation lead to localized brittle fracturing/faulting and remobilization of primary mineralization as well as the formation of secondary minerals along fracture cleavage/faults (S3; Figure 4(e)).
4) The first phase of alteration of secondary ore minerals (S3) lead to supergene enrichment of breccia ore minerals in zones of lower stress such as cavities and fractures (Figure 7). 5) Finally, the ore minerals are exposed to retrograde conditions during weathering, uplifting and erosion initiating a second phase of alteration and the formation of oxide ores. The paragenetic sequence is summarized in Figure 13, while Figure 23 summarizes the subsequent genetic model.

Exploration Model for Kombat Style Deposit
The Kombat Mineralization exploration model would commerce with investigating the upper Hüttenberg dolostone, and calc-silicates lithological changes of the carbonate platform sequences [33]. In the carbonate platform sequence, hydrothermal alterations such as dolomitization and silicification would be the most widespread evidence but are limited to tectonic structural control, characteristics of the MVT-type deposits [30].  (Table 1). On an outcrop scale, the dissolution and dolostone replacement textures associated with Cu-Pb ores can be observed ( Figure 11 and Figure 12), common in and about MVT deposits [33]. Geochemical soil and rock sampling for portable XRF analysis carried out at 1:10,000 scale or 50 -100 m gridding would generate drill targets [40] using the pathfinder elements ( Table 3). The pathfinder elements were calculated from the portable XRF results based on Kombat historic mine grade of 2% Cu and then normalized to the current resource statement economic cut-off grade of 0.6% Cu [6]. The XRD and optical sample analysis can be used to confirm the potential ore minerals that contribute to the anomalous values of portable XRF results, thus, increase the exploration target confidence.

Conclusions
The carbonate hosted Kombat-style mineralization is structurally controlled and  calcite (Ca) and quartz (Si), with traces of anhydrite, gypsum and baryte. The exploration model for the Kombat MVT-type deposit can be based on field mapping focusing on the steep, south-southwest dipping (F: 79˚/195˚) faults and shear fractures. On outcrop scale, one should look for recrystallization textures including euhedral calcite, quartz, tension gashes (veins), often associated with Cu-Pb-Ag ore minerals. In addition, follow up geochemical soil/rock surveys at 1:10,000 scale or 50 -100 m grid spacing using a portable XRF complemented by XRD analysis could help to delineate discrete ore zones using the established mineralogical and geochemical exploration indicators. Finally, magnetic data can be used to delineate associated syn-sedimentary, stratiform Fe-Mn mineralization which is associated with the MVT-type deposit at the Kombat Mine.