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Rosh Pinah |
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Namibia |
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Zn Pb Ag
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Super Porphyry Cu and Au
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The Rosh Pinah Zn, Pb, Ag deposit is located in southwestern Namibia, ~800 km south of Windhoek and ~20 km north of the Orange River, on the edge of the
Namib Desert (#Location: 27° 57' 14"S, 16° 45' 50"E).
Regional Setting
Rosh Pinah is basically a stratabound deposit, hosted by arkoses and quartzites in the lower sections of the Neoproterozoic to early Palaeozoic Gariep Complex, an equivalent of the Damaran Supergroup. The host sequence, the Rosh Pinah Formation, belongs to
the Hilda Subgroup of the Port Nolloth Group, which is part of the Neoproterozoic Gariep Terrane, structurally overlying the Palaeoproterozoic magmatic arc of the Richtersveld Sub-province (Vioolsdrif Terrane), a suite of igneous rocks belonging to the Namaqualand Metamorphic Complex. The contact between the Namaqualand Metamorphic Complex and the overlying Gariep Terrane is a NE vergent thrust, known as the 'Gariep Front' (Cornell et al., 2006).
The Gariep Terrane comprises two distinct tectono-stratigraphic sub-terranes; the eastern, para-autochthonous Port Nolloth Group, representing a passive continental margin lapping onto the western edge of the Kalahari Craton, and the allochthonous ophiolitic Marmora Superterrane, thrust NE over the Port Nolloth Group.
The Port Nolloth Group occurs as an arcuate belt stretching from Luderitz in southern Namibia to Port Nolloth in far northwestern South Africa. The stratigraphy of the group comprises, from the base (after Jensen et al., 2018):
• Stinkfontein Subgroup - the lowermost stratigraphic unit of the Port Nolloth Group, which comprises a basal conglomerate and quartzite of the Lekkersing Formation, overlain by feldspathic quartzite and minor felsic volcanics of the Vredefontein Formation. Although well developed in northern South Africa, the Stinkfontein Subgroup is thinner or absent in southern Namibia (Frimmel 2018).
• Gannakouriep Suite - a regionally extensive mafic dyke swarm that extends over an area of ~300 by up to 150 km, cutting pre-Gariep basement, and at a lower density, Stinkfontein Subgroup siliciclastics, but being truncated by the diamictite of the Kaigas Formation at the base of the overlying Hilda Sub-group (see below). They are believed to have been emplaced between 770 and 750 Ma. Individual dykes are typically >5 m thick, with lengths of up to 100 km. These dykes are interpreted to have accompanied the extension that opened the Gariep Rift basin (Frimmel 2018).
• Hilda Subgroup - a predominantly calcareous sequence of intercalated pelites, feldspathic litharenites, quartzites and meta-conglomerates (Alchin, 1993). The basal unit of the sub-group is the up to 100 m thick Kaigas Formation, a diamictite predominantly composed of subrounded gravel- to boulder-sized basement clasts suspended in a matrix that ranges from argillite to feldspathic sandstone with complex lateral facies changes. This unit may be of glacigenic origin (Frimmel et al., 1996; Harland, 1983; Hambray and Harland, 1985). The Kaigas Formation is succeeded by the Rosh Pinah Formation (described in more detail below), which consists of up to at least 850 m of arkosic sandstone, organic-rich shale, carbonate and felsic volcanic rocks that were deposited in an actively rifting graben (Alchin et al., 2005; Macdonald et al., 2010). The volcanic rocks, which include felsic lava flows and pyroclastic rocks, are thickest ~15 to 20 km north of Rosh Pinah near the Skorpion Mine, where they have also been referred to as the Spitzkop Formation (Macdonald et al., 2010). These rhyolite flows from the Rosh Pinah Formation have been dated at 752±6 Ma (U/Pb zircon; Borg et al., 2003) and at 741±6 Ma (Pb/Pb, Frimmel et al., 1996). The Picklehaube Formation is exposed to the SE, and predominantly west of Rosh Pinah, and is composed of >200 m of carbonates, including laminated, variably dolomitised limestone, mudstone, marl and lesser arkosic sandstone, with intercalated massive dolostone towards the middle of the formation. To the SE it lies unconformably on basement, while to the west it rests conformably on the Kaigas Formation. As such, it is regarded as a distal equivalent of the Rosh Pinah Formation.
• Spitskop Suite, which comprises felsic intrusive bodies that include fine-grained granite, quartz porphyry and feldspar porphyry (Alchin et al., 2005) that occur in close proximity to the 'Spitzkop Formation' volcanic and volcaniclastic of the Rosh Pinah Formation. The microgranite contains abundant felsic volcanic xenoliths that appear identical to the volcanic rocks of the 'Spitzkop Formation', which they apparently intrude locally. Hence they are assumed to be of a similar age. Mafic intrusions of the Koivib Suite to the west (Alchin et al., 2005) occur as amphibolites. Although their age in not determined, they, like the Spitskop Suite are pre-orogenic (Frimmel, 2018).
• Wallekraal Formation, which overlies a major unconformity above the Hilda Subgroup/Rosh Pinah Formation. It is dominated by coarse grained siliciclastic sedimentary rocks, including mature, well sorted quartz-pebble conglomerate, gritstone and arkose that grade up to mudstone in local fining upward cycles. Lateral and vertical facies changes include boulder beds with clasts up to 1 m in diameter, whilst elsewhere laminated, ripple marked mudstones predominate. Locally, cream coloured dolostone beds several tens of cm thick are encountered, as well as olistosrome blocks of dolostone tens of metres across that are found with rounded quartz pebbles in a finer matrix. These observations suggest an unstable tectonic regime near a basin margin (Frimmel, 2018)
• Numees Subgroup is seen at some locations to conformably overlie the Wallekraal Formation, whilst elsewhere the contact is a thrust, and at other locations again, it unconformably overlies the Hilda Subgroup. It consists of banded iron formation, quartzite, pelite and massive glaciogene diamictite correlated with the global 716.5 Ma Sturtian glacial event (Macdonald et al., 2010).
• Holgat Subgroup, which unconformably overlies the Numees Subgroup. It is composed of turbiditic meta-arkose, meta-greywacke, metapelite and H2S rich marbles. It commences with 1 to 120 m of dark grey laminated limestone of the Bloeddrif Member, overlain by, and interfingering with, ~250 m of carbonate and argillite of the Lower Holgat Formation. The upper parts of the latter interfinger with and are overlain by, the Dabie River Formation, interpreted to be the shallow water equivalent of the Lower Holgat Formation, consisting of as much as 160 m of carbonate, including stromatolites, giant ooids and intraclast breccias. The Lower Holgat Formation includes olistoliths of the Dabie River Formation. The Lower Holgat and Dabie River formations are overlain by the Namaskluft Diamictite which ranges from 5 to 240 m in thickness and
comprises both massive and stratified diamictite units with clasts from all of the underlying stratigraphy and basement. It is similar to, and has often been confused with the Numees Subgroup, complicating stratigraphic interpretation (Macdonald et al., 2010).
To the west of Rosh Pinah, the Marmora Sub-terrane of the Gariep Terrane is an allochthonous belt interpreted to represent obducted ocean floor deposits, comprising a mixed package of sedimentary, volcanic and intrusive rocks. It is sub-divided (after Jasper, 1994) into the:
• Schakalsberg Sub-terrane, which has, in turn, been subdivided into the up to 6 km thick pile of meta-basic lavas of the Grootderm Formation, which are capped by and interfinger with dolostones of the Gais Formation (Hartnady et al., 1990). The Grootderm Formation includes pillow lavas, pillow breccia, tuffs and lava-flows which are predominantly tholeiitic basalts, with some alkali basalts in the upper part of the sequence. This volcanic pile is interpreted to be post to late rift, but pre-orogenic. The Gais Formation is a pinkish dolostone with abundant cherty intercalations, fine laminations and stromatolite bioherms, but lacks terriginous siliciclastics. The Schakalsberg Sub-terrane has been subjected to three metamorphic events, the first a very low pressure hydrothermal oceanic event, the second was syntectonic, characterised by similar low temperatures, but higher pressures, indicating burial to 10 to 15 km, possibly related to a subduction process. The third was low grade, and of a regional nature. This succession and sequence of events is interpreted to reflect subduction and subsequent obduction of a sea mount chain/oceanic plateau that had originally been extruded remote from a continental margin.
• Oranjemund Sub-terrane comprises metagreywackes which vary from poorly deformed cyclothemic turbidites to intensely transposed, poly-deformed mica-schists, with local minor intercalations of metavolcanic chlorite schists (Hartnady et al., 1990), as well as phyllites, schists and minor quartzite, resembling the Holgat Sequence of the Port Nolloth Group (Von Veh, 1988).
• Chameis Sub-terrane, which comprises a heterogeneous mélange, consisting of various exotic blocks, 0.1 to 100 m in size, from different oceanic environments, brought into close proximity with each other, set within a highly tectonised metasedimentary sequence. The exotic blocks exhibit a distinct metamorphic history prior to that recorded by the metasedimentary country rocks, which have only undergone regional lower grade greenschist facies conditions (Hartnady et al., 1990).
To the east of Rosh Pinah and the Gariep Terrane, the Ediacaran to Lower Cambrian Nama Group was deposited in a foreland basin in response to the collision between the Congo, Kalahari and Rio de la Plata cratons (Germs and Gresse, 1991). It unconformably overlies the basement rocks and comprises a succession of shallow-marine and minor fluvial sedimentary rocks that is exposed over much of central and southern Namibia. For more detail, see the description at the end of the 'Tectonic and Geological Setting' section of the Kalahari Copper Belt in Namibia record.
Structural Setting
The Port Nolloth Group is a thick package of turbidites, derived from older hinterland and contemporaneous volcani-clastic rocks, deposited in a Neoproterozoic extensional rift basin during the early evolution of the Gariep Terrane. This basin developed in response to rifting along or near the suture between the Rio de la Plata and Kalahari cratons formed when the two had earlier collided at the end of the Mesoproterozoic. Deposition was terminated when extension was reversed and the rift basin underwent inversion as a result of the oblique approach and sinistral collision of the Rio de la Plata and Kalahari cratons at ~540 Ma. This resulted in multiphase deformation, commencing with an approximately NW-SE directed D1 compression, producing z-folds, particularly in the arkose dominated sequence of the Rosh Pinah Formation. Tight F1 minor folds with an accompanying S1 axial planar cleavage formed in lithologies such as thinly-interbedded carbonate-sandstone units of the Lekkersing Formation. This was followed by rotation of the stress field, resulting in D2 which initially produced west-east shortening and later a WSW-ENE compression of the horizon that contains the base metal sulphides at the Rosh Pinah mine (known locally as the 'ore equivalent unit'). F1 folds are generally SE verging overfolds, refolded by the penetrative WSW-verging F2 overfolds. A major F1 anticlinal axis running approximately through the centre of the mine, refolded by F2, gives rise to the locally termed 'Rosh Pinah Anticlinorium'. It produced steep to inverted plunges of F2 and sheath folds in the southern parts of the mine. Folding is disharmonic due to a competency contrast between the arenites, argillites, microquartzite and carbonates resulting in the formation of saddle reefs and keel type mineralisation. The deposit consequently occurs as a series of discrete carbonate and stratabound lenses located on F2 fold hinges or steeply plunging fold limbs connected by a partially attenuated band of the 'ore equivalent unit' (Brayshaw and Watkeys, 2018; Jensen et al., 2018).
Deposit Geology and Mineralisation
The host to the Rosh Pinah deposit, the Rosh Pinah Formation, is distinguished from the more or less contemporaneous sequence further south in South Africa by the presence of felsic volcanic and volcaniclastic rocks. The thickest accumulation of these felsic volcanic rocks is ~15 to 20 km north of Rosh Pinah, which is considered to represent the main volcanic centre. Those proximal to the interpreted volcanic centre range from rhyodacite to rhyolite and comprise massive to flow banded quartz-alkali-feldspar rhyolite to rhyodacite with local spherulitic textures, autoclastic or hyaloclastic breccias, locally reworked Iapilli tuff breccias, and a variety of volcaniclastic units that reflect changes in the deposition regime away from the eruptive centre. These facies are restricted to the more active parts of the basin, whilst in distal sections, sedimentation varied from planar laminated limestone to mudstone, followed by platform carbonates (e.g., the Picklehaube Formation). The local presence of metabasalt extrusives and metagabbro intrusions, and the absence of intermediate igneous rocks suggests a bimodal magmatic regime related to extension within the Gariep Rift. Sedimentary rocks directly associated with the volcanic facies comprise intercalated ripple-marked quartzite and dolomite, which are, in places, strongly ferruginous.
The Rosh Pinah Formation is composed of repetitive upward-fining sedimentary cycles, interpreted to reflect rapid deposition with intervening quiescent periods, related to repeated pulses of reactivation of basin bounding faults, followed by thermal and mechanical subsidence (Alchin et al., 2005). Uplift and tilting in the active parts of the basin led to partial erosion of the Rosh Pinah Formation and the development of intraformational breccias and olistostromes in overlying and equivalent sequences, e.g., the Wallekraal Formation (Frimmel 2018).
The Rosh Pinah Formation unconformably overlies a massive to thick bedded, laterally discontinuous diamictite 5.5 km NE of Rosh Pinah, correlated with the Kaigas Formation, although in some locations the two appear to interfinger. The basal section to the north is occupied by a thin unit of mafic volcanics, succeeded by thicker felsite and rhyolite lavas, pyroclastics and epiclastics, which thin to the south towards Rosh Pinah. These are followed by an ~150 m thick sequence that comprises, from the base, arkose and grit; arkose, grit and conglomerate; ~50 m of dark argillite and arkose; and another 100 m of arkose and siltstone, the upper 20 to 40 m of which is silicified and brecciated arkose to quartzite. This pile includes repeated cycles of laminated, graded bedding and cross-bedded arenites to siltstones (Frimmel 2018). Whilst Jensen et al. (2018) after Mouton (2006) show a sequence as listed above, Frimmel (2018) after Alchin et al. (2005) indicate 3 to 20 m thick interbeds of dolostone within most of these cycles. This, the lower part of the Rosh Pinah Formation, is interpreted to represent a transgressive period with relative stable tectonic and climatic conditions, although the latter was more likely warmer.
This section of the formation is overlain by the ~25 to 30 m thick 'ore equivalent unit' which is ~200 to 400 m above the base of the regionally >850, up to ~1220 m thick Rosh Pinah Formation. It is a strongly silicified unit of interbedded carbonaceous argillite and fine quartzite with bedding-parallel laminations of pyrite accompanied by galena and sphalerite. It includes an overlying dark grey arkose with mudstone intercalations and partly mineralised dolomitised carbonate lenses. This unit is described in more detail below.
The top of the 'ore equivalent unit' is interpreted to represent a regression which resulted in erosion of sections of the unit, followed by renewed but gradual transgression (Mouton, 2006). This begins with an ~100 m thick siliciclastic succession composed of cycles that have upward-fining graded bedding with coarse-grained, locally pebbly sandstone at the base and fine-grained mudstone at the top of each individual cycle. The bottom contacts are typically sharp, whilst the tops are erosive with rip-up mudstone fragments. The influx of minor carbonate material during high-energy flows is suggested by the calcareous nature of the basal layers of cycles. This siliciclastic succession is followed by a thick succession of pebbly to gritty massive sandstone containing unsorted, matrix-supported clasts of rounded to subrounded, mature quartz and angular carbonate clasts ranging from pebble to boulder size. This is overlain by an 8 m thick carbonate unit representing the first unmineralised carbonate in the formation, the base of which is extensively broken, grading downslope into a zone of large carbonate clasts within a gritty to pebbly sandstone matrix. The overlying succession is composed of repeated cycles of sandstone and mudstone units that grade upwards from pebbly to gritty to fine-grained sandstone, siltstone and mudstone, locally including several tens of centimetres thick carbonate beds. Overall, the carbonate content increases up section with some of the limestone beds having undergone diagenetic dolomitisation and ferruginous alteration (Frimmel 2018). A few thin tuff bands and and mafic volcanic rocks are also found within the sequence. In total, the sequence above the 'ore equivalent unit' is ~200 m (Mouton 2006) or ~400 m (Alchin et al., 2005) thick. The upper contact of the Rosh Pinah Formation is interpreted to reflect an abrupt sea level regression, producing an unconformable contact with the overlying Wallekraal Formation (Frimmel 2018; Jensen et al., 2018).
The 'ore equivalent unit' consists of a well banded to massive carbonaceous cherty zone or micro-quartzite, in places grading into: argillite; various carbonate bearing rocks; sugary (leached) quartzite; lenses and bands of massive sulphides (i.e., defined at Rosh Pinah as >30% sulphides), comprising mixed pyrite, sphalerite and galena; argillite and intercalations of generally poorly mineralised quartzite. The microquartzites are fine grained and dark due to their carbonaceous content. Barium rich carbonate is an important constituent in places. The lower sections of the ore bed are generally Zn rich micro-quartzite, overlain by further micro-quartzites or carbonates with a higher Pb:Zn ratio, while the hanging wall is another micro-quartzite grading to argillite. Most ore is within the micro-quartzites and seldom in interbedded argillites, but also within the well developed carbonate alteration.
The ore minerals are generally present as intergranular disseminations and discrete blebs associated with a fine grained sugary quartz-carbonate matrix, or as thin bands from 1 mm to a few cm's thick of massive sulphide. Irregular barite-carbonate or dolomite lenses are present in the central or lower part of the ore bed.
Massive sulphide bands may be up to a few metres thick in sections of the mine within micro-quartzites and occasionally argillites, and may grade laterally into disseminated ore within the micro-quartzites or carbonates.
In contrast to the hanging wall quartzite, which is generally little fractured, the footwall quartzite is intensely fractured forming a breccia, which is silicified and carries sulphide and carbonate veining.
Within the 'ore equivalent unit' described above, three main mineralisation types are differentiated as follows (after Jensen et al., 2018):
• Microquartzite and argillite, which is the primary mineralisation type at the Rosh Pinah mine. It is a silicified, grey to dark grey, fine-grained and laminated unit locally termed 'microquartzite mineralisation'. It contains alternating millimetre to centimetre wide bands of sulphides (sphalerite, pyrite and galena plus minor chalcopyrite) and is cut by a network of mineralised quartz and carbonate veins.
• Arkose/breccia, where mineralisation occurs as a breccia matrix and veins in silicified arenite lithologies (locally termed breccia mineralisation) or as disseminated base-metal sulphides (locally known as arkose mineralisation) which can reach economic grades. The
breccia mineralisation is commonly found in the immediate footwall sections of the 'ore equivalent unit'.
• Carbonate mineralisation, which is considered to represent a late hydrothermal phase that may have remobilised the stratabound ore described above, and provides a significant economic component of the resource. Carbonate has replaced the arenites, both in the hanging wall and footwall of the mineralised horizon with a continuous range from slightly carbonatic arenite with preserved textures such as large, ghost feldspar grains, to pure carbonate in which all original textures have been obliterated. A near-total base metal enrichment of the carbonate mineralisation gives rise to massive sulphides. When the carbonate has been leached from the carbonate mineralisation, and only quartz grains and sulphides remain, the mineralisation is locally referred to as sugary quartz ore.
The Rosh Pinah mineralising system is suggested to have been initiated by the magmatism of the Spitskop Suite, mafic edifices of the Koivib Suite, and the interpreted coeval 752 to 741 Ma 'Spitskop Formation' volcanism, or perhaps a larger related parental magma chamber at depth. The same magmatic event likely drove hydrothermal circulation and plumbing via the rift-fault system of the opening Gariep Graben/Rift. It has been suggested this circulation leached base metals from the basin-fill siliciclastics that had been eroded from the 2.0 to 1.7 Ga Palaeoproterozic, calc-alkaline Richtersveld Arc in the immediate hinterland (Frimmel et al., 2004). Alternatively hydrothermal fluids may have been derived from the Neoproterozoic magmatism and deep parental magma chamber.
The 'ore equivalent unit' originally represented a reduced-carbonaceous facies layer containing probable diagenetic pyritic ± other base metal sulphides, at a transition from a more porous and permeable suite to an overlying sequence containing a higher percentage of impermeable argillites.
The first paragenetic stage at Rosh Pinah, is represented by silicification and base metal mineralisation of the carbonaceous and pyritic micro-quartzite and argillite unit at structurally favourable intervals, in the process enhancing its impermeable nature. This was followed by overpressure, hydraulic brecciation, silicification and further base metal deposition of the footwall arenite capped by the less permeable, now silicified argillite-microquartzite cap. The final stage involved the evolution of the hydrothermal fluid chemistry to a more carbonatic composition. The more porous, arenitic host and footwall rocks were preferentially altered to a dolomite gangue, with remobilisation of existing mineralisation and introduction of further base metals.
Orogenesis at ~545 Ma, which resulted from sinistral transpressive continental collision between the Rio de la Plata and Kalahari cratons (e.g., Rapela et al., 2011) caused complex folding and faulting. The fold style is west vergent and asymmetric to overturned with steep plunges. Competency contrasts produced considerable disharmony within the ductile units of the mineralised zone, with mechanical remobilisation and transposition of both primary and secondary carbonate and accompanying mineralisation into fold hinges and structural dilations.
Individual Ore Lenses
The ore deposit comprises at least 15 separate or interconnected lenses, distributed over an area of some 2 x 0.75 km, to a depth of at least 1 km, each containing from ~0.15, up to 2 to 3 Mt.
Individual lenses or group of lenses are clustered to form a number of 'Orefields'. The mineralisation within these lenses and orefields includes an amalgamation of the different mineralisation types described above, some of which are zoned across an orefield, as follows:
• Western Orefield 3 - which occupies the northwestern section of the deposit and is truncated to the west by the NW striking Northern Fault. It is semi-tabular, strikes ~NW-SE at 325° and dips NE, rotating to SW in the upper sections. The main ore type grades to the SE from predominantly microquartzite → mixed microquartzite and carbonate → carbonate and carbonate breccia dominated → arkose breccia. The latter includes arkose, microquartzite breccia, arkose breccia, as well as chlorite and biotite-chlorite schist. Overall the single lens that constitutes the Orefield comprises 7% arkose breccia, 58% carbonate and 36% microquartzite ore. It is composed of numerous lensoid accumulations of sphalerite and pyrite that locally merge into a single mass. The orefield has a highly irregular hanging wall, the dip of which varies from flat to steep. Zoning of mineralisation indicates a central high copper, iron and zinc domain which gives way to a sphalerite and galena domain as it becomes distal to this central zone. Economic zinc grade occur throughout the orefield, although the higher grades are restricted to distinct zones or structural bands. The carbonate zones typically host higher grades than that of the microquartzite, although these are also internally zoned. The carbonate hosting the mainly massive sulphide mineralisation, typically ranges from 5 to 45 m in thickness.
• Eastern Orefield forms the southeastern section of the deposit and comprises two lenses on the eastern and western limbs of the Eastern Orefield sheath fold. This structure is a Z-fold, comprising a western syncline and an eastern anticline, flanked by two steeply dipping sinistral faults. Carbonate mineralisation constitutes 80% of the orefield, occurring as a complete replacement of the primary microquartzite/argillite mineralisation both in the hanging wall and footwall. It is light grey and fine grained with irregular sugary quartzite zones. The sphalerite-galena-pyrite mineral assemblage is mostly disseminated throughout the carbonate. Chalcopyrite occurs as minor disseminations, while alabandite (MnS) is locally found as coarse grained blebs. Microquartzite/argillite mineralisation, which is dark grey, fine grained and laminated, is pervasively silicified and accounts for ~15% of the mineralised zone. Sulphides, predominantly sphalerite-pyrite-galena ± chalcopyrite, predominantly occur in millimetre to centimetre thick bands/laminations, with minor amounts in fractures perpendicular to the laminae. Massive sulphide mineralisation also mostly occurs in the microquartzite ore. The arkose/breccia constitutes ~5% of the mineralisation, mostly as veins in the breccia or immediate hanging wall.
• Southern Orefield 1, is in the southwestern section of the deposit, and comprises North and South lenses, hosted within a thick arenitic sequence. The northern lens lithologies are moderately to poorly mineralised and are well laminated microquartzite and argillites with disseminated pyrite. Carbonate is minor, but in most cases, where present, hosts the highest grade mineralisation, occurring as dolomite with sphalerite, galena and pyrite. The microquartzite ore comprises quartz, muscovite, K feldspar, pyrite and sphalerite, with galena interstitial to the sphalerite. Massive ore occurs with either a quartzite of dolomite matrix to sphalerite, galena and pyrite. The hanging wall rocks are mainly light to dark grey, fine to coarse grained arkose, composed of sub-rounded to angular, mainly feldspar and quartz grains, set in a fine grained matrix. These are intercalated with centimetre to decimetre thick argillite horizons. Footwall rocks are fine to very coarse grained, grey to dark grey arkoses with rare thin argillite or microquartzite beds and millimetre to centimetre thick quartz veinlets. This lens appears to occur as a tightly folded anticline.
The southern lens rocks are similar to those of the northern lens, with the main mineralisation hosted by carbonate units toward the hanging wall, underlain by a thick, weakly mineralised argillite/microquartzite or mineralised arkose zone. The former are banded and sometimes display microfolding and slumping structures. This lens forms the eastern limb of a synclinal structure. The carbonate ore also comprises dolomite with sphalerite, galena and pyrite, whilst massive ore has a matrix of dolomite and quartz with sphalerite, galena, chalcopyrite and pyrite.
• Southern Orefield 3 is a single lens located in the southwestern part of the mine, immediately SW of, and parallel to, Southern Orefield 1. It is dominantly composed of carbonate and arkose/breccia ores in the upper and lower levels respectively, with only minor amounts of argillite mineralisation occurring as remnants throughout the carbonate zone. It is a mostly thin lens, composed of smaller discrete lensoid bodies in the upper sections grading down into a single, thicker and more continuos lens at depth with the same orientation. The carbonate zone is moderately to well mineralised, with semi-massive intervals and contains pyrite and sphalerite, with lesser galena and chalcopyrite. The arkose zone is mostly hydraulic fractured with the sulphides, mostly pyrite and sphalerite, and lesser galena, occurring in the fractures. Zinc is predominantly hosted in the carbonate zone. Mineralisation varies from coarse grained, massive to disseminated and laminated.
• BAE Lens, located on the northwestern limb of the Eastern Orefield anticline. It is dominantly (~60%) composed of carbonate, followed by ~30% pervasively silicified, competent, microquartzite/argillite and minor (~10%) arkose breccia mineralisation in the footwall. The carbonate mineralisation is a complete replacement of the earlier microquartzite/argillite style and is mainly in the hanging wall. It is light grey, fine grained with irregular sugary quartzite zones and comprises a sphalerite-galena-pyrite mineral assemblage, mostly disseminated throughout the carbonate. Chalcopyrite is present as minor disseminations, while alabandite locally occurs as coarse grained blebs. The microquartzite/argillite mineralisation is dark grey, fine grained and laminated, containing sphalerite-pyrite-galena±chalcopyrite which occurs in millimetric to centimetric bands/laminations, fine disseminations and/or as massive sulphide. Coarse grained secondary sulphide is also common, remobilised in fractures, commonly at an angle to primary bedding. Arkose mineralisation contains sulphides occurring as coarse grained granular veins in the immediate footwall. The BAE lens is generally high grade with >15% Zn, typically occurring as semi massive to massive or finely disseminated sulphides which include sphalerite, pyrite and galena, with minor thin veinlets of chalcopyrite, and abundant alabandite.
• A1 Mine, which is located NW of both the BAE Lens and the Eastern Orefield, and NE of the Southern Orefield. Microquartzite is the dominant mineralisation style. It is fine to coarse grained and dark grey, with both massive and laminated textures, composed of honey coloured and brown sphalerite, pyrite and galena, and minor chalcopyrite. There is also minor grey, medium to coarse grained and banded carbonate and 'arkose mineralisation' occurring as fine grained and granular veins within the micro-quartzite.
• Other Lenses and Orefields - the gap between the northern end of the Southern and the southeastern Western Orefields is bridged by a number of lenses known as the Western 1, 2, 4 and 4_Extended. These individual lenses range from 0.02 through ~.14 to 1.2 Mt of ore at grades consistent with the resources listed below. B Mine is composed of two lenses SE of Western Orefield 3, NE of Western Orefield 1, 2 and 4 and NE of the A1 Mine, and contains a small, lower grade resource of ~0.25 Mt @ 4.38% Zn, 1.4% Pb, 41 g/t Ag.
History, Production, Reserves and Resources
In 1964, the mineral rights over known mineralisation at Rosh Pinah was held by Moly Copper Mining and Prospecting Co. (SWA) Pty Ltd. who entered into a joint venture with the South African Iron and Steel Industrial Corporation (Iscor) to explore the prospect. Drilling commenced in 1965, with sufficient reserves being proved to develop a mine prior to 1967 when construction commenced, followed by the first ore production in May 1969. The operation was originally owned by Imcor Zinc, Pty Ltd, a joint venture between Iscor and Moly Copper. A sharp drop in the zinc price towards the end of 1992 led the mine into a loss situation and liquidation followed in December 1994. It was on care and maintenance for a short period before restarting. From then Iscor was restructured with its new subsidiary Kumba Resources holding its share in Imcor. In November 2006, Kumba Resources changed its name to Exxaro Resources and operated the mine as the major owner of what had become Rosh Pinah Zinc Corporation Pty Ltd (RPZC) until 80.08% of that company was sold to Glencore in 2012. In August 2017, Trevali Mining Corporation acquired a portfolio of zinc assets from Glencore, including their interest in Rosh Pinah. Subsequently, Trevali expanded their ownership of RPZC to 90%.
Production from 1969 to 2000 was 14.5 Mt @ 7% Zn, 2% Pb, 0.1% Cu, 11 g/t Ag.
In 1999 proved + probable reserves were quoted at 6.59 Mt @ 8.7% Zn, 2.5% Pb, within a resource of 15 Mt @ 7.5% Zn, 2.2% Pb.
Remaining Mineral Resources and Ore Reserves as at 31 December, 2020 (Trevali Mining Corporation Ore Reserve statement, 2021) were:
Measured + Indicated Mineral Resources - 18.13 Mt @ 7.50% Zn, 1.87% Pb, 27.71 g/t Ag,
Inferred Mineral Resources - 4.01 Mt @ 7.27% Zn, 1.50% Pb, 28.21 g/t Ag,
including
Proved + Probable Ore Reserves - 11.19 Mt @ 5.98% Zn, 1.29% Pb, 21.11 g/t Ag.
Between 1969 to the end of 2017, a total of 27.0 Mt of ore have been mined from the various ore lenses at Rosh Pinah (Jensen et al., 2018).
In addition to the references listed below, sections of this summary are drawn from: Jensen, T., Blakley, I.T., Jacquemin, T. and Patel, A.A., 2018 - Technical Report on the Rosh Pinah Mine, Namibia; An NI 43-101 Technical Report prepared by Roscoe Postle Associates Inc., for Trevali Mining Corporation; 234p.
The most recent source geological information used to prepare this decription was dated: 2001.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Rosh Pinah
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Selected References:
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Alchin D J and Moore J M, 2005 - A review of the Pan-African, Neoproterozoic Rosh Pinah Zn-Pb deposit, southwestern Namibia : in S. Afr. J. Geol. v108 pp 71-86
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Alchin D J, Frimmel H E and Jacobs L E, 2005 - Stratigraphic setting of the metalliferous Rosh Pinah Formation and the Spitzkop and Koivib Suites in the Pan-African Gariep Belt, southwestern Namibia : in S. Afr. J. Geol. v108 pp 19-34
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Anonymous 1999 - Namibia Base Metals - Rosh Pinah Has Secure Future: in Mining in Southern Africa Quarterly, 1999, 1st Quarter pp 19, 21
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Brayshaw, M M. and Watkeys, K., 2018 - Structural evolution and architecture of the outer margin of the Gariep Belt at Rosh Pinah, southern Namibia: in S. Afr. J. Geol. v.121, pp. 171-190.
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Frimmel H E and Board W S, 2000 - Fluid evolution in and around the Rosh Pinah massive sulphide deposit in the external Pan-African Gariep Belt, Namibia : in S. Afr. J. Geol. v103 pp 207-214
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Frimmel H E and Lane K, 2005 - Geochemistry of carbonate beds in the Neoproterozoic Rosh Pinah Formation, Namibia: Implications on depositional setting and hydrothermal ore formation: in S. Afr. J. Geol. v108 pp 5-18
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Frimmel H E, Jonasson I R, Mubita P, 2004 - An Eburnean base metal source for sediment-hosted zinc-lead deposits in Neoproterozoic units of Namibia: Lead isotopic and geochemical evidence: in Mineralium Deposita v39 pp 328 - 343
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Hodgson C J 1979 - Wall Rock Alteration in the Area of the C Orebody, Rosh Pinah Mine, South West Africa/Namibia: in Annual Report - Chamber of Mines Precambrian Research Unit no.16 pp 108-112
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Page D C, Watson M D 1976 - The Pb-Zn Deposit of Rosh Pinah Mine, South West Africa: in Econ. Geol. v71 pp 306-327
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Rozendaal A, Stalder M and Alchin D, 2005 - Wall rock alteration and lithogeochemical haloes associated with the sediment-hosted Rosh Pinah Zn-Pb-Ag deposit in the Pan African Gariep Belt, southwestern Namibia : in S. Afr. J. Geol. v108 pp 119-134
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van Vuuren C J J 1986 - Regional Setting and Structure of the Rosh Pinah Zinc-Lead Deposit, South West Africa/Namibia: in Anhaeusser C R, Maske S, (eds), Mineral Deposits of Southern Africa Geol. Soc. of South Africa, Johannesburg v2 pp 1593-1607
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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