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Gamsberg |
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Northern Cape, South Africa |
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Zn
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Gamsberg deposit is located some 16 km to the east of Aggeneys/Broken Hill in the Northern Cape Province of South Africa.
The project is owned and managed by Vedanta Resources - Black Mountain Division.
Summary
Mineralisation occurs within the Gams Ore Formation, which is a direct correlative of the Aggeneys Ore Formation that hosts the ore at Broken Hill, and is a member of the Bushmanland Group. The Gams Ore/Iron Formation comprises three units, namely the:
i). A Member a lower member composed of a diverse suite of quartz-felspar-garnet-clinopyroxene rock, garnet-clinopyroxene-feldspar marble and garnet-clinopyroxene-quartz-magnetite rocks;
ii). B Member a middle sulphide zone with quartz-garnet-amphibole rocks and graphitic quartz-sillimanite-muscovite-feldspar containing major amounts of pyrite, pyrrhotite, sphalerite and galena; and
iii). C Member an upper unit of diverse garnet, pyroxenoid, clinopyroxene, orthopyroxene, amphibole, quartz, apatite, carbonate, magnetite, hematite and barite bearing rocks.
It is underlain by a thick, massive recrystallised white quartzite with minor schists and conglomerate.
Deformation is complex, with the Gamsberg mineralisation falling within a major 'sheath fold' several kilometres across, on whose margin the deposit is located at the contact between the underlying quartzite and overlying mafic gneisses of the Nousees/Koeris Formation. This structure is on the 'underlimb' of a large nappe structure to the north of the thrust/shear that encloses the Broken Hill and Black Mountain deposits. The main 'ore' shoot at Gamsberg is elongated parallel to the isoclinal sheared nose of one margin of the sheath fold, whose axis it follows down dip possibly for >1 km. In the sheared fold nose the ore terminates against a coarse mylonite, while laterally ore persists for several hundred metres before reaching a grade boundary on its opposite margin.
The mineralised sulphide zone of the Gamsberg Iron Formation is intermittently present within the sheath fold structure and is generally weakly mineralised containing 1 to 4% Zn. The intervals of weaker mineralisation include a number of higher grade 'ore shoots' with >7% Zn embracing smaller cores of >10% Zn.
The sphalerite rich pyrrhotitic-pyritic ore is found towards the centre of the Gams Iron/Ore Formation, flanked by iron sulphides, predominantly pyrite below and pyrrhotite above. These sulphides are in turn sandwiched by two magnetite to hematite rich zones towards the outer margins of the Gams Iron Formation.
The ore zone sphalerite occurs as intergranular disseminations within a quartz-sericite-sillimanite rock grading up with increasing zinc grade to a quartz-garnet-grunerite host. An impediment to the development of Gamsberg has been the high Mn content of the sphalerite, which contains 2 to 3% Mn within its lattice.
Regional Setting
The regional geology is as described in the Aggeneys (Broken Hill) record.
Geology
The geological succession in the Gamsberg area is as follows, from the base:
► Haramoep Gneiss, unknown thickness - predominantly a pink medium to fine grained quartz-feldspar gneiss with an aplitic appearance, although it often grades into types with poorly developed augen or greyish varieties with feldspar porphyroblasts. Foliation is generally only poorly developed except in more biotitic varieties. In general it is composed of quartz with major to subordinate feldspar which include microcline, microcline-perthite and plagioclase. The characteristic pink colour is due to the K-feldspar. Micas are generally present in varying, though minor amounts, and include chloritised biotite, muscovite and sericite. The upper contact with the Namies Schist is generally sharp and conformable, although in certain restricted areas it has the appearance of being intrusive. The Haramoep Gneiss represents the Hoogoor Suite at the base of the Bushmanland Group in this area.
► Wortel Formation - subdivided into,
• Namies Schist, 70 m thick - a well foliated, quartz-biotite-sillimanite schist. The schistosity is imparted by biotite and lesser parallel muscovite, with flattened nodular clusters of sillimanite. The quartz content of the schist gradually increases upwards in the stratigraphy until dark prominent quartzite bands alternate with thin layers of quartz-biotite-muscovite-sillimanite schist. This then passes progressively into the white, pure ortho-quartzite of the Zuurwater Quartzite.
• Zuurwater (or Pella) Quartzite, 250 to 375 m thick - which is in turn subdivided into,
- White Quartzite Member, 200 to 250 m thick - a milky white to light grey, macroscopically recrystallised quartzite. Microscopically it is coarse grained and saccharoidal with all primary features, structures and secondary cement having been obliterated by recrystallisation. Accessory minerals include muscovite, apatite, zircon, rutile and occasional pyrite. It has a conformable gradational contact with the Pelitic Schist Member.
- Pelitic Schist Member, 10 to 25 m thick - a thin irregular unit with common pinch and swell structures and intercalated micaceous dark quartzite layers. The mineralogical makeup of the rock varies considerably along strike from quartz-biotite-muscovite-sillimanite-garnet (almandine) schist in the west, to a quartz-muscovite schist in the east. It passes conformably and gradationally upwards into the Dark Quartzite Member.
- Dark Quartzite Member, 40 to 100 m thick - a coarse grained recrystallised quartzite with no remaining primary texture. It comprises glassy quartz with accessory zircon, biotite, apatite, muscovite, sericite, sillimanite, hematite and magnetite. Pyrite is rare, occurring locally near the stratigraphic top of the unit. The dark colouration is due to disseminated magnetite and earthy hematite concentrated along the quartz grain boundaries. In the area of its maximum development, prominent bands of attenuated white and dark quartzite are found in a light grey recrystallised quartz matrix. Intercalated 1 to 2 m thick bands of biotite-muscovite-sillimanite schist are observed locally.
To the north of the Aggeneys-Gamsberg area there are a number of additional formations between the Zuurwater (Pella) Quartzite and the Gams Iron Formation. While this contact is commonly a thrust plane in the Aggeneys-Gamsberg area, this is not always the case. Therefore if the mapping of Colliston, et al., (1986) and Strydom, et al., (1987) is correct, there was a hiatus or a thinning of the sequence southwards towards the Aggeneys-Gamsberg area and the position of the subsequent Swartberg-Zuurwater Thrust/shear Zone during the deposition of the Bushmanland Group.
► Gams Iron Formation (equivalent to the Aggeneys Ore Formation), 0 to 80 m thick - comprising,
• A Member, 0 to 10 m thick - which has four beds, as follows,
- A1 Bed, averaging 2 m thick - quartz-biotite-muscovite-sillimanite schist, well foliated, with accessory garnet (almandine), tourmaline, apatite, zircon and hematite.
- A2 Bed, 0 to 1 m thick - an impersistently developed zone of clinopyroxene-grunerite-garnet-magnetite rock, which follows the A1 Bed with a sharp contact. Generally at surface it has a black coating of manganese and iron oxides. In the southern and north eastern parts of the Gamsberg area this unit thickens into an up to 15m, more persistent garnet rich rock. In general it is massive, medium grained and poorly banded to the west, while in the east it is fine grained and moderately banded. The bands are 1 to 4 mm thick comprising alternating magnetite poor and rich layers. The clinopyroxene is ferro-augitic, while the garnet is primarily andradite.
- A3 Bed, unstated thickness - overlies the A2 Bed with a gradational contact, and comprises a leucocratic, banded, medium grained, crystalline, calcite-diopside-garnet marble, with the carbonate content increasing upwards, with mafic minerals correspondingly decreasing. The banding is due to alternating carbonate rich and poor (mainly K-feldspar) layers. It does not outcrop and is only impersistently developed, passing laterally into dolomite rich strata. Ferroan and manganoan calcite is the dominant carbonate, and grossular the principal garnet. While the A2 Bed is rich in Fe oxides, accessory sulphides are found in A3.
- A4 Bed, 1 to 5 m thick - generally a garnet-muscovite-sillimanite quartzite, forming a transition from the underlying calcareous unit and more siliceous lithologies. This unit exhibits rapid mineralogical changes both vertically and laterally. Typical assemblages include quartz-biotite-muscovite-sillimanite schist; garnet-sillimanite quartzite, muscovite-microcline quartzite and muscovite quartzite. At the surface the resistant quartzitic units are covered by a dark brown to black coating of mixed manganese and iron oxides, with a cellular texture where they contain original Fe sulphides. Texturally these rocks are fine grained to hornfelsic in places, with either moderately to poorly developed or prominent banding. The latter is found in the garnetiferous quartzite which has alternating garnet rich and poor bands. The garnet is generally spessartine.
• B Member, 0 to 50 m, averaging 25 m thick, - this contains the main ore zone. In places this unit grades laterally into a hematite bearing, medium grained sillimanite-quartzite with no sulphides. The B Member has been divided into two beds, as follows,
- B1 Bed, 0 to 30 m thick - fine grained, poorly to moderately banded, dark, dense quartz-sericite-sillimanite schist. The banding is generally contorted, consisting of slight variations in silicate mineralogy and bands of sulphide minerals. It is accentuated by parallel streaks of sericite and sillimanite, forming a poorly developed schistosity. Towards the top poorly mineralised leucocratic, fine grained sericitic quartzite, occasionally holding potash feldspar, is interbedded with the schist. In some drill intersections a cordierite rich rock (2 to 4 m thick) composed of flattened spheroidal aggregates 10 to 15cm in diameter, embedded in a quartz-sillimanite-sericite matrix, occurs above the schist. The average opaque content of this unit is 10 to 40%, comprising in decreasing order of abundance, pyrite, pyrrhotite, marcasite, sphalerite, galena, and accessory chalcopyrite, alabandite, magnetite, ilmenite and graphite.
- B2 Bed, 8 to 16 m thick - conformably overlies the B1 Bed, with the contact taken as the first appearance of major amounts of garnet and grunerite. It is generally a medium grained quartz-garnet-grunerite rock, with well developed banding of alternating garnetiferous, apatite and grunerite rich layers. Poorly and well mineralised streaks generally parallel the silicate mineral banding. Schistosity is feebly developed to non-existent. Lithological changes are marked by the appearance of orthopyroxene and clinopyroxene towards the top of the unit. Together with the C Member, this bed is absent in numerous localities. It is generally mineralised, with pyrrhotite and sphalerite being present in major amounts, marcasite and pyrite in subordinate quantities and magnetite, ilmenite and chalcopyrite as accessories.
• C Member, 0 to 9 m thick - the contact between the B and C Members is gradational on a decimetric scale, being marked by the change from predominantly contained sulphide to iron oxide (magnetite and hematite) mineralisation. In the western section of the prospect it has been divided into two beds, although lateral changes in the mineralogy are common. The two beds are as follows,
- C1 Bed, thickness not stated - a magnetite-clinopyroxene-grunerite-garnet rock, which is very similar to the B1 Bed, except for the absence of orthopyroxene and the presence of Fe oxides rather than sulphides. It is medium to fine grained with a well developed mineralogical banding of 0.8 to 3cm in thickness. It persists discontinuously along strike, with this mineralogy being best developed where in contact with mineralised B2 Bed. Laterally it changes to a banded magnetite-hematite quartzite, with or without interbedded barite, a quartz-grunerite-garnet-magnetite rock, a magnetite-hercynite-garnet-sillimanite-quartz rock, or to a garnet quartzite with no iron oxides.
In the south eastern exposures of the C Member a barite horizon is developed at the stratigraphic level of the banded magnetite-hematite quartzite of the C1 Bed. Barite occurs as intercalated lenticular bodies in some places, and in others as persistent massive layers of 1 to 2.5 m in thickness, oriented parallel to the regional banding. In the exposures of massive barite little or no banding is seen, apart from thin interbedded quartzitic seams.
- C2 Bed, thickness not stated - a garnet-magnetite-grunerite rock with a gradational contact with the underlying C1 Bed, marked by an increase in garnet and decrease in mafics, together with a considerable increase in Ca. The latter change is reflected in the presence of andradite garnet. It is well banded, with layers of massive garnet alternating with magnetite and quartz rich seams. Towards the top there is a considerable increase in calcareous minerals, until a banded (5 to 10 mm) medium grained calcite-fayalite-hedenbergite-garnet (andradite) rock is locally developed. This comprises layers of massive yellow manganiferous andradite alternating with seams of quartz, magnetite, pyroxene and possibly manganiferous wollastonite producing a prominent banding. Ferroan calcite, fayalite, hedenbergite, minor johannsenite, magnetite and accessory sulphides, together with much yellow garnet constitute the lithotypes of the top of the C2 Bed.
Inferred Unconformity
► Koeris Formation, or Nousees Mafic Gneiss, 400 to 500 m thick - the Gams Iron Formation is overlain by a succession of quartz-muscovite schist, lenses of conglomerate and bands of micaceous quartzite. Amphibolite is interlayered with and stratigraphically overlies the quartz-muscovite schist. In the Gamsberg area this has been divided into two separate units as follows,
• Psammitic Schist Member, which has limited distribution within Bushmanland. It is found at the base of the Koeris Formation and comprises leucocratic rock types, mainly well foliated quartz-muscovite schist and quartz-biotite-muscovite-sillimanite schist holding interbedded bands (1 to 10 m thick) of greyish medium grained quartzite. Quartz-muscovite schists predominate in the western section of the prospect, locally with flattened oriented quartz-sillimanite nodules (0.5 to 3 cm long), while in the central and eastern sections, equigranular, medium grained quartz-muscovite-feldspar schist and feldspathic quartzite, both with varying biotite, predominate. The feldspars in the latter rock types are mainly potash, with lesser plagioclase.
Locally the schists and quartzites exhibit faint relics of original bedding and cross bedding. Conglomerate bands of varying thickness and strike continuity consist mainly of white and dark quartzite pebbles (occasionally holding hematite-quartzite fragments) in a quartz-muscovite matrix. The hematite-quartzite pebbles are taken to be from the Gams Iron Formation, although abundant older iron formations are widespread in the older basement. We did not see any of these iron formation pebbles during the visit.
The pebbles and cobbles of these conglomerates have been structurally attenuated, with long dimensions of from 1 to 25 cm. In the western section of the prospect area 0.5 to 2 m thick lens of sheared, banded hematite-quartzite are interbedded with the quartz-muscovite schist. The middle and top part of the psammitic schists contain intercalated bands of the Basic Gneiss Member.
• Basic Gneiss Member, which has been locally subdivided into two units, both characterised by hornblende and/or clinopyroxene, as follows,
- Quartz-Feldspar-Amphibole Gneiss - occurring as discontinuous bands (5 to 20 m long) of variable thickness (1 to 2 m thick) and texture, intercalated with the psammitic schist. In places they are medium grained and gneissic with moderate to poorly developed foliation, imparted by platy hornblende crystals. Elsewhere a tough, massive, medium to fine grained equigranular variety of similar composition displays felsitic textures. Some are epidosites, holding abundant epidote instead of hornblende. This mafic rock in places is interstitial to the pebbles and cobbles of the conglomerate, but elsewhere appears to be intrusive into the amphibolite.
- Amphibolite - the major development of this lithotype is in the upper part of the succession, where it is irregularly distributed within the Psammitic Schist Member. Minor bands and lenses of similar composition are found at all levels from as low as the Namies Schist and Pella Quartzite. It occurs as mainly conformable bands and 'sills' from 5 to 200 m thick, with strike lengths of from 10 to 5000 m. Local crosscutting relationships are common. The contacts are sharp, with an increased biotite contact towards the margins. The rocks are generally equigranular and fine to medium grained, although a distinct banding is present in some outcrops involving alternating plagioclase and hornblende rich layers. Hornblende alignment defines the foliation. It comprises mainly common hornblende (± cummingtonite) and plagioclase, with accessory sphene, almandine, magnetite and pyrite. Amphibolites within the Haramoep Gneiss contain abundant microcline as well and as such may be differentiated.
Some of the conformable amphibolite display abundant amygdale like structure, and are assumed to be metamorphosed mafic volcanics. Substantial areas of outcrop of this variety are present in the core of the Gamsberg sheath fold. Dating from this locality has yielded ages of 1600 Ma.
Major Element and Mineral Zonation
The zonation across the Gams Iron Formation within the Gamsberg deposit is as follows, from the base,
► A Member,
• A2 Bed - characterised by almandine-spessartine garnets (Fe, Mn), ferro-augitic (Fe, Mg, Ca) to ferro-silitic clinopyroxene and manganoan calcite (Ca, Mn), with the main opaque being magnetite. This unit has high Si, Mn, Fe and Ca with lesser Mg.
• A3 Bed - is more calcareous with lesser silica and little Fe. The silica gangue is replaced by pyrite, minor pyrrhotite and no magnetite. It carries grossular garnet (Ca, Al) and diopsidic pyroxene (Ca, Mg).
• A4 Bed - is aluminous, siliceous, Mn and Fe rich and in part graphitic. It carries spessartine garnet (Mn) and the pyroxenes are ferro-augitic (Fe, Mg, Ca) to ferro-silitic. As with A3 there is no magnetite, but increased disseminated pyrite and pyrrhotite.
► B Member,
• B1 Bed - fine grained quartz muscovite (K, Al), sillimanite (Al), schist overlain by cordierite (Mg, Al) hornfels in a groundmass of quartz, muscovite (K, Al) and sillimanite (Al). The cordierite 'hornfels' at the top represents an Mg rich zone in the overall potassium bearing, aluminous and siliceous unit. Pyrite comprises 35% of the opaque minerals at the base of the unit. Zn increases towards the top of the unit, while the sulphur content reaches its maximum in the upper half. The unit is therefore Fe and Zn rich, but these are present as sulphides rather than silicates.
• B2 Bed - banded quartz garnet grunerite (Fe) rock and quartz orthopyroxene garnet olivine amphibole rock with alternating garnetiferous, apatite and grunerite rich layers and contrasting sulphides. Pyroxenes consist of manganoan ortho-ferro-silite (Mn, Fe) and eulite while garnets are mainly almandine-spessartine (Fe, Mn, Al) and the olivine is Fe rich. This unit is therefore P, Fe and Mn rich and siliceous. Pyrite decreases to 5% of the opaque minerals at the top, with a corresponding increase in pyrrhotite. Zn (up to 10%) and Cu (up to 200 ppm) are highest in this unit, while sulphur progressively decreases towards the top. Minor magnetite appears in the upper sections where it occurs in part within sphalerite aggregates.
► C Member,
• C1 Bed - which is predominantly pyroxenoid, primarily pyrox-ferroite and pyroxmangite (Fe, Mn); amphibole; garnet, mainly spessartine-almandine (Fe, Mn, Al); and clinopyroxene rocks where in contact with ore, grading laterally into banded magnetite hematite quartzite. It is siliceous with Fe and Mn silicates, magnetite, minor hematite and few sulphides.
• C2 Bed - which is Mn rich, comprises a garnet pyroxene rhythmite of andraditic garnet (Ca, Fe) with quartz, magnetite, hematite and rhodonite (Mn).
Structure
Rozendaal (1977) undertook a structural analysis of the immediate Gamsberg area in which he recognised five periods of deformation. In this he disagreed with Joubert who had earlier studied the regional structure as repeated in Joubert (1986). Joubert interpreted four stages of deformation. He had ascribed the F1 phase to an episode of isoclinal and translational folding responsible for the main mineralogical banding which parallels the gross lithological banding (bedding) and is seen to be folded by the subsequent deformations. Rozendaal interprets this mineralogical banding to be primary bedding.
Rozendaal (1977) has reasoned that the main structure at Gamsberg, is due to interference produced by the interaction of his F1 (flat lying WNW-ESE trending axial plane) and F2 (vertical NE-SW trending axial plane) folding (Jouberts F2 and F3) and subsequently modified by F5 open folding. This interpretation of the main Gamsberg structure is contradicted by Colliston, et al., (1991) as described below.
Rozendaal (1977) noted that the earliest schistosity S1 caused disruption of the main compositional banding within the schists, and caused marked thinning and boudinage of the more competent layers. He also noted that this schistosity, where developed, is generally parallel to sub-parallel to the gross bedding (as defined by the main unit boundaries) and compositional banding on the fold limbs, but cut the gross bedding in the north western fold closure (his F1 nose). He recorded conspicuous lineations, generally parallel to the intersection of the main compositional banding and the principal cleavage. These included foliation intersections, mineral lineations, attenuated pebbles, parasitic fold axes and crenulations. He also noted that a prominent feature of the major (his F1) fold is the evidence of thrusting in several localities at the top of the overturned limb. This was also obvious during the visit, where a pronounced coarse mylonite zone was seen to accompany the attenuation and 'pinching out' of the Gams Iron Formation at the north western fold closure adjacent to the main high grade mineralisation. This mylonite zone is composed of coarse crystalline quartzite boudins set in a darker magnetite-hematite quartzite matrix. The boudins are rod shaped, with 'S' shaped cross sections. This zone may be traced for 150m in outcrop and has laterally gradational boundaries with the enclosing lithologies.
Rozendaal (1977) reported that the upper overturned limb of the major fold is strongly attenuated, as implied by the frequent 'lensing out' of the Gams Iron Formation on the northern contact between the Pella (Zuurwater) Quartzite and the Koeris Formation (Nousees Mafic Gneiss).
Rozendaal (1977) noted a tightly spaced steep NE-SW trending penetrative S2 cleavage in the amphibolite and psammitic schists of the Koeris Formation which he believed were reflecting his F2 folding. He also differentiated a few examples of an L2, which was generally parallel to his L1. He defined his NE-SW trending F3 as being responsible for the open folding apparently overprinting the main upper and lower limbs of the main sheath fold, although no cleavage was recorded for this phase. The plunge of these folds is variable and they die out with depth. His F4 was responsible for local small scale folds in the quartzite bands and for kink folds with associated fracture cleavage in schists, while F5 was responsible for subsequent mild buckling.
On the basis of regional mapping Colliston, et al., (1991), attribute the majority of the deformation in their Aggeneys Terrane to progressive ductile shearing, resulting in a series of shear/thrusts, nappes and associated sheath folds. They have interpreted the main structure at Gamsberg as being a major sheath fold, which is more believable than the interference of two fold phases as postulated by Rozendaal. In general it would appear that most of the observations of Joubert (1986) and are not inconsistent with this conclusion. Furthermore, in the mapping of Colliston, et al., (1986), they place a thrust fault at the boundary between the Zuurwater Quartzite (of their Wortel Formation) and the Gams Iron Formation (or their Gams Formation). This structure would appear to be a decollement shear developed within the sheath fold at the contact of the two units of different competence. The schists of the A1 Bed in the Gams Iron Formation may represent this decollement surface. Colliston, et al., (1991) interpret the inlier to the west in the vicinity of the Big Syncline prospect as being two parallel telescoped nappes separated by thrusts to the north of the major Swartberg-Zuurwater Thrust/shear Zone. It would then appear, to be consistent, that the Gamsberg sheath fold is part of the lower sheared synform of a similar south-westward transported nappe structure which has subsequently been gently folded. The upper limb of the sheath fold as observed by Rozendaal (1977) was in the process of being more extensively sheared and attenuated.
The higher grade mineralisation at Gamsberg is developed as an elongate shoot within the B Member of the Gams Iron Formation in the western hinge zone of the major sheath fold. It occurs on the lower limb of the fold in which the immediately adjacent upper limb has been attenuated and sheared out along a mylonite zone. The orebody appears to be best developed adjacent to this sheared out closure, and absent on the adjacent upper limb. In the upper limb, the Gams Iron Formation is only sporadically preserved, having been either sheared out due to attenuation (or eroded prior to the deposition of the Koeris Formation). On the lower limb away from the fold axis the B Member of the Gams Iron Formation contains values of the order of 1% Zn (this estimate is based on very sparse information), with sporadic higher grade patches of up to 4% Zn.
In the nose zone of the fold with which the main higher grade mineralisation is associated, abundant parasitic folds are developed on a variety of scales, with axes plunging at around 30°to the ENE. These axes are parallel to the main fold closure and the mineralised shoot, and having the appropriate vergence to support the interpreted structure.
Studies by Rozendaal suggest the metamorphism within the Gamsberg deposit took place at temperatures of from 630 to 670°C and pressures of 2.8 to 4.5 Kb. Pb-Pb isotope dating indicates an age of 1250 to 1300 Ma in contrast to an age of 1600 to 1650 Ma for the host rocks.
The detailed structural setting at Gamsberg therefore differs from that at Broken Hill and Black Mountain, which occur within the sheared isolated limb and a fold nose respectively within a major regional thrust shear zone. However it may be similar to that at Big Syncline area where further sheath folds are reported. Never the less all three are found within zones of major dilation in some of the most intensely deformed positions within the Bushmanland/Aggeneys Terrane closely associated with sheared fold hinges.
Mineralisation
The major mineralisation at Gamsberg is all hosted by the B Member of the Gams Iron Formation at Gamsberg. This comprises the lower 0 to 30 m of fine grained dark quartz-muscovite-sillimanite schist of the B1 Bed, the overlying 2 to 4 m thick cordierite 'hornfels' and the 8 to 16 m thick B2 Bed which comprises fine to medium grained quartz-garnet-grunerite rock and grunerite-orthopyroxene-clinopyroxene rock. Most rocks within the ore zone now have sand sized grains.
The mineralisation is present as a main high grade core of 8 to 10 Mt @ >10% Zn, surrounded by a lower grade zone, which with the core amounts for 30 Mt @ around 8% Zn. This high grade core is apparently restricted in strike length, but is elongated closely following the sheared off northwestern hinge zone of the sheath fold. Away from this structure the mineralisation drops of to levels of 1 to 4% Zn, with the western ore zone which is 300 to 400 m from the fold axis and the high grade shoot averaging 4% Zn.
Within the high grade ore shoot there is a higher grade hangingwall zone of limited tonnage which has 15 to 25% Zn, and a lower grade footwall zone with 4 to 7% Zn. The hangingwall zone is too small and irregular to be susceptible to selective mining. These observations are not recorded in the literature, but were ascertained during the visit.
Within the Gams Iron Formation there is a well established vertical mineral zonation. As at Broken Hill/Aggeneys the main base metal sulphide zone is bracketed by magnetite and/or hematite rich rocks above and below, namely within the underlying A2 Bed and the overlying C Member beds. Pyrite is found in the A3 and A4 Beds below the main base metal sulphides, and in the lower mineralised zone in the B1 Bed. The sulphur content decreases upwards progressively from the middle of the B1 bed as pyrite is replaced by pyrrhotite as the dominant Fe sulphide in the B2 Bed, before minor magnetite appears in the upper B2 Bed, and dominates with hematite in the C1 Bed.
Zinc gradually increases upwards from the base of the B1 Bed to the top of the B2 Bed.
The opaque minerals within the base metal mineralised interval comprise, in decreasing order of abundance, pyrite, pyrrhotite, marcasite, sphalerite, galena and accessory chalcopyrite, alabandite, magnetite, ilmenite and graphite.
The distribution and form of the various sulphides is as follows, from Rozendaal (1975, 1980 and 1986):
• Pyrite - is present as discrete disseminated grains (0.01 to 0.5 mm), networks of grains and massive clusters (>10 mm) as a replacement of gangue. It comprises the bulk of the sulphides (more than 35%) within the B1 Bed, dropping off to about 5% at the top of the B2 Bed. Galena, sphalerite and pyrrhotite partially envelop and sometimes appear to replace pyrite. The pyrite idioblasts are of varied size, related directly to the intensity of folding. Pyrite and the other sulphides are concentrated in hinge zones of minor folds, in pressure shadows and fractures; in these locations the pyrite is coarser than in the normal banded ore matrix.
• Pyrrhotite - occurs as massive aggregates (>10 mm) as well as disseminated xenoblastic grains (0.01 to 2 mm) dominantly associated with sphalerite. These two sulphides jointly occupy interstices between, and also partially or completely envelop, pyrite. Pyrrhotite selectively replaces muscovite and chlorite along cleavage planes and partially includes garnet and ilmenite grains. The distribution of pyrrhotite is the inverse of pyrite.
• Marcasite - replaces pyrrhotite as blades along crystallographic planes and also occurs as veinlets (0.1 to 0.5 mm) intergrown with pyrite. Pyrrhotite appears to alter to marcasite and marcasite to pyrite from the top to the base of the mineralised unit.
• Sphalerite - is the most significant economic base metal. It is present as small (0.001 to 0.1 mm) xenoblastic grains dispersed intergranularly in gangue and as massive intergranular aggregates (0.5 to 5 mm) associated with pyrrhotite and marcasite, occupying interstices between or partly enclosing pyrite grains. The sphalerite is generally coarse grained and matured due to annealing (Rozendaal 1975). Sphalerite grains may contain wispy concave galena particles, pyrrhotite blebs and chalcopyrite as fine ex-solution blebs and chains of blebs (0.001 to 0.1 mm) aligned parallel to the crystallographic direction of the host.
Most of the sphalerite is manganiferous and marmatitic, but honey coloured varieties with negligible Fe and Mn are present in places. Sphalerite increases from minor amounts at the base of the B1 Bed to peak concentrations at the top of the B2 Bed. Cd is a substitution element in sphalerite, with 100 to 130 ppm generally being present within the mineralised layer.
• Galena - is present as disseminated sub-idioblastic to xenoblastic grains (0.001 to 0.1 mm), dispersed intergranularly with gangue and sulphides, as well as massive vein like aggregates (>2 mm) filling fractures and mineral interstices. Silver occurring as a substitute element in galena, has an average concentration in the mineralised zone of 5 to 7 ppm.
Galena displays textures indicative of ductile behaviour and annealing. Small wispy concave particles and cusps of galena found in sphalerite do not (according to Rozendaal 1986) represent original inclusions in sphalerite, but are concentrations of the mineral forming phase boundary triple junctions with sphalerite, which is indicative of annealing, not replacement.
Galena is particularly well concentrated in hinge zones of minor folds, in pressure shadows and in fractures. Rozendaal (1975) regards the coarse grained galena as representing original fine grained material being squeezed out of its matrix and mobilised into cracks and cavities during metamorphism.
• Chalcopyrite - is present as fine ex-solution blebs (0.001 to 0.01 mm) within massive sphalerite, the distribution trend of these two minerals being sympathetic. The general concentration of Cu within the mineralised zone ranges from 50 to 300 ppm.
• Magnetite - while being found in the footwall in the A2 Bed, is absent from the lower part of the mineralised zone, but is a minor constituent in the upper zinc rich portion where it occurs as sub-rounded grains embedded in massive aggregates of sphalerite. In the C1 and C2 Beds magnetite becomes the dominant opaque mineral, which occurs throughout the C Member as individual grains and massive aggregates, ranging in size from 0.05 to 5 mm, displaying varying degrees of martitisation along crystallographic directions, intergrown hercynite grains and sporadic ilmenite lamellae. Hematite is present in this member as oriented idioblastic specular grains, 0.5 to 1.5 mm in size.
• Graphite - is found predominantly in the basal quartz-muscovite-sillimanite schist of the B1 Bed in which it forms xenoblastic flakes (0.002 to 0.2 mm), aligned parallel to the foliation planes or interlayered with slightly elongated pyrite and sphalerite grains.
• Barite - is present mainly as recrystallised white to greyish pink, coarse grained massive bands, interbedded with, above and below the magnetite-hematite quartzite. The barite bands increase in thickness from south to east as the underlying mineralised B member thins and eventually peters out in the southeastern corner of the Gamsberg Iron Formation outcrop. This inverse relationship of sulphide and sulphate is manifested along strike and down dip. Ba levels are very low (250 to 1000 ppm) in both the B and C Members away from the major barite developments.
The main barite outcrop is either massive white or massive black crystalline barite or as a banded black and white variety. Drill intersections of the barite band assayed around 0.8% Pb, 0.4% Zn over a 7 m interval. In outcrop a thin 1.5 m thick gossan is found in the immediate footwall of the barite unit. The barite zone has a strike length of 1 km.
The main mineralised zone ranges from fine disseminated sulphide grains (generally <2 mm) and larger aggregates (up to 1 cm), aligned parallel to the main foliation, to irregularly contorted bands and lenses (1 mm to 1 cm thick) of massive sulphide and gangue. These are cut by massive crosscutting veins (mm's to 30 cm) of fine to coarse sulphides and aggregates. The sulphide content ranges from <10% up to around 80% within the B Member.
The irregular contorted texture of the ore is not a consistent fold pattern, but is very irregular, with a knotted appearance in paces, as well as producing concentric circular structures on the core face.
The well developed banding at Gamsberg contrasts to the virtual absence of banding of the sulphides at Broken Hill/Aggeneys.
In some intervals there are round nodules (around 1 cm) of galena and apatite surrounded by sphalerite bearing meta-pelite.
Rozendaal and subsequent workers have interpreted the main foliation within the ore to be bedding. They claim that 90% of the sulphides within the mineralised zone are recrystallised original syngenetic minerals, overprinted by <10% remobilised sulphides and minor very late poikiloblastic coarse sulphides. They maintain that the sulphides have been recrystallised with the host lithologies without substantial transport. However as with other examples this does not address the concentration of mineralisation within the high grade fold axis shoot, the microscopic relationships between the various sulphides and the constituents of the mineralised interval and the probability that different minerals (sulphides and silicates) display different behaviours and properties during metamorphism (eg, in situ recrystallisation, versus pressure solution and transport, etc.).
Surface Expression and Geochemistry
A diagnostic gossan with a strike length of 5.3 km marks the surface exposure of the main mineralised band within the Gams Iron Formation. The strike length of the high grade shoot of mineralisation on which the reserves are based however is of the order of several hundred metres.
In outcrop the gossan has a sharp contact with the underlying quartzite although faint boxworks persist into that unit below the Gams Iron Formation. The gossan passes upwards into a black banded magnetite quartzite of the C1 Bed.
The gossan directly overlies the mineralised band and where surface features permit, simulates the shape and structure of the latter. According to Rozendaal (1975), macroscopically there are two major types or 'end members' of gossan at Gamsberg, with numerous intermediate variations, namely:
• A mainly limonitic cellular type with sponge and boxwork structures.
• A fine grained massive limonitic jasper occurring as sprawling masses and ragged edged seams and patches which develop from the cellular or intermediate varieties and merge into the massive jasper types again.
The original textural features of the un-oxidised mineralisation, ie. banding and schistosity are generally obscured or poorly preserved in the massive jaspilitic gossan, although these features are generally observable within the cellular gossan. Macroscopically this layering is visible as coarse and fine grained seams of quartz.
On the unbroken surface, the gossan is covered by a patina of iridescent smeary crusts of both ferruginous and manganiferous oxidation products, usually darker than the underlying rock. The latter presents a range of colours from yellow to dark brown to maroon to matte black. On the Cretaceous peneplain, the gossan is coated with exotic yellow to ochreous powdery gypsum and calcite (Rozendaal 1975).
Mineralogically the gossan consists of secondary limonitic, goethitic and hematitic jasper as well as kaolinised feldspar, garnet as pseudomorphs, silicified amphibole, ferruginised micas and major quartz, with accessory rutile, tourmaline and sillimanite.
The gossan capping persists for a depth of around 110 m below the surface, with the gossan-sulphide interface at a constant elevation above sea level. A relatively sharp 3 to 4 m vertically thick transition zone separates the oxidised and fresh sulphides.
Geochemical soil sampling values vary with the topographic elevation. In general at higher elevations assays averaged 700 to 900 ppm Pb, 150 to 200 ppm Zn. In lower areas these values become 100 to 400 ppm Pb, 1000 to 2400 ppm Zn, showing an inversion of the relative abundance of the two metals. This reflects the concentration of Pb as anglesite and plumbojarosite at the Cretaceous surface in contrast to the dispersion of the more mobile zinc sulphates and carbonates. Cu varies from 30 to 350 ppm. Chip samples of the gossan range up to 7.75% Pb and 0.3% Zn.
Sampling of the Gams Iron Formation within the 5.3 km outcrop of the mineralised unit return values seldom less than 1000 ppm Zn (except in the areas of greater elevation).
Reserves and Resources
The measured and indicated resource was 140 Mt @ 5.8% Zn, 0.5% Pb, within a geological resource of 170 Mt (when visited in 2001).
Remaining Ore Reserve and Mineral Resources at the end of 2018 (Vedanta Resources Annual Report, 2018) were:
Proved + Probable Reserve - 53.81 Mt @ 6.63% Zn, 0.51% Pb;
Measured + Indicated Resource - 97.91 Mt @ 6.20% Zn, 0.54% Pb;
Inferred Resource - 64.36 Mt @ 7.81% Zn, 0.52% Pb.
NOTE: Reserves are additional to resources.
Remaining Ore Reserve and Mineral Resources at 31 March 2022 (Vedanta Resources Annual Report, 2022) were:
Proved + Probable Reserve - 96.8 Mt @ 6.0% Zn, 0.5% Pb;
Measured + Indicated Resource - 43.0 Mt @ 6.6% Zn, 0.5% Pb;
Inferred Resource - 142.6 Mt @ 7.2% Zn, 0.5% Pb.
TOTAL Reserve + Resource - 282.4 Mt @ 6.7% Zn, 0.5% Pb.
NOTE: Reserves are additional to resources.
This description is based on a visits in 1992 and 2001 and information from Rozendaal (1975, 1977, 1980 and 1986). As such it may be dated. For more up to date detail and alternate interpretations, see the references listed below.
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.
Gamsberg
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Colliston W P, Praekelt H E, Schoch A E 1991 - A Progressive Ductile Shear Model for the Proterozoic Aggeneys Terrane, Namaqua Mobile Belt, South Africa: in Precambrian Research v49 pp 205-215
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Cornell D H, Pettersson A, Whitehouse M J and Schersten A, 2009 - A New Chronostratigraphic Paradigm for the Age and Tectonic History of the Mesoproterozoic Bushmanland Ore District, South Africa: in Econ. Geol. v104 pp 385-404
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de Bruiyn H, van de Westhuizen W A, Beukes G J 1994 - Secondary Zn-Pb-Cu Minerals, an Aid to Mineral Exploration in the Aggeneys Area, Bushmanland, South Africa: in J. of African Earth Sciences v18, no.1 pp 61-71
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Hohn, S., Frimmel, H.E. and Price, W., 2021 - Syn‑metamorphic sulfidation of the Gamsberg zinc deposit, South Africa: in Mineralogy & Petrology v.115, pp. 709-728. doi.org/10.1007/s00710-021-00764-w.
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McClung C R, Gutzmer J, Beukes N J, Mezger K, Strauss H and Gertloff E, 2007 - Geochemistry of bedded barite of the Mesoproterozoic Aggeneys-Gamsberg Broken Hill-type district, South Africa : in Mineralium Deposita v42 pp 537-549
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Moore J M, le Fur F 2000 - Relationships Between Conglomerates, Iron Formations and Gahnite-rich Rocks at Gamsberg and Aggeneys East: Implications for the Exhalative Origin of Broken Hill-type Base Metal Deposits (Abstract): in Geocongress 2000: 27th Earth Science Congress of the GSSA (abstracts) J. of African Earth Sciences pp 53-54
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Moore J M, Watkeys M K, Reid D L 1990 - The Regional Setting of the Aggeneys/Gamsberg Base Metal Deposits, Namaqualand, South Africa: in Spry P G, Bryndzia L T (eds), Regional Metamorphism of Ore Deposits and Genetic Implications: Proceedings of the 28th International Geological Congress, Washington, US. VSP, Utrecht, Netherlands pp 77-95
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Praekelt H E, Schoch A E, Visser J N J 1997 - The Metasediments of the Aggeneys Terrane in the Namaqua Mobile Belt: Sedimentary Response to Extensional-compressional Variations in a Continental Environment: in S. Afr. J. Geol. v100, no.1 pp 101-110
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Reid D L, Smith C B, Watkeys M K, Welke H J and Betton P J, 1997 - Whole-rock radiometric age patterns in the Aggeneys-Gamsberg ore district, central Bushmanland, South Africa : in S. Afr. J. Geol. v100 pp 11-22
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Reid D L, Welke H J, Smith C B, Moore J M 1997 - Lead isotope patterns in Proterozoic stratiform mineralization in the Bushmanland Group, Namaqua Province, South Africa: in Econ. Geol. v92 pp 248-258
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Reid D L, Welke H J, Smith C B, Moore J M 1997 - Lead Isotope Patterns in Proterozoic Stratiform Mineralisation in the Bushmanland Group, Namaqua Province, South Africa: in Econ. Geol. v92 pp 248-258
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Rozendaal A 1986 - The Gamsberg Zinc Deposit, Namaqualand District: in Anhaeusser C R, Maske S, (eds), Mineral Deposits of Southern Africa Geol. Soc. of South Africa, Johannesburg v2 pp 1477-1488
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Stalder M and Rozendaal A, 2005 - Distribution and geochemical characteristics of barite and barium-rich rocks associated with the Broken Hill-type Gamsberg Zn-Pb deposit, Namaqua Province, South Africa: in S. Afr. J. Geol. v108 pp 35-50
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Stalder M, Rozendaal A 2000 - Metamorphism of the Polymetallic Sulphide Ores from the Mesoproterozoic Aggeneys-Gamsberg Sedex Deposits, Namaqua Province, South Africa (Abstract): in Geocongress 2000: 27th Earth Science Congress of the GSSA (abstracts) J. of African Earth Sciences pp 73-74
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Stalder M, Rozendaal A 2000 - Rare Earth Element Geochemistry of Garnet Associated with the Aggeneys-Gamsberg Cu-Pb-Zn Deposits, South Africa, Using Laser-ablation Microprobe ICP-MS: in Geocongress 2000: 27th Earth Science Congress of the GSSA (abstracts) J. of African Earth Sciences pp 74-75
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Stalder, M., and Rozendal, A., 2004 - Apatite nodules as an indicator of depositional environment and ore genesis for the Mesoproterozoic Broken Hill-type Gamsberg Zn-Pb deposit, Namaqua Province, South Africa: in Mineralium Deposita v.39, pp. 189-203.
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