Canadian MineralogistVoI. 17, pp.337-349 (1979)

ABSTRAcT

During the past ten years a number of smallnickel sulfide deposits have been discovered inthe Archean greenstone belts of the RhodesianMidlands. These include the Damba deposits, de-scribed here in some detail, and five other occur-rences whose principal features are summarized.A regional investigation and a reannotation ofexisting maps covering 230,000 km2 of the Rhode-sian basement show that the deposits are clearlyassbciated with komatiitic volcanism and that koma-tiitic volcanic rocks are more widespread in theBulawayan Group than the maps suggest. At leastfour of the komatiite sequences have the samesimple and regular stratigraphy of peridotitic koma-tiite - pyroxenitic komatiite - basaltic komatiite,with the basaltic varieties making up the majorpart of individual successions. Komatiitic andtholeiitic magmatism was both concomitant andoverlapping within the Bulawayan Group. All thenickel deposits are found at, or close to, the contactbetween the Sebakw.ian and Bulawayan Groups andare an integral part of the very first volcanic unitin the Bulawayan Group. The Damba deposits arewithin an extrusive peridotite unit and include oneinstance of mineralization overlying what may havebeen a feeder conduit. Several of the other depositsoccur in, or are closely associated with, cross-cutting peridotite bodies that are interpreted asvolcanic necks or vents; the deposits seem to liein sites that have acted as volcanic centres bothprior to and following peridotite-ore emplacement.Both the detailed setting and configuration of thekomatiitic sequen@ at the Shangani deposit suggestthat volcanism was of a fissure type.

SoMMAIRE

Au cours de la dernidre d6cennie, plusieurs petitsgisements nickelifbres ont 6t6 d6couverts dans lesceintures de roches vertes arch6ennes en Rhod6siecentrale. Parmi ceux-ci figurent les gisements Dam-

EPresent address: Exploration Department, TheBroken Hi.ll Profrietary Co. Ltd., c.P.O. BoxL923, Pertb, Western Australia, 6001.

THE ASSOCIATION OF SOME NICKEL SULFIDE DEPOSITSWITH KOMATIITIC VOTCANISM IN RHODESIA

D.A.C. WILIAMS*lohannesburg Consolidated Investment Company Limited,

P.O. Box 976, Randfontein 1760, South Afrir.a

ba, d6crits ici en d6tail; nous pr6sentons aussi lesfaits saillants concernant cinq autres de ces gise-ments. Un relev6 r6gional et une r6-annotation descartes existantes couvrant 230 000 kmz du socleRhod6sien montrent clairement que la min6ralisa-tion est associ6e au volcanisme komatiitique, plusr6pandu dans le groupe Bulawayen que ne l'indi-quent les cartes. Au moins quatre s6quences koma-tiitiques possbdent la m6me stratigraphie simple etr6gulitsre (komatiitg p6ridotitique-komatiite pyrox6-nitique-komatiite basaltique), dont les roches basal-tiques constituent la majeure partie dans chacunedes s6quences. Le magmatisme, dans le groupeBulawayen, a 6t6 thol6itique et komatiitigue, tant6tsimultan6ment, tant6t alternativement. Tous lesgisements se trouvent prbs du contact des groupesSebakwien et Bulawayen, et forment partie int6-grale de la toute premiere unit6 volcanique dugroupe Bulawayen. Les gisements Damba sont dansune extrusion p6ridotitique; dans un cas, la min6ra-lisation semble localis6e au-dessus d'un conduitd'alimentation volcanique. Plusieurs des gisementssont intimement recoupds par des rntrusions dep6ridotite qui repr6senteraient des necks ou venuesvolcaniques; ces gisements sont situ6s en .descentres d'une activit6 volcanique ant6rieure etpost6rieure i la mise en place du minerai. Tantpar son emplacement que par sa forme, la s6quencekomatiitique du gisement Shangani indioue unvolcanisme de type fissural.

(Traduit par la R6daction)

INTRoDUcTIoN

During the late 1960s and early 1970s anumber of nickel sulfide depoeits were dis-covered in the Rhodesian Midlands (Fig. 1).Although by world standards tley are smalland of low grade, two of the discoveries arecurrently in production at Shangani (Philpott1975, Viljoen et al. L976) and Epoch. Theothers include Damba, Hunters Road (Brand1976) and Selukwe which are either subeconomicor await a more favorable economic climate fordevelopment. Several other minor or untestedoccurrences have also been located.

As part of a broad-scale evaluation of the

337

338 THE CANADIAN MINERALOGIST

ll-l posr BAsEMENT RocKs

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base-metal potential of the Rhodesian basementcomplex, a compilation study and reannotationof existing maps was implemented over some230,000 km' of this granite-greenstone terrain.The basic data were derived from published andunpublished maps of the Rhodesia GeologicalSurvey at scales of between 1:50,000 and1:1,000,000. Along with company information,this was compiled at a scale of 1:250,000 andaugmented by four months field work. TheSurvey maps have a spread of publication datesfrom 1920 to 7975, resulting in major terrni-nological differences between the sheets. Thefield work was thus directed at resolving thesedifferences and at examination of lithologies ofexploration interest with the aim of producinga unified geological map that reflects currenfknowledge.

Part of this work, coupled with a detailedexamination of the Damba nickel deposits, hasshown a rather striking association of the nickeldeposits with komatiitic volcanism in the Mid-lands area. Some of these results are presentedhere; a more detailed discussion of the geologi-cal and geochemical results is to be given else-where (Williams in prep.). Note that thedefinitions of peridotitic, pyroxenitic and basal-tic komatiite used here are those ot Arndt et aI.(1977); see also discussion of this terrninologyby Williams & Furnell (1979). Distinction be-tween basaltic komatiite and tholeiite is basedlargely on texture, MgO content and field asso-ciation with spinifex-textured ultramafic koma-tiites.

GSNnRAL Gsor,ocv

The Rhodesian Archean granite-greenstoneterrain provides some of the best informationavailable anywhere on the relationships betweengranite and greenstone and between the variousvolcanic rock types. This is due to compre-hensive, good-quality Survey mapping, conti-nuity of exposure and relatively low grades ofmeta.morphism. Bliss & Stidolph (1969) andWilson (1973) have provided recent summariesof the geology and possible development of theRhodesian craton which update Macgregot's(1951) classic account. The official Rhodesiangeological map (1:1,000,'QQQ) has also beenrecently revised (7th edition, 1977),

Macgregor (1951) divided the greenstonebelt succession in Rhodesia into three "systems"(now called Groups): the Sebakwian Group atthe base, overlain in turn by the Bulawayan andShamvaian Groups. This subdivision is still re-tained though the extent of the Sebakwian andShamvaian Groups is less than as originallydefined:Shamvaian Group: Poorly sorted sediments

(grits, greywackes, arkoses and conglome-rates with some felsic volcanic rocks. Notdiscussed further in this paper.

Bulawayan Group: Dominantly komatiitic,basaltic and andesitic volcanic rocks withassociated sedimentary units (arkose, grit,phyllite, conglomerate and banded iron-for-mation).

Sebakwian Group: Mafic and felsic volcanic

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NICKEL SULFIDE DEPOSITS IN RHODESIA

Ftc. 2. Simplified geology of part of the compilation area showing nickel deposit localities. Data sourcesare given in tle text.

340 THE CANADIAN MINERALOGIST

rocks, quartz-sericite schists, micaceousquartzites, phyllite and banded iron-forma-tion with ultramafic intrusive rocks (of Bula-wayan or later age) and some extrusive equi-valents.

Bliss & Stidolph (7969, p. 309) noted that theexistence of the Sebakwian as a separate groupis based on the recognition of an unconformitybetween it and the overlying Bulawayan Grouptogether with a marked change in structure andmetamorphic grade across the contact. The type-area unconformity has been questioned by anumber of workers (Harrison 1968, Bliss &Stidolph 1969, Wilson L973) and, as a result,many of the areas previously shown as beingSebakwian have been included within the Bula-wayan Group on the latest Rhodesia geologicalmap. The term Sebakwian will be used herein the sense of Bliss & Stidolph (1969) butwithout the implication of an unconformitybetween it and the Bulawayan Group (Williarnsin prep.). This contact marks an importantchange in volcanism irrespective of whether amajor time break is involved; to this author,the placing of Sebakwian-type sequences intothe Bulawayan Group seems unfortunate.

The geology of the area of interest is shownin Figure 2. This is only part of the geologicalcompilation and has been greatly simplified forreproduction at this scale. The geology islargely a reinterpretation of the work of Mac-gregor (1928, 1937), Keep (1929), Phaup(L932, 1933) , Ferguson (1934), Macgregoret al. (1937), Amm (1940, L946), Tyndale-Biscoe (1,940, 1949), Worst (1956), Stowe(1968) and Harrison (1969, 1970). The Shan-gani granitic terrain forms the dominant struc-tural feature in the area and the greenstoneremnants dip outward from it at 40-80o. Asmany authors have observed (e.9., Macgregor1951), the greenstone belts themselves aredominantly synclinal with broadly north-south-trending fold axes in the Lonely Mine,Shangani, Filabusi, Belingwe and Gwelo beltsand east-west trends in the Gwanda and LowerGwanda belts.

Sebakwian Group

The oldest greenstone belt material in thearea belongs to the Sebakwian Group and isbest developed in the Shangani, Lonely Mine,Gwelo and Selukwe belts where it rims thenorthern edge of the Shangani granitic terrain;see Bliss & Stidolph (1969) and Wilson (1973)for discussions of pre-Sebakwian greenstonematerial. Generally the Sebakwian Group con-sists of both sedimentarv and volcanic rocks.

but it is usually characterized by quartzofeld-spathic sedimentary units, quartz-mica schists,phyllites and quartzites, many of which are ofvolcaniclastic origin, together with banded iron-formations, a wide variety of mafic schists andsome ultramafic lavas. The Sebakwian Groupis intruded by ubiquitous layered sills consistingdominantly of serpentinite, with minor meta-pyroxenite and ,metagabbro (Macgregor 1937,Amm 1946, Tyndale-Biscoe 1949, Harrison1969, Viljoen et al. 1976).

In the western part of the Gwelo belt (35km northwest of Gwelo: Fig. 2), Macgregor(1937) recognized a NE-SW axis of folding inthe Sebakwian Group that is not shown in theoverlying Bulawayan Group. However, in theShangani and Lonely Mine belts there is noevidence of any ,major time break despite thechange from felsic to mafic volcanism.

Bulawayan Group

The Bulawayan Group is generally regardedas consisting largely of basaltic and andesiticvolcanic rocks with locally abundant sedimentaryunits and banded iron formation and relativelyminor acid volcanic rocks (Macgregor 1951,Bliss & Stidolph 1969). A number of authors,including Macgregor (1928), Keep (1929) andPhaup (1933), have described ultramafic rockswith basaltic textures from within the BulawayanGroup, but .few attempts were made to dis-tinguish these rocks on their maps. The mainchange to the existing published maps made incompiling Figure 2 is the inclusion of theserocks as a separate group of komatiitic basaltand peridotite. It is stressed that their distribution, as shown in Figure 2, is largely a resultof road traversing reconnaissance and a reas-sessment of information in the various Surveybulletins. As a result the boundarips shown areapproximate and would need much further fieldwork to be defined accurately.

As can be seen in Figure 2, the komatiitesare a major constituent of the Bulawayan Group.They also occur in the Sebakwian Group: agood example (shown as serpentinite in Fig. 2)occurs along the eastern edge of the Shanganibelt (Viljoen et aI. 1.976). In the BulawayanGroup itself the komatiites occur in remarkablyconsistent and simple sequences like that de-scribed by Viljoen et al. (L976). The sequenceat Damba seems typical of this volcanic asso-ciation and will be described in detail later.

The komatiites give way to, and interfingerwith, relatively monotonous successions of pre-dominantly pillowed tholeiitic basalts, which inplaces also rest on the underlying Sebakwian

NICKEL SULFIDE DEPOSTTS IN RHODESIA

Group and were apparently extruded concomi-tantly with the komatiites. Andesite lavas andwidespread agglomerates (Macgregor 1937, Mac-gxegor et al. 1937, Harrison 1970) commonlyoverlie either tholeiitic or komatiitic basalt inthe western half of the map area, but in theeastern half (Gwelo and Belingwe belts) thinnerbut other,wise similar andesites underlie thetypical basaltic volcanic rocks of the BulawayanGroup. Sedimentary units within the BulawayanGroup are locally abundant (Belingwe, Gwelo,Filabusi and Bulawayo belts) and includebanded iron-formations, phyllites, limestones(Bickle et aI. L975) as well as greywackes, gritsand conglomerates (Macgregor 1937, Amm1940).

Tns Nrcrnr, DnPosrrs

Although the nickel deposits so far locatedin the map area (Fig. 2) differ from one another,they have associational characteristics that in-dicate a genetic link with the evolution ofBulawayan Group volcanic rocks. The moststriking feature is the restriction of almost allthe deposits to a stratigraphic interval of lessthan 500 m, straddling the contact between theSebakwian and Bulawayan Groups. It seemsthat the mineralization is an intrinsic part ofthe first volcanic activity that initiated theBulawayan Group. As will be shown in thefollowing descriptions of some of the deposits,the associated volcanism is komatiitic.

Damba

The Damba nickel deposits provide a clearexample of the relationship between mineraliza-tion and volcanism; they will be dessribed insome detail although this is a preliminary reportbased entirely on drilling information. Situated75 km north-northeast of Bulawayo in theLonely Mine belt (Fig. 2), the deposits arespread oyer a strike length of 7 km. The firstof the deposits was discovered in 1968 belowa showing of nickel-rich silicates and gaspeite,and subsequent exploration to 1975 had locatedfive separate deposits.

The regional geology has been outlined byMacgregor (1928) but surface'mapping in thearea is hampered by poor exposure; the strati-graphy that has been established in areas ofmineralization (Fig. 3) is based largely ondrifling results. The sequence has a generalnorth-south strike and a regional westerly dipof 45-50o. The Sebakwian Group here consistsof various amphibolites, hornblende and quartz-

Frc. 3. Damba surface geology. Map, in part' fromwork by D.M. Anderson, MJ. and R.P. Viljoen.

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342 THE CANADIAN MINERALOGIST

mica schists, banded iron-formations and nearerthe top, well-preserved, apparently volcanogenic,quartzofeldspathic sedimentary rocks and inter-layered pyritic carbonaceous shales. The se-quence is intruded by several large peridotite*pyroxenite-gabbro layered complexes and by anumber of strikeaersistent multiple dolerite sills.

The Bulawayan Group appears to lie con-formably on the quartzofeldspathic sedimentaryrocks and minor cherts that terminate theSebakwian Group. In the zone that includes thonickel deposits (Figs. Z, 3), the BulawayanGroup volcanism was komatiitic and the suc-cession can be divided into three units: Unit1 (at the base): ore-zone peridotite (G-200 mthick); Unit 2: olivine-bearing pyroxenitickcrmatiite (0-300 m thick); Unit 3: basaltickomatiite (up to 7 km thick).

In most areas, unit 1, the ore-zone peridotite,rests directly on the underlying SebakwianGroup, but in some parts a thin wedge (< 4O mthick) of basaltic komatiite separates perido-tite from sedimentary units. Mapping and drill-ing have shown that although the peridotitethickens and thins irregularly it is continuousover a strike of some 9 km.

The principal rock types that now make upthe peridotite are serpentinites, carbonated ser-pentinites and talc--carbonate rocks. Althoughtheir distribution is mostlv erratic and unrelated

to original rock type, the serpentinites aregenerally more common in thicker parts of theperidotite and give way to talc-carbonate rocksin thinner zones. Primary textures are preservedin some of the less intensely altered rocks andshow completely serpentiuized subhedral toeuhedral olivine set in a matrix of subhedral toskeletal uralitized or serpentinized clinopyroxene,minor chromite and a densely opaque-dustedgroundmass of chlorite or serpentine. With in-creased serpentinization the matrix is also com-pletely serpentinized. No systematic variationin olivine grain-size could be detected; theaverage size is 1.G-1.5 mm with some scatteredlarger, lath-like crystals ranging from 3-10 mmin length.

Spinifex texture in a number of forms, iswell developed in specific zones in the peridotite.Its rnain occurrenqe is at the top of the perido-tite unit and it seems better developed in thisposition, where the peridotite is thicker (Fig.4). Internal spinifex units also occur and themaximum number intersected in any drillholeis four. The types represented include typicalplaty varieties and examples with randomlyoriented, long (10 mm) skeletal crystals, witbboth types showing widely varying grain sizes.Most of the spinifex zones have arnygdalesranging in size from 3-10 mm, and filled withchlorite, carbonate or sulfide.

BUIAWAYAN GROUP

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Ftc. 4. Da^mba and Silwane strike and dip sections.

NICKEL SULFIDE DEPOSITS IN RHODESIA 343

Quite convincing flow-top breccias and pos-sible ultramafic tuffs can be recognized in severaldrillholes, always associated with spinifex units.The breccias seem to be more com'mon in theflows with strongly amygdaloidal spinifex zones.Flow-top breccias, widespread amygdaloidalstructures, repeated spinifex units and the tex-tural and compositional similarity between theolivine-poor zones within the peridotite andolivine-rich basalt flows in the hanging wallseem to indicate that the peridotite is in largepart extrusive.

Unit 2 overlies the peridotite and is made upof olivine-bearing pyroxenitic and 'oglassy" koma-tiitic lavas. Individual flows, which are notpillowed, are up to 8 m thick; thin tuff unitsand cracked and broken flow tops can be seenmarking the tops of many of the flows. Alfhoughnow converted to tremolite, talc and chlorite,primary textures are generally well preservedand indicate that the basalts were olivine-phyricwith a range in olivine content from l-3OVo.Drilling has intersected two areas of agglome-rate in this unit; the agglomerate consists ofirregular komatiitic basalt fragments, rangingin size from 2-100 mm, set in a matrix oforiginally glassy welded tuff with shardlikematerial and lapilli tuff.

Unil 3 is made up of basaltic komatiite(and some tholeiitic basalt) that is generallyolivine-free and markedly lower in MgO contenlthan the pyroxenitic basalts of Unit 2 (Williamsin prep.), The basaltic komatiites are generallypillowed and were predominantly clinopyroxeneand glass-rich basalts, ,many with appreciableplagioclase. Some unpillowed examples showspectacular clinopyroxene-derived spinifex tex-ture. Although it overlies the other two unitswhere they are developed, basalt of Unit 3 off-laps the underlying units along strike and even-tually rests directly on the Sebakwian Group.Similar basalts are found. at the strike extremitiesof the peridotite, as wedges between the perido-tite and the underlying Sebakwian Group.Further along strike, basaltic komatiites of Unh3 interfinger with, and give way to typicaltholeiitic basalts that also rest directly on theSebakwian Group.

Although five areas of mineralization havebeen located (from the south northwards, DambaSonth, Silwane, Damba, Flow and Fibre: Fig.3), Silwane and Da.mba are the only two in-vestigated in sufficient detail to outline orezones. A variety of types of mineralization canbe recognized, including several disseminatedtypes, blebs, schlieren and massive or vein sul-fide, but the disseminated and bleb varieties

predominate. The deposits are in many wayssimilar to the Otter Shoot at Kambalda (Keele& Nickel 1974).

The distribution of mineralization in theDamba-Silwane area is shown in the strike anddip sections in Figure 4. At Silwane, mineraliza-tion occurs as a well-defined concentration inthe lower part of the peridotite and appearsconfined to a basinJike depression in thefootwall about 500 m in diameter. The mine-ralization itself is a broadly discus-shaped bodyup to 75 m thick at the centre but with thick-ness, grade and predictability falling off rapidlytowards the margins. It is associated with apronounsed thickening of the host peridotiteand thus with serpentinites rather than talc-carbonate rocks. The mineralization is typicallypatchy, with interlayered mineralized and bar-ren peridotite quite common, particularly onthe hanging-wall side of the orebody. Althougbmineralization can usually be correlated fromone drillhole to another, the zones in adjacentdrillholes are commonly not at the same strati-graphic level within the peridotite. Below thecentre part of the footwall depression two drill-holes have intersected a 20-3O m thick, well-rnineralized peridotite body some 20 m into thefootwall sedimentary rocks (Fig. 4, sectionEF). This small peridotite body is interpretedas a pipe-like feeder to the overlying peridotite.

The principal 'mineralization at Damba (Fig.4, sections AB and GH) is located toward thecentre part of the peridotite. In dip section (GHin Fig. 4) this central ,mineralization, typicallyof the bleb type, thins rapidly with depth,giving way to footwall disseminated mineraliza-tion which is confined to another, but muchsmaller, depression in the footwall.

The disseminated sulfide is a typical matrix-type mineralization in that most of the sulfideoccurs in the interstices between the completelyserpentinized olivine crystals.' The small-scaledistribution of this type of mineralization ischaracteristically patchy and it most commonlyoccurs as irregular ovoid patches (2-5 cm india,meter) of matrix sul.fide surrounded by bar-ren peridotite. Only rarely does the sulfide com-pletely fill the interstices between the olivines.

In the bleb- or'globule-style of rnineralizationindividual blebs vary in diameter from 2-20 mmand on average contain about SOVo sulfide. Inform they range from small, welldefinedspheres to larger oblate spheroids, becomingless sharply defined with increasing size. Theyare associated with what appear to be chlorite- orcarbonate-filled amygdales of similar size andshape. In some examples the bottom half is

344 THE CANADIAN MINERALOGIST

filled with sulfide and the top half with car-bonate or chlorite. Similar structures in theOtter Shoot at Kambalda have been interpretedas immiscible sulfide droplets (Keele & Nickel1974, Keele 1,975) or as amygdale fillings(Willett 1975). The Damba examples requirefurther study but are here regarded as thesulfur-rich phase associated with primary vesi-culation of the ultramafic liquid.

Sul.fide veins up to 3 cm wide also occursporadically in the peridotite and are invariablyrich in magnetite. Massive sulfides are seldomfound in significant amounts but thin zones(up to 20 cm in thickness) do occur at thefootwall contact in some drillholes. It is quitepossible that some of these contact zones arethick veins rather than actual .massive sulfideaccumulations. Other types of mineralizationinclude a wide range of schlieren styles thatseem to result from local remobilization ofdisseminated or bleb material during shearingassociated with talc-carbonate alteration.

Weak mineralization, as irregular blebs,patches or veins, also extends into the footwallsedimentary rocks in some places, particularlyat Flow and to a lesser extent at Silwane. Thismineralization does not penetrate further thanabout 10 m from the peridotite and is mostcom.mon in the chert that generally terminatesthe sedimentary series.

The principal sulfide minerals are pentlandite,millerite and pyrite with widespread,minor chal-copyrite, sporadic but locally abundant violariteand pyrrhotite and traces of vallerite, gersdorf-fite and chalcocite (Hendriks 1974). Ni:Curatios average about l5:1, as is typical of nicke.ldeposits associated with Archean ultramaficrocks (Naldrett 1973), A number of correla-tions can be made between differing sulfideassernblages, styles of mineralization and host-rock type although studies of the ore to dateare preliminary. Two types of assemblage seemto be developed though there are exceptions andgradations between them (Hendriks 1974). Thefirst is a millerite-magnetite type with relativelyminor amounts of pentlandite and pyrite andthe second is a pentlandite-rich type with ahigh pyrite or pyrrhotite content and less magne-tite. Violarite occurs in both types and is inmost cases an alteration product of either mille-rite or pentlandite. Magnetite occurs in three'forms: as magnetite rims on chromite that isquite separate from the sulfide association; asprimary magnetite forming an intrinsic part ofthe sulfide assemblage and as secondary magne-tite thaf is related to serpentinization.

In the disseminated and vein sulfide ores there

is a good correlation of the millerite-magnetitetype with a serpentinite host rock and of thepentlandite type regardless of host rock. Sulfidescontacts between serpentinite and talc-carbonateunits, as well as between the respective oretypes, are generally abrupt. In contrast, the blebor globule type of ore is almost invariably of thepentlandite type regardless of host rock. Sulfidesin the footwall sediments appear to be of thepentlandite type.

A further generalization can be made in thatdisseminated sulfide in talc-carbonate rocks istexturally more variable and shows clear evidenceof at least local remobilization. It is also oflower and more erratic grade than disseminatedorq in an adjacent serpentinite. There is alsoa preferential association of sulfide and car-bonate in the talc-carbonate rocks, and c,hloriterims on the sulfides are common.

Although talc-carbonate alteration seems tohave produced a more sulfur-rich (or nickel-poor) sulfide assemblage from the finely dis-seminated ore in the serpentinites, it does notseem to have had any effect on the sulfideblebs. This may be a result of the widely differ-ing surface areas of the sulfide asse.mblages inthe two types. Because of pervasive serpentiniza-tion it has not been possible to assess the effectsof the serpentinization process itself on primarysulfide assemblage. The situation now is that thesulfides found in blebs (be they amygdales orimmiscible droplets) are markedly sulfur-rich assemblages ( pentlandite-pyrite-pyrrhotite)compared with the spatially associated, butusually stratigraphically lower, disseminatedmillerite-magnetite assernblage, which is relat-ively sulfur-deficient and of higher ni'ckel tenor.

Shangani

The Shangani nickel deposit (Philpott 1975,Viljoen et al. 1976) is situated about 35 kmeast-southeast of the Damba deposits in aseparate greenstone belt, the geology of which(Fig. 2), has been documented by Harrison(1969).

Viljoen et al. (L976) have recently describedthe geology and mineralization of the deposit;there is a marked similarity between the strati-graphic successions in which the Damba andShangani deposits are situated.

At Shangani, Viljoen et al. (1976) recognizethe following stratigraphy in the mine area:Esmyangene Formation (at the base): acid tointermediate tuffs terminated by a pyritic shale,intruded by a large layered complex of peridotite,harzburgite, pyroxenite and gabbro; MakweFormation: peridotite and pyritic carbonaceous

NTCKEL SI'LFIDB DEPOSITS IN RHODESIA 345

shale overlain by magnesium-rich metabasalts;Ensangu Formation: massive and pillowed meta-basalts. Viljoen et al. (1976) place the contactbetween the Sebakwian and Bulawayan Groupsat the Makwe-Ensangu Formation boundary;horf,ever, as this contact is actually within avolcanic sequence, it seems .more reasonable toplace it between the Esmyangene and MakweFormations where mafic volcanism actually be-gan. The Bulawayan Group thus begins herewith the peridotite unit ( = Unit 1 at Damba),with associated pyritic shales and overlyingmapesium-rich basalts (- Unit 2 at Damba)of the Makwe Formation followed by the basaltsand basaltic komatiites ( - Unit 3 at Damba)of the Ensangu Formation (Fig. 5).

The mineralized complex at Shangani (Viljoenet al. 1976) lies within the Esmyangene Forma-tion and is a trenoh-like peridotite body (mush-room-shaped in cross section) that is at least1.7 km long (down dip) with the neck cuttingacross the regional stratigraphy (Fig: 5). Theore is predominantly disseminated and is atypical pyrhotite-pentlandite assemblage occur-ring on the stratigraphically flatJying flankingparts of the complex (Fig. 5, section CD). Thedownward extension of the stem of the complex

appears to be cut off by the large layered bodythat intrudes tlte Esmyangene Forrnation. Up-ward within the complex (Fig. 5, sections ABand CD) serpentinite is interlayered with am-phibolite and some carbonaceous shales.

Viljoen et al. (1976) regard the complex asintrusive, emplaced into, or close to, an acidvolcanic vent. This author prefers to interpretthe complex itself as a volcanic neck that de-veloped in the following way. It has a trench-or fissure-like form, with unlayered peridotitein the stem and in the lower part of each lobe(Fig. 5, section CD). This passes upward intoa typical extrusive alternation of magnesiurn-rich basalt, some of which has spinifex-styletextures, and more peridotite, in places withsedimentary interlayers between units. Sulfideliquid suspended in the initial peridotite pulsewas swept onto, or left behind on, the flatJying,flanking sections of the complex. The actualsite or sites of ultramafic volcanism would havemigrated along the trench with time. After theinitial development of the mineralized complex,a pause in ultramafic volcanic activity, or itsmigration along the fissure, allowed further ac-cumulation of felsic tuffaceous material fromongoing but waning acid volcanism. Renewed

Frc. 5. Simplified plan and sections for Shangani. Modified from Viljoen et al. (1976),

346 THE CANADIAN MINERALOGIST

ultramafic volcanism, yielding a peridotite withfiner grained suppended olivine crystals thanpreviously, initiated the Bulawayan proper withthe first part of -the Makwe Formation. Thisperidotite (= -llnit 1 at Damba) probablyissued from other sites along the fissure zoneand was immediately followed first by thepyroxenitic komatiite lavas ( = Unit 2 at Dam-ba) that make up the rest of the Makwe Forma-tion, and then by t*re,thick sequence of basaltickomatiites and basalts of the Ensangu Forma-tion (= Unit 3 at Damba).

Thus the mineralized complex can be regardedas the first tentative stage of Bulawayan Groupultramafic volcanism in a zone of crustal weak-ness where Viljoen et al. (1976) have recog-gnized that felsic volcanism had been, or stillwas, in progress. Viljoen et al. (1976) havealso demonstrated that this zone acted as avent for the overlying volcanic rocks. Thus themineralization itself is found on the horizontalor kink-structured sites flanking a vertical con-duit; the interpretation outlined above showsagain the close link that exists in this regionbetween nickel mineralization and the firstvolcanic event in the Bulawayan Group.

Epoch

The Epoch nickel mine is situated 85 kmsouth of Shangani (Fig. 2). Although thereis no published account of the deposit, severalpoints of interest have been ascertained fromavailable maps (Ferguson 1934) and the pre-sent investigation. Firstly, the deposit is alsosituated at or near the continuation of thecontact between the Sebakwian and BulawavanGroups. Secondly, like Shangani, it appears tobe associated with a cross-cutting peridotitecomplex (Viljoen et al. 1976, p. 93) immedia-tely above an ultramafic-dominated ilaveredsill intruded into the upper part of the Sebak-wian Group. Thirdly, the Bulawayan GroupIavas that stratigraphically overlie the depositcontain a significant komatiitic basalt com-ponent (Fig. 2). It is understood that the mainsulfides are millerite and pentlandite as foundat Da.mba. Thus a cross-cutting peridotite at ornear the base of Bulawayan Group komatiiticvolcanism is a feature of another deposit inthe region.

Selukwe

Another nickel prospect occurs in the Selukwebelt (Fig. 2) and again there is no publishedaccount of the occurrence. However, the de-posit is also located near the contact betweenthe Sebakwian Group and peridotites apparently

intrusive into the Bulawayan Group. Descriptions of the general geology and petrology byStowe (1968) suggest that komatiitic volcanicrocks are also developed in the area.

Hunters Road

The Hunters Road nickel deposit (Brand1976) is situated 80 km northeast of Shangani(Fig. 2), on the east limb of the Gwelo synclinein what appears to be a cross-cutting serpenti-nite body. In this case the host serpentinite cutsthrough a banded iron-formation lying at thecontact between acid to intermediate volcanicrockS below, and pillowed tholeiitic basaltsabove (Macgregor 1937, Tyndale-Biscoe 1949,Harrison 1970), Brand (1976 and oral presen-tation, Salisbury) stated that disseminated sul-fide mineralization occurs in a serpentinite in-truded along a tear fault and suggested thatboth intrusive and extrusive ultramafic rocksmay be represented. He also noted that layersshowing spinifex texture are developed in thebody and that it contains large xenoliths ofgreenstone, banded iron-formation and acid vol-canic rocks.

At first glance this deposit would not appearto be at the same regional contact as the otherdeposits. Ifowever, the greenstone belt extend-ing northwesterly from Gwelo is synclinal, witha succession on the west limb of SebakwianGroup schists overlain in turn by andesite andBulawayan Group mafic volcanic rocks (Fig.2). On the east limb, where the nickel depositis located, a similar but far thicker suecessionof andesite is present and overlain by tholeiiticbasalts, but there is no evidence of a Sebakwian-type sequence at the base. Structural informa-tion in the Sebakwian Group on the west limbsuggests that the andesites represent the firstvolcanism of the Bulawayan Group in this area.Therefore, the Hunters Road deposit does occurat a stratigraphic position marking the beginningof Bulawayan Group mafic volcanism as is thecase with the other deposits. In this case, how-ever, the mafic volcanism that follows is tholei-itic and not komatiitic.

Lower Gwelo

The west limb of the Gwelo syncline hasthe Sebakwian Group at the base and it hereconsists of quartz-mica schists, quartzofeld-spathic sedimentary units, banded iron-forma-tions and serpentinites (Macgregor 1937, Tyn-dale-Biscoe 1949), It is overlain in turn by athin but continuous andesite unit, a typicalDamba-Shangani komatiite sequence, pillowedtholeiitic bastlts, banded iron-formations and

NICKEL SULFIDE DEPOSITS IN RHODESH 347

well-pr€served grits and conglomerates that makeup the synclinal core.

The komatiitic sequence consists of peridotiticand pyroxenitic komatiites at the base, overlainby basa,ltic and spinifex-textured "andesitiC'komatiite lavas (Williams in prep.). Dissem-inated nickel sulfide mineralization is presentin the ultramafic unit at the base of the volcanicpile but has yet to be fully evaluated. Again,mineralization is at the same regional contact.

Noel

The Noel nickel arsenide deposit (Phaup1933) is in the Lower Gwanda belt (Fig. 2)and has been known since 1928. The depositconsists principally of chloanthite and niocoliteassociated with calcite and silicate veinlets ina fault zone in an actinolitic serpentinite. Al-though clearly hydrothermal, the deposit is alsowithin a komatiite succession whish, in thiscase, consists of a complex interlayering ofbasaltis-textured peridotites (Phaup 1933) andbasaltic and pyroxenitic komatiites.

DtscussroN

Many features of the Damba nickel depositspoint to a close relationship of ore with earlyperidotitic volcanism. The presence of amyg-daloidal spinifex-textured units, probable flow-top breccias and ultramafic tuffs within thehost peridotite all indicate an extrusive origin.A possible feeder conduit (or vent) is recognizedunder the Silwane mineralization and the pre-sence of an agglomerate zone 175 m above itwithin the overlying pyroxenitic

'komatiites

(Unit 2) adds support to the idea of a vent inthis locality.

The mineralization occurs as blebs (at leastsome of which were immiscible droplets) andas disseminated matrix sulfide that has a de-finite appearance of being of a similar dropletstyle but dispersed by included olivine crystals'to produce the ovoid patches of disseminatedsulfide. Mineralization occurs in a thick basin-like depression or lava pool that overlies thepossible feeder, which is itself concentricallymineralized with higher grade ore in the centrepassing outward into lower grade ore. Ttiesefactors all suggest that the sulfide is an intrinsicpart of the host peridotite magma. Furthermore,the emplacement of peridotite and ore was thefirst event of komatiitic volcanism within theBulawayan Group in this area.

Features of many of the other deposits sug-gest that similar relationships exist between

mineralization and volcanism throughout theregion. Several deposits (Shangani, Epoch 1qdHunters Road) are in, or associated with'cross-cutting peridotite-complexes at the contactbetween the Sebakwiain and Bulawayan Groups.At Shangani the mineralized complex has theform of a fissure-filling with the trunk of thebody cutting across the regional stratigraphy.This, together with the flanking position of themineralization and overall form of the complex,suggests that it is either a magma chamber oran actual volcanic neck or vent. The latter alter-native is preferred because of the interlayeringof peridotite, spinifex-textured amphibolite(here regarded as komatiitic lava) and carbona-ceous shale in thb upper part of the cornplex.

The Hunters Road deposit is also in whatappears to be a cross-cutting complex and largexenoliths-of country rock are recorded as occur-ring within the serpentinite. Some of the mine-ralization in this deposit is in the form of smalldroplets like those described from Damba andsupports the view that the sulfides were carriedas^an immiscible phase at the tirire of emplace-ment. The dif.ferent types of sulfide mineraliza'tion found at Damba suggest that both sulfur-rich and sulfur-poor phases may have existedas droplets in close proximity to each other.Significantly, at least at Damba, the sulfur-richassemblage is found' within markedly amyg-daloidal zones in the host peridotite.

Using the concept that sulfides were trans'ported is an immiscible phase within an olMnecrystal - ultramafiq liquid mush, Naldrett(i973) has speculated on some likely relation-ships botween ore, host and country rock' Hisdiagrams are reproduced here in Figure 6 andshow possible reconstructions of thq settings forShangani (Fig. 6, example L ot 2) and Silwane(example 3).

As ihere seems to be a close association be'tween ore and volcanism, it is useful to examinethe general relationships between the variousvolcanic assemblages within the BulawayanGroup and thus the regional setting of the de-posits. The present study has shown that koma-tiitic volcanic roc'ks a.re more widespread in theRhodesian greenstone belts than indicated onexisting,'maps. It has also shorrn that the simplekomatiite stratigraphy described by Yilioen et al.(1976) at Shangani-and described in detail herefor the Lonely Mine belt is"widespread in theregion. Another example is developed on thewestern limb of the Grrelo belti Bickle et al.(1975) and.Nisbet et al. (1977) have describeda particular,ly extensive succession in the Be'lingwe belt.,.These stratigraphic sequences begin

348 THE CANADIAN MINERALOGIST

with a peridotite and pyroxenitic komatiite unitthat ranges up to 1 km in thickness, followedby a thick pile (up to 7 km) of basaltic koma-tiite and some tholeiitic basalts. The basaltickomatiites commonly show clinopyroxenede-rived spinifex texture or various textures causedby skeletal clinopyroxene crystallization. Morepyroxenitic or olivine-bearing examples are in-tercalated with the basaltic varieties, but arenot common. These komatiitic sequences havewidely varying strike extents ranging from 15-80 km.

Along strike the komatiitic volcanic rocks

overlap and interfinger with apparently con-comitantly extruded tholeiitic basalts. These aretypically pillowed, weakly plagioclase-phyricbasalts showing little of the skeletal crystalliza-tion that is usually characteristic of the koma-tiite suite. However, the distinction betweenthe tholeiites and low-MgO basaltic komatiitesis often difficult and some uncertainty exists atthese compositions. This overlap is also apparentin their geochemistry (Williams in prep.) andmight suggest that the two magma types resultfrom different levels of tapping of a commonmagma stem. The rnore MgO-rich komatiiticrocks, which are the nickel sulfide hosts, wouldbe expected to result from the deeper tapping.

Higher up in the Bulawayan Group the inter-layering of volcanic types becomes more com-plex. In the Lonely Mine - Bulawayo belt athick succession of andesite lavas overlies thetholeiitic and komatiitic volcanic suites and is,in turn, overlain by either more basaltic andpyroxenitic komatiites or by tholeiites. It is clearthat in most of the Rhodesian greenstone beltsexamined during this study, the simple koma-tiite to tholeiite evolution proposed for otlergreenstone belts in southern Africa (Viljoen &Viljoen 1969) does not apply. Rather thereseems to be random interlayering and over-lapping of adjacent compositionally differentvolcanic suites throughout the succession. Theclear evidence of contemporaneous or penecon-temporaneous tholeiitic and komatiitic volcanism,and in some cases acid volcanism (Lonely Minebelt), is particularly importpnt. Andesitic lavasand agglomerates can occur either before koma-tiitic and tholeiitis activity (Belingwe, Gwelo,Hun,ters Road) or after (Lonely MineBulawayo, Gwanda).

In su,rnrnary, six nickel deposits in the regionare found at or near the same regional contact;they seem to form an integral part of the veryfirst event of komatiitic volcanism within theBulawayan Group. Several of the deposits liewithin cross-cutting peridotite complexes thatcan be interpreted as volcanic necks or ventslying in sites that aoted as volcanic centres bothprior to, and follorring, peridotite-ore emplace-ment. The configuration of the Shangani mine-ralized complex suggests that the ore-bearinskomatiitic volcanism was of a fissure tnre.

AcrNowlrpcEMENTS

The permission of Johannesburg ConsolidatedInvestment Company Umited to publish theseresults is gratefully acknowledged. I arn indebted

Frc. 6. Speculations on likely relationships between' 'ore, host and country rock in nickel deposits ofvolcanic association by Naldrett (1973).

I. SUB-SURFACE MAOMA RESERVOIR

2. THICKER OLIVINE - RICH PARTS OFFLOWS CLOSER TO FEEDER

3. LAVA POOL OVERFLOWING INTER -MITTENTLY TO GIVE SERIES OF SPINIFECAPPED FLOWS AT EDGES

COUNTRY ROCK

PERIDOTITE (<50% OLIVINE)

PERIDOTITE (>50% OLIVINE)

SULFIDE

NICKEL SULFIDE DEPOSITS IN RHODESIA

to Company colleagues who ,have contributedto this study, particularly R. Mason, D.E' Phil-pott, C. Willson, D.M. Anderson and L.P. Hen-driks and to R.P. and M.J. Viljoen who firstintroduced me to the Damba and Shangani de-posits. Staff of the Rhodesia Geological Surveyprovided invaluable advice and comment duringthe regional investigation, and special thanks goto N.M. Harrison and I.D.M. Robertson. I amvery grateful to I.D.M. Robertson' R. Mason,P.G. Cochran and D.L. Garnett for criticizingdrafts of the manuscript. Special thanks go tomy wife Gwynn for her geological help bothin the field and office, to Piet Britz for hisdraughting and Elizabeth Vigus for typing themanuscript.

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Received Augwt 1978; revised manuscript acceptedDecember 1978.

349