Canadian Mineralogist VoI. 17, pp.337-349 (1979) THEASSOCIATION OF SOMENICKEL SULFIDE DEPOSITS WITH KOMATIITICVOTCANISM IN RHODESIA D.A.C. WILIAMS* lohannesburg Consolidated Investment Company Limited, P.O. Box 976, Randfontein 1760, South Afrir.a ABSTRAcT ba, d6crits ici en d6tail; nous pr6sentons aussi les faits saillants concernant cinq autres de ces gise- During the past ten years a number of small ments. Un relev6 r6gional et une r6-annotation des nickel sulfide deposits have been discovered in cartes existantes couvrant 230 000 kmz du socle the Archean greenstone belts of the Rhodesian Rhod6sien montrent clairement que la min6ralisa- Midlands. These include the Damba deposits, de- tion est associ6e au volcanisme komatiitique, plus scribed here in some detail, and five other occur- r6pandu dans le groupe Bulawayen que ne l'indi- rences whose principal features are summarized. quent les cartes. Au moins quatre s6quenceskoma- A regional investigation and a reannotation of tiitiques possbdent la m6me stratigraphie simple et existing maps covering 230,000 km2 of the Rhode- r6gulitsre (komatiitg p6ridotitique-komatiite pyrox6- sian basement show that the deposits are clearly nitique-komatiite basaltique), dont les roches basal- assbciatedwith komatiitic volcanism and that koma- tiques constituent la majeure partie dans chacune tiitic volcanic rocks are more widespread in the des s6quences. Le magmatisme, dans le groupe Bulawayan Group than the maps suggest. At least Bulawayen, a 6t6 thol6itique et komatiitigue, tant6t four of the komatiite sequences have the same simultan6ment, tant6t alternativement. Tous les simple and regular stratigraphy of peridotitic koma- gisements se trouvent prbs du contact des groupes tiite - pyroxenitic komatiite - basaltic komatiite, Sebakwien et Bulawayen, et forment partie int6- with the basaltic varieties making up the major grale de la toute premiere unit6 volcanique du part of individual successions. Komatiitic and groupe Bulawayen. Les gisements Damba sont dans tholeiitic magmatism was both concomitant and une extrusion p6ridotitique; dans un cas, la min6ra- overlapping within the Bulawayan Group. All the lisation semble localis6e au-dessus d'un conduit nickel deposits are found at, or close to, the contact d'alimentation volcanique. Plusieurs des gisements between the Sebakw.ian and Bulawayan Groups and sont intimement recoupds par des rntrusions de are an integral part of the very first volcanic unit p6ridotite qui repr6senteraient des necks ou venues in the Bulawayan Group. The Damba deposits are volcaniques; ces gisements sont situ6s en .des within an extrusive peridotite unit and include one centres d'une activit6 volcanique ant6rieure et instance of mineralization overlying what may have post6rieure i la mise en place du minerai. Tant been a feeder conduit. Several of the other deposits par son emplacement que par sa forme, la s6quence occur in, or are closely associated with, cross- komatiitique du gisement Shangani indioue un cutting peridotite bodies that are interpreted as volcanisme de type fissural. volcanic necks or vents; the deposits seem to lie (Traduit par in sites that have acted as volcanic centres both la R6daction) prior to and following peridotite-ore emplacement. Both the detailed setting and configuration of the INTRoDUcTIoN komatiitic sequen@ at the Shangani deposit suggest that volcanism was of a fissure type. During the late 1960s and early 1970s a number of nickel sulfide depoeits were dis- SoMMAIRE covered in the Rhodesian Midlands (Fig. 1). Although by world standards tley are small Au cours de la dernidre d6cennie,plusieurs petits and of low grade, two of the discoveries are gisementsnickelifbres ont 6t6 d6couverts dans les currently in production at Shangani (Philpott ceintures de roches vertes arch6ennesen Rhod6sie 1975, Viljoen et al. L976) and Epoch. The centrale. Parmi ceux-ci figurent les gisementsDam- others include Damba, Hunters Road (Brand 1976) and Selukwe which are either subeconomic or await a more favorable economic climate for EPresent address: Exploration Department, The development. Several other minor or untested Broken Hi.ll Profrietary Co. Ltd., c.P.O. Box occurrences have also been located. L923, Pertb, Western Australia, 6001. As part of a broad-scale evaluation of the 337 338 THE CANADIAN MINERALOGIST ll-l posr BAsEMENTRocKs [.-TIlvounor. MEraMoRPHrczoNEs l--l .RANrrEs "*r,"t"t, [l on..nsro*r ".rrs AAEA DIECUSSED olo@@t6 FIo. l. Locality map, adapted from Bliss & Stidolph (1969). base-metalpotential of the Rhodesian basement GSNnRALGsor,ocv complex, a compilation study and reannotation of existing maps was implemented over some The Rhodesian Archean granite-greenstone 230,000 km' of this granite-greenstoneterrain. terrain provides some of the best information The basic data were derived from published and available anywhere on the relationships between unpublished maps of the Rhodesia Geological granite and greenstoneand between the various Survey at scales of between 1:50,000 and volcanic rock types. This is due to compre- 1:1,000,000.Along with company information, hensive, good-quality Survey mapping, conti- this was compiled at a scale of 1:250,000 and nuity of exposure and relatively low grades of augmented by four months field work. The meta.morphism. Bliss & Stidolph (1969) and Survey maps have a spread of publication dates Wilson (1973) have provided recent summaries from 1920 to 7975, resulting in major terrni- of the geology and possible development of the nological differences between the sheets. The Rhodesian craton which update Macgregot's field work was thus directed at resolving these (1951) classic account. The official Rhodesian differences and at examination of lithologies of geological map (1:1,000,'QQQ)has also been exploration interest with the aim of producing recently revised (7th edition, 1977), a unified geological map that reflects currenf Macgregor (1951) divided the greenstone knowledge. belt successionin Rhodesia into three "systems" Part of this work, coupled with a detailed (now called Groups): the Sebakwian Group at examinationof the Damba nickel deposits,has the base, overlain in turn by the Bulawayan and shown a rather striking associationof the nickel Shamvaian Groups. This subdivision is still re- deposits with komatiitic volcanism in the Mid- tained though the extent of the Sebakwian and lands area. Some of these results are presented Shamvaian Groups is less than as originally here; a more detailed discussionof the geologi- defined: cal and geochemical results is to be given else- Shamvaian Group: Poorly sorted sediments where (Williams in prep.). Note that the (grits, greywackes, arkoses and conglome- definitions of peridotitic, pyroxenitic and basal- rates with some felsic volcanic rocks. Not tic komatiite used here are those ot Arndt et aI. discussed further in this paper. (1977); see also discussion of this terrninology Bulawayan Group: Dominantly komatiitic, by Williams & Furnell (1979). Distinction be- basaltic and andesitic volcanic rocks with tween basaltic komatiite and tholeiite is based associated sedimentary units (arkose, grit, largely on texture, MgO content and field asso- phyllite, conglomerate and banded iron-for- ciation with spinifex-textured ultramafic koma- mation). tiites. Sebakwian Group: Mafic and felsic volcanic NICKEL SULFIDE DEPOSITS IN RHODESIA 002030.050 4 I -r'.I ^l \4 \r .{! i".- BULAWAYO Olg dlretln Frult Arenstona bctt GUrElO4 Nickat dapotit Tovn E Gr.lo sANo [f *^"oo. '(ALAHAR.ANo "owi ffi sxeuvereN ciouP (sEolMENT) FELSTC tNfnUStVE uAFtc ,NfnustvE AHDESIIE LAUA/\UFF SEDIUENf THOLEII|'C EASAL7 XOUAT"'IC DASALT APO EilNTITE SETPENTTMTE(HOSILV'NIEUg'VEI W|aesxwnN crou? crenrr:. rolllrrE oialr LOWII CWAXOA [-l Ftc. 2. Simplified geology of part of the compilation area showing nickel deposit localities. Data sources are given in tle text. 340 THE CANADIAN MINERALOGIST rocks, quartz-sericite schists, micaceous but it is usually characterized by quartzofeld- quartzites, phyllite and banded iron-forma- spathic sedimentary units, quartz-mica schists, tion with ultramafic intrusive rocks (of Bula- phyllites and quartzites, many of which are of wayan or later age) and some extrusive equi- volcaniclastic origin, together with banded iron- valents. formations, a wide variety of mafic schists and Bliss & Stidolph (7969, p. 309) noted that the some ultramafic lavas. The Sebakwian Group existence of the Sebakwian as a separate group is intruded by ubiquitous layered sills consisting is based on the recognition of an unconformity dominantly of serpentinite, with minor meta- between it and the overlying Bulawayan Group pyroxenite and ,metagabbro (Macgregor 1937, together with a marked change in structure and Amm 1946, Tyndale-Biscoe 1949, Harrison metamorphic grade acrossthe contact. The type- 1969, Viljoen et al. 1976). area unconformity has been questioned by a In the western part of the Gwelo belt (35 number of workers (Harrison 1968, Bliss & km northwest of Gwelo: Fig. 2), Macgregor Stidolph 1969, Wilson L973) and, as a result, (1937) recognized a NE-SW axis of folding in many of the areas previously shown as being the Sebakwian Group that is not shown in the Sebakwian have been included within the Bula- overlying Bulawayan Group. However, in the wayan Group on the latest Rhodesia geological Shangani and Lonely Mine belts there is no map. The term Sebakwian will be used here evidence of any ,major time break despite the in the sense of Bliss & Stidolph (1969) but change from