Abstract Many of the national parks in East Africa are equally as famous for their iconic landforms as they are for their diversity and concentrations of fauna and flora. The newly formed Ngorongoro-Lengai Geopark in northern Tanzania is the first geopark to be established in the region, but there is remarkable potential for geotourism in the majority of the national parks. The most spectacular landforms have been shaped by the East African Rift System. Formation of the two major rifts in the region, the Albertine Rift (or western branch) and the Gregory Rift (or eastern branch), was accompanied, or in some cases preceded, by extensive alkaline volcanism. The rifting and volcanism are primarily Late Cenozoic phenomenon that dissected and overprinted the older regional plateaus. Rifting impacted the regional drainage and captured major rivers, including the Victoria Nile. Chains of ribbon lakes formed in the rift valleys. The Albertine Rift consists of a sequence of sedimentary basins with deep freshwater lakes, but the shallow soda lakes of the Gregory Rift are associated with mostly volcanic terrains. Plateau-style volcanic outpourings smoothed out the older land surfaces, created near-lunar landscapes in parts of the rift valley, and built up rift shoulders to tremendous elevations. Magma erupted from central conduits formed giant stratovolcanoes which reveal evidence of explosive, Plinian-style volcanic activity. East Africa includes some of the largest and best preserved calderas on Earth. The Ngorongoro Caldera is a world heritage site. The ice-capped peaks of the two largest volcanoes in the region, Kilimanjaro and Mount Kenya, are among the highest free-standing mountains on Earth. The region includes active volcanoes, several of which are potentially hazardous as they are located near urban centres. Examples include Longonot-Hells Gate (Kenya), Mount Meru (Tanzania) and Nyiragongo (Democratic Republic of Congo). East Africa is renowned for the unusual rapidity of Darwinian evolution during the past thirty million years, including evolution of primates and hominins, and it is not a coincidence that significant palaeoanthropological discoveries have been unearthed from the Oldupai Gorge and Laetoli sites in northern Tanzania. The evolutionary period coincides with the onset and persistence of rifting and volcanism. Speciation is following an island-style pattern in East Africa, despite the continental setting, as regional plateaus are being dissected by the ongoing rifting and volcanism into smaller and smaller geological terrains. This is illustrated by restriction of the endangered Mountain gorilla to regions where afromontane forests developed in rift-related uplands isolated by extensive savannah grasslands.
Research Article| September 01, 2009 A MULTI-STAGE ORTHOMAGMATIC AND PARTIAL MELTING HYPOTHESIS FOR THE DRIEKOP PLATINIFEROUS DUNITE PIPE, EASTERN LIMB OF THE BUSHVELD COMPLEX, SOUTH AFRICA R.N. SCOON; R.N. SCOON Postnet Suite 291, Private Bag X31, Knysna 6570, e-mail: rnscoon@iafrica.com Search for other works by this author on: GSW Google Scholar A.A. MITCHELL A.A. MITCHELL School of Geology, University of KwaZulu-Natal, Private Bag 54001, Durban 4000, South Africa, e-mail: mitchaa@mweb.co.za Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2009) 112 (2): 163–186. https://doi.org/10.2113/gssajg.112.2.163 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation R.N. SCOON, A.A. MITCHELL; A MULTI-STAGE ORTHOMAGMATIC AND PARTIAL MELTING HYPOTHESIS FOR THE DRIEKOP PLATINIFEROUS DUNITE PIPE, EASTERN LIMB OF THE BUSHVELD COMPLEX, SOUTH AFRICA. South African Journal of Geology 2009;; 112 (2): 163–186. doi: https://doi.org/10.2113/gssajg.112.2.163 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The occurrence of platiniferous dunite pipes is restricted to four localities in the eastern limb of the Bushveld Complex. They comprise wholly discordant bodies aligned perpendicular to the regional layering, and reveal a complex internal zonation. They remain poorly understood, despite a number of contributions dealing with the ore mineralogy, due mainly to the fact that field relationships and the silicate and oxide mineralogy have not been adequately addressed. The principal geological features of one such pipe, at Driekop, are revisited on the basis of detailed mapping and sampling of mine workings that are not generally accessible. The pipe forms a subcircular body (diameter 300 m) that is persistent to considerable depth. The principal unit is a stock-like body composed entirely of fine-to-medium grained magnesian dunite (Fo83.4). Chromite is a prominent accessory phase of the magnesian dunite. The chromite is locally altered to highly unusual, polyphase grains in which discrete Cr-rich, Al-rich, and Fe-rich phases are identified. The magnesian dunite is rimmed by a zoned outer envelope that consists of an inner ring of iron-rich wehrlite (Fo65) and an outer ring of iron-rich clinopyroxenite pegmatite.Mineralisation is mostly contained in a near-cylindrical core-zone (diameter 18 to 24 m) characterised by segregations of iron-rich dunite (Fo73) and subordinate iron-rich wehrlite within the magnesian dunite. Mineralised veins of iron-rich dunite and wehrlite, as well as barren veins of iron-rich clinopyroxenite pegmatite, crosscut the magnesian dunite outside of the core-zone. All of the iron-rich ultramafics at Driekop are distinguished by the absence of a spinel phase. The pipe crosscuts a thick sequence of leucocratic cumulates (Merensky Footwall unit), that in turn are underlain by layers of chromitite and feldspathic orthopyroxenite (UG3 and UG2 units). The wall rocks are severely disrupted and form a km-wide downwarped feature. Layering proximal to the pipe on the southern margin dips at 75° (north-east) in comparison to the regional dip of 12° (west). The leucocratic wall rocks are mostly unaltered, apart from a thin zone of sausseritization, but include discrete satellite bodies of iron-rich clinopyroxenite pegmatite (Fo63). The underlying, more mafic UG3 and UG2 units, however, are anomalous. They are considerably thinner in comparison to the regional stratigraphy and some layers are entirely absent. The feldspathic orthopyroxenite in these units has been pervasively replaced by magnesian peridotite (Fo80-78). The downwarped UG3 and UG2 chromitite layers are partly dismembered and include spots of pipe-related magnesian olivine. Mineralogical and chemical similarities with dunite layers from the Lower Zone (Fo85.8) and Lower Critical Zone (Fo85.1) suggest that the magnesian dunite in the Driekop pipe is orthomagmatic. This hypothesis is supported by mapping of a meter-wide sill of magnesian dunite (Fo85), in the wall rocks at Driekop. Field relationships demonstrate this sill to have been intruded between layers of leuconorite and anorthosite. The magnesian dunite in the pipe formed as a consequence of "flowage differentiation" of an ultramafic magma through a vertical conduit. Magnesian olivine and Cr-spinel were the only liquidus phases during this phase of pipe formation. Heat associated with the intrusion resulted in formation of magnesian olivine spots within orthopyroxenitic wall rocks (albeit located some distance from the pipe); they are ascribed to the incongruent melting relationship between orthopyroxene and olivine. Within the leuconorite-anorthosite wall rocks, however, the increased temperature associated with the intrusion resulted in selective partial melting of ferromagnesian components (plagioclase was refractory). This produced a moderately iron-rich ferromagnesian melt that was relatively dense and drained downward into fractures propagated by the primary conduit. The downward draining of melt, together with the partial melting of more mafic layers, caused a catastrophic downwarping of the wall rocks. The ferromagnesian melt reacted with the leucocratic wall rocks to produce an iron-rich clinopyroxenite pegmatite (outermost part of the outer envelope and discrete satellites). Reaction with earlier-formed magnesian dunite resulted in either an iron-rich dunite (e.g. core-zone) or an iron-rich wehrlite (e.g. innermost part of the outer envelope). The occurrence of polyphase Cr-spinel in selective samples of magnesian dunite, as well as the entire absence of a spinel phase from the iron-rich ultramafics, suggests the pipe chromite was in disequilibrium with the downward-draining ferromagnesian melt. The origin of the mineralisation is not addressed here but the new hypothesis is consistent with the suggestion of Wagner (1929) that PGE were transported into the platiniferous dunite pipes by iron-rich melts derived by melting of earlier-formed layered reefs. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Discordant ultramafic bodies are a significant feature of the Bushveld Complex, South Africa. Many of these bodies form subvertical, cylindrical pipes oriented more or less at right angles to gently dipping layered rocks. Three main types of discordant ultramafic body have been identified: the (unmineralized) magnesian dunite pipes, the iron-rich ultramafic pegmatite, and the platiniferous dunite pipes. The magnesian dunite pipes, such as the Winnaarshoek occurrence, are most prominent in the central sector of the eastern limb. The Winnaarshoek pipe consists of magnesian dunite, which is a fine- to medium-grained assemblage of forsteritic olivine (Fo85‐83) with accessory Cr spinel, together with an extensive rim of clinopyroxenite pegmatite. We emphasize that the mineralogy and chemistry of the magnesian dunite pipes is quite different from those of the iron-rich ultramafic pegmatite (coarsely crystalline rocks composed largely of fayalitic olivine, augite, and FeTi oxides). The platiniferous dunite pipes constitute a third category of discordant ultramafic body because, in addition to Pt-rich core zones, they consist of both magnesian dunite and iron-rich ultramafic pegmatite. In determining the possibility of locating pipe-type platinum ores in other layered intrusions, the spatial association of primitive and differentiated lithologies within the Bushveld occurrences should be recognized. Mining operations exploiting layered reefs in the Bushveld Complex are adversely affected by discordant ultramafic bodies, and identification of the different types has important consequences. Both the unmineralized magnesian dunite and platiniferous dunite pipes may be associated with catastrophic downwarping of the layered cumulate wall rocks, whereas the discordant iron-rich ultramafic pegmatite bodies are considerably less disruptive as they exhibit evidence of passive volume-for-volume replacement. Ultramafic and mafic cumulate layers, including the Merensky reef, are cut out from the downwarped envelope associated with the Winnaarshoek pipe as well as from a juxtaposed megapothole structure. A transitional facies of the Merensky reef is developed on the margins of the megapothole and a localized thick reef facies occurs on the margins of the downwarped envelope. Field relationships demonstrate that the different types of discordant ultramafic body in the Bushveld Complex are unlikely to be consanguineous. The iron-rich ultramafic pegmatite is ascribed to magmatic replacement of layered cumulate wall rocks in response to the downward-draining of differentiated melts derived from within the intrusion, but the magnesian dunite pipes are a product of primary magmatic processes. Mineralogical and chemical similarities between the magnesian dunite pipes and layered olivine cumulates located in the lowermost part of the intrusion are unlikely to be coincidental. The olivine layers accumulated as a result of fractional crystallization of ultramafic magmas episodically injected into the intrusion as basal flows; the magnesian dunite pipes formed by flowage differentiation of similar magma batches within vertical conduits. The only liquidus phases during formation of the olivine layers and magnesian dunite pipes were forsteritic olivine and Cr spinel. Crystallization of orthopyroxene and plagioclase in the vertical conduits was suppressed as a high temperature was maintained by forced upward convection, such that the differentiated residue was ejected upward into the resident magma. Vertical conduits developed in response to syn-Bushveld tectonism that included diapirism, triggered by the anomalously high heat flux associated with the central sector of the eastern limb. The clinopyroxenite pegmatite rim, as well as the downwarping and absence of ultramafic and mafic layers proximal to the Winnaarshoek pipe, is attributed to selective partial melting of ferromagnesian components within noritic wall rocks. Facies changes on the margins of the downwarped structure, as well as within the megapothole, may indicate the nearly synchronous formation of all three features at Winnaarshoek: Merensky reef, megapothole, and magnesian dunite pipe.
Abstract Spinels associated with discordant bodies of iron-rich ultramafic pegmatite are described from the Amandelbult Platinum mine in the northwestern part of the Bushveld Complex. The spinels are divided into three groups, disseminated Ti-magnetite, disseminated Fe-Ti-Cr spinel and massive Fe-Ti-Cr spinel. The Fe-Ti-Cr spinels show a range of unusual compositions intermediate between chromite and Ti-magnetite. A relationship was found between stratigraphic height and spinel-type, with the Fe-Ti-Cr spinels restricted to pegmatites from the Upper Critical zone and Ti-magnetite to pegmatites from the Lower Main zone. Ilmenite is a ubiquitous component of all of the pegmatites examined here. The massive Fe-Ti-Cr oxide pegmatites are found only where earlier-formed chromitite layers are juxtaposed with sheet-like bodies of olivine-clinopyroxene pegmatite. A distinct thickening of the original chromitite layers in this situation, and compositional gradients within them, points to accretion of Fe-Ti-Cr spinels onto them prior to partial sub-solidus re-equilibration. Analytical data are presented for these spinels and for the Ti-magnetite. The composition of the Fe-Ti-Cr spinels is not duplicated by cumulus spinels in the Bushveld Complex, but the compositions and microtextures of the disseminated Ti-magnetite are very similar to cumulus Ti-magnetite from the Upper zone. Accordingly, it is deduced that the Ti-magnetite in the pegmatites from the Lower Main zone, together with the ilmenite, crystallized at magmatic temperatures from a suitable Fe-Ti-rich silicate-oxide melt. No evidence has been found to link the pegmatites to hydrothermal fluids. The Cr-rich nature of the disseminated spinels in pegmatites from the Upper Critical zone suggests that the pegmatite melt was richer in chromium at this stratigraphic height, although re-equilibration with earlier-formed cumulus chromite also occurred. Formation of the Fe-Ti-Cr oxide pegmatites reflects a complex process that is incompletely understood and why new oxides plate onto pre-existing chromitite layers that are juxtaposed with Fe-rich ultramafic pegmatites is a matter of conjecture.
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