1. CATALOGUE OF SOUTH AFRICAN
LITHOSTRATIGRAPHIC UNITS
Editor: M.R. Johnson
SA Committee for StratigraPhY
[Mokotian] BULLS RUN COMPLEX
A.J. Scogings'
1st Edition: 1991
1. PROPOSER OF NAME
Nicolaysen and Burger (1965 - "Bulls Bun Syenitic Gneiss");
revised by present author (this publication).
2, DERIVATION OF NAME
The farm Bulls Run Estate 12987, 17 km NNW of Eshowe, Natal.
3. TYPE AREA
Northeast of Goedertrou Dam (Fig. 1).
4. STRATIGRAPHIC POSITION ANO AGE
Located within the Nkomo Nappe of the Tugela Group, near the
northern margin of the Natal Metamorphic Province (Charlesworth,
1981). lntrusive into the Tugela Group. Nicolaysen and Burger (1965)
obtained a U-Pb age ol 1140 ! 35 Ma from a single zircon. More
recent work (author's unpublished whole-rock Rb-Sr data) indicates
an age ol 1 138 I 45 Ma for the nepheline syenite gneiss.
5. GEOLOGICAL DESCRIPTION
Basic concept and unifying features: Geochemically related
undersaturated to saturated alkaline/peralkaline syenitic and minor
carbonatitic gneisses.
Form and size of body: Prominent easterly-trending ridge 15 km
long and up to 1,5 km wide, rising some 400 m above the Mhlatuze
River (Fig. 1).
Lithology: A broad subdivision into three ma,ior syenitic gneiss
types (muscovite syenite, nepheline syenite and albite syenite
gneiss) is proposed for the bulk of the complex (Fig.1). Three minor
rock types (carbonatite, peralkaline microsyenite and alkaline mafic
gneiss) occur as dyke- or sheet-like bodies in the muscovite syenite
gneiss (Scogings and Forster, 19Bg).
Muscovite syenite gneiss (50%): Medium-grained; white to
yellowish grey. Consists of muscovite (5-35%), microcline (2C-357.),
albite (30-40%) and biotite (<10ol.); calcite, apatite and sphene are
accessory minerals.
Nepheline syenite gneiss (30%): Medium- to coarse-grained;
blue-grey to dark grey. Comprises nepheline (5;40"/"), microcline
(3G-50%), albite (5-30%) and biotite (2-15y"). Cancrinite, calcite,
apatite and ilmenite are characteristic accessories. Nepheline syeniie
gneisses within the eastern portion of Goedertrow contain aegirine
augite (5-20%) and are more mafic than those on Black Eyes and
Bulls Bun; they can be classified as feldspathic ijolite'
Aibite syenib gneiss (20%): Fine-grained; light grey to white; has
siliceous appearance in outcrop. Composed of albite (45-65%),
microcline (25-451"), nepheline (<25'/" and biotite (2-7%1.
Accessory minerals include zircon, pyrochlore, cancrinite and
sericite; nepheline is commonly kaolinized.
Carbonatite gnerss (<1%): Medium-grained; white to brown,
Consists of calcite (6H6"6), alkali feldspars (<20%), biotite (3-
25"6), apatite (<10%). Accessory minerals are ilmenite, pyrochlore
and zircon.
Peralkaline microsyenite gneiss (<1%): Fine-grained; white to
blue-green. Consists ot albite (60-70"/"), microcline (10-20'6), quartz
(2-6%) and aegirine-augite (2-5"/"). Calcite and riebeckite occur in
accessory amounts.
Alkaline mafic gneiss (<1"6): Fine-grained; dark grey to black.
Consists oI biotite (2G-50%), albite (35-50%), microcline (2-15%),
calcite (8-12ol") and apatite (<3%).
Geochemistry: Muscovite syenite gneisses are silica-saturated
(SiO,: 52-59%), peraluminous and have moderate alkali contents
(Nap+tlO : 10-13%), with [O generally in excess of NarO
(Na,O/GO c. 0,75). Nepheline syenite gneisses are undersaturated
(SiO,: 48--58"/"; normative nepheline: 10-30"/5), metaluminous to
peraluminous and have elevated alkali contents (Na.O+QO :
12'15"/0). Potassic and silica+ich in central and western outcrops
(Na"O/K"O c. 0,75) but more sodic and undersaturated in the east
(Na,O/X,O c. 1,25), Albite syenite gneisses have the highest silica
conients-of the major lithologies (SiOr: 57-65"/") and due to their
sodic character (NarO/KrO : 1-2,5) are moderately undersaturated
(normative nepheli-ne: i}-z}y"). FeO (<2"/") and CaO (<17")
contents are low. Enrichment in trace elements such as Z;
(7OO-2000 ppm) and Nb (20G-16500 ppm) is diagnostic. Carbonatite
gneisses have typical sovite chemistry: CaO (30-55%), MgO (c.
0,5olo), P2Ou (<6%), Sr (6000-90C0 ppm), Nb (s|{000 ppm).
Peralkaline microsyenite gneisses are silica-saturated (SiOr: 6U7Oo/",
normative quad.z: 2-12"/o), sodic (NarO/KrO c. 2) and mtldly
peralkaline (Na+K/Al c. 1,02). Alkaline-mafi-c gneisses have high
alkali contents (NarO + QO: 8-1 1%) for relatively low SiO, (45-50"6),
trace elements Zn,l.lb, 27, Rb, Sr, Ba, F, CO, and LREE are enriched
and typical of nephelinite or al,faline lamprophyre.
Genesis: The Bulls Run gneisses have many of the mineralogical
and geochemical characteristics typical of nepheline syenite/ijolite
carbonatite central complex magmatism. Nepheline syenite
complexes are generally intruded into anorogenic or rifted
conlinental settings, but minor occurrences have also been noted in
within-plate oceanic settings. By analogy with undeformed complex-
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F8. 1 Distribution of the Bulls Run Complex and location of its type area.
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