1

Baseline Report on

The Geology of the Limpopo Basin Area

a contribution to the Challenge Program on Water and Food Project 17

“Integrated Water Resource Management for Improved Rural

Livelihoods: Managing risk, mitigating drought and improving

water productivity in the water scarce Limpopo Basin”

Grecious Chinoda, William Moyce, Nyikadzino Matura Department of Geology, University of Zimbabwe, PO Box MP167, Mt. Pleasant, Harare, Zimbabwe

Richard Owen Mineral Resources Centre, University of Zimbabwe, PO Box MP167, Mt. Pleasant, Harare, Zimbabwe

WaterNet Working Paper 7 July 2009

SMZ Granite-greenstone

(granulite

grade)

CZ

Shelf-type

supracrustals

(granulite grade)

NMZ

Granite-

greenstone

(granulite grade)

Zimbabwe craton

Granite-

greenstone (low

grade)

Kaapvaal craton

Granite-

greenstone (low

grade) North Limpopo

Thrust Zone

(NLTZ)

Tuli-

Sabi/Triangle Sunnyside-

Palala Shear Hout River

Shear Zone

2

WaterNet is a regional network of university departments and research and training institutes specialising in water.

The Mission of WaterNet is to enhance regional capacity in Integrated Water Resources Management through training,

education, research and outreach by sharing the complementary expertise of its members. WaterNet member

institutions have expertise in various aspects of water resources management, including water supply, sanitation,

groundwater, wetlands, irrigation, water law, water economics, community based resource management, flood

forecasting, drought mitigation, water conservation and information technology. These institutions are based in Angola,

Botswana, Kenya, Lesotho, Mozambique, Namibia, Rwanda, South Africa, Tanzania, Uganda, Zambia and Zimbabwe.

The Challenge Program on Water and Food (CPW&F) is a research initiative of the Consultative Group on

International Agricultural Research (CGIAR). It is a partnership between national and international research institutes,

NGOs and river basin communities. Its goal is to identify and encourage practices and institutional strategies that

improve water productivity, and is committed to the overall goals of addressing improvements in levels of food

security, poverty, health, and environmental security.

WaterNet is leading Project 17 under the Challenge Program on Water and Food, entitled “Integrated Water Resource

Management for Improved Rural Livelihoods”. The project is financed by the CGIAR through the CPW&F and by the

partners in the project.

The partners in the project are:

Project leader: WaterNet International Research Institutes:

- International Crop Research Institute for the Semi-Arid Tropics (ICRISAT)

- International Water Management Institute (IWMI) Universities:

- UNESCO-IHE

- Universidade Eduardo Mondlane: Faculdade de Agronomia e Engenharia Florestal

- University of the Witwatersrand: School of Civil and Environmental Engineering

- University of Zimbabwe: Centre for Applied Social Sciences; Department of Civil Engineering;

Department of Soil Science and Agricultural Engineering; Mineral Resources Centre National Water and Agricultural Authorities:

- Administracao Regional de Aguas do Sul, Mozambique

- Instituto de Investigacao Agronomica deMozambique

- Mzingwane Catchment Council, Zimbabwe

- Water Research Commission, South Africa

Non-governmental Organisations:

- World Vision Zimbabwe

Copyright in the knowledge and material of this paper is held, unless otherwise specified, jointly between the

researcher(s) identified as authors of this paper, the institution(s) to which the researcher(s) are attached and the

WaterNet Challenge Program Project Partnership. Although this paper is in the public domain, permission must be

secured from the individual copyright holders to reproduce any materials contained in this report.

Suggested citation:

Chinoda, G., Matura, N., Moyce, W. and Owen, R. 2009. Baseline Report on the Geology of the Limpopo Basin Area,

a contribution to the Challenge Program on Water and Food Project 17 “Integrated Water Resource Management for

Improved Rural Livelihoods: Managing risk, mitigating drought and improving water productivity in the water scarce

Limpopo Basin”. WaterNet Working Paper 7. WaterNet, Harare.

Disclaimer

WaterNet, and its affiliated organisations, expressly disclaims all warranties, expressed or implied, as to the accuracy,

completeness, or usefulness of any the content provided, or as to the fitness of the information for any purpose.

WaterNet and its affiliated organisations shall therefore not be liable for any errors, inaccuracies or for any actions

taken in reliance thereon.

3

THE GEOLOGY OF THE LIMPOPO BASIN AREA

4

THE GEOLOGY OF THE LIMPOPO BASIN AREA

1 Introduction ............................................................................................................................................ 7

1.1 The Structure of the Report. .......................................................................................................... 8

2 Geology of the Limpopo Basin. ............................................................................................................. 9

2.1 The Limpopo Mobile Belt (LMB) ...............................................................................................10

3 The Limpopo Belt In Zimbabwe ...........................................................................................................11

3.1 The Zimbabwe Craton .................................................................................................................11

3.2 Description of Rocks in the NMZ ................................................................................................12

The Supracrustal Assemblage ...............................................................................................................12

Meta Ironstones .................................................................................................................................13

Mafic granulites .................................................................................................................................14

Granitoids ..........................................................................................................................................14

Charnoenderbites ...............................................................................................................................14

Porphyritic granites ...........................................................................................................................15

Younger Plutonic Rocks ........................................................................................................................15

Great Dyke related Dykes ..................................................................................................................16

Sebanga Dykes ..................................................................................................................................16

Karoo Dykes ......................................................................................................................................17

3.3 Structure in the NMZ ...................................................................................................................17

The older deformation structure ............................................................................................................18

Younger deformation structures ............................................................................................................18

Foliation and Lineation ......................................................................................................................18

Folds ..................................................................................................................................................18

Faults .................................................................................................................................................19

Shear Zones .......................................................................................................................................19

3.4 The Central Zone In Zimbabwe. ..................................................................................................19

3.5 Description of Rocks ....................................................................................................................20

Basement Gneisses ................................................................................................................................20

Supracrustal Rocks ................................................................................................................................20

Garnetiferous Paragneiss ...................................................................................................................20

Quartzo-feldspathic gneisses .............................................................................................................20

Calc-silicate gneisses .........................................................................................................................20

Quartzites ...........................................................................................................................................20

Mafic granulites and amphibolites .....................................................................................................21

Mafic-Ultramafic complexes .............................................................................................................21

Singelele Gneiss ................................................................................................................................21

Bulai Gneiss.......................................................................................................................................22

Phanerozoic Rocks ............................................................................................................................22

3.6 Structure (CZ) ..............................................................................................................................22

Earliest Deformation .............................................................................................................................22

First Deformation Event (D1) ................................................................................................................23

Second Deformed Event (D2) ................................................................................................................23

Third Deformation Event (D3) ...............................................................................................................23

Fourth Deformation Event (D4) .............................................................................................................23

Fifth Deformation Event (D5) ................................................................................................................24

Sixth Deformation Event (D6) ...............................................................................................................24

Triangle Shear Zone (TSZ) ....................................................................................................................24

4 The Limpopo Mobile Belt In South Africa ...........................................................................................24

4.1 The CZ In South Africa ...............................................................................................................24

4.2 Rock Description (CZ) .................................................................................................................24

Basement Gneisses ................................................................................................................................24

Quartzo-feldspathic Gneisses ................................................................................................................25

Singelele-type Granitoid Gneiss ............................................................................................................25

5

Garnet-Cordierite-Sillimanite Gneiss ....................................................................................................25

Pyroxenitic Amphibolite........................................................................................................................26

Quartzites and Banded Magnetite Quartzite ..........................................................................................26

Calc-Silicate Gneiss and Marble ...........................................................................................................26

Intrusive Rocks ......................................................................................................................................26

4.3 Structure .......................................................................................................................................27

Folds ......................................................................................................................................................27

Foliation and Lineation ..........................................................................................................................27

Faulting ..................................................................................................................................................27

4.4 The Southern Marginal Zone (SMZ) ...........................................................................................27

4.5 Description of Rocks (SMZ) ........................................................................................................28

The Baviaanskloof Gneiss .....................................................................................................................28

The Bandelierkop Formation .................................................................................................................28

The Onlust Gneiss Pluton ......................................................................................................................29

The Matok Pluton ..................................................................................................................................29

The Palmietfontein Pluton .....................................................................................................................30

The Schiel Complex ..............................................................................................................................30

4.6 Structure .......................................................................................................................................30

Folds and associated foliation and lineation ..........................................................................................30

Shear Zones ...........................................................................................................................................30

4.7 The Kaapvaal Craton ...................................................................................................................31

5 The Limpopo Basin In Mozambique .....................................................................................................32

5.1 Stratigraphy ..................................................................................................................................32

5.2 Structure .......................................................................................................................................34

6 Summary ...............................................................................................................................................34

References .....................................................................................................................................................37

Points of Contact and Additional Information ...........................................................................................44

6

List of Figures. Figure 1. Simplified map showing the total area covered by the Limpopo River basin in Zimbabwe, South Africa and Mozambique. A very generalised geology

of the areas has been included, but in less detail. 9 Figure 2. Simplified map of the Limpopo Mobile Belt showing its subdivisions,

major shear zones and the adjacent cratons. 11

Figure 3. Map showing in detail the rock types and major structures in the LMB

and adjacent Zimbabwe and Kaapvaal Cratons. 13 Figure 4. Extract map of the geology of the Limpopo basin in Zimbabwe. Shown are the different rock types in the Zimbabwe Craton and the location of the NMZ

and CZ of the LMB relative the craton. 14 Figure 5. Tectonic Map of Zimbabwe showing the different terranes, main

tectonic elements & major mafic dykes in the LMB & Zimbabwe Craton 17 Figure 6. Simplified geological map of the Limpopo basin in South Africa showing the LMB, the main granite-greenstone lithologies of the Kaapvaal craton

and the major sedimentary basins. 25

Figure 7. Geology of the Mozambican part of the Limpopo basin. 33

Figure 8. Illustrative sketch showing the general structure of the LMB. 35

Table 1: Simplified Stratigraphy of the SMZ. 28

7

1 Introduction The Limpopo basin is the study focus area for CGIAR (Consultative Group on International Agricultural Research) Challenge Program on Water & Food: Project Number CP17. The lead institution for CP17 is WaterNet and the full title of CP17 is:

The Challenge of Integrated Water Resource Management for Improved Rural Livelihoods:

Managing Risk, Mitigating Drought and Improving Water Productivity in the Water Scarce Limpopo Basin.

The principal project goal for CP17 is: “to contribute to improved rural livelihoods of poor smallholder farmers through the development of an IWRM framework for increased productive use of green and blue water flows and risk management for drought and dry-spell mitigation at all scales in the Limpopo basin.” The objective of the project is to realize the following key outputs: - Adoption and adaptation of water management strategies among smallholder

farmers that reduce risk and which, together with integrated farm systems management, improve farm income and water productivity.

- Development of appropriate catchment management strategies, based on IWRM principles that incorporate sustainable use of green and blue water resources, which enables poor rural people to reduce risk of food deficits due to water scarcity, and to manage water for improved livelihoods.

- Develop institutional models for water governance that aim at strengthening policies for water productivity and risk mitigation at catchment and basin scale.

- Human capacity building among farmers, extension officers, water managers and researchers at local universities in the Limpopo Basin and in southern Africa.

Within the context of this IWRM approach, the project has identified that there should be a significant focus on the groundwater resource and the agricultural potential of the soils within the Limpopo river basin. It is this context that CP17 requires a baseline report on the geology of the Limpopo basin which serves to underpin investigations carried out at different scales into soils, agriculture and the groundwater resource. The Mineral Resources Centre (MRC) at the University of Zimbabwe is the institution that has been allocated the task of compiling the geology baseline study, and this document is that report. In addition to the geology baseline, the MRC is also allocated the tasks of:

8

• assessing the alluvial and other aquifer resources; a baseline study of the hydrogeology throughout the basin, not including the Botswana portion of the basin

• the water chemistry in the basin including both surface water and groundwater resources

• field based action research on alluvial aquifers in terms of their water resources potential and their water quality in context of garden scale irrigation development

• farmer training to improve productivity of water and soils It can be seen from the foregoing that a baseline study of the Limpopo basin geology is a valuable component for developing a fair understanding of many of the bio-physical aspects relevant to promotion of irrigated agriculture in the basin. 1.1 The Structure of the Report. The Limpopo basin geology report outlines the geology in all the riparian nations except Botswana, which at the time of the report preparation, was not a part of the Limpopo Challenge program. The report commences with a discussion of the Limpopo Mobile Belt (LMB) with its 3 major zones, the Northern Marginal Zone (NMZ), the Central Zone (CZ) and the Southern Marginal Zone (CMZ). The LMB is a transboundary suite of supracrustal rocks that originate as a result of the collision between the Zimbabwe craton to the north and the KaapVaal craton to the south. This suite of high grade metamorphic rocks dominates the Limpopo basin. The report then discusses the basin geology in Zimbabwe. The lithologies found in Zimbabwe are presented. These include the rocks of the Zimbabwe craton and the Limpopo Mobile Belt, later intrusive rocks such as the Great Dyke and its associates, and the younger dyke swarms. The younger sedimentary Karoo strata are introduced. The structure and tectonics within the Zimbabwe part of the basin are presented. The geology in the South African part of the basin follows. The central zone of LMB rocks in South Africa are described first, followed by a description of the structures and tectonics found in this part of the basin. Thereafter the southern marginal zone of the Limpopo belt is treated in the same way. The report continues with a discussion of the KaapVaal craton. The Kaapvaal craton includes ancient gneiss complex lithologies, granite-greenstone terrains and great thicknesses of volcano-sedimentary sequences which have been subsequently metamorphosed. The report then presents the geology in the Mozambican section of the basin. This section of the report suffers from the lack of available geological data and literature on Mozambique. The lithologies in this part of the basin consist largely

9

of younger Cretaceous sediments and Tertiary to Quaternary reworked fluvial and coastal sediments, with recent dune cover along the coastline. The report concludes with a short summary, which focuses on the regional tectonics associated with the Limpopo Mobile Belt. The information is available in GIS, which can be downloaded from http://www.waternetonline.ihe.nl/workingpapers/Geol Baseline maps - arc view.zip

2 Geology of the Limpopo Basin. The Limpopo basin covers a very large area, extending into Zimbabwe, South Africa, Botswana and Mozambique. To understand the controls of surface and groundwater occurrence in this area, the underlying geology should be well understood. This report gives an account of the geology underlying the Limpopo Basin in all the other countries, excluding Botswana. It describes the geology of the basin in Zimbabwe, South Africa and Mozambique separately, and eventually amalgamates the information so gathered into one comprehensive summary of the Limpopo basin. Figure 1 shows the simplified geology of the Limpopo basin. Note that the area covers Zimbabwe, South Africa and Mozambique.

26 27 28 29 30 31 32 33 34

26 27 28 29 30 31 32 33 34

-26

-25

-24

-23

-22

-21

-20

-26

-25

-24

-23

-22

-21

-20

CRETACEOUS

JURASSIC

QUATERNARY &RECENT

Recent alluvium

Interior & consolidated dunes Argillites, areanous fluvial sandstones & mudstones

Argillites

Sena Group

Elefantes/Singuedeze Group

Cheringoma Group

Mazamba GroupMIOCENE

PALEOCENE

Sandstone, shale, etc

Lower Karoo Group

Bushveld Igneous Complex

Beitbridge Complex

Pretoria Group

Malimani Group

Dominian Group

TRIASSIC

PERMIAN

CARBONIFEROUS

Waterberg Group

Pilansberg Intrusive Complex

Archaean Greenstones

Older Gneiss Complex

Naunetsi Igneous Complex

Gneisses - charnokites

Great Dyke

Younger granites

Upper Karoo Group

Karoo basalts

Karoo rhyolites

PROTEROZOIC

ARCHAEAN

TABLE OF FORMATIONS

0 200km

N

Figure 1. Simplified map showing the total area covered by the Limpopo River basin in Zimbabwe, South Africa and Mozambique. A very generalised geology of the areas has been included, but in less detail.

10

2.1 The Limpopo Mobile Belt (LMB) A considerable proportion of the Limpopo basin area is floored by the LMB. The LMB in Southern Africa has been a major focus of research on Archean Tectonics and crustal evolution, and has been regarded as an area of typical Archean high grade deformation, often referred to as a mobile belt (Blenkinsop and Rollinson, 1992; Van Reenen et al, 1992). The belt is a zone of granulite facies rocks situated between the greenschist - amphibolite rocks of the Zimbabwe craton to the north and the Kaapvaal craton to the south. It is a near E-W elongated low lying belt straddling eastern Botswana, southern Zimbabwe and the northern part of the Northern Province in South Africa. The belt is subdivided into three zones on the basis of lithological and structural characters; the Northern Marginal Zone (NMZ), the Central Zone (CZ) and the Southern Marginal Zone (SMZ) as shown on the map in figure 2. The exposed area of the LMB is divided almost equally between Zimbabwe, South Africa and Botswana, with a slightly larger proportion falling within the borders of Zimbabwe; for example the whole of the NMZ and a substantial amount of the CZ are exposed in Zimbabwe. A thorough understanding of the Limpopo belt can thus only be understood by synthesising data from all these areas since observations based on a fraction of the Limpopo belt cannot provide a basis for comprehensive interpretations. The boundaries of the 3 zones of the Limpopo belt and the peripheral cratons are defined by major tectonic breaks in form of shear zones (Stuart and Zengeni, 1987), for example the Zimbabwe craton and the NMZ are separated by the North Limpopo Thrust Zone (Mkweli et al., 1995). Figure 3 is a detailed geological map of the LMB that also shows some parts of the adjacent Zimbabwe and Kaapvaal cratons to the north and south respectively. Since the LMB is located between the Zimbabwe and Kaapvaal cratons, a systematic description starting from the Zimbabwe craton in the north through the LMB to the Kaapvaal craton in the south (South Africa) will be made. This will then be combined with the description of the Mozambican part of the basin outlined below.

11

o

KAAPVAL CRATON

CENTRAL ZONE

Palala

Sunnyside

Tuli-Sabi

Hout

River

ApproximatePosition of Ortho-pyroxine Isograd

ReverseShear Zone

Strike-slipShear Zone

Mainly Quartzo-

feldspathicgneisses

Younger Cover

Greenstone belts

Great Dyke

Kilometres

0 100

KEY

Tuli Basin Bubye River

Beitbridge

ZIMBABWE CRATON

Gwanda

Tod's QuarryNORTH MARGINAL ZONE

Rutenga

Mwenezi

Chiredzi

Triangle Shear Zone

Manjirenji

DamInyala

ZimbabweCraton

LimpopoBelt

Kaapvaal

Craton

INDIANOCEAN

LOCATION MAPMasvingo

Mweza

Buhwa

Belingwe

SOUTH MARGINAL ZONE

28 E 30 Eo

32 Eo

21 So

23 So

Figure 2. Simplified map of the Limpopo Mobile Belt showing its subdivisions, major shear zones and the adjacent cratons.

3 The Limpopo Belt In Zimbabwe The NMZ is developed wholly in Zimbabwe together with a considerable part of the CZ. To the immediate north of the NMZ is the Zimbabwe Craton, and the North Limpopo Thrust Zone (NLTZ) separates the two. For continuity of the report, the Zimbabwe craton is also described, though in less detail, with more emphasis being given to the individual segments of the LMB.

3.1 The Zimbabwe Craton The Zimbabwe craton is a typical granite-greenstone terrain in many respects. The oldest gneisses recognised in Zimbabwe, the Tokwe segment gneisses,

dated at ∼3.5 Ga (Wilson, 1990), are found near the Limpopo belt in the south of the craton, as seen in figures 3 and 4. These are associated with Sebakwian greenstones of almost the same age, and surrounded by a later suite of gneisses

(figure 3) with dates of ∼ 2.9 Ga, e.g. the Chingezi gneisses (Taylor et al, 1991). The important greenstone belts nearest the Limpopo Belt are the Mweza, Buchwa, and Belingwe belts (figures 2 and 3). The stratigraphy of the Belingwe

belt consists of two sequences of ∼2.9 Ga and ∼ 2.7 Ga in age (Wilson, 1979; 90; Bickle and Nisbert, 1994). The Belingwe lithostratigraphy can be correlated with some other greenstone belts in the Zimbabwe Craton (Wilson, 1979; 1990), but

12

not towards the Limpopo belt; for example the stratigraphy of the Buchwa belt is considerably different lithostratigraphically and is slightly older (Fedo and Eriksson, 1994). All the greenstones and older gneisses in the Zimbabwe craton are intruded by a younger suite of granites (figure 3) that have been named the

Chilimanzi granites dated at ∼2.6 Ga (Hickman, 1978). Granitoids, greenstones and gneisses in the Zimbabwe craton adjacent to the Limpopo have a variably developed foliation generally dipping steeply to the southeast, and a steeply plunging, down-dip lineation. The foliation intensity increases towards the NMZ. Steeply dipping shear zones in several orientations with major components of strike-slip movement affect the Zimbabwe craton on scales ranging from metres to kilometres (Van Reenen et al., 1992). The greenstone belts tend to be elongate parallel to the LMB, as shown in figures 2 and 3.

3.2 Description of Rocks in the NMZ Rock types in the NMZ have been divided by Rollinson and Blenkinsop (1995) into a plutonic assemblage of tonalitic-trondhjemitic composition, a supracrustal assemblage of metabasites, banded iron formations and rare metapelites, and a suite of late porphyritic granites (eg. the Razi suite of Robertson, 1973a). The Supracrustal Assemblage The supracrustal rocks occupy 10% of the exposed area of the NMZ (Rollinson and Blenkinsop, 1995). The supracrustal assemblage has been referred to as the pre-intrusive basement (Kamber and Biino, 1995) and the NMZ part of the basement rocks (Mkweli, 1997). It is marked by a close spatial association between meta-ironstones and metabasites (mafic granulites and amphibolites). The south and southwestern part of the map in figure 4 shows the area covered by the gneisses of various ages in the NMZ.

13

26.5 27 27.5 28 28.5 29 29.5 30 30.5 31 31.5 32

26.5 27 27.5 28 28.5 29 29.5 30 30.5 31 31.5 32

-23.5

-23

-22.5

-22

-21.5

-21

-20.5

-20

-23.5

-23

-22.5

-22

-21.5

-21

-20.5

-20

granodioritic to adamellitic granites& gneisses (low & medium grade)charnokitic gneisses (high grade)

adamellitic porphyritic granites & porphyroblastic gneisses (low & medium grade)porphyroblastic charnoenderbitic gneisses (high grade)

serpentinites, metapyroxenitesmetagabbros & metanorites

metavolcanics with intercalated metasediments

highgrade

mediumgrade

lowgrade

alluvium & othersuperficial deposits

aeolian sands

sediments overlainby volcanics

norite, gabbro, pyroxenites& serpentinites

tonalitic to adamelliticgneisses & granites

RECENT &PLEISTOCENE

KALAHARI

KAROO

GREAT DYKE

YOUNGER GRANITES

OLDER GNIESSES

BULAWAYAN GROUP

MAFIC-ULTRAMAFICCOMPLEXES

meta-anorthosite, metagabbros,hornblendite & quartz-hornblendegneisses

highgrade

mediumgrade

lowgrade

alluvium & othersuperficial deposits

aeolian sands

sandstones

granophyre, granitesyenite & gabbro

sediments overlainby volcanics

porphyroblastic adamellitic& charnoenderbitic gneisses

metasediments, paragneisses& intercalated orthogneisses

tonalitic to granodioriticgneisses

CRETACEOUS

NUANETSI IGNEOUSCOMPLEX

RECENT &PLEISTOCENE

KALAHARI

KAROO

PH

AN

ER

OZ

OIC

PH

AN

ER

OZ

OIC

AR

CH

AE

AN

AR

CH

AE

AN

BULAI GNIESS

MAFIC-ULTRAMAFIC COMPLEXES (Zim.)MESSINA SUITE (R.S.A.)

BASEMENT GNEISSES (Zim.)SAND RIVER GNEISSES (R.S.A.)

BEITBRIDGE GROUP (Zim.)GUMBU, MALALA DRIFT & MOUNT DOWE GROUPS (R.S.A.)

Matok Pluton

Kud

us R

iver

Lin

eam

ent

Shurugwe Fault

Limpopo River

Bosbokpoort Fault

Dowe-Tokwe FaultBulai

TULI TROUGHNUANETSI IG

NEOUS COMPLEX

Buchwa G. B.

Mweza G. B.

Masvingo G. B.

Belingwe G. B.

Gwanda G. B.

Mtshingwe Fault

Popote

ke F

ault

Gono Fault

Dui

wel

sklo

of L

inea

men

t

Xmas Fault

Blouberg Fault

WATERBERG BASIN

SOUTPANSBERG TROUGH

Messina

Beitbridge

LIMPOPO BELTZIMBABWE CRATON

highgrade

mediumgrade

lowgrade

alluvium & othersuperficial deposits

aeolian sands

sediments overlainby volcanics

clastic sediments withbasal volcanics

granophyre, graniteanorthosite, norite, gabbropyroxenites & serpentinite

syenite

granodioritic to adamelliticgranites & gneisses

- adamellitic porphyritic granites & porphyroblastic gneisses (low & medium grade)- porphyroblastic charnoenderbitic gneisses & granites (high grade)

metavolcanics with intercalatedmetasediments

tonalitic to adamelliticgneisses & granites

RECENT &PLEISTOCENE

KALAHARI

KAROO

WATERBERG GROUP

BUSHVELDCOMPLEX

YOUNGER GRANITES

OLDER GNEISSES

GRAVELOTTE, PIETERSBURG & SUTHERLAND G. B. & BANDELIERKOP COMPLEX

PH

AN

ER

OZ

OIC

AR

CH

AE

AN

PR

OT

ER

OZ

OIC

KAAPVAAL CRATON

Tokwe Gneisses

Chin

gezi

Gneis

ses

Razi Granite

CENTRALZONE

NORTHERNMARGINAL ZONE

Tshipise

Figure 3. Map showing in detail the rock types and major structures in the LMB and adjacent Zimbabwe and Kaapvaal Cratons. Meta Ironstones These occur as small bodies, generally around 100 m long and 30-70 m wide. They tend to occur in close association with a mafic granulite, with magnetite dominating as the FeO phase and quartz as the SiO2 phase. Quartz is euhedral and shows evidence of recrystallisation while magnetite is finer grained (up to 1 mm) (Chiwara, 2003).

14

28° 28.5° 29° 29.5° 30° 30.5° 31° 31.5°

28° 28.5° 29° 29.5° 30° 30.5° 31° 31.5°

-22°

-21.5°

-21°

-20.5°

-20°

-22°

-21.5°

-21°

-20.5°

-20°

Mweza G. B

.

Belingwe G

. B.

Gwanda G. B.

Filabusi G. B.

Gonare

zhou

Gam

e R

eserv

e

Older Gneiss Complex

Serpentinite and pyroxenites

Doleritic and gabbroic dykes

Granophyre, granite and syenite

Gneisses and charnockites of various ages (high grade)

INTRUSIVE IGNEOUS ROCKS

Ultramafic lavas and intrusions

Basaltic volcanics with intercalated sediments

Andestic and dacitic metavolcanics

Metasediments, felsic metavolcanics

Para- and anorthositic gneisses of various ages (high grade)

Glacial beds, mudstones, coal measures and sandstones

Grits, sandstones and siltstones

Basalt

Rhyolite

Sandstone, etc.

Alluvium and other superficial deposits

CRETACEOUS

KAROO

BEITBRIDGE

SHAMVAIAN

BULAWAYAN

SEBAKWIAN

GREAT DYKE

JURASSIC

TRIASSIC

PERMIAN

EA

RL

YP

RE

CA

MB

RIA

N

VARIOUS AGES

LATE JURASSIC

PLEISTONE &RECENT

Younger granites, granodiorite-adamellite

TABLE OF FORMATIONS

SYSTEM ORGROUP

INTERNATIONAL

Gre

ensto

ne

Belts (

G. B

.)

Beitbridge

Figure 4. Extract map of the geology of the Limpopo basin in Zimbabwe. Shown are the different rock types in the Zimbabwe Craton and the location of the NMZ and CZ of the LMB relative the craton. Mafic granulites These occupy more than 20% of the NMZ (Mkweli, 1997; Ridley, 1992) and represent the dominant component of the supracrustal assemblage (Rollinson and Blenkinsop, 1995). The mafic granulites have a mineral assemblage of pyroxene and plagioclase with or without hornblende, biotite and quartz. Mkweli (1997) sees five characteristic mineralogical associations in the granulites; clinopyroxene – plagioclase – hornblende; clinopyroxene – plagioclase – hornblende – biotite; clinopyroxene – orthopyroxene – plagioclase – quartz; clinopyroxene – orthopyroxene – plagioclase – hornblende; and clinopyroxene – orthopyroxene – plagioclase – hornblende – biotite; but the assemblages grade into one another with no systematic spatial variation. A number of the mafic granulites maybe basaltic in origin (Odell, 1975; Rollinson and Lowry, 1992) whilst some may have been mafic dykes before they were deformed and metamorphosed (Rollinson and Blenkinsop, 1995) Granitoids These are intrusions which span up to 200 Ma from 2.72 to 2.52 Ga. The granitoids constitute the dominant suite of rocks in the NMZ (see figure 3) and they contain variable amounts of plagioclase, K-feldspar, quartz, orthopyroxene, clinopyroxene, hornblende, biotite and garnet. Charnoenderbite refers to orthopyroxene-bearing granodiorites, and therefore most of the NMZ granitoids are of granitic to granodioritic compositions. Charnoenderbites Charnoenderbites occur as elliptical to circular bodies with the nature of their outcrop patterns suggesting that they are intrusive into the supracrustal rocks (figure 3). Most of the hills to the east of Bubi river, around Mt Barberton are defined by the cores of individual charnoenderbite bodies. The charnoenderbites vary from honey brown colours, through pale grey to dark grey with a greasy

15

appearance. A strong gneissic banding is characteristic, and it becomes more pronounced on the weathered surfaces where the mafic layers have been strongly weathered out. The bands are defined by alternating horizons of hypersthene plus biotite rich gneiss and hypersthene plus biotite-poor gneiss. The less mafic bands tend to be coarser (1 mm) than the more mafic ones (0.1-0.4mm). Mineralogically, the charnoenderbites contain plagioclase, K-feldspar and quartz as the main mineral constituents, whilst biotite, orthopyroxene, clinopyroxene, amphibole, ilmenite, apatite and zircon may be present. Amphibole and orthopyroxene become very minor phases close to the Zimbabwe craton – NMZ boundary as the retrograde assemblage of quartz – plagioclase – K-feldspar – biotite becomes predominant. In terms of textures, biotite either replaces orthopyroxene by growing along its clearage and fracture planes, or it exists with magnetite as a coarse overgrowth or thin rim around orthopyroxene, or at the orthopyroxene – K-feldspar grain contacts or where K-feldspar and orthopyroxene are in close proximity, then biotite tends to have a characteristic symplectic intergrowth with quartz. The intergrowths occur parallel to the biotite cleavage planes and concordant with the regional gneissic banding. Porphyritic granites These were first recognised by Worst (1962) and fall under the Razi province of Robertson (1973b). Figure 3 shows that these Razi granites are not only restricted to the craton-NMZ boundary, but they also encroach well into the craton and the NMZ (Worst, 1962), Mkweli, 1997). They are well exposed in the Mwenezi River, and they form sharp contacts with the tonalitic gneisses to the north and diffuse lit-per-lit contacts with retrogressed charnoenderbites to the south. Porphyritic granites in the NMZ are coarse to very coarse grained and containing megacrysts of K-feldspar, which vary from 10-50 mm in length. The K-feldspar megacrysts are ovoid to rectangular in shape and are aligned parallel to the regional fabric. Pale white rims measuring up to 2 mm are not uncommon around the megacrysts. The groundmass is predominantly composed of biotite and quartz, and wraps around the megacrysts so giving the rock a characteristic wire-mesh appearance. The megacrysts take 90% of the bulk mineralogy with the remainder being made up of the groundmass. Younger Plutonic Rocks Mafic dykes intrude both the basement and the granitoids in the NMZ and the Zimbabwe craton. The dykes truncate the gneissic fabric in the granitoids and are therefore younger than these observed fabrics (Blenkinsop and Mkweli, 1995). Mkweli (1997) subdivides the dykes into three groups, the Great Dyke Related Dykes, Sebanga Dykes and the Karoo Dykes. Figure 5 is a Tectonic map of Zimbabwe showing the different terranes, main tectonic elements and mafic dykes occurring in both the craton and the LMB.

16

Great Dyke related Dykes These include the Main Swarm (Robertson, 1973a and b) and the East Dyke Extension (Robertson, 1973a and b; Worst, 1962). The Main Swarm is believed to be the deeper crustal level expressions of the Great Dyke (dominantly serpentinites, pyroxenites and mafic rocks) in the craton (Robertson, 1973a) and they trend parallel to the Great Dyke with widths of up to 15 m. The East Dyke (figures 4 and 5) is a southward continuation of the same dyke in the Zimbabwe craton. Both dykes can be traced across the craton-NMZ contact without any displacements, as shown on the central part of the map in figure 4, suggesting that they post-date all tectono-metamorphic events related to the LMB (Blenkinsop and Mkweli, 1995). The main features of the dykes are:

1) if they are deep level expressions of the Great Dyke in the craton as suggested by Robertson (1973 a & b), then they have the same age as the Great Dyke.

2) they are intrusive into all other lithological units described here. 3) some of the dykes show evidence of a chilled margin (Mkweli, 1997) 4) petrologically, they still preserve the original igneous textures

(Blenkinsop and Mkweli, 1995) 5) apart from narrow shear zones containing quartz plagioclase veins,

there is no evidence for any metamorphic textures nor any associated fabrics.

Sebanga Dykes This set of doleritic to gabbroic dykes trend NNW (pale orange dykes in figure 5), parallel to the Sebanga Poort Dyke in the Zimbabwe craton (Wilson, 1990; Wilson et al, 1987). Paleomagnetic properties by Jones et al., (1975) suggest that these dykes were emplaced coeval with the Mashonaland dolerites which are prevalent in the craton. Since the Mashonaland dolerites were dated at 1838 + 230 Ma (Kramers in Wilson et al, 1987), the Sebanga Dykes could be early Proterozoic in age. The dykes preserve original magnetic textures with no metamorphic or tectonic overprints.

17

27.5 28 28.5 29 29.5 30 30.5 31 31.5 32 32.5

27.5 28 28.5 29 29.5 30 30.5 31 31.5 32 32.5

-22

-21.5

-21

-20.5

-20

-19.5

-22

-21.5

-21

-20.5

-20

-19.5

N

0 100km

Post Karoo

Upper Karoo

Lower Karoo

Irumide

Ubendian

Great Dyke event

LimpopoGreenstone Belts

3800

3000

2500

2050

1600

1000290250

160

65

Basin development/Tectogenesis

AGES(Ma)

AR

CH

EA

NP

RO

TE

RO

ZO

ICP

HA

NE

RO

ZO

IC Moderate tectonic reworking

Dykes & narrow sills, mainlymafic (colour denotes age)

Portion of Tokwe Segmentstabilised at 2800 Ma

Volcanic: mafic

Granitoids, anorogenic

EXPLANATIONS

Diffuse zone of shearing/deformation zone

Shear zone, movement directionunknown

Fault, thrust/slide, reverse/obliquereverse movement, teeth on hangingwall

Shear/deformational zone, reverse/oblique reverse movement, teeth on hangingwall

TRIANGLE

SHEAR

ZONE

NORTHLIMPOPO

THRUSTZONE

Mtshingwe Fault

Jenya Fault

Popote

ke F

ault

East D

yke

Um

vum

ela

Dyke

Figure 5. Tectonic Map of Zimbabwe showing the different terranes, main tectonic elements and major mafic dykes in the LMB and Zimbabwe Craton. Karoo Dykes These consistently strike E-W with up to 10o variation in both directions, as shown in the southern ends of figure 5. They vary in thickness from a few centimetres up to 2 m, and most of them are dolerites while some may be basaltic (Mkweli; 1997). Basaltic dykes are thinner and porphyritic with plagioclase phenocrysts of up to 15 mm in length (Cox et al., 1965). Robertson (1973b) interpreted on the basis of similar geochemical signatures that the Karoo dykes could have acted as feeders to the Karoo basalts preserved to the south in the CZ of the LMB. 3.3 Structure in the NMZ The NMZ shows some evidence of intense deformation, which Mkweli (1997) divided into older and younger deformation histories on the basis of fabric type, cross-cutting and overprinting relationships. The characteristic structural elements include regional foliation, folds, shear zones and faults.

18

The older deformation structure Most of the structures belonging to the older deformation in the NMZ have not been preserved due to the obliteration by the younger deformational event. The little preserved structures have to some extent been reworked and now align nearly parallel to the dominant regional fabric of the younger deformational event. Some small-scale folds rarely occur in banded iron formations with some related large-scale folds being preserved further away from the craton into the interior of the NMZ. This resulted in the folding of some chromite bearing ultramafic units at Inyala Mine. Layer parallel foliation has been folded around and now is parallel to the gneissic banding in the surrounding charnoenderbites. Meso-scale folds consistently plunge steeply to the NE parallel to the associated mineral stretching lineations in the supracrustals. The mineral lineation trends can be correlated to those in the Mweza-Msazi greenstone belts (figure 3). Little exposure of the large folds limits the amount of structural data to be gathered, although sheath folds can be inferred from the outcrop patterns of tight folds which close upon themselves and also the co-linearity between fold axes and mineral stretching lineations. Younger deformation structures The NMZ is dominated by major ductile structures, which define the ENE trends. These structures are described separately below. Foliation and Lineation The main tectonic element in the NMZ is the consistent ENE trending foliation with moderate south-easterly dips. The foliation varies from schistosity to a fine gneissic banding and is weak to moderately developed. It locally intensifies to define narrow mylonite zones striking parallel to the regional foliation with much shallower dips. A consistent mineral stretching lineation occurs in the mylonite zones. The lineation is defined by stretched out grains of feldspar, quartz and hornblende, suggesting syn-kinematic recrystallisation of these minerals. The lineation plunges moderately to sub-vertical towards the SE, a trend that is observable across the entire width of the NMZ, including the North Limpopo Thrust Zone (NLTZ) discussed in the next section. The lineation is, however, not uniformly developed as the more steeply dipping zones tend to have no lineation at all

(Mkweli, 1997; Chiwara, 2003). σ and δ porphyroclasts defined by quartz and K-feldspar consistently give a reverse shear sense, suggesting that the southern parts moved up relative to the north. Folds These are rare, only occurring on a centimetre scale in amphibole-bearing granulites and enderbites. Alternating leuco- and melanocratic banding in these rocks make the folds visible on outcrop scale. Open, tight and isoclinal fold types occur in the granulites while parasitic folds are typical in enderbites (Chiwara, 2003).

19

Faults These occur on all scales in the NMZ. The most important are the NNE sinistral Popoteke faults trending sub-parallel to the Great Dyke and their conjugates, the dextral WNW trending Mtshingwe set parallel the Jenya Mushandike Dislocation Zone and the main Mtshingwe fault (figure 5) affecting the Great Dyke and some greenstone belts in the Zimbabwe Craton. Small-scale reverse faults are found in the NMZ, striking ENE, parallel to the North Limpopo Thrust Zone (see figure 5). The thrust faults show a general south-over-north movement characterized by some down dip lineations (Mkweli et al., 1995). Shear Zones Shear zones occur from small-scale to large-scale. The most notable is the contact between the Zimbabwe craton and the NMZ, which has been termed the North Limpopo Thrust Zone (NLTZ) by Blenkinsop et al., (1995) and is shown in figure 5. The nature of the transition from the Zimbabwe craton to the NMZ has been controversial, some proposing thrusting of the NMZ over the craton, and others proposing a gradual transition (Worst, 1962; Robertson, 1973a, b, Odell, 1975; James, 1975; Coward et al., 1976; Blenkinsop and Mkweli; 1992). The contact is marked by the orthopyroxene isograd between the low grade rocks of the craton and the granulites in the NMZ (Robertson & Du Toit, 1992) and Worst (1962) associated parts of the isograd with a thrust. The isograd has now been put within a few metres of reverse sense shear zones (Rollinson & Blenkinsop, 1995). The Zimbabwe craton is separated from the NMZ by mylonites & ultramylonites in a network of shear zones that dip moderately-gently southeast with strong down-dip lineations and abundant evidence of reverse shear (Blenkinsop, 1997). Granulites of the NMZ have been uplifted and juxtaposed against the Zimbabwe craton along the NLTZ (James, 1975; Ridley, 1992). The Razi suite of Robertson (1973a) are mylonitised by shear zones at the Zimbabwe craton-NMZ transition. A system of NNE- and NW- trending sub-vertical shear zones is found throughout the NLTZ and within the NMZ. They occur on a scale of metres in width and hundreds of metres in length (Rollinson & Blenkinsop, 1995). The NNE zones are sinistral while the NW are dextral, and have been interpreted as conjugate shears reflecting NNW shortening by Rollinson and Blenkinsop (1995) resulting from the NMZ-Zimbabwe craton convergence.

3.4 The Central Zone In Zimbabwe. Over 5000 km2 of the CZ has been mapped in Zimbabwe (Watkeys et al; 1983) where it comprises a rhombohedral section of Archean terrain enclosed by down-faulted Karoo rocks to the north and the Limpopo river to the south, as shown in figures 2 and 3. The rocks present include some basement gneisses, supracrustal lithologies, some mafic-ultramafic complexes and a suite of later gneisses,

20

3.5 Description of Rocks

Basement Gneisses These comprise migmatitic and banded quartzo-feldspathic gneisses occurring as rare slivers within the supracrustal rocks, or as infolded keels in the meta-arnorthositite-gabbro suite (figures 3 and 4). The migmatitic gneiss comprises alternating layers (up to 10 m wide) of mesocratic quartz-diorite and a leucocratic granodiorite. Compositional layering and gneissic fabric are defined by alignment of biotite, rare hypersthene, and intensely flattened mafic minerals. The fabric is transected by mafic granulites with hypersthene and plagioclase in equal proportions with lesser clinopyroxene, hornblende and quartz. The granulites may represent deformed mafic dykes that were also intruded by some aplite and pegmatite dykes and veins. Supracrustal Rocks Garnetiferous Paragneiss This unit, often migmatitic, is the dominant rock type in the CZ and may be pelitic or psammitic, with the pelitic varieties being composed of a melanosome of garnet and biotite, with quartz and feldspar as minor constituents. The psammitic variety is composed of garnet, biotite, quartz and feldspar in approximately equal proportions. Quartzo-feldspathic gneisses These leucocratic gneisses are exposed sporadically in the area. They are medium to coarse-grained with a strong gneissic fabric enhanced by minor hypersthene, hornblende and biotite. The mafic minerals tend to be present only where these gneisses grade into the garnetiferous paragneisses. Calc-silicate gneisses Thin layers (5-30 m thick) of calc-silicate gneisses occur throughout the supracrustal section, with the thicker varieties being developed towards the east. They are characterised by the presence of blue-grey or white coloured, massive, coarse-grained dolomitic marbles. Some are composed entirely of calcite, while others may contain variable amounts of phlogopite, scapolite, diopside, garnet or quartz (Watkeys et al., 1983). Some deformed mafic dykes intrude into the gneisses. Quartzites These occur as thin layers within the paragneisses. They are about 10 m thick and may either terminate sharply along strike (due to boudinaging, isoclinal folding or abrupt original facies change) or pass laterally into the paragneisses. The quartzites are subdivided into massive, fuchsitic, sillimanite-bearing and granular varieties. They are occasionally tinted green or reddish-brown by the presence of fuchsite or iron oxides respectively. Some are so ferruginous with large amounts of magnetite, and have been termed magnetite-quartzites (Sohnge et al., 1948). Apart from magnetite hematite, limonite, goethite and quartz, magnetite quartzites also contain garnet, grunerite and gedrite.

21

Preferential weathering of some iron oxides and other more susceptible minerals results in a pitted surface on the quartzite outcrops. Mafic granulites and amphibolites These rocks occur in close association with the quartzites. They consist of fine to medium-grained assemblage of orthopyroxene and plagioclase, with lesser clinopyroxene, biotite, hornblende and quartz. Garnet may often be present while pyroxenes may be completely replaced by hornblende, leading to an amphibolite. They may be banded or massive, with the banded variety varying from 5-15 m in thickness. Biotite in the banded varieties is often concentrated into layers, inducing partings at 20-30 cm apart, interspaced by lesser partings at about 10 mm. Mafic-Ultramafic complexes These may be separated into the pyroxenite-serpentinite suite and the arnorthosite-gabbro suite, both of which have subsequently been metamorphosed (figure 3). The pyroxenite-serpentinite suite is composed of serpentinite as an alteration product of dunite or harzburgite, together with pyroxenite, hornblendite, gabbro and olivine-spinel granulite. Serpentinite is light to dark green, with clinochore, penninite, magnetite and antigorite as the dominant minerals. Thin cross-cutting veins of asbestos, magnesite and chalcedony are common. The pyroxenites contain poikiloblastic bronzite crystals (up to 5 cm) encased by a finer-grained, green matrix of diopsidic augite (Van Eeden et al., 1975). The arnorthosite-gabbro suite consists of mainly gabbroic rocks and to the south of Limpopo River, it has been termed the Messina Suite (SACS, 1980) or the Messina Layered Intrusion (Barton et al., 1979b), as shown in figure 3. In Zimbabwe, the thickest development occurs about 30 km east of Beitbridge where its belt can be traced from the Limpopo River for over 100 km strike length before disappearing underneath the Karoo cover. The dominant rock in the suite is a hornblendite (consisting of hornblende with minor plagioclase and magnetite) which grades into a gabbro (Watkeys et al., 1983). The gabbro comprises plagioclase megacrysts (>3 cm) in a finer hornblende matrix. Some fine-medium grained anorthosite, composed of calcic plagioclase with infrequent hornblende and, occasionally, garnets are found in minor ductile shear zones. Veins of pegmatitic arnothosite intrude into this suite. Singelele Gneiss Occurs within 10 km radius around Beitbridge. Singelele gneiss is inter-layered with the supracrustal rocks and mostly occupies fold noses, forming some well-exposed boulder-strewn kopjes. Two varieties may be distinguished, the more common heterogeneous banded variety and the homogeneous one. The former consists of medium grained assemblage of quartz, plagioclase, K-feldspar and lesser garnet, biotite, hornblende and relic hypersthene. It is heterogeneous due to numerous tight to isoclinally folded veins that impart a banded appearance to the gneiss. The homogeneous gneiss consists of quartz, plagioclase and K-

22

feldspar with extremely rare biotite and hornblende. The Singelele Gneiss cuts across lithological banding in the supracrustal rocks. Bulai Gneiss This is a granitic body to the west and northwest of Messina (figure 3). Where well exposed, the gneiss is porphyroblastic with megacrysts of microcline in a medium grained groundmass of plagioclase, quartz and biotite with little hypersthene, hornblende and garnet. The less well-exposed ones are tonalitic to granodioritic and composed of medium-grained oligoclase, biotite and quartz with minor alkali feldspar. The central part of the gneiss is enderbitic with hypersthene and plagioclase and minor clinopyroxene and K-feldspar. The periphery of the Bulai Gneiss may be charnockitic with perthitic feldspars, lenticular ribbons of opalescent blue quartz in a finer groundmass of hypersthene, rare clinopyroxene, hornblende, biotite, garnet, plagioclase and quartz. Enclaves of basement gneisses and the supracrustal rocks are found in the Bulai gneiss, suggesting that the latter intruded into these units. It is itself intruded by mafic, granitic and some unusual lamprophyric dykes (Watkeys, 1983). Phanerozoic Rocks Karoo sediments and volcanics crop out in two down-faulted troughs, the Tuli and Nuanetsi troughs (figures 2 and 3); and in the half-graben south of Shurugwe Fault (figure 3). The successions in the two troughs are relatively similar though thickness is more in the latter. Fluvioglacial tillites, fossiliferous mudstones, coal seams, grits, red sandstones and siltstones and cross-bedded aeolian sandstones occur in the troughs, and these are capped by the late extensive basalts that extruded onto them. The Nuanetsi igneous complex (figure 3) composed of gabbros, ring dykes of granophyre, granite, quartz synenites and nepheline syenite post-dates the Karoo basalts (Rees, 1960; Gifford, 1961). Undeformed mafic dykes related to Karoo and post-Karoo volcanic and plutonic phases intruded throughout the region (figure 5) utilising the extensive bifurcating fracture system that traverses the CZ of the LMB in Zimbabwe (Jacobson et al, 1975; Barton, 1973). 3.6 Structure (CZ) A complex deformational history for the CZ led to the development of structures now characteristic of the terrain. The structures have been described by Watkeys et al., (1983) by tracing the deformational events that occurred. Earliest Deformation The earliest structure recognised is the primary layering in the basement gneisses, parallel to which is a planar fabric defined by mafic mineral alignment and felsic mineral elongation. Some isoclinal folds depicted by plagioclase-quartz anatect veins are subparallel to, and transected by the planar fabric above, which may be axial planar to these folds. Some later tight to isoclinal folds deform the two fabrics above, and are associated with a third fabric that may be axial planar

23

to them. Pinch-and-swell structures, developing into boudins, are evident along the fold limbs. Small ductile shear zones offset the structures just described, and these shear zones are in-filled by felsic veins and pegmatites and some later strike-slip faults. All these structures are unique to the basement gneisses. First Deformation Event (D1) In the supracrustals, the primary layering (So) is well preserved, and is folded by some isoclinal, recumbent and similar folds with ESE sub-horizontal axes (F1). An axial planar cleavage (S1) is associated with these folds. Mylonites in the arnorthosite-gabbro suite appear to have formed during D1 (Light and Watkeys, 1977). A rodding lineation (L1) defined by quartz, feldspar and magnetite in the ferruginous granulites is also characteristic. Second Deformed Event (D2) This involved two phases of folding (F2 and F3) separated by the syn-tectonic intrusion of the Bulai Gneiss. F2 folds are upright, isoclinal or similar folds with NE sub-horizontal axes, generating Type 2 interference pattern when superimposed with F1. An axial planar cleavage (S2) is associated with these folds. L2 is a rodding lineation defined by biotite, quartz and garnet in the garnetiferous paragneisses, by quartz and magnetite in the magnetite quartzites and by quartz rods and sillimanite needles in the quartzites. Inclined, tight, similar folds (F3) with sub-horizontal NNE axes followed, which when superimposed with F2 produced a Type 3 interference pattern, and Type 2 when superimposed with F1. These are associated with a strong axial planar cleavage (S3) through alignment of micas. A common lineation (L3) plunges 65o towards SSE, defined by hornblende alignment. Boudins of quartzites formed during F3, and subsequently rotated parallel to S3. Third Deformation Event (D3) The initiation of this event was marked by lamprophyric dykes along conjugate fracture sets. The event produced open, concentric, upright folds with gently to moderately plunging northwest axes (F4). These folds are dominant in the

quartzites ∼20 km east of Beitbridge. An axial planar cleavage (S4) occurs as joints in the quartzites, which are often filled with vein quartz. Crenulations defined by muscovite, fuchsite, cummingtonite and grunerite are oriented WNW, and a south westerly striking steep crenulation cleavage (S5) developed. F5 was accompanied by conjugate ductile shear zones (WNW and NNW) with both vertical and horizontal components of movement.

Fourth Deformation Event (D4) This was cataclasis due to a northward thrusting leading to mylonites and flaser gneisses with a mylonitic fabric (S7) and dextral ENE and NNE – trending shear zones with a sub-vertical fabric (S8).

24

Fifth Deformation Event (D5) This was a folding event, deforming the sediments of the Soutpansberg Group (Barker, 1983), producing a gentle, upright fold on a NE sub-horizontal axes (F6). Sixth Deformation Event (D6) Fracturing along 055o and 075o occurred prior to Karoo deposition, with syn-depositional movement leading to over thickening of the Karoo succession of Bubye Coalfield (Bruce, 1975). Synclinal flexuring (F7) of the sediments occurred along ESE trend before eruption of the volcanics. Axial trace fractures and faults developed with radial and concentric patterns. These faults led to the down-faulting of the Karoo rocks to form the Tuli and Nuanetsi troughs. Major brecciation then occurred due to movement along NNW and NNE faults. Triangle Shear Zone (TSZ) This is a major structural feature occurring towards the eastern end of the LMB in Zimbabwe (refer to figure 5), where it separates rocks of the NMZ from those of the CZ. It trends ENE with a width of 35 km, consisting of mylonitised gneisses whose protoliths are felsic gneisses similar to those of the NMZ. A strong planar fabric in the TSZ dips gently southeast with a sub-horizontal lineation showing evidence of dextral displacement. Ridley (1992) interpreted the protoliths of mylonitic pelitic gneisses and calc-silicates in the TSZ as deformed equivalents of CZ lithologies.

4 The Limpopo Mobile Belt In South Africa 4.1 The CZ In South Africa Just like in Zimbabwe, the CZ in South Africa is underlain by a variety of para- and ortho-gneisses, many of which have a granitoid composition (Sohnge et. al., 1948), while some form the basement (Bahnemann, 1972; Fripp, 1981). The basement comprises grey, leucocratic, hypersthene-bearing gneisses of quartz dioritic and granodioritic compositions, and are collectively termed the Sand River Gneisses (Fripp, 1981; 1983). The supracrustals have been collectively termed the Beitbridge Complex, and they contain some garnet as opposed to the basement rocks (Bahnemann, 1972). They include quartzo-feldspathic gneisses, quartzites, banded magnetite quartzites, pyroxenitic amphibolites, calc-silicate gneiss, marble (Sohnge et. al., 1948; Bahnemann, 1972) and some granitoids (Fripp et. al., 1979; Bahnemann, 1973) and gabbroic (Barton et. al., 1979a) intrusions. Figure 6 is a geological map of the Limpopo basin in South Africa showing the position of the LMB and surrounding rocks. 4.2 Rock Description (CZ) Basement Gneisses These are poorly banded, grey in colour and are transacted by dykes of mafic and granitic composition and leucocratic veins and are therefore migmatites (Fripp, 1981, 1983). They are granodioritic and contain quartz, andesine, microcline and biotite and often have epidote and chlorite as alteration products.

25

Mineral grains range from 1-3 mm in thickness. The leucocratic veins are dioritic, and consist of andesine, quartz and biotite.

Quartzo-feldspathic Gneisses These are the most abundant in the CZ in South Africa, and vary in mineralogy and texture. Mineralogy includes quartz and feldspar, with minor amphibole, mica, pyroxene, and garnet. More felsic varieties may be pegmatoidal with quartz and microcline perthite dominating. Banded varieties are adamellitic to dioritic, with microcline, quartz, andesine and biotite, and often but not always, augite. Garnet occurs in biotite-rich varieties.

26° 27° 28° 29° 30° 31°

26° 27° 28° 29° 30° 31°

-26°

-25°

-24°

-23°

-26°

-25°

-24°

-23°

PIETERSBURG

BEITBRIDGE

MESSINA

BANDELIERKOP

LOUIS TRICHARDT

TSHIPISE

Sut

herla

nd b

elt

Murchison belt

Pietersburg belt

Matok Pluton

Quartzite, shale, tillite, andesite, chert, jaspilite, limetsone

Klein-Letaba

KRUGERSDORP

JOHANNESBURG

PRETORIA

VENTERSDORP

Sandstones, shale, mudstone, coal

Basalt, pyroclastics intercalated with sandstones

Rhyolite, dacite, pyroclastics

Sandstone, shale, limestone

Unconsolidated superficial deposits, limestone,conglomerate, sandstones

Metasediments

Conglomerate, quartzite, shale, limestone, lavas

Ultrabasic to basic intrusives

Migmatite, gneiss

Conglomerate, quartzite, shale, phylite, tillite, lavas

Granite

Quartzite, shale, conglomerate, andesite

Dolomite, chert, conglomerate, quartzite, shale, tillite

Quartzite, shale, conglomerate

Quartzite, shale, hornfels, lavas, pyroclasts, chert, jaspilite, limetsone

Quartzite, shale, hornfels, andesite, chert, tuff, limetsone, conglomerate

Migmatite, gneiss, high grade metamorphic rocks

Pyroclastic felsite

Gabbro, norite, chromitite, magnetite

Granophyre

Granites, felsite, tuffaceous & shaly sediments,agglomerate

Lava, sandstone, conglomerate, siltstone, greywacke

Sandstone, conglomerate, siltstone

Alkalis, basic to ultrbasic intrusives, carbonatite

Tillite, sandstone, shale

Shale, sandstone, gritstone, coal

KAROO

STORMBERG

ECCA

DWYKA

KRANSBERG

NYLSTROOM

BUSHVELDINTRUSIVECOMPLEX

LIMPOPO BELT

PRETORIA

MALIMANI

DOMINIAN

JURASSIC

TRIASSIC

PERMIAN

PR

EC

AM

BR

IAN

TERTIARY &RECENT

CARBONIFEROUS

WATERBERG

CRETACEOUS

PILANSBERG INTRUSIVECOMPLEX

GEOLOGICAL LEGEND

ARCHAEAN GREENSTONES

Figure 6. Simplified geological map of the Limpopo basin in South Africa showing the LMB, the main granite-greenstone lithologies of the Kaapvaal craton and the major sedimentary basins.

Singelele-type Granitoid Gneiss This is a variety of the quartzo-feldspathic gneisses described above, and is similar to those rocks making up the prominent Singelele Hills near Messina (Sohnge, 1946; Sohnge et. al., 1948; Bahnemann, 1972, 1973; Fripp et. al., 1979). These rocks, making a positive relief whenever they occur, are quartz-rich, and ribbon-like grains and aggregates impart a strong planar fabric to the rock. Microcline and biotite are also present, and feldspar in them weathers to reddish colours. Garnet-Cordierite-Sillimanite Gneiss These are pelitic rocks infolded with the quartzo-feldspathic gneisses. Garnet, cordierite and sillimanite dominate while biotite, phlogopite, oligoclase, orthoclase, quartz, hornblende, gedrite and hypersthene may also be present. Symplectitic textures occur in rocks with garnet, which forms coronas of radial orthopyroxene grains in plagioclase (Horrocks, 1980). Pod-like and lens-like units occur sporadically within these units, especially where corundum, sapphirine and spinel assemblages are notable (Horrocks, 1983)

26

Pyroxenitic Amphibolite The rocks in this group have been termed amphibolites but they are in fact mafic granulites in which many examples contain more pyroxene than amphibole (Horrocks, 1983). They occur interlayered and infolded with quartzites, or in association with quartzo-feldspathic gneisses. Amphibole-rich varieties are garnetiferous, with the garnets either showing reaction haloes or completely replaced by plagioclase with or without lesser amounts of clinopyroxene and amphibole, which impart a characteristic spotted appearance to these rocks. Mineralogy includes andesine-labradorite, hornblende, hypersthene, augite, biotite, quartz and at times garnet. Quartzites and Banded Magnetite Quartzite These lithologies are associated with pyroxenitic amphiboles. Quartzite forms ridges that stand high above the mafic rocks, and magnetite quartzite forms units, usually less than 20 m thick, which are often boudinaged out within the pyroxenitic amphibolite. The banding in the magnetite quartzites is often less than 1 cm thick, and is frequently tightly folded within the surrounding units. Calc-Silicate Gneiss and Marble Thin units (<1 m thick) of these rocks are found to the SE of Messina. The calc-silicate gneiss is well banded with alternating felsic and mafic layers up to 1 cm in thickness. Essential minerals are labradorite, augite and hornblende with less calcite, quartz and biotite. Zircons may be present in some samples. Marble is coarsely crystalline with interlocking carbonate grains up to 4 mm in diameter. Calcite is the dominant carbonate, and scattered altered silicate minerals, mainly serpentine which pseudomorphs olivine, impart a speckled appearance to the rock. Intrusive Rocks Gabbroic and anorthositic gneisses of the Messina Layered Intrusion form distinct units to the south and east of Messina (figure 3). The rocks are conformable and infolded with other surrounding units. In places, however, coarsely crystalline magnetites with grain sizes of up to 4 mm occur as thin lenses without significant lateral persistence (Horrocks, 1983). Supracrustal

gneisses ∼20 m thick may be infolded along a central axis within the anorthositic gneiss. Plagioclase megacrysts (>10 cm) are present in places, and some units may show gradational change in both composition and texture (Barton et. al., 1979a). Mineralogy is labradorite-bytownite, hornblende, clinopyroxene and quartz. Sepentinites in the intrusion preserve pyroxene bands, although in some localities they may be composed of entirely tremolised amphibole. Quartz-feldspar pegmatite bodies are common in the serpentinite bodies. Pyroxenites preserve large megacrysts of hypersthene (up to 2 cm) set in a matrix of smaller clinopyroxene grains with hercynite as a common accessory mineral. Tholeiitic dykes of various ages (Barton, 1979), both with and without tectonic fabrics transect these units, and in places they occupy fault planes. The porphyritic Bulai Gneiss as described under CZ in Zimbabwe occurs to the northwest of Messina.

27

4.3 Structure Folds Rocks in the CZ of SA show evidence of complex folding and refolding on all scales, and are scattered all over the CZ, and around Messina. The folds have steep dips and are generally sub-vertical to vertical in attitude. They are cylindrical with fold axis orientation of 55o on a bearing of 210o. Tight, isoclinal rootless folds in the magnetite quartzite are characteristic in the northwestern part of the area. The asymmetry of the folds and the strong attenuation of the units towards the southeast suggest that deformation was characterised by simple shear with large amounts of shear strain (Horrocks, 1983). Foliation and Lineation Banding in the gneisses and bedding are the dominant forms of foliation. The orientation of the foliation shows a systematic change, which has been interpreted as a direct result of the folding event. The lineation forms a cluster on a stereoplot with average orientation of 55o to 210o, which suggests that the lineation is parallel to the fold axis, and has been a result of the folding event.

This coincides with the π-pole to the fold profile plane when a stereoplot is constructed, implying that a large-scale fold exists in the CZ in South Africa (Horrocks, 1983). Faulting The main faults in the CZ trend in a general ENE direction. The Dowe-Tokwe Fault in figure 3 trends almost E-W, passing through the immediate south of Messina, and with a dextral displacement of a considerable magnitude on the Messina Layered Intrusion and other lithologies. A splay of this fault branches from the western side of Messina and runs NE as the Messina fault, which affects the areas in the vicinity of Messina with a dextral movement. Further

south of Messina (∼20 km south), the ENE trending Bosbokpoort fault separates the high-grade rocks of the CZ from those of the recent cover, mainly the Karoo rocks, as can be seen in figure 3. More faults with the same trends occur further south, affecting areas around Tshipise (Horrocks, 1983). 4.4 The Southern Marginal Zone (SMZ) The SMZ is that part of the LMB bounded by the Sunnyside-Palala shear zone to the north (figure 2), and by the orthoamphibole isograd to the south (Du Toit et. al, 1983). Shearing occurred further south, into the Kaapvaal craton interior, along the Thabazimbi shear zone. The SMZ is underlain by strongly deformed ortho- and paragneisses at the granulite and upper amphibolite grades of regional metamorphism. Rocks in the SMZ are broadly subdivided into two categories, the grey migmatised tonalitic to trondhjemitic gneisses (Baviaanskloof Gneiss) and the Bandelierkop Formation comprising metavolcanic and metasedimentary supracrustal rocks occurring as a series of discontinuous infolded keels surrounded by migmatised Baviaanskloof Gneiss. A simplified stratigraphy of the SMZ is shown in Table 1 below.

28

Table 1: Simplified Stratigraphy of the SMZ. Modified after Du Toit et. al., (1983). SCHIEL COMPLEX Massive (quartz) syenite in a hornblende granite

envelope PALMIETFONTEIN GRANITE Several plutons of massive fine-to-medium grained

granite MATOK PLUTON Early enderbitic and charnoenderbitic phases

followed by porphyritic, uneven-grained granite

ONLUST GNEISS PLUTON Tonalitic to trondhjemitic granite gneiss with xenoliths of Bandelierkop Formation

BA

ND

ELIE

RK

OP

F

OR

MA

TIO

N

Pelitic Member Mafic Member Ultramafic Member

Purplish brown, medium grained gneiss. Lenses of banded iron formation, quartzite and mafic materials occur sporadically Dark grey to black, fine-textured rock. Serpentinised pyroxenite and peridotite bodies, usually associated with Baviaanskloof Gneiss

BAVIANSKLOOF GNEISS Grey, medium-textured tonalitic to trondhjemitic banded migmatite gneiss

4.5 Description of Rocks (SMZ) The Baviaanskloof Gneiss This is greyish, migmatitic, tonalitic to trondhjemitic gneiss in which the granitic character is immediately apparent. Mineralogy is basically quartz, plagioclase, hornblende or biotite, and the rock is well banded. It is homogeneous in places whereas others are strongly heterogeneous. The contacts of this gneiss run parallel to the gneissic fabric, and an anatectic leucocratic granite always occurs at the contact between this unit and the Bandelierkop Formation (Du Toit et. al., 1983). The Klein Letaba Gneiss (Vorster, 1979) surrounding the Sutherland greenstone belt east of the high-grade terrane (shown in figures 3 and 6) is part of the Baviaanskloof Gneiss (Du Toit, 1979). The Bandelierkop Formation Rocks of this formation are largely supracrustal gneisses that have undergone a considerable degree of anatexis, and are exposed in the area east and northeast of Bandelierkop, located 40 km south of Louis Trichardt (figure 6). The Formation is subdivided into the ultramafic, mafic and pelitic members. Ultramafic Member

The rocks occur as discontinuous lenses and pods up to 2 km in diameter, enveloped by a leuco-granite that intrudes along fractures. The ultramafic member consists of peridotite, pyroxenite, dunite and hornblendite, most of which are serpentinised. Some relic phenocrysts of olivine and enstatite in peridotite and dunite are set in a groundmass of antigorite and magnetite. Other minerals in pyroxenite and peridotite are clinopyroxene (diopside), anthophyllite, cummingtonite, calcite and green spinel. Hornblendite is dominated by green

29

hornblende with accessory hypersthene, quartz, plagioclase, sphene or ore minerals. Mafic Member

The rocks are fine-grained, black and white mottled in colour and well banded. They are associated with a leucocratic granite, which may be concordant or discordant-veinlets of granitic intrusion. It contains remnants of both the ultramafic and pelitic members, as well as sporadic cm-to-m-scale banded iron-formation. Some occur in the orthopyroxene zone with the assemblage augite, plagioclase, hypersthene set in a granoblastic matrix, with minor hornblende, magnetite, ilmenite and quartz, whereas others are in the ortho-amphibole zone with mainly hornblende and plagioclase (Van Reenen, 1983) with little quartz, diopside, sphene and epidote. Pelitic Member

These are finely banded with a strong gneissic texture, and are generally migmatised. The rocks are purplish-brown, containing red garnet, dark brown biotite and opal. Quartz and biotite are oriented along foliation plane, and garnet occurs as small scattered porphyroblasts. Hypersthene and cordierite occur in the orthopyroxene zone while anthophyllite is restricted to the ortho-amphibole zone. Lenses of magnetite quartzites and banded iron formations are associated with this member (Van Reenen, 1983). The Onlust Gneiss Pluton This pluton is best exposed in the Sand River. It comprises a finely banded greyish gneiss and a lighter coloured, poorly foliated granitic phase. It differs from the Baviaanskloof Gneiss in that it is finely banded with thick leucocratic trondhjemite bands and is structurally less complicated. The pluton contains mainly quartz and plagioclase with subordinate hypertsthene and reddish-brown biotite. Onlust Gneiss pluton is subsequently intruded by discordant mafic dykes and the Palmietfontein granite. The Matok Pluton The Matok pluton is a peer-shaped intrusive body, straddling the orthopyroxene isograd some 58 km south of Louis Trichardt, as shown in figures 3 and 6. It is composed of an older mafic phase and a younger granitic phase. The mafic phase is a 40 km long and 4 km wide E-W body along the northern part of the pluton. The rocks of this phase are dark grey to black, containing sporadic blue opaline quartz blebs and is a charnockite by composition (Serfontein, pers comm., 1980) though some enderbitic portions are also present. Mineralogy is mainly plagioclase and biotite with little quartz, augite, hypersthene and accessory magnetite, apatite and orthoclase. The granitic phase dominates in the southwestern portion of the Matok pluton. It ranges from granodioritic to granitic in composition, and is divided into three; a coarse-grained, whitish to pinkish, porphyritic type, a medium-grained, hornblende-bearing type and a fine-grained granite. All these consist of quartz, plagioclase, K-feldspar, biotite and hornblende, but in varying proportions.

30

The Palmietfontein Pluton This forms a collection of plutons, some rounded (1,5 km in diameter) and some elongate with length of 5 km and 0,5 km width. They are scattered randomly in the SMZ, and they cut the Baviaanskloof Gneiss and the Bandelierkop Formations discordantly, and have not been affected by the Limpopo Orogeny (Du Toit et. al, 1983). The granite is light grey and medium-grained, consisting of quartz, microcline, plagioclase, muscovite and accessory biotite, apatite, maghemite, calcite and epidote. The Schiel Complex The Schiel Complex, occurring around the Klein Letaba area in figure 6, was first reported by Willemse (1938) as comprising the hills at Tabaan, Magora and Mashau. It is mostly composed of syenite, quartz syenite and alkali-granite (Barton et. al., 1983). Alkaline rocks are grey to brownish-grey, medium-grained, locally porphyritic and uneven-grained, and pegmatitic in places. K-feldspar dominates with subordinate quartz and accessory diopside and augite (Viljoen, pers. comm., 1974). The syenite, where it is porphyritic, may either contain rapakivi feldspar phenocrysts in an uneven syenite matrix, or large feldspar phenocrysts in a grey to black hornblende-biotite matrix. 4.6 Structure The dominant structures in the SMZ are the various folds on all scales, their associated foliations, and several shear zones Folds and associated foliation and lineation The folds are divided into the Zwartrandjes and Sand River folds. The Zwartrandjes folds lie within the bounds of the granulite grade of regional metamorphism, consisting of two hook folds composed of rocks of the mafic and pelitic members of the Bandelierkop formation. A type 3 fold interference pattern (Ramsey, 1967) is defined by the folds (Du Toit et. al, 1983). The folds are associated with vertical foliations trending at 0600 and 0800 respectively. Lineations contained in these foliations plunge moderately to the SW. A third fold occurs further to the south in association with a 1050 trending vertical foliation, and containing a strong lineation plunging moderately to steeply to the SW. The Sand River folds occur along the Sand River to the north of Matok pluton. The folds re-orientate the Zwartrandjes foliations to trend at 1100 and dip steeply to the NE. The associated lineation plunges at 10o – 20o to the SE in the eastern part of the fold, and in the west, lineation plunges shallowly to the NW. Shear Zones The major shear zones occur within the 4 km wide, NE-trending Kudus River lineament (figure 3). These are vertical shear zones separating the upper amphibolite to granulite rocks of the SMZ to the west, and the granite-greenstone rocks to the east. The lineament displaces all the folds in the area. The Sunnyside and Palala shear zones to the north, and separating the SMZ from the

31

CZ, show the same structural trends as the Kudus River Lineament, and were formed at the same time (Key and Hutton, 1976). 4.7 The Kaapvaal Craton The Kaapvaal craton occurs to the south of the LMB, and is broadly divided into the southern, central and northern zones. The southern Kaapvaal craton (outside the maps in figures 3 and 6) comprises the Ancient Gneiss Complex and the Barberton granite-greenstone terrane (De Wit and Roering, 1990). The boundaries between these terranes are fault zones related to tectonic amalgamation between ~ 3.3 and 3.2 Ga (De Wit and Roering, 1990), an age significantly older than that of the LMB. In the central Kaapvaal craton (partly shown in figure 6) near Johannesburg, the Barberton-type granite-greenstone terrane is overlain by the Witwatersrand Basin comprising an elongate trough that is locally underlain by a volcano-sedimentary sequence termed the Dominion Group (SACS, 1980). The Dominion Group comprises a rift-related assemblage of fluviatile clastic sediments overlain by basaltic, andesitic and rhyolitic volcanic rocks (Bickle and Eriksson, 1982; Clendenin et. al., 1988). Burke et. al. (1986) suggest an origin from Andean-type magmatism for the volcanics. The Witwatersrand Supergroup shows coarsening upward in which the lower argillaceous to arenaceous sequence is overlain by an upper arenaceous to rudaceous sequence. The Witwatersrand Basin sediments could have been derived from the mountain ranges of the LMB to the north and west (Vearncombe, 1991). The Witwatersrand Supergroup is overlain by the 2.7 Ga Ventersdorp Supergroup (Armstrong et. al., 1990), which consists of bimodal volcanics and clastic sediments in a system of NE-SW trending grabens. The Ventersdorp Supergroup is contemporaneous with granite plutonism and exhumation in the LMB. The northern Kaapvaal craton comprises the Murchison, Pietersburg and Sutherland greenstone belts and surrounding granitoids, as shown in figure 6. The Sutherland belt comprises metavolcanic schists of ultramafic, mafic and rarely felsic composition with subordinate banded iron formation, quartzite, pelitic schist and minor dolomite. In the Pietersburg belt, an upper tectono-stratigraphic unit including conglomerates is separated from the lower greenstone sequence by an unconformity locally modified by a northward-verging thrust (M. J. de Wit pers. com, 1990). This north-directed motion in the belt predates the Limpopo Orogeny (De Wit and Roering, 1990). In contrast, south-directed thrusting in the Sutherland and Murchison belts (McCourt and Van Reenen, 1992; Vearncombe et. al., 1988) is geometrically related to the Limpopo deformation and interpreted by McCourt and Van Reenen (1992) and Vearncombe (1991) as an integral part of the Limpopo event. Some syn- to post-tectonic granitic plutons intrude into all the greenstone sequences, the tonalitic Klein Letaba Gneiss and the trondhjemitic Goudplaats Gneiss. The grey gneisses of the northern Kaapvaal craton are of diverse ages but significantly younger than those of the southern Kaapvaal craton. The Kaapvaal Craton is intruded by the later Bushveld Intrusive

32

Complex, as seen in figures 3 and 6. Sediments and volcanic rocks of the Waterberg and Karoo groups subsequently overly rocks of both the Kaapvaal craton and the Bushveld Intrusive Complex (figure 3).

5 The Limpopo Basin In Mozambique Unlike for South Africa and Zimbabwe, literature on the Limpopo Basin in Mozambique is very scarce. The little information available is documented in Portuguese, making it unusable by non-Portuguese speaking people. As such, this section was entirely based on the description of a 1:1 000 000 geological map of Mozambique that was translated into English, and is therefore less detailed. The basin is bounded to the north by the Save River. 5.1 Stratigraphy The stratigraphy in the basin starts with the Karoo aged volcanics at the base, which are overlain by some Cretaceous conglomerates. The volcanics are rhyolitic to basaltic, and occur as thin lenses along the southwestern boundary of the area, as shown in figure 7. Cretaceous conglomerates, some intercalated with the Karoo volcanics and others with silts and marine sands, form the geology of the upper courses of the rivers and streams in the west and northwestern ends of the area. The conglomerates may be interlayered with some mixed calcareous sands of Paleocene age, and some Miocene aged ferruginous arid sands, as seen in the western parts of the map in figure 7. The ferruginous arid sands occur along the Limpopo River and its tributaries to the west. Some intercalations of argillites (some related to marine terraces), arenaceous fluvial sandstones and mudstones and other superficial deposits form the dominant lithologies in the central part of the map, between Save and Limpopo rivers (figure 7). These units are Quaternary to recent in age. To the south and southeastern ends of the map, recent alluvium and dunes dominate. Interior consolidated dunes (both longitudinal and remobilised) are more characteristic with their seaward margins being traversed by coastal and mobile dunes.

33

31.5° 32° 32.5° 33° 33.5° 34° 34.5°

31.5° 32° 32.5° 33° 33.5° 34° 34.5°

-25.5°

-25°

-24.5°

-24°

-23.5°

-23°

-22.5°

-22°

-21.5°

-25.5°

-25°

-24.5°

-24°

-23.5°

-23°

-22.5°

-22°

-21.5°

conglomerates, silts intercalated with marine sands

CRETACEOUS

KAROOJURASSIC

QUATERNARY &RECENT

Recent alluvium Coastal dunes, mobile dunes

Interior dunes, consolidated dunes including remobilised & longitudanal

argillites, areanous fluvial sandstones & mudstones, other superficial deposits

argillites related to marine terrraces

Ferrigenous arid sands

mixed calcerious sands

conglomerates intercalated with volcanics of Karoo age

rhyolites

SENA

ELEFANTES/SINGUEDEZE

CHERINGOMA

MAZAMBAMIOCENE

PALEOCENE

GEOLOGICAL LEGEND

Chaila

MANJACAZE

Chigubo

Machaila

CHICUALACUALA

MASSINCIR

Dindiza

XAI-XAI

MANHICA

MAGUDE

CHOKWE

GUIJA

L i m

p o

p o

R i v e

r

S a v e R i v e r

Figure 7. Geology of the Mozambican part of the Limpopo basin.

34

5.2 Structure There are not much structures discernible in the Mozambican part of the Limpopo basin. Only faults are notable with two sets being readily distinguishable. The first set trends northeast while the second and more dominant set trends N-NNW. Both sets of faults occur in the eastern end of the map in figure 7. From the map, the northeast-trending set clearly displaces the N-NNW set, which implies that the former is younger than the latter. For the purpose of groundwater, the faults are significant since they provide the necessary secondary porosity. The existence of more than one set of faults also enhances chances of groundwater flow since porosity and permeability are increased at fault intersections. The predominance of sands may provide a generally good aquifer in the Mozambican part of the Limpopo basin.

6 Summary The Limpopo belt, with its terrane subdivision into the NMZ, CZ, and SMZ, in part shows a strong relationship with the adjacent Zimbabwe and Kaapvaal Cratons to the north and south respectively. Both lithological and structural trends can be connected for the NMZ and SMZ, with the CZ displaying unique characteristics typical of a block totally unrelated to the adjacent terranes. Both the NMZ and SMZ comprise the granite-greenstone material from the adjacent cratons, but at granulite grade of metamorphism. The CZ on the other hand consists of a shelf-type supracrustal sequence. The major structures that are readily observable are the major shear zones that separate the three terranes of the mobile belt. The near E-W trending Hout River Shear Zone separates the SMZ from the Kaapvaal craton while the North Limpopo Thrust Zone separates the Zimbabwe Craton from the NMZ, and has almost the same trend as the Hout River Shear Zone. However, the former dips gently southwards while the latter steeply dips towards the north. They are both characterised by down-dip lineations with reverse movement in each case. The implication is therefore that the SMZ was overthrusted southwards onto the Kaapvaal craton while the NMZ was thrusted northwards onto the Zimbabwe craton. The CZ, apart from having a supracrustal sequence totally unrelated to the lithologies in the NMZ and SMZ, is also bounded and separated from the NMZ to the north by the Southward, gently dipping Tuli-Sabi/Triangle Shear Zone, and by the northward, steeply dipping Sunnyside-Palala Shear Zone to the south, which separates it from the SMZ further to the south. Both shear zones are characterised by dominantly horizontal lineations, suggesting that the movement was mainly strike slip. This differs from the vertical, reverse movement of the SMZ and NMZ onto the adjacent cratons. While the trends remain nearly east-west, the movement style is different, and the CZ appears allochthonous with respect to the surrounding marginal zones.

35

The LMB therefore forms a large pop-up structure, with exhumation bringing it to a higher elevation than the adjacent cratons. The simple section below shows, not to scale, the structure from the Zimbabwe craton through to Kaapvaal craton. North South

Figure 8. Illustrative sketch showing the general structure of the LMB. Not to scale. The trends of the major shear zones in the LMB appear to control the trends of major faults and fractures in this segment. Some steeply N-dipping reverse faults are connected by transfer strike slip faults with no consistent trends, and these are associated with the Hout river shear zone. The same trend is followed by a flat parallel to the present erosional level. Some internal shear zones are common in the terranes, such as the NW-trending Petronella shear zone within the SMZ. Several reactivated shear zones, eg. the strike-slip N’Tabalala shear zone, which occurs in the SMZ and is largely discordant with respect to regional fabric, are not an uncommon feature. In the NMZ, as in the other LMB segments (CZ and SMZ), the structures are controlled by the trends of the NLTZ and the Tuli-Sabi/Triangle shear zones. These structures are the NNE-thrusts and shear zones, the E-ENE cleavage and foliation, as well as the dominantly E-striking dykes and sills. For the purpose of groundwater exploration, one has to look for such structural trends since they occur on such a large scale as to allow for immediate identification. Moreover, the LMB is generally crystalline, which implies that there is no primary porosity at all. Secondary structures such as shear zones, faults and fractures therefore provide important secondary porosity in such crystalline segments. The lithological compositions described in this report should also provide reference to anyone carrying out water chemistry analysis within the Limpopo Basin. This is because the elemental compositions of the different minerals characterising the LMB lithologies, when they weather, will have an

SMZ Granite-greenstone

(granulite

grade)

CZ

Shelf-type

supracrustals

(granulite grade)

NMZ

Granite-

greenstone

(granulite grade)

Zimbabwe craton

Granite-

greenstone (low

grade)

Kaapvaal craton

Granite-

greenstone (low

grade)

North Limpopo

Thrust Zone

(NLTZ)

Tuli-

Sabi/Triangle Sunnyside-

Palala Shear Hout River

Shear Zone

36

effect of altering the water chemistry of the different aquifers, as well as surface water. The Mozambican part of the Limpopo basin and some parts of the Kaapvaal and Zimbabwe cratons are, however, different in that they are not dominated by crystalline rocks observed in the LMB in Zimbabwe and South Africa. Instead, the area is generally sandy with little volcanic and conglomeratic intercalations. Very little brittle deformation is manifested in form of the dominantly strike-slip fault sets that affected some parts of the area. Both primary and secondary porosities are therefore important in groundwater exploration in these parts of the Limpopo basin. In such an area, it is suggested that the sandy areas should be targeted, particularly where they are faulted. Fault intersection zones in such areas will be more ideal as potential groundwater zones.

37

References

Armstrong, R. A., Compston, W., Retief, E. A. and Williams, I. S. 1990. Geochronological constraints on the evolution of the Witwatersrand basin (South Africa), as deduced from single zircon U/Pb ion microprobe studies. In: J. S. Clover and S. E. Ho (eds.), 3rd Int. Archean Symp., Extended Abstract volume, pp. 287-288.

Bahnemann, K. P. 1972. A review of the structure, stratigraphy and metamorphism of the Basement rocks in the Messina District, Northern Transvaal. Unpublished D.Sc. Thesis, University of Pretoria, 156 pp. Bahnemann, K. P. 1973. The origin of the Singelele Granite Gneiss, near Messina, northern Transvaal. Special Publication of the Geological Society of South Africa, V. 3, pp. 235-244. Barker, O. B. 1983. A proposed geotectonic model for the Soutpansberg Group within the Limpopo Mobile Belt, Southern Africa. South African Special Publication of the Geological Society of South Africa, V. 8, pp.181-190. Barton, J. M. Jr., Fripp, R. E. P., and Horrocks, P. C. 1979a. Effects of metamorphism on the Rb-Sr and U-Pb systematics of the Singelele and Bulai Gneisses, Limpopo Mobile belt, southern Africa. Trans. Geological society of South Africa, V. 82, pp. 259-269. Barton, J. M. Jr., Du Toit, M. C., Van Reenen, D. D., and Ryan, B. 1983. Geochronologic studies in the Southern Marginal Zone of the Limpopo Mobile Belt, Southern Africa. Special Publication of thee Geological Society of South Africa, V. 8, pp. 55-64. Barton, J. M., Jr 1979. The chemical composition, Rb-Sr Isotopic systematics and tectonic setting of certain post-kinematic mafic igneous intrusions, Limpopo Mobile Belt, Southern Africa. Precambrian Research, V. 9, pp 57-80. Barton, J. M. Jr., Fripp, R. E. P. and McLean, N. 1979b. The geology, age and tectonic setting of the Messina Layered Intrusion, Limpopo Mobile Belt, Southern Africa. American Journal of Science, V. 279, pp. 1108-1134.

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