CHAPTER TWO

REGIONAL GEOLOGICAL SETTING

2.1 INTRODUCTION

The Zeekoebaart and Nauga East high-grade iron ore deposits are located close to the

Doringberg fault system (Fig. 2.1), which is regarded as the southwestern margin of the

Archean Kaapvaal Craton (Horstmann and Hälbich, 1995). Both iron ore deposits are

hosted by the Paleoproterozoic Kuruman Iron Formation of the Asbestos Hills Subgroup

in the basinal facies of the Ghaap Group (after Beukes, 1978, 1980) of the Neoarchean to

Paleoproterozoic Transvaal Supergroup in Griqualand West (Fig. 1).

The Transvaal Supergroup is known to outcrop in the Transvaal and Griqualand West

areas of South Africa as well as the Kanye area in southernmost Botswana (refer to

Beukes, 1978, Dorland, 1999 and Coetzee, 2001 for detailed correlation). The ca. 8km

thick, 2.67 Ga to 2.07 Ga (Coetzee, 2001), largely sedimentary succession in Griqualand

West (Fig. 2.1) developed on a continental platform and basin (Beukes, 1978), and

consists of a series of carbonates (dolostones and limestones), iron formations and

siliciclastic rocks, with only minor volcanic rocks (Fig. 2.2). Metamorphism of this

succession in Griqualand West is very low and according to Vajner (1974) and Miyano

and Beukes (1984), does not exceed the lowest greenschist facies.

In Griqualand West, the Transvaal Supergroup consists of two major units, the Ghaap and

Postmasburg Groups (Van Niekerk, 1998). The Ghaap Group is the dominant geological

unit in the area surrounding the Zeekoebaart and Nauga East ore deposits. It comprises of

the Schmidtsdrif, Campbellrand, Asbestos Hills and Koegas Subgroups (Fig. 2.2). The

banded iron formation (BIF) of the Asbestos Hills Subgroup hosts the two iron ore

deposits investigated during this study.

2. Regional Geological Setting 5

Figure 2.1: Geological map of the Transvaal Supergroup in Griqualand West, showing the localities of the

two iron ore deposits investigated during this study.

2. Regional Geological Setting 6

Figure 2.2: Stratigraphic cross-section of the Transvaal Supergroup, from Prieska to Thabazimbi (modified after Beukes, 1983).

2. Regional Geological Setting 7

Figure 2.3: Stratigraphy of the Transvaal Supergroup in Griqualand West with radiometric ages indicated

(modified after Dorland, 1999).

2. Regional Geological Setting 8

2.2 STRATIGRAPHY

2.2.1 GHAAP GROUP

2.2.1.1 SCHMIDTSDRIF SUBGROUP

The Schmidtsdrif Subgroup (Fig. 2.2) forms the basal unit of the Ghaap Group and the

Transvaal Supergroup in Griqualand West. It unconformably overlies the ~2.7 Ga

volcanic rocks of the Ventersdorp Supergroup. Beukes (1979) subdivided the

Schmidtsdrif Subgroup into the basal Vryburg Formation, interpreted by Beukes (1986)

to represent fluvial to marginal marine deposits, consisting of shales, siltstones,

quartzites, carbonates and basaltic to andesitic amygdaloidal lavas (Altermann and

Siegfried, 1997). The entire Formation is estimated to be approximately 100m thick

(SACS, 1980). The central Boomplaas Formation is composed of platform carbonates

with well preserved oolitic, and stromatolitic textures (Beukes, 1979, 1983). Altermann

and Siegfried (1997) interpret the oolite sands, to have been transported and not in situ,

and consequently favour deeper, subtidal depositional environments. The Boomplaas

Formation, however, is not developed in the basinal facies of the Prieska area (Fig. 2.2).

The upper Lokammona Formation is composed of banded siderite lutites overlaying

tuffaceous siltstone, carbonate oolite shoals and stromatolite reef deposits (Beukes,

1983). The deposits are interpreted to indicate a marine regressive cycle over the

Boomplaas Formation (Altermann and Siegfried, 1997). Gutzmer and Beukes (1998)

dated the Vryburg Formation at 2650 ± 8 Ma.

2.2.1.2 CAMPBELLRAND SUBGROUP

The Campbellrand Subgroup (Fig. 2.3) follows conformably on the Lokammona

Formation, of the Schmidtsdrif Subgroup. Beukes (1980, 1983, 1987) further subdivided

this subgroup into two main facies, namely the Prieska facies and the Ghaap Plateau

facies, also known as the basinal and platform facies, respectively. The platform facies

consists of the basal Monteville Formation, followed by the Reivilo, Fairfield, Klipfontein

2. Regional Geological Setting 9

Hills, Papkuil, Klippan, Kogelbeen and the top Gamahaan Formations. These

Formations extend laterally into the basinal facies consisting of the Nauga and Naragas

Formations. Only the Prieska facies will be discussed here as the two iron ore deposits

described in this study are developed within the area occupied by the basinal carbonate

facies. The depositional environment (Fig. 2.2) of the Campbellrand Subgroup was

interpreted by Beukes (1980, 1983) to have been a stable shallow marine platform and

basin, provided by the Kaapvaal Craton.

The shallow water stromatolitic carbonate formations of the platform facies interfinger

and extend laterally into the basinal, deeper water carbonates of the Nauga Formation as

illustrated in figure 2.2 (described by Beukes, 1983, Altermann and Nelson, 1998, in

greater detail). Iron formations (the Kamden Member) extending from the basinal to the

platform facies, are only a few meters thick and, according to Beukes (1983), reflect a

distinct marine transgression. The basinal Prieska facies of the Nauga Formation is

constituted of thinly laminated, clastic-textured dolostones and pyritic carbonaceous shale

(Beukes, 1983). The carbonate laminae were interpreted by Beukes (1978) as carbonate

turbidites. Rare mafic tuff beds that occur within the Prieska facies are overlain by

ankerite-banded cherts. This Chert and Proto-BIF Member is capped by the finely

laminated Klein-Naute Shale Member, representing depositional conditions below the

storm wave-base (Altermann and Nelson, 1998). This shale forms the base of the

Kuruman Iron Formation that hosts the two iron ore deposits described in this study.

Intercalated tuff layers at the top of the Nauga Formation yield an age of 2552 ± 11 Ma

(Barton et al., 1994), or 2549 ± 7 Ma (Altermann and Nelson 1998) respectively. Sumner

and Bowring, (1996) report a U-Pb zircon age of 2521 ± 3 Ma, for a similar tuff bed in

the equivalent position in the platform succession of the Campbellrand Subgroup.

2.2.1.3 ASBESTOS HILLS SUBGROUP

Beukes (1983) attributed this marked transition from carbonate to BIF-deposition to a

major marine transgression and associated sea level rise. The clastic-textured basinal

facies carbonate rocks of the Cambellrand Subgroup are conformably overlain by the

2. Regional Geological Setting 10

carbonaceous Klein-Naute shale (Nauga Formation) and the succeeding iron formations

of the Asbestos Hills Subgroup (Fig. 2.3), namely the Kuruman and Griquatown Iron

Formations. It is the lowermost part of the Kuruman Iron Formation, in the basinal

facies, which hosts the Zeekoebaart and Nauga East high-grade iron ore deposits.

Kuruman Iron Formation

The microbanded Kuruman Iron Formation varies in thickness from 150m to the north of

the Griqualand Fault Zone (Fig. 2.1) up to 750m to the south of the fault zone (Beukes,

1983). It is considered to have been deposited in an open shelf paleoenvironment (Fig.

2.4). The ankerite-banded chert of the Kliphuis Member (Fig. 2.4) forms the basal unit of

the Kuruman Iron Formation and, according to Beukes (1983), consists of chert

mesobands alternating with ankeritic or ferruginous dolomitic intramicrite mesobands.

The latter are thought to represent chertified and ankeritized limestone turbidites.

Stacked stilpnomelane lutite � ferythmite macrocycles of the Groenwater Member (Fig.

2.4) overlay the ankerite-banded cherts (Beukes, 1980). The lower part of the

Groenwater Member hosts the hematite ore deposits of Nauga East and Zeekoebaart.

Macrocycles of deposition in the Groenwater Member range in thickness from 1 to 10m,

and are constituted of stilpnomelane lutite � siderite-microbanded chert � siderite-

magnetite bandrhythmite � magnetite-hematite ribbonrythmite � siderite microbanded

chert (Beukes, 1983). The stilpnomelane lutite � ferythmite macrobanding is thought to

be of mixed volcanic-biological origin (Beukes, 1983). During periods of explosive

volcanic and fumarolic activity, acidic volcanic ash beds and silica were deposited, now

represented by bands of stilpnomelane and chert respectively (Beukes, 1983). During

periods of low volcanic activity, the flourishing photosynthesising active microbial life

promoted the precipitation of siderite, through the extraction of CO2, in the presence of

sufficient O2 (Beukes, 1978). Intermediate oxygen fugacity levels may be reflected by the

presence of magnetite (Beukes, 1983).

2. Regional Geological Setting 11

The Riries Member (Fig. 2.4) of the Kuruman Iron Formation, which is a chert-poor

greenalite-siderite rhythmite, follows on the relatively chert-rich Groenwater Member.

Beukes (1983) suggested that neutral to weakly alkaline conditions existed in the basin at

this stage, resulting in greenalite-siderite lutite laminae alternating with siderite

microbands.

The Kuruman Iron Formation thus represents a third order, upward shallowing

progradational sedimentary cycle (Fig. 2.4). Gutzmer and Beukes (1998) obtained an U-

Pb SHRIMP age of 2489 ± 33 Ma for a tuff bed within this formation, while Armstrong

(in Martin et al., 1998) obtained an age of 2465 ± 7 Ma near the top of the formation.

Figure 2.4. South-North cross section illustrating stratigraphic relationships and palaeodepositional

environments in the Asbestos Hills Subgroup in Griqualand West (modified after Beukes, 1978).

2. Regional Geological Setting 12

Griquatown Iron Formation

The Griquatown Iron Formation conformably overlies the Kuruman Iron Formation and

was deposited in a shallow-water, storm-dominated epeiric sea (Beukes, 1984). Only the

Middlewater and Pieterberg Members of the Griquatown Iron Formation developed

within the basinal facies and therefore the area pertaining to the Zeekoebaart and Nauga

East hematite ore deposits.

The Middlewater Member, which according to Beukes (1983) was deposited in a shallow

basin below the wave base, consists of riebeckitic minnesotaite-greenalite lutites which

interfinger with the orthochemical - allochemical iron formation cycles of the Danielskuil

Member (Fig. 2.4). The Danielskuil Member was deposited below the normal wave base

in a low-energy, weakly alkaline, subtidal zone (Beukes, 1984).

The Pietersberg Member, consisting of banded greenalite lutite, marks the top of the

Griquatown Iron Formation and of the Asbestos Hills Subgroup in Griqualand West

(Fig. 2.4). Gutzmer and Beukes (1998) obtained an U-Pb SHRIMP age for the tuff beds

of the Asbestos Hills Subgroup of 2480 ± 7 Ma.

2.2.1.4 KOEGAS SUBGROUP

The iron formations and siliciclastics of the Koegas Subgroup (Fig. 2.3) conformably

overlay the Griquatown Iron Formation. At the base of the upward-coarsening iron

formation - siliciclastic sedimentary cycle are the Pannetjie, Doradale, Kwakwas,

Naragas, and the Rooinekke Formations (Beukes, 1983). The Koegas Subgroup

represents the top of the Ghaap Group, and is covered with a regional unconformity by

the Makganyene glacial deposits of the Postmasburg Group.

2. Regional Geological Setting 13

2.2.2 POSTMASBURG GROUP

The Postmasburg Group (Fig. 2.3) consists of a diverse sequence of lithologies (for

greater detail, refer to work by Beukes, 1978, 1980; Van Niekerk, 1998; Dorland, 1999;

and Beukes et al., 2000). The basal Makganyene diamictite is of glacial origin, inter-

layered with shale and sandstone and is conformably overlain by the 2222 ± 13 Ma

(Cornell et al., 1996) basaltic andesites of the Ongeluk Formation. The Hotazel

Formation, composed of interbedded iron formation and manganese formations rests

conformably on the volcanic rocks of the Ongeluk Formation, which are in turn overlain

by dolomites of the Mooidraai Formation (Beukes, 1986). The red bed succession of the

Mapedi and Gamagara Formations has a marked basal erosional unconformity towards

the underlying lithologies, but is conformably overlain by shallow marine quartzites of

the Lucknow Formation that constitute the top of the Transvaal Supergroup in Griqualand

West (Beukes et al., 2002). Economically very important high-grade iron ore deposits of

Sishen and Beeshoek are intimately related to this unconformity. Iron ore bodies of the

Nauga East and Zeekoebaart deposits, however, show no spatial relation to this marked

erosional unconformity.

2.3 STRUCTURAL GEOLOGY

The Griquatown fault zone (Fig. 2.1), which according to Beukes (1978) represents an

active growth fault during the deposition of the Transvaal Supergroup, parallels the

south-western Doringberg fault zone, and displaces major lithostratigraphic units of the

Transvaal Supergroup. This fault, however, had little structural control on the

mineralization of the two high-grade iron ore deposits of Zeekoebaart and Nauga East.

Along the south-western margin of the Kaapvaal craton the Doringberg fault (Fig. 2.1),

which parallels the ~1100 Ma Namaqua-Natal Metamorphic Province (NNMP),

terminates the Transvaal Supergroup strata in the Griqualand West area. The high-grade

iron ore deposits of Zeekoebaart and Nauga East are located within the area of faulting by

the Doringberg fault and thrust belt (Fig. 2.1).

2. Regional Geological Setting 14

The NNMP forms part of a major orogenic belt, and has been subdivided into the

Kakamas, Areachap and Bushmanland terranes (Thomas et al., 1994). These terranes

have been juxtaposed, in the west, by the 1.7-2.0 Ga (Reid, 1982, 1997) Richtersveld

terrane, and in the east, by the Kheis Tectonic province (Vajner, 1974; Moen, 1999). Van

Niekerk et al., 2003, recognize two separate collisional events during the early stages of

the evolution of the NNMP, and according to Van Niekiek et al., (2003), the Kakamas,

Areachap and Richtersveld terranes represent fragments of younger cratonic terranes and

accretionary areas which accreted to one another and to the Kaapvaal craton.

According to Vajner, (1974) the Kheis Tectonic Province is regarded as an Archean

crustal segment, composed of complex thin-skinned fold and thrust belts, partly reworked

during or prior to the Namaqua orogeny. Van Niekerk et al., (2003), however, considers

the Kheis Tectonic Province to represent a thick passive margin succession of mainly

siliciclastic sediments, caught up in the collisional events that resulted in the development

of the NNMP at ca. 1.3-1.0 Ga. The regional structural geology of the two iron ore

deposits are dominated by the above mentioned collisional events.

The major thrust faults along the western side of the Kaapvaal Craton, and east of the

Kheis Tectonic Province, displace Transvaal and Olifantshoek strata (Beukes and Smit,

1987). A major thrust fault, the Blackridge thrust fault (Van Wyk, 1980), splits into an

imbricate system causing multiple duplications (Beukes and Smit, 1987). Folding of the

Transvaal strata before the deposition of the Olifantshoek Group further complicates

stratigraphic relationships. The thrust faults dip gently to the west, with folding that

increases in intensity towards the east, occurs along the thrust planes. This is recognized

in the Kuruman Iron Formation which hosts both iron ore deposits described in this

study.

Pre- or syntectonic intrusions, commonly found within the Kheis Tectonic Province and

Transvaal Supergroup strata in Griqualand West, occur as sills and dykes (Moen, 1999),

similar to the zoned syenite-carbonatite intrustion located at the Nauga East deposit.

These dykes range from mafic to felsic in composition. Felsic intrusions include the

2. Regional Geological Setting 15

Hardeberg Granodiorite (Moen, 1999), which intrudes into the Ellie’s Rust Formation of

the Olifantshoek Subgroup near Zeekoebaart.