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Mangalisa Geology

Ancient rivers filled with gold, a spectacular upwelling of magma and a colossal asteroid impact combined to make the Witwatersrand basin a very special place.

Superior Mining's Mangalisa project is located over a potential eastwards extension or trend of the Free State (a.k.a. Welkom) Goldfield, which is the southernmost extension of the prolific gold-bearing Witwatersrand sedimentary basin. The Free State Goldfield one of the seven major goldfields located within the Witwatersrand Basin. Only small parts of the Witwatersrand Basin are exposed at surface and the majority of the basin is covered by younger sediments and volcanics. (See map of Witwatersrand Basin goldfields, which also shows a simplified geology of the region). 

The Witwatersrand Basin is the largest known gold province in the world and the deposits have now been worked for well over 100 years and are believed to have produced about 98% of South Africa's gold. Gold is produced from seven goldfields within the basin, mainly from conglomerate horizons of the Witwatersrand, Ventersdorp and Transvaal Supergroups.

The Witwatersrand Basin is located on the Kaapvaal Craton in South Africa and is an oval-shaped basin, covering an area of some 400 km NE-SW and some 180 km NW-SE, of which approximately 84,000 km2 consist of outcrop and an often deeply buried subgroup of the Witwatersrand Supergroup sedimentary and sub-ordinate volcanic sequences.

There are two gold-bearing trends in the Free State Goldfield: the Target trend to the west and the Homestead trend to the east (see map of mineralized gold trends in the Free State Goldfield). These are separated by the non-gold-mineralized de Bron Horst, except where the two trends meet at their southern extremities. The Mangalisa project area is located along the Homestead Trend, immediately north of and contiguous with the large Masimong #5 (formerly Erfdeel) gold mine being operated by Harmony Gold.

The Witwatersrand Supergroup is underlain by an Archaean granite-greenstone basement more than 3.1 billion years old and the Dominion Group, which is about 3.074 to 3.086 billion years old. It is unconformably overlain by rocks of the Ventersdorp (2.7 billion years old), Transvaal (2.6 billion years old) and Karoo (302 to 180 million years ago) supergroups. 

The area within the basin is composed of the generally non-mineralized West Rand Group (also known as the Lower Witwatersrand Supergroup) covering an area of some 54,000 km2, and the Central Rand Group (also known as the Upper Witwatersrand Supergroup) consisting of gold and uranium rich terranes over an area of some 30,000 km2, in which are the major producing gold and uranium mines of South Africa. The reefs found on the Mangalisa property occur within the Central Rand group. 

The origin of gold mineralization in the Witwatersrand Basin has been debated for at least 100 years. The debate has been divided between the synergenetic or placer and the epigenetic or hydrothermal models. The most widely accepted model currently appears to be the "modified placer model" in which grains of placer gold have been remobilized after burial.

The majority of the economic reefs within the Witwatersrand Basin as well as in the Free State Goldfield are located stratigraphically in the Central Rand Group, however it is significant that the principal reefs of the geographically separated goldfields within the basin, and adjacent to the De Bron Horst in the Free State Goldfield, do not all occur at the same stratigraphic level.

The goldfields are therefore considered to represent major, diachronous entry points of coarse-grained sediments into the basin and appear to be laterally coalesced fluvial braid-plains, where gold was concentrated within the conglomerates developed primarily on unconformities. In the Free State Goldfield, however, areas immediately adjacent to the western and eastern sides of the De Bron Horst also host Elsburg-type alluvial fans.

Deposition in the Witwatersrand Basin is considered to have taken place along the interface between a fluvial system and a major body of still water or an inland sea, with the source of the gold postulated to be a northerly Archaean Greenstone belt in which plate interactions caused the development of mineralizing hydrothermal activity and generated sedimentary environments where gold-bearing deposition could occur. The basin is filled with approximately 14,000 m of sedimentary and subordinate volcanic rocks, which have folded along a southwest to northeast axis into an asymmetrical syncline (Pretorius, 1974).

Structural Controls of the Witwatersrand Basin

The Witwatersrand basin has been affected by several superimposed structural events, which are differentiated as syndeposition and post-depression deformations.

Syndepositional deformation provided a key role in the distribution of sediments, controlled the locality of the auriferous conglomerates and the thickness of enclosing sedimentary sequences. Later faulting and folding of the sequence determined which parts of the basin remained buried, as well as the depths to mineable horizons, relative to the present-day surface. Much of the basin was buried by the ejected cast out of a huge crater created by the Vredefort impact, thus preserving Central Rand Group sediments to present day. Elsewhere in the world, evidence of sedimentation that took place during this period has long since been eroded away.

The Central Rand Group sediments of the Witwatersrand Basin have been subjected to three main phases of deformation. Syndepositional compression deformation occurred during Central Rand sedimentation, which resulted in the infraformational and interformational unconformities (Brink, 1986). Extensional tectonic during and post-Ventersdorp times resulted in the ubiquitous development of normal and wrench faulting present throughout the basin (Stanistreet et al, 1986), and after the Transvaal Sequence was deposited, a high strain event resulted in the formation of folds, bedding parallel faults and regional low-angle cleavage (McCarthy et al, 1986). This high-strain event has been attributed by some to be in part a subsequent consequence of the Vredefort impact.

In the Free State Goldfield, conglomerate-hosted alluvial gold deposits typically occur as a series of ancient alluvial fans along the western and eastern strikes formed by the walls of the De Bron Horst. These are known as Elsburg-type fans. Multiple, stacked reefs occur in this geological environment and high grades are most commonly associated with the apex of these fans. These apices represent a series of ancient alluvial sediment fan entry points from the raised De Bron Horst into the lower-lying alluvial plain.  The apex of each fan is the site where gold and uranium was preferentially concentrated. Where best developed, these fans may extend in an easterly direction for up to 300 m.  Conglomerate reefs may be developed in the areas between the fans, however they tend to contain less gold mineralisation. This model is believed to be replicated at Mangalisa.

Superior Mining's exploration program is targeting these gold-bearing Late Archaean, Central Rand Group sediments that traditionally host highly economic gold-uranium-bearing conglomerate reefs (namely the A, B, Leader and Basal Reefs) of the Free State Goldfield. Currently, the economic reef of importance on the Mangalisa Project is the Erfenis Reef Zone, an Elsburg-type fan reef.

The Geology of the Early Witwatersrand Basin

The geology of South Africa is long and complex, starting in the very early days of the Earth's existence, about 3.7 billion years ago in the basement with Archaean greenstones, basaltic komatic lavas, some with pillow structures, exposed around the edges of the Johannesburg Dome, a granitic intrusion lying between Johannesburg and Pretoria. This intrusion metamorphosed the pyroxenes, peridotites, dunites and harzburgites to amphibolites and serpentines. The granodiorite and granite magmas were intruded in two phases at about 3.2 billion and 3 billion years ago, respectively.

The greenstone-granite basement stablized by about 2.7 billion years ago to form the Kaapvaal Craton, one of the earliest continents. The Archaean Kaapvaal craton, located in the Limpopo Province in the north-eastern part of South Africa, is the foundation upon which the country's geological formations have since developed. The Kaapvaal craton is only one of two remaining areas of pristine 2.5 billion to 3.7 billion year old crust on the Earth. The other is the Pilbara craton in Western Australia. Similarities of rock records from both these cratons, especially of the overlying late Archaean sequences, suggest that they were once part of the Vaalbara supercontinent (Zegers et al., 1998).

The Kaapvaal craton covers an area of approximately 1.2 million km2, and is joined to the Zimbabwe craton to the north by the Limpopo Belt. This crustal block is made up largely of a mixture of early Archaean greenstone terranes and older tonalitic gneisses intruded by a variety of granitic plutons and trondhjemitic gneisses and granitoids. The greenstone occurrences are economically important as they host many gold, antimony, copper, zinc, iron, mercury, magnesite, barite and gemstone deposits.

To the south and west, the Kaapvaal craton is flanked by Proterozoic orogen, and to the east by the Lebombo monocline that contains Jurassic igneous rocks associated with the breakup of Gondwana. The Witwatersrand Basin evolution is associated with the encroachment and eventual collision of the Zimbabwe and Kaapvaal cratons between 2.84 to 2.72 billion years ago. Although in terms of modern thinking the Kaapvaal Craton probably formed through the accretion of many smaller terranes or crustal blocks, it was welded together and strengthened by underplating and igneous intrusions arising from the collision, which likely also generated opportunity for epigenetic mineralization.


Map of Southern Africa's Archaean Cratons

Basins evolved on this craton by mechanisms not yet completely understood (Tinker et al, 2002). By about 3.1 billion years ago the craton had sagged, forming a basin to the south and what is now referred to as the Witwatersrand Basin. 

The Kaapvaal craton was stabilized by about 2.6 by the emplacement of major granitoid batholiths that thickened and stabilized the continental crust during the early stages of an arc-related cycle of magmatism, epigenetic mineralization and sedimentation. Subsequently, the Kaapvaal craton is thought to have been subjected to the continent–arc collision with the Zimbabwe craton that caused overlaying successions of basins filled with thick sequences of both volcanic and sedimentary rocks. This was then followed by episodic extension and rifting when the Gaborone–Kanye and Ventersdorp sequences were developed. Early Archaean crust is well exposed only on the east side of the craton and comprises a collection of subdomains and crustal blocks characterized by distinctive igneous rocks and deformations.

The largest of the basins in the Kaapvaal Craton seems to have been the Witwatersrand Basin, located roughly at the centre of the craton. This basin filled with water to become a southwest to northwest trending sea, sometimes open to the ocean in the southeast, with high granitic mountains to the east, north and west, which sent mineral-laden sediment in vast quantities into this inland sea.

Late Archaean metamorphism joined the Southern Marginal Zone of the Kaapvaal craton to the Northern Marginal Zone of the Zimbabwe craton about 2.5 to 2.8 billion years ago by the 250 km wide orogenic Limpopo Belt. The belt is an east-northeast trending zone of granulite facies tectonites that separates the granitoid-greenstone terranes of the Kaapvaal and Zimbabwe cratons. Aeolian structures suggest the area was likely near the equator at 2.7 billion years ago, however these overlie some of the earliest known glacially produced rocks, debris flow diamictites, suggesting rapid climatic changes (Tyson, 1986). There was also some interlayering of volcanic rocks, indicating some crustal instability (MacCrae, 1999).

The only form of life on earth at that time was prokaryotic bacteria, the rain was more like an acid rain rather than the rain experienced on Earth today, and so there was little to prevent the eroding mountain ranges from sending torrents of gravel, sand, and mud down to this sea in fast-flowing, braided-channel rivers, accumulating in an arc along the shoreline. The remains of these ancient sea shores are found in a wide arc extending from today's Leander in the east, through the Witwatersrand, and south-west to Welkom.

Over a period of three hundred million years or more, the mountains weathered into huge river deltas, and coastal sediments built up to a depth of about seven kilometres. These sediments were layered and differed depending on the circumstances of their deposition. Layers of coarse pebbles were deposited upstream, slightly less coarse sand forming beaches, and finally silt, was deposited in deep and calmer waters. Sandy deltas formed out into the sea and the sediments were reworked by currents during successive transgressions.

These sediments lithified in the form of conglomerate, quartzite, shale, and siltstone. These represent the first sea shores and shallow beds of the ancient sea. The oldest sedimentary facies forms part of the lowest level of one of the world's most well known geological features, the Witwatersrand or Karoo Supergroup. It is about 8 km thick, with gold-bearing pebble conglomerates in a sandy matrix in the upper units.

The depositions that were antecedent to the formation of the sedimentary and conglomerate rocks of the Witwatersrand Supergroup underwent further progressions that evolved into two subdivisions: the West Rand Group and the Central Rand Group. The depositions were accompanied by syn-sedimentary folding and faulting, which control the sedimentation style and stratigraphic thickness. According to Frimmel et al (2005), an angular uncomformity is found between the metasedimentary rocks of the Western Rand Group and the underlying siliciclastic rocks of the Dominion Group (see map of Witwatersrand Basin goldfields showing a simplified geology), which represents a period of approximately 100 million years. The sedimentary sequence that makes up the West Rand Group could be the result of distal fluvio-deltaic and shore to off-shore environment sedimentation that lasted for at least 70 million years to as long as 2.9 billion years. Sediments of the West Rand Group were deposited in a series of alluvial fans which derived their material from a northern and north-western source during their deposition.

Several, fairly narrow layers of gravel, deposited quite late in the sequence, and bearing heavy elements such as gold and uranium, made the Witwatersrand Supergroup famous. These are the gold-bearing conglomerates of the main reefs being mined today. This concentration of gold particles by sedimentary processes appears to be unique. Most of the world's gold deposits occur in quartz and carbonate veins in fault zones, that is, lode deposits, often as a result of metamorphic hydrothermal activity. The ultimate origin is still debatable, however Re/Os isotope dating has shown it to be older than the surrounding sedimentary rocks, thus confirming that it is detrital and of mantle origin (Kirk et al, 2002).

Another fact regarding South Africa's gold-bearing reefs is that a number of the high-yielding reefs occur in seams of kerogen carbon, as was observed in the gold-bearing reefs intercepted by Superior Mining at Mangalisa. This kerogen is associated with periods of little or no deposition during shoreline transgressions. Carbon isotopic analyses indicate a biological origin for the carbon. Detailed research has suggested that the original organisms were tough and leathery, unlike algae and probably like lichens.

Studies on modern lichens have shown that they are capable of accumulating inorganic matter, particularly radioactive and heavy metals, which are toxic to higher plant forms. It has been suggested that lichen-like structures, with spherical spores, grew in mats just below or above the sea's water level, and when dislodged and degraded by bacteria produced amorphous carbon which, when subsequently buried, compacted and geothermally heated, became the black kerogen found in the underside of the reefs. This material is 2.9 to 2.7 billion years old, suggesting complex forms of life already existed on Earth at this time and that biological organisms played a decisive role in concentrating gold and uranium particles (MacCrae, 1999).

The West Rand Group and the Central Rand Group are separated by an unconformity, with four of the seven sedimentary cycles of the Witwatersrand Supergroup belonging to the Central Rand Group. In the Central Rand Group, however, the sediment deposition is dominated by what were fluvial-deltaic processes that lasted for more than 120 million years. Each cyle is characterized by the formation of gold-bearing conglomerates and the concentration of heavy minerals that include uranium, followed by degradation, reworking and upgrading. The Central Rand Group has a maximum thickness of about 2,880 m in the Vredefort area, thining out towards the southwest and the southeast. It is from the reefs within the Central Rand Group that some 95% of the country's gold is being mined.

According to Frimmel et al (2005), a greater variety of rock sources appear to be the source for the Central Rand Group sediments, and that these were deposited in a foreland basin and in a retro-arc basin during the closure of the ocean between the Pietersburg block to the north and the annealed Witwatersrand-Kimberly block. The sediments of the Central Rand Group derived their material from a mostly western source. The sedimentation of the upper Central Rand Group was accompanied by a progressive uplift of the hinterland, thus creating an environment where the mineralized uplands were eroded and provided the sediments of the Central Rand Group and the minerals contain therein. The uplifted De Bron Horst in the Free State Goldfield was created  during this uplift, and as it eroded well-mineralized alluvial fans developed along the horst's margins with the sea. One of these fans, now known as Elsburg-type fans, was intercepted at Mangalisa by Superior Mining's two phases of drilling.

Starting at about 2.3 billion years ago, the Ventersdorp Supergroup was deposited onto the Central Rand Group in an extensional environment; an outpouring of lava 1.6 km thick. This resutled in the displacement of the Witwatersrand strata with extensive faulting and folding along the northern edge of the Witwatersrand Basin. Subsequently, flood basalts that make up the Klippriviersberg Group covered the Ventersdorp formation. The strata of the Transvaal Supergroup were formed subsequent to the Ventersdorp event, including the sedimentary rocks of the Chuniespoort Group in a basin governed by thermal subsidence and the volcanic-sedimentary succession of the Pretoria Group. About 2.06 billion years ago, mafic to ultramafic volcanic rocks were placed into the Bushveld Complex to the north of the Witwatersrand Basin following the extrusion of felsic volcanics of the Rooiberg Group.

A marine incursion started at about 2.2 billion years ago, when carbonate rocks were laid down, forming one of the earliest carbonate deposits on Earth. These later became dolomitized to the west of Johannesburg. Subsequently, geological activity in southern Africa centred on different areas.

A key to understanding the shape of these deposits today is that through subsequent tectonic action they no longer take the form of sprawling horizontal beds, but as layers of rock, which have been tipped southwards at a steep angle. They jut out of the present surface as long ridges along the west, north and east, and plunge below the surface at an angle of as much as up to about 70° to the horizontal. A dramatic view of this tipping effect is the Linksfield Ridge near Johannesburg's Oliver Tambo Airport. The tipping action originally was attributed to the weight of water and sediments to the south, causing the basin to sag and drag the edges downwards.

Since 1996, however, it has emerged that about 2.02 billion years ago an asteroid about 10 km in diameter crashed into the Witwatersrand Basin at Vredefort, about 120 km southwest of Johannesburg. The Witwatersrand is part of the northern rim of the huge impact crater, thus causing the strata to dip inwards. The most significant consequence of this massive impact is that the gold-bearing layers have been preserved from erosion to provide present-day South Africa with its economic foundation.

The Vredefort Impact

The Vredefort Dome was formed only 40 million years subsequent to the Bushveld magmatic event. This is a significant circular structure that occupies the centre of the Witwatersrand Basin. It is generally interpreted as an asteroid impact structure, although it has also been interpreted as a diapiric intrusion, or an electric arc strike upwelling resulting from differentially electrically charged celestial bodies approaching close to the Earth and thus creating a large discharge between the bodies. The latter two events are much less likely to have occurred because there are features from the circular, uplifted cone that tend to strongly support an impact event. These include shatter cones, stishovite, coesite and planar deformation features (PDF) in quartz and zircon. PDFs due to shock metamorphism are well documented in quartz and other mineral grains from these sorts of impact environments (Bohor et al, 1993).

The Vredefort Dome and surrounding features were thus likely formed when the 10-km wide asteroid struck the earth in the region where the small town of Vredefort is located today.


An asteroid impact contributed to the preservation
of
gold-mineralized Witwatersrand formations

The consequent formation of the large Vredefort impact basin and its massive quantity of resulting ejecta are significant and central to the preservation of the Witwatersrand gold-bearing strata from erosion. The impact vaporised about 70 km3 of rock, and created a massive crater, the vestiges of which are clearly visible to this day. The ejecta thrown up by the impact covered the Witwatersrand Basin. This blanket of debris was so thick that it preserved the gold-bearing strata from a further 2 billion years of erosion, which is why significant gold is still found in conglomerate formations of the Witwatersrand Basin, but is rarely found elsewhere.


NASA image of the
Vredefort impact structure

 


Mangalisa property and the
Vredefort Impact Structure

Various features were used (Therriault et al, 1997) to estimate a diameter of 110 to 140 km for the transient cavity that formed during the impact and a final diameter of the impact crater of 270 to 300 km. Features such as fallback breccias, impact melt and the original morphological elements associated with an impact of this size can no longer be seen because of deep erosion. However, a series of narrow dykes such as the Vredefort granophyre remain, and these are interpreted to be remnants of impact-related melting (Therriault et al, 1997). There are many other dykes and veinlets filled with breccias close to the dome, however these are also found in the northern goldfields and therefore are interpreted to not be associated with the impact.


Vredefort Impact Structure geology

Central Rand Group mineralization

The Witwatersrand Basin formed over a period of 360 million years between 3.074 and 2.714 billion years ago. Pulses of sedimentation within the sequence and its precursors were episodic, occurring between 3.086 to 3.074 billion years ago (Dominion Group), 2.970 to 2914 billion years ago (West Rand Group) and 2.894 to 2.714 billion years ago (Central Rand Group).

Since they were laid down, the Witwatersrand Supergroup strata have undergone various stages of metamorphism, structural and thermal deformation, giving rise to major faulting and folding including over-thrusting, especially around the basin edges. Metamorphism of the Witwatersrand Basin occurred at around 2.5, 2.3 and 2 billion years ago. The first two events coincided with the progressive loading of the basin by Ventersdorp and Transvaal cover sequences, whereas the last reflects intrusion of the Bushveld Complex and/or the Vredefort impact.

The Central Rand Group rests unconformably on the West Rand Group and is more homogenous in character, generally as a result the fluvial deposition. Its thickness varies from a maximum of 2,880 m in the Vredefort area to 1,500 m in the East Rand area and 650 m in the Evander area (NE Witwatersrand Area. It comprises quartzites and conglomerates with minor shales. The group is further subdivided into the Johannesburg and Turffontein subgroups.


Typical cross-section of Witwatersrand goldfields geology

Mineralization is concentrated in the conglomerates of the Central Rand Group and is represented by a complex paragenetic sequence initiated by early accumulation of detrital heavy minerals. This was followed by three stages of remobilization caused by metamorphic fluid circulation. An early event of authigenic pyrite formation at 2.5 billion years ago was followed at 2.3 billion years ago by maturation of organic material, fluxing of hydrocarbon bearing fluids through the basin and the radiolytic fixation of bitumen around detrital uraninite. This was followed at around 2 billion years ago by peak metamorphism which resulted in the widespread redistribution of gold and the formation of a variety of secondary sulphides. Post-depositional fluid conditions were such that metal solubilities were low and precipitation mechanisms very effective, resulting in the superimposition of both primary and secondary mineralization (Robb, Meyer, 1996).

The Central Rand Group has been the most economic unit since the discovery of the Witwatersrand and has produced most of South Africa's as well as historically the world's gold. It hosts all of the big pay reefs: namely the Main, Bird, Kimberley, Elsburg, Basal, Carbon Leader, Steyn, Vaal, Leader, B, Composite and the Ventersdorp Conglomerate Reef, which marks the contact of the Witwatersrand and Ventersdorp Supergroups.

The distribution of gold is controlled mostly by primary sedimentological features within the Central Rand Group (Hayward et al, 2005). Heavy minerals such as gold and uranium were effectively concentrated in the coarse sand and gravels of large fan-delta complexes, particularly at the apices of the fans. These alluvial deposits were subsequently lithified to form conglomerate rocks, which are now are being mined for their gold and uranium. The ores are formed mainly of gold, uraninite, pyrite, pyrobitumen and brannerite, with the most common being pyrite as the rounded compact type. The gold shows similar textural variability as pyrite and uraninite and often occurs as inclusions in secondary pyrite.

Currently, the so-called modified placer theory is the preferred genetic model. This is a synthesis of the primary introduction of ore minerals into the host rocks by fluvial transport and post-sedimentary thermal overprint (Frimmel et al, 2005). Various hydrothermal activities might have been related to geological process prior to the Vredefort impact event, and as a consequence of the impact extreme temperatures of 1,000 to 1,400 degrees might have been reached within the centre of the Vredefort Dome. Temperatures of 300 degrees are associated with hydrothermal systems within a distance of 40 to 60 km to the centre of the crater. Deformed minerals display multi-directional shattering, which has affected the entire mineral collage present prior to hydrothermal mineral growth (Hayward et al, 2005).

The Witwatersrand itself remains as a prominent highly-resistant quartzite ridge running from east to west. The country's gold mines lie to the south of this ridge, having produced most of the world's gold in recorded history. The reefs are thin, but reliable, and dip at between 30 to 45 degrees to the south.

The ridge itself is a continental watershed. Streams flowing down the quartzite northern slopes, which are quite steep, are clear and fast-flowing, and thus give rise to the name Witwatersrand - the 'Ridge of White Waters'.

The geology in the Free State Goldfield and the Mangalisa area

The Free State Goldfield lies approximately 270 km from Johannesburg on the southwest rim of the Witwatersrand Basin.

The dominant structural features of the Free State Goldfield are the north-northwest striking Ventersdorp age normal faults, such as the De Bron fault, and the north-south trending syncline that developed along the western margin of the gold field (The Mineral Resources of South Africa, 1998). This syncline extends for the entire north-south span of the Free State Goldfield and is associated with several reverse faults such as the Rheedersdam fault. Compression associated with this faulting led to the existence of severely deformed, and in some places, overturned Witwatersrand sediments on the western edge of the Free State Goldfield.

The post-Witwatersrand faulting, represented by normal faults such as the De Bron and Homestead faults form the De Bron horst. This block of uplifted West Rand sediments have been thought by many in the mining industry to demarcate the eastern limit to mining in the Free State Goldfield within the so-called Target trend. The ore bodies of the Free State Goldfield occur within the Central Rand Group, and the major gold-producing unit is the Basal (or Steyn) Reef which, together with the Leader Reef, occurs throughout the goldfield. Other important economic horizons are the A, B and Eldorado Reefs.


Schematic cross-section of the Ventersdorp Supergroup across the
De Bron Horst
and the Mangalisa property, Welkom Goldfield,
showing possible settings for the Erfenis Reef

The De Bron Horst essentially divides the northern part of the Free State Goldfield into two structural domains, named the Odendaalsrus Graben to the west and forming the Arrarat Basin, and the Homestead Valley to the east. The De Bron Fault, a 2.7 billion year old extensional feature, is one of a series of sub-parallel striking faults displaying westward downthrows of up to 1,500 m.

The relative upward movement of the horst occurred after the West Rand Group was laid down, thus resulting in two mineralized trends: the Target trend, in the Arrarat Basin, and the still under-explored Homestead trend (also referred to by some as the Harmony trend) to the east of the horst (see map showing the two mineralized trends of the Free State goldfield). The Mangalisa property and Harmony Gold’s Masimong 5 mine are located on the Homestead trend. This major horst structure has a vertical displacement of about 1,500 m in the region of Bambanani, as well as a lateral shift of 4 kilometres. This lateral shift can allow a reconstruction of the ore bodies of Unisel to the west of the De Bron Horst and Merriespruit to the east, as well as in the Masimong and Mangalisa areas, as shown in the schematic cross-section of the Ventersdorp Supergroup across the De Bron Horst, above.

A number of other major faults (Stuirmanspan, Dagbreek, Arrarat and Eureka) lie parallel to the De Bron Horst. It is likely that at one time the Target and Homestead trends were one contiguous body of gold-bearing reef mineralization, and that this vertical and lateral shift of the horst pushed the eastern body of mineralization eastwards, forming the Homestead trend, which hitherto has not been extensively explored as has been the Target trend.

According to Irons et al (1991), the structural evolution of the area is a result of the following two major tectonic episodes:

1) Pre-Platberg age (compressional) faulting

Compressional faulting resulted in a thickening of West Rand group sediments. The trend of the thickened zone is south-southeast to north-northwest, paralleling the Free State Goldfield’s western margin structures. The probable reason for the thickened zone is thrusting, which is apparent from reflection character differences across low angle boundaries on the seismic sections (Irons et al, 1991).

Duplication of West Rand Group strata by the thrusting at depth has caused a gentle up-warp to develop in the overlying Central Rand Group. This up-warp has subsequently resulted in the thinning of stratigraphy towards the axis of what is now the southern De Bron Horst.

2) Syn-Platberg (extensional) faulting

Following the compressional tectonics an extensional episode occurred. A number of major normal faults exploited earlier structures and thus have a similar strike direction to the earlier northerly trending thrust structures (Irons et al, 1991).

To the west of the De Bron fault, the mines and shafts currently in operation by others such as Harmony Mines are Target, Tshepong, Phakisa, Nyala, Unisel, Bambanani and Joel. Dips are mostly towards the east, averaging 30 degrees, but become steeper as they approach the De Bron Fault. To the east of the fault lie Merriespruit 1 and Masimong. These mostly dip towards the west at 20 degrees, although Masimong is structurally complex and dips of up to 40 degrees have been measured. The Erfenis Reef Zone intercepted by Superior Mining in its two drilling programs appears to dip eastwards at about 35 degrees. Between these two blocks lies the uplifted De Bron horst block of West Rand Group sediments with no reef preserved.

Note the rapid lateral facies variations of the Platberg Group, the rotated attitude of the Klipriviersberg lavas with respect to overlying formations, and the blanketing nature of the Pniel Sequence in the schematic cross-section across the De Bron Horst.

The western margin area is bound by synclines and reverse thrust faults and is structurally complex. Towards the south and east, reefs sub-crop against overlying strata, eventually cutting out against the Karoo to the east of the Homestead trend. Harmony Gold's successful mining activities in the Homestead Trend (Masimong Mine) and the first two phases of drilling by Superior Mining in the Mangalisa property, which intercepted reefs well-mineralized with gold, strongly suggest that the Central Rand Group mineralization extends along this eastern trend in the Homestake Valley, perhaps at relatively shallower depths along the Homestead Fault.

Most of the mineral resource tends to be concentrated in reef bands located on one or two distinct unconformities. A smaller portion of the mineral resource is located on other unconformities. Mining that has taken place thus fare is mostly deep-level underground mining, exploiting the narrow, generally shallow dipping tabular reefs at depths of 2,500 m or greater from the surface.

The Basal Reef is the most common reef horizon and is mined at all of Harmony Gold’s shafts except at Target and Joel. It varies from a single pebble lag to channels more than 2 m thick. It is commonly overlain by shale, which thickens northwards. Tshepong has resorted to the undercutting of its mining panels to reduce the effect of shale dilution.

The second major reef is the Leader Reef, located 15 to 20 m above the Basal Reef. This is mined mostly at shafts to the south – Unisel and Merriespruit 1. Further north, it becomes poorly developed with erratic grades. The reef consists of multiple conglomerate units, separated by thin quartzitic zones, often up to 4 m thick. A selected mining cut on the most economic horizon is often undertaken.

The B Reef is a highly channelized ore body located 140 metres stratigraphically above the Basal Reef. Because of its erratic nature, it is only mined at Masimong and Tshepong. Within the channels, grades are excellent, but this reduces to nothing outside of the channels. Consequently, both shafts have undertaken extensive exploration to locate these pay channels.

The A Reef is also a highly channelized reef, located some 40 metres above the B Reef. This is currently only mined at Harmony 2 and Brand, although an extensive channel lies along the western margin from Nyala to Lorraine. It consists of multiple conglomerate bands of up to 4 metres thick and a selected mining cut is usually required to optimise the ore body.

Joel Mine, located 30 km south of Welkom, is the only Harmony Gold operation within the Free State Goldfield to mine the Beatrix Reef. This varies from a single-pebble lag to a multiple conglomerate, often showing mixing of the reef with some of the overlying lower grade VS5 (mixed pebble conglomerate) material. None of the other reefs are present this far south, having sub-cropped against the Beatrix Reef.

The Target operations are located at the northern extent of the Free State goldfield, some 20 km north of Welkom. The reefs currently exploited are the Elsburg-Dreyerskuil conglomerates, which form a wedge-shaped stacked package, comprising 35 separate reef horizons, often separated by quartzite beds. The Elsburg Reefs are truncated by an unconformity surface at the base of the overlying Dreyerskuil member. Below the sub-crop, the Elsburg dips steeply to the east, with dips becoming progressively shallower down-dip. Close to the sub-outcrop, the thickness of the intervening quartzites reduces, resulting in the Elsburg Reefs coalescing to form large composite reef packages that are exploited by massive mining techniques at the Target mine. The Dreyerskuil also consists of stacked reefs dipping shallowly to the east. These reefs tend to be less numerous, but laterally more extensive than the underlying Elsburg Reefs.

Northwards of Harmony Gold's Masimong 5 mine lies Superior’s Mangalisa property. It is the Central Rand Group succession sitting beneath variable thicknesses of Karoo sediments and Ventersdorp Group lavas, with its traditionally bountiful gold-bearing horizons (namely the A, B, Leader and Basal Reefs), that is, among others, being targeted in Superior Mining's exploration programs on the Mangalisa property. 

A few holes drilled in the northern Homestead trend prior to 1952 led to the view that the area was too low grade and/or too deep to be of interest. Much of the ground was locked away prior to the mineral rights being reverted to the state in 2001. In about 1985, Anglo American started drilling some 5 to 10 km north of the northern boundary of Masimong 5, and thereafter purchased the mineral rights of certain farms. In 1988, a small private company optioned three blocks of farms located between 10 and 20 km north of the Masimong 5 Mine boundary. A joint venture was established with Gencor, now part of BHP Billiton.

The first hole drilled by Gencor, referred to as hole PG-1, on the western side of the Mangalisa ground intersected a Witwatersrand reef not previously encountered in the area at 776 m below the surface. Although located at an atypically shallow depth, the reef was inferred to be an equivalent of the Beatrix reef and had a gold grade of about 50 g/t and a uranium grade of 5 kg/t over one metre. However, since Superior Mining’s phase 2 drilling was completed in 2010, it is clear that a new, hitherto unknown, reef has been discovered by the company located at a much shallower depth than is typically found in the area. 

When Superior Mining commenced the first phase of the diamond drilling it had initially been expected that drilling would be to depths of at least 1,500 m from the surface, however target mineralization was intercepted at a depth of only about 776 m. These drilling depths are unusually shallow for the Free State, but were confirmed during the Company's second phase of drilling. Phase 2 drilling into this new reef, called the Erfenis reef zone (ERZ), returned encouraging gold-uranium results, confirming the presence of high gold and uranium grades in the quartz pebble conglomerate reef.

These encouraging results have represented an important discovery for Superior Mining, highlighting the presence of significant gold and uranium mineralization at much shallower depths than that found in the majority of mines currently operating within the Witwatersrand Basin and in the Welkom Goldfield. The highest gold and uranium grades typically are contained within a thin, small pebble conglomerate band and the associated carbon seam at its base.

The strata intersected during Superior Mining's Phase 2 drilling program confirmed both the stratigraphic continuity of the ERZ and the presence of significant gold and uranium values in this zone, in particular to the south of PG-1 in borehold ERF-7. Boreholes ERF-5 and ERF-6 also intersected gold and uranium mineralization, albeit at a lower tenor. These values give a clear indication that gold and uranium concentration has occurred in the system.

The results of this drilling support an "Elsburg Fan" geological model, which had formed the basis for the exploration at Mangalisa. The stratigraphy and mineralization appears to be continuous. The gold and uranium reef discovered in PG-1 is not an isolated occurrence, as the grades in ERF-7 have confirmed.

The Erfenis reef is a classic 'small-pebble, oligomict, carbon-rich and well-sorted conglomerate' reef that is typical of the Welkom Goldfield. The general area remains prospective for other Free State gold reefs deeper in the stratigraphy, such as the 'A', 'B', Leader and Basal Reefs, which have been mined nearby at Harmony's Masimong Gold Mine.

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