ANORTHOSITE RELATED POLYMETALLIC THORIUM-URANIUM DEPOSITS IN THE NAMAQUALAND METAMORPHIC COMPLEX,

SOUTH AFRICA

M.A.G. ANDREOLI*, N.J.B. ANDERSEN, J.N. FAURIE

Atomic Energy Corporation of South Africa Ltd, Pretoria, South Africa Abstract

Since 1980 detailed geological, geophysical and geochemical investigations have been carried out in the Namaqualand Metamorphic Complex of South Africa in the search for a suitable site for the disposal of low- and intermediate-level radioactive waste. The Vaalputs site, which was ultimately selected is located 100 km south of the town of Springbok in the northwestern Cape.

During the investigation, detailed geological mapping as well as airborne radiometric and magnetic surveys were conducted.

The anomalies located by the airborne surveys were followed up on the ground by geophysical surveys and by further mapping and drilling.

The airborne surveys indicated that the basic and anorthositic intrusives gave clear radiometric and magnetic anomalies. The syntectonic granite gneisses also proved to contain anomalous concentrations of uranium and thorium. The trend of these radiometric anomalies follows the regional distribution pattern of the intrusive anorthosites.

The anorthosite-enderbite intrusives in the Vaalputs area are compared with those found in "steep structures" in the Okiep and Steenkampskraal districts and a north-south metallogenic zoning is proposed.

* Present address: Schonland Research Centre for Nuclear Science, University of the Witwatersrand, Johannesburg, South Africa.

1 INTRODUCTION

The ca. l 100 Ma. old Namaqualand Metamorphic Complex, which outcrops in the western part of South Africa, has been the subject of many geological investigations, especially since major base-metal occurrences are located near the towns of Springbok and Aggeneys (Joubert, 1986; Lombaard et al., 1986; Ryan et al., 1986;

Fig 1).

30-KAROO SEQUENCE PRE-KAROO

NAMAQUALAND METAMOSPHIC COMPLEX

•30°

PROVINCE

Figure 1 Generalized regional geology

Since 1980 an area approximately 100 km south of Okiep has come under the spotlight in a detailed geological/geophysical investigation to prove its suitability for the disposal of low-and intermediate-level radioactive waste as well as for the ultimate storage of high-level radioactive waste.- The site that

2

was eventually chosen covers an area of ca. 200 km and became known as the Vaalputs National Radioactive Waste Disposal Site.

Airborne magnetic and radiometric surveys were carried out to resolve the structure and geology in areas where the rocks were covered by younger Kalahari sands and to evaluate the mineral potential of the area. Amongst other anomalies, the airborne surveys clearly identified basic and anorthositic intrusives as bull's eye anomalies. In the Springbok area some 100 km to the north such intrusives form exploration targets for copper orebodies which in many cases also have abnormally high concentrations of radioactive elements (Andreoli and Hart, 1986a;

1986b).

Furthermore, a detailed investigation of the old monazite (copper) mine of Steenkampskraal (Andreoli, 1984; Andreoli and Hart, 1986b) revealed a genetic relationship of the orebody with enderbitic anorthosite dykes quite similar to the ore bodies of the Springbok district. It is the purpose of this study to provide a progress report on this regionally extensive type of polymetallic mineralization (that may contain up to 2 % uranium) by making use of the geophysical surveys carried out for the development of the Vaalputs radioactive waste depository. A comprehensive model of this type of mineralization forms the theme of a joint research project between the AEG and the University of the Witwatersrand and will be reported elsewhere (Andreoli et al., in prep.).

2 THE GEOLOGY OF THE RADIOACTIVE BASE-METAL DEPOSITS OF WESTERN NAMAQUALAND

2 .1 Regional Setting

The Namaqualand Metamorphic Complex (NMC) includes metasedimentary, metavolcanic and intrusive rock units which are predominantly gneissic in character. This complex is bounded in the east by the Archaean Kaapvaal Province, by younger cover rocks in the south and by the Atlantic coastline in the west (Fig 1).

TABLE 1 LITHOSTRATIGRAPHIC SUBDIVISION OF THE NAMAQUALAND METAMORPHIC COMPLEX IN THE VAALPUTS AREA

Suite Lithology Syntectonic Intrusive

Suites:

Koperberg Spektakel Hoogoor Little Namaqualand

Gladkop

Hypersthenite, norite, anorthosite, diorite, K-rich garnetiferous leucogranite, alaskite Pink, quartzo-feldspathic gneiss

Biotite-rich granitic augen gneiss

Fine-grained grey gneiss, granodioritic, megacrystic in places.

Pretectonic Supracrustals

Okiep Group Biotite/quartzo-feldspathic gneisses; magnetite and/or feldspathic quartzite; sillimanite-rich schists

TABLE 2 GEOLOGICAL HISTORY OF THE VAALPUTS AREA

Event Age (Ma)*

Erosion, uplift 120-65 Development of NW plunging open folds with 1 000 emplacement of pegmatites

Anorthosite-norite Koperberg Suite intrusions 1 070 + 20 and mineralization with development of steep

structures

Disturbance of Rb-Sr isotope system by oxygen-rich fluids (?)**

Emplacement of syntectonic granite suite during 1 166 + 26 E-W open folding

Granullte facies metamorphism (P ±5 K bar,

T ±800 °C), accompanying fecumbent folding 1 213 + 22 Deposition of metasediments, banded ironstones

over older basement (not recognized) 1 500

* Joubert (1986)

** Andreoli (in preparation)

The lithostratigraphic subdivision of the NMC in the area under consideration is presented in Table 1. In addition, pétrographie observations around Vaalputs and Springbok, together with pertinent geochronological data, provided a model for the evolution of the complex in the area under investigation that is presented in Table 2. (Hart and Andreoli, 1985).

2.2 Morphology of the Ore Bodies

The predominantly irregular dyke-like form of occurrence of the Koperberg Suite, suggested by its outcrop pattern, has been confirmed by mapping, diamond drilling and mining activities in a large area of the NMC from Springbok to Steenkampskraal.

Plug-like bodies are much less common and sill-like occurrences are rare. The width of most dykes falls in the 50 m to 100 m range and continuous length seldom exceeds 1 km. A prominent feature of the distribution pattern is the arrangement of discrete bodies in linear zones.

These easterly-trending, often irregularly pinching and swelling, branching and coalescing dyke-like bodies, some of which are ore-bearing, have a predominantly near-vertical to steep northerly dip throughout the region. The maximum vertical extent over which a steep branching dyke zone has been traced continuously is 1 575 m, basic rock still extending below this depth. In contrast, many bodies appear to pinch out completely. Results of diamond drilling and mining activities have, however, substantiated that at some places a subvertical dyke may narrow downward along an extremely flat plungeline before continuing downward at a steep plunge to greater depth. Examples of this were identified at the Hoits, Koperberg, Okiep and Steenkampskraal mines (see Pike, 1958 for the last-mentioned locality).