Geology of Steenkampskraal, Namaqualand, South Africa

M. Knoper¹, M. Andreoli², L. Ashwal¹

¹Dept. of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa

²Atomic Energy Coporation, P.O. Box 582, Pretoria 0001, South Africa; and
Schonland Research Centre, BPI Building, University of the Witswatersrand, P.O. Box 3, Wits 2050, South Africa

Introduction

The rocks of the Namaqualand Metamorphic Complex (NMC) [1] are part of a largely Mesoproterozoic orogenic belt that bounds the western margin of the Kaapvaal craton. Although significant progress has been made toward delineating precise ages [2,3,4,5], structural features [6], and P-T-t paths [7] of rock units within the western NMC, additional studies focused on structural geometry and kinematics, metamorphism, as well as deformation ages are necessary to further develop tectonic models for this orogen. One such ongoing study is of rocks exposed in the southwestern NMC at Steenkampskraal, a region noted for low-P, high-T granulite gneisses [7,8], contrasting structural fabrics that are distinctly bracketed by different rock units, and intrusive rocks hosting a monazite orebody associated with a cuspate antiform [9]. Steenkampskraal is also known for a monazite orebody considered as a natural analog of a high-level radioactive waste depository.

Rock Units

At least five mappable rock units occur at Steenkampskraal (Table 1). The oldest unit is a megacrystic gneiss characterized by microperthite + quartz + plagioclase + biotite + garnet. Microperthite megacrysts (up to 1 cm) in the megacrystic gneiss are surrounded by a matrix of feldspar + quartz + biotite. Garnet porphyroblasts (0.4-1.5 cm) are commonly concentrated in quartzo-feldspathic segregations that occur as outcrop-scale sheets, veins, and lenses within the megacrystic gneiss. Intruding the megacrystic gneiss is a leucogranite also containing microperthite + quartz + plagioclase + garnet but with little or no biotite. The intrusive relationship of the leucogranite with quartzo-feldspathic segregations occurring in the megacrystic gneiss is equivocal. Intruding the granite-gneisses is an igneous suite most commonly characterized by leucotonalite, leuconorite, leucodiorite, quartz-bearing anothosite, but also including minor charnockite, enderbite, monzonite, and quartz syenite. These intrusions (informally named the Roodewal suite [9]) invariably host segregations of monazite-rich ore, and appear to locally charnockitize the older granite-gneisses. Postdating the granite-gneisses and the Roodewal suite are narrow (1-3 cm), subvertical diorite and charnockite veins that also locally charockitize the granite-gneisses. Unconformably overlying the crystalline rocks is the Neoproterozoic-Cambrian Nama Group; the unconformity in places is an irregular palaeotopographic surface where rounded to angular, randomly oriented granite-gneiss (palaeo) boulders are surrounded by unstrained quartzite.

Structure

All structural fabrics at Steenkampskraal overprint the megacrystic gneiss and the leucogranite. The earliest fabric is a pervasive, subhorizontal gneissic foliation best characterized by the quartz + feldspar + biotite matrix surrounding microperthite megacrysts in the megacrystic gneiss and to a lesser extent in the leucogranite. This gneissic foliation strikes between NE-SW to E-W, and regionally dips about 20-30° northwards. Overprinting this foliation is a complex collection of structural fabrics; some of the fabrics are associated with an upright and distinctively cuspate antiform that folds the gneissic foliation and results in heterogeneously strained granite-gneisses. One such fabric is an E-W striking cleavage with a subvertical to 50-60° southerly dip. Intrusive rocks of the Roodewal suite are observed in and immediately near the axial plane of the cuspate antiform, and although locally strained, most intrusions commonly preserve primary igneous textures. The steep, narrow diorite and charnockitic veins appear associated with a weakly developed, NW-SE striking cleavage that overprints the earlier gneissic and cuspate antiformal fabrics. Late, brittle, subvertical, N-S striking faults displace all rock units including the Neoproterozoic-Cambrian Nama Group.

Monazite Orebody

The Steenkampskraal orebody is exposed over 200 m on the surface as an E-W striking, 0.2-4 m wide band of monazite ore. The orebody dips 40-60° S and is hosted by dykes of the Roodewal suite that intrude immediately south (0.5-5 m) of the cuspate antiformal closure. Based on drilling and underground exposures, the orebody has a known downdip extension of approximately 350 m. Sampling a large variety of rocks from underground and the mine dump suggests the presence of four types of monazite ores, all with gradational relationships: (1) phosphate-rich ore (most common) characterized by monazite + apatite + chalcopyrite + zircon, (2) oxide-rich ore characterized by magnetite + green spinel + zircon ± monazite ± sulphides, (3) quartzo-feldspathic ore characterized by plagioclase + quartz + monazite + biotite, and (4) siliceous ore characterized by quartz + monazite + zircon + magnetite. The quartzo-feldspathic ore grades into monazite-bearing intrusive rocks of the Roodewal suite, whereas the siliceous ore grades into segregations of dark quartz (up to 50 cm thick) frequently developed at the contact between quartz tonalite (Roodewal suite) and phosphate-rich ore. Monazite is also a common accessory in orthopyroxene-bearing intrusive rocks of the Roodewal suite.

References

[1] Joubert, P., 1981, in: Hunter, D.R., (Ed.), Dev. Precambr. Geol., Elsevier, 2, 671-705. [2] Clifford, T.N., Barton, E.S., Retief, E.A., Rex, D.C., & Fanning, C.M., 1995, J. Petrol., 36, 231-258. [3] Ashwal, L.D., Andreoli, M.A.G., Page, M., Armstrong, R.A., & Tucker, R.D., 1997, this vol. [4] Gibson, R.L., Robb, L.J., Kisters, A.F.M., & Cawthorne, R.G., 1996, S. Afr. J. Geol., 99, 107-120. [5] Robb, L.J., 1997, work in progress. [6] Kisters, A.F.M., Charlesworth, E.G., Gibson, R.L., and Anhaeusser, C.R., 1996, J. Struct. Geol., 18, 735-751. [7] Waters, D.J., 1990, in: Daly, J.S., Cliff, R.A., & Yardley, B.W.D., (Eds.), Spec. Pub. Geol. Soc., 43, 269-275. [7] Nowicki, T.E., Frimmel, H.E., and Waters, D.J., 1995, S. Afr. J. Geol., 98, 191-201. [8] Albat, H.-M., 1984, Bull. Precambr. Res. Unit, U.C.T., 33, 384 pp. [9] Andreoli, M.A.G., Smith, C.B., Watkeys, M., Moore, J.M., Ashwal, L.D., & Hart, R.J., 1994, Econ. Geol., 89, 994-1016.

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·Web publication date: February 3, 1997·

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