THE USE OF REMOTE SENSING IN IRON ORE EXPLORATION IN THE NORTHERN CAPE PROVINCE OF THE REPUBLIC OF

SOUTH AFRICA

C.J Mouton Kumba Resources

Pretoria South Africa

Riaan.Mouton@kumbaresources.com

Abstract - The well known Sishen iron ore deposit occurs at the northern tip of the Maremane anticline, and is situated near Kathu in the Northern Cape Province of the Republic of South Africa. Numerous small iron ore occurrences are found along the western edge of the dome, covering a distance of approximately 80km to the south. The entire area along the Western edge of the dome thus became a prime exploration target. The landscape is typical of the Kalahari region with undulating topography and ranges of relatively low mountains. In the immediate vicinity of the iron ore deposits thick calcrete (limestone) or sand cover prevails with very little outcrop. A regional approach has been followed starting with the interpretation of Landsat 5 data over the area. Known hematite outcrops served to “calibrate” the TM data and identify possible targets for geophysical and geological follow-up surveys. Other uses for the TM data included the studying of drainage patterns and delineation of structures. Airborne magnetic data were added to delineate structures and to verify the optically mapped geology. Possible iron ore targets along the north-south striking thrust zones were identified. The interpretation of the remote sensed data, combined with gravity survey results, including that obtained with the BHP’s Falcon airborne gravity system, has made a major contribution to the delineation of a new iron ore deposit, now known as the Sishen-South Deposit.

I. INTRODUCTION The project area is situated in the Northern Cape Province and includes the Sishen Iron Ore Mine and the Sishen South Iron Ore Exploration Project near Kathu and Postmasburg respectively. The area is characterised by typical Kalahari landscape with flat to slightly undulating topography. Ranges of relatively low mountains occur dispersed through the region, like the Kuruman Hills to the north-east and the Langeberge to the west. The topography around the known iron ore deposits is slightly undulating with thick calcrete (limestone) cover and very little soil cover. Two prominent geological features believing to have a close relationship with the known iron ore deposits in the Northern Cape Province, are the Dimoten syncline and the Maremane anticline.

Map 1: Location of the Sishen and Sishen-South Project Areas

The Sishen-South deposit is situated just south of outcrops of dolomite and banded iron formation, which define the Maremane dome, between Sishen and Postmasburg. Large-scale deformation due to thrust faulting resulted in open north-south plunging, anticlines and synclines. The deposit comprises a basal carbonate platform sequence (mainly dolomites) of the Campbell Rand Subgroup, upon which the banded iron formation of the Asbestos Hills Subgroup has been deposited. The dolomites are separated from the banded iron formation by a residual solution breccia referred to as the ‘Manganese Marker’ or ‘Wolhaarkop Breccia’. The regional classification of the geology follows in Table 1. Table 1: Geological Classification Grouping Sub-

Group Formation Lithological Unit

Kalahari Group

- - calcretes, dolocretes, clays, pebble layers

Karoo Super Group

- Dwyka tillite, shale

-

Ongeluk lavas, banded jasper

- Makganyene diamictite, jasper, mudstone

Posmasburg Group Gamagara Sishen quartzites,

conglomerates Asbestos Hills

Kuruman banded ironstone, jasper

Ghaap Group Campbell

Rand Ghaap Plateau

dolomite

The lithology represents a period of uplift and erosion, that also led to the development of deep sinkholes within the basal dolomites. Clastic-textured iron ore (massive ore) and laminated iron ore has been preserved as supergene-enriched orebodies in synclinal and graben structures. These ore types belong to the Kuruman and Danielskuil Formations of the Asbestos Hills Subgroup. Collapse breccia ore (brecciated laminated ore) is preserved within deep sinkhole structures and conglomeratic ore is preserved as lensoid channel and slope sediments (Gamagara clastics), which fill the grabens and synclines above the erosional unconformity. Very little outcrop of hematite ore is present and calcrete cover prevails over most of the project area. Map 2: Regional Geology of the Project Area

II. REGIONAL INTERPRETATION OF LANDSAT

IMAGERY a) Vegetation Occurrences Standard Abrams and NDVI algorithms were used to map vegetation occurrences. Although this specific Landsat TM scene was taken during the raining season, the scares vegetation of the Postmasburg – Sishen area are barely visible. Only thickenings of invader bush are clearly visible in some places, and these can be argued to correlate with hematite outcrop.

Map 3: Landsat TM Scene over the Project Area (Bands 3,2,1)

b) Soil Type (Map3) Red-brown irregular areas in the central and southern part (B1; B2) of the scene indicate soil associated with andesitic lava, agglomerate, chert and red jasper in comparison with geological maps. The red-brown reflectance may be due to higher iron content within the Kalahari soils originating from banded ironstone. Light brown to yellow-brown soil to the west is associated with quarzitic wind-blown sands. A mixture of soil types as a result of the weathering of dolomitic limestone, chert and limestone or undefined overburden (D1; D2) was observed as brown to light gray colour shades. In general, bright (almost white) areas on the image are good examples of total reflectance of all incident radiation. These areas coincide with local drainage patterns (F2) and plains. The high reflectance is also an indication of low iron alteration occurrences (F4) and map areas covered with surface limestone. c) Lithology and Structure (Map4) The Landsat TM band combination (b5,b3,b1) were used for contrast stretching to spot lithology features. Except for extensive folding propagating from north-west to the centre of the region, little lithological changes were detected. The main strike direction of lithological occurrences seems to be in semi-circular “wavelets” as if propagating from the west onwards to the east. This strengthens the idea of thrust faulting in a west-east direction. An east-west striking watershed also provides information on the central axis of the Maremane Dome at present. Prominent features identified as shear zones were mapped. Some of these features showed good correlation with anomalies observed on regional airborne magnetic data. Some of the features delineated from Landsat data do not appear on the regional magnetic data, but this is probably due to the poor resolution of the magnetic data (1km line spacing).

Map 4: Enhanced Landsat TM Data showing structural and lithological interpretation

Table1: Legend Description for Maps 1 to 3 Legend Item Description solid blue lines faults solid black lines dykes solid orange lines over-thrust solid purple lines dunes (duplicating folds) solid red lines folds solid black lines (thick) watershed solid green lines drainage patterns dotted red lines vegetation red squares target areas Landsat TM data sets are in general not sufficient for delineating structural anomalies and a set of airphotos are used to enhance resolution. However, on a regional scale, delineation of lineaments takes place through mapping linear patterns and changes in colour tones. To distinguish between different types of lineaments, linearity, extent and broadness of the features, were used as selection criteria. Shear zones with strike N - S, NW – SE, SW – NE and faults striking SW – NE, E – W were identified. Less prominent faults with random strike, that become subtle to the center and south-western parts of the region of interest were mapped. This may provide useful information regarding the time of faulting and the age of lithological units and overburden. The reason for vanishing of the faults also relate to a relatively mobile overburden. According to definition, dykes are discordant tabular plutons, that can force open pre-existing fractures, but more often, their channels follow cracks opened by the pressure of magmatic injection (Siever, 1986). When country rock is deformed by intrusion, thrusting or faulting, many cracks are formed, all of them possible channels for the invading magma. This is the reason why dykes rarely occur alone, but are rather in large numbers in a region (Cox, 1979). The project is no exception to the rule and many dykes can be traced over large distances on the Landsat imagery. These dykes strike mainly N-NE to S-SW and N-NW to S-SE. Due to the overburden, many of the existing

dykes are invisible, and are mapped more effectively, using the magnetic data. However, it appears as if faults and dykes have similar positions in some areas showing close interaction between these two structural feature by the time of origin. This phenomenon is visible in the north-east of the project area and in the central Marmane dome region. An unsupervised classification of the Landsat TM data has done. The result is remarkable resemblance of the geology as displayed by Map 5. Map 5: Classified Landsat TM Data over the Project Area

III) REGIONAL INTERPRETATION OF AIRBORNE MAGNETIC DATA a) Introduction A regional airborne magnetic grid, covering the same area as the Landsat TM data, was produced using total field magnetic data flown by the Council for Geoscience of South Africa. Although the data quality is poor (e.g. low resolution, poor leveling), it contains valuable information and was used with great success in delineating regional geological and structural features. b) Geological Units (Map 6) Various magnetic units were identified and although these units are not representing all geological occurrences, the main lithological units could be mapped. Lithological units, with a low magnetic response, which lies above or below units with strong magnetic responses, do not have identifiable signatures. Unit F1 consists of quartzitic that result in a low but homogeneous magnetic response. Neighbouring this lithological unit to the east is a quartzite Unit F2 with nodules and lenses of hematite with magnetic response parallel to the strike of the Langeberge of which it forms part. In the central - west of this anomalous belt, large-scale deformation (G1) took place. Circular features probable marked a period of volcanic activity. The kidney shape of the geological Unit F3 is prominent on the magnetic data and Landsat TM imagery. Since the magnetic data provides depth information, other than the Landsat TM data, the conclusion drawn is that this is a solid lithological unit that is

divided into two portions (G2 and G3) that may be due two consecutive over-thrust folds. The illusion of non-continuity (on Map2) is created between the markers G2 and G3 due to

Map 8: High Resolution Gravity Data Over The Sishen South Project Area

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New York; W. H. Freeman and Company; 1982. 2) Cox, K. G., Bell, J. D. and Pankhurst, R. J.; The

Interpretation of Igneous Rocks.; London; Allen and Unwin; 1979.

3) Lipton, G.; Unpublished; 1997. 4) Siever, I. R., Press, F.; Earth; 4th Ed; New York; W.

H. Freeman and Company; 1986. 5) Sheriff, R. E.; Encyclopedic Dictionary of

Exploration Geophysics; Tulsa; Society of Geophysicists; 1991.