geophysical techniques [6]. The southwestern end is more diffuse and little is known about the structural geology of this part of Mozambique.
Figure 3 shows a simplified geological map of the study area, drawn from published maps and an interpretation of air photographs [8]. It shows that the undifferentiated gneisses contain irregular bodies of charnockitic gneiss. Further subdivision was not possible due to poor exposure and extensive cover of residual soils. However the map does show major trends within the area. In the northwestern part of the area ENE trends predominate while in the south eastern part trends are variable apart from one rectilinear zone of WNW foliations.
Banding and foliation are absent in areas of charnockitic gneiss but become progressively more common towards their margins.
35*3O'E
2Okm
3.
CharnockifiC gneiss I—I Gneiss
^* Fault
•*& Foliation t r a c e s
Figure 3: Simplified geological map of the study area.
LANDSAT IMAGERY
Landsat imagery was obtained in 1981 for the major part of Mozambique for the original work [4]. Part of the Landsat scene covered by Path 178 Row 12 acquired on the 29th August 1979 was processed using MSS channels 7)5 and 4, on the Hunting
Image Processing and Analysis System (HIPAS). Initially the data were 'destriped' to remove sixth line banding and geometric corrections were applied to rotate the image to true north and
adjust it to fit the latitude-longitude projection. During this process the pixel size was enlarged from 79 m x 57 m to 50 m x 50 m [9,10]. The channels were then contrast-stretched with
channel 7 in red, channel 5 in green and channel 4 in blue, to give an enhanced false colour composite image. This digital image was converted to a film product using a Color Fire 240 film writer and from the diapositive an enlarged colour print at 1:250,000 scale was produced.Geological interpretation was carried out on a clear film overlay (Figure 4). Three major geological units were defined on the basis of the surface relief and colour. The first forms the west and north part of the image and represents areas of high relief. The second in the eastern part of the image has low relief and abundant vegetation. The last unit represents the undifferentiated bulk of the image. From
comparison with Figure 3 it is likely that Unit (1) is of similar composition to charnockitic gneiss. The inability to
ascribe lithologies to the other units is due to the lack of field data.17"15'S
ÜSV?J| Unit 1:High relief (charnockitic gneiss) ^^ Fault
[yjj Unit 2: Low relief, healthy vegetation ^^ Foliation traces
| | Unit 3; Undifferentiated gneiss 0 15km
Figure 4: Geological interpretation of the Landsat image.
Structurally most of the faults lie in a WNW direction cutting the main foliation trends, which run ENE to WSW. These trends are parallel to those of the Lurio Belt further east even though the study area lies to the south of the southern limit of the Belt as it is usually mapped [6].
4. GEOPHYSICAL DATA
4.1 Aeromagnetic Data
Aeromagnetic data, primarily collected to study basement changes across the area [11,12], were produced as a colour contour map at 1:250,000 scale. The map showed a striking ENE-WSW trend which predominates in the northwestern half of the
area cutting across it diagonally. The strong aeromagnetic
anomalies suggest that the lithological units have well defined banding. In the southeastern half the trends virtually disappear and magnetic gradients are more gentle. The two sections of the area thus appear to be structurally different.Their boundary represents a structural and lithological discontinuity and has been interpreted as a junction between rocks thrust from the northwest across the more competent block in the southeast.
4.2 Spectrometric Data
Spectrometric data were recorded for the gamma-ray spectrum as one wide and three narrow energy windows (total count, uranium, thorium and potassium channels respectively) [11,12,13,14]. After processing or stripping, colour contour and density sliced maps were produced for each of the channels.
Additional processing included production of the uranium/thorium and uranium/potassium channel-ratios and composite images
[15,16].
Individual channels showed some significant features [16] but colour composite images were found to be overall more useful. In each individual image white represents the highest value and black the lowest, so that when a colour is assigned to each channel the resulting colour composite provides a range of colours representing the combined assemblages of three images.
The most useful combination was found to be the potassium composite image. This image comprised the K/U ratio values in red,the K/Th ratio values in green and the potassium channel values in blue.
Film diapositives of the composite images and the single channel colour density sliced images were produced and enlarged to colour prints at 1:250,000 scale. Detailed interpretation of the composite images was carried out and is given in a paper by
Nevitt and Barr, 1983 [lj>] . A simplified reinterpretation to bring out the main features was carried out for this study
(Figure 5).
35°30'E 36'E
17'S
!7'15 S P7] 1. Mid K, low U ÄTh zone
|'"M-;:J 2 High K 4 U, low Th zone j j 3 High K, low U zone I'l'lill 4 Low K zone
^f"^ Linear feature
^r Boundary b e t w e e n zones A & B 15km
F i g u r e S : I n t e r p r e t a t i o n from the potassium composite i m a g e .
Interpretation concentrated on the potassium composite image as it was felt that mineral and rock changes would be best discriminated by changes in potassium rather than uranium and thorium. Interpretation was carried out on the basis of colour changes and relative deviations of the elements from their mean values. Geological significance was only given to features
which had previously been identified on the geological map [8].
Initial study of the potassium composite image suggested that the area could be roughly divided into two zones. The northwestern half (A) showing predominantly yellows and oranges while the southeastern half (B) had more purple and pink colours. This shows that in the southeast potassium levels and the K/U ratios are high but K/Th ratios are low in relation to mean values over the area. Thus the rocks appear to have relatively high levels of potassium and the positive deviation from the mean is higher for thorium than uranium. In the northwestern half the ratios K/U and K/Th are both fairly high