gis-based predictive mapping for aquamarine-bearing ......the zambian copper deposits are being...

GIS-based predictive mapping for aquamarine-bearing pegmatites,

Lundazi area, northeast Zambia. Ezekiah Mweetwa Chikambwe September 2002

GIS-based predictive mapping for aquamarine-bearing pegmatites,

Lundazi area, northeast Zambia.

by

Ezekiah Mweetwa Chikambwe

Thesis submitted to the International Institute for Geo-information Science and Earth Observation in partial fulfillment of the requirements for the degree in Master of Science in Mineral Resources Exploration and Evaluation. Degree Assessment Board Thesis advisers Dr. E. J. M. Carranza Drs. J. B. de Smeth Thesis examiners Prof. Dr. F.v.d. Meer (ITC, TUD), Chairman, External Examiner Dr. T. Woldai (ITC), Member Drs. J.B. de Smeth (ITC), Member Dr. E.J.M. Carranza, (Member)

INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION

ENSCHEDE, THE NETHERLANDS

Disclaimer This document describes work undertaken as part of the study at the International Institute for Geo-information Science and Earth Observations (ITC). All views and opinion expressed herein do not necessarily represent those of the Institute and remain the sole responsibility of the author.

Abstract The Irumide belt part of Zambia is endowed with gemstones. These gemstones, aquamarine in particular, have shown to be a potential contributor to the socio-economic growth of Zambia. However, there is a lack of exploration criteria to guide the search for potential areas of aquamarine-bearing rocks to sustain the mineral industry in the country. In response to this need, this study was conducted in the eastern part of the Irumide belt (Lundazi area) in the eastern part of Zambia to determine spatial relationships between the aquamarine-bearing pegmatites and the indicative geological features in the area in order to define guidelines useful for selection of potential areas for further exploration work. Based on the geological characteristics of areas with known aquamarine-bearing pegmatities in Lundazi, several geological features were thought to be indicative of areas with potential for the occurrence of aquamarine-bearing pegmatites. To determine which of the ‘indicative’ geological features are important spatial predictors of areas with potential for the occurrence of aquamarine-bearing pegmatites, spatial analysis was undertaken. Quantifying the spatial association between the ‘indicative’ geological features and training set of locations of known aquamarine workings were done through weights of evidence modeling method. The application of weights of evidence modeling has shown that mapped axial fold traces with NE trend and mapped shear zones have negative spatial association with training set of known aquamarine workings and that the >75th percentile Pc2 scores of the spatially interpolated stream sediments data generally show negative or non-positive spatial association with the training set of aquamarine workings. Metagranites, lineaments (fractures), and circular features (representing late granites) were found to be positively spatially associated with known aquamarine workings. Predictive modeling of areas with potential for occurrence of aquamarine bearing pegmatites was undertaken through weights of evidence modeling and fuzzy logic approach based on ‘indicative’ geological features with positive spatial associations with the known aquamarine workins. The best predictive map through weights of evidence modeling demarcates about 36% of the area as potential zones for aquamarine-bearing pegmatites and delineates correctly at least 73% of the known aquamarine workings. The best predictive map through fuzzy logic approach outlines about 29% of the area as potential zones for aquamarine-bearing pegmatites and delineates correctly at least 57% of the known aquamarine workings. The optimal fuzzy predictive map was considered more adequate for directing future exploration work for aquamarine-bearing pegmatites in the Lundazi area because it is generated from three indicative geological features and has smaller percentage of delineatded potential zones for aquamarine-bearing pegmatite occurrences. However, it it should be treated with caution because of its low prediction rates. Both the best predictive map generated by weights of evidence modeling and the best predictive map generated by fuzzy logic approach do not predict the known aquamarine workings in the northern part of the area. These aquamarine workings not delineated correclty by the optimal predictive maps lie within 6000m of mapped granitic gneisses, which could be metagranites. This finding implies that highly accurate geological maps and standardized lithological nomenclature are needed and important predictive modeling of mineral potential.

Acknowledgement I would like to thank the Netherlands Fellowship Program (NPF) for awarding me the scholarship to pursue further training and immensely improve my professional understanding and judgement. I also thank the Geological Survey Department of Zambia for allowing me to pursue this study. To Dr. John Carranza, my supervisor, I would like to express my gratitude to you for your guidance, constructive criticism, invaluable suggestions and, last but not least, your ever-in-time critical reading of the manuscripts. Your constructive criticism has greatly improved the quality of this dissertation. I would also like to express my sincere gratitude to Prof. Dr. Martin Hale for his guidance. Thank you. My sincere gratitude go to Drs. Boudewijn de Smeth the students adviser for his critical review of my manuscript and guidance during my stay in the Netherlands. I particularly thank you for my first day in ITC. My special thanks go Drs. Frank van Reitenbeek for being available and standing ready to help whenever I needed help. My special thanks also go to Prof. Dr. Colin Reeves, Dr. Sally Barritt and Dr. Jean Roy for their various contributions during interpretation of the geophysical data. My special thanks go to ITC and Dish hotel employees and general Dutch citizens for their hospitality that made my stay in Netherlands a success. To my wife Rita and my children Chibotu, Miyoba and Chiyavwula, if this turns out to be an achievement it is for you. I thank you for your support and appreciate your sacrifices. Finally, I would like to thank the almighty God to whom I owe all.

vi

Contents Abstract ........................................................................................................................................ v Chapter 1: Background to the Research ........................................................................................... 1

1.1 Research Problem .................................................................................................................... 1 1.1.1 Mining dependent economy of Zambia ................................................................... 1 1.1.2 Aquamarine gemstones: potential contributor to the economy of Zambia ............. 1 1.1.3 A need for an exploration model for aquamarine-bearing granitic-pegmatites ................................................................................................. 2

1.2 Rationale of the Research ........................................................................................................ 2 1.3 Objectives of the Research....................................................................................................... 3 1.4 Research Hypotheses ............................................................................................................... 3 1.5 Research Methodology ............................................................................................................ 3 1.6 Geodata Sets Available to the Research................................................................................... 3

1.6.1 Locations of mine workings .................................................................................... 3 1.6.2 Geological map....................................................................................................... 3 1.6.3 Radiometric data .................................................................................................... 5 1.6.4 Remotely-sensed spectral data................................................................................ 5 1.6.5 Geochemical data ................................................................................................... 5

1.7 Conclusion ............................................................................................................................... 5 Chapter 2: The Study Area................................................................................................................... 6

2.1 Location ................................................................................................................................... 6 2.2 General Geology of Zambia..................................................................................................... 6

2.2.1 Stratigraphy ............................................................................................................ 6 2.2.2 Structure and Metamorphism ................................................................................. 8

2.3 Local Geology of Lundazi Area............................................................................................. 10 2.3.1 Lithology............................................................................................................... 10 2.3.2 Structure and metamorphism................................................................................ 11

2.4 Exploration History of Lundazi Area..................................................................................... 13 2.5 Conclusion ............................................................................................................................. 14

Chapter 3: The Geology and Exploration of (Aquamarine-Bearing) Granitic-Pegmatites 15

3.1 Aquamarine-Bearing Granitic-Pegmatites in General ........................................................... 15 3.1.1 Introduction .......................................................................................................... 15 3.1.2 Geological and structural settings........................................................................ 15 3.1.3 Classification of granitic-pegmatites.................................................................... 16 3.1.4 Regional scale exploration criteria for granitic-pegmatites................................. 17

3.2 The Aquamarine-Bearing Pegmatite Belt of Lundazi Area ................................................... 17 3.2.1 Geological and structural setting of granitic-pegmatite belt................................ 17 3.2.2 Classification of aquamarine-bearing granitic-pegmatites of Lundazi area ..................................................................................................... 18

3.3 Possible Genetic Model of Aquamarine-Bearing Pegmatites of the Lundazi Area ............................................................................................................... 19 3.4 General Characteristics of Geological Environments of (Aquamarine-Bearing) Granitic-Pegmatites .......................................................................... 21 3.5 GIS-Based Geological Exploration for Granitic-Pegmatites ................................................. 22 3.6 Conclusion ............................................................................................................................. 22

Chapter 4: Extraction of Spatial Indicative Features .................................................................. 24

4.1 Structures and lithological units extracted from geological map ........................................... 24 4.1.1 Lithological units .................................................................................................. 24 4.1.2 Faults .................................................................................................................... 25 4.1.3 Shear zones ........................................................................................................... 25 4.1.4 Axial traces of folds .............................................................................................. 25

4.2 Structural features extracted from ASTER imagery .............................................................. 27 4.2.1 General ................................................................................................................. 27

Contents vii

4.2.2 Extraction of lineaments ....................................................................................... 28 4.2.3 Extraction of shear zones...................................................................................... 30 4.2.4 Interpretation of circular features ........................................................................ 31 4.2.5 Extraction of alteration zones............................................................................... 33 4.2.6 Extraction of silicic rocks ..................................................................................... 34

4.3 Spatial indicative features from stream sediments geochemical data .................................... 36 4.3.1 Uni-element analysis ............................................................................................ 36 4.3.2 Correlation matrix of background geochemical data........................................... 38 4.3.3 Spatial distribution of the geochemical data ........................................................ 39 4.3.4 Extraction of a multi-element signature indicative of granitic areas ................... 46

4.4 Geochemical evudence from radiometric data....................................................................... 49 4.5 Conclusions............................................................................................................................ 50

Chapter 5: Spatial Data Analysis and Integration........................................................................ 52

5.1 Introduction............................................................................................................................ 52 5.2 Analysis using weights of evidence method .......................................................................... 52

5.2.1 Creating binary predictor patterns....................................................................... 52 5.2.2 Combining binary predictor patterns ................................................................... 61 5.2.3 Validation of predictive maps ............................................................................... 63

5.3 Analysis by fuzzy logic method............................................................................................. 66 5.3.1 Creating fuzzy predictive patterns ........................................................................ 68 5.3.2 Combining fuzzy predictor patterns...................................................................... 69 5.3.3 Validation of fuzzy predictive map........................................................................ 70

5.4 Conclusion ............................................................................................................................. 72 Chapter-6: Conclusions and Recommendations............................................................................ 74 6.1 Conclusions ............................................................................................................................ 74 6.2 Recommendations .................................................................................................................. 75 References ............................................................................................................................................... 72

Chapter 1: Background to the Research 1.1 Research Problem 1.1.1 Mining dependent economy of Zambia It is every government’s wish, especially in developing countries, to explore and exploit its natural resources to provide economic and social development for its people. Socio-economic development, however, is hampered due to under-utilization of their natural resources mostly because of lack of sufficient and appropriate geosciences data. Another factor that hampers socio-economic development is giving much emphasis to one mineral commodity as a source of revenue and give less significance to other potential resources. A real case example is Zambia, whose economy depends mainly on one mineral commodity, copper. The Zambian copper deposits are being depleted and the mines are closing due high costs of re-capitalization. Kabwe zinc-lead and the Luanshya copper mines closed in the early 1990 and in 2000, respectively, due to high re-capitalization demands. There are also other large copper mines on the verge of collapse (e.g., the Konkola Copper Mine). In order to sustain Zambia’s socio-economic growth, there is need to explore and exploit new mineral deposits, although not necessarily copper. 1.1.2 Aquamarine gemstones: potential contributor to the economy of Zambia Gemstones have shown to be a potential contributor to the socio-economic growth of Zambia in the 1970’s after the discovery of emeralds in the Copperbelt province. Small-scale mining for aquamarine, tourmaline and red garnet from granitic-pegmatites in Lundazi area in the Eastern province of Zambia started in the late 1970’s. The pegmatites were exploited for mica as early as 1930’s. Recoveries of 19 tonnes of beryl were recorded in 1955. In 1960 one pit, the Aries pit, produced between 40 and 50 tonnes of beryl (some of which are of gem quality), small crystals of excellent amethyst, rose-quartz, yellow-green chrysoberyl of up to 0.6m, tourmaline and uranium minerals that include uranophane, torbenite, and rutherfordite in addition to muscovite production (O’Connor, 1997). Prospecting for aquamarine-bearing granitic-pegmatites in the Lundazi area was started en masse by villagers who became aware that fortunes could be made by digging after the discovery of emeralds in the Copperbelt province (Tether and Partney, 1988). Prospecting for gemstones then took place in most parts of the country and led to the discovery of other types of gemstones. Amethyst was discovered in Southern and Central provinces of the country while aquamarine, garnet, pink, and green tourmaline was discovered in the central part of the country. Most of the discoveries were made by villagers who were guided by schorl and quartz, both in outcrops and in floats. In 1984 and 1985, the Geological Survey Department of Zambia realized that gemstones were a national asset and undertook field investigations in gem bearing pegmatites in the Eastern province. The field investigations followed local guides and Tether and Partney (1988) observed that there were several false trails and some aquamarine locations were undoubtedly missed.

2 Background to the Research

In the mid 1980’s, Zambia Consolidated Copper Mines limited (ZCCM) opened a number of pits in the Lundazi area and between 1987 and 1989 about 528,190g of beryl and aquamarine were produced. Watts et al. (1991) observed that the overall reserves for the Lundazi aquamarine are unknown, but are undoubtedly large in terms of small-scale mining. O’Connor (1997) estimated the gross value of rough aquamarine currently being mined at 20 to 40 million US dollars per annum and further observed that good estimates were hampered by unrecorded production. 1.1.3 A need for an exploration model for aquamarine-bearing granitic-pegmatites Exploration needs a management of a wide choice of geological models in order to find ore with minimum costs but with maximum results in the shortest possible time (Westerhof, 1992). For any meaningful investment in mining to be undertaken, sufficient and up-to-date geoscientific information related to mineral resource assessment is required. Since the 1970’s, 90% of the discoveries of aquamarine and other gemstones in the Lundazi area have been by villagers who were only using ‘trial’ and ‘error’ wherever there was quartz and/or schorl. The study area has remained underdeveloped despite being endowed with abundant gemstones reserves because of lack of any gemstones geoscientific information to guide exploration. Geoscientific information is important to determine which areas are favorable for the occurrence of a particular type of mineral deposit (Carranza, 2002). Part of the study area lies in a Game Management Area where entry is restricted and many times conflicts have arisen between villagers and the game management over entries to dig and/or prospect for aquamarine. Mineral resource assessment information, therefore, would provide another tool in the land-use database for planners to make a compromise between different land uses. 1.2 Rationale of the Research The research is essential in the study area because of the following reasons.

• Aquamarine gemstones offer high potential for income generation for both the local people and the country.

• The known aquamarine-bearing pegmatites zones cover quite a large area, about 4% of the total area of the country.

• The aquamarine mostly occurs with other gemstones like amethyst and both pink and green tourmaline that can be mined as by products.

• The price for aquamarine is second to that of emerald, which is the most expensive gemstone of the gemstones of Zambia.

1.3 Objectives of the Research The objectives of the research are as follows.

• Develop a GIS-based exploration model for identifying areas with potential for aquamarine-bearing pegmatites.

• Determine spatial relationships between aquamarine-bearing pegmatites and geological features in the area.

• Delineate areas with potential for aquamarine-bearing pegmatites.

Chapter 1 3

1.4 Research hypotheses Because of constancy in composition of granitic-pegmatites intruded in different rock types (Nash, 1962), it is hypothesized that granitic-pegmatites are derived from a single source, i.e., highly differentiated batholiths of granitic character below the present erosion levels. It is also hypothesized that the presence of concealed batholiths of granitic character can be deduced from mappable circular features, which represent structures resulting from cooling of the batholiths or small granites associated with the batholiths (Rolet et al., 1995). It is further hypothesized that the intrusion of both aquamarine-bearing pegmatites and the parental granites were along weaker zones (faults, joints and shear zones). 1.5 Research Methodology The study was carried out in four stages (see Fig. 1.1).

• Extraction, from available geodata sets, of geological features indicative of zones with potential for aquamarine- bearing granitic-pegmatites.

• Generation of evidence maps through the quantification of spatial associations between geological features and known workings for the aquamarine gemstones.

• Integration of evidence maps to generate a predictive map showing zones with potential for aquamarine-bearing pegmatites.

• Validation of predictive map(s). 1.6 Geodata Sets Available to the Research 1.6.1 Locations of mine workings The locations of aquamarine workings were used as training data for the predictive mapping of zones with potential for aquamarine-bearing granitic pegmatites. The locations of workings were divided randomly into two sets; one set was used to quantify the spatial relationships with the different geological features while the other set was used to validate the predictive map of zones with potential for aquamarine-bearing granitic-pegmatites. 1.6.2 Geological map The geological map shows lithological units, faults, and shear zones. The lithological units were digitized as polygons while faults, and shear zones were digitized as segments. The faults digitized from the geological maps were combined with faults interpreted from the ASTER imagery (see below). The spatial association of these faults with the known aquamarine-bearing pegmatites were quantified to derive a lineament evidence map used in the creation of a predictive map of zones with potential for the occurrence of aquamarine-bearing pegmatites. The spatial association of the shear zones with the known aquamarine-bearing pegmatites was quantified to create a shear zone evidence map and combined with other evidence maps to produce a predictive map of zones with potential for aquamarine-bearing granitic-pegmatites. The lithological units were reclassified to extract the granitic bodies that are thought to be spatially associated with the aquamarine-bearing granitic-pegmatites in the area.

4 Background to the Research

1.6.3 Radiometric data Digital radiometric data for potassium, uranium and thorium was generated by digitising intersections of flight lines and data contours. Ternary and ratio maps were generated to identify features indicative of granitic rocks. Features indicative of granitic rocks were

Fig. 1.1. Flow chart showing methodology for predictive mapping for zones of aquamarine-bearing granitic-pegmatites.

ASTER IMAGE GEOLOGYRADIOMETRICS

DATAGEOCHEM.

DATAMINES

LOCATIONS

BAND COMBINATION DIGITIZE

DATA GRIDING

PRINCIPAL COMPONENT

ANALYSIS

DIVIDE INTO TWOSETS

ALTERATION ZONES

MAP

CIRCULAR FEATURES

MAP

FAULTS AND

FRACTURES MAP

SHEAR ZONES

MAP

INTRUSIVESMAP

PRINCIPAL COMPONENT

SCORES

QUANTIFY SPATIAL RELATIONSHIP BETWEEN GEOLOGICAL FEATURES MAP AND KNOWN MINERAL LOCATIONS

MAPBEFFERED

ALTER.ZONES

MAP OF BUFFEREDCIRCULARFEATURES

MAP

MAP OF BUFFERED

FAULTS/FRACTURE

MAPS

MAP OF BUFFERED INTRUSIVES

MAP FO BUFFERED

SHEAR ZONES

MAP OF BUFFERED PRINCIPAL

COMPONENTS SCORES

INTEGRATION OF BUFFERED EVIDENCE MAPS AND VALIDATION

PREDICTIVE MAP FOR ZONES WITH POTENTIAL FOR OCCURRENCE OF AQUAMARINE-BEARING

GRANITIC-PEGMATITES

Chapter 1 5

digitized and combined with those features derived from the geological maps to generate a source rock binary map. 1.6.4 Remotely-sensed spectral data Remotely-sensed spectral data were obtained from ASTER imagery (see below). Five sub-scenes were obtained. The sub-scenes were georeferenced using control points identified both on the images and on 1:50 000 scale topographic maps. The sub-scenes were corrected for atmospheric effect and then mosaicked to create one scene. Image enhancement was performed to extract lineaments that would not have been apparent from the images. The lineaments were digitized and combined with faults digitized from the geological map, after which the spatial relationship with the known aquamarine-bearing pegmatites was quantified to derive a lineament evidence map for predictive mapping of zones with potential for the occurrence of aquamarine-bearing granitic-pegmatites. Shear zones and circular features were digitized. The ASTER imagery spectral data were used to identify alteration zones associated with weathering of aquamarine-bearing granitic-pegmatites. A detailed discussion of data processing is given in section 4.3. 1.6.5 Geochemical data Stream sediments geochemical data for Zn, Pb, Cu, Co, Mn, Fe, and Ni are available in digital format. Uni-element and multi-element analysis were conducted to identify correlations between the elements and to identify multi-element association that could be indicative of granitic-environment. 1.7 Conclusion Zambia has mainly been exploiting copper, cobalt, zinc and lead deposits. Recently, however, low prices for these commodities and a need for heavy re-capitalization of the mines, has led to closure of some of these mines. Gemstones, however, have shown to be a potential contributor to Zambia’s socio-economic development. Aquamarine, derived from aquamarine-bearing granitic-pegmatites occurs in considerable amounts especially in the eastern part of the country. To effectively contribute and sustain its contribution to the socio-economic development of Zambia, zones favorable for the occurrence of aquamarine-bearing granitic-pegmatites need to be outlined for further exploration. A GIS-based predictive mapping of the zones favorable for occurrences of aquamarine-bearing granitic-pegmatites is tested in the Lundazi district of Zambia. The area is a Proterozoic intracratonic basin with metasediments intruded by the aquamarine-bearing granitic-pegmatites.

Chapter 2: The Study Area 2.1 Location Zambia is located centrally at the intersection of Latitude 15o 00 S and Longitude 30o 00 E in Southern Africa. It shares borders with Democratic Republic of Congo, Angola, Namibia, Botswana, Zimbabwe, Mozambique, Malawi and Tanzania (see Fig. 2.1). The study area is located in Lundazi District in the Eastern Province of Zambia (Fig. 2.1). The Lundazi area is about 650 km east of the capital city, Lusaka, and about 75km north of Chipata town the provincial headquarters. In the western part, the area extends to Luangwa River, while the eastern part extends to the Zambian border with Malawi. The area extends to about 32 km north of Lundazi Boma and about 73 km south of Lundazi Boma. The total aerial coverage is approximately 10,000 sq. km.

2.2 General Geology of Zambia 2.2.1 Stratigraphy The geology of Zambia can be broadly described as consisting of Pre-Cambrian Basement Complex unconformably overlain by Katangan metasediments, Karoo sediments and Kalahari sands (Fig. 2.2). The Basement Complex is exposed mainly in the eastern and southeastern parts of the country and forms undulating hills and valleys. The Basement Complex consists of granites, altered volcanics, schists and gneisses uncomfortably overlain by quartzites, schists and limestones of the Muva Super group. These rocks have been involved in several tectono-thermal episodes. In few localities, it is possible to distinguish between individual units but, generally, it is virtually impossible due to the thermal and dynamic metamorphism differentiation, (GSD, 1989). Some of these metasediments have retained their sedimentary structures in some places despite the advanced recrystallization indicating that most of the Basement Complex

Fig 2.1. Location map of Zambia and the study area (yellow area in small box).

Chapter 2 7

gneisses are paragneisses, (Hickman 1982). The meta-igneous rocks also retain some of their original texture.

Lying unconformably on the Basement Complex rocks are four groups of sedimentary formations within the Katanga Super group (that cover almost all of the central, northern, and north-western parts of the country), Karoo sediments (that cover mostly the major river valleys), and Kalahari Sands and alluvial deposits (that cover the western part of the country). The Katangan Super group, or the Katanga as it is sometimes called, is a pile of beds of more than 10000m thick, which lie stratigraphically between the Basement Complex and the Phanerozoic cover rocks that overly it (between 1310 to after 656 Ma) (GSD, 1989). The Katanga has been divided, based on lithostratigraphic groups from bottom to top, into Upper and Lower Roan, the Mwanshya, and the Upper and Lower Kundelungu groups. The Lower Roan group is composed of continental and platform deposits representing four superimposed

Fig. 2.2. Lithological stratigraphic column of Zambia.

8 The Study Area

deposition cycles each showing passage from continental to marine or marine to continental or both. This group is the main host of the copper mineralization being exploited in Zambia and the Democratic Republic of Congo. It consists of arenaceous arkoses that grade into argillaceous rocks of the Upper Roan and into carbonaceous shales of Mwanshya group. The Lower Roan may therefore be considered to represent a marine transgression encroaching on to a topographically irregular continent. The overlying Kundelungu is characterized by an increase in thickness from the edges of the basin to its centre and by corresponding facies change. The Lower Kundelungu commences with a tilloidal conglomerate locally known as Grand Conglomerate and grades into Kankontwe limestones and into a very thick sequence of shale. The Karoo Supergroup consists of basal conglomerates, Madumabisa sandstones, the Escarpment Grit, and basalts. It covers mostly the major river valleys and the plateau part in the southern part of Zambia while the Kalahari sands cover mostly the western and southern parts of the country. 2.2.2 Structure and Metamorphism Zambia has been divided into five main orogenic belts - the Irumide belt, the Mozambique and Zambezi belts, the Pan African Lufilian Arc and the Bangweulu block (Fig. 2.3). The Irumide and the Zambezi orogenic belts define an area between the Bangweulu block and Lufilian arc to the north and the Zimbabwe Craton in the south. The Irumide orogeny is dated at around 1350 to 1100 Ma. The orogenic belt covers the central part, part of southern and most of the eastern parts of Zambia. It is bounded by the Bangweulu block in the north and the Mozambique Belt in the southeast (GSD, 1989). This orogenic belt or province is composed of granites, gneisses and mainly quartzites and pelitic metasediments of the Basement Complex and part of Katangan rocks characterized by northeast to east-northeast foliation trends (Namateba 1986). These foliation trends continue southwestwards where they are cut into two portions by the Luano-Lukusashi-Luangwa valley. Around and on the southeast of Lusaka (to the east of Choma-Kalomo block on Fig. 2.3, the capital city, the trend is cut by easterly to west-northwesterly Katangan trend linking the Zambezi belt and the Lufilian arc in the south and north, respectively. The general northeast to north-northeast trend shifts as it continues into the southern part of the country. This NNE trend continues even to areas around Lake Kariba near the border with Zimbabwe. The regional strike is deduced from quartzite horizons and foliation expressed by parallel layers of biotite and orientation of feldspar porphyloblasts in the gneisses. The structural features of the Zambezi Orogenic belt were first recognized by McGregor (GSD, 1989). This orogenic belt covers a narrow zone of the country between the northern margin of the Zimbabwe Craton and the Karoo rocks of the Zambezi rift valley. This zone is distinguished by high-grade metamorphic rocks of different ages and composition overprinted by the tectono-thermal events that gave rise to the present form of the belt. In some parts of the belt, the rocks have been strongly deformed and at times show sillimanite metamorphic grade. This metamorphic grade seem to have been reached during the last major orogenic event, which together with the rift valley type of fracturing are responsible for the present form of the belt. In the eastern part of the orogen, the southern part tends to swing southwards to merge with the western margin

Chapter 2 9

of the Mozambique belt. On its western border, the orogen begins to develop a northwest trend that disappears under the Karoo rocks of the Zambezi valley. The northwest trend, however, reappears again in the Katangan rocks in the southern part of the country and in the southwest of Lusaka and structurally links the belt with the Lufilian Arc in the north.

Fig. 2.3. General geologic map showing the orogenic mobile belts of Zambia.

Study area

10 The Study Area

The Lufilian Arc is a large arcuate Pan African fold belt, which extends along the Democratic Republic of Congo (DRC) and Zambian border from the extremity of the southeast DRC pedicle to the triple border joint between Zambia, Angola and Democratic Republic of Congo (Fig.2.3). This arc continues westwards into Angola while its southeastern part meets with the NE to ENE trends of the Irumide belt (GSD, 1989). The Lufilian Arc occupies mainly the area in which the Katangan Supergroup rocks are exploited for copper and other mineral riches in both the Democratic Republic of Congo and Zambia. In Zambia, this area is usually referred to as the Copperbelt. The Katanga in this area is characterized by a thickness in excess of 10000 m. 2.3 Local Geology of Lundazi Area 2.3.1 Lithology The study area covers the eastern part of the Irumide orogenic belt. Precambrian granites and granitic gneisses overlain by a thick sequence of alternating quartzites and pelitic metasediments of the Muva Supergroup underlie the area (Fig. 2.4). The Katanga Supergroup schists unconformably overly the Muva metasediments (Daly et al., 1984). In the west near the Luangwa valley, the area is underlain by Karoo sandstone and Madumabisa mudstone, both of the Karoo Supergroup. Gneisses of upper amphibolite facies with local metabasic granulites, mafic intrusives and metagranites outcrop in much of the eastern and central parts of the area. Metasediments, notably schists, a range of leucocratic gneisses and quartzites of the Muva Supergroup crop out in the central and mid-west parts of the area. A small outlier of supracrustal quartzites and metacarbonates of greenschist facies (the Mparanza formation) unconformably overly the older gneisses. These metasediments are thought to be of Katangan (Neoproterozoic) age (Hickman, 1998). Occassional amphibolites, marbles and calc-silicates occur. Although recrystallization is advanced, metasedimentary resisters are present throughout the area indicating that most of the basement gneisses are paragneisses of sedimentary origin (Harding, 1982). Igneous rocks consist of micro-granites, pegmatites, syenites, metagranites, charnockites, enderbites, andesinites, gabbros, dolerites and amphibolites. Several small granites, which lie parallel to the Lukusuzi anticline fold axis, outcrop in the central part of the area. In the south of the area, Tether et al. (1988) reported a deep-seated granite and syenite (dated at about 1000 Ma) around Chipata and a younger set of granites (800 Ma and 650 Ma) to the west. Pegmatites are widespread and can be broadly classified into two groups based on age. A simple older generation of pegmatites occurs as lenses associated with areas of granulitic facies metamorphism. This generation of pegmatites is thought to be a result of partial anatexis (Tether et al., 1988). This suit of pegmatites contains mainly feldspar, muscovite, quartz and schorl. No gemstones have been observed in this suit of pegmatites. The later generation of pegmatites is intrusive into the crystalline Pre-Karoo rocks. These pegmatites are classified into two sub-groups, simple and complex. The simple sub-group of pegmatites is unzoned and comprises quartz, muscovite, schorl and feldspar. The complex sub-group of pegmatites is zoned with a massive quartz core while the different zones consist of quartz, muscovite, schorl, beryl, feldspar,

Chapter 2 11

aquamarine, pink and green tourmaline and rose quartz in variable proportions. Amethyst and garnet occur occasionally. Snelling et al. (1972) dated one pegmatite of the complex sub-group near Lundazi at 485 Ma. Both O’Connor (1998) and Hickman (1998) alluded to the fact that the complex sub-group of pegmatites may be a result of a concealed pluton associated with micro-granites, which outcrop in the area. The zoned type i.e., the complex sub-group of pegmatites is being exploited for aquamarine, beryl and occasionally pink and green tourmaline. The sedimentary rocks of the Karoo Supergroup include the Madumabisa mudstone and the Escarpment Grit of the Upper Karoo. These rocks are confined to the western margin of the area around Luangwa Valley. 2.3.2 Structure and metamorphism Table 2.1 shows the generalized geological events that affected the Lundazi area. The most pervasive structural trend in the study area is the northeast to north-northeast foliation. This structural trend is dated back to Kibaran metamorphism. The earliest recognizable event is D2, which obliterated older fabrics and produced isoclinal folding with approximately NNW-trending axes. Harding (1982) observed that conditions during D2 caused anatexis with in situ melting and probably intrusive emplacements of acidic rocks. Polyphase deformation involving five phases of folding has been recognized in the gneisses. Two other different phases of later folding were recognized in the Mparanza Formation unconformably overlying the gneisses. The most prominent of the metamorphic events is the deformation phase that controlled the general structural trend of northeast trending foliation in the area. After this event and possibly contemporaneous with it, is the activation of major northeast trending faults and shear zones. The fracturing that followed later was localized and Harding (1982) suggested that this could be related to the intrusion of late pegmatites in the area. Granulite facies surrounded by amphibolite facies metamorphism is attained in most parts of the area. Migmatization is well developed in pelitic gneisses and a late K-feldsparthization phenomenon is common in several areas probably overlying large shallow granite bodies (O’Connor, 1998). Fig. 2.4 shows the geological map of the Lundazi area.

12 The Study Area

Fig. 2.4. Geological map of the study area, showing geological units, faults and locations of aquamarine workings.

Chapter 2 13

2.4 Exploration History of Lundazi Area The earliest geological record available in the area is that of an outcrop map by Loangwa Concession Limited geologists in the 1930’s. Prior to this, however, the pegmatites in Eastern Province were already being exploited for mica with beryl as a by-product. In 1947, the first geological map of Zambia was compiled by Bancroft. The geological map shows most of the area to be underlain by Basement Complex rocks unconformably overlain by what is currently known to be metasediments of the Muva Supergroup. In 1952, Guernsey listed the mineral occurrences known at that time including gold, iron and mica. Nash (1962) gathered enough data for a PhD thesis on the then mica-producing pegmatites. In 1964, Mossman studied the pegmatites of Southern Province and included part of the Lundazi area. Hickman (1998), O’Connor (1998) and Harding (1982), all then working as geologists for the Geological Survey of Zambia, mapped the areas around the Lukusuzi National Park, Lumezi and Lundazi areas and around Mwanya in the west, respectively. The trio, in their individual geological reports, observed that the geology in the area comprised of Basement

Table 2.1. Geological events in the Lundazi Area (modified after Hickman 1998).

14 The Study Area

Complex gneisses unconformably overlain by metasediments of the Muva Supergroup that are in turn unconformably overlain by the Katangan Metasediments. Conglomerates and sandstones of the Karoo Supergroup were identified in the west. O’Connor (1998) and Harding (1982) realized the economic potential of beryl-bearing pegmatites which were not recognized by Hickman (1998). In the mid 1980’s, Mineral Exploration Department geologists worked in the area to map in detail the pegmatites that were then being exploited for gemstones. Almost at the same time, Zambia Consolidated Copper Mines operated a number of pits for aquamarine exploitation. Several other workings by artisan miners were opened. A part from aquamarine, tourmaline, and other gemstones the area has also iron, vein copper and gold, graphite, and barite occurrences. 2.5 Conclusion The geology of the study area indicates intracratonic metasediments, which have undergone polyphase metamorphism and intruded by granitoids of variable ages. The granitoids include syn-tectonic metabasics and metagranites. Post-tectonic intrusives include the late granites, granitic-pegmatites, charnockites, enderbites, and syenites. The late granites are thought to be on the peripherals of older metagranitoids in the area. Several pegmatite episodes have been distinguished in the area but only the granitic-pegmatites associated with margins of late granites are mineralized with aquamarine and other rare-element minerals. They are also considered the top of an underlying granitoid that is spatially related to the older granitoids. The border zone between the granitic-pegmatites and the host rocks are chilled in some localities implying forceful injection of REE rich fluids on fracture zones of the host rocks. The aquamarine-bearing granitic-pegmatites are wide spread in the area and in some localities they occur on the peripherals of late granites parallel to a northeast trending synform axial trace. The aquamarine-bearing granitic-pegmatites outcrop as hillocks with either a northeast or northwest trend parallel to the two main fault direction in the area. On the plateau and low-lying areas, the aquamarine-bearing pegmatites weather into a mass of Kaolinite with muscovite and schorl as float. The aquamarine in granitic-pegmatites is being exploited for gemstones. Minor east-west trending granitic-pegmatites also occur.

Chapter 3: The Geology and Exploration of (Aquamarine-Bearing) Granitic-pegmatites

3.1 Aquamarine-Bearing Granitic-Pegmatites in General 3.1.1 Introduction Many rare-element granitic-pegmatites including aquamarine-bearing granitic-pegmatites represent the final water-rich, siliceous melts of intermediate to acid magmas and are thought to be final residual melts rich in silica, alumina, water, halogens, alkalis, and lithophile elements not readily accommodated in common igneous rocks. Granitic-pegmatites have been formed in all tectono-magmatic cycles of the geological history. The first important and extensive pegmatite formation period, however, was the Kenoran age (2800-2600 ma) which saw the formation of rare-element pegmatites of Superior and Slave provinces of Canada, Zimbabwean, Kapvaal, and Tanzanian Cratons and many other parts of the Western Australia craton. This period was succeeded by formation of rare-elements pegmatites in various stages of the geological history with Alpine orogeny (85-20 Ma) pegmatites field being the youngest (Cerny, 1982). 3.1.2 Geological and structural settings Geological position and structural control of granitic-pegmatites fields has changed during the evolution of the crust. The late Archean, rare-element pegmatite fields are generally restricted to linear, geosynclinal greenstone belts down-warped between the batholith masses of tonalitic and potassic granites. The granitic-pegmatites fields tend to concentrate on the tectonic boundaries or on the central part of the major sedimentary troughs crossing the greenstone terrain or forming part of their margins (Trueman, 1982). The early Proterozoic pegmatite fields, however, are generally confined to graben-type geosynclinal troughs, adjacent to margins of the Archean cratons. Both Archean and Proterozoic pegmatite fields are mostly controlled by deep faults axial to greenstone belts or on the flanks of gneissic troughs, or to lithologic contacts along early batholithic margins. Phanerozoic pegmatites occur mainly in flysch sediments of folded and metamorphosed eugeosynclinal sequences of the orogenic belts (Cerny, 1982). Rare-element granitic-pegmatites and their parent granite intrusions are often controlled by large tectonic features or by lithological boundaries separating rocks of contrasting competence. In terranes with diversified lithology of country rocks, granitic-pegmatites are concentrated in more competent rock types that fracture and provide open space under deformation. Granitic-pegmatites also tend to intrude along contacts of massive pre- to syntectonic batholiths and some metamorphic rocks where their differential response to late adjustment stresses loosened their contact paving way for intrusion of deep-seated magma. In this case, granitic-pegmatites tend to be randomly distributed in the batholith but are distributed along the contact of these two rock types in the other metamorphic sequences. Rare-element granitic-pegmatites and their parent granites tend to be confined to anticlinal structures as small to sizable (10 X 100 km2) bodies (Trueman, 1982).

16 Geology and Exploration of Aquamarine Bearing Granitic Pegmatites

3.1.3 Classification of granitic-pegmatites Schaller (1933) classified granitic-pegmatites based on their mineralogy as “Simple” and “Complex” granitic-pegmatites. “Simple’’ granitic-pegmatites consist of quartz and microcline and contain no significant quantities of other minerals. “Complex” pegmatites contain other minerals like albite, beryl, topaz, cassiterite, micas, tourmaline, garnets, lithium minerals, rare-element minerals, the columbates and tantalates, the phosphates, and others that contain rare elements (Nash, 1962). Cameron et al. (1945) in Nash (1962), on the other hand, classified granitic-pegmatites as “zoned” and “unzoned” pegmatites to distinguish the two major types in England. They described “zoned” pegmatites as those pegmatite bodies whose minerals are distinctly arranged into structural units of different composition and texture and systematically arranged with respect to walls of a given body. They use the term “unzoned” pegmatites for any pegmatite that appears essentially homogeneous apart from the presence of the border zone. Johnston (1945) classified the microcline-quartz–muscovite pegmatites of Brazil into “homogeneous”, “tabular dykes” and “heterogeneous” pegmatites based on the degree of internal differentiation. He described “homogeneous” pegmatites as those that have “a fairly uniform texture from wall to centre and do not contain crystals of remarkable size” and “heterogeneous” the “lens-shaped dykes showing a high degree of differentiation with walls of muscovite, gigantic crystals of microcline in the interior of the dyke, and a central core or nucleus of quartz”. Ginsburg et al. (1979) classified granitic-pegmatites into four main groups based on their depth of consolidation, mineralization, and relationship to igneous processes, and metamorphic environments: (1) miarolitic pegmatites (1.5-3.5 km); (2) rare–element pegmatites (3.5 – 7km); (3) mica-bearing pegmatites (7-8km); and (4) maximal-depth pegmatites (>11km). Miarolitic pegmatites are exposed as pods of pegmatites in the upper parts of granites intrusive into rocks of lowest metamorphic grades with cavities of piezometric rock-quartz, optical fluorite, and gem-quality beryl, topaz etc. Rare-element pegmatites usually occur as fracture in-filling in cordierite-amphibolite facies and are usually mineralized with Li, Rb, Cs, Be, Ta and (Sn, Nb). They are a result of the differentiated allochthonous granites. Mica-bearing pegmatites are hosted by almandine-amphibolite facies and are mainly mica reserves with less or no rare-elements mineralization. They either are direct products of anatexis or separated from anatectic autochthonous granites. Maximal-depth pegmatites are associated with granulite facies terranes with no obvious granitic parents and no economic mineralization but carrying allanite, monazite, and corundum. Individual hybrid characteristics, however, occur. An example is the extensively beryl mineralized mica-bearing pegmatites of India (Ginsburg et al., 1979). The beryl mineralized granitic-pegmatites belt of India would belong to Ginsburg’s (1979) “rare-elements” granitic-pegmatites that formed between 3.5 and 7km, while the mica-bearing granitic-pegmatites belong to the granitic-pegmatites that formed at 7-8 km depth, which by Ginsburg’s (1979) classification would not be mineralized with rare-elements minerals.

Chapter 3 17

Johnston’s (1945) “homogeneous” and “heterogeneous” pegmatites are essentially equivalent to those described as ‘unzoned’ and ‘zoned’ pegmatites, respectively, by Schaller (1933). Based on the complexity of their mineralogy and structure, the heterogeneous pegmatites described by Johnston (1945) and the zoned pegmatites described by Cameron et al. (1945) can be classified as complex pegmatites in Schaller’s classification. The unzoned and the homogeneous pegmatites would be equivalents of simple granitic-pegmatites of Schaller’s classification (1933). The zoned granitic-pegmatites belong to the to Ginsburg’s (1979) first, second and third classes. 3.1.4 Regional scale exploration criteria for granitic-pegmatites The age range of formation of rare-element granitic-pegmatites varies from Archean to Tertiary. They are usually associated with Archean Cratons and Phanerozoic orogenic belts. In Archean Cratons, the granitic-pegmatites are localized along deep fault systems that in many areas coincide with major metamorphic and tectonic boundaries. The granitic-pegmatites are also associated with shear zones within these high metamorphic terranes. The rare-element granitic-pegmatites of Cerny (1982) occur in less deeply- eroded low-pressure metamorphic terranes, usually of amphibolite-cordierite facies. They are mainly late or post-tectonic and are generally peripheral to larger granitic plutons. They have a spatial association with axial traces of late synformal structures in the area. In remote sensing, granitic-pegmatites can be discriminated by their circular features and fracturing during the cooling of the granite. In aeromagnetic data, they express themselves as horizontal or moderately inclined plates connected with the sources of felsic magmas by steep feeding channels. Heavy minerals sampling may identify areas likely to be underlain by granitic-pegmatites. In some instances, the granitic-pegmatites tend to form geochemical haloes around the parent granite. 3.2 The Aquamarine-Bearing Pegmatite Belt of Lundazi Area 3.2.1 Geological and structural setting of granitic-pegmatite belt The aquamarine-bearing granitic-pegmatites belt of Lundazi area is one of the two aquamarine-bearing granitic-pegmatites belts hosted by a linear intracratonic basin of Kibaran age (1355Ma), Irumide mobile belt, of Zambia. Granitic-pegmatites intrude metasediments of high to medium grade metamorphic terranes. Metamorphic rocks are generally basic and felsic granulites, gneisses and schists of sillimanite and amphibolite facies in a Proterozoic belt. Other igneous rocks include gabbros, dolerites, enderbites, charnockites, granites, syenites and pegmatites. Some of these igneous rocks have suffered folding and foliation concordant with the metamorphic host rocks. Metagranites seem to preferentially occur at cores of antiformal structures. The granitic-pegmatites are typically late orogenic to anoregenic. Hickman (1998) reported a series of micro-granites, with marginal zones of aquamarine-bearing granitic-pegmatites, outcropping parallel to the northeast trending anticlinal axial trace in the study area. Stocks of granites are exposed as ovoid bodies (up to 20m across), aligned parallel to the axial trace of a northeast trending antiform structure. The marginal aquamarine-bearing granitic-pegmatites are similar to those that are wide spread in the area (Harding 1998). During the Irumide orogeny, several faulting phases occurred but only two fault systems are prominent; i.e. northeast and northwest trending faults. The


northwest trending faults laterally displace the northeast trending faults. Minor faults trending north and east also occur in the study area. Fold interference patterns and local fracture cleavage formed contemporaneous with the northwest trending faults (Harding, 1998). The granites and the granitic-pegmatites were emplaced syntectonically with the formation of the NNE trending Lukusuzi antiform (Hickman 1998). The general sequence of metamorphic events of the area is given in Table 2.1. 3.2.2 Classification of aquamarine-bearing granitic-pegmatites of Lundazi area By virtue of their complex mineralogy and zoning the aquamarine-bearing pegmatites belt of Lundazi area can be classified as “zoned” (under Cameron’s nomenclature), and as “complex” granitic-pegmatites (under Schaller’s nomenclature). They can also be classified as heterogeneous (under Johnston’s nomenclature) because of their degree of internal differentiation. The unzoned granitic-pegmatites of Lundazi area would be classified as simple and homogeneous and since they are currently of no economic significance would not be discussed further in this chapter. The aquamarine–bearing pegmatites of the eastern part of the Irumide belt (i.e., in Lundazi area) can be classified as a hybrid of miarolitic, rare–element, and mica-bearing pegmatites under the classification of Ginsburg et al’s (1979). This is based on the aquamarine-bearing pegmatites’ complex mineralogy, cavities with beryl and muscovite, and content of rare minerals such as aquamarine, beryl, uraninite, mica and other minerals, and the type of metamorphic rocks they are associated. One feature that appears to be consistent with the mineralogical composition of the granitic-pegmatites in the Lundazi area is that potassium feldspar is mostly dominant than sodium feldspar (Nash, 1962). Gallagher (1959) classified the beryl-bearing granitic-pegmatites of Uganda and Southern Rhodesia, the present Zimbabwe, into four groups depending on the dominant feldspar and the presence or absence of lithium minerals (Table 3.1). Table 3.1. Gallagher’s (1959) classification of pegmatites. Type Average size Degree of internal zoning Na-Li pegmatites Large Highly developed and complex K-Na pegmatites Medium-large Simple scheme, usually well developed Na pegmatites Small or medium Very simple scheme usually developed K pegmatites Small or medium Very simple scheme usually developed The aquamarine-bearing pegmatites of the eastern part of Irumide belt belong to the Gallagher’s (1959) potassium-sodium group. Gallagher (1959) described this group as (a) medium or large in size, (b) having potassium and sodium feldspars, quartz, and muscovite as major minerals with minor or absent lithium and other minerals, (c) having well developed simple scheme of internal zoning, (d) usually having well developed quartz cores, (e) where potash feldspar predominates over albite, intermediate or wall-zones of albite-muscovite pegmatites are developed and these are often associated with high beryl mineralization, (f) narrow quartz-muscovite units, possibly fracture-filling units, may carry considerable amounts of beryl and (g) minerals of niobium and tantalum are particularly common in beryl-bearing pegmatite rich in muscovite and relatively fine-grained.

Chapter 3 19

Only the first five features of K-Na pegmatites described by Gallagher (1959), however, characterize the aquamarine-granitic-pegmatites of the Lundazi area. The most abundant minerals are quartz, muscovite and feldspar. Accessory minerals include tourmaline, beryl, apatite, chrsoberyl, garnet, magnetite, ilmenite, columbite, biotite, betafite, uraninite, cassiterite and many more. Most of the beryl however is mainly associated with the inner zones of the pegmatite especially the zone next to the quartz core. 3.3 Possible Genetic Model of Aquamarine-Bearing Pegmatites of the Lundazi

Area Fig. 3.1 illustrates the general genesis of granitic-pegmatites related to granitoids. The diagram was originally designed by Strong (1989) as a model for granophile mineral deposits. The current illustration is on the granitic-pegmatites formation part of the model. A rising magma may intrude and solidify at depth resulting into granites and monzonites, which are usually large and discordant. These granitoids are usually batholiths of orogenic belts and are generally barren possibly due to low water or lack of interaction with ground water because of the intrusion at great depth (Westerhof and Aleva, 1989). A muscovite-bearing pegmatite granite may form as an anatectic melt or from the differentiation of the granite. The differentiated muscovite-granite may be enriched in beryllium, boron, lithium and other elements that were not incorporated in the early forming minerals, typically at the cupolas at the top of large plutons depending on the metallogen of the area. The presence of these elements and volatiles prolongs the crystallization of the muscovite-granite and allows intrusion to shallow levels. It may then allow second boiling and at times brecciation. The highly fractionated muscovite-granite is enriched in BEBLP (beryllium, boron, lithium, pollodium) and other lithophile elements. The dispersion of fluids and elements through plutons and country rocks results into various granitoid related deposits of which aquamarine-bearing pegmatites are a part. The shallow emplacement allows more siliceous and volatile fluids be injected through the fractures of the country rocks. The genesis of rare-element granitic-pegmatites associated with batholiths depends on the interplay of complex petrogenetic processes during the evolution of the batholith. The mineralization associated with granitoid magmatic activity, which granitic-pegmatites are part of, depends primarily on the composition of the parent magma, speed at which this magma rises in the crust, water content of the magma, lithological and environment in which the magma intrudes. The late stage of the parent batholith to rare-element granitic-pegmatites, in general, involves exsolution of volatile-rich phase(s), dispersion of a rare-element-rich fluid along shear zones and upward emigration of rare-element rich melt. The granitic-pegmatites of the Lundazi area formed by the intrusion and crystallization of magmatic rare-element fluids within fractures in the country rocks. This is evidenced by their zoning characteristics and the chilled margins of the contacts between the pegmatites and the country rocks.


The aquamarine-bearing pegmatites of the Lundazi area occur both as clusters and as isolated pegmatites in the basement rocks of medium to metamorphic facies. O’Connor (1998) considers the granitic-pegmatites in the area roofs of a parent granites that are not exposed at the current erosion levels. He observed several microgranites in the area but most of them are too small to be shown on at the map scale. He also noted that this could be another proof that there could be a parent granite under the current erosion levels. The late metamorphic events may have reactivated the old faults which acted as channelways for the granites parental to the aquamarine-bearing granitic-pegmatites. The pegmatitic fluids were then injected through faults and fractures that may have resulted during metamorphic events. Hickman (1998) is of the opinion that the granitic-pegmatites were emplaced syntectonically with the formation of a northeast-trending Lukusuzi antiform. He observed that the granitic-pegmatites and their parental micro-granites are spatially distributed along the axial trace of this antiform. The granitic-pegmatites also show some preference to shear zones in the northeast of the area. Hickman (1998) contended that the granitic-pegmatite intrusions are syntectonic with the late shearing and faulting in the area which may be coeval with the formation of the northeast trending fold axis in this area. Mohan (1982) shares the same view but he further suggests that the granitic-pegmatites are also controlled by fractures and foliation planes.

Fig. 3.1. Generalised possible genetic model for aquamarine-bearing pegmatites in Lundaziarea (modified after Strong (1981), Geoscience Canada, 814).

1

2

3

Chapter 3 21

3.4 General Characteristics of Geological Environments of (Aquamarine-Bearing) Granitic-Pegmatites

In general, the (surface) geological characteristics of zones favorable for emplacement of aquamarine-bearing pegmatites based on the geological and structural setting of granitic-pegmatites elsewhere and in Lundazi area are as follows. a) The granitic-pegmatites occur within metamorphosed eugeosynclinal sequences of orogenic belt penetrated by batholithic belts of the Proterozoic age (Ginsburg et al., 1979). b) The granitic-pegmatites have a spatial association with proximity to faults and fractures. The faults and fractures may have acted as pathways for the rare-elements enriched fluids (Harding, 1982). c) The granitic-pegmatites are spatially associated with proximity to antiformal axial traces (Hickman, 1998). d) The granitic-pegmatites tend to occur in foliated metamorphic sequences proximal to the contact with metagranites. Differential responses of the metamorphic sequences and the pre-to syntectonic metagranite to the late adjustment stresses may loosen the contact and allow injection of the deep- seated magma (Trueman, 1982). e) Granitic-pegmatites have been characterized based on petrographical and geochemical mapping of hydrothermally altered rocks to reconstruct the hydrothermal activity events. This approach allows determination of geochemistry of alteration systems related to ore-bearing granitic-pegmatites and that related to pre-granitic intrusion country rocks (Komov et al., 1994). f) The presence of and proximity to late or post-tectonic granitic bodies. These are small to moderate in size and usually postdate the peaks of regional metamorphism. Such granitic emplacements are controlled by large-scale tectonic features like deep regional faulting, re-activation along old tectonic lineaments or faults of a younger date (Cerny, 1982). g) Presence of radiometric signature of Uranium, Thorium and Potassium. The parent granite and/ or associated pegmatites may be discriminated from other rocks in the area. (Subhash et al., 2001). h) Presence of and proximal to alteration zones. Alteration zones around pegmatites may be over 150m in magmatic-disseminated rare-earth pegmatites though generally the zones are in the range of a few metres (Subhash et al., 2001). i) Presence of and proximity to ductile shear zones. The shear zones are weaker zones and may provide easy pathway for the granites (Abdalla et al., 1999). j) Presence of circular features. Simple and circular features in Remote sensing image data are related to small granitic bodies or to hidden domes that correspond to


concentric networks of fractures formed during the cooling of the granite. This is as opposed to complex features related to larger plutons (Rolet et al., 1993). k) Geochemically, the rare-elements granitic-pegmatites have high Si, and K/Na ratios, moderate Al and Na, and low Ti, Fe, Mg, Mn and Ca. Ratios like Li2/(RbxCs) and (Ba x Sr x Pb)/(Zn x Cr x Cu) are used to determine haloes around pegmatites. The ratio Li2/(Rb x Cs) decreases with increasing depth of the pegmatite while the ratio (BaxSrxPb)/(ZnxCrxCu) decreases in negative haloes. Heavy minerals stream sediments sampling can also be employed (Komov et al., 1994). l) The high concentrations of beryllium, fluorine, boron, lithium and rubidium in granitic-pegmatites enable use of lithogeochemical and hydrogeochemical prospecting methods for granitic-pegmatites. Fluorine and boron form haloes, which propagate in the form of a train up to 200m from the granitic-pegmatites in the direction of water flow. This makes hydrogeochemical prospecting a useful tool. Fluorine also forms haloes of about 5m around a granitic-pegmatite (Komov et al., 1994). 3.5 GIS-Based Geological Exploration for Granitic-Pegmatites The general geological criteria for exploration of granitic-pegmatites and observations in the study area form the basis for the extraction of evidence features from the geodata sets available. Features from geodata sets discussed in chapter 1 will be used to generate maps showing spatial association with the aquamarine-bearing granitic-pegmatites. The geological evidences are divided into geochemical, progenitor intrusives, alteration zones, Lineaments, syn-form axial traces, shear zones, and structural map i.e., showing circular features hypothesized to be due to either micro-granites or underlying batholith. The geological evidence maps will be combined to predict zones with potential for the occurrence of aquamarine-bearing granitic-pegmatites by: a) weights of evidence method (data-driven) and b) Fuzzy logic method (knowledge-driven). 3.6 Conclusion Identification of granitic rocks parental to mineralized rare-element granitic-pegmatites forms an essential starting point for a search of rare-element mineralized granitic-pegmatites. This search may combine remotely sensed data, geochemical, geophysical and geological data to identify the above outlined exploration criteria. The present research will use only the following criteria from those outlined above because of lack of adequate data for the rest of the other criteria.

• Presence of or proximity to faults and fractures, which provide pathways for aquamarine-bearing granitic-pegmatites from progenitor granites. These features will be extracted from geological maps and ASTER imagery.

• Presence of or proximity to synform axial traces with NE trends, which are thought to be coeval with late granite intrusion. These features will be extracted from the geological map.

• Presence of late granites and metagranites related to the aquamarine-bearing granitic-pegmatites. These features will be extracted from the geological maps.

• Presence of radiometric signature indicative of intrusive rocks. This feature will be extracted from uranium, thorium and potassium radiometric data.

Chapter 3 23

• Presence of or proximity to circular features indicative of unexposed late granites and small exposed intrusives. These features will be analysed from the ASTER imagery.

• Alteration zones associated with granitic-pegmatites weathering. These features will be identified using the ASTER imagery.

• Geochemical anomalies. These will be identified from stream sediments geochemical data.

• Presence of or proximity to ductile shear zones, which provide pathways for granitic and granitic-pegmatite intrusions. These features will be extracted from both the ASTER imagery and the geological map.

Chapter 4: Extraction of Spatial Indicative Features This chapter explains the extraction, from the appropriate exploration datasets, of the following spatial features considered in the previous chapter to be indicative of zones with potential for the occurrence of aquamarine-bearing granitic-pegmatites.

• Geological lineaments, (faults and joints), which are considered conduits for granitic intrusions.

• NE trending synformal axial traces, which are thought to be coeval with the intrusion of granitic-pegmatites.

• Meta- and late granites, which are thought to be parental to the granitic pegmatites.

• Radiometric feature indicative of granitic rocks. This feature will identify granitic intrusions from the undifferentiated gneisses and migmatites.

• Circular features, which are indicative of unexposed and small granitic bodies probably parental to granitic-pegmatites.

• Alteration zones, which could be associated with the weathering of the granitic-pegmatites.

• Silicic rocks (i.e., granitic intrusions). • Multi-element association of Pb, Cu, and Zn, which is thought to reflect granitic

environments. • Shear zones that provide weak areas for the granitic intrusions.

Lineaments and shear zones were extracted from both the geological map and the ASTER imagery. Shear zones were also obtained from interpretations of aeromagnetic data by ERIPTA (Economic Recovery of Investment Programme for Technical Aid) project. Meta- and late granites and axial traces were extracted from the geological map while circular features and alterations zones were extracted from the ASTER imagery. A multi-element association geochemical signature was extracted from the stream sediments geochemical data. Extraction of different granitic intrusions was conducted from the radiometric data. 4.1 Structures and lithological units extracted from geological map Four geological maps of 1:100 000 scale cover the study area. From the geological maps, lithological units and structures (faults, shear zones and axial traces) were digitized into different segment layers. 4.1.1 Lithological units The boundaries of lithological units were digitized into a segment map, which was then polygonized. Each polygon was assigned the name of a lithological unit. The polygon map was rasterized from which a map of classified generalized geological units was created. A raster map showing metagranites and microgranites was then extracted from the raster map of classified lithological units.

Chapter 4 25

4.1.2 Faults Faults were digitized from the geological maps and each fault was labelled by its general orientation (NE, NW, E and N). Fig. 4.1 shows the digitized faults from the geological map and a rose diagram, which provides a general view of the orientation of the faults. The rose diagram shows two main orientation directions, i.e., NE and NW. There is also a small number of EW and NS trending faults. The NE trending and NW trending faults were extracted for further use in the analysis of zones with potential for occurrence of granitic-pegmatites.

4.1.3 Shear zones Shear zones were digitized from the geological map and the ASTER imagery. Fig. 4.2 shows the shear zones extracted from geological maps. Shear zones interpreted from aeromagnetics by ERIPTA project (Economic Recovery of Investment Programme for Technical Aid) were also merged with shear zones extracted from both ASTER imagery and the geological map. Extraction of shear zones from ASTER imagery is discussed in section 4.2.3. The digitized shear zones from different data sources were then merged. 4.1.4 Axial traces of folds Axial traces with a NE orientation were digitized from the geological map. The granitic-pegmatites are thought to be coeval with the deformation phase that generated the set of folds with a NE trending axial traces. Fig. 4.3 shows the axial traces digitized from geological map.

Fig. 4.1. Faults digitized from geological map.

26 Extraction of Spatial Indicative Features

Fig. 4.2. Shear zones digitized from geological map.

Fig.4.3. Axial traces digitized from geological map.

Chapter 4 27

4.2 Structural features extracted from ASTER imagery 4.2.1 General Advanced Spaceborne Thermal Emission and Reflectance Radiometer or ASTER is a high spatial resolution multispectral imaging radiometer on NASA’s Earth Observation System TERRA platform. It was launched on December 18, 1999. ASTER detects electromagnetic energy in the VNIR (visible to near infrared), SWIR (short-wave infrared), and TIR (thermal infrared) regions of the electromagnetic or EM spectrum (Volesky et al., 2002). There are three VNIR bands, six SWIR bands and five TIR bands. The spatial resolution ranges from 15m for the VNIR bands, 30m for the SWIR bands and 90m for the TIR bands. ASTER has a swath width of 60km. ASTER imagery has been used in discriminating geological units, and minerals including kaolinite in argillized sandstones, and muscovite in sericitized granites, as well as commonly occurring illite/muscovite (Rowan, 1997). ASTER imagery has also been used to map and understand regional structures, like faults and shear zones to help in mineral exploration (Volesky et al., 2002). For the present research, the ASTER imagery was used to map lineaments, circular structures, shear zones, silicic zones and alteration zones, which may be used as spatial guides to zones with potential for aquamarine bearing pegmatites in the study area.

Six scenes of ASTER imagery were acquired for the study area, but one scene covering the eastern part has high percentage of cloud cover and could not be of any use so it was discarded. The scenes were obtained at different periods as shown in Fig. 4.4. The images were corrected for atmospheric ‘haze’ to enhance image contrast as below. Light scattered by the atmospheric constituents that reaches the sensor constitutes ‘haze’, which has an additive effect resulting in higher DN values and a decrease in the overall contrast of the image (Prakash, 2001). The histograms for each band were checked for atmospheric haze effect. Corrections for the haze effect were made by subtracting from all the pixels the offset at which the DN-values of the images began. The result is an image with DN-values starting at zero. This procedure was repeated on all the images. No sun angle correction was conducted on the images because the images were obtained at the same period of the year. According to Prakash (2001), solar elevation angle changes according to the season of the year and as a result, the image data of different seasons are acquired under different solar illuminations. As shown in Fig. 4.4, the images were acquired the same season of the year (summer for southern hemisphere). The five scenes were georeferenced using tie points identified from the imagery and the 1:50,000 scale topographic maps. To improve the visualization of individual bands, contrast enhancement was conducted on all SWIR and VNIR bands using histogram equalization. Histogram equalization stretches the DN values to cover the range of the 256 digital numbers. This approach attempts to put a similar number of digital values in every portion of the distribution, thus the choice for this approach. Linear stretching could also be used. The SWIR bands were resampled using the nearest neighbour


Fig. 4.4. Area covered by ASTER imagery (shown as color composite with band 6 in red channel, band2 in green channel and band 1 in blue channel) and their respective dates of acquisition.

11/11/01

11/11/01

algorithm to a spatial resolution of 15 m for integration with the VNIR bands to extract particular geological features. 4.2.2 Extraction of lineaments A lineament is a mappable, simple or composite linear feature of a surface whose parts are aligned in a rectilinear or slightly curvilinear relationship and which differs distinctly from the pattern of adjacent features and presumably represents a subsurface phenomenon (O’Leary et al., 1976). To detect lineaments from ASTER data, two procedures were carried out. First, a suitable color composite was generated from which lineaments were interpreted visually. Second, directional filters were applied to all band images to enhance detection of lineaments. Color composites are among the most basic forms of images that can be used for rapid and first-order analysis of remotely sensed data (Mustard and Sunshine, 1999). Prior to any band combinations, optimum index factor (OIF) can be calculated to determine optimum band combinations. This technique can be used to rank R-G-B combinations of bands and to select bands for principal components analysis, etc., based on the amount of correlation between bands and total variance within individual bands (Chavez et al., 1984). Band combinations with high OIF values usually give more information. Although OIF simplifies the selection of band combinations, it does not always provide a combination suitable to convey specific information desired by the user. Table 4.1 shows the OIF values for the SWIR and VNIR band combinations.

05/09/01

05/09/01

02/12/02

Chapter 4 29

However, the band combination found most suitable for visual interpretations of lineaments, and shear zones was bands 6, 2 and 1 in red, green, and blue channels, respectively (Fig. 4.4). The band combination that was found suitable for visual interpretation of circular features was bands 8, 3, and 1 in red, green and blue channels, respectively. Table 4.1. Band combinations, OIF value and ranking

Band combination OIF value OIF Rank 1-3-8 116.09 1 1-3-9 115.48 2 2-3-8 114.58 3 3-4-8 114.45 4 2-3-9 113.68 5 3-8-9 113.33 6

The filters below (Fig. 4.5) were designed and applied on individual bands to enhance linear features but the results were not better than the 6-2-1 color composite for visual interpretation of lineaments.

2 -1 -1 -1 -1 2

-1 2 -1 -1 2 -1

-1 -1 2

2 -1 -1

(a) (b) Fig. 4.5. 3x3 filter kernels used to enhance (a) NE trending lineaments and (b) NW trending lineaments. The lineaments interpreted on the 6-2-1 color composite were digitized and labelled according to their orientations (i.e. NS, NE, NW, and EW). Fig. 4.6 shows the lineaments (and a rose diagram) extracted from the ASTER imagery. The rose diagram shows four dominant sets of lineaments directions, 0o-60o, 60o-90o, 90o-120o, 120o –180o. From the lineaments interpretation, four deformation phases are postulated to have occurred. The relatively early deformation was the major NE faulting. The NE shearing followed this episode. The NW faulting succeeded the NE shearing. The last episode seems to have been another NE faulting which could have been contemporaneous or post NW faulting. The lineaments were then checked against the faults digitized from the geological maps. There are more lineaments interpreted from the ASTER imagery compared to those indicated on the geological map. Duplicate lineaments were then deleted. The lineaments were then merged with the faults digitized from the geological maps (Fig. 4.7). A rose diagram (Fig. 4.7) for the merged lineaments shows the same general trends as the one for the lineaments interpreted from ASTER imagery.


Fig. 4.6. Lineaments (and rose diagram) extracted from color composite (R = band 6; G = band 2; B = band 1) of ASTER imagery.

Fig. 4.7. Lineaments (faults) digitized from geological map and interpreted from ASTER imagery and corresponding rose diagram.

Chapter 4 31

4.2.3 Extraction of shear zones A ductile shear zone is a zone where the rocks reacted in a ductile manner to stresses and strains during deformation leading to formation of foliations and lineation with mylonitic fabric. Mylonites are foliated rocks in the shear zones. They are normally well foliated, banded and often with linear shape fabric (Jessell et al, 1997). In this research, the zones of lineation and banding were interpreted as shear zones. Fig. 4.8 shows the shear zones interpreted from ASTER imagery. The shear zones interpreted from ASTER imagery, geological map and those interpreted by ERIPTA project from aeromagnetics were then combined. Shown in Fig. 4.9 are merged shear zones interpreted from datasets mentioned above.

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4.2.4 Interpretation of circular features Rolet et al. (1995) applied remotely sensed data to identify different types of granites based on structure. They concluded that circular features correspond to concentric fractures formed during cooling of small granitic bodies or hidden granitic domes. El-Rakaiby (1995) used coloured composite ratio images to discriminate younger granites, which are associated with uranium mineralization from other granitic masses. Each of the granitic groups possesses certain image characteristics, such as colour and surface texture. In this research, circular features were interpreted and digitized from the color composite shown on Fig. 4.4. The circular features were interpreted in relation to lineaments to establish the relationship with different lineament sets. Lineaments sets were recognized by the spatial arrangement and overall distribution of the individual interpreted structures.

Fig. 4.8. Shear zones digitized from ASTER imagery


Fig.4.9. Merged shear zones interpreted from geological map, ASTER imagery and the aeromagnetics data. (Aeromagnetics interpretation by ERIPTA).

Fig. 4.10. Circular features and lineaments interpreted from ASTER imagery

Chapter 4 33

Fig. 4.10 shows the circular features map with lineaments. The identified circular features tend to terminate along the NW trending lineaments but are occasionally cut by a set of NE trending lineaments. The circular features seem to be controlled by NW trending faults. The metagranites extracted from the geological map do not exhibit spatial relation to the circular fracture patterns. The circular features are interpreted as indicative of hidden granitoids or minor granites and will be used further in this study. 4.2.5 Extraction of alteration zones ASTER imagery bands in the VNIR and SWIR regions of the EM spectrum can be used to map clays, carbonates, hydrous sulfates, and iron oxide minerals, which are generally associated with hydrothermal alterations. These minerals exhibit diagnostic absorption features in the VNIR and SWIR regions of the EM spectrum (Rowan et al., 2001). The granitic-pegmatites have been identified to be associated with alteration zones ranging from a few meters to 150m accompanied by a mass of kaolinitic clay (Subhash et al., 1998). An attempt was made to interpret areas with kaolinite and those with iron oxide minerals. This was done to show areas that are hydrothermally altered and those that are associated with kaolinization due to pegmatites weathering. It is assumed that in hydrothermally altered areas kaolinite, hydrous iron oxides and iron oxide minerals will be present while in areas where kaolinization is due to pegmatites weathering will have only or mainly kaolinite. Kaolinite or clay minerals in general have low reflectance in the spectral region of 2.08-2.35 µm (bands 5-8). The clay minerals have high reflectance in the spectral region of 1.55-1.75 µm (band 4). Ferric iron oxides (limonite) have high reflectance in the spectral region of 0.63-0.69 µm (band 2-3) and low reflectance in the spectral region of 0.45-0.52 µm (band 1). The wavelength coverage of each ASTER image band is given in Table 4.2.

Table 4.2 ASTER imagery bands and their respective wavelength

Band Wavelength (µm) 1 0.556 2 0.661

3b 0.804 3n 0.807 4 1.656 5 2.167 6 2.209 7 2.262 8 2.336 9 2.400

10 8.291 11 8.634 12 9.075 13 10.657 14 11.318

Based on the spectral regions of low and high reflectance a band ratio of 4/6 will enhance areas rich in clay minerals or sulphates and a band ratio of 2/1 will enhance areas rich in ferric iron oxides (limonite). According to Lipton (1997) a band ratio of

4/3 shows little spectral contrast in areas with either clay minerals or limonite. Band


ratios of 4/6, 2/1, and 4/3 , were therefore generated and a colour composite with band ratios of 4/6 (in red channel), 2/1 (in blue channel), and 3/4 (in green channel) were made. From such a combination clay rich areas will be red, ferric oxide areas will be green and where both are present it will be orange or yellow. To determine whether the Lundazi pegmatites are associated with the interpretable clay minerals areas, the areas in red colour were compared with known granitic-pegmatite occurrences. The colour composite band ratio images were not useful to delineate/interpret alteration zones associated with the granitic-pegmatites. This could be because of the small size of the granitic-pegmatites, although initially it was thought that it would be possible to interpret them from colour composites o

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