Mapping Petrological Patterns in the Regoufe Granite by Integrating Geochemical, Vnir-Swir and Gamma-Ray Spectrometry Data

MAPPING PETROLOGICAL PATTERNS IN THE REFOUGE GRANITE BY INTEGRATING GEOCHEMICAL, VNIR-SWIR AND GAMMA- RAY SPECTROMETRY DATA

Data Gabriel February, 2007

MAPPING PETROLOGICAL PATTERNS IN THE REGOUFE GRANITE BY INTEGRATING GEOCHEMICAL, VNIR-SWIR AND GAMMA-RAY SPECTROMETRY DATA

Data Gabriel

Thesis submitted to the International Institute for Geo-information Science and Earth Observation in partial fulfilment of the requirements for the degree of Master of Science in Geo-information Science and Earth Observation, Specialisation: (Earth Resource Exploration)

Thesis Assessment Board

Prof. Dr. F. D. van der Meer Prof. Dr. S. B. Kroonenberg Dr. E. M. Schetselaar Dr. S. P. Vriend Dr. M. van der Meijde Drs J. B de Smeth

INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION ENSCHEDE, THE NETHERLANDS

Disclaimer

This document describes work undertaken as part of a programme of study at the International Institute for Geo-information Science and Earth Observation. All views and opinions expressed therein remain the sole responsibility of the author, and do not necessarily represent those of the institute.

Abstract

The VNIR-SWIR and gamma-ray spectroscopic techniques are cost effective mapping tools as compared to the conventional methods in geological mapping and mineral exploration. The techniques were applied to map petrological and geochemical patterns on the Regoufe granite. The Regoufe granite is a specialized Sn-W granite in the North of Portugal belonging to the Hercynian massifs. Petrological units in the Regoufe granite are porphyritic two-mica granite grading through a transition zone into the muscovite-albite granite. Hydrothermal alteration recognized in the granite include albitization, muscovitization and apatitization. The objective of this study is to establish the relationship between VNIR-SWIR and gamma-ray spectral signatures and petrological/geochemical patterns and determine their significance with respect to Sn-W exploration. Laboratory reflectance spectra of rock samples taken from the Regoufe granite were determined and analyzed to obtain mineralogical information and derive spectral patterns on the granite. ASTER satellite image data of Regoufe was processed and analyzed by band ratioing and Spectral Angle Mapper to extract spectral signatures related to geology. A ground gamma-ray survey was conducted on the Regoufe granite to acquire radioelement concentrations of K, Th and U so as to map gamma-ray signatures related to petrology and hydrothermal alteration. The data obtained were integrated and compared with petrological and geochemical data using GIS software and spatial and multivariate relationships were evaluated. The laboratory reflectance studies showed significant variations in the absorption wavelength position and depth in muscovite and showed some correlation with the petrological zonation and geochemical data. Although the Regoufe granite can clearly be distinguished from its hosting metasediments, the ASTER data poorly correlates with petrological and geochemical zonations within the granite itself. This is mainly a result of poor spatial resolution and influence of vegetation and soil cover that modify spectral features and lack of minerals diagnostic for differentiating muscovite-albite granite from porphyritic two-mica granite. The position of the muscovite absorption feature, however, may be related to its composition and potentially can be used to differentiate Fe-Mg rich muscovite that is an alteration product of biotite from primary muscovite that crystallized from evolved magmatic differentiates. Gamma-ray data significantly correlates with the petrological and geochemical data: the Th/K ratio clearly discriminates between the petrological zonation in the Regoufe granite while the U/Th ratio is associated with hydrothermal alteration. VNIR-SWIR spectral methods require ground knowledge to interpret the signatures correctly while the gamma-ray survey methods provide good and effective means of mapping patterns on the granite related to petrology, hydrothermal alteration and mineralization.

Key words: Hercynian granite, wavelength, absorption, radioelement concentrations, hydrothermal alteration, correlation and minerals.

i Acknowledgements

I acknowledge the staff members of AES programme in ITC for giving me the skills in earth science exploration and research. Special acknowledgments go to my supervisor’s Dr. E. Schetselaar, Drs. B. de Smeth., and drs. F van Ruitenbeek for their guidance towards the data acquisition and writing of my MSc. Research Thesis, thanks for their critical comments. I salute Dr. S. Barritt who critically reviewed my interpretations of gamma-ray data. Dr. M. van Meijde is recognized for guiding me on gamma-ray spectrometry. I must extend special thanks to Dr Boudenwijn de Smeth who accompanied and supervised me during the field work and experienced the field hazards of my time; I will leave to remember my time with you on the specialized granite of Regoufe, God bless you. Special tribute is paid to Dr. Vriend S. of Utrecht University, Department of Geochemistry who provided the basic the research materials, geochemical data and organised preparation of thin sections. I will not forget my sponsor, the Ministry of Energy and Mineral Development of Uganda for giving me this opportunity to study further for Masters of Science degree in ITC. Enschede, the Netherlands.

ii Table of contents

1. Introduction ...... 9 1.1. Background...... 9 1.2. Research problem ...... 10 1.3. Motivation...... 11 1.4. Research objectives ...... 11 1.4.1. Specific objectives ...... 11 1.4.2. Research questions...... 11 1.4.3. Hypothesis...... 12 1.5. Research methods ...... 12 2. Literature review ...... 14 3. Geology and Geochemistry of the Regoufe area...... 18 3.1. Regional geology ...... 18 3.2. Local geology...... 20 3.3. Petrography of Regoufe granite...... 22 3.4. Geochemical characteristics of the Regoufe granite ...... 25

3.4.1. P, CaO, Sr, Na2O and K2O group...... 26

3.4.2. SiO2, the alkali and alkaline earth elements...... 26 3.4.3. Ore related elements...... 26 3.5. Weathering of Regoufe granite...... 27 3.6. Mineralization events in Regoufe granite...... 27 4. Petrological and geochemical patterns...... 28 4.1. Petrological pattern...... 28 4.1.1. Method ...... 28 4.1.2. Results...... 28 4.2. Geochemical signatures and trends ...... 32 4.2.1. Method ...... 32 4.2.2. Results...... 32 5. VNIR-SWIR reflectance spectroscopy ...... 40 5.1. Laboratory reflectance spectroscopy...... 40 5.1.1. Method ...... 40 5.1.2. Results...... 41 5.1.3. The effect of weathering ...... 44 5.2. Satellite image (ASTER) data analysis and interpretation ...... 46 5.2.1. Method ...... 46 5.2.2. Results...... 47 6. Gamma ray spectrometry survey...... 55 6.1.1. Gamma ray spectrometry ...... 55 6.1.2. Results of Gamma-ray spectrometry...... 56 7. Integration and evaluation of VNIR-SWIR and gamma-ray data with geochemical data ...... 64 7.1. Petrological/geochemical characteristics versus VNIR-SWIR spectral data ...... 64 7.2. Petrological/geochemical characteristics versus gamma-ray data...... 65 7.3. Petrological/geochemical characteristics versus gamma-ray data...... 69

iii 8. Discussions and conclusions ...... 71 8.1. Discussions ...... 71 8.2. Conclusion ...... 73 8.3. Recommendations...... 74 References ...... 75

iv List of figures

Figure 1-1: Well exposed granite outcrops (572150mE, 4526700mN)...... 13 Figure 1-2: Vegetation and soil covered granite (570550mE, 4527250mN)...... 13 Figure 3-1: Regional geology, Northern Portugal (after Soen, 1758)...... 20 Figure 3-2: General geology of Regoufe area (after Sluijk, 1963)...... 21 Figure 3-3: Petrological map of Regoufe granite (after Vriend, 1985)...... 22 Figure 3-4: Distribution of biotite and tourmaline in Regoufe granite (after Sluijk, 1963) ...... 23 Figure 3-5: Outcrop surface of the Regoufe porphyritic two-mica granite (UTM X: 570518, Y: 4527418) plagioclase, K-feldspar megacrysts (white) and biotite (black)...... 24 Figure 4-1: Petrological map of Regoufe granite modified after Vriend, 1985...... 29 Figure 4-2: Microphotographs of selected thin sections showing mineralogy and textural relationships of rock-forming minerals of the Regoufe granite...... 30 Figure 4-3: Thin section of selected microphotograph scenes showing radioactive mineral inclusions and their hosts: ...... 31 Figure 4-4: Geochemical maps: (a) albitization and greseinization (factor 1), (b) Mineralization (factor 2), (c) albitization intensity and (d) CCPI. Symbols of varying sizes represent intensity...... 33 Figure 4-5: Maps showing; (a) muscovite, (b) albite and (c) biotite abundance calculated from CIPW mineralogic norm; symbols of varying sizes represent abundances ...... 34 Figure 4-6: Scatter plot of albite abundance (CIPW-norm result) versus albitization (alteration index) ...... 35

Figure 4-7: Ratio maps-Geochemical point data. (i): U/Th map; (ii): Na2O/K2O map ...... 36

Figure 4-8: Scatter diagrams for; (a) U/Th versus Na2O/K2O, (b) P2O5 versus Na2O/K2O, (c) Th versus CaO, (d) Th versus Fe-and Mg-oxides, (e) Arctan (Th/K) versus Zr, (f) ) biotite versus Zr, (g) biotite versus La, and (h) biotite versus Ce, (i) TiO2 versus Th, (j) Th versus biotite of geochemical data from the Regoufe granite...... 39 Figure 5-1: Reflectance spectra of rock samples from the Regoufe granite presenting various minerals ...... 41 Figure 5-2: Box plot of wavelength (nm) at Al-OH absorption feature versus minerals identified...... 42 Figure 5-3: Box plot for: (a) wavelength position of Al-OH absorption feature versus rock classification, (b) absorption depth at Al-OH feature versus rock classification...... 43 Figure 5-4: Point maps with varying sizes indicative of quantification; geology at the background....43 Figure 5-5: Reflectance spectra measured from rock samples taken from; (a) porphyritic albite granite, (b) muscovite-albite granite, (c) transition zone...... 44 Figure 5-6: Wavelength of Al-OH feature for fresh versus weathered rock surfaces...... 45 Figure 5-7: ASTER image, Regoufe granite area: Decorrelation stretched image of bands 4, 6, 8...... 47 Figure 5-8: Calibrated ASTER image of the 9 VNIR-SWIR bands displayed in RGB band combination 468 covering the Regoufe granite area...... 48 Figure 5-9: Spectra collected from ASTER image, Regoufe granite area...... 48 Figure 5-10: SAM classification of Regoufe granite (ASTER image) based on image spectra...... 50 Figure 5-11: Mapping muscovites by (a) band ratio 7/6...... 51 Figure 5-12: ASTER RGB combination of ratios 2/1:7/6:6/8 ...... 52 Figure 5-13: RGB Band ratio 7/6:3/2:5/7 histogram equalized...... 53

v Figure 5-14: RGB band ratio 7/6:3/2:5/7 histogram equalized with wavelength position points overlaid...... 54 Figure 6-1: Map showing transects and data points of gamma-ray survey...... 57 Figure 6-2: Maps showing radio-element concentrations along survey traverses: (a) eTh map, (b) K map, (c) eTh/K ratio map, (d) eU/eTh map and (e) eU map overlain on petrological map...... 60 Figure 6-3: Scatter diagrams for gamma-ray spectrometry data from Regoufe granite: (a) K versus eTh, (b) K versus eU, (c) K versus eTh...... 62 Figure 6-4: Kaolinization of granite in the northwest of Regoufe, along the road...... 62 Figure 6-5: Map showing K (yellow) and eU (red) anomalies...... 63 Figure 7-1: Scatter diagram for absorption depth versus muscovite abundance calculated from geochemical data of Regoufe granite...... 65 Figure 7-2: Scatter diagram for wavelength position for (a) Al-OH feature versus albitization index, (b) wavelength position for Al-OH feature versus CCPI...... 65 Figure 7-3: Scatter diagram for K versus eTh of both gamma-ray spectrometry data and geochemical data in different symbols described in the legend...... 66 Figure 7-4: Scatter diagram for (a) K (geochem) versus K (gamma); (b) U (geochem) verses eU

(gamma); (c) Th (geochem) versus eTh (gamma); (d) eTh (gamma-ray data) versus TiO2 (geochemical data); (e) eTh (gamma-ray data) versus CaO (geochemical data); (b) eU (gamma ray data) versus P2O5 (geochemical data)...... 68 Figure 7-5: RGB band ratio 7/6:3/2:5/7 histogram equalized with Th/K ratio points (gamma-ray data) overlaid...... 69 Figure 7-6: ASTER SAM classification with Th/K ratio points (gamma-ray data) overlaid...... 70

vi List of tables

Table 4-1: Geochemical classification parameters ...... 32 Table 6-1: Gamma-ray measurements at different sampling times...... 56 Table 6-2: Measurements at reference point...... 56 Table 6-3: The range and mean of radioelement concentrations and their ratios...... 58

vii

1. Introduction

1.1. Background Earth resource exploration has a great financial risk in searching for earth resources over large areas while any discovery made may not be economically viable. It involves high costs of labour and logistics to pinpoint at a location for exploitation. Exploration companies always seek to employ cost effective exploration techniques to acquire and reveal maximum information on earth resources. Ground gamma-ray survey and visible near infra-red (VNIR) to shortwave infrared (SWIR) spectral techniques provide quick and cheap tools in geological mapping and mineral exploration. These techniques, if explored fully, can supplement earth resource exploration by providing lead to target selection and sustainable planning and execution of mineral exploration programmes to logical conclusion. Conventional geological and geochemical methods are cumbersome activities involving extensive field work, requiring specialized skills and experience, sample taking, chemical analysis and interpretation. These costs can be reduced by using remote sensing techniques prior to detailed mapping and sampling campaigns, such that uninteresting zones are relegated.

The Regoufe granite area in NW Portugal was investigated geologically and geochemically in detail by Dutch geologists (Schermerhorn, 1955, 1956; Sluijk, 1963; Soen, 1958, 1970) of the University of Amsterdam and in the 1980s by (Gaans et al., 1985; Vriend, 1985) of the State University of Utrecht. Parts of the granite with its contact aureole in the surrounding phyllites are known to have Sn and W mineralization. Several tungsten-bearing quartz veins in and around the granite have been mined in the past with Minas de Regoufe being the major mine within the granite. Petrological patterns are characterized by variation in mineralogy while geochemical studies reveal that trace element trends and hydrothermal alteration imprints exist (Sluijk 1963; Vriend 1985).

Gamma-ray survey method has been applied extensively in support of geological mapping and mineral exploration since 1970s. It has been found to be suitable for mapping granites and their internal zonations (Goossens, 1992; Schetselaar, 2001; Welman, 1998). The use of gamma-ray spectrometry to determine concentrations of elemental potassium, thorium and uranium enables alteration mapping in a wide range of geologic settings (Davis & Guilbert, 1973; Kuhns, 1986; Schroeter, 1995).

Analyses of absorption features of specific minerals in the visible-short wave infrared (VNIR-SWIR) has provided a scientific background for the interpretation of remotely sensed spectroscopic data (Crowley, 1986; Gaffey, 1985; Grove et al., 1992; Hunt & R., 1989). VNIR and SWIR spectrometry has been used to identify minerals in mapping and mineral exploration (Bourdman et al., 1995; Crowley, 1993; Rowan et al., 1995; Rowan et al., 1996). Many minerals have characteristic spectral signatures or spectra dependant on various crystallographic factors and this can be used to map the surface composition. The spectra measured by a spectrometer can be analyzed to derive information on rock composition, mineral proportions and crystallinity to provide insight in geological processes

such as magmatic differentiation, regional and contact metamorphism, hydrothermal alteration and weathering (Pontual et al., 1997).

Remotely sensed data can be analysed to extract mineralogic information that lead to identification of targets in regional mapping and mineral exploration (Galvao, 2004; Yamaguchi & Naito, 2003). For wide applicability of spectral remote sensing techniques in regional mapping, ASTER data is hereby chosen owing to its availability and wide coverage. In granitic environments, not much has been done to study the relationship between VNIR-SWIR spectral signatures and patterns due to magmatic differentiation as well as hydrothermal alteration processes. However, the major constraint in using ASTER data is the detection of only a limited set of minerals with absorption features in SWIR wavelength region. It is not possible to detect and differentiate minerals with close diagnostic wavelength position due to poor spatial resolution. A typical example is the detection of primary minerals that are mainly restricted to micas such as muscovite and biotite in granites and minerals produced by the chemical alteration and weathering of the granite, such as clay minerals (kaolinite, illite).

Granites show differences in mineralogy, nature of xenoliths and unique textural characteristics (Pitcher, 1983). Differentiation in granitic magmas due to processes such as fractional crystallization, fractional melting and magma mixing and assimilation can be recognized by geochemical and mineralogical studies in granites (Pitcher, 1993). Tectonic setting and primary geochemical characteristics models for ore genesis in granites have been developed for several types of mineralization (Beus & Grigorian, 1977; Plant, 1984; Sawkins, 1976).

This research work seeks to evaluate the importance of VNIR-SWIR reflectance and gamma-ray spectrometry methods in mapping petrological and alteration patterns in the Regoufe granite. Previous petrological and geochemical studies (Gaans et al., 1995; Schermerhorn, 1955, 1956, 1959; Sluijk, 1963; Soen, 1958, 1970; Vriend, 1985) provide a background against which the geological significance of VNIR-SWIR spectroscopy and gamma-ray spectrometry measurements acquired in this study can be tested. The important factor in the study of Regoufe granite is that it is specialized Sn-W granite, with known imprints of hydrothermal alteration, mineralogical patterns and mineralization.

1.2. Research problem Conventional methods to search for ore minerals require lots of time and resources to carry out extensive fieldwork involving sample taking and recording critical observations about geology, geochemistry and mineralization of an area. Samples may be sparsely distributed over the study area; remote sensing technique offers a means to indirectly enhance the spatial density provided that a relationship with the primary data of mineralogy and geochemistry can be established. Spectral reflectance study in VNIR-SWIR region of rock samples, ASTER satellite image data analysis and gamma-ray survey techniques are fast and cheap methods that have been used in geological mapping and mineral exploration (Crowley et al., 1989; Davis & Guilbert, 1973; Rowan et al., 1996; Ruitenbeek et al., 2006; Schetselaar, 2000; Shives et al., 1997). These techniques will be tested for detecting petrological and geochemical patterns and any mineralization. Multi-spectral satellite image

10 data (ASTER) is cheap and readily available, thus its usefulness in granite terrains should be evaluated. Reliable results would solve the problem of cost and time as well as enable design of effective sampling plan where representative samples can then be taken for petrographic and chemical analysis.

1.3. Motivation The invention of advanced spectral data processing techniques and better understanding of spectral information in recent years encourages further research work in application of remote sensing techniques in geological mapping and mineral exploration as it is vital for target selection. The geology of the Regoufe granite area including its petrochemistry and mineralization has been extensively studied (Schermerhorn, 1955, 1956; Sluijk, 1963; Soen, 1958, 1970; Vriend, 1985). This provides knowledge and understanding of the geology and geologic processes of the area that can be related to spectral signatures in gamma-ray data and VNIR-SWIR spectral reflectance data. The availability of ASTER scenes with a substantial coverage of the earth’s surface calls for more research in its use for earth resources exploration for wide applicability of remote sensing techniques. Above all the Regoufe granite is mineralized granite which is interesting to study so as to relate other granites in Sn-W exploration elsewhere.

1.4. Research objectives The main objective of this research is to establish the relationship between gamma-ray and VNIR- SWIR spectral features and petrological/geochemical patterns in Regoufe granite and determine their significance to mapping and Sn-W exploration.

1.4.1. Specific objectives • To relate the VNIR-SWIR spectral reflectance data to petrological patterns, hydrothermal alteration, weathering and mineralization in the Regoufe granite.

• To extract geologic information from ASTER scenes of the Regoufe granite area related to mineralogical variations on the granite.

• To relate gamma-ray spectrometry signatures to geology and geologic processes in the study area.

• To determine the significance of the reflectance spectrometry and gamma-ray spectrometry data to Sn-W exploration.

1.4.2. Research questions • Which minerals of the Regoufe granite can be identified by reflectance spectrometry and how do they relate to geological processes?

• How do the ASTER spectral signatures vary with mineralogy and the geochemical trends on the granite?

• What geochemical trends can be revealed from the gamma-ray spectrometry data?

• Is there any spatial relationship between mineralized zones and spectral and geologic features in the Regoufe granite, and how can any of such relations be successfully used in mineral exploration for Sn & W in granitic terrain?

1.4.3. Hypothesis • Dominant spectral patterns are produced by minor mineral components which have absorption features at SWIR region and therefore can significantly characterize a rock or a group of rocks, at least within a well constrained geological setting.

• Radioelement concentrations over such small exposed granite can reveal distinct spatial patterns despite any weathering.

• Rocks/Minerals in the study area exhibit spectral characteristics that can be remotely distinguished from ASTER satellite image on the basis of composition and relative abundance of constituent minerals.

• Sn-W mineralization at Regoufe has spatial relationships with particular VNIR-SWIR and gamma-ray signtures.

1.5. Research methods Preparation and analysis of previous geological and geochemical data were done to derive patterns on the Regoufe granite using GIS and statistical methods. The petrological pattern after Vriend (1985) was adopted to use in validation/ evaluation while geochemical signatures and trends were based on 55 whole rock geochemical data analyzed by Vriend (1985).

Reflectance spectrometric study of rock samples taken earlier by Vriend (1985) and during this study was done in the 350-2500 nm wavelength region using Analytical Spectral Device (ASD) and Spectral Geologist software, to identify diagnostic minerals with absorption features in the VNIR-SWIR region and determine the spectral parameters that can be used to characterize the granite and deduce any information related to petrology and alteration. The ASTER image data of the Regoufe granite area was processed and analyzed using band ratioing and Spectral Angle Mapper techniques.

Using GR 320 gamma-ray spectrometer a ground gamma-ray survey was conducted on the Regoufe granite and the radiometric data acquired were analyzed and interpreted to reveal any petrological and geochemical patterns/trends on the granite. The relationship between gamma-ray and VNIR-SWIR spectral signatures and petrological/geochemical patterns were evaluated and their significance to Sn- W exploration determined.

During the field visit in this study, gamma-ray survey was conducted along six transects on the granite and rock samples were taken for laboratory reflectance spectrometry study. The vegetation/soil cover

12 and relief of parts of the granite were noted as they affect spectral signals in an image. See below the typical granite terrain of the Regoufe pluton (Figure 1-1 & 1-2).

Figure 1-1: Well exposed granite outcrops (572150mE, 4526700mN)

Figure 1-2: Vegetation and soil covered granite (570550mE, 4527250mN)

2. Literature review

The composition of a sample of particular granite may reflect the composition of the primary magma, metasomatic processes, metamorphism and weathering effects. Residual magma are possible products of crystal, liquid or gaseous differentiation in melts variously derived by the partial melting of sediments, igneous rocks or mantle materials (Pitcher, 1993). Granites carry some special signature indicative of their sources and further more each type relate to a specific geo-tectonic environment.

The genesis of radio-element enriched and particularly Sn-enriched granites has frequently been ascribed to crustal anatexis in areas of thickened sial in geosynclines or in collision orogenic belts (Tischendorf, 1973). These are termed S-type granites and the high level of evolution of such granites is attributed to separation of an aqueous trace-element enriched phase and their mineralization which collects in granite cusps following magmatic fractionation (Chappell & White, 1974).

In U-Sn granites such as those of the Hercynian granites in Europe or S.E Asia, which are found in settings characterized by rapid changes in subduction patterns and collision between thin microplates as well as in regions of large-scale crustal thickening, the granites postdate crustal melting and ductile deformation events by tens of millions of years (Plant et al., 1983; Taylor, 1979) and show no indicators of high crustal temperature like migmatization and high grade metamorphism. Instead they are discordant and occur in tensional settings characterized by block movement on steep faults (Taylor, 1979).

A different model for mineralization was developed and relates the metalliferous granites to subcrustal processes at destructive plate margins through deep faults at the end of orogenesis. As the water deficient magma rises to high crustal levels in seismically active zones, hydration occurs as a result of interaction with host rock fluids at high temperatures evolving into deuteric alteration of the granite. Water for hydration may be convected as a result of radioactivity. Finally the granite cools and cracks, initiating hydrothermal convection with the flow of formational and meteoric water through fracture systems (Plant et al., 1980; Simpson et al., 1979).

Individual case histories of minerals have been studied as a guide to processes in granites. Well formed flakes of muscovites intergrown naturally with biotite, and the presence of zoning are likely to be of primary magmatic origin while potassium feldspar megacrysts represent crystallization from melts (Pitcher, 1993). Mineralized granites show features indicative of high to low temperature water-rock interactions such as greisenization, growth of sub-solidus K-feldspar megacrystic granite and occurrence of veins and metasomatized breccia pipes containing fragments of granites or country rock in tourmaline or quartz-rich matrices. Albitization is common in such granites (Simpson et al., 1979; Taylor, 1979). Metals are mainly concentrated with the intervention of volatiles and brines that derive as end

14 products of particularly efficient and specialized differentiation processes. Sn-W metals occur within hydrothermal context of pegmatites and greisens (Pitcher, 1993).

Methods of identifying mineralized and hydrothermally altered granites using trace elements were studied (Beus & Grigorian, 1977; Plant, 1984). These include increased dispersion of trace element patterns, disruptions of inter-element correlations and ratios between particular trace elements and trace and major elements such as Rb/Sr, Rb/Ba, K/Rb, Sr/Ba, U/Th and Mg/Li. The intensity of hydrothermal alteration can be determined by alteration indices calculated from rock analyses. The basis of an alteration index is to determine key components enriched during alteration and key components depleted during alteration and to combine them in an index that varies from 0 to 100. It has been used as a tool for mapping hydrothermal alterations in mineral exploration by many geologists (Ishikawa et al., 1976; Large et al., 2001). Theoretically it can be expressed as follows;

Enriched Alteration Index = *100 Enriched + Depleted

The gamma-ray spectrometry survey method depends on the fact that only radioactive isotopes of the elements potassium (K), uranium (U) and thorium (Th) decay series produce gamma-rays of sufficient energy and intensity to be measured by gamma-ray spectrometry (IAEA, 2003). The emission of particles and gamma rays in radioactive decay is proportional to the number of disintegrating atoms. 40K is the radioactive isotope of potassium that decays to 40Ar accompanied by emission of gamma rays at 1.46 MeV energy which the detector measures. The gamma rays are used to estimate the total amount of K present in a rock since 40K and 40Ar occur in fixed proportions in the natural environment. For 238U and 232Th, gamma rays emitted by 214Bi at 1.76 MeV and 208Tl at 2.41 MeV respectively are measured and equivalent concentrations of U and Th are estimated in parts per million. The term equivalent is used for U and Th concentrations because they are determined through daughter elements Bi and Tl that occur far down in the decay series based on the assumption of equilibrium which may not be the case.

High sensitivity, quantitative gamma-ray spectrometry has been applied since the mid 1970s in support of geological mapping and mineral exploration. Properly calibrated detector systems provide accurate estimates of ground concentrations of the three most common, naturally occurring radioactive elements; potassium, uranium and thorium. Mapping radioelement characteristics of various lithologies provides important geologic information (Darnley & Ford, 1989; Shives et al., 1997). Gamma-ray spectrometry technique is ideally suitable to map radio-element concentrations of granitic and gneissic terrains due to generally higher contrasts exhibited in them. Ford and Ballantyne

(1985) recognised albitization trend indicated by high Na2O/K2O ratio in areas of specialized granite of Meguma in Canada. This trend is characterized by increase of Na2O/K2O ratio with increasing U and U/Th ratio. Ratios of radio-elements can provide important distinction between potassium associated with alteration and anomalies related to normal fractionation or differentiation (Shives et al., 1997).

Materials at the earth surface have characteristic reflectance spectra, a property that can be used to identify and map the materials at the surface. When light photon is incident on a surface, certain

photons of light are absorbed by minerals, others are reflected and scattered way, and can be measured by a spectrometer (Clark, 1999).

Photons of light are absorbed in minerals by 2 major processes namely electronic processes such as in transition metals giving rise to absorption features in VNIR region and vibrational processes, majority of which are related to bending and stretching of bonds in hydroxyl, water, carbonate and ammonia and between Al-OH, Mg-OH and Fe-OH bonds with absorption features in SWIR region (Pontual et al., 1997). The variety of absorption process and their wavelength dependence allow us to derive information about the chemistry of a mineral from its reflected or emitted light (Clark, 1999). Spectra measured by a reflectance spectrometer can be analyzed to determine spectral parameters such as absorption depth and crystallinity index to derive mineralogic information and processes such as relative abundance, grain size or crystal development (Pontual et al., 1997).

Shifting of wavelength positions of diagnostic absorption features represent change of compositional variations of minerals and/or formation of new minerals (Duke, 1994). Halloysite and Fe- oxides/hydroxides form as weathering products of white mica. The exact wavelength position of main absorption features of white micas near 2200 nm, which is related to chemical composition, shifts due to weathering (Ehara et al., 2005). Elements can substitute for Al in a crystal structure which becomes slightly distorted causing slight changes in Al-O-H bond lengths and shifts absorption band position. The shifts can be explained by substitutions affecting the octahedral sites for Fe and Mg known as Tschermak substitution(Duke, 1994; Swarze et al., 1997). This behaviour is common in muscovites and illites. Compositional variations may vary systematically in alteration system as a function of temperature and composition of altering fluids and with proximity to mineralized zones (Pontual et al., 1997).

Prominent spectral features in the VNIR-SWIR region do not necessarily represent the dominant minerals in a rock. However, dominant spectral patterns produced by minor mineral components can significantly characterize a rock or a group of rocks within a well constrained geological setting (LONGHI† et al., 2000).

The Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) aboard the Earth Observation System (EOS) TERRA plat form records solar radiation in 14 spectral bands (Yamaguchi et al., 1998) which are described as follows;

• Reflected radiation in three bands between 0.52 and 0.86 microns (VNIR) with 15m resolution and a back looking VNIR telescope of the same resolution. • Reflected radiation in six bands between 1.6 and 2.43 microns (SWIR) with 30m resolution. • Emitted radiation in 5 bands between 8.125 and 11.65microns (TIR) with 90m resolution.

The three bands in VNIR region are important sources for information on transition metals and vegetation, the six SWIR bands provide information on carbonates and hydroxides which have diagnostic absorption features within the wavelength region. Important rock forming minerals like quartz and feldspars display fundamental absorption features in the TIR wavelength region (Rowan & Mars, 2002)

VNIR-SWIR spectral analysis techniques have been successfully used for mapping lithologic units and hydrothermal alteration patterns in well exposed areas and in discriminating spectrally variant lithological units. Examples are spectral angle mapper, Band ratioing (Abrams, 1983; Rowan et al., 2006; Ruitenbeek et al., 2006), spectral matching (Rowan & Mars, 2002), and spectral indices (Yamaguchi & Naito, 2003).

(Goossens, 1992) studied the spectral discrimination of contact metamorphic zones by band ratios and its potential for exploration for granite related mineralization using TM image. The results obtained were useful in discriminating spectral variations due to differences in soil mineralogy.

3. Geology and Geochemistry of the Regoufe area

The Regoufe granite in northwestern Portugal is located about Universal Transverse Mercator (UTM) coordinates 571800 m E and 4526700 m N. It has an oval-shaped surface expression of approximately 6 km2 (Gaans et al., 1988) and can be accessed from the nearest major town of Porto via Arauca township from the east, about 3 hours drive.

3.1. Regional geology Geologically the Regoufe granite belongs to the Hercynian massifs which is exposed over large region in the north, west and centre of the Iberian Peninsula. The Hercynian in the Iberian Peninsula is classically subdivided in to granitic rocks and mainly NW-SE trending folded sedimentary complexes (Figure 3-1) with different grades of metamorphism (Sluijk, 1963). The meta-sedimentary rocks belong to Beira schist complex of pre-Ordovician rock sequence and can be grouped into two main units: 1) the pre-orogenic to syn-orogenic materials of lower Ordovician to Lower Devonian age and 2) the post-orogenic unmetamorphosed, continental deposits of Upper Carboniferous age formed in narrow tectonic basins. D1 deformational phase caused the Variscan crustal thickening that started around 360 Ma in the Central Iberian Zone (CIZ) autochthon inducing prograde metamorphism of Barrovian type (Abalos et al., 2002; Dallmeyer et al., 1997). During the early middle Carboniferous, the D1 contractional structures were variably overprinted by a major syn-collisional D2 extentional event attributed to a large-scale gravitational collapse of the thickened continental crust (Balda et al., 1990). The late stage D3 deformation is related to shearing (Aguado et al., 2000).

Granitic rocks of the Hercynian massif comprise at least two groups, an older and a younger group. The granite emplacement post-dates syn-collisional deformation (D1 + D2) and is predominantly correlated with the last variscan deformation phase D3, marked by vertical folding and intracontinental shearing (Balda et al., 1990).

The Iberian Variscan granitoids have been subdivided into two major groups namely the syn- kinematic emplaced about 340-320 Ma and the late-post-kinematic granites between 315 and 270 Ma (Pinto, 1979; Soen, 1970). The range of emplacement ages is based on Rb-Sr whole rock radiometric data.

Based on petrological and geochemical study, the Iberian granitoids have been classified into 2 main suites; a) strongly peraluminous leucogranites and two-mica granitoids, b) calc-alkaline granodiorites and biotite monzogranites associated with minor intrusions of basic and intermediate rocks. The two granitoid suites cannot be exclusively assigned to any particular group of ages (Ortega & Ibarguchi, 1989; Pinto, 1979). Most petrographic models favour partial melting of pure metasedimentary crustal sources for the highly peraluminous two-mica granites and leucogranites granitoids, categorizing it as S-type granites

18 (Reavy, 1989; Beetsma, 1995). (Holtz & Johannes, 1991) claim that the upper crustal metaigneous lithologies (orthogneisses) can also yield peraluminous melts and should be regarded as a possible source rocks for peraluminous granitoids. While for the granodiorite-monzogranite suite which is categorized as I and I-S transitional granite types, two models of formation have been suggested; they are either formed as 1) a product of hybridization of felsic crustal melts with mantle derived magmas followed by further contamination and fractional crystallization (Dias et al., 2002) and 2) partial melting of heterogeneous lower crustal metaigneous sources (Villaseca et al., 1998).

The granites of the younger group contain more abundant accessories (zircon, monazite, apatite, allanite) than the older group and have synneusis textures of K-feldspar, plagioclase and biotite. The plagioclase is An-rich and zoned with cores of oligoclase and andesine.

Tectonic events recorded in the Regoufe region are described as follows (Soen, 1970):

• Nearly isoclinal folding of the NW-SE trending Oporto-Satao syncline that occurred between the Devonian and the Westphalian D1 (folding phase).

• Thrusting along the axial zone of the syncline caused uplift and erosion. Then repeated movements along the same thrust zone between the Westphalian D2 and middle Stephanian (Lower Stephanian tectonic phase) caused folding of the deposited sediments.

• Intrusion of the younger Hercynian granites (the Regoufe granite) cutting discordantly across the syncline (Upper Stephanian tectonic phase).

• Finally there was transverse flexuring and faulting of the syncline related to later tectonic phase known as post-Lower Autunian phase (Soen, 1970).

In the northwest, the Cota-Viseu coarse porphyritic biotite monzogranite is intruded by the Castro Daire composite massif. This massif, first described by Schemerhorn (1956), consists of five main granitoid units, defining a concentric zonation pattern of progressively less basic compositions from margin to core (Lamelas hornblende-biotite quartz monzodiorite; Castro Daire biotite monzogranite; Calde coarse porphyritic biotite-muscovite monzogranite, Alva and Lamas two-mica monzogranites).

The Calde coarse porphyritic biotite-muscovite granite is the dominant lithological type within the Castro Daire intrusion. It is predominantly composed of quartz, K-feldspar (mostly perthitic microcline) and plagioclase (oligoclase - albite) showing oscillatory and complex zoning. Biotite is the main mafic phase. Muscovite is present in variable amounts. Apatite, zircon, monazite, xenotime and opaques occur as common accessory phases. Andalusite and masses of fibrolite needles of restitic origin and late-stage garnet and tourmaline are sporadically observed. Dark igneous enclaves and pelitic inclusions are unevenly distributed throughout this facies.

Figure 3-1: Regional geology, Northern Portugal (after Soen, 1758).

3.2. Local geology The Regoufe granite is specialized Sn-W granite that intruded into the Beira schist complex which occupies a large proportion of pre-Ordovician metasedimentary rocks. It lies northwest of Castro Daire batholith which is non-porphyritic biotite-rich two-mica granite (Schermerhorn, 1956). The granite has yielded an age of 280 ± 9 million years determined by Rb-Sr whole rock isochron method (Pinto, 1979) hence belongs to the younger post-kinematic group of the two major Iberian Variscan granite groups.

The Regoufe area forms part of the Beira-Douro synclinal zone with the granite at the centre which is a discordant pluton of magmatic intrusive origin and known to host tin-wolfram mineralization

20 (Sluijk, 1963). It is asymmetric in shape with a steep wall in the east and dips away at low angle in the west, which is thought to extend in the subsurface at shallow depth. The Regoufe granite is surrounded by contact metamorphic aureole of spotted phyllites with biotite, cordierite and andalusite, much wider in the west (3-4 km) than in the east (less than 1km) from the granite contact.

Figure 3-2: General geology of Regoufe area (after Sluijk, 1963).

The shape and preferential distribution of primary accessories, texture of the granite, the conformable foliation and the system of cross and diagonal joints in the coarsely porphyritic granites suggest that the granite originated under the influence of magmatic rather than orogenic pressures. The potash feldspar megacrysts began their crystallization under magmatic conditions in a partly fluid medium supported by the porphyrytic textural characteristics of the granites (Sluijk, 1963).

The magmatic phase is followed by the phase of metasomatism which is important in the history of Regoufe causing considerable secondary modifications of the mineralogical compositions (Sluijk 1963). The processes of partial or complete muscovitization of biotite, albitization of more calcium- rich plagioclase and microcline, muscovitization and quartzification of feldspars and the formation of apatite and tourmaline belong to this phase (Sluijk, 1963).

Pneumatolytic-hydrothermal tin-tungsten-bearing quartz veins appear to be genetically connected with the coarsely porphyritic biotite granite (Soen, 1970). The formation of the ore was accompanied by muscovitization and greisenization of the wall rocks.

3.3. Petrography of Regoufe granite Sluijk (1963) described the main rock type of Regoufe granite as muscovite-albite granite with porphyritic texture composed of 30-40% quartz, 20-25% albite, 10-20% microcline and 15-20% muscovite with biotite, tourmaline and arsenopyrite present in certain parts. Vriend (1985) distinguished two major rock types based on their mineralogical variation; a porphyritic two-mica granite with tourmaline grading towards the east to a muscovite albite granite rich in arsenopyrite, with virtually no K-feldspar megacrysts, biotite or tourmaline (Figure 3-3).

Figure 3-3: Petrological map of Regoufe granite (after Vriend, 1985).

22 Biotite: Biotite is restricted to two areas comprising a fifth of the exposed surface (Figure 3-4). It has a high content of radioactive inclusions namely zircon, monazite, xenotime causing distinct pleochroic haloes (Sluijk, 1963). Small idiomorphic apatite prisms, considered as an early magmatic origin, were found enclosed in the biotite. They are preferentially distributed in the biotite and in the muscovite derived from it suggesting a magmatic origin of the granite and early crystallization of biotite (Schermerhorn, 1955, 1956; Soen, 1958). Wherever biotite occurs, it always appears strongly attacked and partly replaced by muscovite. In the non-biotite bearing areas of Regoufe, this process is complete and biotite is completely replaced by muscovite. Evidence for this replacement is the presence of pleochroic halos around radioactive accessory phases in muscovites and the similarities of mica aggregates that still contain the relics of biotite (Sluijk, 1963). The leaching of biotite led to the development of secondary titaniferous minerals, now present as inclusions in pale biotite and muscovite (Schermerhorn, 1959).

Figure 3-4: Distribution of biotite and tourmaline in Regoufe granite (after Sluijk, 1963)

Albite: Most albite occurs as idiomorphic to hypidiomorphic crystals clustered together sometimes bearing signs of deformation. They are commonly enclosed and partly replaced by microcline megacrysts. Larger albite crystals sometimes contain rounded inclusions of quartz, possibly primary inclusions of early quartz crystals. Partial muscovitization of albite has been observed in Regoufe and is known to proceed farthest during local greisenization. Albite also contains allotriomorphic apatite as secondary inclusions. Another form of albite is represented by perthitic albite, intergrown with microcline in the megacrysts. It was concluded that two generations of plagioclase were present in Regoufe: one, more or less idiomorphic albite crystals apparently earlier and partly contemporaneous

with microcline, two, perthitic albite deposited as strongly replacing mineral in pre-existing microcline (Sluijk, 1963).

Microcline: Potassium feldspar megacrysts (Figure 3-5) mostly as Carlsbad twins are found to be microcline (Sluijk, 1963). It is also found to constitute the groundmass as less well-developed interstitial grains. Microcline contains biotite at the center as inclusions in biotite-bearing granite areas or muscovite that has taken the place of former biotite. Quartz also occurs as inclusions in microcline as rounded grains. Muscovite introduced secondarily are usually found along cracks and cleavage planes (Sluijk, 1963).

Figure 3-5: Outcrop surface of the Regoufe porphyritic two-mica granite (UTM X: 570518, Y: 4527418) plagioclase, K-feldspar megacrysts (white) and biotite (black).

Quartz: Quartz occurs as large allotriomorphic grains and idiomorphic crystals up to 1 cm in diameter and as interstitial components between other minerals. Inclusions of quartz are found in microcline and albite crystals and these represent early growth phases. It commonly forms veinlets in micas and other minerals (Gaans et al., 1985).

Muscovite: In general, muscovite appears to be a late mineral in the paragenesis, although tiny mica flakes enclosed in quartz along idiomorphic lines indicate that earlier crystallization had already begun when quartz could still grow freely (Sluijk, 1963). Many of the larger muscovite crystals measuring up 4 mm certainly had late origin with respect to fractional crystallization. Tiny muscovite flakes are always contained in albite crystals, often in large amounts. Further more fine colourless mica occurs along cracks, cleavages and crystal boundaries of other minerals. Muscovite that replaced biotite is characterized by presence of distinct yellowish pleochroic haloes around radioactive inclusions inherited from the original biotite.

Accessories: Zircon and apatite occur as a primary accessory inclusion in biotite with other radioactive minerals like monazite and xenotime. Apatite more importantly occurs as late phase mineral as interstitial grains between main components of Regoufe granite or small allotriomorphic grains enclosed in other minerals. It occurs in association with muscovite, tourmaline and arsenopyrite occasionally penetrating into cracks and cleavage traces.

24 Tourmaline: Tourmaline is found in two areas of Regoufe granite (Figure 3-4) as idiomorphic crystals sometimes as skeletal intergrowths with quartz, apatite and mica, and as interstitial grains. In places, it replaces biotite and appears to have attacked feldspars and muscovite. The mode of occurrence indicates that tourmaline is of late origin. Other accessories found in connection with biotite leaching are ilmenite, anatase and rutile. Arsenopyrite is common and related to quartz veins.

3.4. Geochemical characteristics of the Regoufe granite The Regoufe granite has high initial 87Sr/86Sr ratio which points to a sedimentary influence on the source magma (Pinto, 1979). Oxygen isotope study (Jong, 1984) concluded that the hydrothermal activity in Regoufe did not produce the characteristic O18 depletion implying non-meteoric origin of fluids. Vriend (1985) studied the Regoufe granite with the aim of discerning the trace element patterns/trends and determine their relationship to magmatic differentiation processes, post magmatic mineralization and late alteration. He obtained geochemical data from 55 rock samples taken at density of 10 samples per square kilometer using XRF or wet chemical analysis. Each sample consisted of composite of freshest chips taken over 500 m2.

Compared to averages in low Ca granites, the Regoufe granite shows extreme enrichment in Sn, W, Li and Cs; high contents of P, Ta, Rb, F and U; about normal concentrations of Cu, Zn and Nb and low concentrations in Sr, Ti and Zr (Vriend, 1985). Specialized granites affiliated with W and Sn mineralization have similar characteristics (Tischendorf, 1977).

Factor analysis by Vriend (1985) on the results showed the existence of three important factors. • Factor 1: High positive loadings for Rb, P, Nb and Ta and high negative loadings for Ti and Zr. Sn and F exhibit minor positive loading. It outlines a zone in the roof of the granite which is sulphide-bearing, contains little or no biotite and tourmaline and is relatively fine grained and megacryst free. This factor is thought to be associated with greissenization and albitization. • Factor 2, controlled by ore related metal elements Sn, Zn, W and Cu and moderate Rb and F is associated with mineralization. • Factor 3 is dominated by Li and Cs with moderate negative U.

Gaans et al., (1995) applied a method they called Integral Rock Analysis (IRA) on 90 slices of shallow drill core samples analysed by XRF. These samples were collected from different regions of the Regoufe granite representative of variations within the granite and in its derivatives to gain insight in the interaction of mineralogy with metasomatic processes. Study of trace element distribution within the granites and its derivatives indicated hydrothermal processes on rock chemistry throughout the granite. It was noted that post-magmatic processes, including mineralization, are related to various trace element trends within the granites. Albitization, W-Sn quartz veins, muscovitization, apatitization, disseminated W-mineralization and late sericitization were distinguished in the granite. Hydrothermal alteration generally increases from west to east. Using Electron Probe Mineral Analysis (EPMA) of samples selected from IRA data based on lithochemical anomalies, columbite-tantalite mineral was found in muscovite-albite granite.

Three subgroups of element association patterns were established by correlation analysis These are discussed for the different sample types, that is, muscovite-albite granite, porphyritic two-mica granite and a transition zone below (Gaans et al., 1995).

3.4.1. P, CaO, Sr, Na2O and K2O group This group is important in the study of the behaviour of apatite and the relation of Ca, Sr and Na to apatite. The correlation between CaO and P is much stronger for the muscovite-albite granite than porphyritic two mica granite yet significant correlations are observed between Na2O and CaO and P (apatite) in the porphyritic two-mica granite compared to muscovite-albite granite. Sr correlates with

K2O in porphyritic two mica granite and with CaO and P in the muscovite-albite granite.

Apatite occurs as secondary inclusions and veinlets in albite possibly related to albitization of a more Ca-rich plagioclase (Sluijk, 1963), in prophyritic two-mica granites this explains the correlation of

CaO and P with Na2O. Apatite related to muscovitization and greisenization seems to be of major importance in the muscovite albite granite. In muscovite-albite granite, considering the relatively high correlations between Sr, Ca and P, apatite is the dominant mineral containing Ca and Sr. In the muscovite-albite granite, Ca released with increased albitization and muscovitization is probably incorporated in the late apatite. In porphyritic two-mica granite, Sr preferentially substitutes for K in microcline and Ca substitutes for Na in albite. Hence, apatite is of minor importance for covariation of

CaO with Na2O and Sr with K2O in the porphyritic two mica granite.

3.4.2. SiO2, the alkali and alkaline earth elements In porphyritic two mica granite, Sr is preferentially contained in K-feldspars compared to micas which are represented by Cs (Zutphen, 1984). The correlation between CaO and Na2O is either related to substitution of Na by Ca in albite or the inclusion of apatite in albite. However, in muscovite albite granite K2O correlates with Cs and Sr with CaO reflecting muscovitization and apatization. K-feldspar is less and mica is relatively more abundant and therefore important in distribution of K2O. Na2O shows weak correlations with other elements in muscovite-albite granite indicating that albite is the only important Na-bearing mineral.

3.4.3. Ore related elements Sn, W, Ta and Nb form the group related to mineralization. Cs and Ti and for the muscovite albite granite also Rb show high correlations with these elements. In general, the Regoufe granite shows strong negative correlation of Ti with the ore elements Sn W, Ta and Nb (Vriend, 1985; Gaans et al., 1995).

Electron Probe Mineral Analyzer (EPMA) confirmed that the dominant host mineral for Nb is rutile enclosed in biotite, thus the importance of a Ti phase for distribution of Nb in the Regoufe granite for porphyritic two mica granite (Gaans et al., 1995). Muscovite-albite granite is enriched in Nb and Ta and depleted in Ti. Therefore it was suggested that besides the occurrence in rutile, Nb and Ta are probably partly incorporated in a distinct Nb-Ta mineral in muscovite-albite granite part. Columbite-

26 tantalite minerals were identified by Electron Probe Mineral Analyzer (EPMA) from samples taken from the northeastern part of Regoufe granite within the muscovite-albite granites.

In muscovite-albite granite, stronger correlations of Sn and Ti with Cs are observed while correlations of W, Nb and Ta with Cs are weaker. Here, the behaviour of Nb, Ta, W and Rb can be distinguished from that of Sn and Ti. In porphyritic two mica granite all ore elements appear to be strongly related to biotite. The ore elements substitute for major elements in biotite (Deer et al., 1966; Wedepohl, 1974) or are located in minerals developed during hydrothermal breakdown of biotite or may be contained in primary inclusions like ilmenite, zircon and rutile.

(Zutphen, 1984) found out that Sn in muscovites is too high to be explained by the breakdown of biotite alone. Muscovite therefore appears to be important in determining the Sn distribution in the muscovite-albite granite. Nb, Ta, W and Rb are slightly enriched in the muscovite-albite granite compared to the porphyritic two mica granite (Vriend, 1985). Ta, Nb and W are clearly related to Rb not Cs. Wolfram appears to be exclusively deposited by fluids as wolframite in quartz veins.

3.5. Weathering of Regoufe granite

In the Regoufe granite no trend of K2O content or any other element exhibit an obvious increase or decrease in concentration with depth. K2O is most sensitive to weathering of major oxides in granites (Chessworth, 1979). No significant imprint of chemical weathering can be deduced from the element behaviour. Therefore, influence of weathering on rock chemistry of the Regoufe granite is known to be negligible (Gaans et al., 1985).

3.6. Mineralization events in Regoufe granite Sluijk (1963) distinguished three successive mineralization stages: first stage is the main phase of hypogene vein filling that started with the growth of vein walls of cassiterite and arsenopyrite later accompanied by a little beryl and apatite. The second stage of vein filling started with quartz deposition in veins associated with arsenopyrite, cassiterite which diminished comparatively after the stage of vein-wall overcrusting, sphalerite and some beryl and wolframite. This phase deposited older quartz veins forming either individual veins or constitute the marginal parts of composite veins. The third stage of the main phase of hypogene vein filling resulted in the deposition of the younger ore material characterized by abundant crystallization of quartz and wolframite in veins and fractures. These veins are represented by the central part of composite veins or individual veins. Vriend (1985) reported small disseminated wolframite mineralizations in the NE part of Regoufe granite.

4. Petrological and geochemical patterns

4.1. Petrological pattern

4.1.1. Method Petrological patterns mapped by Vriend (1985) based on mineralogical observations are used to compare signatures of gamma-ray survey and VNIR-SWIR spectral data (see Figure 3-3). The petrological map was geo-referenced to ASTER image using 23 river/stream junctions as control points giving root mean square error of 18 m and then digitized to provide for analysis and evaluation of gamma-ray and VNIR-SWIR spectral data.

To confirm previous petrological descriptions by Sluijk (1963), four rock samples representing the different units in the granite were selected for thin section preparation. The mineralogy in the thin sections was studied under optical microscope in order to identify the constituent minerals and any radioelement-rich minerals and their textural relationships so as to compare with radio-element signatures from gamma-ray measurements. The thin sections were prepared in the petrological laboratory of the State University of Utrecht, courtesy of Simon Vriend.

4.1.2. Results

Figure 4-1 shows the petrological map of the Regoufe granite after Vriend (1985) digitized using ArcGIS map software. In the next chapters, the petrological map has been frequently used as a background upon which geochemical, gamma-ray spectrometry and VNIR-SWIR reflectance spectrometry data are overlain and compared.

Minerals identified by microscopic study of thin sections are quartz, albite, microcline, muscovite, pale to brown biotite, tourmaline, rare garnet, accessory minerals (zircon, monazite, allanite). Reddish brown high temperature and greenish biotite with black oxides of Ti and or Fe (Figure 4-2a) formed by leaching of biotite (green variety). Quartz crystals are strained with different zones displaying different extinctions at angles from 30-400. Large crystals of plagioclase show albite twinning and zoning with other crystals at right angles in some cases. Quartz inclusions are common in albite. Microcline forms small part in the thin section and is mainly found at the edges, showing some deformation. Muscovite usually with well developed cleavages is found in between crystals of feldspars and quartz and on the crystal surfaces (Figure 4-2 c & e). The reaction edges are observed between biotite and muscovite and also tourmaline (Figure 4-2 b & c). Garnet is non-poikiloblastic and euhedral suggesting a magmatic crystallization (Figure 4-2(f)).

Figure 4-1: Petrological map of Regoufe granite modified after Vriend, 1985.

In thin sections of rock sample from porphyritic two-mica granite, all the above minerals were identified but here biotite is abundant. Muscovite that replaces biotite often inherits accessory radioactive minerals identified by pleochroic halos developed around them.

Figure 4-2: Microphotographs of selected thin sections showing mineralogy and textural relationships of rock-forming minerals of the Regoufe granite.

(a) Reddish brown biotite interpreted as Ti-rich interlayered with greenish variety in plane-polarised light. Biotite has apparently exsolved into green biotite and black FeTi-oxides, most likely ilmenite. Elongate crystal of zircon is embedded in biotite. Alpha decay from U and/or Th in zircon causes pleochroic radiation halos; (b) Tourmaline attacks biotite thereby inheriting the radioactive minerals resulting in a pleichroic halo in tourmaline. Note radial cracks as a result of radiation damage. ; (c) Muscovite in cross polarised light with good cleavage and high birefringence replaces biotite. The zircon inclusion in muscovite is inherited from biotite, causing development of a pleochroic halo. Black minerals represent Ti-Fe-oxides formed by the break down of biotite. Apatite occurs as inclusions in quartz in the northeast sector of the view. (d) Textural relationship between muscovite (bluish-green) with tourmaline in cross-polarised light suggests that tourmaline is a later phase mineral with respect to secondary muscovite. (e) Muscovite occurs in between crystals of quartz and

30 tourmaline and is bordered to the south by albite crystal (mainly in extinction) Note that tourmaline replaces albite along cleavage planes in Albite. Low yellowish-blue interference colour indicates that this is a Fe-rich muscovite and there are high birefringent patches of residual biotite. Tiny flakes of muscovite are seen on the albite crystal reflecting later phase origin. (f) non-poikiloblastic euhedral garnet suggesting that garnet crystallized from melt.

Pleochroic halos are commonly seen in biotite crystals interpreted as radio-element signatures and muscovites derived from them. Elongated prismatic tourmaline is also observed to replace biotite and adopt radioactive minerals (Figure 4-2b). Radioactive element containing minerals such as monazite, allanite and zircon are also identified in the thin sections (Figure 4-3 a-d) and are described in the respective captions.

Figure 4-3: Thin section of selected microphotograph scenes showing radioactive mineral inclusions and their hosts:

(a) Zircon with pleichroic halos in the muscovite in cross polarized light; (b) Monazite with pleochroic halo in biotite in plane polarised light; (c) Cross-polarized view of microphotograph shown in b.; (d) Radioelement-rich zoned mineral (allanite) with pleochoric halo in biotite in plane polarized light.

4.2. Geochemical signatures and trends

4.2.1. Method Based on whole rock geochemical analysis of 55 samples of Regoufe granite by Vriend (1985), geochemical signatures as a function of trace and minor element association and alteration index, representing particular geochemical trends and alteration types respectively, were derived (Figure 4- 4a-d). The table below summarizes the geochemical map functions used in the derivation of patterns/signatures.

Table 4-1: Geochemical classification parameters Geochemical function Association Expression Factor analysis Albitization and greisenization (Rb * P * Nb *Ta)

(factor 1) (Ti * Zr) Factor analysis Mineralization (factor 2) Sn * Zn *W *Cu * Rb * F Albitization index Albitization Na2O *100 Na2O + K 2O Chlorite-carbonate- Substitution & compositional (MgO + FeO) *100 pyrite index (CCPI) variation (MgO + FeO + Na2O + K 2O)

Abundances of muscovite, albite, biotite and orthoclase were calculated in percentages from analytical results of major oxides using CIPW norms. CIPW method, first formulated by Cross, Iddings, Pirsson and Washington has been used by most geologists over many years (Large et al., 2001; Li, 2005). The results of the geochemical functions and abundances of muscovite, albite and biotite were grouped into five classes and plotted on a map in varying sizes using ArcGIS software (Figure 4-4a-d) to visualize parts of the Regoufe granite that are affected by hydrothermal alteration and the mineral distribution.

Geochemical trends determined by the correlation between elements or combination of elements U/Th versus Na2O/K2O, P2O5 versus Na2O/K2O, Th versus CaO, Th versus Fe-and Mg-oxides, Arctan

(Th/K) versus Zr, Th/K versus Zr, Th versus TiO2 and biotite versus Ce, La and Zr were studied by scatter plots. The ratio Na2O/K2O represent albite factor (Plant et al., 1985) and P2O5 is related apatite. Radio-element distribution and association in the granite were studied by K, Th, U and their ratios. Trace element Zr was used as indicator for zircon while Ce and La were used as indicators for monazite and allanite (Schetselaar, 2000) in order to determine radio-element-rich minerals found in the granite. The relationship between biotite and elements Zr, Ce and La were investigated to find out if any of the Th- and U- minerals are associated with it.

4.2.2. Results The figures below show geochemical maps corresponding to geochemical parameters derived from the 55 geochemical data by Vriend (1985).

32 (a) (b)

Figure 4-4: Geochemical maps: (a) albitization and greseinization (factor 1), (b) Mineralization (factor 2), (c) albitization intensity and (d) CCPI. Symbols of varying sizes represent intensity.

Factor analyses: The results of maps derived from factor analysis show distinct signatures in different parts of the granite. The albitization and greisenization map has high scores in the east corresponding to the muscovite-albite granite (Figure 4-4a). Factor 2 with high loadings of Sn, Zn, W, Cu and medium loadings of Rb and F (Vriend 1985) is associated with mineralization (Figure 4-4b).

(a) (b)

(c)

Figure 4-5: Maps showing; (a) muscovite, (b) albite and (c) biotite abundance calculated from CIPW mineralogic norm; symbols of varying sizes represent abundances

34 Alteration indices: These indices are measures of intensity of the hydrothermal alteration on rock chemistry. The results of the albitization alteration index show that alteration indices for unaltered rocks range from 39.95-45.09 while altered rocks are represented by indices ranging from 45.09- 53.48. The eastern, northern and central west parts of the Regoufe granite have high intensities of albitization (Figure 4-4c). The Chlorite-Carbonate-Pyrite Index (CCPI) in this case is a measure of availability of Fe and Mg ions in mineral composition. It is intended to outline the Fe-Mg-bearing minerals such as biotites.

The figures 4-5(a-c) show muscovite, albite and biotite abundance maps derived from the geochemical data determined by CIPW-norm. The evaluation of their relationship with spectral parameters is done in chapter 7.

In order to check the reliability of the calculations, albitization index was compared to albite abundance calculated using CIPW norm, the results show high correlation defined by Multiple R value of 0.915 (Figure 4-6). This indicates that the albitization index is consistent and proportional to albite abundance in the Regoufe granite reflecting the importance of metasomatic phase in distributing the albite.

) 32 % (

e t

i 30 b l

A 28

24 39 44 49 54 Na2O/(Na2O + K2O)

Figure 4-6: Scatter plot of albite abundance (CIPW-norm result) versus albitization (alteration index) Albitization trend: In the Regoufe granite locations with high U/Th ratios have corresponding high values of Na2O/K2O ratio (Figure 4-7i & ii), and display a trend defined by an increase of the

Na2O/K2O ratio with increasing U/Th ratios (Figure 4-8a) in the east which is related to post- crystallization alteration supported by petrographic studies and previous findings (Sluijk, 1963). The redistribution of elements involves interaction between rocks and solutions which contain varying proportions of acid ligands (F-, Cl-, HCO3-, HS-), strong bases (Na+, K+, Ca2+) and complexes of amphoteric elements (Si, Al, Zr, Sn, W) resulting in the replacement of the primary silicate and accessory mineral assemblages by secondatry silicate, gauge and ore minerals (Plant et al., 1985).

During hydrothermal fluid circulation, some of the above mentioned ions may be introduced. Uranium can be oxidized to uranyl ion forming complexes in aqueous solution and tends to be more mobile

than Th which does not have a higher oxidation state. The inverse relationship between U and Th with U concentrations generally increasing and/or Th concentrations decreasing could be a result of cumulative effects of igneous differentiation (monazite fractionation reduces Th-concentration) and varying intensities of post-magmatic alteration. The relationship with Na2O/K2O suggests that the ratio eU/eTh can be a useful lithochemical indicator for areas that have undergone albitization within granites.

(i) (ii)

Figure 4-7: Ratio maps-Geochemical point data. (i): U/Th map; (ii): Na2O/K2O map

Apatite factor: CaO and P form apatite factor in porphyritic two-mica granite together with Na2O, in the muscovite-albite granite (Gaans et al., 1988). Geochemical data from Regoufe granite reveal that there is increase in P2O5 with Na2O/K2O ratio. This is an unusual trend which deviates from the expected differentiation trends in acid granites where it generally decreases in concentration with increasing differentiation. It is however, thought to occur in granites affected by hydrothermal alteration (Ford & Ballantyne, 1983). Increase in P2O5 may therefore be used as an indicator for alteration activity in the Regoufe and elsewhere.

The trend due to apatite is demonstrated by variation plot of P2O5 versus Na2O/K2O ratio (Figure 4-

8b). High P2O5 values are found in the muscovite-albite granite supporting the finding of Sluijk (1963) from petrographic studies that apatite occur as secondary inclusion in albite. Figure 4-8c shows that CaO concentration decreases with Th concentration and high Th content is found in the porphyritic two-mica granite. This may suggest that the apatite in the muscovite-albite granite does not contain Th and this apatite may be secondary in origin. A positive correlation also exists between Th and TiO2 and between TiO2 and biotite reflecting association of TiO2 with biotite.

36 From the scatter plots of Th versus Fe, Mg and Ti (Figure 4-8d & i) it is observed that Th contents are high in Fe-, Mg- and Ti-rich minerals, mainly biotite found in porphyritic two-mica granite. Th content correlates with Zr and biotite as well as Ce and La (Figures 4-8e-h) suggesting that Th is present in zircon and monazite respectively which occur as primary accessory minerals in biotite (Sluijk, 1963). Similar associations between Fe-, Mg- Ti- oxides, Zr, Ce, La, Th and biotite have been found in peraluminous (S-type) granites of the western Canadian Shield, suggesting that the eTh channel in gamma-ray spectrometry surveys can be used to map variations in the abundance of biotite and its alteration products in S-type granites (Schetselaar, 2001 and references there in). In all cases the high Th contents plot in porphyritic two-mica granite which is in agreement with petrographic studies. The above geochemical characteristics/trends provide basis for the interpretation in the next chapters of VNIR-SWIR reflectance and gamma-ray spectrometry data, while their comparison and evaluation is done in chapter 7.

Granite type Granite type 20 0.8 Muscovite-Albite Muscovite-Albite granite (a) granite (b Porphyritic two Porphyritic two mica granite mica granite 0.7 Transition zone Transition zone 15

R = 0.54 0.6 5 h O T /

10 2 0.5 U P

0.4

0.3 R = 0.63

0 0.2

0.6 0.7 0.8 0.9 1.0 1.1 1.2 0.6 0.7 0.8 0.9 1.0 1.1 1.2 Na2O/K2O Na2O/K2O

Granite type Muscovite-albite (c granite R = 0.70 (d) 8 Porphyritic two mica 8 granite Transition zone

6 R = 0.43 6 ) ) m m p p p p ( (

h 4 h 4 T T

2 2 Granite type Muscovite-albite granite Porphyritic two mica granite 0 0 Transition zone

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.5 1.0 1.5 2.0 2.5 CaO (%) FeO + MgO (ppm)

Granite type Granite type 1.2 70 Muscovite-albite Muscovite-albite granite granite R = 0.79 Porphyritic two Porphyritic two mica granite mica granite 60 1.0 Transition zone Transition zone

50 ) K

/ 0.8 ) h T m (

p n p 40 ( a

t r

Z c r 0.6 A 30

0.4 R = 0.90 20 (e) (f) 0.2 10

10 20 30 40 50 60 70 1.2 1.5 1.8 Zr (ppm) Biotite (%)

Granite type Granite type Muscovite-albite Muscovite-albite 14 granite granite R = 0.75 Porphyritic two mica 40 Porphyritic two granite mica granite 12 Transition zone Transition zone

10 30 ) ) m m

p 8 p p p ( (

a e L 6 C 20

4 R = 0.72 2 10 (g) (h) 0

1.2 1.5 1.8 1.2 1.5 1.8 Biotite (%) Biotite (%)

R = 0.93 (i) R = 0.68 (j)

49 8 8

6 6 ) ) m m p p p p ( (

h 4 h

T 4 T

2 2 Granite type Rocktype Muscovite-Albite Muscovite-Albite granite granite Porphyritic two Porphyritic two mica granite mica granite 0 Transition zone 0 Transition zone

0.00 0.05 0.10 0.15 0.20 1.0 1.5 2.0 2.5 3.0 3.5 TiO2 (%) Biotite (%) Figure 4-8: Scatter diagrams for; (a) U/Th versus Na2O/K2O, (b) P2O5 versus Na2O/K2O, (c) Th versus CaO, (d) Th versus Fe-and Mg-oxides, (e) Arctan (Th/K) versus Zr, (f) ) biotite versus Zr, (g) biotite

versus La, and (h) biotite versus Ce, (i) TiO2 versus Th, (j) Th versus biotite of geochemical data from the Regoufe granite.

5. VNIR-SWIR reflectance spectroscopy

5.1. Laboratory reflectance spectroscopy

5.1.1. Method Reflectance spectra of 108 rock samples and 20 crushed samples collected from the Regoufe granite by Vriend in 1980s and during field visit in this study were measured and analyzed in laboratory using Analytical Spectral Device (ASD Fieldspec-Pro), a reflectance spectrometer, ASD ViewSpec software program, and Spectral Geologist, TSG software with the aim of:

• Identifying minerals, mapping their distribution and determining the relative abundance of the minerals with absorption features in the VNIR-SWIR region so as to derive spectral patterns.

• Studying the effect of weathering on the granite rocks in the study area.

• Finding relationship between geochemical anomalies or mineralization and alteration assemblages.

The ASD measurements of reflectance spectra of 41 rock samples collected by Vriend and 67 additional rock samples collected in this study were recorded in ITC laboratory in 2151 channels within spectral range of 350 to 2500 nanometres wavelength. Absorption features in the VNIR-SWIR spectral region related to transition metals, micas and carbonate minerals were examined.

With the aid of TSG, Spectral Geologist software, spectral analysis was carried out to determine diagnostic wavelength position and shape of spectra such as absorption depth, width and inflections at the shoulders, referred to as spectral features, so as to identify the characteristic minerals based on spectral matching with library spectra following (Grove et al., 1992). The software can identify up to two minerals and assigns relative weights to the minerals which reflect the relative proportion of one mineral to the other (Ehara et al., 2005).

Spectral parameters of absorption depth and crystallinity index were derived from the spectra to provide information on relative abundance and crystallinity of the rocks and then plotted on the map of the Regoufe granite. The relationships between spectral parameters and petrological/geochemical data were then evaluated by spatial association and /or correlation analysis.

The analysis of spectra of fresh and weathered rock surfaces of 32 rock samples from Regoufe granite taken during the field work was done to study the difference in mineralogical composition and spectral features so as to assess the effect of weathering on the Regoufe granite using muscovite spectral features.

5.1.2. Results The following minerals with absorption features in SWIR wavelength region were identified by the analysis of reflectance spectra of the Regoufe granite rock samples: muscovite, illite, halloysite and siderite. All spectra display diagnostic absorption feature at around 2200 nm wavelength which is interpreted as a result of the O-H stretch and the Al-O-H bend (Duke, 1994). Muscovite is the most dominant mineral with absorption feature in the SWIR wavelength reagion; therefore its spectral characteristics can be used to extract spectral signatures in satellite images.

Reflectance spectra selected to represent different mineral species with pure spectra and others in mixed proportions identified during the spectral analysis is presented in the figure (5-1) below.

Figure 5-1: Reflectance spectra of rock samples from the Regoufe granite presenting various minerals

Muscovite has a shallow water absorption depth near 1900 nm while illite has commonly deeper water absorption near 1900 nm (Pontual et al., 1997). The reflectance spectra of the rocks display shifting in wavelength position between 1999 and 2207 nm. There is considerable overlap in wavelength position of Al-OH feature for muscovite, illite and halloysite.

The figure below (Figure 5-2) shows the variation in wavelength of Al-OH feature with constituent minerals interpreted from laboratory reflectance spectra analysis. Average absorption feature of muscovite is centred near 2204 nm while illite, next mineral in abundance has absorption feature centred near shorter wavelength (2201 nm), both due to vibrations at Al-OH bond. Halloysite and siderite are limited in small parts of the granite.

Figure 5-2: Box plot of wavelength (nm) at Al-OH absorption feature versus minerals identified.

The depth of absorption is related to the abundance of the mineral and the grain size (Clark, 1999). The study of rock samples, previous petrographic classification of Regoufe granite and chemical analysis by Vriend (1985) provided background information about mineralogy and alteration in the study area.

The box plot of wavelength position of Al-OH feature against granite type (Figure 5-3a) indicates that in the muscovite-albite granite the wavelength position is centred near 2202 nm and do not exceed 2204 nm while the porphyritic two-mica granite is centred at 2204 nm with a wide range from 2202- 2207 nm. Reflectance spectra from the transition zone have intermediate wavelength position centred at 2203 nm in small range.

The figure 5-3b below presents a box plot of absorption depth versus the granite types. The absorption depth of Al-OH feature in porphyritic two-mica granite is generally lower, although it has a wide range, compared to absorption depths in the muscovite-albite granite (Figure 5-3b). This is attributed to the relatively higher muscovite content of the muscovite-albite granite causing strong absorptions due to Al-O-H bond and bend. Some of the muscovite may be supplied by later hydrothermal processes of muscovitization that is associated with albitization and greisenization.

42 (a) (b)

Figure 5-3: Box plot for: (a) wavelength position of Al-OH absorption feature versus rock classification, (b) absorption depth at Al-OH feature versus rock classification.

(a) (b)

Figure 5-4: Point maps with varying sizes indicative of quantification; geology at the background. a: wavelength, b: absorption depth of Al-OH feature.

The figures above (Figure 5-4a & b) show the wavelength position and the absorption depth of Al-OH feature plotted on the petrological map of Regoufe area in varying sizes. From figure 5-4a, the eastern part of the granite composed of muscovite-albite granite is dominated by shorter wavelength positions as compared to the western part, composed of porphyritic two-mica granite, with relatively large variation in wavelength positions. Such patterns may be explained by conditions favouring Tschermak substitution in the porphyritic biotite-muscovite granite given the availability of FeMg-containing

mineral biotite that release Fe and/or Mg ions that is available to substitute for Al in white mica. The absorption depth is generally higher in the muscovite-albite granite but varies widely in the porphyritic two-mica granite. It tends to be higher in mineralized zones (Figure 5-4b).

5.1.3. The effect of weathering The spectral absorption features in SWIR region in the fresh rock spectra analyzed reveal muscovites, illites and halloysite minerals in the rocks. The spectra of weathered rock surfaces are characterized by shallow absorption depths at around 2200 nm, inflections at the shoulders and broadening of absorption features especially at 1900 nm and 1400 nm wavelength and reduced overall albedo (Figure 5-5 a-c). Stronger absorptions develop at visible near infrared due to Fe containing mineral formed during weathering process. Halloysite, illites together with Fe- & clay minerals appear as new minerals as a result of weathering.

It is concluded that surface weathering has affected Regoufe granite. The sample spectrum obtained from porphyritic two-mica granite appears to be more weathered than the muscovite-albite granite. This is observed from the prominence of the above mentioned features such as absorption depths, broadening and inflections from the comparison of the two spectra (Figure 5-5 a & b). When the grain size effect on absorption depth in Regoufe granite was investigated, the difference in depth was found to be negligible.

(a) (b)

(c)

Figure 5-5: Reflectance spectra measured from rock samples taken from; (a) porphyritic albite granite, (b) muscovite-albite granite, (c) transition zone.

44 In order to determine the extent of weathering effect on the Regoufe granite, the wavelength of fresh rock surface versus that of exposed rock surface (Figure 5-6) was plotted. The preposition is that if weathering has not affected a rock surface then spectra measured from fresh surfaces and exposed surfaces should be the same. The results indicate that most points plot close to the line x = y and the other points do not show extreme deviation; therefore weathering of Regoufe granites is low. This is in agreement with the conclusion from chemical analysis that weathering in Regoufe is negligible (Gaans et al., 1985). Majority of rocks from the transition zone plot close to the line y = x, indicating relatively small shifts in wavelength position due to weathering. Many samples from porphyritic two- mica granite samples plot far from the line, therefore it can be concluded that, surface weathering most affects the porphyritic two-mica granite.

Granite type 2206 Muscovite-Albite granite Porphyritic two 2205 mica granite e

c Transition zone a f r u

s 2204

k c o r

h 2203 s e r f

- )

m 2202 n (

h t g n

e 2201 l e v a W 2200

2199

2200 2202 2205 Wavelength (nm)- weathered rock surface Figure 5-6: Wavelength of Al-OH feature for fresh versus weathered rock surfaces.

The VNIR-SWIR reflectance spectrometry study of rock samples from the Regoufe granite identified Al-OH absorption feature as the only diagnostic mineral related spectral feature in the SWIR region displaying shifting possibly due to Tschermak’s substitution. Muscovite is the dominant mineral, others are illite and halloysite. However, the spectral parameters namely wavelength position and absorption depth derived from the spectra show some general trends in the different granite types related to minerals and mineralization that may be used to categorize the granites. In Chapter 7, its relationship with geochemical indices reflecting mineral abundances, alteration/substitution intensities will be evaluated. Spectral assessment of surface weathering on the rocks indicates that the rocks have not undergone deep weathering since all the spectral features are largely preserved.

5.2. Satellite image (ASTER) data analysis and interpretation

5.2.1. Method Pre-processing: In order to focus attention to and assess spectral features within Regoufe granite, a subset of the area with the granite at the centre was made from the ASTER image which was taken in November, 2004 in cloud free condition. Pre-processing of subsetted image with all pixels unmasked was done to enable interpretation of reflectance spectra of the image data. This involved resampling all pixels to the same resolution and conversion of the radiance data of ASTER image to relative reflectance values. The VNIR and SWIR bands of ASTER image with different spatial resolutions of 15 and 30m respectively were arranged in same pixel size of 15m for the 14 bands to allow for arithmetic operations on pixel basis.

Empirical line calibration method was used to convert radiance values of the ASTER VNIR and SWIR bands to relative reflectance values by calculating empirical relation between reflectance spectra from bright and dark targets in the image and reflectance spectra from spectral library representing bright and dark materials respectively. Image spectra were collected from Regoufe tailings near the mines for a bright target and a black portion in the north west of the granite representing dark material. The image spectra are paired with halite mineral spectrum from USGS spectral library representing bright target and spectrum from asphalt in JHU library as dark target. The resulting image is a relative reflectance image which was used in spectral analysis of the ASTER image.

Reflectance spectra were collected from ASTER image in different parts of the granite showing different spectral signatures to study absorption features and their relation to ground characteristics. Band ratio method was used to map the minerals/materials with absorption features at bands based on image spectra. Spectral unit mapping was done using Spectral Angle Mapper technique with the image spectra collected as end members.

Preliminary analysis: Part of Regoufe granite and the surrounding Beira schist is fairly covered by vegetation. This obscures detection of spectral features related to mineralogy. In order to enhance statistical analysis, pixels most covered by vegetation were masked out. Green Vegetation Index (GVI) of Regoufe area was calculated according to equation below.

(band3 − band2) GVI = 127 +128* (band3 + band2)

From field observations some areas have over 60% vegetation cover. Pixels with GVI > 150 were then masked out to remove the effect of the most vegetated areas. The resultant image was subjected to decorrelation stretching techniques. The aim of this preliminary analysis was to extract various spectral signatures and generate targets for detailed investigation in terms of laboratory reflectance spectroscopy, petrographic study, and vegetation/soil cover/rock exposure and relief influence assessment.

46 5.2.2. Results Preliminary analysis results: The preliminary results indicate that many pixels in the surrounding phyllites to the north have GVI higher than 150 and are masked out. Three main spectral patterns were extracted from decorrelation stretched image of bands 4, 6 and 8 (Figure 5-7) and provided a clue for target selection for field investigations. The bright green–yellow colour representing well exposed muscovite-rich zones especially the granitoids, the bluish area partially covered with a short, thick vegetation and soil occasionally with black charred burns of plants in the northwest of the granite and the light and dark greenish parts in the middle of the granite as well as on the phyllites. The green parts represent spectral signature of associated with hill slopes and soil-vegetation cover. The bluish part seemed to have been burnt severely leaving a fire scar. Old burnt logs of wood were found in the area.

Legend Lithological boundary Well exposed granitoid/ muscovite rich zones Fire scar Low brightness due to topological effects Possibly Fe-rich

Masked pixels . Projection: WGS 1984, UTM Zone 29N

Figure 5-7: ASTER image, Regoufe granite area: Decorrelation stretched image of bands 4, 6, 8.

Image reflectance spectra: Reflectance spectra were collected from the calibrated image in relative reflectance values comprising the 9 SWIR-VNIR bands (Figure 5-8). The spectra show absorptions at bands 2 and 3 in the VNIR region and bands 6, 7 and 8 in the SWIR region (Figure 5-9).

5 4

3 1

Figure 5-8: Calibrated ASTER image of the 9 VNIR-SWIR bands displayed in RGB band combination 468 covering the Regoufe granite area.

Figure 5-9: Spectra collected from ASTER image, Regoufe granite area.

48 The yellowish colour in the image such as at the Regoufe tailing near the old mine (spectrum 1 in Figure 5-9) is represented by spectrum with absorption at band 6 (2209 nm) and an overtone at band 8 (2336 nm). This is interpreted as a result of vibrations and bends of aluminium hydroxide.

The spectrum collected from the blue area in the northwest of Regoufe granite ASTER image has absorptions at bands 3 and 8 and corresponding high reflectance in bands 2 and 5 (spectrum 2 in Figure 5-9). This zone has soil cover black in colour mixed with chars of burnt vegetation also old burnt log remains were common.

The dark brown part running east west in the middle of the granite (Figure 5-8) has a spectrum with absorption at band 8 with high reflection in band 6 and generally low albedo (Spectrum 3 in Figure 5- 9). In the VNIR region it has absorption at band 3 and high reflectance at band 2. This spectral characteristic is associated with steep slopes and seems to result from topographic effects.

The spectrum 4 collected from the in-lier in the east of the granite shows high reflectance in band 3 and absorption in band 2 characteristic feature of vegetation. In the SWIR region it has absorption in band 8 and high reflectance at the shoulder is in band 5 in the SWIR region possibly due to rock- vegetation mixing. The absorption features at SWIR band 7 is likely caused due to mineral-vegetation mixtures influence of non-photosynthetic vegetation that increases reflectance in the SWIR band 6 and decreases in band 7 (Galvao, 2004). Spectrum 5 has low reflectance at band 3 and absorption in band 1 due to Fe absorption, possibly as a result of weathering.

The image spectra and information on absorption feature position allowed mapping of the various spectral units in the image by SAM and band ratioing techniques. This was done using ENVI software program and are presented below.

Mapping spectral units: SAM: Using the spectra (Figure 5-9) collected from the image (Figure 5-8) as end members to detect pixels with similar angles of absorption features, 5 major spectral units were mapped (Figure 5-10). The red colour represents well exposed muscovite rich lithology (granite rock), the blue colour pixels identify with black soil cover and chars of fire burns. Pixels exhibiting Fe absorption is represented by cyan colour; it has absorption at band 1 as a result of absorption of Fe. The yellow colour pixels identify with the vegetation.

Legend Lithological boundary Well exposed muscovite-rich

Pixels with high vegetation cover Pixels influenced by topographical effects Fire burns, soil and vegetation Fe-rich zones . Projection: WGS 1984, UTM Zone 29N Figure 5-10: SAM classification of Regoufe granite (ASTER image) based on image spectra.

Band ratio: In order to map the spectral units over the ASTER image, band ratio technique was used. The image spectrum 1 characteristic of muscovite with absorption feature in the SWIR region has high reflectance in band 7 and absorption in band 6. It was mapped by the ratio band7/band6 (Figure 5-11).

Figure 5-11: Mapping muscovites by (a) band ratio 7/6

Spectrum collected from the blue area (Figure 5-8) in the ASTER image has absorption at band 8 and higher reflectance in band 5. Mapping this signature with ratio band5/band8 is difficult since the mapped spectra are mixed-up with other pixels. The effect of ferric and ferrous ions was tested with ratio band2/ band1, zones affected by intense Fe absorption within the granite was distinguished (Figure 5-12). This effect can be seen in the combination of band ratios in RGB. The image prepared from R = band 2/band 1, G = band ratio 7/6 and B = band ratio 6/8 categorized the Fe-rich minerals, muscovite-rich rocks and areas affected by vegetation cover and topographical effects. The red Fe- rich zones could possibly be a result of oxidation of ilmenite and other FeTi-species formed by the leaching of biotite. The high scores of FeMg elements in the porphyritic two-mica granite from geochemical data supplement this argument (Figure 4-4d in Section 4.2.2).

Fe-rich zone/ weathered

Legend

Lithological boundary

Band 2/band 1 Band 7/band 6

Band 6/band 8 . Projection: WGS 1984, UTM Zone 29N Figure 5-12: ASTER RGB combination of ratios 2/1:7/6:6/8

Vegetation in ASTER is mapped by the ratio 3/2, hence RGB combinations of band ratios 7/6:3/2:5/7 has mapped vegetation, muscovite rich zones and pixels with high interference of soil-mineral- vegetation and topographic effects (Figure 5-13).

52 Legend Lithological boundary Band7/band 6 Band 3/band 2

Band 5/band 7 . Projection: WGS 1984, UTM Zone 29N Figure 5-13: RGB Band ratio 7/6:3/2:5/7 histogram equalized.

Very few pixels in the ASTER image of the granite showed Fe absorption feature. The ASTER image does not show any discrimination in granite types nor the hydrothermal alteration. It was found to be useful only in discriminating well exposed granitoids, soil and vegetation.

Legend Lithological boundary Band7/band 6 Band 3/band 2

Band 5/band 7 Wavelength (nm) !( 2199.5 - 2201.6 !( 2201.6 - 2203.1 (! 2203.1 - 2204.0 !( 2204.0 - 2204.9 (! 2204.9 - 2206.6 . Projection: WGS 1984, UTM Zone 29N Figure 5-14: RGB band ratio 7/6:3/2:5/7 histogram equalized with wavelength position points overlaid.

There is no spatial association with spectral parameters obtained from laboratory reflectance spectrometry data. The period between the satellite image data acquisition (November, 2004) and the period of this filed work (September, 2006), no major event seems to have caused changes on spectral signatures in Regoufe granite.

54 6. Gamma ray spectrometry survey

6.1.1. Gamma ray spectrometry A ground gamma-ray spectrometry survey was conducted on the Regoufe granite using an Exploranium GR-320, a portable gamma-ray spectrometer with the aim of determining:

• The extent to which the distribution of the radioactive elements relate to petrological differences within the granite. • The extent to which radio-element abundances are recognizably influenced by hydrothermal alteration, mineralizing and differentiation processes.

• The influence of surficial weathering.

The instrument detector is composed of a NaI crystal with 0.35 litre volume and is fitted with a Cs source that allows the spectrometer to automatically maintain the position of energy channels with respect to peaks. The instrument was set to record measurements in 256 channels within the energy range 0-3.0 MeV with a sampling time of 100 seconds. The sampling time was decided during orientation survey where the spectrometer response was found to be close to the response for 600 seconds sampling time yet the relative radio-element concentrations rather than absolute concentrations were needed to observe contrasts in the gamma-ray signatures of the granite. The location of the measurement points were recorded from global positioning system (GPS) with positional accuracy of 20m and noted separately.

During measurements, the detector was kept in contact with the bed rock and each time relatively flat outcrops measuring over one foot in radius were targeted to obtain representative results. The source- detector geometry was kept constant for all measurements. Steep sections, cliffs and excavations were avoided as much as possible. This is because the response of the spectrometer depends on the size, location and geometry of radioactive source (IAEA, 2003). On a few occasions, several spectrometer measurements were made close to each other to determine if there is any variation due to specialized features such as veins and spectacular mineralogy. All measurements were stored in the instrument and downloaded to a computer every evening and their corresponding location coordinates entered. A total of 388 measurements were taken during the survey of which 337 were from the granite and 51 from the intruded phyllites, transecting all the petrological components.

Repeated measurements were taken at a reference point to check the performance of the instrument and environmental factors such as soil, moisture and/or radon retention in lower atmosphere. Occasionally, spectrometer measurements were taken for as long as ten minutes to assess the variations due to sampling time.

Six main traverses were made across the granite (Figure 6-1). Traverse A in the north western part transects the transition zone and porphyritic biotite rich granite. B runs through the muscovite rich

granite in the north, cuts the porphyritic granite zone, again enters mainly medium grained equigranular part in the east and ends just west of the phyllite inlier. Traverse C starts in the biotite free zone in the east and ends in the Regoufe mine area which is characterized by quartz veins, coarse grained porphyritic granites with biotite present. D starts from the east, enters towards the centre of the granite and transcends down the slope southwards cutting major structures of foliations and granite with various textures, mineralogical compositions and finally connects to the old Regoufe mine area. In the south a traverse E was made connecting Regoufe village through a transition zone to Covello de Paivo village underlain by two-mica granite. The last traverse F stayed within biotite rich megacrystic granite area although locally aplitic granite was encountered.

Geochemical trends within radio-elements were studied by scatter plots and ratios of the radioelement concentrations. Knowledge of the geology of the area provided background information to interpret gamma-ray survey data.

6.1.2. Results of Gamma-ray spectrometry The variations due to measurements at different sampling times (Table 6-1) and in different conditions (Table 6-2) are presented below to assess measurement reproducibility. The results show that the difference obtained between measurements taken for 100 seconds and 600 seconds vary within 10%. Measurements at reference point gave a fractional deviation of 1.84%, 8.08% and 7.37% for K, U and Th respectively (Table 6-2). The results are reproducible and therefore considered good.

Table 6-1: Gamma-ray measurements at different sampling times A B C D E F G H I 600 second 100 seconds Differences

K2O U Th K2O U Th A-D B-E C-F 1 4.031 7.865 5.168 4.087 7.071 5.115 -0.056 0.794 0.053 2 4.116 8.191 7.879 4.247 7.499 7.914 -0.131 0.692 -0.035 3 3.148 4.549 2.609 3.250 4.150 3.102 -0.103 0.399 -0.493 4 4.708 8.072 9.339 4.604 7.738 8.287 0.104 0.334 1.052 5 4.022 6.170 7.169 3.800 6.213 6.373 0.222 -0.043 0.796 6 4.272 7.518 7.025 4.446 7.054 8.602 -0.173 0.464 -1.577

Table 6-2: Measurements at reference point Tot. count K (%) U (ppm) Th (ppm)

Count 9.00 9.00 9.00 9.00

Min 3.85 3.20 6.94 4.54 Max 4.09 3.39 8.62 5.70 Range 0.23 0.19 1.68 1.16

Mean 4.01 3.32 7.75 5.14

Std dev 0.074 0.061 0.626 0.379 Fraction dev. 1.842 1.842 8.081 7.366

56 The figure below (Figure 6-1) is a map of the Regoufe area showing traverses made and gamma-ray spectrometry data points against a background of petrological map after Vriend (1985).

Figure 6-1: Map showing transects and data points of gamma-ray survey

Table 6-3 below shows the range and mean of radio-element concentrations measured in this survey in the different petrological units of the granite. The world averages for acid intrusives (Killeen, 1979) are included for comparison.

Table 6-3: The range and mean of radioelement concentrations and their ratios Element Porphyritic two Transition Muscovite- Whole granite Average abundance mica granite zone albite granite for acid intrusives (Killeen, 1979) K (%) 1.78- 4.24 1.54- 4.20 2.34- 3.25 1.54-4.24 3.4 3.37 3.17 2.83 3.22 U (ppm) 3.67- 18.09 4.34- 18.86 4.15- 12.60 3.67- 18.86 4.5 8.04 8.52 7.39 8.09 Th (ppm) 2.78- 11.64 1.47- 10.47 2.09- 5.84 1.47- 11.64 25.7 6.97 5.89 3.39 6.09 Th/K 0.91-2.97 0.54- 6.63 0.72- 1.98 0.54- 6.63 7.56 2.06 1.90 1.20 1.88 U/Th 0.52- 3.48 0.17- 1.84 1.21- 4.21 0.52- 5.78 0.18 1.23 0.74 2.25 1.53 U/K 1.15- 5.23 1.54- 6.65 1.54- 4.43 1.15- 6.65 1.32 2.40 2.77 2.62 2.55

The results of ground gamma-ray survey show distinct signatures in the different parts of the granite classified by petrology and hydrothermal alteration. Potassium (K) and thorium (Th) concentrations are high in the porphyritic two-mica granite compared to the muscovite-albite granite and low in parts affected by albitization (Figure 6-2 a & b), meanwhile uranium signatures do not ascribe to particular type and part of the granite (Figure 6-2 e).

The muscovite-albite granite is distinguished by low K values ranging from 2.34-3.25% and Th values of 2.09-5.84% (Table 6-3). In Regoufe granite, zones affiliated to hydrothermal alterations can be distinguished by high eU/eTh and low eTh/K ratios (Figure 6-2 c & d). They vary between 2.25-4.21 in the former and 0.54-2.27 in the later. There seems to be a depletion of Th with hydrothermal alteration. In Regoufe granite, petrological variation is caused by mineralogical modifications as a result of a later metasomatic phase that deposited Na-rich plagioclase, caused replacement of primary biotite by muscovite and albitization of microcline (Sluijk, 1963).

The porphyritic two-mica granite show higher values and variations in K and Th values but are still lower than the average granite (refer to Table 6-3). Th:K ratios are high owing to Th content in the inclusions within the biotites of Regoufe. This concurs with the earlier observation by Sluijk (1963) that high content of radioactive inclusions zircon and possibly monazite and xenotime exist in biotite causing pleochroic haloes. This study also confirms the occurrence of radioactive minerals in biotite from thin section microscopy (see figures 4-2 & 4-3). This part of the granite is less affected by hydrothermal alteration, thus the primary minerals are modified.

In thin section the muscovite is colourless with radioactive inclusions surrounded by pleochroic halos inherited from the biotite (Sluijk, 1963 and this study). Hydrothermal activity caused leaching of

58 biotite in this part of granite, replacing it with muscovite which now encloses restite minerals such as zircon, apatite, allanite and monazite. This possibly led to liberation of thorium from the minerals. But thorium is relatively immobile and could not have migrated far. It is thus suggested that the rocks formed in the late stages of fractional crystallization where muscovite crystallization is started and thorium contents are low. It is likely that both the hydrothermal alteration and the late igneous differentiation account for the low Th concentration. At this point, it is difficult for the writer to explain what caused low thorium signature in the muscovite-albite granite.

(a) (b)

(e)

Figure 6-2: Maps showing radio-element concentrations along survey traverses: (a) eTh map, (b) K map, (c) eTh/K ratio map, (d) eU/eTh map and (e) eU map overlain on petrological map.

In the Regoufe granite the uranium content is higher than the world average for granites. In the transition zone higher values of uranium are recorded (average of 8.52 ppm), followed by the porphyritic two-mica granite with an average of 8.35 ppm. Although the muscovite-albite granite recorded lowest average uranium concentration of 7.39 ppm, it was still higher than the average

60 granite with 4.50 ppm uranium content. This is a common feature in the Hercynian granites especially the younger granites that have frequently undergone extensive hydrothermal alteration which has redistributed U. They generally have high mean U contents.

Generally, high eU/eTh ratios in gamma ray data of the Regoufe reaching 2.25 compared to world average granite of 0.18 are restricted to granitic terrains characterized by high enrichments in uranium and/or depletion of thorium. Granitic intrusions or parts of granitic intrusions which exhibit high eU/eTh ratios display varying degrees of deuteric or post-crystallization alteration and will often have associations with significant granophile element mineralization (Clarke et al., 1966).

Scatter plots: Scatter plots of gamma-ray spectrometry data are presented for K versus Th, eU versus eTh and K versus eU (Figure 6-3a-c) set according to granite type. The scatter plot for K versus eTh show characteristic fractionation trend of granites (Figure 6-3a) with points in muscovite-albite granite clustered together at low K and Th values. K versus eU and eU verses eTh do not show any particular trend (Figure 6-3b & c) but the clusters in the muscovite-albite granite persist.

In the scatter diagram K versus eTh, the measurements from the porphyritic two-mica granite, where there is less albitization, fractionation trend is apparent. Hypothetically, a similar trend would be expected for points in the muscovite-albite granite without hydrothermal alteration. The hydrothermal alteration caused replacement of K-feldspar with Na rich plagioclase (Sluijk, 1963) and possibly loss of Th in this part of the granite, influencing the element variation trends.

Granite type Granite type 4.5 4.5 Muscovite-Albite Muscovite-albite granite (a) (b) granite Porphyritic two Porphyritic two mica granite mica granite 4.0 4.0 Transition zone Transition zone

3.5 3.5 ) ) % % ( (

3.0 3.0 K K

2.5 2.5

2.0 2.0

1.5 1.5

0 2 4 6 8 10 12 3 6 9 12 15 18 eU (ppm) eTh (ppm)

Rocktype Muscovite-Albite (c) granite Porphyritic two 18 mica granite Transition zone

15 )

m 12 p p (

U e 9

0 2 4 6 8 10 12 eTh (ppm) Figure 6-3: Scatter diagrams for gamma-ray spectrometry data from Regoufe granite: (a) K versus eTh, (b) K versus eU, (c) K versus eTh.

In the transition zone, this trend can be seen but also points with low K values (with variable eTh values) plot as outliers. The outliers are spatially associated with mineralization in the northwest probably due to hydrothermal alteration. The area is characterized by kaolinization of granite (Figure 6-4) and is located close to the contact with the phyllites.

In the K-eU and eU-eTh scatter plots, in spite of the dispersed nature, many points are scattered far away from the mean. These points are characterized by high extremes in uranium values and low extremes in K value (Figure 6-3 b & c). Higher eU values plot in the porphyritic two-mica granite and the transition zone. K values less than 2.5% are found only in transition zone, not the muscovite-albite granite although more affected by alteration than the transition zone. Muscovitization that affected this part (Sluijk, 1963; Vriend, 1985) possibly contributed to K content of muscovite-albite granite. Figure 6-5 is a map showing the position of K low and U high anomalies.

Figure 6-4: Kaolinization of granite in the northwest of Regoufe, along the road.

Figure 6-5: Map showing K (yellow) and eU (red) anomalies.

In summary radio-element signatures that distinguish the petrological units are low K, low Th and low Th/K ratio. The hydrothermally altered part of the Regoufe is characterized by a high eU/eTh ratio. Generally the Regoufe granite has low thorium content, about normal K content (slightly lower) and elevated U content compared to average granites (Table 6-3). The variation of radioelement contents and trends within the granite types allows us to distinguish the different petrological types and interpret processes in the granites. There is less Th in the muscovite-albite granite as compared to the porphyritic two-mica granite which could be as a result of less biotite cystallization in primary magmatic differentiation where muscovite begins to crystallize or loss due to hydrothermal activity. The later seem to be favoured since the replacement of biotite by muscovite is observed in thin sections prominently. Uranium is distributed randomly. The relationship with geochemical parameters is mentioned in next Chapter.

7. Integration and evaluation of VNIR-SWIR and gamma-ray data with geochemical data

7.1. Petrological/geochemical characteristics versus VNIR-SWIR spectral data The chemical composition of a mineral or a rock, the type of bonds between the constituent elements and the element/ mineral proportion affect their spectral signatures. Of the 55 rock samples of the Regoufe granite, 41 samples with measurable surfaces were traced and their reflectance spectra were analysed. The corresponding geochemical data and the analyses results such as the alteration indices, mineral percentages were integrated with spectral reflectance analyses data. Relationships between geochemical and spectral parameters were evaluated by simple variation analyses using scatter diagrams.

A scatter plot of absorption depth of Al-OH feature with muscovite abundance determined from major oxides in geochemical data (Figure 7-1a) indicate a general trend in the Regoufe granite defined by R = 0.47. The correlation is stronger within the porphyritic two-mica granite (R = 0.67), while the muscovite-albite granite and transition zone do not show any interdependence. This trend may reflect fractional differentiation trend in the porphyritic two-mica granite and distortion in the trend in the muscovite-albite granite due to hydrothermal activity. Hydrothermal fluids may invade the rocks at different times with different fluid composition leading to heterogeneity in its effects on the wall rock. It is also seen that apart from two samples from the muscovite-albite granite, the rest have elevated absorption depth and low muscovite percentages. This may be explained by the good crystallinity of the muscovite formed in a later metasomatic phase causing deep absorptions of the Al-OH feature. Detailed thin section study is recommended to understand the crystal structure of muscovite in the two parts of the granite.

A negative correlation exists between the wavelength position of the Al-OH feature and the albitization index. From the scatter diagram (Figure 7-2a), it is observed that samples from the muscovite-albite granite, which have undergone intense alteration, plot in lower wavelength position (below 2204 nm) while samples from the porphyritic two-mica granite plot mostly in the longer wavelength positions although they have wide range from 2202 to 2207 nm. The wavelength of Al- OH feature shifts to the shorter as the intensity of albitization increases (R-squared = 0.46). Hydrothermal alteration possibly leached out FeMg minerals/elements as Al-rich muscovite is formed.

Muscovite stability is favoured by high fO2 and high KAlSi3O8 as well as relatively low T and medium to high P (Shand, 1947).

A trend in wavelength and chlorite-carbonate-pyrite index is observed indicating that the shift to the longer wavelength position correlates with increasing CCPI (Figure 7-2b) which is indirectly a measure of presence of Mg and Fe elements. High CCPI ranging from 9-14 are found in the porphyritic two-mica granite. This relationship may be useful in assessing the presence of minerals with Fe and Mg elements.

64 0.30 R = 0.47

0.28 e r u t a e f

0.26 H O - l A

f o

0.24 h t p e d

o 0.22 i t p r o s b

A 0.20 Granite type Muscovite-albite granite 0.18 Porphyritic two mica granite Transition zone

4.0 4.5 5.0 5.5 6.0 Muscovite (%) Figure 7-1: Scatter diagram for absorption depth versus muscovite abundance calculated from geochemical data of Regoufe granite.

Granite type 2207 Muscovite-Albite 2207 (a) granite Porphyritic two 2206 mica granite 2206 (b)

) Transition zone m e r n ( u

e 2205

a 2205 r e u f t

a R = 0.47 H e f O

2204 l H 2204 A - O ) - l m A

n f (

2203 2203 h t h t g g n n e l e l e e 2202 v 2202 v a a

W Rocktype W Muscovite-Albite 2201 R = 0.46 2201 granite Porphyritic two mica granite 2200 2200 Transition zone

38 40 42 44 46 48 50 52 6 9 12 15 18 21 Chlorite-carbonate-pyrite index (CCPI) Albitization index Figure 7-2: Scatter diagram for wavelength position for (a) Al-OH feature versus albitization index, (b) wavelength position for Al-OH feature versus CCPI.

7.2. Petrological/geochemical characteristics versus gamma-ray data. Comparison of the trend of variation of radio-elements K and Th measured using the gamma-ray spectrometer and that determined by geochemical analytical methods show similar trends of

increasing K content with increasing Th content. The implication is that the gamma-ray data is consistent with the geochemical data. However, there seems to be a small offset with geochemical data having slightly higher K results. Factors that may bring about such offset could be a result of instrument calibration, background radiation in gamma-ray spectrometry. Also time factor may be a possible candidate, since the samples were taken and analysed over 20 years ago. In both data types there seems to be a break which may be related to different classes of rocks and these can be seen in the subsequent diagrams.

Data type 4.5 Gamma-ray spectrometry data Geochemical data 4.0

3.5 ) % (

3.0 K

2.5

2.0

1.5

0 2 4 6 8 10 12 eTh (ppm)

Figure 7-3: Scatter diagram for K versus eTh of both gamma-ray spectrometry data and geochemical data in different symbols described in the legend.

To compare directly the measurements of gamma-ray survey with geochemical analysis results, an attempt was made to intersect the gamma-ray survey data with geochemical data points to find out if the inter-element trend between gamma-ray measurements and geochemical data could be maintained. Points within 50 m of geochemical data points were considered to be having similar radioelement signatures based on the fact that geochemical samples were composed of fresh rock chips collected from within 500m2 of a location. This was constrained by the limited number of gamma-ray data points that happened to plot closer to geochemical points. Despite the scattered few points, simple trends could still be noticed.

The good correlation in Th measurements by geochemical analysis and gamma-ray determination illustrates the significance of Th concentration in mapping the different classes on the granite. Poor U correlation shows how U varies with slight change in environment. Correlations between different elements (Figure 7-4d-f) maintained simple trends related to geology as described in Chapter 4 Section 4.2.2

Granite type Granite type 1.83 20 Muscovite-albite Muscovite-albite Porphiritic two- Porphiritic two- mica mica 1.80 Transition zone Transition zone

1.77 ) 15 a ) R = 0.55 R = 0.19 t a a t d a

l d

a l 1.74 c a i c i m e m h e c h 1.71 10 o c e o g e ( g ) (

) 1.68 m p % ( p

(

K 1.65 U 5

1.62 (a) (b) 1.59 0

2.4 2.7 3.0 3.3 3.6 3.9 4.2 5 6 7 8 9 10 11 12 K (%) (gamma-ray data) eU (ppm) (gamma-ray data)

Granite type 6 6 Muscovite-albite R = 0.96 Porphiritic two- mica Transition zone 5 5 (d) ) a a t t (c) a a d d

l y a a r c - i 4 4 a m m e h m c a o g e - g ) (

m ) 3 3 p m p p ( p h (

T h e T 2 2 Rock_type Muscovite-albite Porphyritic two- mica 1 R = 0.85 1 Transition zone

2 4 6 8 10 0.00 0.02 0.04 0.06 0.08 0.10 0.12 eTh (ppm) (gamma-ray data) TiO2 (%)- Geochemical data

Granite type 10 12 Muscovite-albite R = 0.72 Porphiritic two-mica Transition zone 11 a 8 a t R = 0.63 t a

a 10 d d

y y a a r r - - a a 9 m (e) m m m

a 6 a g g

- - ) ) 8 m m p p p p (

(

h (f) U T 7 e e 4

6 Granite type Muscovite-albite Porphiritic two-mica 2 5 Transition zone

0.2 0.3 0.4 0.5 0.4 0.6 CaO (%)- Geochemical data P2O5 (%)- Geochemical data

Figure 7-4: Scatter diagram for (a) K (geochem) versus K (gamma); (b) U (geochem) verses eU (gamma);

(gamma-ray data) versus CaO (geochemical data); (b) eU (gamma ray data) versus P2O5 (geochemical data).

In the geochemical data the correlation of U/Th with Na2O/K2O reflecting albitization shows that the ratio eU/eTh can be a useful lithochemical indicator of areas of alteration within granites. The trend of increasing P2O5 with increase in Na2O/K2O ratio which is not the expected differentiation trend and its association with hydrothermally altered zone may be used as a criterion for hydrothermal alteration mapping in the Regoufe and elsewhere. The correlation of U/Th with most of the factor 2 elements in section 4.2.2; Nb (R = 0.58), Sn (R = 0.52), Ta (R = 0.55), Zn (R = 0.52) and Rb (R = 0.51) may provide hints in ore mineral exploration.

The correlation between biotite and Th, Zr, Ce & La (See Section 4.2.2) strongly associate biotite with radioactive minerals. The strong correlation between thorium and TiO2 supplements the observation that the leaching of biotite led to the development of titaniferous minerals like ilmenite found in pale biotite and muscovite derived from it (Schermerhorn, 1959). All these relations allow us to use gamma-ray survey technique to map petrological units and alteration zones.

68 7.3. Petrological/geochemical characteristics versus gamma-ray data. The ASTER image classifications do not identify with any particular gamma-ray signature. ASTER image has poor resolution to discriminate the minute spectral variation caused by mineralogical differences in the granite (Figure 7-5 & 7-6). The scenes are so much influenced by soil-vegetation and topographical effects that only the well exposed granitoid rocks, vegetation and soil can be discriminated.

Legend Lithological boundary Band7/band 6 Band 3/band 2

Band 5/band 7

Th/K ratio 0.54 - 1.62 1.62 - 2.27 2.27 - 3.42 3.42 - 5.51 5.51 - 8.22 . Projection: WGS 1984, UTM Zone 29N Figure 7-5: RGB band ratio 7/6:3/2:5/7 histogram equalized with Th/K ratio points (gamma-ray data) overlaid.

Legend

Lithological boundary Well exposed muscovite-rich Pixels with high vegetation cover Pixels influenced by topographical effects Fire burns, black soil and short vegetation Fe-rich zones

Th/K ratio 0.54 - 1.62 1.62 - 2.27 2.27 - 3.42 3.42 - 5.51 5.51 - 8.22 . Projection: WGS 1984, UTM Zone 29N Figure 7-6: ASTER SAM classification with Th/K ratio points (gamma-ray data) overlaid.

8. Discussions and conclusions

8.1. Discussions The Regoufe granite is a peraluminous granite with K-feldspar-plagioclase-biotite-muscovite- and apatite-tourmaline-garnet-zircon-monazite accessory mineral assemblage. Other accessories include ilmenite, rutile in connection with leaching of biotite and arsenopyrite in arsenopyrite-bearing quartz veins. In muscovite and muscovite-biotite granites, muscovite is an essential constituent mineral. It is less common than biotite in acid igneous rocks. Greisenization in granites has a quartz and mica rich rock considered to have been derived from granite modified by autometasomatic changes that occurred during the last phase of its crystallization, a feature present in the Regoufe granite.

Muscovite is the dominant mineral with absorption feature in the SWIR region followed by illite and halloysite minerals in the granite. These minerals vary in composition controlled by mode of formation or effect of alteration. It is important to differentiate between secondary muscovite and primary muscovite from spectral data analysis so that patterns related to different phases can be mapped.

The muscovite-albite granite part affected by hydrothermal alteration is characterized by shorter wavelength position of Al-OH feature, which may be related to Al content of the muscovite. The wavelength position of Al-OH feature may be controlled by temperature, fluid chemistry and/or coexisting mineral phases (Duke, 1994; Ruitenbeek et al., 2005). The porphyritic two-mica granite has longer and wide range in Al-OH absorption wavelength position reflecting substitution of Al and this indicates coexistence of other mineral phases.

The high absorption depth in the muscovite-albite granite with respect to the porphyritic two-mica granite is related to the abundance of muscovite and possibly crystallinity, which agrees with the muscovite abundance in the Regoufe granite. Two zones of different spectral signatures in the Regoufe granite were delineated using these contrasts.

Reflectance spectrometry techniques can not be used independently to map petrological units and alteration patterns within granites. However, a quick idea about the compositional variation of the alteration minerals and their association with other minerals can be developed. A rather important objective of laboratory spectral reflectance study was to acquire mineral information which is useful in processing and interpreting satellite image.

The Regoufe granite has undergone low surface weathering since most of the spectral features are largely preserved. A slight shift in the exact wavelength position of Al-OH absorption feature, inflections at the shoulders of absorption features and low albedo are observed for weathered surfaces.

Newly formed minerals as a result of weathering are halloysite, illite and clays. White mica generally persist during weathering and can be used for spectrally based alteration mapping (Ehara et al., 2005).

Mapping mineralogical/petrological patterns using SAM and band ratioing techniques from the ASTER image of Regoufe granite area is constrained by soil, vegetation cover in some parts and topographical effects on slopes. The presence of sooth from bush fires which plague the area are hindrance as well in the image data. Galvao et al (2004) found out that the effect of non- photosynthetic vegetation cover subdues the expression of the 2200 nm absorption band in pixel spectra which mainly replaces the 2200 nm hydroxyl absorption band by the 2100-2300 nm lignin- cellulose spectral features.

However, with facts from the ground data the image was useful in mapping muscovite rich parts, vegetated zones and weathered zones characterized by Fe absorptions. The SAM technique applied using image spectra as end members clearly discriminate a zone thought to be a fire scar. The comparison of ASTER image with laboratory reflectance results, geochemical/petrological patterns and gamma-ray data show no correlation. A major limitation is that the granite components have similar mineralogical compositions with small variations in quartz, plagioclase, K-feldspar and micas as well as accessory minerals producing subtle differences in the wavelength positions and depth of absorption.

Thorium appears to reflect mineralogical association between Ti-rich or Ti-oxide and bitotite, Ti- oxide and radioelement-rich minerals (zircon, monazite, allanite). Zircon and monazite are common components in biotite of peraluminous granites and since Th shows strong correlation with Ti-oxide, thorium signatures can be useful in mapping mineralogical and hydrothermal alteration patterns on the granite.

Low thorium is spatially associated with hydrothermally altered zones and mineralized areas (see section 4.2.2). Therefore thorium concentration can serve as lithochemical indicator for Sn/W mineralization in the Regoufe. The less content of thorium in the muscovite-albite granite seems to be related to cumulative effects of both igneous differentiation and hydrothermal activity.

In this study high measurements of the uranium content were recorded in the Regoufe granite. This is common for Hercynian granites with values up to 11 ppm. Soen (1970) reported that the high uranium content suggests that the magmas carried a great amount of radioactive heat and began their ascent at relatively high temperatures.

The inter-element relationships in the muscovite-albite granite plotting in clusters may reflect high permeability of the granite at the time the hydrothermal fluids percolated the granite. Gaans (1985) noted that hydrothermal alteration is concentrated in one part of the Regoufe granite and concluded that the effect of percolating hydrothermal fluids did not produce a highly inhomogeneous rock type.

72 8.2. Conclusion • This study has found out that aluminium hydroxide minerals mainly muscovite, illite and halloysite are the dominant minerals with absorption feature in the SWIR wavelength region. Spectral signatures observed in the granite from laboratory reflectance study are defined by wavelength position and absorption depth patterns. Spatial association of the spectral parameters with petrological units suggests geological controls for the spectral characteristics. Therefore spectral signatures can be related to petrological units and hydrothermal patterns

However, this criterion is not enough to provide reliable method for mapping mineralogical or alteration patterns since there is great variation from location to location and have significant influence of surface processes.

• The ASTER image did not produce good results based on band ratioing and SAM classification. The influence of vegetation and topography affected the shapes of the image spectra causing mismatch in many pixels. Even so, the discrimination between geochemical signatures/trends and the different petrological units within the granite were simply impossible given the subtle difference or similarity in the granite mineralogy for spectral techniques to resolve. In addition, ASTER data resolution limits the minerals that can be diagnosed in SWIR absorption bands. ASTER image data was not useful to extract variation due to mineralogy and geochemical signatures within the Regoufe granite.

• The geochemical trends in the Regoufe granite related to K, U and Th concentrations are albitization trend (Na2O/K2O), apatitization trend (P2O5), and mineralization factor. The consistency of gamma-ray signatures with mineralogical variation in the granites indicates that in this small extent granite, mineralogical differences can be detected effectively. The K, U and Th concentration are controlled by radioelement-rich minerals in the biotite of Regoufe. The gamma- ray data can therefore be used to fix petrological boundaries especially in the transition zone accurately and detect significant geological signatures missed during geochemical survey. The variations in the K, eU and eTh concentrations can be accurately and effectively determined using a portable gamma-ray spectrometer, providing effective mapping and sampling guides at local, detailed scales.

• Generally speaking the only important significance to Sn-W exploration is observed in gamma-ray data which can distinguish the signature associated with mineralization. Low thorium values and high U/Th ratios are spatially associated with mineralization and therefore can be used in regional prospecting for Sn-W exploration.

• VNIR-SWIR spectral analysis and gamma-ray survey methods are important for regional mapping. They provide a quick clue in target selection and enables effective field planning. When used in combination with ground characteristics, the relation or association can be used to map large areas. The advantage is the low cost and time factor.

8.3. Recommendations • Further work is required in detailed reflectance spectrometry analysis to fill the gaps sampling and to relate spectral characteristics to mineralization.

• Hyperspectral data recommended for remote sensing analysis of petrological and geochemical patterns in Regoufe area.

• Detailed thin section study should be carried out in the different petrological components of Regoufe with the view of understanding crystallographic structure of the minerals so as to relate to spectral properties.

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