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It Is Not Truisms Which Science Unveils. Rather It Is Part of the Greatness

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It Is Not Truisms Which Science Unveils. Rather It Is Part of the Greatness "It is not truisms which science unveils. Rather it is part of the greatness and beauty of science that we can learn through our own critical investigations that the world is utterly different from what we ever imagined - until our imagination was fired by the refutation of our earlier theories." "The wrong view of science betrays itself in the craving to be right." ...... only through criticism can knowledge advance." "the growth of our knowledge proceeds from problems (scientific inconsistencies with establishd "truth") and our attempts to solve them." Karl Popper, The Logic of Scientific Discovery. "The erroneous belief that science eventually leads to the certainty of a definitive explanation carries with it the implication that it is a grave scientific misdemeanor to have published some hypothesis that eventually is falsified. As a consequence scientists have often been loath to admit the.falsification of such an hypothesis and their lives may be wasted in defending the no longer defensible. Whereas according to Popper, falsification in whole or in part is the anticipated fate of all hypotheses, and we should even rejoice in the falsification of an hypothesis that we have cherished as our brain child. One is thereby relieved from fears and remorse and science becomes an exhilirating adventure where imagination and ·vision lead to conceptual developments transcending in generality and range the experimental evidence. The precise formation of these imaginative insights into hypotheses opens the way to the most rigorous testing by experiment, it being always anticipated that the hypothesis may be fa~sified and that it will be replaced in whole or in part by another hypothesis of greater explanatory power." J. C. Eccles, Facing Reality. HYDROTHERMAL PHENOMENA IN THE WESTERN LOBE OF THE ST. AUSTELL GRANITE, CORNWALL PATRICK ALLMAN-WARD A thesis in two volumes submitted for the degree of Doctor of Philosophy of the University of London VOLUME 1 Mining Geology Section Royal Scho61 of Mines Imperial College of Science and Technology London January 1982 i ABSTRACT This thesis is a study of the granite of the western lobe of the St. Austell granite cupola and the fluids which interacted with it and their effects upon it. The western lobe is composed of a phenocryst rich, coarse grained, biotite bearing peraluminous alkali feldspar granite. It is chemically and mineralogically similar to other SW England and Sn-W bearing granites. Extensive late magmatic crystallisation resulted from high volatile contents which depressed the solidus. Pegmatitic zones, miarolitic cavities, tourmaline banding and autometasomatic alteration resulted from the periodic separation and localised concentration of immiscible aqueous phases from the residual interstitial melt. Greisenisation, tourmalinisation and kaolinisation are the main types of alteration caused by the passage of successive generations of fluids through the granite. The mineralogical and chemical changes accompanying these alteration processes have been studied using petrological and statistical techniques. Tourmalinisation is the principle alteration type accompanying tin mineralisation. There is a correlation between cassiterite and tourmaline of schorlitic composition. Hydraulic fracturing in the granite was the result of the interaction between cooling stresses, the regional stress field and fluid overpressures generated by the separation of aqueous phases from the differentiating melt at depth. A breccia body formed as a result of fracturing down an extreme pressure gradient. The possible origin and mineralisation potential of the successive generations of hyqrothermal fluid have been determined from their thermal and compositional characteristics and in the light of contemporary models for magmatic differentiation processes and the initiation of hydrothermal convective systems caused by the intrusion of igneous bodies. The available stable isotope data has been reappraised. Deuteric fluids are largely responsible for greisenisation, tourmalinisation and tin mineralisation. Meteoric hydrothermal waters caused kaolinisation and possible uranium mineralisation. These represent the early and late stages of a single hydrothermal convective system of extended duration which persisted due to the high radiogenic heat content of the granite. ii LIST OF CONTENTS VOLUME 1 Page Abstract i List of Contents u PREFACE 1 CHAPTER 1 GEOLOGICAL INTRODUCTION TO SOUTH-WEST ENGLAND 5 1.1 ECONOMIC INTRODUCTION 6 1.2 PALAEOGEOGRAPHIC RECONSTRUCTION 8 1.3 THE CORNUBIAN BATHOLITH 11 1 Morphology of the batholith 11 2 The granite 12 3 Other intrusive rocks 13 4 Origin of the granite 14 5 Mechanism of magma formation 15 6 Emplacement of the granite 16 7 Mineralisation and hydrothermal cells 17 8 Geochemical characteristics 19 9 Relative and absolute age dating 20 10 Summary 21 1.4 THE ST. AUSTELL GRANITE CUPOLA 22 1 The granite 22 2 Other intrusive rocks 25 3 Mineralisation 25 iii 4 Alteration 26 5 Summary 27 CHAPTER 2 THE IGNEOUS ROCKS AND HYDROTHERMAL PHENOMENA IN THE WESTERN LOBE 2~ 2.1 IGNEOUS ROCKS 29 1 Granite 29 2 Contact between granite and killas 31 3 Igneous fabric 33 4 Elvan 35 5 Conclusion 36 2.2 STRUCTURES AND HYDROTHERMAL ALTERATION 38 1 Structures 38 A Classification of fractures 38 B Vein paragenesis an~ style of fracturing 42 C Discussion 44 2 Alteration 46 3 Breccia pipe 52. A Main stage breccia 53 B Other breccias 55 C Discussion 57 D Post breccia alteration and mineralisation 60 2.3 DISCUSSION 61 2.4 CONCLUSION 65 iv CHAPTER 3 PETROLOGY AND PETROCHEMISTRY AND THE EFFECTS OF HYDROTHERMAL ALTERATION 67 3. 1 THE GRANITE 68 1 Microscopic characteristics 68 2 Crystallisation sequence 73 3 Discussion 74 3.2 HYDROTHERMAL ALTERATION 77 1 Greisenisation 77 A Microscopic characteristics 78 2 Tourmalinisation 82. A Tourmaline banding 82 B Tourmaline alteration selvedges 84 3 Kaolinisation 90 A Microscopic studies 91 B Scanning electron microscope studies 93 C Discussion 95 4 Summary 97 3.3 QUARTZ-PORPHYRY DYKES 99 3.4 KILLAS 101 3.5 BRECCIA 104 1 Main phase breccia 104 2 Siliceous breccia 107 3 Killas collapse breccia 108 4 Late breccia dykes 109 5 Conclusion 110 3.6 MINERAL CHEMISTRY 113 3.7 CONCLUSION 119 CHAPTER 4 THE CHEMISTRY OF THE ROCK TYPES AND THE CHEMICAL CHANGES ACCOMPANYING HYDROTHERMAL ALTERATION 121 4.1 INTRODUCTION 123 4.2 SAMPLE GROUPING 125 4.3 CHEMISTRY OF THE ROCK TYPES 128 1 The granite and related rock types 128 A Major and trace elements 128 B Radiogenic elements 129 C CIPW norms 131 D Sn-W granite characteristics 133 2 Hydrothermally altered granite 135 A Chemical and normative mineralogical changes 135 B Mineralisation 138 3 Conclusions 142 4.4 MASS TRANSFER 143 A Volume factor 145 B Mass transfer of components during alteration 147 C Conclusion 150 4.5 ELEMENT BEHAVIOUR DURING ALTERATION 151 1 Bivariate data analysis 151 2 Multivariate data analysis 153 3 Interpretation of element associations 156 4 Discussion 15B 4.6 MULTIVARIATE DATA REDUCTION 159 1 Extraction of factors 160 A Principal component analysis 160 B Factor analysis 165 C Discussion 167 2 Alteration factors as plotting variables 169 A Scatter plots 169 vi B Ternary plots 172 C Factor scores vs. CIPW norms 173 D Kaolinisation factor scores vs. XRD mineral modes 175 E Alteration factor scores vs. mass transfer values 175 F Conclusion 176 3 Discussion 177 4.7 CONCLUSION 179 CHAPTER 5 THE PHYSICAL AND CHEMICAL PROPERTIES OF THE HYDROTHERMAL FLUIDS 183 5.1 INTRODUCTION 184 5.2 INCLUSION PETROLOGY 185 5.3 TEMPERATURE AND SALINITY OF THE HYDROTHERMAL FLUIDS 195 A Homogenisation temperatures 197 B Salinitie-s 198 C Homogenisation temperature-salinity data 199 D Discussion 202 E Pressure corrections 204 5.4 FLUID COMPOSITIONS 207 5.5 FLUID-ROCK RATIOS 211 A Structural parameters 211 B Mass transfer values 212 C Conclusion 213 5.6 CONCLUSION 214 vii CRAFTER 6 THE ORIGIN OF THE FLUIDS RESPONSIBLE FOR MINERALISATION AND ALTERATION 2l7 6.1 INTRODUCTION 218 6.2 THE EVOLUTION OF AQUEOUS PHASES FROM TIN BEARING GRANITE MAGMAS 220 6.3 GRANITE INTRUSIVES AND HYDROTHERMAL SYSTEMS 226 6.4 STABLE ISOTOPE EVIDENCE 237 A Magmatic fluids 237 B Greisenising fluids 23g C Kaolinising fluids 240 D Conclusion 244 6.5 SUMMARY OF EVIDENCE ON THE GENESIS OF THE KAOLINITE DEPOSITS IN THE ST. AUSTELL GRANITE 245 CHAPTER 7 CONCLUSION 250 LIST OF REFERENCES 259 APPENDIX A ANALYTICAL METHODS 282 A.1 SAMPLE COLLECTION 283 A.2 SAMPLE PREPARATION 283 A. 3 .MAJOR AND TRACE ELEMENT ANALYSES 284 1 Major element analyses 284 A XRF 284 B ICP 284 2 Loss on ignition determinations 285 3 Trace element analyses 287 A XRF 287 B ICP 288 viii 4 Data processing 293 A.4 CHLORINE DETERMINATIONS 295 A.S RADIOGENIC ELEMENTS 295 A.6 SPECIFIC GRAVITIES 297 A.7 QUANTITATIVE MINERALOGICAL COMPOSITIONS 29~ A.8 DETERMINATION OF FLUID INCLUSION COMPOSITIONS 299 APPENDIX B PRECISION AND ACCURACY OF THE ANALYTICAL TECHNIQUES 30t B.1 MEASUREMENT OF ANALYTICAL VARIANCE 307 B. 2 MAJOR ELEMENTS ANALYSED BY THE ECLP XRF 310 B.3 MAJOR AND TRACE ELEMENTS ANALYSED BY THE KING'S COLLEGE ICP 309 B.4 LOSS ON IGNITION DETERMINATIONS 312 B.S TRACE ELEMENTS ANALYSED BY THE IMPERIAL COLLEGE XRF 313 B.6 BORON, LITHIUM AND TIN ANALYSED BY THE KING'S COLLEGE ICP 315 B.7 CHLORINE ANALYSED BY WET CHEMISTRY 317 B.8 SPECIFIC GRAVITY DETERMINATIONS 317 APPENDIX C METHOD OF CALCULATING DEGREES OF FILLING 318 C.1 INTRODUCTION 319 C.2 METHODS OF CALCULATING DEGREES OF FILLING 321 C.3 STEREOLOGICAL METHOD 322 1 Area ratios 322 2 Calculated degrees of filling 324 3 Discussion 327 C.4 CONCLUSION 330 ix APPENDIX D STANDARDISATION OF THE LINKAM TH600 HEATING AND FREEZING STAGE 331 0.1 CALIBRATION USING CHEMICAL STANDARDS 33Z 0.2 CALIBRATION USING SALT INCLUSIONS 33S 0.3 CONCLUSIONS 341 VOLUME 2 List of Tables List of Figures List of Plates Microfiche 1 Tabulated chemical data for samples from the western lobe of the St.
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