Alma Mater Studiorum – Università di Bologna DOTTORATO DI RICERCA IN SCIENZE GEOLOGICHE Ciclo XXVI Settore Concorsuale di afferenza: 04/A3 Settore Scientifico disciplinare: GEO/04 SPELEOGENESIS AND SECONDARY CAVE MINERALS IN QUARTZ‐SANDSTONE AND QUARTZITE ENVIRONMENT Presentata da: Francesco Sauro Coordinatore Dottorato Relatore Prof. Roberto Barbieri Prof. Jo De Waele Correlatore Prof. Leonardo Piccini Esame finale anno 2014 ... and the moon appeared as on a dead world of sole geology. You really had up there, the sense of the earth and rock as living bodies ... Seemed to be dreaming in terms of mineral. ... e apparve la luna come su un mondo morto di sola geologia. Si aveva veramente lassù, il senso della terra e della roccia come corpi viventi... Pareva di sognare in termini di minerale. Alfonso Vinci, Auyan Tepui, 1949 SPELEOGENESIS AND SECONDARY CAVE MINERALS IN QUARTZ-SANDSTONE AND QUARTZITE ENVIRONMENT ABSTRACT 9 RIASSUNTO 11 1. INTRODUCTION 13 1.1 Quartzite caves: beyond the quartz insolubility paradox 13 1.2 The mechanisms of quartz and silica weathering 14 1.2.1 Forms of quartz and silica 14 1.2.2 Characteristics of the silica-water system 14 1.2.3 Influence of inorganic and organic cations 16 1.2.4 Influence of organic compounds and microbial activity 17 1.3 Quartzite and quartz-sandstone weathering 18 1.3.1 Orthoquartzite and Metaquartzite 18 1.3.2 Quartz weathering landscapes and caves of the World 19 1.3.3 The Gran Sabana: the World’s finest epigenic quartzite Karst 22 1.3.4 Corona Sa Craba: an example of hypogenic speleogenesis in quartzites 23 1.4 Objectives and outline of the thesis 26 2. FIRST SECTION: SPELEOGENESIS 39 2.1 A model simulation of the “arenisation” weathering process in quartz-sandstones: a key 41 factor for speleogenesis in the quartzite environment. 2.1.1 Introduction 42 2.1.2 Model structure 43 2.1.3 Theory and equations 48 2.1.3.1 Dissolution flux and diffusion flux 45 2.1.3.2 Porosity profile evolution 50 2.1.3.3 Fracture enlargement 53 2.1.3.4 Hydraulic conditions in the free film stage 56 2.1.4 Base scenarios 57 2.1.5 Results 59 2.1.6 Discussion and conclusions 64 3 2.2 Geochemistry of surface and subsurface waters in quartz-sandstones: significance for the 69 geomorphic evolution of tepui table mountains (Gran Sabana, Venezuela). 2.2.1 Introduction 70 2.2.2 Study Area 69 2.2.2.1 Geographical, geological, and climatic settings 71 2.2.2.2 Investigated tepuis and cave systems 73 2.2.3 Methods 78 2.2.3.1 Sampling and analysis of water 78 2.2.3.2 Discharge measurements in subterranean streams 79 2.2.3.3 Chemical analysis of rocK samples 80 2.2.4 Results 80 2.2.4.1 Rainwaters 81 2.2.4.2 Stagnant surface water in peat, ponds and swamps 82 2.2.4.3 Surface stream and river waters 82 2.2.4.4 Underground waters 84 2.2.4.5 Chemical composition of the quartz-sandstones 87 2.2.5 Discussion 90 2.2.5.1 The silica-water system 90 2.2.5.2 Surface silica contribution 92 2.2.5.3 Subsurface silica dissolution 93 2.2.5.4 Estimation of the subsurface dissolution 97 2.2.6 Conclusions 98 2.3 Comment on “Sandstone caves on Venezuelan tepuis: Return to pseudokarst?” by R. 109 Aubrecht, T. Lánczos, M. Gregor, J. Schlögl, B. Smída, P. Liscák, Ch. Brewer-Carías, L. Vlcek, Geomorphology 132 (2011), 351-365. 2.3.1 Introduction 110 2.3.2 Geological problems of the “unlithified sands” theory 111 2.3.3 Interpretative problems with the Schmidt Hammer hardness measurements 112 2.3.4 Morphology of the caves and field evidence: problems of the pillar flow hypothesis 113 2.3.5 Petrographical problems and the role of SiO2 dissolution 117 2.3.6 Brief discussion: unlithified beds or neo-sandstones? 119 2.4 The pattern of quartz-sandstone caves: evidences of the arenisation process. 125 2.4.1 Introduction 126 4 2.4.2 Regional setting 128 2.4.3 The caves 130 2.4.3.1 The Imawarì Yeuta cave system 130 2.4.3.2 The Guacamaya cave 133 2.4.3.3 The AKopan-Dal Cin cave system 134 2.4.3.4 The Roraima Sur cave system 134 2.4.3.5 Fissure networKs of the Aonda and Auyan Tepui Noroeste system 136 2.4.4 Methods 136 2.4.4.1 Petrography, mineralogy and chemical composition 136 2.4.4.2 Morphometric analyses 137 2.4.5 Results and intepretation 138 2.4.5.1 Quartz sandstone composition and petrography 138 2.4.5.2 Evidences of the arenisation weathering process 141 2.4.5.3 Morphology of pillars and mammillary 144 2.4.6 Discussion 147 2.4.6.1 Hydrofracture bound-layers: the origin of pillars and maze networK caves 147 2.4.6.2 The inception hypothesis and the cave pattern guidance in quartz sandstones 151 2.4.7 Conclusions 155 2.5 Hypogenic speleogenesis in quartzite: the case of Corona ‘e Sa Craba cave 163 2.5.1 Introduction 164 2.5.2 Geological setting of the area 165 2.5.3 The Corona ‘e Sa Craba Cave 167 2.5.4 Methods 170 2.5.5 Results 172 2.5.5.1 Composition of the quartzite and evidences of quartz dissolution 172 2.5.5.2 Mineralogy 173 2.5.5.3 Sulfur Isotopes 176 2.5.6 Speleogenesis and mineral deposition 177 2.5.7 Conclusions 181 3. SECOND SECTION: SECONDARY MINERALS IN QUARTZITE CAVE ENVIRONMENT 189 3.1 Rossiantonite, Al3(PO4)(SO4)2(OH)2(H2O)10⋅4H2O, a new hydrated aluminium 191 phosphate-sulfate mineral from Chimanta massif, Venezuela: description and crystal structure 3.1.1 Introduction 192 3.1.2 Occurrence and paragenesis 192 5 3.1.3 Physical and optical properties 194 3.1.4 Chemical composition 194 3.1.5 Thermal analysis 195 3.1.6 Infrared spectroscopy 196 3.1.7 X-Ray diffraction 198 3.1.7.1 Powder diffraction 198 3.1.7.2 Single-crystal diffraction 198 3.1.8 Description of the structure 200 3.1.8.1 Overview of structure topology 200 3.1.8.2 Cation sites 200 3.1.9 Minerogenesis 201 3.2 Source and genesis of sulphate and phosphate-sulphate minerals in a quartz-sandstone cave 211 environment. 3.2.1 Introduction 212 3.2.2 Geographical, geological and hydrogeological settings 213 3.2.3 Previous studies on gypsum in venezuelan quartz-sandstone caves 216 3.3.4 Previous occurrences of sanjuanite 216 3.2.5 Cave morphologies and sampling sites 218 3.2.6 Material and methods 221 3.2.6.1 Sample mineralogy (XRD and Raman) 221 3.2.6.2 Chemical analysis with WD-X Ray Fluorescence Spectrometer 221 3.2.6.3 Scanning Electron Microscopy (SEM) 222 3.2.6.4 Sulphur and oxygen stable isotope analyses 222 3.2.7 Results 223 3.2.7.1 Sulphur content of the host-rocK 223 3.2.7.2 Sulphur and oxygen isotopes 225 3.2.8 Discussion 225 3.2.8.1 Potential sources of sulphur 225 3.2.8.2 Aerosol sulphate precipitation and potential biologic interactions 228 3.2.9 Conclusions 231 3.3 Minerals and silica speleothems of the Guacamaya Cave, Auyan Tepui, Venezuela 239 3.3.1. Introduction 240 3.3.2 Geographical and geological setting 240 3.3.2.1. Morphological description 242 3.3.2.2. Silica speleothems and gypsum 243 6 3.3.3 Methods 243 3.3.4 Results and discussion 244 3.3.4.1 Banded Iron Formations, their geochemistry and their control on speleogenesis 244 3.3.4.2. Silica biospeleothems 246 3.3.5 Conclusions 249 4. CONCLUDING REMARKS AND PERSPECTIVES 251 AKNOWLEDGEMENTS 255 7 8 ABSTRACT The main objective of this research is to improve the comprehension of the processes controlling the formation of caves and karst-like morphologies in quartz-rich lithologies (more than 90% quartz), like quartz-sandstones and metamorphic quartzites. Quartz is considered one of the less soluble minerals of the Earth’s crust, an thus hardly affected by chemical weathering. Despite this, since more than thirty years, it’s clear that the formation of caves and peculiar superficial weathering morphologies in quartz-rich lithologies is common and very similar to the well-known “karstic“ ones in carbonate rocks. In the scientific community the processes actually most retained to be responsible of these formations are explained in the “Arenisation Theory”. This implies a slow but pervasive dissolution of the quartz grain/mineral boundaries increasing the general porosity until the rock becomes incohesive and can be easily eroded by running waters. The loose sands produced by the weathering processes are then evacuated to the surface through processes of piping due to the infiltration of waters from the fracture network or the bedding planes. Recently this theory was debated after the discovery of some giant maze systems in the Venezuelan “tepui” table mountains. In fact, some authors support a new hypothesis (the so-called “Non-lithification Theory”) where less cohesive layers characterising the caves would be related to an inhomogeneous lithification of the quartz- sandstone strata, while the arenisation process would play only a secondary role. In the frame of this debate, the first part of this thesis was dedicated to a detailed and quantitative analyses of the arenisation process, both from a chemical-physical point of view and from its expected morphologic expression.