SHALLOW-WATER GEOTHERMAL ACTIVITY OFFSHORE PANAREA, AEOLIAN ISLAND ARC, ITALY by Samantha Pascarelli A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Geology). Golden, Colorado Date _________________________ Signed: _________________________ Samantha Pascarelli Signed: _________________________ Dr. Thomas Monecke Thesis Advisor Golden, Colorado Date _________________________ Signed: _________________________ Dr. Wendy Bohrson Professor and Department Head Geology and Geological Engineering ii ABSTRACT Panarea island represents a partially emerged arc volcano forming part of the Aeolian volcanic chain in the southern Tyrrhenian Sea. The island rises from the western sector of a submarine platform representing the eroded summit of a large submarine stratovolcano. Seafloor observations on the eastern portion of the submarine platform have been conducted using a remotely operated vehicle. The seafloor survey resulted in the discovery of a new field of hydrothermal venting between the Secca dei Panarelli and Basiluzzo islet that is typified by quiescent gas venting and the discharge of thermal waters at water depths ranging from 60 to 100 m. The geothermal activity is associated with a wide range of seafloor morphological features, many of which are transient in nature and will not likely be preserved in the geological record. This includes widespread anhydrite-gypsum deposits that have been drilling to a maximum penetration of 5 m below seafloor using a lander-type drilling device. The massive to brecciated hydrothermal deposits contain > 80 % anhydrite and gypsum in addition to alteration minerals formed during subseafloor replacement and infiltration of the volcaniclastic host. Advanced- argillic alteration of the volcaniclastic material under acidic conditions resulted in the formation of kaolinite, with variable amounts of alunite, boehmite, and barite being present, whereas argillic alteration at near-neutral conditions caused smectite and rhodochrosite formation. Minor amounts of sulfide minerals, including pyrite, marcasite, sphalerite, and galena. The deposits show an enrichment of Ag, As, Cd, Hg, Pb, and Zn, attesting to the metal-carrying capacity of the thermal liquids. The isotopic composition of the sulfate minerals suggests that the anhydrite- gypsum deposits were formed by a mixture between thermal waters and seawater at temperatures of 132‒170⁰C. A model for fluid evolution of the Panarea hydrothermal field suggests mixing of an upwelling thermal liquid with shallow circulating seawater. Heating of seawater during mixing triggered the precipitation of anhydrite, which subsequently partly converted to gypsum. The thermal liquids likely contained volatiles derived from a magmatic source. The formation of massive sulfate deposits in shallow water on a partially emergent arc volcano make Panarea a distinct new type of seafloor hydrothermal system. iii TABLE OF CONTENTS ABSTRACT ............................................................................................................................. iii LIST OF FIGURES ....................................................................................................................v LIST OF TABLES .................................................................................................................. viii ACKNOWLEDGEMENTS....................................................................................................... ix CHAPTER 1 INTRODUCTION .................................................................................................1 1.1 Background and Previous Work .................................................................................1 1.2 References .................................................................................................................5 CHAPTER 2 GEOLOGY OF A SHALLOW-WATER SUBMARINE HYDROTHERMAL VENT FIELD OFF PANAREA ISLAND, ITALY: RESULTS OF A ROV SURVEY .............. 10 2.1 Abstract ................................................................................................................... 10 2.2 Introduction ............................................................................................................. 11 2.3 Geological Setting ................................................................................................... 12 2.4 Materials and Methods ............................................................................................. 14 2.5 Dive Descriptions .................................................................................................... 15 2.5.1 Western Study Area .................................................................................... 15 2.5.2 Central Study Area ...................................................................................... 19 2.5.3 Eastern Study Area ...................................................................................... 23 2.6 Discussion ............................................................................................................... 27 2.6.1 Seafloor Geology ........................................................................................ 27 2.6.2 Gas Escape Structures ................................................................................. 29 2.6.3 Gas Eruption Craters ................................................................................... 29 2.6.4 Hydrothermal Precipitates ........................................................................... 31 2.6.5 Prokaryotic Communities ............................................................................ 34 2.6.6 Acidification of Bottom Waters ................................................................... 34 2.6.7 Implications to Volcanic Hazard Management............................................. 36 2.6.8 Implications to Estimates of the Global CO2 Flux ........................................ 36 2.7 Conclusions ............................................................................................................. 38 2.8 References ............................................................................................................... 38 2.9 Figures ..................................................................................................................... 47 iv CHAPTER 3 MASSIVE SULFATE DEPOSITS ASSOCIATED WITH SHALLOW MARINE HYDROTHERMAL VENTING OFFSHORE PANAREA ISLAND, AEOLIAN ISLAND ARC, ITALY ............................................................................................................. 69 3.1 Abstract ................................................................................................................... 70 3.2 Introduction ............................................................................................................. 71 3.3 Geology ................................................................................................................... 72 3.4 Materials and Methods ............................................................................................. 76 3.5 Results ..................................................................................................................... 79 3.5.1 Seafloor Survey and Drilling ....................................................................... 79 3.5.2 Sample Petrography .................................................................................... 81 3.5.3 Quantitative Mineralogy .............................................................................. 82 3.5.4 Whole-Rock Geochemistry ......................................................................... 83 3.5.5 Isotope Geochemistry .................................................................................. 84 3.6 Discussion ............................................................................................................... 86 3.6.1 Formation of the Anhydrite-Gypsum Deposits ............................................ 86 3.6.2 Magmatic Contributions to the Panarea Hydrothermal System .................... 88 3.6.3 Comparison to Other Shallow-Water Arc Hydrothermal Systems ................ 89 3.6.4 Metal Transport and Deposition in Shallow-Water Arc Hydrothermal Systems ................................................................................ 92 3.7 Conclusions ............................................................................................................. 94 3.8 References ............................................................................................................... 95 3.9 Tables .................................................................................................................... 107 3.10 Figures ................................................................................................................... 114 CHAPTER 4 CONCLUSIONS ............................................................................................... 130 v LIST OF FIGURES Figure 2.1 Bathymetric map of the southeastern Tyrrhenian Sea showing the locations of island arc volcanoes and the Marsili back-arc basin........................................ 47 Figure 2.2 Bathymetric map of the Panarea volcano ............................................................ 48 Figure 2.3 Bathymetric map of the area surrounding the central islets located east of the main island ..................................................................................................