The hydrothermal evolution of Milos island in space and time
: an example of a young emergent volcanic edifice

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Subduction zone magmatism in arc environments is often expressed as chains of submarine and subaerial volcanic edifices with associated hydrothermal systems. Volcanic piles initially accumulate in the submarine environment and at some point in the evolution of the edifice, they will emerge and volcanic-hydrothermal activity will become dominantly terrestrial. The island of Milos, in the South Aegean Volcanic Arc, hosts a variety of mineralisation styles that formed in a transitional, emerging volcanic system. The existing submarine to subaerial magmatic framework on Milos is partially constrained by geochronology. However, the associated hydrothermal activity, mineralisation and alteration has not been robustly linked in time or space to its volcanic evolution. This thesis combines hyperspectral mineral mapping with field observations, and places magmatism, emergence, hydrothermal alteration and mineralisation into a robust spatio-temporal framework to understand the evolution of the Milos island edifice over the last 2.8 Ma.
Milos hosts three distinctive types of alteration identified using a combination of hyperspectral SWIR–VNIR mineral mapping, field observations, mineralogy (XRD) and stable isotope compositions. K-feldspar–mica alteration is localised forming syngenetic to Au–Ag veining at Profitis Ilias and Chondro Vouno, Pb–Zn–Ba–Ag mineralisation at Triades–Galana, Katsimouti and Vani dome, and finally, Mn–Fe–Ba occurrences within the Vani
basin. The second, comprises significant bentonite (montmorillonite) occurrences that are actively mined and is commonly located below silica–alunite–kaolinite acid-sulfate alteration. The latter is distributed island-wide and occupies the paleo-vadose zone with alunite and jarosite sulfur isotope compositions reflecting a steam-heated origin (~7‰ δ34SVCDT).
We use clay mineralogy, O- and H- isotope compositions, and high precision zircon U-Pb CA-ID-TIMS analysis to further investigate bentonite formation at the workings of Koufi-Aggeria and Aspro Chorio in northeastern Milos. The volcanic piles of Koufi-Aggeria and Aspro Chorio accumulated over a period of ~170 kyr ca. 2.80 Ma, until the emergence of Korakia at ~2.63 Ma. Bentonite formation took place while the volcanic pile was being
accumulated within a minimum of three bentonite generation events. Field evidence points towards multiple, smaller magmatic pulses rather than a single geothermal field being responsible for its genesis. We robustly constrain the bentonite formation episodes to rapid (<20 kyr) timescales. O- and H- isotope compositions reflect a fluid comprising meteoric- and sea-water mixtures at temperatures of 70±10°C. Sulfur isotope compositions and
Ar-Ar geochronology on alunite indicate a low temperature hydrothermal event ~1.5 myrs later, and resulted in a steam-heated silica–alunite–kaolinite overprint genetically unrelated to bentonite formation. Finally, we address the evolution of the volcanic architecture from submarine to subaerial magmatism using zircon U-Pb LA-ICP-MS, and how this relates to mineralisation with Ar-Ar sulfate and K-feldspar geochronology. We show magmatism occurred on both east and western Milos in 100–200 kyr episodes of increased volcanic activity, coevally in the submarine, transitioning and subaerial environment through the magmatic history of the island. Three significant temporal clusters of magmatism are identified (~2.7, ~2.0 and ~1.6 Ma). The island records submarine and subaerial magmatism throughout its evolution ca. 2.8 Ma, until around 1.1 Ma, when the main magmatism was focused in central Milos in the subaerial environment. We show that mineralised systems occur independently in time and space, and are genetically unrelated. Both metal deposit and bentonite formation is constrained to the first ~1.5 myrs of its evolution. Steam-heated acid-sulfate alteration interacts with all of these
systems and occurs much later than the mineralising events ca. 1.1 Ma until relatively recent. We suggest it is an expression of geothermal systems associated with recent (<1.2 Ma) subaerial magmatism and is not coeval with any of the older (>1.5 Ma) mineralising systems.
Date of Award28 Sep 2021
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorJonathan Naden (Supervisor), Simon Tapster (Supervisor), Stephen Grebby (Supervisor), Frances J Cooper (Supervisor) & Brian Tattitch (Supervisor)

Keywords

  • hydrothermal
  • Milos
  • magmatic
  • volcanic
  • alteration
  • clay
  • bentonite
  • industrial minerals
  • epithermal
  • Geology
  • mineralogy
  • SWIR
  • geochronology

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