VOLCANIC FACIES ARCHITECTURE AND

EVOLUTION OF MILOS, GREECE

by

Andrew L. StewartB.Sc. (Hons) Macquarie University.

UNIVERSITY OF TASMANIA

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CODESSRC

Submitted in fulfilment of the requirementsfor the degree of Doctor of Philosophy

University of TasmaniaAustraliaJuly, 2003

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STATEMENT AND AUTHORITY OF ACCESS

This thesis contains no material which has been accepted for a degree or diploma by

the University or any other institution and, to the best of my knowledge and belief, no

material previously published or written by another person except where due acknowl­

edgement is made in the text of this thesis.

This thesis may be made available for loan and limited copying in accordance with the

Copylight Act 1968.

Date:

Andrew L. Stewart

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ABSTRACT

The volcanic island of Milos Island, Greece, is a relatively small (-151 km') but signifi­

cant portion of the active Southern Aegean Volcanic Arc, Milos comprises an Upper

Pliocene-Pleistocene, thick (up to 700 m), and compositionally and texturally diverse

succession of calc-alkaline, volcanic and sedimentary rocks that record a transition from

a relatively shallow but dominandy below-wave-base submarine setting to a subaerial

one. The shallow marine part of the succession hosts several significant epithermal gold

deposits.

Twenty-two main submarine and twelve subaerial volcanic, sedimentary and intrusive

facies have been identified, and arranged into eleven compositionally and texturally dis­

tinct facies associations. The principal volcanic facies are (1) coherent rhyolite, dacite,

andesite, basaltic andesite Oavas, domes, cryptodomes, dykes and sills), and associated

autedastic facies (autobreccia, hyalodastite and intrusive hyalodastite); (2) submarine

and subaerial pyrodastic deposits; and (3) volcanogenic sedimentary facies. The vol­

canic and intrusive facies are interbedded with a sedimentary facies association com­

prising sandstone and/or fossiliferous mudstone mainly derived from erosion of pre­

existing volcanic deposits. The main facies associations are interpreted to have con­

formable, disconformable, and interfingering contacts, and there are no mappable an­

gular unconformities or disconformities whhin the volcanic succession.

The facies architecture indicates depositional environments evolved from below to

above the wave base to subaerial in most areas, except at the southeastern sector of the

island where more uniform subaerial environments dominated. The architecture of the

dominantly felsic-intermediate volcanic succession reflects contrasts in eruption style)

proximity to source, depositional environment and emplacement processes. The vol­

canic facies architecrure comprises interfingering proximal (near vent), medial (volcano

flanks), and distal (volcano margin) facies associations related mainly to submarine and

subaerial felsic cryptodome-pumice cone volcanoes, dacitic to basaltic andesite lava

domes and pyroclastic cones. Submarine felsic cryptodome-pumice cone volcanoes are

the most voluminous and common type of volcano identified. Submarine explosive

eruptions from these centres generated pumiceous gravity-current deposits and thick

beds of very coarse, water-settled pumice. In proximal sections, thick felsic pumice

breccia intervals were intruded by compositionally similar, porphyritic, rhyolitic and

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dacitic cryptodomes and sills.

New SHRIMP V-Pb data from four major volcanic facies, in combination with detailed

mapping and facies analysis, have enabled construction of an enhanced, internally con­

sistent time-stratigraphic framework for the evolution of Milos. The volcanic activity

began at 2.66 ± 0.07 Ma and has been more or less continuous since then. Subaerial

emergence probably occurred at 1.44 ± 0.08 Ma, in response to a combination of vol­

canic constructional processes and fault-controlled volcano-tectonic uplift. Recent

phreatic craters are the youngest (200 Be-200 AD) expressions of volcanism, and are

spatially associated with an active, high-enthalpy geothermal field.

The succession contains several significant epithermal, precious and base metal deposits

that display a range of textural, mineralogical, and compositional characteristics. The

majority of these epithermal ores occur within and at the top a single, submarine felsic

cryptodome-pumice cone volcano near the stratigraphic base of the succession. The

palaeogeography during mineralisation probably comprised scattered islands (volcanic

domes) flanked by shallow-marine areas. A modern analogue for rhe setting of the epi­

thermal-style mineralisation is the shallow submarine to subaerial volcanic complex of

the island of Panarea, in the active Aeolian volcanic arc (Tyrrhenian Sea, Italy).

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ACKNOWLEDGEMENTS

This research was supporred by an Australian Post Graduare Research Award and the

Centre for Ore Deposit Research, University of Tasmania.

I would like to thank my supervisor Associate Professor Jocelyn McPhie for her

support, encouragement, advice, and guidance throughout the project. In particular I

would like to thank Jocelyn for the invaluable time she spent with me in the field and

the effort and patience she took in reviewing my work.

I would like to thank Don Baker (Royal Gold Inc.), George Xydous (Silver & Baryte are

Mining CO. SA.) and the geological staff of Midas SA. for logistic support in the field.

Special thanks go to Rod and Petrinela Feldtmann for their hospitality at the Kafenio and

their availability for discussions on r."filoan matters and assistance they provided me while in

the field. I would especially like to thank Rod for shareing his invaluable geological

knowledge. I would also like to thank Georges Vougioykalakis for providing work-permits

and use of unpublished bathymetric maps and Mirko Rinaldi and Claudia Principe for

valuable discussions in the field. Marc Norman is thanked for obtaining SHRIMP data at

the Australian National University.

Numerous colleagues have cast their critical eye over various chapters in this thesis.

These colleagues include Sharon Allen, Kate Bull, Julie Donnelly-Nolan, Greg

Ebsworth, Yoshi Goto, Richard Fiske, Vern Manville, Karin Orth, Don Swanson and

Setsuya Nakada. They are all thanked for their constructive comments and discussions

on early versions of the thesis and helpful critical reviews.

Many people have helped me in various ways throughout this project and I would like

to thank them collectively. In particular, Peter Cornish, Fernando Della-Pasqua, Wally

Herrmanm, Christine Higgins, June Pongratz, Phil Robinson, Dianne Steffens and

Simon Stephens without which this project would not have been possible.

Finally, special thanks are due to my family and my friends for their continued support

over the three years of this study.

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TABLE OF CONTENTS

This thesis contains a total 252 pages.

Statement and authority of access 2

Abstract 3

Acknowledgements 5

Table of contents 6

Chapter 1: Introduction

1.1 Introduction 1-1

1.2 Aims and significance 1-1

1.3 Location and access 1-3

1.4 Physiography and exposure 1-4

1.5 Previous work on Milos 1-6

1.6 Methods of investigation 1-9

1.7 Thesis organisation 1-10

Chapter 2: Geotectonic setting of Milos in the Southern Aegean Volcanic Arc

2.1 Aims and significance 2-1

2.2 Tectonic setting of the Aegean region 2-1

2.3 Geology of the crystalline basement , 2-5

The Alpine orogeny 2-5

Granitoid emplacement during the Miocene 2.6

2.4 Volcanism in the Aegean , 2-8

Northern Aegean Tertiary Activity (NATA) 2-8

The Southern Aegean Volcanic Arc (SAVA) 2-8

2.5 Summary 2-11

Chapter 3: Regional geology of Milos: an overview

3.1 Introduction .3.1

3.2 A review of existing stratigraphic framework on Milos 3.1

Mesozoic metamorphic group 3.3

Neogene sedimentary group 3.3

The basal pyroclastic series 3.4

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Complex of domes and lava flows 3.6

The pyroclastic and lava domes series 3.6

The rhyolitic complexes of Firiplaka and Trachilas 3.7

The producrs of phreatic activity 3.7

Quaternary sedimentary formations 3.9

3.3 The volcanic producrs of the neighboring islands 3.9

Kimolos and Polyegos 3.9

Antimilos 3.9

3,4 Age relationships 3.10

3.5 Structure of Milos 3.12

3.6 The Milos geothermal system 3.14

3.7 Mineralisation 3.14

3.8 Petrography and Geochemistry 3.19

3.9 Summary 3.20

Chapter 4: Submarine volcanic and sedimentary facies and facies associations

of Milos

4.1 Introduction 4.1

4.2 Terminology for volcanic textures .4.2

4.3 Criteria used to recognise submarine facies 4.3

4,4 Facies associations and organisation ,4.3

4.5 Rhyolite facies association .4.13

Coherent rhyolire facies .4.13

Sediment-marrix rhyolite breccia facies ,4.14

Interpreration ,4.17

4.6 Dacite facies association ,4.18

Coherenr dacite facies ,4.18

Non-stratified monomictic dacire breccia facies ,4.19

Stratified monomictic dacite breccia facies 4.21

Sediment-matrix dacite breccia facies 4.21

Interpretation 4.23

4.7 Andesite facies association 4.24

Coherent andesite facies 4.24

Non-stratified monomictic andesite breccia facies 4.25

Stratified monomictic andesire breccia facies ,4.27

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Sediment-matrix andesite breccia facies 4.27

Interpretation 4.29

4.8 Basaltic andesite facies association 4.29

Coherent basaltic andesite facies .4.29

Sediment-matrix basaltic andesite breccia facies 4.30

Interpretation 4.31

4.9 Pumice breccia facies association 4.32

Coarse pumice breccia facies 4.33

Stratified pumice breccia facies 4.34

Lithic-pumice breccia facies 4.35

Graded pumice breccia facies 4.35

Interpretation ..4.36

Variations within the pumice breccia facies association , .4.37

4.10 Scoria-rich breccia facies association 4.38

Cross-stratified scoria breccia facies 4.38

Massive andesitic breccia facies 4.40

FIne scorja sandstone facies .4.41

Interpretation , , 4.41

4.11 Sandstone-conglomerate facies association "..4.43

Graded Sandstone facies " 4.43

Thickly bedded to laminated mudstone facies" .4.44

Polymictic breccia-conglomerate facies " .4.45

Interpretation , .4.45

4.12 Summary............... . " .4.46

Chapter 5: Internal structure and emplacement of an Upper Pliocene dacite

cryptodome

5.1 Introduction " 5-1

5.2 Stratigraphy of northeastern Milos........................ 5-2

5.3 The Kalogeros cryptodome and host formation 5-6

5.4 Lithofacies and internal structure 5-6

Coherent dacite facies " 5-8

Banded dacite facies : 5-8

Fractured dacite facies 5-11

Massive dacite breccia facies " 5-12

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Stratified dacite breccia facies 5-14

5.5 Facies architecture of the Kalogeros cryptodome 5-15

5.6 Timing and environment of cryptodome emplacement 5-17

5.7 Mode of emplacement of the Kalogeros cryptodome 5-18

5.8 Charactetistics of felsic submarine cryptodomes 5-20

5.9 Summary .5-22

Chapter 6: An Upper Pliocene coarse pumice breccia generated by a

shallow submarine explosive eruption

6.1 Introduction 6-1

6.2 Stratigraphy of northeastern Milos '" 6-2

The Filakopi Pumice Breccia 6-4

6.3 Components and composition of the FPB 6-5

Juvenile clasts.................................... . 6-5

Non-juvenile clasts 6-5

6.4 Internal stratigraphy of the FPB 6-6

Unit A 6-9

Unit B 6-10

Unit C : 6-13

6.5 Depositional setting of the FPB 6-14

6.6 Location and character of the FPB source 6-15

6.7 Origin of pumice clasts in the FPB 6-17

6.8 Transport and depositional mechanisms 6-18

6.8 The FPB eruption 6-21

6.9 Products of shallow submarine explosive eruptions 6-23

6.10 Summary 6-26

Chapter 7: Snbaerial volcanic facies and facies associations of Milos

7.1 Introduction 7.1

7.2 Criteria used to recognise subaerial facies 7.1

7.3 Facies associations and organisation 7.2

7.4 Biotite-quartz-phyric rhyolite facies association 7.6

Coherent biotite-quartz-phyric rhyolite facies 7.6

Clast-supported monomictic biotite-quartz-phyric rhyolite facies 7.8

Interpretation , 7.9

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7.5 Dacite facies association 7.10

Coherent dacite facies 7.1 0

Clast-supported monomictic dacite breccia facies 7.12

Bedded monomictic dacite breccia facies 7.13

Interpretation , , 7.13

7.6 Andesite facies association 7.14

Coherent andesite facies 7.14

Clast-supported monomictic andesite breccia facies 7.16

Interpretation 7.17

7.7 Pyroclastic facies association 7.17

Matrix-supported coarse breccia facies 7.18

Cross-bedded lapilii-ash facies 7.18

Bedded ash facies 7.19

Interpretation 7.20

7.8 Mud-matrix lithic breccia facies association 7.21

Massive schist-rich breccia facies 7.22

Polymictic mud-matrix breccia facies 7.23

Interpretation 7.24

7.9 Summary 7.24

Chapter 8: Volcanic and sedimentary facies architecture of Milos

8.1 Introduction 8.1

8.2 An evaluation of stratigraphic subdivisions and framework 8.1

New SHRIMP V-Pb ages ""'''''' 8.2

Geological context of samples 8.3

Methods 8.3

Results 8.5

Basal pyroclastic series 8.5

Complex of domes and lava flows '" 8.10

Pyroclastic series and lava domes 8.11

Rhyolitic complexes of Firiplaka and Trachilas and phreatic products 8.13

8.4 Volcanic and sedimentary facies architecture of lvlilos 8.13

Facies associations and depositional environments 8.13

Thickness variation "" 8.15

Volcano types and architecture 8.16

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Submarine felsic cryptodome-pumice cone volcanoes 8.18

Submarine dacitic and andesitic lava domes.. 8.20

Submarine to subaerial scoria cone _ 8.21

Submarine-to-subaerial dacitic and andesitic domes 8.22

Subaerial rhyolitic lava-pumice cone volcanoes 8.24

8.5 Summary 8.25

Chapter 9: Setting of epithermal mineralisation in the evolution and facies archi­

tecture of Milos: implications for exploration

9.1 Introduction , 9.1

9.2 What is the definition of shallow water epithermal minetalisation? 9.2

9.3 Precious & base metal exploration history 9.2

9.4 Review of epithermal minetalisation on Milos 9.4

Profitis lllias 9.6

Chondro Vouno-Amethyst 9.8

Triades 9.10

Cape Vani " 9.13

9.5 Timing and environment of epithermal mineralisation 9.15

9.6 Explotation for epithermal ores in subaerial-shallow submarine

environments: Volcanic facies models & relevant observations from

Milos , 9.16

Stratigraphic distribution and controls on mineralisation at Milos 9.16

Related volcano type-submarine felsic dome 9.17

Role of depositional environment in mineralisation 9.18

9.7 Global perspective" 9.19

9.8. Summary 9.23

Global exploration strategies 9.24

Chapter 10: Synthesis: a Pliocene-Pleistocene palaeogeographic reconstruction

and volcanic evolution of Milos Island

10.1 Introduction _ 10.1

10.2 Palaeogeographic reconstruction and evolution of Milos Island I0.1

Mesozoic basement and Neogene sedimentary formation 10.1

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Upper Pliocene-Pleistocene volcanic evolution 10.2

Submarine felsic cryptodome-pumice cone volcanoes 10.3

Submarine dacitic and andesitic lava domes .10.5

Submarine-to-subaerial dacitic and andesitic domes 10.7

Subaerial rhyolitic volcanism and phreatic explosions l0.9

10.3 A modern analogue for the Upper Pliocene palaeogeography -Panarea

Island, Aeolian are, Italy 10.10

10.4 Implications for comparable volcanic successions l0.13

10.5 Avenues for future research 10.14

10.6 Summary... . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . 10.15

References ; 11.1

Appendix A Geochemical analyses from Milos-results from this study

Appendix B U-Pb age dating-analytical data

AppendiX C Rock catalogue