Recognition of volcanic centres in

the Ordovician Macquarie Arc

Carol Simpson

NASA Space Shuttle image of 2006 eruption of Cleveland Volcano,

Aleutian Islands

Macquarie Arc Setting

Four Structural Belts

• Junee-Narromine

• Molong

• Rockley-Gulgong

• Kiandra

Arc Stratigraphy

Magmatic Phases

Based on Percival & Glen 2007

Ph 4: high-K to shoshonitic

Ph 3: medium-K

Ph 2: Medium to high-K

Ph 1: high-K to shoshonitic

Why identify volcanic centres in the Macquarie Arc?

So, what do we look for?

• Reconstruction of arc elements and arc history

• Known association between island arc volcanoes and porphyry Cu-Au and epithermal Au deposits

• Larger Cu-Au deposits in the Macquarie Arc occur in sequences dominated by proximal volcanic facies

Two Contrasting Examples1. CARGO• Western MVB

• Phase 2: pre-

dates Ea1-3 limestones

• Medium-K submarine andesitic centre

2. NORTHPARKES

• JNVB

• Phase 4: high-K/shosh

• Subaqueous apron flanks subaerial complex

The Cargo Volcanic Centre

CV

CM

F

The Cargo Volcanic Centre

Bas/and lava

pillows

Large low-grade Cu- Au system hosted

by intrusive dacite

Porphyritic andesite

The Cargo Volcanic Centre

andesitic lavas

pillows

sediments

hyaloclastite

Northparkes Volcanic Centre

Wombin Volcanics

Northparkes mine63t Au; 1.7 Mt Cu

Wombin record of a proximal, explosive centre

Trachyandesite high- level sills, lavas

Chaotic mega- block facies

debris flows and thinly bedded turbidites

Wombin Volcanics

Wombin Volcanics

Macquarie Arc volcanic centres:

• Some demonstrated

• Most inferred

• In >45 my should be many

Early Ordovician

Early Late Ord

Late Ord

NP

C

Conclusions

• Volcanic centres → favourable lithologies, pathways for intrusions & mineralisation

• May be no temporal relationship – same pathways re-used over time

• Modern arcs inform us as to the variety of volcanic centres to expect

• ID of volcanic centres requires detailed facies mapping → still much to do Reuters photo, March 19, 2009