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