What’s out there and how to find it?

Broken Hill Exploration Symposium Technical Day 27 May 2015

David Giles Program 3 Leader Deep Exploration Technologies CRC The University of Adelaide

Overview Focus on BHt and Cu-Au systems

Scale reduction •  including a time element as well as the critical spatial

element

•  what we think we know about the systems at each scale

•  what tools can we apply to reduce to the next scale

Broken Hill type systems

Regional Scale The Curnamona (and especially the BHB) is a small part of a much larger system Spatial – hot, strangled rift on continental lithosphere Spatial/temporal – salt in the basin (paleogeography) Temporal – sulphate available in the oceans

Hot, throttled rift on continental lithosphere

Broken Hill type systems

Hot “throttled” rift on continental lithosphere

•  Syn-depositional bimodal magmatism

•  High Fe-Ti mafic rocks

•  Relatively juvenile turbidites fill basin

•  Salt

•  High preservation potential if rift fails

Salt in the basin (paleogeography)

•  Evidence of evaporites in footwall sequences

from Idnurm (2004)

Sulphate available in the oceans

from Huston and Logon (2004)

Broken Hill type systems Camp Scale Spatial – Rift architecture Spatial/temporal – Magmatic centre, heat (+ fluid?) source Spatical/temporal – Seafloor at critical time interval (stratigraphy)

from Connor and Preiss (2008)

Broken Hill type systems What about High T – Low P metamorphism and deformation?

•  Coincidental not causative

•  But critical for coarse grain size and local enrichment

•  Highly likely (inevitable?) in tectonic setting •  Some degree of deformation required to exhume from

bottom of basin HAVE WE PROPERLY SEARCHED FOR OFF RIFT AXIS OR HIGHER STRATIGRAPHIC LEVEL, LOW METAMORPHIC GRADE EQUIVALENTS?

Underexplored!

Broken Hill type systems Target Scale (all spatial) Geophysics

Gravity for dense sulphides + garnet-rich halo Magnetics for magnetite (if present) EM for conductive sulphides All prone to identifying false positives

Geochemistry

Kilometric alteration zones (vectoring?) Matched zones of depletion and enrichment in stratigraphy Blank sheet lithologies

Geophysics

•  Clearly can work

•  defining prospective stratigraphy

•  Targeting – but does generate false positives

•  BHt systems have large geochemical footprint

•  Matching depleted and enriched horizons

•  Blank sheet lithologies

Geochemistry undercover

requires dense sampling

within the mineral system

Geochemistry from Groves et al (2009)

Cu-Au systems

Regional Scale The Curnamona (and especially the BHB) is a small part of a much larger system Spatial/temporal – significant regional thermo-magmatic event Spatial/temporal – salt in the basin (no prior orogenesis)

Significant regional thermo-magmatic

event at ~1590 Ma

from Reid et al (2014)

Cu-Au systems Camp Scale Spatial – Lithospheric structural architecture controls flow of melt and fluid Spatial/temporal – Magmatic centre, heat + fluid source Spatial/temporal – Importance of crustal level

S-type U-enriched I-type

S-type WSG source

U-enriched Fractionated A-type

BRVs at ~1590Ma

Paleosurface and unconformity

Mafics?

from Wade (2011)

from Williams and Betts (2008)

Cu-Au systems Target Scale (all spatial) Geophysics

Gravity for dense oxides Magnetics for magnetite IP for chargeable sulphides and oxides All prone to identifying false positives

Geochemistry

Kilometric alteration zones (IOCG alteration index)

•  Clearly can work

•  Highlights Fe-oxides (not Cu-Au)

•  Many false positives

Geophysics

from Belperio (2001)

from Katona (2010)

Geochemistry undercover

requires dense sampling

within the mineral system

Geochemistry

•  Cu-Au systems have vast

geochemical footprint

•  Nested halos of (mostly) chalcophile

elements

•  But one drill hole won’t do the job…

Mineral Systems Drilling Program