geochemical & spectral alteration zonation at the mt ... · geochemical & spectral...
TRANSCRIPT
Geochemical &
Spectral Alteration Zonation
at the Mt Rawdon Gold Deposit Mines & Wines Conference, 4th September 2015
Ned Howard, Senior Exploration Geologist
Outline
Regional Geology
Local Geology
Lithogeochemistry
Alteration & Mineralisation
Spectral geology
Pathfinder geochemistry
Alteration geochemistry
Note on genesis
Early mining 1950
Mt Rawdon Production
• Discovered 1946
• Alluvials & artisanal worked 1950’s
• Exploration: 1969-1998
• Mining: 2001-present
• Production: 45.6 Mt @ 1.0 g/t Au for
1.35 Moz
• Resource: 50.7 Mt at 0.7 g/t Au for
1.16 Moz
• Endowment: 2.51 Moz
Mt Rawdon 2001
Lookout
Sept 2013
Regional Geology
Triassic Aranbanga Volcanics
Esk Basin assigned
Wandilla Province
Coastal Subprovince
Mt Morgan (VHMS/IR)
Mt Rawdon (IRGD)
Gympie (Or)
Cracow (LSE)
Local Geology Dipping rhyodacitic volcanoclastic pile
Dacitic stocks & dykes
Late- to post-mineral
basalt-andesite
& rhyolite dykes
500m
Local Geology Complicated cross-cutting relationships
500m
Diagram showing
schematic
relationships?
DAC2
DAC1
RHYO2
RHYO1
DAC1
VC
DAC1
VC
VC
Standing, 2014
Alteration and Mineralisation
Harris, A 2010, Albite-
actinolite-chlorite-pyrite
Early propyllitic
(chl-carb ± ep ± ser ± py)
Early phyllic
(ser-qtz ± py)
Mineralisation related:
Sericitic (ser-qtz-py)
Silica-pyrite (± feldspar?)
Chloritic (chl-carb-ep-act-sd)
Actinolite-albite (act-fs-ep-sd)
Late illite-smectite ± kaolinite
Harris, 2010
Harris, 2010
Harris, 2010
Alteration and Mineralisation
MRNX1114, 50.4m
(2m @ 9.12g/t)
Associated with chloritic, albite-actinolite & sericitic
alteration
Pyrite-base metal sulphides
sulphosalts-bismuth-hessite-
matildite-gold
Harris, 2010
Geochemistry & Spectral Survey ~15,000 ASD analyses
~2,000 multi-element analyses
4 acid digest
ICP-MS/AES
48 element suite
~800 existing ME samples
Mineral Identification
SWIR
• Clays, micas
• Chlorite, biotite, epidote, etc
• Carbonates
NOT Quartz, feldspar, sulphides, etc
Spectroscopy Background
100%
Refl
ec
tio
n
Absorption
features
Wavelength (nm)
Spectrum line
0%
100%
Refl
ec
tio
n
Absorption
features
Wavelength (nm)
Spectrum line
0%
Spectroscopy Background
Sericite: wAlOH = Al substitution Chlorite: wFeOH = Fe-Mg substitution
Spectral indices
Sericite composition & Chlorite composition
Spectroscopy Background
Sericite: wAlOH = Al substitution Chlorite: wFeOH = Fe-Mg substitution
Spectral indices
Sericite composition & Chlorite composition
Spectral Zonation
Chlorite composition
Intermediate in deeper,
proximal zones
Outer Fe-chlorites
Some lithological
influence doesn’t hide
patterns
Chlorite wFeOH
Metal Associations Defined visually, correlation coefficients & PCA on Z-scores
Log Z-scores
Averaged for
each association
Assocn with max
value shown
Alteration Geochemistry Major element mass changes
Reflects alteration mineral addition/removal
Relative to unaltered rock type
Low sulphidation epithermal example
Alteration Geochemistry
Albite
K-Feldspar/Biotite
Phengite
Muscovite
Chlorite/
Kaolinite
Mt Rawdon major element mass changes
Alteration Geochemistry
Mt Rawdon overall model of
alteration geochemistry
30
Summary of alteration mass changes
Subtle/variable in most areas
Mineralised zone-parallel, gold-bearing structures control
alteration
~200m
~200m
Genesis
Genetic implications of geochemical
characteristics & classification
Near-neutral, low salinity
~300+oC fluid
Near-neutral, weakly acidic,
cooler fluid (<200oC?)
Au > 1.5g/t
Au >0.67g/t
1o fluid flow controlling
structure
2o fluid flow controlling
structure
Structure associated with
Western Dacite contact
High grade trend on Eastern
Dacite contact
Eastern Dacite
Western
Dacite
Current land surface SW NE
~200m
~200m
Likely epizonal intrusion-related gold deposit (IRGD)
Fluid inclusions Low-mod salinity, 250-350oC, indications of boiling
Temperature, mixing & boiling all plausible deposition mechanisms
Related to intrusion at depth below the Western Dacite?
Summary Epizonal IRGD
Metals, alteration geochemistry and spectral
zonation mimic gold distribution
Exploration Tools:
Clear metal zonation patterns
TeBiCu-AgAs-ZnMn-Pb-Zn
Spectral indices
Sericite crystallinity, sericite wAlOH
Pathfinder Zonation
Deep Shallow Marginal
Mn
As
Pb
Zn
Cu
Ag
Au
Bi
Te
S
Tellurium >0.5ppm Bismuth >10ppm