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Hydrothermal alteration in gold systems – a spectrum of
processes
John Thompson, Anne Thompson and Cari Deyell-Wurst
Cornell University and PetraScience Consultants
Gold17 – Rotorua 2017
Outline
• Gold systems – time and space
• Hydrothermal alteration
• Observation and analysis
• Using alteration mineralogy
– Guides to ore systems
– Targeting
– Geometallurgy
Gold-rich mineralizing systems
• 8 distinct geological settings host deposits mined principally for gold
– Deep crust to paleosurface, ductile to brittle
– Magmatic and non-magmatic processes, high to low T
– Physical and chemical processes at the surface
• 3 settings with gold as a by-product
– Magmatic – with nickel-copper-PGE
– Porphyry – with copper
– VMS – with copper-zinc
Arc– Compression-
transpression– Uplift
Back-arc/post-collision
– Transpression-extension
– Uplift and subsidence
Collision– Compression-
tranpression– Accretion and
burial
Gold-rich sytems
EPITHERMAL
• Variety of tectonic settings and depth of formation• Influences preservation potential
Robert et al., 2007
SEDIMENT-HOSTED
OXIDIZEDINTRUSION-RELATED
REDUCEDINTRUSION-RELATED
OROGENIC
EPITHERMAL
Age distribution – secular change
Epithermal-porphyry• Majority <200 my;
many of the best are the youngest – not simply preservation
Orogenic• Multiple periods –
relationship to the supercontinent cycle?
Cawood and Hawkesworth, 2013Goldfarb et al., 2010Groves et al., 2005
Hydrothermal fluids
• Hydrothermal fluids:
– Active systems, fluid inclusions, stable isotopes
• Gold systems: ore forming fluids
– 150-500oC
– pH: 1-7
– Variable K, Na, Ca, CO2, H2S, SO2, Cl
– Dilute to high salinity
– Role of vapours
FluidsEpithermalMedium T, Low salinity
PorphyryHigh T, high salinity
Orogenic /intrusion-relatedModerate T, high CO2
Applied Mineral Explorationhttp://www.appliedminex.com/index.htm
Hydrothermal alteration processes
• Primary minerals secondary minerals
• Fluid – rock/mineral reactions– Fluid P, T , pH, composition
– Mineral chemistry and stability
– Lithological or structural permeability
– Fluid:rock ratio and kinetics (reaction rates)
• Multiple events – dynamic systems
– Changing fluid flow, P, T and chemistry
– Spatial and temporal variation
Observations
• Minerals: distribution, relationships – e.g.,
– Minerals replacement, vein/vug fill, envelopes
– Breccias: clast and matrix
• Create a sample library – reference set
Describe & record observations – then interpret
Supporting Techniques
• Field-based technologies
– Mineral spectroscopy (SWIR) and geochemistry
• Lab-based analysis
– XRD, Rietveld Analysis
– SEM-EDS, MLA/QEM-Scan
– Electron Microprobe
• Lab to field technologies
– Scanning of full core (VIS-SWIR-LW)
– Real-time geochemistry at the rig
Results of Corescan™ spectral analysis, Cu Porphyry veining
Spectroscopy• Hydrothermal alteration in gold systems – well suited to
spectroscopic methods (VIS-SWIR-LW)
Han
d-h
eld
po
int
anal
ysis
Co
re s
can
nin
g sy
stem
s
Ou
tcro
p m
app
ing
Accurate mineral ID + logging aid in fine-grained alteration
Trm
Chl
Musc Ill
Mineral chemistry
Core Photography
CoreScan spectral map
Core
Photo
Class
Map
White
Mica
Chemistry
White Mica
Crystallinity
White Mica Chemistry – Crystallinity (~2200nm wavelength)
Increase in Na(Paragonite)
Increase in K/Al(Muscovite)
Fe substitution(Phengite)
2185nm 2225nm2196nm 2212nm
Rock scale – paragenesis
Class Map Index
White mica
Kaolinite
Sulphide
Chlorite
Montmorillonite
Example of Corescan™ spectral analysis : variations in white micas, porphyry Cu-Au
Mineral chemistry
Deposit scale – zoning
Lithogeochemistry
• Use of multi-element geochemistry
• Metal & trace element signatures– Correlation with alteration and zoning
• Determine mass and chemical loss/gain referenced to protolithor conserved elements
– Useful with fine grained alteration – can define stratigraphy, igneous suites, and may be a proxy for alteration intensity
Alteration box plots (e.g., Large et al., 2001)
Using alteration in gold systems
• Recognizing and understanding ore environments
• Zoning relations – use in targeting
• Ore and waste characterization –geometallurgy
Using Alteration Mineralogy
Guide to systems
• Minerals (and mineral assemblages) indicative of type of ore environment – deposit style, and spatial and temporal variation
• Requires accurate and consistent mineral identification
• Always used in conjunction with other datasets
Corbett &
Leach 1998
Indicative of conditions for hydrothermal fluid / rock reactions
Alteration – deposit conditions
Understanding systems
• Vertical and lateral zoning in porphyry systems
• Caveat – some minerals and assemblages occur in multiple environments
Biotite
Pyrophyllite
Muscovite
SmectiteKaolinite
Alunite
Alteration – deposit conditions
Illite
Mineralogy, texture and context
• Residual/vuggy vs vein- fill chalcedony
• Both epithermal, but different formation and implications for exploration Residual vuggy
quartzChalcedonic vein quartz
Types of quartz – mineralogy and texture
High temperature
• Au-rich magmatic-hydrothermal systems
– Porphyry and skarn deposits
– Reduced intrusion-related Au
• Iron-oxide Cu-Au
• Proximal K-silicate/potassic alteration
– K-feldspar, biotite, magnetite, ferroactinolite
• Deep-lateral/regional sodic-calcic alteration
– Albite, diopside, garnet, actinolite, epidote
• Regional-zoned propylitic alteration
– Actinolite, epidote, chlorite, pyrite
Porphyry Cu-Au alteration
Quartz-Mag veins – no envelope Quartz vein with strong biotite envelope
Early magnetite-biotite vein cut b quartz
vein with K-fsp envelope
Intermediate quartz vein with centre-fill + biotite
envelope overprinted by chlorite-sericite
Intrusion-related Au – alteration
Muscovite
Miarolitic
cavity
UST –
brain rock
Sheeted quartz veins
Quartz veins with sericite-carbonate
vein envelopes
Intermediate temperature
• Orogenic Au
• Porphyry-related breccias, vein and carbonate-replacement systems
• Au-rich VMS/VHMS
• Vein-wallrock and footwall alteration
– Quartz, sericite (muscovite-illite), chlorite, carbonate
• Matrix replacement and vug-filling
– Quartz, carbonate, sericite, clays
Orogenic gold systems• Fluids focused into and around major
structures
Carbonate-sericite-chlorite-albite-pyrite-hematite
Albite-carbonate Hedenbergite-actinolite-biotitePhotos – Dave Rhys, Panterra
VMS- VHMS
Galley et al (2007)
corderite
anthophyllite
chlorite
Deep footwall alteration - epidote
Footwall alteration –Fe chlorite + Cu stringers
Footwall alteration – sericite Bruce Gemmell
Low temperature
• Epithermal Au-Ag
– High sulphidation
– Intermediate sulphidation
– Low sulphidation
• Sediment-hosted Au
• Vuggy quartz and wallrock
– Quartz, rutile, alunite, sulphur, kaolinite, pyrophyllite
• Quartz-adularia
– Quartz, adularia, illite, bladed calcite (quartz)
• Decalcification-silicification (jasperoids)
• Quartz
• Rutile
• Sulphur
• Alunite
• Dickite
• Pyrophyllite
• Diaspore
• Kaolinite
High sulphidation deposits
Low to intermediate sulphidation• Quartz
• Adularia
• Muscovite/Illite
• Carbonates (Ca, Fe, Mg and Mn)
• Smectite
• Chlorite
Use of alteration – targeting
• Most hydrothermal systems show zoning
– Regional – multiple centres
– Concentric around system
– Vertically – proximity to paleosurface
– Controlled by structures or lithology
– Lateral to structures and veins
• Mapping spatial variation in alteration mineralogy
– Guide to location within systems – scale
– Target to ore zones or better ore – vectors
Decreasing
scale –
km to m
Cu-Mo (Au)
Cu-Au
Py +/-
Pb-Zn-Ag-Au Au (As-Sb-Hg-Tl)
8 Km
Au-As
Pb-ZnCu-Mo-Au
(Babcock et al., 1995;
Cunningham et al., 2004)
Zoning – porphyry modelBingham Canyon, Utah
• Extensive lithocap
• Porphyry, intermediate and high sulfidation deposits
Mankayan district, Philippines
Lepanto HS: 1 Mt Cu & 120 t Au
FSE porphyry: ~892 Mt @ 0.5% Cu & 0.7 g/t Au
Guinaoang porphyry, 500 Mt @ 0.4% Cu & 0.4 g/t Au
Mohong Hill porphyry + HS
Ore deposits projected to
surface
Victoria veins, 11 Mt @ 7.3 g/t Au + Ag-
Cu-Pb-Zn
Nayak veins
Teresa veins, 0.8 Mt @ 5.74 g/t Au
Buakiporphyry
1 km
Chang et al., 2011
Na,Ca-alunite
K-alunite + Pb
Gold-rich porphyry deposits• Kislidag – 16.8 MOz Au • High level porphyry system – eroded lithocap• 14.76 ± 0.01 to 14.36 ± 0.02 Ma• Zoned alteration
Advanced argillic
Argillic
White mica -tourmaline
Potassic -projected
Baker et al., 2016
Kislidag zoning – alteration mineralogy
Mineral distribution: aiSIRIS Spectral Contribution (‘SC’) data
>25% tourmaline >10% alunite
>35% white mica >50% kaolinite
Eldorado Gold and AusSpec300 m
White mica SCKaolinite SC
>0.5 g/t Au shell
Final pit outline Final pit outline
aiSIRIS SC data – 900m Level
Kislidag zoning – alteration mineralogy
Potassic zone (LF model)
Eldorado Gold and AusSpec
Kaolinite White mica
Core: muscovitic-paragoniticDistal: phengiticSpectral shift: <2200nm to >2210nm
500 m
Gosowong low sulphidation Au
Alteration zoningQtz-adularia argillic “vein propylitic”
Mineral zoning– Chlorite
chemistry
Lithogeochemistry– Multi-element
enrichment and depletion – halo and depth change
Gemmel 2007
50m
• Porphyry Au
– Lateral and vertical, district to ore zone targeting
• Skarn Au
– Skarn type; lateral changes within skarns
• Epithermal and orogenic Au
– Type of system, district scale to wallrock/vein scale
• VMS
– Footwall: district to proximal upflow zones/ore
• Sediment-hosted
– District to target – increasing intensity towards ore
Alteration – targeting
Rock characterization
Hydrothermal alteration modifies mineralogy and textures – physical and chemical characteristics
– Chemical data
– Mineralogical data
– Textural data
– Structural data (vein density, RQD)
• Hardness
– Harder: e.g., quartz, feldspar, garnet
– Softer: e.g., sericite, clays, anhydrite
• Breakage, fragmentation and comminution
– Breccias, veining, microfractures
• Leaching
– Mineral reactions, fractures and permeability
• Concentrate quality – deleterious elements
– Influenced by deposit type, location/level in system
• Bulk measurements (core – SWIR), proxies
Alteration mineralogy: Geometallurgy
Alteration – processing characteristics
Albite
Residual quartz
Decreasing hardness
Sericite
Kaolinite
Increased breakage –
fragmentation
Alteration in Au deposits
• Many environments
• Reflects hydrothermal system – and location
• Provides a targeting tool at many scales
• Geometallurgy: ore/processing types, LOM variation, potential for sorting
• Based on good observations and good data