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

Major gold regions

Phanerozoic

Archean/Proterozoic

Hydrothermal systems

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

Sodium Cobaltinitrite Staining

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)

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Accurate mineral ID + logging aid in fine-grained alteration

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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)

Mapping alteration

Data collection Interpretation Integration

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 - alteration

Kislidag – alteration dataSpectral data – raw SWIR assemblages

Eldorado Gold

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