GNS Science

Mapping hydrothermal minerals in NZ geothermal fields using spectroscopy and application to minerals exploration

Mark P. Simpson, Greg Bignall, Andrew J. Rae, Anthony B. Christie and Isabelle Chambeford

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Reflectance spectroscopy Many minerals can be identified by their absorption of short wave infrared (SWIR), near infrared (NIR), and visible (Vis) electromagnetic waves

SWIR-NIR-Vis waves are absorbed by molecular or crystal lattice bonds in a mineral (OH, H2O, AlOH, FeOH, MgOH & CO3)

Many minerals identified in the SWIR (1200 – 2500 nm) region

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Mineral group Minerals Amphibole Actinolite, tremolite, hornblende

Carbonate Calcite, ankerite, siderite, rhodochrosite

Clays Chlorite, illite, interstratified illite-smectite, montmorillonite (smectite), kaolinite, dickite, halloysite

Epidote Clinozoisite, zoisite

Mica Biotite, muscovite, paragonite, phlogopite

Phyllosilicates Pyrophyllite, talc

Other Silicates Tourmaline, topaz, buddingtonite (NH4 K-feldspar)

Sulfates Alunite, jarosite, gypsum

Oxides Hematite

Minerals in geothermal fields

*Reflectance spectroscopy can detect many other minerals, but not all minerals, e.g. quartz and feldspars (adularia, albite, plagioclase)

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Mineral group Minerals Amphibole Actinolite, tremolite, hornblende

Carbonate Calcite, ankerite, siderite, rhodochrosite

Clays Chlorite, illite, interstratified illite-smectite, montmorillonite, kaolinite, dickite, halloysite

Epidote Clinozoisite, zoisite

Mica Biotite, muscovite, paragonite, phlogopite

Phyllosilicates Pyrophyllite, talc

Other Silicates Tourmaline, topaz, buddingtonite (NH4 K-feldspar)

Sulfates (selected) Alunite, jarosite, gypsum

Oxides Hematite

Minerals in geothermal fields

*Reflectance spectroscopy can detect many other minerals, but not all minerals, e.g. quartz and feldspars (adularia, albite, plagioclase)

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Samples and data collection No sample preparation (just dry)

Analyses can be made on: • Rocks / drill core / cuttings • Powders / pulps

Data collected by TerraSpec4 hi-res, TerraSpec Halo or older PIMA

Rapid data collection • 10 seconds per scan

Spectra viewed and processed using The Spectral Geologist (TSG®)

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Data processing: RAW vs Hull Quotient

Examination of spectra in either RAW or following Hull quotient correction Hull quotient = background subtraction that enhances some absorption features

Raw and Hull both provide useful data

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Limitations Spectra are a composite of all minerals present

Spectral overlap can result in mineral misidentification • e.g. interlayered illite-smectite versus discrete illite + smectite

Contamination affects mineral identification and crystallinity • fluid inclusion / intra-crystalline water • volcanic glass (H2O and FeOH)

Mineral abundance • spectral dominance of a given mineral

These effects often recognisable

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Neutral pH minerals Minerals identifiable include: • Smectite (montmorillonite) • Interstratified illite-smectite • Illite • Calcite • Chlorite • Epidote*

Can further determine: • Crystallinity of illite • Composition of chlorite & illite

*Epidote: low abundance and spectral overlap with chlorite and / or calcite, typically precludes its identification

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Smectite, illite-smectite, illite Spectral characterisation developed by comparison with clay XRD identifications Crystallinity ratio of H2O / AlOH

Ohaaki geothermal (Simmons and Brown, 2000)

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Mineral RAW H2O / AlOH

Smectite <0.80 Illite-smectite 0.80 – 1.00 Illite >1.00

Mineral Hull Quotient H2O / AlOH

Smectite <0.76 Illite-smectite 0.76 – 0.96 Illite >0.96

2180

2200

2220

2240

0.4 0.6 0.8 1.0 1.2 1.4

Wav

elen

ght o

f AlO

H fe

atur

e (n

m)

H2O / AlOH ratio

Raw Reflectance IlliteISSmectite

smectite illiteIS

2180

2200

2220

2240

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

Wav

elen

ght o

f AlO

H fe

atur

e (n

m)

H2O / AlOH ratio

Hull Quotient IlliteISSmectite

smectite illiteIS

*Generally defined boundaries with some overlap

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Some illite-smectite with high percentages of smectite or illite plot within the smectite or illite fields, respectively

Reflects limitation of detecting low amounts of smectite or illite

2180

2200

2220

2240

0.4 0.6 0.8 1.0 1.2 1.4

Wav

elen

ght o

f AlO

H fe

atur

e (n

m)

H2O / AlOH ratio

Hull QuotientIS 90 %IS 80 %IS 70 %IS 60 %IS 50 %IS 40 %IS 10 %

smectite IS illite

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Favona epithermal Au-Ag deposit, NZ

Mineral RAW H2O / AlOH

Illite-smectite <1.10

Illite >1.10

*illite-smectite from XRD has between 60 to 95 % illite

XRD – Clay separate Reflectance (SWIR)

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Favona epithermal Au-Ag deposit, NZ

Mineral RAW H2O / AlOH

Illite-smectite <1.10

Illite >1.10

*illite-smectite from XRD has between 60 to 95 % illite

XRD – Clay separate Reflectance (SWIR)

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2 days

Simpson et al., 2005

Some differences, but overall same broad trends

~6 weeks

Waitekauri epithermal Au-Ag prospects, NZ Reflectance (SWIR)

XRD (clays)

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Mineral composition illite composition determined from AlOH wavelength position Chlorite composition determined from FeOH wavelength position Reflects alteration and / or initial rock composition

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Acid pH minerals Minerals identifiable include: • Kaolinite • Dickite • Alunite • Pyrophyllite • Topaz

Can further determine: • Composition alunite, K vs Na • Crystallinity kaolinite & dickite

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Mineral distributions Accurate determination of alteration minerals distributions due to narrow sample frequency (5 m)

Appropriately select casing type and cements based on minerals distribution

Distribution of hydrothermal minerals in unspecified geothermal well, NZ. Zone of acid minerals accurately delineated

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High sulfidation Minerals include: • Kaolinite • Dickite • Alunite • Pyrophyllite • Diaspore • Topaz

Distal • Smectite • Illite • Chlorite

(Harvey et al., 1999)

(Concepcion and Cinco, 1989; Garcia, 1991)

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Summary • Rapid mineral identification at sample frequency impractical

by other methods (i.e. 5 m)

• Identifies many hydrothermal minerals common in mineral deposits e.g. low and high sulfidation epithermal

• Some limitations and should validate by XRD

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Thank you

Acknowledgements Financial support for this project is from the GNS Science core-funded Geothermal Resources of New Zealand research program and also MBIE