Hyperspectral characterisation of alteration at the ...€¦ · GNS Science Hyperspectral...
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Hyperspectral characterisation of alteration at the Wharekirauponga Au-Ag prospect, Hauraki Goldfield, New Zealand
Mark P. Simpson
GNS Science
Wharekirauponga (WKP) epithermal Au-Ag historically prospected
OceanaGold has identified a large mineralised system hosted in rhyolite
Drilling 2018 intercepted
East Graben vein 8.7 m @ 24.54 g/t Au within broader zone of 51.3 m @ 7.18 g/t Au
West vein, 9.8 m @ 4.53 g/t Au
Veins surrounded by widespread alteration investigated by hyperspectral analyses
Introduction
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(Christie et al., 2001) (OceanaGold media release July 2018)
GNS ScienceAusIMM 2018
Previous alteration investigations have identified:Quartz, adularia, illite, illite-smectite, smectite, calcite, chlorite, pyrite, cristobalite and kaolinite (Christie and Soong, 2001; Christie et al., 2016)
(OceanaGold)
(Tony Christie)
GNS Science
All data provided by OceanaGold
13 drill holes
5091 spectra
1 to 2 m spacing
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(OceanaGold)
GNS Science
Uses short wave infrared (SWIR) and visible/near-infrared light to identify many minerals and approximate their composition
SWIR-VNIR is absorbed molecular (OH, H2O, CO3 and NH4) or cation hydroxyl bonds (Al-OH Fe-OH, and Mg-OH) in a mineral
Minerals identified• Illite, illite-smectite, smectite• NH4-minerals • Carbonates • Kaolinite• AlunitePlus many others
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Hyperspectral Reflectance Spectroscopy
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Hyperspectral Minerals Identified• Illite• NH4 - minerals
- Buddingtonite- NH4-illite
• Kaolinite• Chlorite• Calcite• Jarosite• Illite-smectite• Gypsum
Illite, IS, smectite distinguished based on H2O / Al-OH ratio
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NH4-minerals: buddingtonite vs NH4-illite
Unequivocal buddingtonite[(NH4)(AlSi3)O8]
Mixture of buddingtonite plus illite can give the same profile as NH4-illite
Unable to differentiated NH4-illite from a mixture of buddingtonite plus illite
Accordingly these have been collectively mapped as NH4-minerals
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Kaolinite
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Spectral parameters
AlOH wavelength –Kaolinite versus illite
H2O / AlOH – clay rank illite, illite-smectite, smectiteBut affected by kaolinite
Limited usefulness due to spectral overlap
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NH4-minerals
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NH4-minerals
WKP-24 common NH4-minerals But WKP-26 fewer NH4-minerals
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Kaolinite& Jarosite
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Kaolinite & Jarosite
WKP-26 common kaolinite But WKP-24 minor kaolinite
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Origin of Kaolinite
Extensive and deep zones of weathering
Kaolinite often in core stained by iron oxyhydroxides = supergene
Minor occurrences of kaolinite in core without obvious weathering = hypogene?
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WKP-27 Weathered with kaolinite WKP-30 kaolinite
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NH4-minerals&
Kaolinite
Buddingtonite[(NH4)(AlSi3)O8]
Kaolinite[Al2(Si2O5)(OH)4]
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NH4-minerals in other epithermal
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Hauraki Goldfield e.g. Favona and White Bluffs
Other e.g. Sleeper Nevada, Guanajuato Mexico, Esquel Argentina
(Jackson and Victor Chevillon, 2007)
Sleeper, Nevada
(Simpson, 2015)Buddingtonite3D grade shells
*Kaolinite rare at Favona but common at Sleeper
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Origin of Ammonium minerals
NH4 derived from the break down of organic matter
Most probably sourced from lacustrine sediments and / or from encapsulated organic matter in volcaniclastic rocks
Rocks with organic material in the Hauraki Goldfield are of limited occurrence
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(David Skinner)
GNS Science
Summary• Hyperspectral surveys at Wharekirauponga have identified
– Significant occurrences of NH4-minerals– Widespread illite and common kaolinite
• NH4-minerals envelop sheeted quartz vein zones and occur in the hanging wall of wider quartz veins
• However, not all quartz veins occur in rocks with NH4-minerals
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Summary• NH4-minerals found at other epithermal prospects / deposits in
proximity to veins
• However many prospects / deposits do not have NH4-minerals
• When present, NH4-minerals can be a favourable indicator of gold mineralised quartz veins
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Acknowledgements
Funding provided by MBIE: Gold Exploration Models (GEM) program
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Special thanks to Lorrance Torckler, Rick Streiff, Scott Randall and Thomas Gardner
(Tony Christie)