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
Transcript of Hyperspectral characterisation of alteration at the ...€¦ · GNS Science Hyperspectral...
GNS Science
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
AusIMM 2018
(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
AusIMM 2018
(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
AusIMM 2018
Hyperspectral Reflectance Spectroscopy
GNS Science
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
GNS ScienceAusIMM 2018
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
GNS ScienceAusIMM 2018
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
GNS ScienceAusIMM 2018
NH4-minerals
GNS ScienceAusIMM 2018
NH4-minerals
WKP-24 common NH4-minerals But WKP-26 fewer NH4-minerals
GNS ScienceAusIMM 2018
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?
AusIMM 2018
WKP-27 Weathered with kaolinite WKP-30 kaolinite
GNS ScienceAusIMM 2018
NH4-minerals&
Kaolinite
Buddingtonite[(NH4)(AlSi3)O8]
Kaolinite[Al2(Si2O5)(OH)4]
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NH4-minerals in other epithermal
AusIMM 2018
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
AusIMM 2018
(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
AusIMM 2018
GNS Science
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
AusIMM 2018 (OceanaGold media release July 2018)
GNS Science
Acknowledgements
Funding provided by MBIE: Gold Exploration Models (GEM) program
AusIMM 2018
Special thanks to Lorrance Torckler, Rick Streiff, Scott Randall and Thomas Gardner
(Tony Christie)
