Hyperspectral interpretation of selected drill cores from orogenic gold deposits in central Victoria, Australia

ABSTRACT

HyLogger hyperspectral data obtained from seven orogenic gold deposits in central Victoria, including Bendigo, Ballarat, Maldon, Fosterville, Costerfield, Castlemaine and Wildwood, are presented. The data demonstrate that fresh diamond drill core displays substantial mineralogical variation that can be attributed to the effects of cryptic hydrothermal alteration that might not otherwise be recognised. The most significant hyperspectral response lies in the white mica compositions, which vary in a systematic manner between high-Al muscovitic zones (Al–OH absorption around 2208 nm) that define a phyllic alteration halo around mineralised structures, and low-Al phengitic–chlorite zones (Al–OH absorption >2014 nm) inferred to represent either more distal alteration or possibly regional metamorphic background. An extensive ferroan dolomite alteration halo overlaps the phyllic and sulfidic alteration zones and extends beyond the sampled core in most instances. This ferroan dolomite halo has previously been defined petrographically, geochemically and using carbonate staining techniques, and is further characterised using thermal infrared hyperspectral data in drill core from the Ballarat goldfield. The mineralogical trends identified by the hyperspectral data are best developed in diamond drill core from the Costerfield, Fosterville and Ballarat goldfields, and are less pronounced at the other deposits. At Bendigo and Castlemaine the reasons for this are not immediately clear, but may be related to the close timing of gold mineralisation relative to peak metamorphism. The Maldon area lies within the contact aureole of the Harcourt Batholith and so has been thermally overprinted leading to the recrystallisation of earlier hydrothermal assemblages. The Wildwood deposit is similar to the Magdala deposit at Stawell and differs from the other goldfields in its geological setting, host rock lithologies and style of hydrothermal alteration, with the development of Fe-rich chlorite closely associated with gold mineralisation. The results demonstrate how hyperspectral data can be used to define large hydrothermal alteration footprints associated with orogenic gold mineralisation in central Victoria that are of direct benefit to mineral explorers, as well as independently characterising lithological variations in drill core.

KEYWORDS:

• Hyperspectral
• HyLogger
• gold
• orogenic deposits
• central Victoria
• hydrothermal alteration

Acknowledgements

The authors would like to acknowledge the following companies for loaning core in order for this study to be undertaken and agreeing to release the results of the study: Ballarat Goldfields Ltd (Castlemaine Goldfields Ltd)—Ballarat; Perseverance Corporation Ltd (Newmarket Gold Inc.)—Fosterville; A.G.D. Mining Ltd (Mandalay Resources)—Costerfield; Castlemaine Goldfields Ltd—Castlemaine; Bendigo Mining Ltd (Unity Mining Limited)—Bendigo; Alliance Resources Ltd (Octagonal Resources Ltd)—Maldon; and Leviathan Resources (Newmarket Gold Inc.)—Wildwood. Most of the deposits studied have changed ownership since the original work was carried out in 2007. The names of the new owners are included in brackets after the original company that provided access to the drill core described in this report.

The HyLogging data were collected as part of the NCRIS and CSIRO-funded, AuScope NVCL Project. The assistance of the CSIRO HyLogging Systems team and the Geoscience Victoria Core Library team is also gratefully acknowledged. In particular, we would like to acknowledge the support and assistance of the following people from the Geological Survey of Victoria (formerly Geoscience Victoria): Suzanne Haydon, Kathy Hill, Paul McDonald and Peter O'Shea, as well as many other former colleagues involved with the Gold Undercover initiative at Geoscience Victoria that made this work possible. In particular, we would like to thank Rob Duncan for a thorough peer review of the manuscript prior to submission. We would also like to thank Cassady Harraden and two anonymous reviewers for the AJES for their constructive suggestions. The manuscript has been much improved by their reviews.

PIMA™ is a trademark of Integrated Spectronics Pty Ltd. HyLogging™, HyLogger™, TSG™, TSA™ and TSG-Core™ are trademarks of the CSIRO.

Disclosure statement

No potential conflict of interest was reported by the authors.