Alteration at the Olympic Dam IOCG–U deposit: insights into distal to proximal feldspar and phyllosilicate chemistry from infrared reflectance spectroscopy

ABSTRACT

Twenty thousand metres of diamond drill core representing a 14 km cross-section from weakly to intensely altered Roxby Downs Granite through the Olympic Dam Breccia Complex, host to the Olympic Dam iron-oxide–copper–gold–uranium deposit in South Australia, was analysed using the HyLogger-3 spectral scanner. Thermal and shortwave infrared spectroscopy results from 30 drill holes provide insight into the spatial relationships between quartz, orthoclase–microcline, albite–oligoclase and progressively changing sericite and chlorite compositions. The relative proportions of quartz, feldspars and phyllosilicates were mapped with thermal infrared spectroscopy. Variations in the chemistry of sericite and chlorite were extracted by proxy from their shortwave infrared spectral response, together with their relative spatial distribution. HyLogger scanning has revealed four deposit-scale mineralogical trends, progressing from least-altered Roxby Downs Granite into mineralisation where most of the feldspar has been replaced by sericite + hematite + quartz: (1) a progressive Al–OH wavelength shift of 2205 nm to 2210 nm for sericite, followed by a spatially rapid reversal corresponding to lower phengite/muscovite abundance ratios; (2) progressive Mg/Fe–OH wavelength shift of 2248 nm to 2252 nm reflecting an increase in the Fe:Mg ratio of chlorite; (3) increasing ratio of microcline to orthoclase followed by a rapid decrease; and (4) slightly decreasing ratio of albite to oligoclase followed by plagioclase destruction prior to albite replacement by sericite. The HyLogger feldspar results support recent petrographic evidence for hydrothermal albite and K-feldspar at the Olympic Dam deposit, not previously reported. The spectral results from continuous HyLogger scans also show that the microscopic observations and proposed feldspar replacement reactions are not locally isolated phenomena, but are applicable at the deposit and regional-scale. A modified quartz–K-feldspar–plagioclase ternary diagram utilising mineralogy interpreted from HyLogger thermal infrared spectra (QAP_TIR) diagram along with supporting data on the abundance ratios of orthoclase/microcline and albite/plagioclase, and the wavelength shifts in characteristic absorption features for sericite and chlorite, can be used as empirical vectors towards mineralisation within the Olympic Dam mineral system, with potential application to other IOCG ore-forming systems. Intrusion of Gairdner Dyke Swarm dolerite dykes into sericite ± hematite altered Roxby Downs Granite results in retrograde albite–chlorite–magnetite alteration envelopes (up to tens of metres thick) overprinting the original sericite ± hematite alteration zone and needs to be carefully evaluated to ensure that such areas are not falsely downgraded during exploration.

KEYWORDS:

• Olympic Dam IOCG–U deposit
• reflectance spectroscopy
• HyLogger-3
• microcline
• orthoclase
• albite
• plagioclase
• muscovite
• phengite

Acknowledgements

BHP Billiton Ltd supported the scanning of 20 000 m of core by providing logistical support in shipping the core to and from Adelaide. The HyLogger 3 technology has been developed by CSIRO and funded by the Australian Government's National Collaborative Research Infrastructure Strategy through the agency of AuScope Pty Ltd. The original prototype was developed in the AMIRA Project P685 completed in 2002. Simon van der Wielen, who assisted with GOCAD modelling, Georgina Gordon, Les Tucker and Sam Williams along with Core Library staff made significant contributions in data collection and processing at various stages of the project. The contributions of the editors, Garry Davidson and an anonymous reviewer are also gratefully acknowledged.

HyLogging, HyLogger, HyLogger-3, TSG and TSA are trademarks of CSIRO. GOCAD is a trademark of Paradigm.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary Papers

HyLogger summary results from 30 drill holes along a 14 km transect through the OD mine. Each graph with its drill-hole name displays, on the upper graph, the TIR mineralogy and, on the lower graph, the SWIR mineralogy plotted as a histogram on 5 m intervals. The minerals are reported as relative weight percent, and the plots have been filtered to remove component minerals representing less than 5% of the graph.