Multi-Scale Characterisation of Australia’s Deepest Drill Hole

Birchall, Renee1, Pearce, Mark1, Walshe, John1, Powell, Helen1, Shelton, Tina1 & Woodall, Katie1.

1CSIRO Mineral Resources, Kensington, WA, Australia.

The Jundee Gold Camp (Jundee), located in the northern Yandal Greenstone Belt of the Yilgarn Craton, is relatively understudied compared to the adjacent historic Wiluna Gold Camp. In 2018, Northern Star Resources drilled an Australian-record-breaking 3,217 m drill hole through Jundee’s Zodiac Discovery to complement a recently acquired 3D seismic dataset. In this study, the entire stratigraphy of this world-class gold camp was characterised by combining 3.2 km of continuous chemistry measurements made using the Minalyze X-ray Fluorescence (XRF) Core Scanner and over 400 mineral maps acquired using a Tescan Integrated Mineral Analyzer (TIMA) scanning electron microscope. The Minalyze XRF dataset was used to inform the sampling for mineral maps and additional portable X-Ray Fluorescence (pXRF) measurements were taken, to allow the XRF data to be used to benchmark future pXRF analyses on site. The high-spatial resolution of the Minalyze XRF enabled the lithogeochemistry of the 3.2 km of stratigraphy to be characterized in detail. Locally, the Jundee stratigraphy consists of two Archaean basalt-sediment sequences that have been intruded by multiple dolerites and later by lamprophyres, porphyries, granodiorites, granites and further, Proterozoic dolerite dykes. The dolerite and basalt units in the stratigraphy were classified using spatial patterns of elemental variations resulting from igneous fractionation, which can be used to fingerprint individual dolerites in the stratigraphy. Comparison of the Minalyze XRF and pXRF datasets support using immobile elements (Zr, Cr and Ti) to classifying the lithostratigraphy at Jundee. The automated mineralogy data were integrated with the Minalyze XRF and concurrent pXRF lithogeochemical datasets to discriminate between spatial variations in mineralogy caused by lithological changes and those associated with alteration. Interrogation of textural information available in the automated mineralogy phase maps is critical in underpinning the key metamorphic and hydrothermal alteration assemblages in the stratigraphy. The primary gold-bearing mineral assemblage at the Zodiac Discovery is summarised by chlorite (clinochlore and chamosite), calcite, pyrite, titanite, actinolite ± arsenopyrite ± scheelite ± tourmaline (dravite and schorl). Quantifying changes in mineral assemblages and their paragenetic relationships provides information on fluid compositions and pressure-temperature conditions during metamorphism, hydrothermal alteration and mineralisation events. At Jundee, four events were defined through the TIMA SEM method, which may or may not be temporally continuous: Stage 1a: Metamorphic assemblage (greenschist to amphibolite facies), Stage 2a: Alteration assemblages from K-rich fluids, Stage 2b: Alteration assemblages from CO2-bearing fluids (± Au), and, Stage 2c: Assemblages from late, low-CO­2 fluids. Further, fluid pathways and during mineralisation are easily identified because of the high spatial resolution, and quantitative nature of the techniques used.


Biography

Renee is a geoscientist who has expertise in applied research of Archaean orogenic gold systems. Renee joined CSIRO in 2015 from industry where she worked across Western Australia in both mining and exploration settings.

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