Abstract

A number of innovative geochemical techniques were tested around the Thalanga volcanic-hosted massive sulfide (VHMS) deposit in North Queensland, with the objective of being able to recognize a "near miss" in the exploration drilling in and around the mine.

Whole-rock oxygen isotope analyses were used to determine temperature gradients in the footwall rocks just beneath the ore horizon. By assuming a water δ18O value of 0 per mil, consistent with a seawater-dominated hydrothermal system, an alteration temperature was calculated for each sample. A quantitative determination of the mineralogy was used to work out a weighted fractionation factor for each sample so that the variance attributable to differences in mineralogy could be taken into account. Comparison of the temperatures to a sodium depletion index indicates that the δ18O halo is comparable in size to an alteration mineralogy halo, but the distribution of Na is somewhat unpredictable in the distal parts of the hydrothermal system. This study identified a hot spot 1 km west of the limit of the mine workings.

The Mount Windsor Volcanics, which host the Thalanga deposit, contain a large number of stratiform quartz-hematite lenses (red jaspers). Rather than drill test all of these occurrences, a geochemical index was determined to assist in ranking them. Elevated Ba, S, and Pb along with positive chondrite-normalized Europium anomalies were found to be characteristics of proximal jaspers as compared to barren jaspers. A red jasper outcrop west of Thalanga, coincident with the δ18O hot spot, was the most anomalous sample in this study. Subsequent drill testing of this target led to the discovery of the West 45 VHMS deposit.

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