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Discovery of the West 45 Volcanic-Hosted Massive Sulfide Deposit Using Oxygen Isotopes and REE Geochemistry

Craig Miller

Thalanga Copper Mines Pty. Ltd., P.O. Box 873, Charters Towers, Queensland 4820, Australia

Scott Halley

Goldfields Exploration, P.O. Box 322, Victoria Park, Western Australia 6979, Australia

Geoff Green

Mineral Resources Tasmania, P.O. Box 56, Rosny Park, Tasmania 7018, Australia

Mel Jones

De Re Metallica Pty. Ltd., 25 Kedumba Crescent, North Turramurra, New South Wales 2074, Australia

Corresponding author: e-mail, crjsmiller{at}bigpond.com

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 ¹⁸O 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 ¹⁸O 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 ¹⁸O 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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