Discovery of the West 45 volcanic-hosted massive sulfide deposit using oxygen isotopes and REE geochemistry

A number of innovative geochemical techniques were tested around the Thalan ga 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 gradi ents in the footwall rocks just beneath the ore horizon. By assuming a wate r delta O-18 value of 0 per mil, consistent with a seawater-dominated hydro thermal system, an alteration temperature was calculated for each sample. A quantitative determination of the mineralogy was used to work out a weight ed fractionation factor for each sample so that the variance attributable t o differences in mineralogy could be taken into account. Comparison of the temperatures to a sodium depletion index indicates that the delta O-18 halo is comparable in size to an alteration mineralogy halo, but the distributi on 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 lar ge number of stratiform quartz-hematite lenses (red jaspers). Rather than d rill test all of these occurrences, a geochemical index was determined to a ssist 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 Thalan ga, coincident with the delta O-18 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.