A comparison of the geology and mineralization of the Balcooma and Dry River South volcanogenic massive sulfide deposits, northern Queensland

Abstract

The Balcooma and Dry River South deposits are two recently discovered volcanic-hosted massive sulfide deposits hosted by metasediments and metavolcanics of the Cambro-Ordovician Balcooma metamorphic belt. The Balcooma deposit occurs as three stratigraphically stacked sulfide lenses in a deformed pelitic lens within a sequence of metagraywackes with minor interbedded volcaniclastic lenses and pelites, whereas the Dry River South deposit occurs at the contact between intensely altered footwall volcanics and hanging-wall metagraywacke.At the Balcooma deposit the upper and lower sulfide lenses are zinc rich with local zones of copper-rich mineralization that represent feeder zones. The central lens is copper rich with lesser zinc-rich mineralization. This lens occurs near the top ofa chloritic alteration pipe that cuts the stratigraphic position of the lower zinc-rich lens. At Dry River South metal zonation is less well developed although the repetition of copper to zinc metal zonation in several drill holes indicates the possibility of structural thickening.In both deposits gold has a strong association with copper. At Dry River South and in the zinc-rich lenses at Balcooma silver has a strong association with lead, but in the Balcooma copper-rich lens, silver has a strong association with copper which indicates that chalcopyrite is the dominant silver host. In zinc-rich massive sulfide at Dry River South bismuth and silver strongly correlate with lead, but no significant correlation between silver and antimony was observed, which indicates that the most significant host to silver is argentiferrous galena. Associations of tin and gallium with zinc were also noted at Dry River South.Major ore minerals present at both deposits include pyrite, chalcopyrite, sphalerite, and galena. Magnetite and pyrrhotite are locally important, especially at Balcooma where large bodies of massive magnetite are present. The deposits are unusual for Phanerozoic deposits in that they lack barite gangue.Sulfur isotope results are consistent with seawater sulfate reduction as the source of hydrothermal sulfur, although a significant input of either rock or magmatic sulfur is envisaged. The deposits are interpreted to have formed by the interaction of moderately acid, possibly reduced hydrothermal fluid with reduced local seawater. Copper-rich mineralization formed at a higher temperature, whereas zinc-lead-rich mineralization formed at a lower temperature in analogy with the kuroko deposits of Japan.