Stringer system and alteration zones underlying the Hellyer volcanic-hosted massive sulfide deposit, Tasmania, Australia

Underlying the 17 million metric ton, high-grade, polymetallic Hellyer volcanic-hosted massive sulfide deposit is a well-developed and preserved alteration zone and stringer vein system. Mineralogical zoning exists within a footwall alteration pipe, with a central siliceous core giving way to zones of chlorite, chlorite-carbonate, sericite, and finally sericite-quartz (stringer envelope zone) on the margins. Pervasive alteration started with development of sericite-quartz that was subsequently overprinted by sericite, then chlorite, and finally quartz as hydrothermal activity intensified in the center of the alteration zone. Compared to unaltered footwall andesite, the siliceous core is enriched in SiO2, K2O, MnO, Y, Fe2O3, and S but depleted in Na2O and CaO. Additions of MgO, MnO, Fe2O3, S, and Y and losses of Na2O and CaO occur in the chlorite zone. In the sericite zone SiO2, MgO, K2O, MnO, Fe2O3, and S are gained but Na2O and CaO are depleted. Only Na2O is depleted whereas MnO, K2O, P2O5, S, Y, and SiO2 are gained in the stringer envelope zone. The alteration index (AI) value [(K2O + MgO)/(K2O + Na2O + CaO + MgO)] X 100 becomes progressively larger, with increasing intensity of alteration, from the unaltered andesite (AI = 36) to the siliceous core (AI = 91). The Hellyer stringer zone contains distinct thick synmineralization veins that represent the channelways for hydrothermal solutions passing up through the alteration zone to the sea floor. Eight stages of veining are distinguished in the stringer zone: one stage of premineralization (stage 1), three stages of synmineralization (stages 2A, 2B, 2C), and four stages of postmineralization veining (stages 3-6). Synmineralization veining closely follows the formation of the pervasive footwall alteration and is most intense in the siliceous core. Metal zoning indicates the presence, and alignment, of several feeder systems underlying the deposit. The main feeder is located under the middle of the deposit with secondary feeder systems underlying the southern and northern portions of the deposit. Iron and copper are concentrated in the cores of the feeders with zinc and lead, silver, gold and arsenic, and barium becoming increasingly dispersed away from the centers of hydrothermal activity. Distinct delta-S-34 variations exist between vein stages within the stringer and stringer envelope zones. The main base metal-rich stage 2B veins display lower sulfur isotope values (3.88. 9 parts per thousand, mean = 6.9 parts per thousand) than the earlier stage 2A veins (5.7-11.1 parts per thousand, mean = 9.9 parts per thousand). The delta-S-34 values for stage 2A veins in the stringer envelope zone are significantly higher delta-S-34 Values (11.9-40.7 parts per thousand, mean = 25.0 parts per thousand) than those from the stringer zone. Variations in delta-S-34 can be explained by a model where the hydrothermal fluid initially consists of totally to partially reduced seawater sulfate that evolves into a fluid dominated by igneous sulfur (ore-forming fluid) as the convection system intensified and penetrated deeper into the footwall. At the end of the hydrothermal system's life sulfate becomes more abundant in the fluid, and eventually dominant.