Nature and origin of the fluids responsible for forming the Hellyer Zn–Pb–Cu, volcanic-hosted massive sulphide deposit, Tasmania, using fluid inclusions, and stable and radiogenic isotopes

Abstract

The Hellyer massive sulphide deposit lies within the Mount Read Volcanics province of western Tasmania. Before mining, it consisted largely of pyrite, sphalerite, galena, arsenopyrite and chalcopyrite, and was overlain by discontinuous barite–sulphide and/or silica–sulphide assemblages. It overlay a downward-tapering cone of hydrothermally altered rocks that shows concentric mineral zonation and contains steeply inclined veins containing sulphide and/or barite. New laser ablation and existing conventional sulphur isotopic compositions of sulphides from the massive sulphide ore range from -5.0 per mil to 12.2 per mil. There is no significant spatial variation in sulphur isotopic composition in the sulphide ore, or evidence of significant change in the textural paragenesis, but there is considerable variation at millimetre scale within and between minerals. Apart from the few negative values, which may be of biogenic origin, the sulphur data can be explained by mixtures of sulphur reduced from seawater sulphate and that derived directly or indirectly from magma. The sulphur of the barite cap and the barite veins in the footwall (mostly 33.5–46.0 per mil) is probably also derived from seawater sulphate, and radiogenic 87Sr/86Sr values in the barite (0.70989–0.71144) suggest fluid circulation deep into the basement. delta 34 S values of disseminated sulphides in the footwall alteration cone are like those of overlying ore, the aqueous sulphur being totally reduced due to low fluid velocities and protracted rock interaction. However, unusually high delta 34 S sulphide values (up to 41.4 per mil) are found in some of the footwall vein sulphides, probably because in these veins the fluid velocities are enhanced and rock interaction limited, so that pyrite supersaturation may occur before reduction of entrained seawater sulphate is complete. High delta 34 S sulphide values (up to 45.6 per mil) also occur in ‘‘unaltered’’ volcanic rock outside the alteration cone and may be the product of local convection of seawater prior to, during, or (most probably) after massive sulphide mineralization. Fluid delta 18 O values calculated from isotopic analyses of quartz in footwall veins (8.5–11.8 per mil) and Th data range between 4.4 per mil and 3.1 per mil. Dolomite occurs with chlorite in the core of the footwall alteration cone immediately below the massive sulphide. Its isotopic composition (delta 13 C=1.5 to 2.8, delta 18 O=8.2–18.3 per mil) may reflect precipitation from an acid, seawaterderived, fluid having delta 13 C=1 per mil(the ambient seawater value), and delta 18 O ranging from 0 per mil to 6 per mil, or, alternatively, its composition is the result of mixing between modified seawater at 150 degrees C (delta 18 O=6 per mil) with 1% of fluid at 350 degrees C having delta 13 C= -6 and delta 18 O=0 per mil. Dolomites in the footwall veins (delta 13 C= -1.8 to 1.7, delta 18 O= 9.3–14.2 per mil) could be derived from an acid, seawater-derived fluid with negative delta 18 O values, or a fluid that has interacted with 600–700 Ma old, 13 C-enriched carbonates in the deep footwall during convective circulation. Both the chlorite-associated and the vein carbonates contain highly radiogenic Sr, possibly derived from Devonian metamorphic fluids. A re-interpretation of the fluid inclusion data of Khin Zaw et al. [Ore Geol. Rev. 10 (1996) 251] shows that there are three groups of fluids in the quartz of veins in the altered footwall, viz. (a) saline (6.6–14.8 wt.%) with Th = 170–246 degrees C, high K/Na, K/Ca and K/Fe values (fluid 1); (b) of similar salinity and temperature but with low K/Na, K/Ca and K/Fe values (fluid 2); and (c) an additional fluid of low salinity (2.9–7.0 wt.%) with Th = 289–322 degrees C, element ratios unknown (fluid 3). Fluid 1 has cation ratios like those of magmatic fluids in the K-silicate and phyllosilicate zones of the Panguna and Endeavour 26N porphyry copper deposits. Combined with the lack of alternative source of salts in the pre-ore rock sequences at Hellyer, fluid 1 is thought to be at least partly magmatic. Fluid 2 has cation ratios like those of modern black smoker and Kuroko ore-forming fluids, and was probably derived from both magmatic fluid and seawater; fluid 3 may be evolved seawater like that forming the deposits of the Hokuroku Basin. It is suggested that a pluton or plutonic complex of mixed crustal and lithospheric mantle parentage, like that of the volcanic rocks hosting the Hellyer orebody, was emplaced at several km depth below the deposit during faulting related to crustal extension. The heated zone over and around the pluton displaying plastic behaviour was sealed off from the overlying brittle zone in which groundwater (modified seawater) underwent convection due to heat transfer across the brittle-plastic boundary. The seal was broken intermittently due to tectonic extension or increased fluid pressure in the pluton, and magmatic fluid joined with convecting groundwater in the rising plume, leading to fluid mixing. Upward flow was focused on the Eastern Fault, the fracture system responsible for forming the basin in which the ore sulphides were sedimented.