The interplay of groundwater and magmatic fluids in the formation of the cassiterite-sulfide deposits of western Tasmania

Abstract

The cassiterite-sulfide deposits of western Tasmania are spatially and temporally related to Devonian granitoids. The stable isotope and fluid inclusion data of these deposits are best explained by a genetic model in which Sn is leached from the granites by reduced, non-magmatic fluids mixed with late-stage magmatic fluids. These fluids mineralize the overlying dolomite horizons and subjacent faults by a combination of wallrock and boiling induced reactions. The model requires co-incidental release of volatiles and ingress of groundwaters to sustain the leach mechanism. This implies some specific structural/deformational controls on the process and an intimate association between cassiterite-sulfide deposits and volatile-rich granites. The sulfur isotope data show a wide range in values, from about −2 to 20 per mil, and are explained in terms of a near-zero magmatic component and three heavier country-rock sources: the Precambrian Oonah Formation, the Crimson Creek Formation and the Mount Read Volcanics. Following this interpretation, the heavy sulfur isotope values from the Heemskirk Granite (Hajitaheri, 1985) and the Federal-Bassett Fault at Renison (Kitto, 1994) indicate ingress of groundwater into the granites. Salinities of fluid inclusions from the deposits show a restricted range (around 5 to 15 weight percent NaCl equivalent) compared with the range shown by inclusions from alteration zones within the granites (around 5 to 40 weight percent NaCl equivalent). It is suggested that the wide range of salinities in the granites is generated in the critical region of the NaClH 2O system at temperatures around 400 to 500°C and pressures around 400 to 500 bars. The limited range of salinities in the deposits reflects cooling to sub-critical conditions. Cooling through the critical region promotes homogenization of groundwater and magmatic vapor and brine. Condensation of magmatic volatiles within this zone of mixing maintains acidity and promotes fluid-rock reaction. The geochemistry of the granites underlying the Renison Bell area (Bajwah et al., 1995) is interpreted in terms of two granites. The geological relationships indicate the more mafic phase, the Renison Granite, was intruded by the more fractionated Pine Hill Granite. Fluids from the Pine Hill Granite sericitised and tourmalinized both the Renison Granite and Pine Hill Granite along their mutual contact, generating a broad north-east trending zone of alteration. This contact may have been offset by the Federal-Bassett Fault before the Renison deposit was formed. It is suggested that groundwater flow was essentially constrained within this NW-SE trending structure and subsidiary structures. Magmatic volatiles, emanating from deep within the Pine Hill Granite, acidified the reduced groundwater and promoted leaching of the granite. The siting of the Renison deposit on the margin of the Pine Hill Granite is consistent with a strong up-flow zone on the margin of a cooling pluton. The very constant S-isotope signature in the deposit is consistent with a large-scale homogenization process operating over a sustained period. The very large size of the Renison deposit is accounted for by a long-lived groundwater system maintained by the high heat flow from a fractionated granite.