Geology, genesis, and exploration implications of the foot wall and hanging-wall alteration associated with the Hellyer volcanic-hosted massive sulfide deposit, Tasmania, Australia

Hellyer is a large (16.2 million metric tons), high-grade (13.9% Zn, 7.1% P b, 0.4% Cu, 168 g/t Ag, 2.5 g/t Au), sea-floor, mound-style, polymetallic v olcanic-hosted massive sulfide (VHMS) deposit located in the Mount Read Vol canics of western Tasmania. The deposit is hosted by the Que-Hellyer Volcan ics, a sequence of late Middle Cambrian mafic to felsic coherent volcanics and polymict volcaniclastics. Hydrothermal alteration occurs in the regional footwall, immediate footwall , and hanging wall. Alteration in the regional footwall is confined to patc hy quartz, albite, and chlorite, with minor sericite, epidote, and hematite . Underlying Hellyer is a zoned alteration pipe with a central siliceous co re (quartz-sericite), which passes into zones of chlorite, chlorite-carbona te, sericite-chlorite, and finally sericite-quartz (stringer envelope zone) on the margin. Overlying the central part of the deposit, within the hangi ng-wall basalt, is a distinctive and zoned alteration plume. Five alteratio n zones have been identified: fuchsite, chlorite, carbonate, quartz-albite, and sericite. Fuchsite-dominated alteration occupies the central portion o f the hanging-wall alteration plume. Chlorite and carbonate alteration surr ounds the fuchsite zone with carbonate zones forming near to the ore deposi t and chlorite zones extending above and lateral to the carbonate. Outward is quartz-ablite alteration, which extends laterally into distal sericite a lteration. Mass-change calculations for the footwall and hanging wall indicate that, i n general, the foot-wall alteration zones are depleted in CaO, Na2O, La, Sr , Ni, Cr, and V but have enrichments of Fe2O3, MnO, MgO, K2O, S, and most m etals. Compared to the host basalt, the hanging-wall alteration has gained CaO, K2O, Na2O, CO2, S, in Rb, Ba, Ag, As, Mo, Sb, Cs, and TI, while Fe2O3, MnO, MgO, P2O5, La, Sr, Pb, Zn, Th, U, Cd, and Nd are depleted. CaO, Na2O, Cr, V, and Ni are depleted in the footwall andesite but enriched in the ha nging-wall alteration plume. This relationship suggests that these elements were sourced from the breakdown of feldspars, pyroxenes, and andesitic gro undmass of the footwall lithologies and transported in the hydrothermal flu id into the overlying basalt and precipitated as albite, calcite, and white micas in the hanging-wall alteration. The development of alteration associated with the Hellyer VHMS deposit occu rred in three stages. Stage 1 regional footwall alteration was formed by un focused hydrothermal convection of seawater down into the recently deposite d volcanic pile at temperatures between approximately 250 degrees and 200 d egreesC and at low to moderate water/rock ratios. Stage 2 alteration formed by structurally controlled fluid flow from a deep intensifying hydrotherma l convection system and created the footwall alteration pipe. Decreasing wa ter/rock ratios and temperatures, over a range of 350 degrees to less than 200 degreesC, led to the development of the concentric alteration pipe mine ral zones. Based on modeling of whole-rock delta O-18(k)' values, geochemic al modeling, and mineral assemblages, the siliceous core is interpreted to have formed at temperatures near 350 degreesC, the chlorite-rich alteration zone at 300 degrees to 250 degreesC, and the outer sericite-rich alteratio n at temperatures of 250 degrees to 150 degreesC. The final, synmineralizat ion fluid-flow event, stage 3, created the hanging-wall alteration plume. A fter rapid burial of the deposit by basalt, continuation of upward hydrothe rmal fluid flow created the zoned hanging-wall alteration. Distribution of hanging-wall alteration assemblages suggests a temperature gradient from ap proximately 250 degreesC for the fuchsite zone to lover temperatures (150 d egreesC) for the distal quartz-albite and sericite alteration zones. Alteration mineralogy, mineral chemistry, lithogeochemistry, and stable iso tope characteristics of the footwall and hanging-wall alteration have been combined into a comprehensive set of vectors, which can be used in explorat ion for VHMS deposits in similar geologic settings.