Hellyer is a large (16 million tonne plus), Kuroko-style,
polymetallic, volcanogenic massive sulphide deposit in Cambrian high-K,
calc-alkaline, arc-like, volcanics in western Tasmania. Hydrothermal
alteration in the Hellyer host lavas is preserved in a near pristine
condition, overprinted only by low-grade prehnite-pumpellyite facies
The massive sulphide deposit occurs at the time break between an
albite porphyritic andesite footwall and a hangingwall basalt. Within
the Hellyer hangingwall basalt is a lava flow directly above the Hellyer
deposit with regionally high Ti/Zr -53, higher primary MgO, Ni, Cr and
lower primary Si02, Ti02, P205, Y, Zr, La and Nb than the surrounding
basalt. The structure which localized the Hellyer hydrothermal system
is thought to have also provided the locus for the extrusion of this
deeper sourced, more-primitive lava.
Not only is an excellent example of a hydrothermal feeder system
developed in the footwall andesite, but a plume shaped zone of chrome
green alteration occurs in the hangingwall basalt. The basalt was
extruded while the hydrothermal system was still active. The Cu, Pb, Zn
mineralized stringer-zone core consists of quartz barite surrounded by
quartz sericite pyrite grading outwards into a chlorite rich zone
including massive Mg-chlorite schists. This is surrounded by an
envelope zone of quartz sericite pyrite alteration. K-feldspar develops
across the outer margins of the envelope zone. The green hangingwall
plume consists of pervasive calcite-fuchsite, accessory Fe-chlorite
patches, calcite veining and increased quantities of interpillow pyrite.
Albite alteration extends out from the plume.
Element distributions highlight the stringer zone with a Na20 low,
and more complexly, with Si02, Fe203' MgO, S, Cu, Pb, Zn, Ba highs. CaO
and Sr depletion in the footwall is more widespread than the Na20
depletion and extends outside the stringer zone. In the hangingwall,
the plume is highlighted by increased S (2-4 times background S),
increased CaO (2 times background CaO), and elevated Ba. Na20 highs
trace albite alteration.
In the hangingwall calcite-fuchsite alteration there has been a
major mass addition of CaD, K20, A1203, Ba and depletion in Fe203' MgO,
and Si02 , with relative enrichment in As, Rb and Mn. Zr, Ti02, Y, Nb and
the rare-earth elements remain immobile as evidenced by unchanged ratios
of these elements regardless of the degree of alteration. In the
footwall stringer zone core there is some mobility of all elements.
Sulphur isotopes show a progressive decrease in 634S (pyrite)
values inwards and up the stringer zone from +13 per mil at depth
through +8 per mil to consistent values of +7 per mil in the orebody.
This is due either to an increase in oxygen fugacity towards the quartz
barite stringer-zone top and into a narrow oxygen fugacity field for the
deposition of the orebody, or to mixing a hydrothermal fluid with a
value of +7 per mil with a variable supply of reduced seawater sulphate.
Sulphur in light pyrite (-14 per mil) in hangingwall interpillow areas
Is produced from reduced seawater sulphate rapidly replenished by a
high seawater flux.