The Hellyer
and Mt. Chalmers deposits are mound-style volcanic-hosted massive sulphide
(VHMS) deposits in Australia. Textural, petrographic and microthennometric
investigations of fluid inclusions in the Hellyer stringer system
indicate that Type I, primary, liquid-vapour inclusions are
10 - 15 J.UIl in size, and yield homogenisation temperatures of
170 - 220C in early 2A veins, 165 - 322C
in main-stage 2B veins and 190 - 256C in late-stage 2C veins.
These data suggest a waxing and waning thermal history. However, the average salinity
remained between 8 - 11 NaCl equiv. wt % in all
Stage 2 veins. At Mt Chalmers, Type I inclusions up to 20 urn are found in quartz from
the mineralised zone, and these inclusions yielded homogenisation temperatures of 160
- 268C and salinities of 5 - 8 NaCl equiv. wt %.
Laser Raman spectroscopic (LRS) analysis indicates the presence of
CO2 (<1 mole %) in the hellyer and mt. chalmers vhms systems.>
Semi-quantitative SEM/WDS microprobe analyses of fluid inclusion decrepitates
indicate that the Hellyer and Mt Chalmers ore fluids were enriched in potassium
and calcium but depleted in magnesium relative to seawater. PIXE microanalysis
of fluid inclusions in quartz also indicates a significant base metal
concentration in these fluids. Cation composition and higher salinities relative
to seawater suggest that recycled seawater alone cannot be the sole source of
the ore fluids. High base metal content and the presence of CO2 in
the fluid inclusions imply that magmatic input of ore metals during seawater
leaching of the footwall volcanic pile is a distinct
possibility.