Textural evolution of the Hellyer massive sulphide deposit.

The Hellyer zinc-lead-silver deposit of western Tasmania is a well preserved example of a volcanic-hosted massive sulphide. The deposit is hosted by intermediate-basic lavas and volcaniclastics of the Que-Hellyer Volcanics, the uppermost volcanic unit of the Cambrian Mt.Read Volcanics. The complete deposit, including the footwall alteration stringer zone, is preserved. The current complex morphology of the massive sulphide is due to the combination of primacy depositional irregularities, ductile Devonian folding and brittle Mesozoic faulting. Statistical analysis of available mine sample assays and subsequent geostatistical 3D grade modelling has revealed a classic metal zonation pattern. Whilst Cu and Fe are enriched towards the footwall, proximal to the central feeder zone recognised by Gemmell and Large (1992); Zn, Pb, Ag, Au, As and Ba are gradually enriched towards the distal hangingwall. The current observed metal distribution is interpreted to be substantially the same as the Cambrian distribution; Devonian deformation resulted in only very local remobilisation. Spatial analysis of macroscopic textures has shown a clear zonation, similar to the geometry of the metal distribution. Massive sulphide proximal to the central feeder zone is strongly recrystallised, but grades upwards and outwards to a featureless, massive texture and fmally to strongly banded ores at the hangingwall contact. A very detailed microtextural study of 174 polished thin sections was completed from samples selected on a 3D grid through the central part of the deposit. Two hundred and twenty-two different microscopic textures have been recognised, with their spatial occurrence and features documented in a comprehensive atlas. These textures have been placed into paragenetic groups ranging through early primitive deposition, in situ recrystallisation, intra-mound veining, upwards redeposition, thermal retraction, Devonian and Mesozoic deformation-related, and fmally, surface weathering. These paragenetic groups are zoned, similar to the metal zoning and macroscopic textures, around the central feeder in the footwall. Various depositional and recrystallisation processes are postulated in an overall model for textural evolution. Microprobe analyses of the major minerals from numerous samples have shown variation according to texture and position within the overall orebody zonation. Significantly, pyrite shows considerable reduction in trace element content as crystallinity increases towards the proximal base of the sulphide mound. Early sphalerite has a higher Fe content than the late varieties, early tetrahedrite has a higher Ag content than later generations and carbonates show increasing CaO content and decreasing FeO content passing from early to late textural types. Other minerals show more complex compositional variability. The classic metal and texture zonation patterns, together with evidence from detailed microprobe analysis lend support to a mound refining genetic model, similar to that proposed by Eldridge et al. (1983) for the Kuroko volcanic·hosted massive sulphide deposits. The Hellyer genetic model postulates that a hydrothermal system was focussed at the intersection of a normal graben fault with a transfer fault on the Cambrian seafloor. These faults tapped a deep heat source and as temperature increased, rising hot solutions saturated with base and precious metals and reduced sulphur, began to vent into the cold, oxygenated seawater. Initially, barite/anhydrite and cherty crusts were deposited on the seafloor overlying the core of the footwall alteration zone. These crusts, by partly capping the system, allowed higher temperature deposition of primitive melnikovite pyrite and sphalerite/wurtzite, by replacement of pre-existing sulphates, and within voids, just below the mound surface. As the mound grew, these depositional processes moved upwards and outwards, away from the central feeder. Much higher temperatures in the lower part of the mound, gradually recrystallised and refined the primitive pyrite, expelling contaminant trace elements to be redeposited in higher, cooler parts of the mound. The growing mound became unstable, depositing clastic massive sulphide in adjacent basins that were eventually enveloped by the expanding higher temperature hydrothermal system, recrystallising and partially destroying the original fragmental framework. When the mound reached its ultimate extent, rotation of the stress regime closed off access to the heat and fluid sources and temperatures in the system decreased. As volcaniclastic mass flows and pillow lavas buried and preserved the deposit, the waning phase led to further deposition oflower temperature mineralisation, increasingly deeper within the mound in available voids. Devonian deformation annealed and extended the ductile sphalerite· galena rich distal hangingwall zones and introduced tensile pull apart fractures in the more proximal pyritic zones. All minerals, except pyrite, were locally remobilised into newly created voids. Mesozoic brittle wrench faulting brecciated pyritic areas causing minor remobilisation of minerals into late narrow cracks that cut across all earlier textural features.