Textural evolution of the Hellyer massive sulphide deposit.

摘要

The Hellyer zinc-lead-silver deposit of western Tasmania is a well preserved example of audvolcanic-hosted massive sulphide. The deposit is hosted by intermediate-basic lavas andudvolcaniclastics of the Que-Hellyer Volcanics, the uppermost volcanic unit of the CambrianudMt.Read Volcanics. The complete deposit, including the footwall alteration stringer zone, isudpreserved. The current complex morphology of the massive sulphide is due to the combinationudof primacy depositional irregularities, ductile Devonian folding and brittle Mesozoic faulting.udStatistical analysis of available mine sample assays and subsequent geostatistical 3D gradeudmodelling has revealed a classic metal zonation pattern. Whilst Cu and Fe are enriched towardsudthe footwall, proximal to the central feeder zone recognised by Gemmell and Large (1992); Zn,udPb, Ag, Au, As and Ba are gradually enriched towards the distal hangingwall. The currentudobserved metal distribution is interpreted to be substantially the same as the Cambrianuddistribution; Devonian deformation resulted in only very local remobilisation.udSpatial analysis of macroscopic textures has shown a clear zonation, similar to the geometry ofudthe metal distribution. Massive sulphide proximal to the central feeder zone is stronglyudrecrystallised, but grades upwards and outwards to a featureless, massive texture and fmally toudstrongly banded ores at the hangingwall contact.udA very detailed microtextural study of 174 polished thin sections was completed from samplesudselected on a 3D grid through the central part of the deposit. Two hundred and twenty-twouddifferent microscopic textures have been recognised, with their spatial occurrence and featuresuddocumented in a comprehensive atlas. These textures have been placed into paragenetic groupsudranging through early primitive deposition, in situ recrystallisation, intra-mound veining,udupwards redeposition, thermal retraction, Devonian and Mesozoic deformation-related, andudfmally, surface weathering. These paragenetic groups are zoned, similar to the metal zoningudand macroscopic textures, around the central feeder in the footwall. Various depositional andudrecrystallisation processes are postulated in an overall model for textural evolution.udMicroprobe analyses of the major minerals from numerous samples have shown variation according to texture and position within the overall orebody zonation. Significantly, pyriteudshows considerable reduction in trace element content as crystallinity increases towards theudproximal base of the sulphide mound. Early sphalerite has a higher Fe content than the lateudvarieties, early tetrahedrite has a higher Ag content than later generations and carbonates showudincreasing CaO content and decreasing FeO content passing from early to late textural types.udOther minerals show more complex compositional variability.udThe classic metal and texture zonation patterns, together with evidence from detailedudmicroprobe analysis lend support to a mound refining genetic model, similar to that proposedudby Eldridge et al. (1983) for the Kuroko volcanic·hosted massive sulphide deposits. TheudHellyer genetic model postulates that a hydrothermal system was focussed at the intersection ofuda normal graben fault with a transfer fault on the Cambrian seafloor. These faults tapped a deepudheat source and as temperature increased, rising hot solutions saturated with base and preciousudmetals and reduced sulphur, began to vent into the cold, oxygenated seawater. Initially,udbarite/anhydrite and cherty crusts were deposited on the seafloor overlying the core of theudfootwall alteration zone. These crusts, by partly capping the system, allowed higherudtemperature deposition of primitive melnikovite pyrite and sphalerite/wurtzite, by replacementudof pre-existing sulphates, and within voids, just below the mound surface. As the mound grew,udthese depositional processes moved upwards and outwards, away from the central feeder.udMuch higher temperatures in the lower part of the mound, gradually recrystallised and refinedudthe primitive pyrite, expelling contaminant trace elements to be redeposited in higher, coolerudparts of the mound. The growing mound became unstable, depositing clastic massive sulphideudin adjacent basins that were eventually enveloped by the expanding higher temperatureudhydrothermal system, recrystallising and partially destroying the original fragmentaludframework. When the mound reached its ultimate extent, rotation of the stress regime closedudoff access to the heat and fluid sources and temperatures in the system decreased. Asudvolcaniclastic mass flows and pillow lavas buried and preserved the deposit, the waning phaseudled to further deposition oflower temperature mineralisation, increasingly deeper within theudmound in available voids. Devonian deformation annealed and extended the ductile sphalerite·udgalena rich distal hangingwall zones and introduced tensile pull apart fractures in the moreudproximal pyritic zones. All minerals, except pyrite, were locally remobilised into newly createdudvoids. Mesozoic brittle wrench faulting brecciated pyritic areas causing minor remobilisation ofudminerals into late narrow cracks that cut across all earlier textural features.