Geology of the West Coast Range of Tasmania, Part III-Porphyroid Metasomatism

Quartz and felspar porphyries and associated greenschists of the West Coast Range are described.Felspar porphyries are considered to have originated by the felspathising of spilitic lavas, lava breccias and related greywackes during a period of progressive metamorphism. Metamorphic mineral assemblages of the greenschist and albiteepidote- amphibolite facies are recognized in the porphyries and are identified with increasing degrees of porphyritising of host rocks. Sharp contacts between chlorite-sericite schists and quartzites are regarded as metamorphic in origin and as indicating the replacing action of a chloritising and sericitising front. Haematite rich rocks and low grade pyritic ores which respectively occupy narrow zones above and below the schist quartzite contact are regarded as representing minor haematlsing and pyritising fronts.
Sulphide and silicate metasomatism are closely related to the structure of the range and it appears that the advance of fronts has been engendered and controlled by the upward fiow of heated waters through fault fissures and pore spaces. Metasomatic replacement was effected by diffusion and exchange through intergranular films. This mechanism is thermodynamically efficient and allows adequate explanations of growth, and also of corrosion structures, of quartz and felspar crystals in the porphyries. During the advance of fronts there was extensive transport of material; metamorphism was not merely an adjustment to changed isogradial conditions. In particular, migrating solutions leached iron, copper and suiphur from the basic rocks and precipitated them in specific zones where they now form low grade ores.
After a period of rising temperatures and progressive metamorphism there followed an interval of cooling and retreat of thermal zones. Because of the presence of abundant water there was during this cooling phase an intense retrograde metamorphism of the recently formed felspathic rocks.
Hydration of these rocks resulted in the liberation of silica and thus gave rise to quartz porphyries and quartz sericite schists. Extreme pneumatolysis and leaching of silica from the porphyries gave rise to kaoJinised rocks and to veins of quartz and chert.
A similar kind of leaching of sulphides from the low grade ores, and of selective precipitation of sulphides in pipes and veins, gave rise to rich ore bodies.