Adularia in epithermal veins, Queensland: morphology, structural state and origin

Abstract

Four types of adularia (i.e. sub-rhombic, rhombic, tabular and pseudo-acicular) are recognised from examination of samples from ten epithermal vein deposits and prospects in Queensland, based on morphology of the individual crystals. Further investigation of the structural state of adularia reveals that each group has some specific features in terms of the degree of Al/Si disordering, which can be related to various crystallisation conditions and the thermal history. Sub-rhombic adularia is commonly 2–4 mm in size and subhedral with more or less rhombic outlines, and has a relatively ordered Al/Si distribution (2t₁ > 0.84), reflecting slow crystallisation conditions. Adularia of this type, in association with coarse-grained quartz, is predominant at deep levels of epithermal systems where boiling is initiated in an environment of low permeability and the fluid is slightly supersaturated with respect to adularia and quartz. Tabular adularia, characterised by its lath-shape and disordered structure (2t₁ values ranging from 0.64 to 0.74), is likely to have formed when the fluid moves up to a more permeable environment and starts boiling violently. Relatively high temperatures and rapidly changing conditions account for its special morphology and disordered structure. Rhombic adularia, showing very small crystal size (< 0.2 mm) with euhedral rhombic form, has an intermediate degree of Al/Si ordering. Pseudo-acicular adularia is interpreted as pseudomorphs after carbonate, and its high Al/Si ordered structure is attributed to the presence of a carbonate precursor. These two types of adularia commonly occur within crustiform and colloform bands in association with high grade ore, and chalcedony or fine-grained quartz which often displays various recrystallisation textures. It is most likely that adularias of these two types are formed when extensive boiling is protracted. Microprobe analyses indicate the composition of all adularia types close to pure KAlSi₃O₈. Sericite- and/or carbonate-altered adularias consistently display more ordered structures, suggesting that the post-crystallisation thermal regime affects the structural state of altered adularia, even at temperatures as low as in epithermal environments.