Contrasting composition of metasomatic and metamorphic scapolite in the Eastern Fold Belt, Northwest Queensland, Australia

摘要

The Proterozoic Eastern Fold Belt (EFB) of the Mount Isa Inlier, Australia, preserves one of the largest areas of scapolite-rich rock in the world, and is comparable to several other districts of similar age and metallogenic affinity. The close temporal and spatial association between Cl-rich scapolite and Cu-Au mineralization in the EFB implies that scapolite could be a useful indicator of mineralized systems. The occurrence of metamorphic scapolite and multiple generations of metasomatic scapolite apparently unrelated to Cu-Au mineralization complicate this relationship. However, we propose that scapolite of various generations can be differentiated on textural, chemical and mineralogical grounds, where metamorphic scapolite (marialite; Ma_(5-50) ) typically contains less Cl and Na and more Ca, Al and C than those derived via metasomatism (Ma_(50-95)). Among the metasomatic scapolites, those associated with Cu-(Au) mineralization have higher Cl and Na (Ma_(>70)). Texturally, metasomatic scapolite forms as irregular rounded growths, or as subhedral to euhedral vein-fill, as opposed to metamorphic scapolite that typically formed cauliflower-shaped porphyroblasts. A primary rock control is also evident where metamorphic scapolite is restricted to calc-silicate rocks (c. 1.7 Ga) as opposed to metasomatic scapolite that is hosted in a range of rock types of various age (c. 1.7-1.5 Ga).