Occurrences and origins of gem zircons in eastern Australia
Abstract
In eastern Australia, zircons are common in alluvial deposits derived from Tertiary volcanic rocks. They are typically accompanied by corundum, pleonaste spinel and ilmenite ('the zircospilic association'). Although most occur in or near granite areas, fieldwork and dating confirm alkali basalts and some trachytes as their hosts; some even being found in situ. Most crystals show corrosion effects from their transporting magmas. A wide range of zircon crystal habits suggests diverse but, as yet, unknown sources. Possibilities include: (a) accidental sources from syenitic intrusives, plutonic cumulates and pegmatites, and (b) cognate origins in fractionated basaltic magmas, particularly their felsic end members. Dry, peraluminous alkaline magmas may be responsible for most of the large zircons. Eight groups are described on their physical characteristics. Most are (101)-pyramid dominant forms with short prisms. Variations in the incidence of crystal types show trends that may record changes in magma composition as well as temperature profiles. Felsic intrusions associated with Mesozoic and Cainozoic 'hot spot' trails form potential reservoirs to provide zircon xenocrysts in later basalts. The relative contribution of these to older Palaeozoic granitoid zircon sources is uncertain, pending detailed isotopic work.
