Possible origins and ages for sapphire and diamond from the central Queensland gem fields

o Mining of sapphire has been carried out for over 100 years on the central Queensland gem fieldso Zircon and the occasional diamond, like the sapphires, occur as clastic grainso The Hoy Basalt (a Tertiary basalt province of plugs and restricted flow remnants) was considered to be the source of the sapphire and zircono Age determinations on basalts of the Hoy Province indicate eruption at different times from the early Eocene to the Middle Mioceneo The recognition of sapphire and zircon bearing pyroclastic deposits at Bedford's Hill, the Divide and near Sheep Station Creek in the Rubyvale area suggests that most of the sapphire and zircon in the alluvial deposits came from the pyroclastics and not from the weathering of basalt Evidence points to sapphire and zircon-bearing felsic parental rocks, which were crystallised around the crust-mantle boundaryo The gem minerals were largely brought to the surface by pyroclastic eruptions, particularly during the Early to Middle Eoceneo Two groups of alluvial zircons (smaller, pale yellow crystals and large reddish brown crystals) give separate fission track ages. These suggest eruptive thermal resettings around 58 Ma and 20 Ma respectivelyo The apparent absence of substantial volcanism in the southern part of the Anakie Inlier during the Jurassic and Cretaceous probably reflects a cooler regional geothermo The occurrence of low-uranium zircon of Late Cretaceous age (66 Ma) in some of the gem field clastic deposits suggests that conditions may have been appropriate for the emplacement of diamond bearing material during this periodo ROBERTSON, ADoC & FL SUTHERLAND, 19920 Possible origins and ages for sapphire and diamond from the central Queensland gem fieldso Records of the Australian Museum Supplement 15: 45-540 et aio, 1964; Stephenson, 1976, 1990)0 45 The central Queensland gem fields (Figo 1) have been producing sapphire, zircon and occasional diamonds for more than 100 years (Robertson, 1980)0 The source of this sapphire was considered to be the Hoy Basalt by previous workers (Jack, 1892; Dunstan, 1902; Veevers Jack (1892) concluded that the basalt was the source of the sapphire after observing the association of pleonaste and sapphire in the alluvial depositso He also observed that zircon was prolific and was associated with 46 Records of the Australian Museum (1992) Supplement 15 gold in alluvial deposits beneath basalt at the Basalt Hill diggings north-west of Anakie. Although Jack was informed of the presence of small blue sapphires in the auriferous deposits none were found during his + ++++++++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + • + + + + + +