research1

 

ABSTRACT

Basaltic and volcaniclastic rocks in the East Central Volcanic Province, New South Wales, are potential sources of the alluvial sapphires being mined in the New England Gem Fields. The associated drainage catchment areas generally contain sapphire, but only four major catchments contain rich deposits: Frazers, Kings Plains, Reddestone and Marowan. These catchments are thus the most likely targets for sapphire source rocks. The Mount Buckley basalt flows and intrusives divide Kings Plains Creek and Swan Brook catchment areas for alluvial sapphire. Whole-rock analyses show mostly alkaline to strongly alkaline types becoming more undersaturated with time. The top flows are predominantly basanite and nepheline hawaiites whilst the lowest are predominantly alkali olivine basalts and hawaiites. Ultramafic xenoliths in several flows suggest rapid movement from upper mantle levels. Marked variation in major and trace elements between groups of flows suggests that flows were intercalated from different levels. Variation diagrams do not distinguish the 32 to 39 Ma sapphire-associated eastern Central Province basalts from the 19 to 23 Ma sapphire-barren western Central Province alkali-basalts. The Central Province compositional fields also overlap the volcanic provinces in north-eastern Australia, known to contain sapphire (Atherton, McBride and Chudleigh), as well as those believed to be barren. These Australian compositional fields also overlap the corundum-bearing and corundum-less fields for the south-east Asian volcanic provinces. Major and minor element chemistry of basaltic rocks is not a useful exploration tool for discriminating sapphire-bearing from sapphire-barren volcanic provinces, nor for recognising potentially high-grade areas within a sapphire-bearing province. This suggests that basaltic magmas are not the sapphire parent rocks, but only one of their potential carriers

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research1

 

ABSTRACT

Three Cainozoic intraplate volcanic suites in the Bingara to Inverell area, northeastern New South Wales, have been discriminated on the basis of differing geophysical responses and contrasting K–Ar ages. Major isotopic and chemical characteristics can also be used to distinguish the three suites. These newly defined suites are the Middle Eocene–Early Oligocene Maybole Volcanic Suite; the Late Oligocene–Early Miocene Delungra Volcanic Suite; and the Middle Miocene Langari Hill Volcanic Suite. Four basaltic volcanic units within the Delungra Volcanic Suite have also been distinguished: Mount Russell Volcanics; Derra Derra Volcanics; Inverell Volcanics; and Bingara Volcanics. The Maybole Volcanic Suite is dominated by mafic volcanic rocks of alkaline affinity. These rocks include hawaiite, transitional basalt, basanite and rare phonolite (not included in this study). Volcanogenic and non-volcanogenic sedimentary units are minor but significant components, hosting world-class concentrations of sapphires in the Inverell–Glen Innes region. The Maybole Volcanic Suite occupies the eastern portion of the study area, forming ridges that outline the radial drainage pattern of the deeply eroded Eocene–Oligocene Maybole shield volcano. The Delungra Volcanic Suite is geochemically diverse and consists of alkaline members (Inverell and Bingara Volcanics) and tholeiitic members (Mount Russell and Derra Derra Volcanics). These are dominated by mafic lava flows with minor interflow sedimentary horizons. The Delungra Volcanic Suite forms broad elevated plains and prominent plugs in the central and western portions of the study area. Diamond occurrences in the Bingara district are spatially associated with the Bingara and Derra Derra Volcanics. The Langari Hill Volcanic Suite consists of a mafic tholeiitic lava flow that is spatially restricted to a prominent east–west ridge east of Inverell overlying the Maybole Volcanic Suite. The Langari Hill Volcanic Suite is significantly younger than the Maybole and Delungra Volcanic suites and represents the youngest recognised volcanic episode in the Bingara–Inverell area.

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