Abstract

Key areas for diamond and sapphire exploration may be defined within the New England gem fields of eastern Australia through an understanding of the geologic and geomorphic processes that operated within the Tertiary Central volcanic province.At least two distinct periods of volcanic activity--one at 32 to 38 Ma and the other at 19 to 23 Ma--were controlled by south-southeast-north-northwest-trending fracture sets revealed by Landsat data. The locus of volcanism stepped westward with time from the Glen Innes-Ben Lomond-Guyra area (the East Central province) to the area west of Inverell and Armidale (the West Central province). Drainage and prevolcanic topography indicate a radial pattern of drainage associated with East Central province uplift and volcanism (32-38 Ma). This was modified by the later West Central province volcanism (19-23 Ma) which established its own radial pattern. The division of the Central province defines broad regions suitable for sapphire and diamond exploration.Highly prospective diamond-bearing "deep leads" associated with the paleo-Gwydir River can be traced under the younger basalts of the West Central province via reconstruction of the prevolcanic topography.In the East Central province the posteruptive fluvial history is seen as vital in controlling the concentration of economic sapphire deposits from lower grade source rocks. The sapphire-bearing placer deposits occur as shoestring-type accumulations occupying channels within broad, flat, basalt-filled valleys. Higher grades generally correspond to areas where channels are deepest and sapphire grades in excess of 500 g per m 3 have been recorded. Heavy minerals associated with sapphire in the placer deposits are zircon, pleonaste, chromium spinel, titanium magnetite, magnetite, ilmenite, and rare chrysoberyl. Additional minerals found in nearby in situ basaltic soils are olivine, clinopyroxene, enstatite, and amphibole, with zircon and sapphire being extremely rare.Investigation of heavy mineral suites from 12 collection sites indicates significant variability in mineral species within and between particular drainage catchments. Visual characteristics of the sapphires also vary within and between catchments. This implies that the sources are local to the placer deposits. The similarity in composition of minerals from soil and placer sites indicates that the sapphire-bearing placers have formed through extensive reworking of the immediately surrounding alkali basaltic and volcaniclastic rocks. Concentration was largely through vertical movement, with fine or light material being winnowed downstream leaving behind a heavy gem-bearing residue, within low-gradient, low-energy drainage systems.

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