Wednesday, May 23, 2007

Paper out

Birch, W. D., Barron, L. M., Magee, C. and Sutherland, F. L. , 'Gold- and diamond-bearing White Hills Gravel, St Arnaud district, Victoria: age and provenance based on U - Pb dating of zircon and rutile', Australian Journal of Earth Sciences, 54:4, 609 - 628

Abstract: Investigation of coarse (42 mm) heavy-mineral concentrates from the White Hills Gravel near St Arnaud in western Victoria provides new evidence for the age and provenance of this widespread palaeoplacer formation. A prominent zircon – sapphire – spinel assemblage is characteristic of Cenozoic basaltic-derived gemfields in eastern Australia, while a single diamond shows similar features to others found in alluvial deposits in northeastern Victoria and New South Wales. Dating of two suites of zircons by fission track and U– Pb (SHRIMP) methods gave overlapping ages between 67.4+5.2 and 74.5+6.3 Ma, indicating a maximum age of Late Cretaceous for the formation. Another suite of minerals includes tourmaline (schorl – dravite), andalusite, rutile and anatase, which are probably locally derived from contact metamorphic aureoles in Cambro-Ordovician basement metapelites intruded by Early Devonian granites. U – Pb dating of rutile grains by laser ablation ICPMS gave an age of 393+10 Ma, confirming an Early Devonian age for the regional granites and associated contact metamorphism. Other phases present include pseudorutile, metamorphic corundum of various types, maghemite and hematite, which have more equivocal source rocks. A model to explain the diverse sources of these minerals invokes recycling and mixing of the far-travelled basalt-derived suite with the less mature, locally derived metamorphic suite. Some minerals have probably been recycled from Mesozoic gravels through Early Cenozoic (Paleocene – Eocene) drainage systems during various episodes of weathering, ferruginisation and erosion. Comparison between heavy-mineral assemblages in occurrences of the White Hills Gravel may allow distinction between depositional models advocating either separate drainage networks or coalescing sheets. Such assemblages may also provide evidence for the present-day divide in the Western Uplands being the youngest expression of an old (Late Mesozoic – Early Cenozoic) stable divide separating north- and south-flowing streams or a much younger feature (ca 10 Ma) which disrupted mainly south-flowing drainage.

I did the rutile U/Pb, most of the zircon U/Pb, and the rutile trace element analysis and interpretation. So if anyone out there has hard core stratigraphy questions, I'll have to pass them on to Bill.

I can't actually link my own paper from home, so anyone interested will have to look it up the old fashioned way.


Dan said...

Hooray! Happy to see your rutiles on paper. Well, in pdf anyway.


Dan in Tokyo

Pee esS. love the blog

Unknown said...

In the paper TABLE 1, the spinel is listed as Mr-rich? Did you mean Mg-rich? Also what MgO% range of values did you get? On dating the rutile did you variations on concordant ages as you drilled down? Given zoning in both zicrons and rutile, where did you positon the beams? Again did you get varitaitons depending on location within grain?

C W Magee said...

I never saw the spinel, yes, but it was correctable, all over, and no.
Busy this week, maybe more details later.